A flashlight (usually called a torch outside North America) is a hand-held electric-powered light source. Usually the light source is a small incandescent lightbulb or light-emitting diode (LED). Typical flashlight designs consist of the light source mounted in a reflector, a lens to protect the light source and reflector, a battery or other power source, and a switch.
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.
Contents |
In 1896, 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 odorless, 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, American Electrical Novelty and Manufacturing Company obtained U.S. Patent No. 617,592 (filed 12 March 1898) from David Misell, its English inventor.[1] 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.[2][3] The company donated some of these devices to the New York City police, who responded favorably to them.[4]
These early flashlights ran on zinc鈥揷arbon batteries, which could not provide a steady electric current and required periodic 'rest' to continue functioning.[5] 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 popular name flashlight.[3]
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 around 1906, 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 lack of flame, heat, smoke and odor, meant that hand-held electric lights began to replace combustion-based lamps such as the hurricane lantern. [6] 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.[7] 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 flashlight use a halogen lamp where the bulb contains trace of halogen 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 life of a bulb may only be 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[8] 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 have existed for decades, mainly used as low-power indicator lights. In 1999, Lumileds Corporation of San Jose, California United States, 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. An LED flashlight will have a longer battery life than a comparable incandescent flashlight. LEDs are also less fragile than conventional glass lamps. LED lamps have a different spectrum of light compared to incandescent sources, and are made in several ranges of color temperature. Since the LED has a very long expected operating life compared to the usual life of a flashlight, very often the LEDs are permanently installed.
A single or two-cell flashlight requires a boost converter to provide the higher voltage required by a white LED, which need around 3.4 volts to function. Since electronics are usually required for this reason, some flashlights electronically regulate the voltage supplied to the LEDs to stabilize light output as the batteries discharge. By contrast, the light output of non-regulated flashlights declines as battery voltage declines. LEDs have the advantage of maintaining nearly constant color temperature regardless of input voltage or current, while the color temperature of an incandescent bulb rapidly declines as the battery discharges. 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 in an incandescent flashlight 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. Heat dissipation for the LED often dictates that LED flashlights have aluminum bodies to dissipate heat; they can become warm during use.[9]
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. 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 ticketholders. Infrared LEDs can be used for illuminators for night vision systems. LED flashlights may be specified to be compatible with night vision devices.
Another less common type of flashlight uses a High Intensity Discharge (HID) lamp as the light source. HID is a type of gas discharge lamp that uses a mixture of metal halide salts and argon as a filler.
HID lamps produce more light than an incandescent flashlight using the same amount of electricity. The lamp will last longer and is more shock resistant than a regular incandescent bulb, since it lacks the relatively fragile electrical filament found in incandescent bulbs. However, they are much more expensive, due to the ballast circuitry required to start and operate the lamp. An HID lamp requires a short warm-up time before it reaches full output.
A typical HID flashlight would have a 35 watt lamp and produce more than 3000 lumens.
Certain accessories for a flashlight allow the color of the light to be altered or allow light to be dispersed differently. Translucent colored plastic cones slipped over the lens of a flashlight increase the visibility when looking at the side of the light. Such marshalling wands are frequently used for directing automobiles or aircraft at night. Colored lenses placed over the end of the flashlight are used for signalling, for example, in railway yards. Colored light is occasionally useful for hunters tracking wounded game after dusk, or for forensic examination of an area. A red filter helps preserve night vision after the flashlight is turned off, and can be useful to observe animals (such as nesting Loggerhead sea turtles) without disturbing them.
A penlight is a small, pen-sized flashlight, often containing two AA batteries or AAA batteries. In some types the incandescent light bulb has an integral lens that focuses the light, so no reflector is built into the penlight. Others use incandescent bulbs mounted in reflectors. LED penlights are becoming increasingly common. Low-cost units may be disposable with no provision for the user to replace batteries or bulbs, and are sometimes imprinted with advertising for promotional purposes.
A headlamp is designed to be worn on the head, often having separate lamp and battery components. The battery pack may be attached to the back of the head or in a pocket to improve balance. Headlamps leave the user's hands free, making them popular for recreational and occupational activities. A miniature headlamp can be clipped to the brim of a hat, or built into a hat, instead of using straps; other types resemble eyeglass frames. Similar to the headlamp, an angle-head flashlight emits light perpendicular to the length of the battery tube; it can be clipped to a belt or webbing to leave the user's hands free, or set on a flat surface. Some types allow the user to adjust the angle of the head.
Sometimes a tactical light is mounted to a handgun or rifle.[10] Such lamps are designed to withstand the impact of recoil, and to be easily controlled while holding the weapon. They are small enough to be easily rail-mounted to a gun barrel.
Although most flashlights are designed for user replacement of the batteries and the bulb as needed, fully sealed disposable flashlights, such as inexpensive keyring lights, are made. When the batteries are depleted or the bulb fails, the entire product usually is thrown away or recycled and a new one purchased to replace it.
