>The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 ×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

The energy density striking an object is given in lumens per square meter, generally known as lux.

This unit of invisible light in transit is the lumen. The official definition of the lumen, the unit of luminous flux, is: The luminous flux dF of a source of luminous intensity I (cd) in an element of solid angle dR is given by dF = IdR. In plain English: The flux from a light source is equal to the intensity in candela multiplied by the solid angle over which the light is emitted, taking account of the varying intensity in different directions.

2. Explanation of Luminance Units

Active illuminants like LED and incandescent lights adopt Candle Power（CD）as the unit of luminous intensity, while reflective or penetrating illuminants adopt Lumen (lm) as the unit of luminous flux. Lux is the SI unit of luminance and luminous emittance. It is used in photometry as a measure of the intensity of light, with wavelengths weighted according to the luminosity function, a standardized model of human brightness perception.

One footcandle is approximately equal to 10.764 lux. The footcandle (or lumen per square foot) is a non-SI unit of illuminance. Like the BTU, it is obsolete but it is still in fairly common use in the United States, particularly in construction-related engineering and in building codes. Because lux and footcandles are different units of the same quantity, it is perfectly valid to convert footcandles to lux and vice versa.