Illuminance
The illuminance Ev (“v” for visual) is the photometric equivalent of the irradiance Ee in radiometry. Photometric quantities and units describe visible light according to the physiological perception of brightness by the human eye. Accordingly, the illuminance is equal to the irradiance in the spectral range of the human eye. Colloquially, it describes the “intensity” or “brightness” of lighting.
The SI unit is lux (lx) = luminous flux per area = lumen (lm)/m², where the luminous flux is the radiant power weighted by the sensitivity of the eye.
To express the “intensity” for illumination in image processing, the term irradiance Ee is always correct, as UV, IR and NIR light (which are not visible) are also used. However, if lighting is used for only visible light (white, RGB), the term illuminance is also correct here.
