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 to 40 percent more efficient than standard incandescent lamps designed for a 2000-hour life. Depending on size, voltage, and designed life, small low-voltage halogen lamps can have 70% higher efficacy than large line-voltage lamps. The high operating temperature of halogen bulbs may be a safety hazard in some applications. A compact fluorescent lamp (CFL) uses a fluorescent lamp tube which is curved or folded to fit into the space of an incandescent bulb and contains a compact electronic ballast in the base of the lamp. Compared to general-service incandescent lamps giving the same amount of visible light, CFLs use one-fifth