Lighting Metrics

This section covers the vocabulary of light: luminous flux, luminous intensity, illuminance, luminance and exitance [1].

Luminous Flux “The outward flow of luminous energy from a light source in all directions per unit of time.” Gross total quantity of light given off by a source regardless of the direction. It is measured in lumens. Symbol: Φ
Luminous Intensity The directional force causing “luminous flux to be emitted from a source” in a given direction. It is measured in candelas. Symbol: I
Illuminance The “density of luminous flux” received by a surface is illuminance. It is measured in lux (lumens per square meter) or fc (lumens per square foot) Symbol: E
Luminance The intensity of light that is received by the eye of an observer. This is dependent upon the direction. It is measured in candelas per square feet or candelas per square meters. Symbol: L
Exitance The amount of luminous flux that leaves a surface in all directions. This is non-directional. It is measured in lumens per square feet or lumens per square meters. Symbol: M