Diving lamps must be watertight under pressure and are used for night diving and supplemental illumination where surface light cannot reach. The battery compartment of a dive lamp may have a catalyst to recombine any hydrogen gas emitted from the battery, since gas cannot be vented in use.
People working in hazardous areas with significant concentrations of flammable gases or dusts, such as mines, engine rooms of ships, chemical plants or grain elevators, use "non-incendive", "intrinsically safe" or "explosion proof" flashlights. These are constructed so that any spark in the gas or dust that leaks into the flashlight is not likely to set off an explosion outside the light. The flashlight may require approval by an authority for the particular service and particular gases expected. The external temperature rise of the flashlight must not exceed the autoignition point of the gas, so substitution of more powerful lamps or batteries may void the approval.
Most flashlights are cylindrical in design, with the lamp assembly attached to one end. However, early designs came in a variety of shapes. Many resembled modern day lanterns, consisting largely of a box with a handle and the lamp attached to the front. Some others were made to have a similar appearance to candles. Portable hand-held electric lanterns can provide larger reflectors and lamps, and more powerful batteries than tubular flashlights meant to fit in a pocket. They are often designed for lighting the broad area immediately around the lantern, as opposed to forming a narrow beam; they can be set down on a level surface or attached to supports. Some electric lanterns use miniature fluorescent lamps for higher efficacy than incandescent bulbs.
Flashlights may be temporarily mounted on handlebars for bicycle lighting, although permanently mounted bicycle lamps are common.
| Other styles of flashlight | ||||||||||||
|
||||||||||||
Multifunction portable devices may include a flashlight as one of their features; for example, a portable radio/flashlight combination. Many smartphones have a button or software application available to turn up their screen backlights to full intensity, or to switch on the camera flash or video light, providing a "flashlight" function.
The most common power source for flashlights is the battery. Many types of batteries are suitable for use in flashlights, such as button cells, carbon-zinc batteries in both regular and heavy duty types, alkaline, lithium and rechargeable lead acid batteries, NiMH, NiCd batteries and lithium ion batteries. The choice of batteries will depend on the light source used, and will usually play a determining role in the size and shape of the flashlight.
Primary cells are most economical for infrequent use; some types of lithium primary cell can be stored for years, where flashlights are required only in emergencies. Flashlights used for extended periods every day may be more economical to run on rechargeable (secondary) batteries. Flashlights adapted for use with rechargeable batteries may include features to allow charging without removing the batteries from the light; for example, a light kept in a vehicle may be trickle-charged and always ready when needed. Power-failure lights are designed to keep their batteries charged from a wall plug and to automatically turn on after an AC power failure; the power-failure light can be removed from the wall socket and used as a portable flashlight. Solar powered flashlights use energy from a solar cell to charge an on-board battery for later use.
At least one manufacturer makes a rechargeable flashlight that uses a supercapacitor to store energy. The capacitor can be recharged more rapidly than a battery and can be recharged many times without loss of capacity; however, the running time is limited by the relative bulk of capacitors compared to electrochemical cells.
One type of dynamo-powered flashlight has a winding crank and spring connected to a small electrical generator that charges a capacitor. Others generate electricity using electromagnetic induction. They use a strong permanent magnet that can freely slide up and down a tube, passing through a coil of wire as it does. Shaking the flashlight will charge a capacitor or a rechargeable battery that supplies a current to a light source. Such flashlights can be useful during an emergency, when utility power and batteries may not be available.
To concentrate the light emitted by the bulb into a directed beam, a reflector is used, with an approximately parabolic shape. This reflects the light emerging in all directions from the bulb into an approximately parallel beam. Some flashlights allow the user to adjust the position of the lamp, giving a variable-focus effect from a wide floodlight to a narrow beam. The reflector is covered to keep out dirt and moisture, sometimes with just a flat transparent cover, but some designs use a plastic or glass "bulls-eye" lens to form a more concentrated beam. The lens or reflector cover must resist impacts and the heat of the lamp, and must not lose too much of the transmitted light to reflection or absorption. Reflectors may be made of polished metal, or glass or plastic with an aluminized reflective finish. Some manufacturers use a pebbled, instead of a smooth, reflector, to improve the uniformity of the light beam emitted. Where multiple LEDs are used, each one may be put in its own parabolic reflector.
Very small flashlights may not have a reflector or lens separate from the lamp. Some types of penlight incandescent bulb have a built-in glass lens. Small light-emitting diodes have a domed package that also acts as a lens.
The original 1890's flashlights used a metal ring around the fiber body of the flashlight as one contact of a switch, and the second contact was a moveable metal loop that could be flipped down to touch the ring, completing the circuit. A wide variety of mechanical switch designs using slide switches, rocker switches, or side-mounted or end-mounted pushbuttons have been used in flashlights. A common combination is a slide switch that allows the light to be left on for an extended time, combined with a momentary button for intermittent use or signalling. Since voltages and currents are low, switch design is limited only by the available space and desired cost of production. Switches may be covered with a flexible rubber boot to exclude dirt and moisture. Weapon-mounted lights may have remote switches for convenience in operation.
