Defining the brightness of an LED lighting or other light source in terms of the luminous flux (the light output) of a is more of a theoretical measure. Luminous flux, measured in lumens (lm), describes the total light output emitted by the lamp in all directions.

More important for the user is the luminous intensity, which is measured in candela (cd). It defines the luminous flux that is emitted in a specific spatial direction.

The illuminance is of special importance. This is given in lux (lx) and is a measure of how much light arrives from our light source on a given surface. The illuminance depends on the illuminated solid angle and the working distance from the light source. The illuminance decreases with the square of the distance.