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 mercury contamination from CFLs have been raised, but CFLs can be shown to result in less mercury in the environment compared to traditional incandescent bulbs due to the significant reductions in power plant emissions. Thus, use of energy-efficient bulbs can actually reduce the amount of mercury released into the environment. Compact fluorescent lamps start poorly when very cold, and most types cannot be dimmed. A few specialist applications are unsuitable for CFLs. Light emitting diode (LED) lamps are used for both general and special-purpose lighting. Compared to fluorescent bulbs, their advantages are that they contain no mercury, they turn on