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.

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.[PAGEBREAK]

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.

LEDs

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.

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.[PAGEBREAK]

Power Supply

Most flashlights are powered by disposable or rechargeable batteries. Your best choice between these should consider how you plan to use the light. If the light is your primary, use-it-every-day tool, rechargeable batteries are probably your best option.

Nickel-metal hydride (NiMH) batteries, like the Eneloop from Sanyo, look and work just like disposable batteries and will hold a charge for up to a year. They're cheap enough that you can rotate through three sets (one in use, one spare, one in the charger) and probably never run out of light. NiMH cells are generally good for about 500 charge-discharge cycles. It's better to charge them when they're only partially run down, rather than flat. They are not prone to battery "memory," where charging the battery before it's completely flat will reduce its capacity. This is a problem with nickel-cadmium (NiCad) cells.

If the light is a spare, or one that won't be used much, replaceable/disposable lithium or alkaline batteries are a better bet. These batteries will hold a charge for up to 10 years. Battery capacity is measured in watt-hours (1 watt-hour is one watt of power delivered for one hour). Use this to compare different brands. Lithium cells are more costly, but deliver more watt-hours.

One drawback of both disposable and rechargeable batteries is temperature sensitivity. In my patrol days, I started each winter night shift by starting the patrol car, turning the defroster to full blast, and placing my rechargeable halogen light on the dashboard next to the defroster vents so it would warm. If I didn't do that, I had an aluminum paperweight. Once the light was warmed up, it would usually stay warm enough next to my body to keep it alive for the entire shift.

Whichever you choose, make sure you dispose of used batteries properly. Most hardware and electronics stores will accept batteries for recycling at no charge.

Supercapacitors

One unique flashlight that doesn't use batteries at all is the Light for Life from 5.11 Tactical. Instead of a battery, the Light for Life contains a supercapacitor that stores enough power to keep its LEDs on their 90-lumen medium-bright setting for half an hour, and at 20 lumens for almost an hour more. Holding the switch down produces a high beam of 270 lumens, either steady-burning or strobe.

No matter how much or how little juice remains in the supercapacitor, placing it in the supplied AC or DC car charger brings it to full power in less than 90 seconds. A flashing blue LED on the charger starts blinking when the light is inserted. The blinking becomes more rapid until it stays lit, indicating a full charge. The supercapacitor is rated for 50,000 charge-discharge cycles.

I received one of these lights as a product sample almost two years ago, and I was skeptical of the technology. The light has been traveling in my car, plugged into its charger, for all that time. It has never failed to work, and it still takes and holds a charge as well as it did on Day One. Had I tried this with a conventional rechargeable light, the battery would be fried by now.

Supercapacitors actually work better as the temperature drops, so that's not a problem. The light might not burn as long at the brightest setting as a conventionally powered light, but then again, it can be recharged to full capacity by the time you turn the corner from your last traffic stop. This innovation could become the next new standard in law enforcement lighting.

Tim Dees is a retired police officer and the former editor of two major law enforcement Web sites. He can be reached via editor@policemag.com.