This xenon lamp runs an IMAX projector. Note the absence of a filament between the electrodes. The light is produced from a cloud of plasma that arcs from the cathode to the anode. Photo: Atlant
Conventional incandescent, halogen, and xenon arc lamps all have the same Achilles' heel: heat. They are grossly inefficient, with as little as 2 percent of their energy devoted to light output. Most of the power consumed by these lights is cast off as heat. One of the unsung victims of the transition from incandescent to compact fluorescent (CFL) lamps in homes and businesses is the Hasbro Easy-Bake Oven many girls grew up with. The heat source for these was a 100-watt incandescent light bulb. The newer models have their own self-contained heating elements.
Halogen-bulb rechargeable flashlights were new, cutting-edge equipmen t when I started in law enforcement in 1979. They were far brighter than the multi-cell conventional flashlights most cops used up until then, and, of course, you didn't have to replace the batteries every week or so. The increased heat output from the lamp was both a blessing and a curse. When I was trying to lift latent prints in cold weather, placing the lens of my lit flashlight over the lift tape before I pulled it off the latent surface warmed up the adhesive and gave me a cleaner lift. On the downside, several officers melted the vinyl covers of their car seats when a light was left on accidentally.
Now light-emitting diodes (LEDs) are replacing xenon and halogen lamps in many flashlights. LEDs produce light via electroluminescence, rather than from a glowing filament or plasma cloud. Electrons still pass from an anode to a cathode, but when the electrons meet a "hole" in the junction between the electrodes, it falls into a lower energy level and releases that energy in the form of a photon, or light particle. Not as much light is produced as there would be from a hot filament or plasma cloud, but the process doesn't use very much energy, either. The shortfall is usually made up by using lots of LEDs and focusing their light output carefully with a reflector.
LEDs have been around for almost 100 years, but until the 1970s they were too expensive for most commercial applications. The first inexpensive LEDs were red, and used for calculator and digital watch displays. They were almost unreadable in sunlight. Now LEDs are available in a variety of colors, and with greatly increased light output.
Several manufacturers are advertising flashlights made with "CREE LEDs," labeling them in a way that implies this is a new type of LED. In fact, Cree Inc. is a semiconductor manufacturer based in North Carolina. They make, among other things, high-brightness LEDs used for various lighting applications.
LEDs have become the new standard in most flashlights, including many weapon-mounted lights, such as this model from Elzetta. Photo: Elzetta.
Lumens, Candelas, and Candlepower
The advertised light output of many flashlights is often characterized in terms of lumens, candelas, and/or candlepower. These terms are not interchangeable, and can be misleading, even when comparing like units of measurement. Candlepower, as the term implies, is a measure of radiated light in all directions equivalent to that from a single candle. A single common candle emits light with a luminous intensity of one candela.
With flashlights, we're seldom concerned with how much light is radiated in all directions; we intend to direct light in a fairly narrow beam. Therefore the lumen, or luminous flux, is a better measurement of this type of light, as it defines the brightness of light perceived by the human eye. Some lights are characterized as emitting X "peak lumens." The peak lumen measurement is the brightest point of the light's beam. One candela is roughly equivalent to 12.57 lumens. You can read with 10-20 lumens directed onto a sheet of paper. For lighting up the interior of a car or searching a building, you probably want 100 lumens or more.