Electronic controls allow the user to select variable output levels or different operating modes such as pre-programmed flashing beacon or strobe modes. Electronic controls may be operated by buttons, sliders, magnets, or rotating control rings. One model of flashlight includes an acceleration sensor to allow it to respond to shaking. At least one manufacturer allows user programming of the features of the flashlight through a USB port.
Early flashlights used vulcanized fiber or hard rubber tubes with metal end caps. Since their invention, many other materials have been used. Drawn steel, plated brass, copper, silver, even wood and leather have been used. Modern flashlights are generally made of plastic or aluminum. Plastics range from low-cost polystyrene and polyethylene to more complex mixtures of ABS or glass-reinforced epoxies. Some manufacturers have proprietary plastic formulations for their products. [11] A desirable plastic for manufacturing flashlights allows for ease of molding and adequate mechanical properties of the finished flashlight case. Aluminum, either plain, painted or anodized, is a popular choice. It is electrically conductive, can be easily machined, and dissipates heat well. Several standard alloys of aluminum are used. Other metals include stainless steel and titanium, which can be polished to provide a decorative finish. Zinc can be die-cast into intricate shapes. Magnesium and its alloys provide strength and heat dissipation similar to aluminum and can also be die cast.
Industrial, marine, public safety and military organizations develop specifications for flashlights in specialized roles. Typically, light output, overall dimensions, battery compatibility and durability are required to meet minimum limits. Flashlights may be tested for impact resistance, water and chemical resistance, and for the life span of the control switch. Flashlights intended for use in hazardous areas with flammable gas are tested for external temperature, and are tested to ensure they cannot set off an explosion. [12] Flashlights approved for flammable gas areas will have markings indicating the approving agency (MHSA, ATEX, etc.) and symbols for the conditions that were tested.
Regulations for ships and aircraft will specify the number and general properties of flashlights included as part of the standard safety equipment of the vessel. Flashlights for small boats may be required to be waterproof and to float. Uniformed services may provide particular models of issue flashlight, or may only provide minimium performance standards for the member to purchase his or her own flashlights.
In the United States, ANSI in 2009 published voluntary standard FL1 Flashlight basic performance standard, which standardizes the test procedures for total light output, beam intensity, working distance, impact and water resistance, and battery running time to 10% of initial light output. The FL1 standard gives standard definitions for terms used in marketing flashlights, and standard test conditions and procedures, with the intention of allowing the consumer to compare products tested to the standard. [13] The standard recommends particular graphic symbols and wording for the product package, to allow the consumer to identify if the product has been tested to the standard. A test laboratory may charge US $2000 or more for a complete set of FL1 tests. [14]
The FL1 standard requires measurements reported on the packaging to be made with the type of batteries packaged with the flashlight, or with an identified type of battery. Initial light output is measured with an integrating sphere photometer, 30 seconds after the light is switched on with fresh (or newly charged) batteries; the total light emitted is reported in lumens. Beam intensity is determined by measuring the brightest spot in the beam produced by the flashlight, in candelas; since this is a measure of the light emitted in a solid angle, the beam intensity is independent of distance. The working distance is defined as the distance at which the maximum illuminance (light falling on a surface) would fall to 0.25 lux; this is a level of illuminance comparable to a full moon on a clear night. It is calculated from the square root of the beam intensity in candelas divided by 0.25 lux; for example, a beam intensity of 1000 candelas produces a working range rating of 63 metres. The result is reported in metres or converted to feet.
Run time is measured using the supplied or specified batteries and letting the light run until the intensity of the beam has dropped to 10% of the value 30 seconds after switching on. The standard does not evaluate the behavior of the flashlight output during run time; a regulated flashlight may run at only a slowly declining output and then abruptly cut off, but unregulated types may have steeply-declining light output after only a short time.
Impact resistance is measured by dropping the flashlight in six different orientations and observing that it still functions and has no large cracks or breaks in it; the height used in the test is reported. Water resistance, if specified, is evaluated after impact testing; no water is to be visible inside the unit and it must remain functional after either a brief immersion (IP Code IPX7) or after 30 minutes immersion at 1 metre or more (IP X8); the depth is reported if greater than 1 metre.
The consumer must decide how well the ANSI test conditions match his requirements, but all manufacturers testing to the FL1 standard can be compared on a uniform basis. The light measurements more directly represent the usefulness of flashlights instead of a measure such as the nominal power input to the lamp (watts), since different LED and incandescent lamp types vary widely in the amount of light produced per watt. Where two flashlights have similar total light (lumen) measures, the unit with the higher candela rating produces a tighter,smaller, more concentrated beam of light, more suitable for lighting distant objects; it will also have a higher working distance shown. If two lights have similar candela ratings, the light with higher lumen value will produce a wider beam and will light a wider area overall.