ARTIFICIAL LIGHT SOURCES [Film Technology]

artificial light sources
[Film Technology]

     In the absence of adequate sunlight, photographers use artificial light to illuminate scenes, both indoors and outdoors. The most commonly used sources of artificial illumination are the electronic flash, or “strobe”; tungsten lamps called photofloods; and quartz lamps. Another once-popular light source, the flashbulb, a disposable bulb filled with oxygen and a mass of fine magnesium alloy wire that fired only once, is largely obsolete, having been replaced by inexpensive, economical electronic flash units. The electronic flash (effectively a kind of stroboscope) consists of a glass quartz tube filled with an inert gas and a halogen gas at extremely low pressure. When high voltage is applied to the electrodes sealed at the ends of the tube, the gas ionizes and produces an intense burst of light of very short duration, a flash. Although large, special-purpose units can produce a flash of about 1/100,000 of a second, most produce flashes lasting from 1/5,000 to 1/1,000 of a second. Flash units must be synchronized with the shutter of the camera so that the burst of light covers the entire scene. Synchronization is achieved through an electrical connection between camera and flash unit, either a bracket mounted on top of the camera, called a “hot shoe”, or a cord called a “synch cord” that runs from the camera's synchronization socket to the flash. Automatic flash units are equipped with sensors, photocells that automatically adjust the duration of the flash for a particular scene. The sensor, which measures the intensity of the flash as it occurs, cuts off the light when adequate illumination is obtained. The dedicated flash, a newer type of automatic flash, is designed to function as a unit with a particular camera. The electronic circuitry of the flash and camera are integrated. The sensor is located inside the camera and gauges the amount of light at the film plane, allowing more accurate measurement of flash intensity. Flash units vary in size from small camera-mounted units to large studio flash units. Generally speaking, the larger the unit, the greater the intensity of light produced. Camera-mounted flashes are adequate for illuminating small scenes, but to illuminate a large scene evenly, and with a single burst of light, a powerful studio unit is needed. Photofloods, incandescent bulbs with filaments thinner than those used in ordinary light bulbs, provide continuous light. For normal colour rendition in colour photography, photofloods must be used with either tungsten-balanced film or a light-balancing filter. Quartz lighting, the standard of the television industry because of the great intensity of light produced and relative longevity of the bulbs when compared to tungsten sources, is also popular among still photographers.

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