Stephen Strahan III, age 13, of Dodge City, Kan., for his question.

WHAT IS A CANDELA?

A candela is the unit of measurement of luminous intensity, the amount of light produced in a certain direction by a glowing object. The more luminous intensity a light has, the brighter the light appears. Luminous intensity is sometimes called candlepower.

One candela is the amount of light that shines out through a hole in one side of a ceramic box after the box has been heated to 3,222 degrees Fahrenheit. The box is called a blackbody radiator and the hole measures one sixtieth of a square centimeter.

The radiator is wrapped in platinum and heated until the metal begins to melt. Then the box is cooled until the platinum hardens again. When the platinum begins to harden, the temperature is 3,222 degress Fahrenheit. At such a temperature, the ceramic inside the box glows with intense light. This light shines out through the hole in the box.

The candela is used to calculate other units of light measurement. These units include lumens and footcandles. The unit of measurement for luminuous intensity was once the candle, the amount of light produced by a certain kind of candle. But scientists found this unit too difficult to standardize. In 1948,the International Commission on Illumination adopted the candela. One candela is slightly less than one candle.