Basic definitions related to light
Photometry describes optical radiation while accounting for the spectral sensitivity of human vision. Learn the quantities used to organise light measurements.
Flux, intensity, illuminance and luminance
Luminous flux describes the total photometrically weighted power and is expressed in lumens (lm). Luminous intensity, in candelas (cd), describes flux emitted in a direction per unit solid angle. Illuminance describes flux incident on a surface and is expressed in lux (lx). Luminance describes light leaving a surface in a given direction and is expressed in cd/m².
Why are these concepts confused?
The same luminaire may produce high flux while illuminance at a workplace also depends on distance and geometry. Two surfaces under equal illuminance may have different luminance because they reflect light differently. The L22 laboratory helps select the correct quantity and method for a test objective.
Frequently asked questions
Are lumen and lux the same?
No. Lumen is the unit of luminous flux, while lux is the unit of illuminance; 1 lx equals 1 lm/m².
What does photometry measure?
Photometry measures optical radiation weighted using a model of the spectral sensitivity of human vision.