Illuminance is the luminous flux incident on a surface of unit area.
Illuminance measures how much energy has fallen on a surface. This quantity is useful for describing the level of illumination incident on a surface without making the measurement dependent on the size of the surface itself.
The International System (SI) unit of illuminance is the lux (lx) which is equal to 1 lumen per square meter. The corresponding American System (AS) unit is the footcandle (fc), equivalent to 1 lumen per square foot.
Illuminance is a function of the distance from the light source. To specify the illuminance of a light, you must enter a value in lx, and the distance at which that illuminance is measured.
In 3ds Max, illuminance is one way to specify the intensity of a photometric light. See Intensity Color Attenuation Rollout (Photometric Lights).