Incandescent light bulb Small variations in resistivity along the filament cause "hot spots" to form at points of higher resistivity; a variation of diameter of only 1% will cause a 25% reduction in service life. These hot spots evaporate faster than the rest of the filament, which increases the resistance at that point—this creates a positive feedback that ends in the familiar tiny gap in an otherwise healthy-looking filament. Irving Langmuir found that an inert gas, instead of vacuum, would retard evaporation. General service incandescent light bulbs over about 25 watts in rating are now filled with a mixture of mostly argon and
Phase-out of incandescent light bulbs the technology becomes available. Utah has also undertaken a number of LED based lighting projects to regain efficiency. In December 2007, the federal government enacted the Energy Independence and Security Act of 2007, which contains legislation to make incandescent light bulbs more efficient by setting maximum wattage requirements for all general service incandescent light bulbs producing 310–2600 lumens of light. Light bulbs outside of this range are exempt from the restrictions. Also exempt are several classes of specialty lights, including appliance lamps, rough service bulbs, 3-way, colored lamps, stage lighting, plant lights, candelabra lights under 60 watts, outdoor post lights