Lighting Terminology


Luminous Flux (lm):



Luminous Efficacy (lm/w):

Luminous efficacy is a figure of merit for light sources. It is the ratio of luminous flux (lm) to power (W). It is measured in lumens per watt (lm/W).

Luminous Intensity (cd):

Measure in Candela (cd), luminous intensity is an expression of the light emitted by a light source in a given direction per unit solid angle.


Illuminance (lx):

Measure in Lux (lx), illuminance is the quantity of light incident on a surface from a light source or sources. Lux is defined as the light energy incident on a surface area of one square meter at a distance of one meter from a point light source, in one second.


Colour Temperature (K):

A method of expressing the colour content and quality of light measured in Kelvin (K).


Colour Rendering Index (Ra):

The effect of a light source to reproduce the colors of various objects on a scale of 1 to 100 where 100 represents no color distortion. The higher the index number, the better the quality of illumination and the richer that colors will generally appear.



*Note CRI measures for a pair of light sources can only be compared if they have the same color temperature.

Example: