Luminous Energy

Luminous energy is a photometrically weighted radiant energy. It is measured in lumen per seconds (lm sec^-1).

Luminous Flux Density (Illuminance and Luminous Exitance)
Luminous flux density is a photometrically weighted radiant flux density. Illuminance is the photometric equivalent of irradiance, whereas luminous exitance is the photometric equivalent of radiant exitance.

Luminous flux density is measured in lux (lx) or lumens per square meter (lm m^-2).


Luminance
Luminance is a photometrically weighted radiance. In terms of visual perception, we perceive luminance. It is an approximate measure of how “bright” a surface appears when we view it from a given direction. Luminance used to be called “photometric brightness.” Luminance is measured in lumens per square meter per steradian (lm m^-2 sr^-1).