Luminance
What is luminance?
Put simply, luminance describes how much light is emitted by a luminous or illuminated surface in a particular direction. It is one of the quantities in photometry and is denoted by the symbol Lᵥ.
The SI unit of luminance is candela per square metre (cd/m²).
Unlike luminous intensity, luminance takes into account the size of the area under consideration and its orientation.
Luminance and perceived brightness
Luminance is closely related to how bright a luminous or illuminated surface appears to an observer. However, it must not be equated with subjectively perceived brightness.
The human eye’s perception of brightness is also influenced by factors such as the surroundings, contrasts and the eye’s state of adaptation. A surface with a specific luminance may therefore be perceived as having different levels of brightness under different conditions.
What is the difference between luminance and illuminance?
Illuminance, measured in lux, describes how much light strikes a surface. Luminance, on the other hand, describes the light emitted by a surface towards the observer or a measuring instrument.
An illuminated surface reflects part of the incident light. Its luminance therefore depends not only on the illuminance, but also on the properties of the surface and the direction from which it is viewed.
What is the significance of luminance in vehicle lighting?
In vehicle lighting, luminance plays a role, amongst other things, in the perception of illuminated and lit surfaces.
Luminance is also relevant when assessing glare. A high luminance within a small area can be distracting to the eye, even though the total luminous flux of a headlamp need not be exceptionally high.
Luminance thus complements quantities such as lumen, candela and lux, but describes a different property of light.