The quantity of light produced by any lamp may be expressed in three ways:
Lumens are the units used to measure light quantity as the light leaves the lamp. In incandescent lamps, if one would screw a plain incandescent lamp into a plain socket with no reflector, the light would be emitted at 360 degrees. If one would place a lumen meter at any point around the lamp, the meter would measure the amount of light in that direction. A lumen can be described as the amount of light passing a given point at a given time, irrespective of direction.
Initial lumens are the amount of light in lumens (for a fluorescent lamp), after about 100 hours of operation.
Mean lumens are the amount of light the lamp produces after it has operated for approximately 40 percent of its rated life. Lamp light reduces as the lamp continues to be used, hence mean lumens have a lower value than initial lumens.
Foot-candles is a measurement of the amount of light that reaches the work surface. Not all the light a lamp produces reaches the work plane. Some light is lost in the fixture and some is absorbed by room surfaces before it reaches the work surface.
The Illuminating Engineering Society (IES), a group of professional lighting engineers and designers, establishes the standards for the amount of light (foot-candles), which should be provided for certain activities to be performed in a given area.
Candle-power is the term given to the measurement of light when one puts a light bulb into a reflector, or uses a reflector lamp. Candle-power is a directed lumen, or light that is forced, by a reflector, in a certain direction. This measurement usually records the maximum beam candle-power, which means the amount of light directly in the center of the beam. This beam can be a spot, a narrow beam, or a flood.