Luminance (cd/m²)

Luminance is the luminous intensity per unit area leaving a surface in a given direction, measured in candela per square metre. It corresponds to what the eye actually perceives as brightness. Road lighting for motorised traffic is designed in luminance (EN 13201 M-classes: 0.3–2.0 cd/m² average) because drivers see the road surface, and identical illuminance produces different luminance on light concrete versus dark asphalt. Luminance also underlies glare metrics: a fixture with high luminance in the field of view causes discomfort regardless of how well the task is lit. Displays and tunnels are further domains where luminance, not lux, is the governing quantity.