Luminous Flux - A unit of measure: lumen (lm). All of the radiated power emitted by a light source and perceived by the human eye.

Luminous Intensity - Luminous intensity is a unit measure of the luminous flux in a solid angle.

Luminous Efficiency - Unit of measure: Lumens per watt (lm/W). Indicates the efficiency with which the power consumed is converted to light.

Lumen - A unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. The most common measurement of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens. For example, a R30 65-Watt indoor flood lamp may have a rating of 750 lumens. Similarly, a light fixture's output can be expressed in lumens. As lamps and fixtures age and become dirty, their lumen output decreases (i.e., lumen depreciation occurs). Most lamp ratings are based on initial lumens (i.e., when lamp is new).

Lux - The metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter. One lux equals 0.0929 foot-candles.