Robert Way, age 11, of Delmar, New York, for his question:

How is the lumen measured?

A 40 watt bulb sheds about 392 lumens of light. The lumen is a small unit for measuring the luminosity from a source of light. It is related to larger units called the candela and the candlepower, both of which are used to measure the mighty beams of lighthouses. The candlepower was invented first, though the method used to measure it was not very precise. Fine crystalline wax was taken from the sperm whale to make a candle 7/8th of an inch thick. The amount of light from this source was called the British Standard Candle Power.

Nowadays we use the more accurate candela unit. It is measured by heating one square centimeter of black material to 2042 degrees Centigrade. The light from this precise source gives the measure of one candela    which happens to be slightly less than the old candlepower unit. The candela unit equals 12.57 smaller units called lumens. A 60 watt bulb in prime condition sheds almost 62 and 3/4 candelas.