Fundamental Theory

Luminous Flux

The rate of flow of luminous energy emitted by a “self-generated light” object. It is measured in lumens (lm).

Luminous Intensity

Describes the power of the source to emitted light in a particular direction measured in candela (cd).


It refers to the quantity of light falling onto an object or surface. This is measured in lux (lumens per square meter) and helps in determining the level of lighting within a room. Illuminance refers only to the light which falls onto particular surfaces and does not give any indication of the room’s brightness.


It relates to the observed brightness of an object. It is determined by how much light is reflected back and is measured in candelas per square meter (cd/m). Since a surface’s reflective properties can vary widely, similar illuminance can result in different luminance. For example, if a black and a white room were lit to the same illuminance level, the luminance of the black room would be very low and dull compared to that of the white room.