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 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 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.