History of the telescope of the IRAS satellite in 1983 revolutionized infrared astronomy from space. This reflecting telescope which had a mirror, operated for nine months until its supply of coolant (liquid helium) ran out. It surveyed the entire sky detecting 245,000 infrared sources—more than 100 times the number previously known. Although optical telescopes can image the near ultraviolet, the ozone layer in the stratosphere absorbs ultraviolet radiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternative coatings such as magnesium fluoride or lithium fluoride are used
Ultraviolet index numerical ranges, the recommendations for protection are as follows: When interpreting the UV Index and recommendations, be aware that: According to the U.S. Environmental Protection Agency, a helpful method to estimate the amount of UV radiation people are exposed to is by observing the length of their shadow. A taller shadow may mean a lower UV exposure while a shorter one could translate to exposure to higher levels of UV radiation. Ultraviolet index The ultraviolet index or UV Index is an international standard measurement of the strength of sunburn-producing ultraviolet (UV) radiation at a particular place and time. The scale