A flashlight (torch in Commonwealth English) is a portable hand-held electric light. Usually, the source of the light is a small incandescent light bulb or light-emitting diode (LED). A typical flashlight consists of a light bulb mounted in a reflector, a transparent cover (sometimes combined with a lens) to protect the light source and reflector, a battery, and a switch. These are supported and protected by a case.
The invention of the dry cell and miniature incandescent electric light bulbs made the first battery-powered flashlights possible around 1899. Today flashlights use mostly incandescent lamps or light-emitting diodes and run on disposable or rechargeable batteries. Some are powered by the user turning a crank or shaking the lamp, and some have solar panels to recharge a battery.
In addition to the general-purpose hand-held flashlight, many forms have been adapted for special uses. Head or helmet-mounted flashlights designed for miners and campers leave the hands free. Some flashlights can be used underwater or in flammable atmospheres.
Early flashlights ran on zinc-carbon batteries, which could not provide a steady electric current and required periodic ‘rest’ to continue functioning. Because these early flashlights also used energy-inefficient carbon-filament bulbs, “resting” occurred at short intervals. Consequently, they could be used only in brief flashes, hence the common North American name flashlight.
In many English-speaking countries the word torch continued to be used for portable lighting devices even when they became battery-powered rather than based on a flame, though the terms “flashlamp” and “flashlight” were also understood.
In 1887, the first dry cell battery was invented. Unlike previous batteries, it used a paste electrolyte instead of a liquid. This was the first battery suitable for portable electrical devices, as it did not spill or break easily and worked in any orientation. Portable hand-held electric lights offered advantages in convenience and safety over (combustion) torches, candles and lanterns. The electric lamp was odourless, smokeless, and emitted less heat than combustion-powered lighting. It could be instantly turned on and off, and avoided fire risk.
On January 10, 1899, British inventor David Misell obtained U.S. Patent No. 617,592, assigned to American Electrical Novelty and Manufacturing Company. This “electric device” designed by Misell was powered by “D” batteries laid front to back in a paper tube with the light bulb and a rough brass reflector at the end. The company donated some of these devices to the New York City police, who responded favorably to them.
The angle-head flashlight on the left uses an incandescent bulb, while the adjustable angle-head flashlight on the right uses LEDs to give white, red, blue, and infrared light
Carbon-filament bulbs and fairly crude dry cells made early flashlights an expensive novelty with low sales and low manufacturer interest. Development of the tungsten-filament lamp in 1904, with three times the efficacy of carbon filament types, and improved batteries, made flashlights more useful and popular. The advantage of instant control, and the absence of flame, meant that hand-held electric lights began to replace combustion-based lamps such as the hurricane lantern. By 1922 several types were available; the tubular hand-held variety, a lantern style that could be set down for extended use, pocket size lamps for close work, and large reflector searchlight-type lamps for lighting distant objects. In 1922 there were an estimated 10 million flashlight users in the United States, with annual sales of renewal batteries and flashlights at $20 million, comparable to sales of many line-operated electrical appliances. Flashlights became very popular in China; by the end of the 1930s, 60 companies made flashlights, some selling for as little as one-third the cost of equivalent imported models. Miniature lamps developed for flashlight and automotive uses became an important sector of the incandescent lamp manufacturing business.
Incandescent flashlights use incandescent light bulbs which consists of a glass bulb and a tungsten filament. The bulbs are under vacuum or filled with argon, krypton or xenon. Some high-power incandescent flashlights use a halogen lamp where the bulb contains a halogen gas such as iodine or bromine to improve the life and efficacy of the bulb. In all but disposable or novelty flashlights, the bulb is user-replaceable; the bulb life may be only a few hours.
The light output of an incandescent lamp in a flashlight varies widely depending on the type of lamp. A miniature keychain lamp produces one or two lumens. A two D-cell flashlight using a common prefocus-style miniature lamp will produce on the order of 15 to 20 lumens of light and a beam of about 200 candlepower. One popular make of rechargeable focusing flashlight uses a halogen lamp and produces 218 lumens. By comparison, a 60-watt household incandescent lamp will produce about 900 lumens. The luminous efficacy or lumens produced per watt of input of flashlight bulbs varies over the approximate range of 8 to 22 lumens/watt, depending on the size of the bulb and the fill gas, with halogen-filled 12 volt lamps having the highest efficacy
Powerful white-light-emitting diodes (LED)s are increasingly replacing incandescent bulbs in practical flashlights. LEDs existed for decades, mainly as low-power indicator lights. In 1999, Lumileds Corporation of San Jose, California, introduced the Luxeon LED, a high-power white-light emitter. This made possible LED flashlights with power and running time better than incandescent lights. The first Luxeon LED flashlight was the Arc LS, designed in 2001. White LEDs in 5 mm diameter packages produce only a few lumens each; many units may be grouped together to provide additional light. Power LEDs, drawing more than 100 milliamperes each, simplify the optical design problem of producing a powerful and tightly-controlled beam.
LEDs can be significantly more efficient than incandescent lamps, with white LEDs producing on the order of 100 lumens for every watt, compared to 8-10 lumens per watt of small incandescent bulbs. An LED flashlight will have a longer battery life than an incandescent flashlight with comparable output. LEDs are also less fragile than glass lamps. LED lamps have different spectra of light compared to incandescent sources, and are made in several ranges of color temperature and color rendering index. Since the LED has a long life compared to the usual life of a flashlight, very often it is permanently installed.
LEDs generally must have some kind of control to limit current through the diode. Flashlights using one or two disposable 1.5 volt cells require a boost converter to provide the higher voltage required by a white LED, which need around 3.4 volts to function. Flashlights using three or more dry cells may only use a resistor to limit current. Some flashlights electronically regulate the current through the LEDs to stabilize light output as the batteries discharge. LEDs maintain nearly constant color temperature regardless of input voltage or current, while the color temperature of an incandescent bulb rapidly declines as the battery discharges, becoming redder and less visible. Regulated LED flashlights may also have user-selectable levels of output appropriate to a task, for example, low light for reading a map and high output for checking a road sign. This would be difficult to do with a single incandescent bulb since efficacy of the lamp drops rapidly at low output.
LED flashlights may consume 1 watt or much more from the battery, producing heat as well as light. In contrast to tungsten filaments, which must be hot to produce light, both the light output and the life of an LED decrease with temperature. Heat dissipation for the LED often dictates that small high-power LED flashlights have aluminium or other high heat conductivity bodies, reflectors and other parts, to dissipate heat; they can become warm during use.
Light output from LED flashlights varies even more widely than for incandescent lights. “Keychain” type lamps operating on button batteries, or lights using a single 5 mm LED, may only produce a couple of lumens. Even a small LED flashlight operating on an AA cell but equipped with a power LED can emit 100 lumens. The most powerful LED flashlights produce more than one thousand lumens and may use multiple power LEDs.
LEDs are highly efficient at producing colored light compared with incandescent lamps and filters. An LED flashlight may contain different LEDs for white and colored light, selectable by the user for different purposes. Colored LED flashlights are used for signalling, special inspection tasks, forensic examination, or to track the blood trail of wounded game animals. A flashlight may have a red LED intended to preserve dark adaption of vision. Ultraviolet LEDs may be used for inspection lights, for example, detecting fluorescent dyes added to air conditioning systems to detect leakage, examining paper currency, or checking UV-fluorescing marks on laundry or event ticket holders. Infrared LEDs can be used for illuminators for night vision systems. LED flashlights may be specified to be compatible with night vision devices.
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