Flashlight
Early flashlights ran on zinc–carbon batteries, which could not provide a steady electric current and required periodic "rest" to continue functioning.
Portable hand-held electric lights offered advantages in convenience and safety over (combustion) torches, candles and lanterns.
[3] 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.
Development of the tungsten-filament lamp in 1904, with three times the efficacy of carbon filament types, along with improved batteries in varying sizes 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.
[5] By 1907, several types of flashlights were available: the tubular hand-held variety, a lantern style that could be set down for extended use, pocket-size penlights for close work, and large reflector searchlight-type lamps for lighting distant objects.
A two-D-cell flashlight using a common prefocus-style miniature lamp produces on the order of 15 to 20 lumens of light[11] and a beam of about 200 candlepower.
[citation needed] White LEDs in 5 mm diameter packages produce only a few lumens each; many units may be grouped together to provide additional light.
Higher-power LEDs, drawing more than 100 milliamperes each, simplify the optical design problem of producing a powerful and tightly controlled beam.
"Keychain" type lamps operating on button batteries, or lights using a single 5 mm LED, may only produce a few lumens.
Colored LED flashlights are used for signalling, special inspection tasks, forensic examination, or to track the blood trail of wounded game animals.
HID lamps produce more light than a traditional incandescent flashlight using the same amount of electricity, though not as much as high power LEDs.
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.
Similar to the headlamp, an angle-head flashlight emits light perpendicular to the length of the battery tube; it can be clipped to a headband, belt, or webbing or set on a flat surface.
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 "nonincendive", "intrinsically safe", or "explosion-proof" flashlights constructed so that any spark in the flashlight is not likely to set off an explosion outside the light.
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.
[17] Otoscopes and ophthalmoscopes are medical instruments that combine a hand-held light source and magnifying lenses for examination of the ear canal and eyes, respectively.
Portable hand-held electric spotlights can provide larger reflectors and lamps and more powerful batteries than tubular flashlights meant to fit in a pocket.
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.
One type of mechanically powered flashlight has a winding crank and spring connected to a small electrical generator (dynamo).
Some flashlights allow the user to adjust the relative position of the lamp and reflector, giving a variable-focus effect from a wide floodlight to a narrow beam.
The reflector may have a flat transparent cover to keep out dirt and moisture, but some designs have a plastic or glass "bulls-eye" lens to form a 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.
Some models of flashlight include an acceleration sensor to allow them to respond to shaking, or to select modes based on what direction the light is held when switched on.
[21] Metal cases provide better heat dissipation for the LED, but plastics are not electrically conductive and may resist corrosion and wear.
Flashlights for hazardous areas may be designed to automatically disconnect the lamp if the bulb is broken, to prevent ignition of flammable gas.
[23] Regulations for ships and aircraft specify the number and general properties of flashlights included as part of the standard safety equipment of the vessel.
Manufacturers of headlamps may use a different standard which rates run times until light output falls to 1 lux at 2 meters distance; this value is not comparable to the FL 1 runtime measurement.
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.
In 2018, in the United States, Underwriter's Laboratories published UL standard 1576 for flashlights and lanterns, outlining safety requirements and performance tests.
