Light Meter

Most light meters include a photo cell made of cadmium sulfide or gallium arsenide. The cell, which is attached to a battery, resists the flow of electric current from the battery by an amount that depends on the brightness of the light. The current is measured by a current meter. When brighter light falls on the cell, the resistance decreases. More current is thus allowed to flow, which produces a larger reading on the current meter.

A less sensitive type of light meter uses a cell of selenium. When light shines on it, the cell produces a small voltage, which is measured by a voltage meter. The voltage and the meter reading it increase with the brightness of the light.

The brightness of light can be expressed in terms of foot-candles, lumens, luxes, or other units. Many photographers' meters measure brightness in arbitrary units chosen by the meter's manufacturer. The units correspond to exposure settings that are on a photographer's camera.

Many cameras feature built-in light meters. Such light meters measure the amount of light entering the camera lens from a narrow range of angles in front of the camera. When the camera points toward an object that reflects light well, the meter registers bright light and so indicates the need for a short exposure. If the rest of the scene is dim, however, the picture may be badly exposed. For this reason, most professional photographers prefer to use a separate light meter that can be directed at the source of light. Such meters are better indicators of the appropriate exposure because they measure the light that illuminates the scene rather than measure the light reflected by individual objects.

Contributor: Jearl Walker, Ph.D., Prof. of Physics, Cleveland State Univ.

See also Candela; Foot-Candle; Lighting; Photography.

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