The U.S. Air Force is a military service organized within the Department of the Air Force, one of the three military departments of the Department of Defense. The Air Force is headed by the civilian Secretary of the Air Force, who reports to the Secretary of Defense, and is appointed by the President with Senate confirmation. The highest-ranking military officer in the Department of the Air Force is the Chief of Staff of the Air Force, who exercises supervision over Air Force units, and serves as a member of the Joint Chiefs of Staff. Air Force combat and mobility forces are assigned, as directed by the Secretary of Defense, to the Combatant Commanders, and neither the Secretary of the Air Force nor the Chief of Staff have operational command authority over them.


Recently, the Air Force refined its understanding of the core duties and responsibilities it performs as a Military Service Branch, streamlining what previously were six distinctive capabilities and seventeen operational functions into twelve core functions to be used across the doctrine, organization, training, equipment, leadership, and education, personnel, and facilities spectrum. These core functions express the ways in which the Air Force is particularly and appropriately suited to contribute to national security, but they do not necessarily express every aspect of what the Air Force contributes to the nation. It should be emphasized that the core functions, by themselves, are not doctrinal constructs.


Assure/Dissuade/Deter is a mission set derived from the Air Force's readiness to carry out the nuclear strike operations mission as well as from specific actions taken to assure allies as a part of extended deterrence. Dissuading others from acquiring or proliferating WMD, and the means to deliver them, contributes to promoting security and is also an integral part of this mission. Moreover, different deterrence strategies are required to deter various adversaries, whether they are a nation state, or non-state/transnational actor. The Air Force maintains and presents credible deterrent capabilities through successful visible demonstrations and exercises which assure allies, dissuade proliferation, deter potential adversaries from actions that threaten US national security or the populations and deployed military forces of the US, its allies and friends.


Nuclear strike is the ability of nuclear forces to rapidly and accurately strike targets which the enemy holds dear in a devastating manner. If a crisis occurs, rapid generation and, if necessary, deployment of nuclear strike capabilities will demonstrate US resolve and may prompt an adversary to alter the course of action deemed threatening to our national interest. Should deterrence fail, the President may authorize a precise, tailored response to terminate the conflict at the lowest possible level and lead to a rapid cessation of hostilities. Post-conflict, regeneration of a credible nuclear deterrent capability will deter further aggression. The Air Force may present a credible force posture in either the Continental United States, within a theater of operations, or both to effectively deter the range of potential adversaries envisioned in the 21st century. This requires the ability to engage targets globally using a variety of methods; therefore, the Air Force should possess the ability to induct, train, assign, educate and exercise individuals and units to rapidly and effectively execute missions that support US NDO objectives. Finally, the Air Force regularly exercises and evaluates all aspects of nuclear operations to ensure high levels of performance.


Nuclear surety ensures the safety, security and effectiveness of nuclear operations. Because of their political and military importance, destructive power, and the potential consequences of an accident or unauthorized act, nuclear weapons and nuclear weapon systems require special consideration and protection against risks and threats inherent in their peacetime and wartime environments. The Air Force, in conjunction with other entities within the Departments of Defense or Energy, achieves a high standard of protection through a stringent nuclear surety program. This program applies to materiel, personnel, and procedures that contribute to the safety, security, and control of nuclear weapons, thus assuring no nuclear accidents, incidents, loss, or unauthorized or accidental use (a Broken Arrow incident). The Air Force continues to pursue safe, secure and effective nuclear weapons consistent with operational requirements. Adversaries, allies, and the American people must be highly confident of the Air Force's ability to secure nuclear weapons from accidents, theft, loss, and accidental or unauthorized use. This day-to-day commitment to precise and reliable nuclear operations is the cornerstone of the credibility of the NDO mission. Positive nuclear command, control, communications; effective nuclear weapons security; and robust combat support are essential to the overall NDO function.


Offensive Counterair (OCA) is defined as "offensive operations to destroy, disrupt, or neutralize enemy aircraft, missiles, launch platforms, and their supporting structures and systems both before and after launch, but as close to their source as possible" (JP 1-02). OCA is the preferred method of countering air and missile threats, since it attempts to defeat the enemy closer to its source and typically enjoys the initiative. OCA comprises attack operations, sweep, escort, and suppression/destruction of enemy air defense.


Defensive Counterair (DCA) is defined as "all the defensive measures designed to detect, identify, intercept, and destroy or negate enemy forces attempting to penetrate or attack through friendly airspace" (JP 1-02). A major goal of DCA operations, in concert with OCA operations, is to provide an area from which forces can operate, secure from air and missile threats. The DCA mission comprises both active and passive defense measures. Active defense is "the employment of limited offensive action and counterattacks to deny a contested area or position to the enemy" (JP 1-02). It includes both ballistic missile defense and air breathing threat defense, and encompasses point defense, area defense, and high value airborne asset defense. Passive defense is "measures taken to reduce the probability of and to minimize the effects of damage caused by hostile action without the intention of taking the initiative" (JP 1-02). It includes detection and warning; chemical, biological, radiological, and nuclear defense; camouflage, concealment, and deception; hardening; reconstitution; dispersion; redundancy; and mobility, counter-measures, and stealth.


Space superiority is "the degree of dominance in space of one force over another that permits the conduct of operations by the former and its related land, sea, air, space, and special operations forces at a given time and place without prohibitive interference by the opposing force" (JP 1-02). Space superiority may be localized in time and space, or it may be broad and enduring. Space superiority provides freedom of action in space for friendly forces and, when directed, denies the same freedom to the adversary.


Space Control is defined as "operations to ensure freedom of action in space for the US and its allies and, when directed, deny an adversary freedom of action in space. This mission area includes: operations conducted to protect friendly space capabilities from attack, interference, or unintentional hazards (defensive space control); operations to deny an adversary's use of space capabilities (offensive space control); and the requisite current and predictive knowledge of the space environment and the operational environment upon which space operations depend (space situational awareness)" (JP 1-02).


This is the passive, active, and dynamic employment of capabilities to respond to imminent or on-going actions against Air Force or Air Force-protected networks, the Air Force's portion of the Global Information Grid, or expeditionary communications assigned to the Air Force. Cyberspace defense incorporates CNE, computer network defense (CND), and CNA techniques and may be a contributor to influence operations. It is highly dependent upon ISR, fused all-source intelligence, automated indications and warning, sophisticated attribution/characterization, situational awareness, assessment, and responsive C2.


Cyberspace Support is foundational, continuous, or responsive operations ensuring information integrity and availability in, through, and from Air Force-controlled infrastructure and its interconnected analog and digital portion of the battle space. Inherent in this mission is the ability to establish, extend, secure, protect, and defend in order to sustain assigned networks and missions. This includes protection measures against supply chain components plus critical C2 networks/communications links and nuclear C2 networks. The cyberspace support mission incorporates CNE and CND techniques. It incorporates all elements of Air Force Network Operations, information transport, enterprise management, and information assurance, and is dependent on ISR and all-source intelligence.


Command and control is "the exercise of authority and direction by a properly designated commander over assigned and attached forces in the accomplishment of the mission. Command and control functions are performed through an arrangement of personnel, equipment, communications, facilities, and procedures employed by a commander in planning, directing, coordinating, and controlling forces and operations in the accomplishment of the mission" (JP 1-02). This core function includes all of the C2-related capabilities and activities associated with air, space, cyberspace, nuclear, and agile combat support operations to achieve strategic, operational, and tactical objectives.


Planning and Directing is "the determination of intelligence requirements, development of appropriate intelligence architecture, preparation of a collection plan, and issuance of orders and requests to information collection agencies" (JP 2-01, Joint and National Intelligence Support to Military Operations). These activities enable the synchronization and integration of collection, processing, exploitation, analysis, and dissemination activities/resources to meet information requirements of national and military decision makers.


Special Operations are "operations conducted in hostile, denied, or politically sensitive environments to achieve military, diplomatic, informational, and/or economic objectives employing military capabilities for which there is no broad conventional force requirement. These operations may require covert, clandestine, or low-visibility capabilities. Special operations are applicable across the ROMO. They can be conducted independently or in conjunction with operations of conventional forces or other government agencies and may include operations through, with, or by indigenous or surrogate forces. Special operations differ from conventional operations in degree of physical and political risk, operational techniques, mode of employment, independence from friendly support, and dependence on detailed operational intelligence and indigenous assets" (JP 1-02).


Airlift is "operations to transport and deliver forces and materiel through the air in support of strategic, operational, or tactical objectives" (AFDD 3–17, Air Mobility Operations). The rapid and flexible options afforded by airlift allow military forces and national leaders the ability to respond and operate in a variety of situations and time frames. The global reach capability of airlift provides the ability to apply US power worldwide by delivering forces to crisis locations. It serves as a US presence that demonstrates resolve and compassion in humanitarian crisis.


