Luminance is a photometric measure of light emitted, reflected or transmitted by a surface in a particular viewing direction. Its SI unit is candela per square metre (cd/m²); one nit is one cd/m².
What the camera sees
A surface’s luminance depends on the light reaching it and how it reflects or transmits that light. Two surfaces under the same illuminance can have different luminances. Viewing angle matters for glossy materials, screens and other directional surfaces.
Practical example
A white card and a black card placed together under the same key can receive similar lux readings from an incident meter. A reflected-light or luminance measurement distinguishes the light coming back from each card. That difference helps explain why equal illumination does not make both surfaces equally bright in the image.
Screens and exposure
Display specifications often use nits. Those figures need conditions such as window size, picture mode and sustained versus peak output. A luminance value is a physical measurement; perceived brightness also depends on the surroundings and adaptation, while recorded exposure depends on the camera and lens settings.
Related terms
Lux · Candela · Photometrics · Reflected Light Meter
References
Konica Minolta: photometric quantities · NIST: SI base and derived units