Because the power is applied directly to the bulb, bulbs are designed for certain voltages, for instance 3 volts. A common type of flashlight bulb is a flanged PR bulb used in Maglite C and D cell flashlights. Some lights require bulbs that are threaded. Smaller bulbs, like the the xenon light included in the Mini Maglite are "bi pin" and have two pins that fit into sockets.
To achieve the maximum brightness from an incandescent bulb, the filament is designed to get very hot, but this greatly decreases the overall life of the bulb. Under-driven bulbs will last for a long time but produce much less light.
While incandescent lights may last only a few hundred hours and aren't very efficient, the advantage is the bulbs are generally pretty inexpensive, don't require any supporting electronics or drivers, and give a white light with a broad spectrum and a high color rendering index (CRI).
Protecting the filament
Incandescent lights are all about maximizing the output of the filament while protecting it from burning out. There are three dangers to the filament:
- oxidation: if any oxygen gets to the filament, the tungsten will oxidize and burn out
- evaporation: as the filament gets hot, parts of it want to cook off which will concentrate heat in the weakened area and burn it out. Evaporating filament can sometimes deposit on the glass bulb itself, darkening it.
- heat loss: the key to producing the light is keeping the filament hot so any lost heat makes the light dimmer and requires more power to keep the filament hot
A vacuum will get rid of oxygen and is an excellent insulator to prevent heat loss, but it won't do much about evaporation. Therefore chemically inactive (inert) gases are added. There is a family of inert elements called "noble gases" which includes helium, neon, argon, krypton, and xenon. Because they are chemically inactive, they will not oxidize or react with the filament. In fact they help prevent evaporation of the filament, but they do allow some heat loss. They have varying levels of performance with xenon performing better than krypton, which in turn performs better than argon. Because flashlight bulbs are small, the cost of the gas does not make much difference and therefore xenon is fairly common despite it being more expensive than krypton.
There is another family of elements called halogens that includes chlorine, iodine, and bromine. Unlike the noble gases, these elements are chemically active. By adding a small amount of a halogen gas (usually iodine, but most of the rest of the gas in the bulb would still be inert krypton or xenon) inside a bulb, a process sets up where tungsten evaporating off of the filament is actually re-deposited. This allows the filament to be driven harder and achieve longer life than would be possible without the presence of the halogen and also increases efficiency of the bulb. However halogen lights operate at extremely high temperatures.
Don Klipstein's lighting website has some great information about all kinds of light written in a way that is easy to understand. The xenon lamp article has some good information about incandescent lights, particularly as they relate to flashlights.