Aeromedical Evacuation is "the movement of patients under medical supervision to and between medical treatment facilities by air transportation" (JP 1-02). JP 4-02, Health Service Support, further defines it as "the fixed wing movement of regulated casualties to and between medical treatment facilities, using organic and/or contracted mobility airframes, with aircrew trained explicitly for this mission." Aeromedical evacuation forces can operate as far forward as fixed-wing aircraft are able to conduct airland operations.


Personnel Recovery (PR) is defined as "the sum of military, diplomatic, and civil efforts to prepare for and execute the recovery and reintegration of isolated personnel" (JP 1-02). It is the ability of the US government and its international partners to effect the recovery of isolated personnel across the ROMO and return those personnel to duty. PR also enhances the development of an effective, global capacity to protect and recover isolated personnel wherever they are placed at risk; deny an adversary's ability to exploit a nation through propaganda; and develop joint, interagency, and international capabilities that contribute to crisis response and regional stability.


Humanitarian Assistance Operations are "programs conducted to relieve or reduce the results of natural or manmade disasters or other endemic conditions such as human pain, disease, hunger, or privation that might present a serious threat to life or that can result in great damage to or loss of property. Humanitarian assistance provided by US forces is limited in scope and duration. The assistance provided is designed to supplement or complement the efforts of the host nation civil authorities or agencies that may have the primary responsibility for providing humanitarian assistance" (JP 1-02).


Building Partnerships is described as airmen interacting with international airmen and other relevant actors to develop, guide, and sustain relationships for mutual benefit and security. Building Partnerships is about interacting with others and is therefore an inherently inter-personal and cross-cultural undertaking. Through both words and deeds, the majority of interaction is devoted to building trust-based relationships for mutual benefit. It includes both foreign partners as well as domestic partners and emphasizes collaboration with foreign governments, militaries and populations as well as US government departments, agencies, industry, and NGOs. To better facilitate partnering efforts, Airmen should be competent in the relevant language, region, and culture.


The U.S. War Department created the first antecedent of the U.S. Air Force in 1907, which through a succession of changes of organization, titles, and missions advanced toward eventual separation 40 years later. In World War II, almost 68,000 U.S airmen died helping to win the war; only the infantry suffered more enlisted casualties. In practice, the U.S. Army Air Forces (USAAF) was virtually independent of the Army during World War II, but officials wanted formal independence. The National Security Act of 1947 was signed on on 26 July 1947 by President Harry S Truman, which established the Department of the Air Force, but it was not not until 18 September 1947, when the first secretary of the Air Force, W. Stuart Symington was sworn into office that the Air Force was officially formed.


The act created the National Military Establishment (renamed Department of Defense in 1949), which was composed of three subordinate Military Departments, namely the Department of the Army, the Department of the Navy, and the newly created Department of the Air Force. Prior to 1947, the responsibility for military aviation was shared between the Army (for land-based operations), the Navy (for sea-based operations from aircraft carriers and amphibious aircraft), and the Marine Corps (for close air support of infantry operations). The 1940s proved to be important in other ways as well. In 1947, Captain Chuck Yeager broke the sound barrier in his X-1 rocket-powered aircraft, beginning a new era of aeronautics in America.


During the early 2000s, the USAF fumbled several high profile aircraft procurement projects, such as the missteps on the KC-X program. Winslow Wheeler has written that this pattern represents "failures of intellect and – much more importantly – ethics." As a result, the USAF fleet is setting new records for average aircraft age and needs to replace its fleets of fighters, bombers, airborne tankers, and airborne warning aircraft, in an age of restrictive defense budgets. Finally in the midst of scandal and failure in maintaining its nuclear arsenal, the civilian and military leaders of the air force were replaced in 2008.


Since 2005, the USAF has placed a strong focus on the improvement of Basic Military Training (BMT) for enlisted personnel. While the intense training has become longer, it also has shifted to include a deployment phase. This deployment phase, now called the BEAST, places the trainees in a surreal environment that they may experience once they deploy. While the trainees do tackle the massive obstacle courses along with the BEAST, the other portions include defending and protecting their base of operations, forming a structure of leadership, directing search and recovery, and basic self aid buddy care. During this event, the Military Training Instructors (MTI) act as mentors and enemy forces in a deployment exercise.


In 2007, the USAF undertook a Reduction-in-Force (RIF). Because of budget constraints, the USAF planned to reduce the service's size from 360,000 active duty personnel to 316,000. The size of the active duty force in 2007 was roughly 64% of that of what the USAF was at the end of the first Gulf War in 1991. However, the reduction was ended at approximately 330,000 personnel in 2008 in order to meet the demand signal of combatant commanders and associated mission requirements. These same constraints have seen a sharp reduction in flight hours for crew training since 2005 and the Deputy Chief of Staff for Manpower and Personnel directing Airmen's Time Assessments.


On 5 June 2008, Secretary of Defense Robert Gates, accepted the resignations of both the Secretary of the Air Force, Michael Wynne, and the Chief of Staff of the United States Air Force, General T. Michael Moseley. Gates in effect fired both men for "systemic issues associated with declining Air Force nuclear mission focus and performance." This followed an investigation into two embarrassing incidents involving mishandling of nuclear weapons: specifically a nuclear weapons incident aboard a B-52 flight between Minot AFB and Barksdale AFB, and an accidental shipment of nuclear weapons components to Taiwan. The resignations were also the culmination of disputes between the Air Force leadership, populated primarily by non-nuclear background fighter pilots, versus Gates. To put more emphasis on nuclear assets, the USAF established the nuclear-focused Air Force Global Strike Command on 24 October 2008.


Due to the Budget sequestration in 2013, the USAF was forced to ground many of its squadrons. The Commander of Air Combat Command, General Mike Hostage indicated that the USAF must reduce its F-15 and F-16 fleets and eliminate platforms like the A-10 in order to focus on a fifth-generation jet fighter future. In response to squadron groundings and flight time reductions, many Air Force pilots have opted to resign from active duty and enter the Air Force Reserve and Air National Guard while pursuing careers in the commercial airlines where they can find flight hours on more modern aircraft.


Specific concerns include a compounded inability for the Air Force to replace its aging fleet, and an overall reduction of strength and readiness. The USAF attempted to make these adjustments by primarily cutting the Air National Guard and Air Force Reserve aircraft fleets and their associated manpower, but Congress reversed this initiative and the majority of the lost manpower will come from the active forces. However, Congress did allow for $208 million of reprogramming from fleet modernization to enable some portion of the third of the grounded fleet to resume operations.


The Department of the Air Force is one of three military departments within the Department of Defense, and is managed by the civilian Secretary of the Air Force, under the authority, direction, and control of the Secretary of Defense. The senior officials in the Office of the Secretary are the Under Secretary of the Air Force, four Assistant Secretaries of the Air Force and the General Counsel, all of whom are appointed by the President with the advice and consent of the Senate. The senior uniformed leadership in the Air Staff is made up of the Chief of Staff of the Air Force and the Vice Chief of Staff of the Air Force.


The organizational structure as shown above is responsible for the peacetime organization, equipping, and training of aerospace units for operational missions. When required to support operational missions, the Secretary of Defense (SECDEF) directs the Secretary of the Air Force (SECAF) to execute a Change in Operational Control (CHOP) of these units from their administrative alignment to the operational command of a Regional Combatant Commander (CCDR). In the case of AFSPC, AFSOC, PACAF, and USAFE units, forces are normally employed in-place under their existing CCDR. Likewise, AMC forces operating in support roles retain their componency to USTRANSCOM unless chopped to a Regional CCDR.


"Chopped" units are referred to as forces. The top-level structure of these forces is the Air and Space Expeditionary Task Force (AETF). The AETF is the Air Force presentation of forces to a CCDR for the employment of Air Power. Each CCDR is supported by a standing Component Numbered Air Force (C-NAF) to provide planning and execution of aerospace forces in support of CCDR requirements. Each C-NAF consists of a Commander, Air Force Forces (COMAFFOR) and AFFOR/A-staff, and an Air Operations Center (AOC). As needed to support multiple Joint Force Commanders (JFC) in the COCOM's Area of Responsibility (AOR), the C-NAF may deploy Air Component Coordinate Elements (ACCE) to liaise with the JFC. If the Air Force possesses the preponderance of air forces in a JFC's area of operations, the COMAFFOR will also serve as the Joint Forces Air Component Commander (JFACC).


AFSCs range from officer specialties such as pilot, combat systems officer, missile launch officer, intelligence officer, aircraft maintenance officer, judge advocate general (JAG), medical doctor, nurse or other fields, to various enlisted specialties. The latter range from flight combat operations such as a gunner, to working in a dining facility to ensure that members are properly fed. There are additional occupational fields such as computer specialties, mechanic specialties, enlisted aircrew, communication systems, cyberspace operations, avionics technicians, medical specialties, civil engineering, public affairs, hospitality, law, drug counseling, mail operations, security forces, and search and rescue specialties.


Beyond combat flight crew personnel, perhaps the most dangerous USAF jobs are Explosive Ordnance Disposal (EOD), Combat rescue officer, Pararescue, Security Forces, Combat Control, Combat Weather, Tactical Air Control Party, and AFOSI agents, who deploy with infantry and special operations units who disarm bombs, rescue downed or isolated personnel, call in air strikes and set up landing zones in forward locations. Most of these are enlisted positions augmented by a smaller number of commissioned officers. Other career fields that have seen increasing exposure to combat include civil engineers, vehicle operators, and Air Force Office of Special Investigations (AFOSI) personnel.


Training programs vary in length; for example, 3M0X1 (Services) has 31 days of tech school training, while 3E8X1 (Explosive Ordnance Disposal) is one year of training with a preliminary school and a main school consisting of over 10 separate divisions, sometimes taking students close to two years to complete. Officer technical training conducted by Second Air Force can also vary by AFSC, while flight training for aeronautically-rated officers conducted by AETC's Nineteenth Air Force can last well in excess of one year.


USAF rank is divided between enlisted airmen, non-commissioned officers, and commissioned officers, and ranges from the enlisted Airman Basic (E-1) to the commissioned officer rank of General (O-10). Enlisted promotions are granted based on a combination of test scores, years of experience, and selection board approval while officer promotions are based on time-in-grade and a promotion selection board. Promotions among enlisted personnel and non-commissioned officers are generally designated by increasing numbers of insignia chevrons. Commissioned officer rank is designated by bars, oak leaves, a silver eagle, and anywhere from one to four stars (one to five stars in war-time).[citation needed]


Air Force officer promotions are governed by the Defense Officer Personnel Management Act of 1980 and its companion Reserve Officer Personnel Management Act (ROPMA) for officers in the Air Force Reserve and the Air National Guard. DOPMA also establishes limits on the number of officers that can serve at any given time in the Air Force. Currently, promotion from second lieutenant to first lieutenant is virtually guaranteed after two years of satisfactory service. The promotion from first lieutenant to captain is competitive after successfully completing another two years of service, with a selection rate varying between 99% and 100%. Promotion to major through major general is through a formal selection board process, while promotions to lieutenant general and general are contingent upon nomination to specific general officer positions and subject to U.S. Senate approval.


During the board process an officer's record is reviewed by a selection board at the Air Force Personnel Center at Randolph Air Force Base in San Antonio, Texas. At the 10 to 11 year mark, captains will take part in a selection board to major. If not selected, they will meet a follow-on board to determine if they will be allowed to remain in the Air Force. Promotion from major to lieutenant colonel is similar and occurs approximately between the thirteen year (for officers who were promoted to major early "below the zone") and the fifteen year mark, where a certain percentage of majors will be selected below zone (i.e., "early"), in zone (i.e., "on time") or above zone (i.e., "late") for promotion to lieutenant colonel. This process will repeat at the 16 year mark (for officers previously promoted early to major and lieutenant colonel) to the 21 year mark for promotion to full colonel.


Although provision is made in Title 10 of the United States Code for the Secretary of the Air Force to appoint warrant officers, the Air Force does not currently use warrant officer grades, and is the only one of the U.S. Armed Services not to do so. The Air Force inherited warrant officer ranks from the Army at its inception in 1947, but their place in the Air Force structure was never made clear.[citation needed] When the Congress authorized the creation of two new senior enlisted ranks in 1958, Air Force officials privately concluded that these two new "super grades" could fill all Air Force needs then performed at the warrant officer level, although this was not publicly acknowledged until years later.[citation needed] The Air Force stopped appointing warrant officers in 1959, the same year the first promotions were made to the new top enlisted grade, Chief Master Sergeant. Most of the existing Air Force warrant officers entered the commissioned officer ranks during the 1960s, but small numbers continued to exist in the warrant officer grades for the next 21 years.


Enlisted members of the USAF have pay grades from E-1 (entry level) to E-9 (senior enlisted). While all USAF military personnel are referred to as Airmen, the term also refers to the pay grades of E-1 through E-4, which are below the level of non-commissioned officers (NCOs). Above the pay grade of E-4 (i.e., pay grades E-5 through E-9) all ranks fall into the category of NCO and are further subdivided into "NCOs" (pay grades E-5 and E-6) and "Senior NCOs" (pay grades E-7 through E-9); the term "Junior NCO" is sometimes used to refer to staff sergeants and technical sergeants (pay grades E-5 and E-6).


The USAF is the only branch of the U.S. military where NCO status is achieved when an enlisted person reaches the pay grade of E-5. In all other branches, NCO status is generally achieved at the pay grade of E-4 (e.g., a Corporal in the Army and Marine Corps, Petty Officer Third Class in the Navy and Coast Guard). The Air Force mirrored the Army from 1976 to 1991 with an E-4 being either a Senior Airman wearing three stripes without a star or a Sergeant (referred to as "Buck Sergeant"), which was noted by the presence of the central star and considered an NCO. Despite not being an NCO, a Senior Airman who has completed Airman Leadership School can be a supervisor according to the AFI 36-2618.


The first USAF dress uniform, in 1947, was dubbed and patented "Uxbridge Blue" after "Uxbridge 1683 Blue", developed at the former Bachman-Uxbridge Worsted Company. The current Service Dress Uniform, which was adopted in 1993 and standardized in 1995, consists of a three-button, pocketless coat, similar to that of a men's "sport jacket" (with silver "U.S." pins on the lapels, with a silver ring surrounding on those of enlisted members), matching trousers, and either a service cap or flight cap, all in Shade 1620, "Air Force Blue" (a darker purplish-blue). This is worn with a light blue shirt (Shade 1550) and Shade 1620 herringbone patterned necktie. Enlisted members wear sleeve insignia on both the jacket and shirt, while officers wear metal rank insignia pinned onto the coat, and Air Force Blue slide-on epaulet loops on the shirt. USAF personnel assigned to Base Honor Guard duties wear, for certain occasions, a modified version of the standard service dress uniform, but with silver trim on the sleeves and trousers, with the addition of a ceremonial belt (if necessary), wheel cap with silver trim and Hap Arnold Device, and a silver aiguillette placed on the left shoulder seam and all devices and accoutrement.


In addition to basic uniform clothing, various badges are used by the USAF to indicate a billet assignment or qualification-level for a given assignment. Badges can also be used as merit-based or service-based awards. Over time, various badges have been discontinued and are no longer distributed. Authorized badges include the Shields of USAF Fire Protection, and Security Forces, and the Missile Badge (or "pocket rocket"), which is earned after working in a missile system maintenance or missile operations capacity for at least one year.


Officers may be commissioned upon graduation from the United States Air Force Academy, upon graduation from another college or university through the Air Force Reserve Officer Training Corps (AFROTC) program, or through the Air Force Officer Training School (OTS). OTS, previously located at Lackland AFB, Texas until 1993 and located at Maxwell Air Force Base in Montgomery, Alabama since 1993, in turn encompasses two separate commissioning programs: Basic Officer Training (BOT), which is for line-officer candidates of the active-duty Air Force and the U.S. Air Force Reserve; and the Academy of Military Science (AMS), which is for line-officer candidates of the Air National Guard. (The term "line officer" derives from the concept of the line of battle and refers to an officer whose role falls somewhere within the "Line of the Air", meaning combat or combat-support operations within the scope of legitimate combatants as defined by the Geneva Conventions.)


The Air Force also provides Commissioned Officer Training (COT) for officers of all three components who are direct-commissioned to non-line positions due to their credentials in medicine, law, religion, biological sciences, or healthcare administration. Originally viewed as a "knife and fork school" that covered little beyond basic wear of the uniform, COT in recent years has been fully integrated into the OTS program and today encompasses extensive coursework as well as field exercises in leadership, confidence, fitness, and deployed-environment operations.


The US Air Force Fitness Test (AFFT) is designed to test the abdominal circumference, muscular strength/endurance and cardiovascular respiratory fitness of airmen in the USAF. As part of the Fit to Fight program, the USAF adopted a more stringent physical fitness assessment; the new fitness program was put into effect on 1 June 2010. The annual ergo-cycle test which the USAF had used for several years had been replaced in 2004. In the AFFT, Airmen are given a score based on performance consisting of four components: waist circumference, the sit-up, the push-up, and a 1.5-mile (2.4 km) run. Airmen can potentially earn a score of 100, with the run counting as 60%, waist circumference as 20%, and both strength test counting as 10% each. A passing score is 75 points. Effective 1 July 2010, the AFFT is administered by the base Fitness Assessment Cell (FAC), and is required twice a year. Personnel may test once a year if he or she earns a score above a 90%. Additionally, only meeting the minimum standards on each one of these tests will not get you a passing score of 75%, and failing any one component will result in a failure for the entire test.


