Definition of luminance

Luminance - symbol L, unit candela per square metre (cd/m²)

Definition of luminance

Luminance is the concept for the luminous intensity emitted per unit of area of a surface in a specific direction. The surface can itself be light-emitting, or transmitting - like the surface of a lamp or the sun - but it can also reflect light from another source (like a road reflecting light from a streetlight and thus act as a secondary light source). It is designated by the symbol L. The unit is candela per square metre (cd/m²).

Luminance can be defined as:The ratio of the luminous intensity from a surface in given direction to the apparent area of that surface.

Apparent area are different for different viewing angles

By apparent area is meant the projection of the area of the surface in question on a plane that is at right angles to the direction of view. For a sphere, for example, the total apparent area in any direction is the area of the cross-section of the sphere.

Surfaces with different reflecting properties will, with the same illuminance, radiate different luminous intensities, and therefore have different luminance.

As both the luminous intensity and the apparent area are independent of distance,luminance is also independent of distance. On the other hand, luminance is generally dependent on the direction of observation, unless the surface has perfectly diffuse reflecting or emitting properties.

The most important lighting quality

What we really 'see' in life are luminances, or rather luminance variations in the field of view. It is therefore the most important quantity in lighting engineering, although the other three - luminous flux, luminous intensity and illuminance - are generally easier to work with when performing calculations or measurements.

Some practical examples :