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
Incandescent light bulb sources are radiant efficiencies and efficacies. A similar chart in the article on luminous efficacy compares a broader array of light sources to one another. The spectrum emitted by a blackbody radiator at temperatures of incandescent bulbs does not match the sensitivity characteristics of the human eye. Most of the radiation is not in the range of wavelengths to which the eye is sensitive. Tungsten filaments radiate mostly infrared radiation at temperatures where they remain solid – below . Donald L. Klipstein explains it this way: "An ideal thermal radiator produces visible light most efficiently at temperatures around . Even