The ground-attack aircraft of the USAF are designed to attack targets on the ground and are often deployed as close air support for, and in proximity to, U.S. ground forces. The proximity to friendly forces require precision strikes from these aircraft that are not possible with bomber aircraft listed below. They are typically deployed as close air support to ground forces, their role is tactical rather than strategic, operating at the front of the battle rather than against targets deeper in the enemy's rear.


In the US Air Force, the distinction between bombers, fighters that are actually fighter-bombers, and attack aircraft has become blurred. Many attack aircraft, even ones that look like fighters, are optimized to drop bombs, with very little ability to engage in aerial combat. Many fighter aircraft, such as the F-16, are often used as 'bomb trucks', despite being designed for aerial combat. Perhaps the one meaningful distinction at present is the question of range: a bomber is generally a long-range aircraft capable of striking targets deep within enemy territory, whereas fighter bombers and attack aircraft are limited to 'theater' missions in and around the immediate area of battlefield combat. Even that distinction is muddied by the availability of aerial refueling, which greatly increases the potential radius of combat operations. The US, Russia, and the People's Republic of China operate strategic bombers.


The service's B-2A aircraft entered service in the 1990s, its B-1B aircraft in the 1980s and its current B-52H aircraft in the early 1960s. The B-52 Stratofortress airframe design is over 60 years old and the B-52H aircraft currently in the active inventory were all built between 1960 and 1962. The B-52H is scheduled to remain in service for another 30 years, which would keep the airframe in service for nearly 90 years, an unprecedented length of service for any aircraft. The B-21 is projected to replace the B-52 and parts of the B-1B force by the mid-2020s.


Cargo and transport aircraft are typically used to deliver troops, weapons and other military equipment by a variety of methods to any area of military operations around the world, usually outside of the commercial flight routes in uncontrolled airspace. The workhorses of the USAF Air Mobility Command are the C-130 Hercules, C-17 Globemaster III, and C-5 Galaxy. These aircraft are largely defined in terms of their range capability as strategic airlift (C-5), strategic/tactical (C-17), and tactical (C-130) airlift to reflect the needs of the land forces they most often support. The CV-22 is used by the Air Force for the U.S. Special Operations Command (USSOCOM). It conducts long-range, special operations missions, and is equipped with extra fuel tanks and terrain-following radar. Some aircraft serve specialized transportation roles such as executive/embassy support (C-12), Antarctic Support (LC-130H), and USSOCOM support (C-27J, C-145A, and C-146A). The WC-130H aircraft are former weather reconnaissance aircraft, now reverted to the transport mission.


The purpose of electronic warfare is to deny the opponent an advantage in the EMS and ensure friendly, unimpeded access to the EM spectrum portion of the information environment. Electronic warfare aircraft are used to keep airspaces friendly, and send critical information to anyone who needs it. They are often called "The Eye in the Sky." The roles of the aircraft vary greatly among the different variants to include Electronic Warfare/Jamming (EC-130H), Psychological Operations/Communications (EC-130J), Airborne Early Warning and Control (E-3), Airborne Command Post (E-4B), ground targeting radar (E-8C), range control (E-9A), and communications relay (E-11A)


The fighter aircraft of the USAF are small, fast, and maneuverable military aircraft primarily used for air-to-air combat. Many of these fighters have secondary ground-attack capabilities, and some are dual-roled as fighter-bombers (e.g., the F-16 Fighting Falcon); the term "fighter" is also sometimes used colloquially for dedicated ground-attack aircraft. Other missions include interception of bombers and other fighters, reconnaissance, and patrol. The F-16 is currently used by the USAF Air Demonstration squadron, the Thunderbirds, while a small number of both man-rated and non-man-rated F-4 Phantom II are retained as QF-4 aircraft for use as Full Scale Aerial Targets (FSAT) or as part of the USAF Heritage Flight program. These extant QF-4 aircraft are being replaced in the FSAT role by early model F-16 aircraft converted to QF-16 configuration. The USAF has 2,025 fighters in service as of September 2012.


The USAF's KC-135 and KC-10 aerial refueling aircraft are based on civilian jets. The USAF aircraft are equipped primarily for providing the fuel via a tail-mounted refueling boom, and can be equipped with "probe and drogue" refueling systems. Air-to-air refueling is extensively used in large-scale operations and also used in normal operations; fighters, bombers, and cargo aircraft rely heavily on the lesser-known "tanker" aircraft. This makes these aircraft an essential part of the Air Force's global mobility and the U.S. force projection. The KC-46A Pegasus is undergoing testing and is projected to be delivered to USAF units starting in 2017.


In response to the 2007 United States Air Force nuclear weapons incident, Secretary of Defense Robert Gates accepted in June 2009 the resignations of Secretary of the Air Force Michael Wynne and the Chief of Staff of the Air Force General T. Michael Moseley. Moseley's successor, General Norton A. Schwartz, a former tactical airlift and special operations pilot was the first officer appointed to that position who did not have a background as a fighter or bomber pilot. The Washington Post reported in 2010 that General Schwartz began to dismantle the rigid class system of the USAF, particularly in the officer corps.


Daniel L. Magruder, Jr defines USAF culture as a combination of the rigorous application of advanced technology, individualism and progressive airpower theory. Major General Charles J. Dunlap, Jr. adds that the U.S. Air Force's culture also includes an egalitarianism bred from officers perceiving themselves as their service's principal "warriors" working with small groups of enlisted airmen either as the service crew or the onboard crew of their aircraft. Air Force officers have never felt they needed the formal social "distance" from their enlisted force that is common in the other U.S. armed services. Although the paradigm is changing, for most of its history, the Air Force, completely unlike its sister services, has been an organization in which mostly its officers fought, not its enlisted force, the latter being primarily a rear echelon support force. When the enlisted force did go into harm's way, such as members of multi-crewed aircraft, the close comradeship of shared risk in tight quarters created traditions that shaped a somewhat different kind of officer/enlisted relationship than exists elsewhere in the military.


Cultural and career issues in the U.S. Air Force have been cited as one of the reasons for the shortfall in needed UAV operators. In spite of an urgent need for UAVs or drones to provide round the clock coverage for American troops during the Iraq War, the USAF did not establish a new career field for piloting them until the last year of that war and in 2014 changed its RPA training syllabus again, in the face of large aircraft losses in training, and in response to a GAO report critical of handling of drone programs. Paul Scharre has reported that the cultural divide between the USAF and US Army has kept both services from adopting each other's drone handing innovations.


Many of the U.S. Air Force's formal and informal traditions are an amalgamation of those taken from the Royal Air Force (e.g., dining-ins/mess nights) or the experiences of its predecessor organizations such as the U.S. Army Air Service, U.S. Army Air Corps and the U.S. Army Air Forces. Some of these traditions range from "Friday Name Tags" in flying units to an annual "Mustache Month." The use of "challenge coins" is a recent innovation that was adopted from the U.S. Army while another cultural tradition unique to the Air Force is the "roof stomp", practiced by Air Force members to welcome a new commander or to commemorate another event, such as a retirement.


The United States Air Force has had numerous recruiting slogans including "No One Comes Close" and Uno Ab Alto ("One From On High"). For many years, the U.S. Air Force used "Aim High" as its recruiting slogan; more recently, they have used "Cross into the Blue", "We've been waiting for you" and "Do Something Amazing", "Above All", and the newest one, as of 7 October 2010, considered a call and response, "Aim high" followed with the response, "Fly-Fight-Win" Each wing, group, or squadron usually has its own slogan(s). Information and logos can usually be found on the wing, group, or squadron websites.


Recent developments in LEDs permit them to be used in environmental and task lighting. LEDs have many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Light-emitting diodes are now used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes and lighted wallpaper. As of 2015[update], LEDs powerful enough for room lighting remain somewhat more expensive, and require more precise current and heat management, than compact fluorescent lamp sources of comparable output.


Electroluminescence as a phenomenon was discovered in 1907 by the British experimenter H. J. Round of Marconi Labs, using a crystal of silicon carbide and a cat's-whisker detector. Soviet inventor Oleg Losev reported creation of the first LED in 1927. His research was distributed in Soviet, German and British scientific journals, but no practical use was made of the discovery for several decades. Kurt Lehovec, Carl Accardo and Edward Jamgochian, explained these first light-emitting diodes in 1951 using an apparatus employing SiC crystals with a current source of battery or pulse generator and with a comparison to a variant, pure, crystal in 1953.


