Luminance
The brightness of a surface as seen from a given direction, measured in candela per square metre.
Formula / rule
L = I / A
Measured in
cd/m² (nits)
In depth
Luminance is what a reflected-light meter and a camera sensor actually respond to: it combines how much light falls on a surface with how much of it that surface sends toward the lens. It is also the unit used to specify monitor brightness, which is why print-matching workflows usually call for a display calibrated to 80-120 cd/m².
Examples in practice
A clear midday sky measures around 8,000 cd/m²; a well-calibrated editing monitor is set near 100 cd/m².