George Sharp Jr., age 14, of Bowling Green, Ohio, for his question:

HOW DOES A LIGHT METER WORK?

A light meter is an instrument used to measure the strength of light. Most light meters used today are photoelectric light meters. They use photo cells made of selenium, a substance which produces a weak electric current when light shines on it.

As more light falls on the photo cell, more electric current flows. By measuring this current with a sensitive electric meter, the strength of the light is indicated.

Other photoelectric light meters use a cadmium sulfide cell to control an electric current supplied by a battery. The amount of current the cells passes depends on how much light shines on it. This current then runs an electric meter.

Different light meters have been invented for use by astronomers, illumination experts and photographers.

Astronomers use light meters to measure light from stars. Illumination experts use light meters called illuminometers, foot candle meters or luxmeters to measure the lighting in homes, schools, and offices.

Photographers use light meters, called exposure meters, to tell them how to set their cameras to expose film correctly. Some cameras have built in exposure meters that automatically set the camera for the correct exposure.

Photoeletric light meters are replacing photometers and other light meters because they are easier to use. A photometer is a light meter that measures the strength of one light by comparing it with another light of known strength. Both lights illuminate separate parts of a viewing screen.

On the viewing screen, the known light is dimmed by moving it away from the screen or with a wedge that absorbs light. When the illumination on both parts of the screen is equal, the strength of the unknown light can be found.

A scale on the wedge, or the distance the known light was moved, will give the strength of the unknown light.

Light meter scales give readings in foot candles, lumens, luxes or a number than can be translated into other units. Photographers' meters give readings in camera settings for different light strengths.

Foot candle is a unit of measurement of illumination, the amount of light that falls on an object. The foot candle is part of the customary, or English, system of measurement.

There are two types of photoelectric light meters used by photographers: incident light meters and reflected light meters.

An incident light meter measures the amount of light on the subject. The meter must receive the same light as the subject to give an accurate reading.

A reflected light meter measures the light that it reflected from the subject toward the camera. To take a reading, point the meter toward your subject. Hold the meter about six inches from the subject's face. The meter will measure light reflected from the face instead of light from the sky or background.