In 1957, Braunstein further demonstrated that the rudimentary devices could be used for non-radio communication across a short distance. As noted by Kroemer Braunstein".. had set up a simple optical communications link: Music emerging from a record player was used via suitable electronics to modulate the forward current of a GaAs diode. The emitted light was detected by a PbS diode some distance away. This signal was fed into an audio amplifier, and played back by a loudspeaker. Intercepting the beam stopped the music. We had a great deal of fun playing with this setup." This setup presaged the use of LEDs for optical communication applications.


In September 1961, while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared (900 nm) light emission from a tunnel diode they had constructed on a GaAs substrate. By October 1961, they had demonstrated efficient light emission and signal coupling between a GaAs p-n junction light emitter and an electrically-isolated semiconductor photodetector. On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, which described a zinc diffused p–n junction LED with a spaced cathode contact to allow for efficient emission of infrared light under forward bias. After establishing the priority of their work based on engineering notebooks predating submissions from G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the U.S. patent office issued the two inventors the patent for the GaAs infrared (IR) light-emitting diode (U.S. Patent US3293513), the first practical LED. Immediately after filing the patent, Texas Instruments (TI) began a project to manufacture infrared diodes. In October 1962, TI announced the first LED commercial product (the SNX-100), which employed a pure GaAs crystal to emit a 890 nm light output. In October 1963, TI announced the first commercial hemispherical LED, the SNX-110.


The first visible-spectrum (red) LED was developed in 1962 by Nick Holonyak, Jr., while working at General Electric Company. Holonyak first reported his LED in the journal Applied Physics Letters on the December 1, 1962. M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. In 1976, T. P. Pearsall created the first high-brightness, high-efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths.


The first commercial LEDs were commonly used as replacements for incandescent and neon indicator lamps, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, as well as watches (see list of signal uses). Until 1968, visible and infrared LEDs were extremely costly, in the order of US$200 per unit, and so had little practical use. The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators. Hewlett Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Readouts in calculators were so small that plastic lenses were built over each digit to make them legible. Later, other colors became widely available and appeared in appliances and equipment. In the 1970s commercially successful LED devices at less than five cents each were produced by Fairchild Optoelectronics. These devices employed compound semiconductor chips fabricated with the planar process invented by Dr. Jean Hoerni at Fairchild Semiconductor. The combination of planar processing for chip fabrication and innovative packaging methods enabled the team at Fairchild led by optoelectronics pioneer Thomas Brandt to achieve the needed cost reductions. These methods continue to be used by LED producers.


The first high-brightness blue LED was demonstrated by Shuji Nakamura of Nichia Corporation in 1994 and was based on InGaN. In parallel, Isamu Akasaki and Hiroshi Amano in Nagoya were working on developing the important GaN nucleation on sapphire substrates and the demonstration of p-type doping of GaN. Nakamura, Akasaki and Amano were awarded the 2014 Nobel prize in physics for their work. In 1995, Alberto Barbieri at the Cardiff University Laboratory (GB) investigated the efficiency and reliability of high-brightness LEDs and demonstrated a "transparent contact" LED using indium tin oxide (ITO) on (AlGaInP/GaAs).


The attainment of high efficiency in blue LEDs was quickly followed by the development of the first white LED. In this device a Y
3Al
5O
12:Ce (known as "YAG") phosphor coating on the emitter absorbs some of the blue emission and produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. However using different phosphors (fluorescent materials) it also became possible to instead produce green and red light through fluorescence. The resulting mixture of red, green and blue is not only perceived by humans as white light but is superior for illumination in terms of color rendering, whereas one cannot appreciate the color of red or green objects illuminated only by the yellow (and remaining blue) wavelengths from the YAG phosphor.


A P-N junction can convert absorbed light energy into a proportional electric current. The same process is reversed here (i.e. the P-N junction emits light when electrical energy is applied to it). This phenomenon is generally called electroluminescence, which can be defined as the emission of light from a semi-conductor under the influence of an electric field. The charge carriers recombine in a forward-biased P-N junction as the electrons cross from the N-region and recombine with the holes existing in the P-region. Free electrons are in the conduction band of energy levels, while holes are in the valence energy band. Thus the energy level of the holes will be lesser than the energy levels of the electrons. Some portion of the energy must be dissipated in order to recombine the electrons and the holes. This energy is emitted in the form of heat and light.


In September 2003, a new type of blue LED was demonstrated by Cree that consumes 24 mW at 20 milliamperes (mA). This produced a commercially packaged white light giving 65 lm/W at 20 mA, becoming the brightest white LED commercially available at the time, and more than four times as efficient as standard incandescents. In 2006, they demonstrated a prototype with a record white LED luminous efficacy of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with luminous efficacy of 150 lm/W at a forward current of 20 mA. Cree's XLamp XM-L LEDs, commercially available in 2011, produce 100 lm/W at their full power of 10 W, and up to 160 lm/W at around 2 W input power. In 2012, Cree announced a white LED giving 254 lm/W, and 303 lm/W in March 2014. Practical general lighting needs high-power LEDs, of one watt or more. Typical operating currents for such devices begin at 350 mA.


The most common symptom of LED (and diode laser) failure is the gradual lowering of light output and loss of efficiency. Sudden failures, although rare, can also occur. Early red LEDs were notable for their short service life. With the development of high-power LEDs the devices are subjected to higher junction temperatures and higher current densities than traditional devices. This causes stress on the material and may cause early light-output degradation. To quantitatively classify useful lifetime in a standardized manner it has been suggested to use L70 or L50, which are the runtimes (typically given in thousands of hours) at which a given LED reaches 70% and 50% of initial light output, respectively.


Since LED efficacy is inversely proportional to operating temperature, LED technology is well suited for supermarket freezer lighting. Because LEDs produce less waste heat than incandescent lamps, their use in freezers can save on refrigeration costs as well. However, they may be more susceptible to frost and snow buildup than incandescent lamps, so some LED lighting systems have been designed with an added heating circuit. Additionally, research has developed heat sink technologies that will transfer heat produced within the junction to appropriate areas of the light fixture.


The first blue-violet LED using magnesium-doped gallium nitride was made at Stanford University in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering. At the time Maruska was on leave from RCA Laboratories, where he collaborated with Jacques Pankove on related work. In 1971, the year after Maruska left for Stanford, his RCA colleagues Pankove and Ed Miller demonstrated the first blue electroluminescence from zinc-doped gallium nitride, though the subsequent device Pankove and Miller built, the first actual gallium nitride light-emitting diode, emitted green light. In 1974 the U.S. Patent Office awarded Maruska, Rhines and Stanford professor David Stevenson a patent for their work in 1972 (U.S. Patent US3819974 A) and today magnesium-doping of gallium nitride continues to be the basis for all commercial blue LEDs and laser diodes. These devices built in the early 1970s had too little light output to be of practical use and research into gallium nitride devices slowed. In August 1989, Cree introduced the first commercially available blue LED based on the indirect bandgap semiconductor, silicon carbide (SiC). SiC LEDs had very low efficiency, no more than about 0.03%, but did emit in the blue portion of the visible light spectrum.[citation needed]


In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, Dr. Moustakas at Boston University patented a method for producing high-brightness blue LEDs using a new two-step process. Two years later, in 1993, high-brightness blue LEDs were demonstrated again by Shuji Nakamura of Nichia Corporation using a gallium nitride growth process similar to Dr. Moustakas's. Both Dr. Moustakas and Mr. Nakamura were issued separate patents, which confused the issue of who was the original inventor (partly because although Dr. Moustakas invented his first, Dr. Nakamura filed first).[citation needed] This new development revolutionized LED lighting, making high-power blue light sources practical, leading to the development of technologies like BlueRay, as well as allowing the bright high resolution screens of modern tablets and phones.[citation needed]


Nakamura was awarded the 2006 Millennium Technology Prize for his invention. Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the invention of the blue LED. In 2015, a US court ruled that three companies (i.e. the litigants who had not previously settled out of court) that had licensed Mr. Nakamura's patents for production in the United States had infringed Dr. Moustakas's prior patent, and order them to pay licensing fees of not less than 13 million USD.


By the late 1990s, blue LEDs became widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative In/Ga fraction in the InGaN quantum wells, the light emission can in theory be varied from violet to amber. Aluminium gallium nitride (AlGaN) of varying Al/Ga fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form the active quantum well layers, the device will emit near-ultraviolet light with a peak wavelength centred around 365 nm. Green LEDs manufactured from the InGaN/GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems, but practical devices still exhibit efficiency too low for high-brightness applications.[citation needed]


With nitrides containing aluminium, most often AlGaN and AlGaInN, even shorter wavelengths are achievable. Ultraviolet LEDs in a range of wavelengths are becoming available on the market. Near-UV emitters at wavelengths around 375–395 nm are already cheap and often encountered, for example, as black light lamp replacements for inspection of anti-counterfeiting UV watermarks in some documents and paper currencies. Shorter-wavelength diodes, while substantially more expensive, are commercially available for wavelengths down to 240 nm. As the photosensitivity of microorganisms approximately matches the absorption spectrum of DNA, with a peak at about 260 nm, UV LED emitting at 250–270 nm are to be expected in prospective disinfection and sterilization devices. Recent research has shown that commercially available UVA LEDs (365 nm) are already effective disinfection and sterilization devices. UV-C wavelengths were obtained in laboratories using aluminium nitride (210 nm), boron nitride (215 nm) and diamond (235 nm).


