There are essentially four types of flashlights: those with incandescent bulbs, fluorescent bulbs, LED bulbs and HD bulbs. Flashlights with incandescent bulbs are the most common type because incandescent bulbs are easy to manufacture and cheap to produce. The bulbs are fragile and must be replaced frequently. They also produce a relatively weak output for the energy used, (although the amount of light produced can be improved by introducing a noble gas such as krypton or xenon into the bulb).
Fluorescent bulbs hold a noble gas (argon, krypton, xenon or neon) mixed with mercury vapor. Electric current excites the mercury vapor, which gives off weak ultraviolet light. This UV light causes a special coating on the interior of the glass to give off light. Fluorescent bulbs are very efficient, but they do contain mercury and can harm the environment. They are mostly used as portable lanterns and not as flashlights. The newest categories of bulbs used in flashlights are LED bulbs and HD bulbs.
LEDs (light emitting diodes) are not really bulbs at all; they are semiconductors with a very special property. They pass electrons through in only one direction and emit photons (light) as this process occurs. LEDs have become cheaper to produce in recent years and new semiconductor combinations now produce brighter light in more useful wavelengths. While early LEDs were only available with weak red glows, modern LEDs produce brilliant yellow, blue and white light. They use less energy than other bulb types and are extremely durable. LEDs are more costly to produce than standard bulbs, but the difference is negated by their extreme longevity. LED lights are the current choice for professional devices.
HD bulbs, more correctly referred to as HID (high intensity discharge) bulbs, are the newest, brightest and most expensive light bulbs on the market. HD bulbs combine xenon gas with metal salt vapors. Electrical current generates a reactive arc which creates tremendously bright light in a very wide portion of the visible spectrum.