For example, a woman may use the same light for reading a newspaper and for hemming a black skirt with black thread. But her eyes need almost seven times as much light for this job as for reading. Engineers use either the foot-candle or the lux to measure the amount of light that falls on a surface. The foot-candle is a unit in the customary system of measurement, and the lux is a metric unit. A sensitive instrument called a light meter records how much light a surface receives at that point.
Scientists measure the intensity of light coming from any source in units called lumens.
Watts tell the amount of electricity used by a lamp, not how much light it produces. According to a principle known as the inverse square law, the amount of light an object receives depends on its distance from the source. An object placed 2 feet from a lamp receives only a fourth as much light as when it is placed 1 foot from the lamp. Dark colors absorb light, and so dark carpets, ceilings, curtains, furniture, or walls may limit the amount of light in rooms. But pale-colored walls and furnishings reflect light back into a room.