White light can be formed by mixing differently colored lights; the most common method is to use red, green, and blue (RGB). Hence the method is called multi-color white LEDs (sometimes referred to as RGB LEDs). Because these need electronic circuits to control the blending and diffusion of different colors, and because the individual color LEDs typically have slightly different emission patterns (leading to variation of the color depending on direction) even if they are made as a single unit, these are seldom used to produce white lighting. Nonetheless, this method has many applications because of the flexibility of mixing different colors, and in principle, this mechanism also has higher quantum efficiency in producing white light.[citation needed]


There are several types of multi-color white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different methods, include color stability, color rendering capability, and luminous efficacy. Often, higher efficiency will mean lower color rendering, presenting a trade-off between the luminous efficacy and color rendering. For example, the dichromatic white LEDs have the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficacy. Trichromatic white LEDs are in between, having both good luminous efficacy (>70 lm/W) and fair color rendering capability.


Multi-color LEDs offer not merely another means to form white light but a new means to form light of different colors. Most perceivable colors can be formed by mixing different amounts of three primary colors. This allows precise dynamic color control. As more effort is devoted to investigating this method, multi-color LEDs should have profound influence on the fundamental method that we use to produce and control light color. However, before this type of LED can play a role on the market, several technical problems must be solved. These include that this type of LED's emission power decays exponentially with rising temperature, resulting in a substantial change in color stability. Such problems inhibit and may preclude industrial use. Thus, many new package designs aimed at solving this problem have been proposed and their results are now being reproduced by researchers and scientists.


This method involves coating LEDs of one color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; the resultant LEDs are called phosphor-based or phosphor-converted white LEDs (pcLEDs). A fraction of the blue light undergoes the Stokes shift being transformed from shorter wavelengths to longer. Depending on the color of the original LED, phosphors of different colors can be employed. If several phosphor layers of distinct colors are applied, the emitted spectrum is broadened, effectively raising the color rendering index (CRI) value of a given LED.


Phosphor-based LED efficiency losses are due to the heat loss from the Stokes shift and also other phosphor-related degradation issues. Their luminous efficacies compared to normal LEDs depend on the spectral distribution of the resultant light output and the original wavelength of the LED itself. For example, the luminous efficacy of a typical YAG yellow phosphor based white LED ranges from 3 to 5 times the luminous efficacy of the original blue LED because of the human eye's greater sensitivity to yellow than to blue (as modeled in the luminosity function). Due to the simplicity of manufacturing the phosphor method is still the most popular method for making high-intensity white LEDs. The design and production of a light source or light fixture using a monochrome emitter with phosphor conversion is simpler and cheaper than a complex RGB system, and the majority of high-intensity white LEDs presently on the market are manufactured using phosphor light conversion.


Among the challenges being faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. As of 2010, the most efficient yellow phosphor is still the YAG phosphor, with less than 10% Stoke shift loss. Losses attributable to internal optical losses due to re-absorption in the LED chip and in the LED packaging itself account typically for another 10% to 30% of efficiency loss. Currently, in the area of phosphor LED development, much effort is being spent on optimizing these devices to higher light output and higher operation temperatures. For instance, the efficiency can be raised by adapting better package design or by using a more suitable type of phosphor. Conformal coating process is frequently used to address the issue of varying phosphor thickness.


White LEDs can also be made by coating near-ultraviolet (NUV) LEDs with a mixture of high-efficiency europium-based phosphors that emit red and blue, plus copper and aluminium-doped zinc sulfide (ZnS:Cu, Al) that emits green. This is a method analogous to the way fluorescent lamps work. This method is less efficient than blue LEDs with YAG:Ce phosphor, as the Stokes shift is larger, so more energy is converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both methods offer comparable brightness. A concern is that UV light may leak from a malfunctioning light source and cause harm to human eyes or skin.


A new style of wafers composed of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids the typical costly sapphire substrate in relatively small 100- or 150-mm wafer sizes. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. It is predicted that by 2020, 40% of all GaN LEDs will be made with GaN-on-Si. Manufacturing large sapphire material is difficult, while large silicon material is cheaper and more abundant. LED companies shifting from using sapphire to silicon should be a minimal investment.


Quantum dots (QD) are semiconductor nanocrystals that possess unique optical properties. Their emission color can be tuned from the visible throughout the infrared spectrum. This allows quantum dot LEDs to create almost any color on the CIE diagram. This provides more color options and better color rendering than white LEDs since the emission spectrum is much narrower, characteristic of quantum confined states. There are two types of schemes for QD excitation. One uses photo excitation with a primary light source LED (typically blue or UV LEDs are used). The other is direct electrical excitation first demonstrated by Alivisatos et al.


The structure of QD-LEDs used for the electrical-excitation scheme is similar to basic design of OLEDs. A layer of quantum dots is sandwiched between layers of electron-transporting and hole-transporting materials. An applied electric field causes electrons and holes to move into the quantum dot layer and recombine forming an exciton that excites a QD. This scheme is commonly studied for quantum dot display. The tunability of emission wavelengths and narrow bandwidth is also beneficial as excitation sources for fluorescence imaging. Fluorescence near-field scanning optical microscopy (NSOM) utilizing an integrated QD-LED has been demonstrated.


High-power LEDs (HP-LEDs) or high-output LEDs (HO-LEDs) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can emit over a thousand lumens. LED power densities up to 300 W/cm2 have been achieved. Since overheating is destructive, the HP-LEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HP-LED is not removed, the device will fail in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.


LEDs have been developed by Seoul Semiconductor that can operate on AC power without the need for a DC converter. For each half-cycle, part of the LED emits light and part is dark, and this is reversed during the next half-cycle. The efficacy of this type of HP-LED is typically 40 lm/W. A large number of LED elements in series may be able to operate directly from line voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named as 'Acrich MJT', capable of being driven from AC power with a simple controlling circuit. The low-power dissipation of these LEDs affords them more flexibility than the original AC LED design.


Alphanumeric LEDs are available in seven-segment, starburst and dot-matrix format. Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrix displays typically use 5x7 pixels per character. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but rising use of liquid crystal displays, with their lower power needs and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.


Digital-RGB LEDs are RGB LEDs that contain their own "smart" control electronics. In addition to power and ground, these provide connections for data-in, data-out, and sometimes a clock or strobe signal. These are connected in a daisy chain, with the data in of the first LED sourced by a microprocessor, which can control the brightness and color of each LED independently of the others. They are used where a combination of maximum control and minimum visible electronics are needed such as strings for Christmas and LED matrices. Some even have refresh rates in the kHz range, allowing for basic video applications.


An LED filament consists of multiple LED dice connected in series on a common longitudinal substrate that form a thin rod reminiscent of a traditional incandescent filament. These are being used as a low cost decorative alternative for traditional light bulbs that are being phased out in many countries. The filaments require a rather high voltage to light to nominal brightness, allowing them to work efficiently and simply with mains voltages. Often a simple rectifier and capacitive current limiting are employed to create a low-cost replacement for a traditional light bulb without the complexity of creating a low voltage, high current converter which is required by single die LEDs. Usually they are packaged in a sealed enclosure with a shape similar to lamps they were designed to replace (e.g. a bulb), and filled with inert nitrogen or carbon dioxide gas to remove heat efficiently.


The current–voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can cause a large change in current. If the applied voltage exceeds the LED's forward voltage drop by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is to use constant-current power supplies to keep the current below the LED's maximum current rating. Since most common power sources (batteries, mains) are constant-voltage sources, most LED fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of three-volt coin cells combined with the high differential resistance of nitride-based LEDs makes it possible to power such an LED from such a coin cell without an external resistor.


The vast majority of devices containing LEDs are "safe under all conditions of normal use", and so are classified as "Class 1 LED product"/"LED Klasse 1". At present, only a few LEDs—extremely bright LEDs that also have a tightly focused viewing angle of 8° or less—could, in theory, cause temporary blindness, and so are classified as "Class 2". The opinion of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) of 2010, on the health issues concerning LEDs, suggested banning public use of lamps which were in the moderate Risk Group 2, especially those with a high blue component in places frequented by children. In general, laser safety regulations—and the "Class 1", "Class 2", etc. system—also apply to LEDs.


