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 instantly at any temperature, their extremely long lifetime is unaffected by cycling on and off, they have no glass to break, and they don't emit UV rays that fade colored materials. LED lamps radiate almost no heat, and can be either multi-directional or unidirectional, eliminating the need for a mirrored reflector in the bulb or fixture. LED lamps can emit saturated colored light. Disadvantages include spectrum limitations due to discrete emission colors, and higher up-front cost than incandescent bulbs. Botswana has an exchange program distributing CFL bulbs. Of the members of ECOWAS, Cape Verde, Guinea, Mali, Nigeria, Benin and Togo