In lighting technology, a number of technical terms and units are used to describe the properties of light sources and the effects that are formed.

  • Luminous flux

This describes the total amount of light emitted by a light source. This radiation could basically be measured or expressed in watt. This does not describe the optical effect of a light source adequately since the varying spectral sensitivity of the eye is not considered. To include the spectral sensitivity of the eye the luminous flux is measured in lumen.

  • Luminous efficacy

This describes the luminous flux of a lamp in relation to its power consumption and is therefore expressed in lumen per watt (lm/W).

  • Luminous Intensity

A measure of the amount of light that a point source radiates in a given direction. It is expressed by the luminous flux leaving the source in that direction per unit of solid angle. The unit for measuring luminous intensity in candela(cd).

  • Illuminance

Illuminance is the means of evaluating the density of luminous flux. It indicates the amount of luminous flux from a light source falling on a given area. Illuminance need not necessarily be related to a real surface. It can be measured at any point within a space. Illuminance can be determined from the luminous intensity of the light source. Illuminance decreases with the square of the distance from the light source.

  • Luminance

Luminance describes the measurement of the amount of light emitting, passing through or reflected from a surface from a solid angle. It indicated how much luminous power can be perceived by the human eye. This means that luminance indicates the brightness of light emitted or reflected off of a surface.