While LEDs have the advantage over fluorescent lamps that they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. Regarding the toxicity of LEDs when treated as waste, a study published in 2011 stated: "According to federal standards, LEDs are not hazardous except for low-intensity red LEDs, which leached Pb [lead] at levels exceeding regulatory limits (186 mg/L; regulatory limit: 5). However, according to California regulations, excessive levels of copper (up to 3892 mg/kg; limit: 2500), lead (up to 8103 mg/kg; limit: 1000), nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg; limit: 500) render all except low-intensity yellow LEDs hazardous."


One-color light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' navigation lights or lanterns (chromacity and luminance standards being set under the Convention on the International Regulations for Preventing Collisions at Sea 1972, Annex I and the CIE) and LED-based Christmas lights. In cold climates, LED traffic lights may remain snow-covered. Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.


Because of their long life, fast switching times, and their ability to be seen in broad daylight due to their high output and focus, LEDs have been used in brake lights for cars' high-mounted brake lights, trucks, and buses, and in turn signals for some time, but many vehicles now use LEDs for their rear light clusters. The use in brakes improves safety, due to a great reduction in the time needed to light fully, or faster rise time, up to 0.5 second faster[citation needed] than an incandescent bulb. This gives drivers behind more time to react. In a dual intensity circuit (rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED will appear if the eyes quickly scan across the array. White LED headlamps are starting to be used. Using LEDs has styling advantages because LEDs can form much thinner lights than incandescent lamps with parabolic reflectors.


Assistive listening devices in many theaters and similar spaces use arrays of infrared LEDs to send sound to listeners' receivers. Light-emitting diodes (as well as semiconductor lasers) are used to send data over many types of fiber optic cable, from digital audio over TOSLINK cables to the very high bandwidth fiber links that form the Internet backbone. For some time, computers were commonly equipped with IrDA interfaces, which allowed them to send and receive data to nearby machines via infrared.


In the US, one kilowatt-hour (3.6 MJ) of electricity currently causes an average 1.34 pounds (610 g) of CO
2 emission. Assuming the average light bulb is on for 10 hours a day, a 40-watt bulb will cause 196 pounds (89 kg) of CO
2 emission per year. The 6-watt LED equivalent will only cause 30 pounds (14 kg) of CO
2 over the same time span. A building’s carbon footprint from lighting can therefore be reduced by 85% by exchanging all incandescent bulbs for new LEDs if a building previously used only incandescent bulbs.


Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used for this purpose, and this is likely to remain one of their major uses until the price drops low enough to make signaling and illumination uses more widespread. Barcode scanners are the most common example of machine vision, and many low cost products use red LEDs instead of lasers. Optical computer mice are an example of LEDs in machine vision, as it is used to provide an even light source on the surface for the miniature camera within the mouse. LEDs constitute a nearly ideal light source for machine vision systems for several reasons:


The light from LEDs can be modulated very quickly so they are used extensively in optical fiber and free space optics communications. This includes remote controls, such as for TVs, VCRs, and LED Computers, where infrared LEDs are often used. Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low-voltage sensor circuit (usually battery-powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.


Many sensor systems rely on light as the signal source. LEDs are often ideal as a light source due to the requirements of the sensors. LEDs are used as motion sensors, for example in optical computer mice. The Nintendo Wii's sensor bar uses infrared LEDs. Pulse oximeters use them for measuring oxygen saturation. Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Further, its sensors only need be monochromatic, since at any one time the page being scanned is only lit by one color of light. Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used, for example, in a touchscreen that registers reflected light from a finger or stylus. Many materials and biological systems are sensitive to, or dependent on, light. Grow lights use LEDs to increase photosynthesis in plants, and bacteria and viruses can be removed from water and other substances using UV LEDs for sterilization.


LEDs have also been used as a medium-quality voltage reference in electronic circuits. The forward voltage drop (e.g. about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Red LEDs have the flattest I/V curve above the knee. Nitride-based LEDs have a fairly steep I/V curve and are useless for this purpose. Although LED forward voltage is far more current-dependent than a Zener diode, Zener diodes with breakdown voltages below 3 V are not widely available.


The term "great power" was first used to represent the most important powers in Europe during the post-Napoleonic era. The "Great Powers" constituted the "Concert of Europe" and claimed the right to joint enforcement of the postwar treaties. The formalization of the division between small powers and great powers came about with the signing of the Treaty of Chaumont in 1814. Since then, the international balance of power has shifted numerous times, most dramatically during World War I and World War II. While some nations are widely considered to be great powers, there is no definitive list of them. In literature, alternative terms for great power are often world power or major power, but these terms can also be interchangeable with superpower.


A great power is a sovereign state that is recognized as having the ability and expertise to exert its influence on a global scale. Great powers characteristically possess military and economic strength, as well as diplomatic and soft power influence, which may cause middle or small powers to consider the great powers' opinions before taking actions of their own. International relations theorists have posited that great power status can be characterized into power capabilities, spatial aspects, and status dimensions. Sometimes the status of great powers is formally recognized in conferences such as the Congress of Vienna or an international structure such as the United Nations Security Council (China, France, Russia, the United Kingdom and the United States serve as the body's five permanent members). At the same time the status of great powers can be informally recognized in a forum such as the G7 which consists of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States of America.


Early writings on the subject tended to judge states by the realist criterion, as expressed by the historian A. J. P. Taylor when he noted that "The test of a great power is the test of strength for war." Later writers have expanded this test, attempting to define power in terms of overall military, economic, and political capacity. Kenneth Waltz, the founder of the neorealist theory of international relations, uses a set of five criteria to determine great power: population and territory; resource endowment; economic capability; political stability and competence; and military strength. These expanded criteria can be divided into three heads: power capabilities, spatial aspects, and status.


All states have a geographic scope of interests, actions, or projected power. This is a crucial factor in distinguishing a great power from a regional power; by definition the scope of a regional power is restricted to its region. It has been suggested that a great power should be possessed of actual influence throughout the scope of the prevailing international system. Arnold J. Toynbee, for example, observes that "Great power may be defined as a political force exerting an effect co-extensive with the widest range of the society in which it operates. The Great powers of 1914 were 'world-powers' because Western society had recently become 'world-wide'."


Other important criteria throughout history are that great powers should have enough influence to be included in discussions of political and diplomatic questions of the day, and have influence on the final outcome and resolution. Historically, when major political questions were addressed, several great powers met to discuss them. Before the era of groups like the United Nations, participants of such meetings were not officially named, but were decided based on their great power status. These were conferences which settled important questions based on major historical events. This might mean deciding the political resolution of various geographical and nationalist claims following a major conflict, or other contexts.


Lord Castlereagh, the British Foreign Secretary, first used the term in its diplomatic context, in a letter sent on February 13, 1814: "It affords me great satisfaction to acquaint you that there is every prospect of the Congress terminating with a general accord and Guarantee between the Great powers of Europe, with a determination to support the arrangement agreed upon, and to turn the general influence and if necessary the general arms against the Power that shall first attempt to disturb the Continental peace."


Of the five original great powers recognised at the Congress of Vienna, only France and the United Kingdom have maintained that status continuously to the present day, although France was defeated in the Franco-Prussian War and occupied during World War II. After the Congress of Vienna, the British Empire emerged as the pre-eminent power, due to its navy and the extent of its territories, which signalled the beginning of the Pax Britannica and of the Great Game between the UK and Russia. The balance of power between the Great Powers became a major influence in European politics, prompting Otto von Bismarck to say "All politics reduces itself to this formula: try to be one of three, as long as the world is governed by the unstable equilibrium of five great powers."


Over time, the relative power of these five nations fluctuated, which by the dawn of the 20th century had served to create an entirely different balance of power. Some, such as the United Kingdom and Prussia (as the founder of the newly formed German state), experienced continued economic growth and political power. Others, such as Russia and Austria-Hungary, stagnated. At the same time, other states were emerging and expanding in power, largely through the process of industrialization. These countries seeking to attain great power status were: Italy after the Risorgimento, Japan after the Meiji Restoration, and the United States after its civil war. By the dawn of the 20th century, the balance of world power had changed substantially since the Congress of Vienna. The Eight-Nation Alliance was a belligerent alliance of eight nations against the Boxer Rebellion in China. It formed in 1900 and consisted of the five Congress powers plus Italy, Japan, and the United States, representing the great powers at the beginning of 20th century.


