Photo: iStockphoto.com.
To the casual observer, a discussion of "flashlight technology" seems like gilding the lily. You push a button, a light comes on; you push it again, the light goes off. Not very technical. Forty years ago, that would have been the extent of the discussion. Today, our flashlights use a wide variety of power sources and types of illumination, and generally cost a lot more than the $1.98 one would pay for a two-cell light at the hardware store.
Up until relatively recently, the incandescent bulb was the source of most artificial light, both in the home and in the typical flashlight. A standard incandescent bulb uses a tungsten wire, aka a filament, strung between two electrodes. The filament is contained within a sealed glass chamber (the bulb) from which the air has been extracted to a near-vacuum and partially replaced with nitrogen, a halogen gas, or a mixture of the two. When current is passed through the filament, it heats up and glows brightly. The absence of oxygen inside the bulb keeps it from burning up. Break that vacuum seal while the light is on, and you'll see a brief white flash as the filament oxidizes and evaporates.
It's difficult to maintain a complete vacuum inside a thin-walled glass bulb. For this reason, nitrogen was introduced to replace the air evacuated from the bulb. This was an imperfect solution, as the tungsten would gradually evaporate over the life of the bulb, and deposit itself on the inside walls of the glass. The bulb would get increasingly dim as the glass was coated black, and needed replacement before it burned out.
Halogen and Xenon Lamps
The use of a halogen (HAL-low-gen) mixed with nitrogen solved this problem. Halogens are the elements that take up the next-to-the-last column of the periodic table of the elements you had on the wall of your high school chemistry classroom: fluorine, chlorine, bromine, iodine, and astatine-only the first four occur in nature. In the presence of a halogen (usually iodine or bromine), the evaporated tungsten redeposits itself on the filament instead of on the interior walls of the glass bulb. This allows halogen lamps to burn brighter and longer than conventional incandescent lamps.
Halogen lamps produce light with a higher color temperature than conventional incandescent lamps. The light from a conventional bulb appears white, but it actually has a yellow cast that isn't apparent until it's compared with light of a higher color temperature. Shine the light from a halogen-bulb flashlight onto the wall of a room lit with standard light bulbs, and the difference becomes evident.
Halogen lamps produce a very white light, but also put out a lot of heat. Photo: Cool Koon
Even brighter and hotter is the light from a xenon arc lamp. Xenon (ZEE-nahn) is one of the elements in the far right column of the periodic table, one of the so-called "noble gases." The elements in this group are generally inert and nonreactive, refusing to combine with other elements. This property makes them ideal for sustaining the incredibly hot process that makes xenon arc lamps function.
Xenon lamps don't use a filament in the way incandescent lamps do. There are tungsten electrodes inside the lamp, but the light is created from a tiny cloud of plasma that results when an electron stream arcs from the cathode to the anode. This light is very close to the color temperature of daylight, and appears almost blue. High-intensity discharge (HID) headlights, with their blue-white color cast on certain high-end cars, are easily distinguished from those with conventional lamps. Most of those are xenon arc lamps; a few are cheaper "pretender" conventional incandescent lamps with blue filters.