Mary McKinney, age 14, of East Lansing, Michigan, for her question:

What is a foot candle?

It is easy to see that the light from a 40 watt bulb is dimmer than that shed by a 100 watt bulb. But in order to distinguish between a 40 watt and a 60 watt, you might have to depend upon the small print on top of the bulb. The problem of measuring the brilliance of light in fine detail is a difficult one. The basic unit used for the fob is the candle power. It is the intensity of light shed by a standard candle. The light shed by a 40 watt electric bulb equals 32 candle power units.

One end of a light beam comes from a source such as a lamp. Its intensity is measured in candle power units. The other end of the beam falls upon a surface, shedding a certain amount of illumination. The light beam spreads out from its source and its strength diminishes with distance. A unit for measuring illumination must take this into account. This unit is the foot candle. It is the light or illumination that falls upon a spot exactly one foot away from a source supplying one unit of candle power.