Shifts of international power have most notably occurred through major conflicts. The conclusion of the Great War and the resulting treaties of Versailles, St-Germain, Neuilly, Trianon and Sèvres witnessed the United Kingdom, France, Italy, Japan and the United States as the chief arbiters of the new world order. In the aftermath of World War I the German Empire was defeated, the Austria-Hungarian empire was divided into new, less powerful states and the Russian Empire fell to a revolution. During the Paris Peace Conference, the "Big Four"—France, Italy, United Kingdom and the United States—held noticeably more power and influence on the proceedings and outcome of the treaties than Japan. The Big Four were leading architects of the Treaty of Versailles which was signed by Germany; the Treaty of St. Germain, with Austria; the Treaty of Neuilly, with Bulgaria; the Treaty of Trianon, with Hungary; and the Treaty of Sèvres, with the Ottoman Empire. During the decision-making of the Treaty of Versailles, Italy pulled out of the conference because a part of its demands were not met and temporarily left the other three countries as the sole major architects of that treaty, referred to as the "Big Three".


The victorious great powers also gained an acknowledgement of their status through permanent seats at the League of Nations Council, where they acted as a type of executive body directing the Assembly of the League. However, the Council began with only four permanent members—the United Kingdom, France, Italy, and Japan—because the United States, meant to be the fifth permanent member, left because the US Senate voted on 19 March 1920 against the ratification of the Treaty of Versailles, thus preventing American participation in the League.


When World War II started in 1939, it divided the world into two alliances—the Allies (the United Kingdom and France at first in Europe, China in Asia since 1937, followed in 1941 by the Soviet Union, the United States); and the Axis powers consisting of Germany, Italy and Japan.[nb 1] During World War II, the United States, United Kingdom, and Soviet Union controlled Allied policy and emerged as the "Big Three". The Republic of China and the Big Three were referred as a "trusteeship of the powerful"  and were recognized as the Allied "Big Four" in Declaration by United Nations in 1942. These four countries were referred as the "Four Policemen" of the Allies and considered as the primary victors of World War II. The importance of France was acknowledged by their inclusion, along with the other four, in the group of countries allotted permanent seats in the United Nations Security Council.


Since the end of the World Wars, the term "great power" has been joined by a number of other power classifications. Foremost among these is the concept of the superpower, used to describe those nations with overwhelming power and influence in the rest of the world. It was first coined in 1944 by William T.R. Fox and according to him, there were three superpowers: the British Empire, the United States, and the Soviet Union. But by the mid-1950s the British Empire lost its superpower status, leaving the United States and the Soviet Union as the world's superpowers.[nb 2] The term middle power has emerged for those nations which exercise a degree of global influence, but are insufficient to be decisive on international affairs. Regional powers are those whose influence is generally confined to their region of the world.


During the Cold War, the Asian power of Japan and the European powers of the United Kingdom, France, and West Germany rebuilt their economies. France and the United Kingdom maintained technologically advanced armed forces with power projection capabilities and maintain large defence budgets to this day. Yet, as the Cold War continued, authorities began to question if France and the United Kingdom could retain their long-held statuses as great powers. China, with the world's largest population, has slowly risen to great power status, with large growth in economic and military power in the post-war period. After 1949, the Republic of China began to lose its recognition as the sole legitimate government of China by the other great powers, in favour of the People's Republic of China. Subsequently, in 1971, it lost its permanent seat at the UN Security Council to the People's Republic of China.


According to Joshua Baron – a "researcher, lecturer, and consultant on international conflict" – since the early 1960s direct military conflicts and major confrontations have "receded into the background" with regards to relations among the great powers. Baron argues several reasons why this is the case, citing the unprecedented rise of the United States and its predominant position as the key reason. Baron highlights that since World War Two no other great power has been able to achieve parity or near parity with the United States, with the exception of the Soviet Union for a brief time. This position is unique among the great powers since the start of the modern era (the 16th century), where there has traditionally always been "tremendous parity among the great powers". This unique period of American primacy has been an important factor in maintaining a condition of peace between the great powers.


Another important factor is the apparent consensus among Western great powers that military force is no longer an effective tool of resolving disputes among their peers. This "subset" of great powers – France, Germany, Japan, the United Kingdom and the United States – consider maintaining a "state of peace" as desirable. As evidence, Baron outlines that since the Cuban missile crisis (1962) during the Cold War, these influential Western nations have resolved all disputes among the great powers peacefully at the United Nations and other forums of international discussion.


China, France, Russia, the United Kingdom and the United States are often referred to as great powers by academics due to "their political and economic dominance of the global arena". These five nations are the only states to have permanent seats with veto power on the UN Security Council. They are also the only recognized "Nuclear Weapons States" under the Nuclear Non-Proliferation Treaty, and maintain military expenditures which are among the largest in the world. However, there is no unanimous agreement among authorities as to the current status of these powers or what precisely defines a great power. For example, sources have at times referred to China, France, Russia and the United Kingdom as middle powers.


Japan and Germany are great powers too, though due to their large advanced economies (having the third and fourth largest economies respectively) rather than their strategic and hard power capabilities (i.e., the lack of permanent seats and veto power on the UN Security Council or strategic military reach). Germany has been a member together with the five permanent Security Council members in the P5+1 grouping of world powers. Like China, France, Russia and the United Kingdom; Germany and Japan have also been referred to as middle powers.


In addition to those contemporary great powers mentioned above, Zbigniew Brzezinski and Malik Mohan consider India to be a great power too. Although unlike the contemporary great powers who have long been considered so, India's recognition among authorities as a great power is comparatively recent. However, there is no collective agreement among observers as to the status of India, for example, a number of academics believe that India is emerging as a great power, while some believe that India remains a middle power.


Milena Sterio, American expert of international law, includes the former axis powers (Germany, Italy and Japan) and India among the great powers along with the permanent members of the UNSC. She considers Germany, Japan and Italy to be great powers due to their G7 membership and because of their influence in regional and international organizations.  Various authors describe Italy as an equal major power, while others view Italy as an "intermittent great power" or as "the least of the great powers".


With continuing European integration, the European Union is increasingly being seen as a great power in its own right, with representation at the WTO and at G8 and G-20 summits. This is most notable in areas where the European Union has exclusive competence (i.e. economic affairs). It also reflects a non-traditional conception of Europe's world role as a global "civilian power", exercising collective influence in the functional spheres of trade and diplomacy, as an alternative to military dominance. The European Union is a supranational union and not a sovereign state, and has limited scope in the areas of foreign affairs and defence policy. These remain largely with the member states of the European Union, which include the three great powers of France, Germany and the United Kingdom (referred to as the "EU three").


Referring to great power relations pre-1960, Joshua Baron highlights that starting from around the 16th century and the rise of several European great powers, military conflicts and confrontations was the defining characteristic of diplomacy and relations between such powers. "Between 1500 and 1953, there were 64 wars in which at least one great power was opposed to another, and they averaged little more than five years in length. In approximately a 450-year time frame, on average at least two great powers were fighting one another in each and every year." Even during the period of Pax Britannica (or "the British Peace") between 1815 and 1914, war and military confrontations among the great powers was still a frequent occurrence. In fact, Joshua Baron points out that, in terms of militarized conflicts or confrontations, the UK led the way in this period with nineteen such instances against; Russia (8), France (5), Germany/Prussia (5) and Italy (1).


Birds (Aves) are a group of endothermic vertebrates, characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. They rank as the class of tetrapods with the most living species, at approximately ten thousand, with more than half of these being passerines, sometimes known as perching birds or, less accurately, as songbirds.


The fossil record indicates that birds are the last surviving dinosaurs, having evolved from feathered ancestors within the theropod group of saurischian dinosaurs. True birds first appeared during the Cretaceous period, around 100 million years ago. DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Paleogene extinction event that killed off all other dinosaurs. Birds in South America survived this event and then migrated to other parts of the world via multiple land bridges while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group Avialae, have been found dating back to the mid-Jurassic period. Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.


Birds have wings which are more or less developed depending on the species; the only known groups without wings are the extinct moas and elephant birds. Wings, which evolved from forelimbs, give most birds the ability to fly, although further speciation has led to some flightless birds, including ratites, penguins, and diverse endemic island species of birds. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly the aforementioned flightless penguins, and also members of the duck family, have also evolved for swimming. Birds, specifically Darwin's finches, played an important part in the inception of Darwin's theory of evolution by natural selection.


Some birds, especially corvids and parrots, are among the most intelligent animals; several bird species make and use tools, and many social species pass on knowledge across generations, which is considered a form of culture. Many species annually migrate great distances. Birds are social, communicating with visual signals, calls, and bird songs, and participating in such social behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Birds produce offspring by laying eggs which are fertilized through sexual reproduction. They are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as hens, lay eggs even when not fertilized, though unfertilized eggs do not produce offspring.


Many species of birds are economically important. Domesticated and undomesticated birds (poultry and game) are important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertilizer. Birds prominently figure throughout human culture. About 120–130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.


Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.


Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs, and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity.


The consensus view in contemporary paleontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these form a group called Paraves. Some basal members of this group, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores.


The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.