Luminance (cd/m2) is the concept for the luminous intensity emitted per unit area of a surface in a specific direction. It is defined as the luminous intensity of the light emitted or reflected in a given direction from an element of the surface, divided by the area of the element projected in the same direction.
This brings in the concept of apparent or projected area; this is the apparent area of an object from a given viewpoint. The diagram below shows the various apparent areas for three different viewpoints around a pyramid. Depending on the viewing direction we can see a greater or lesser amount of the surface area. The greater the intensity per unit area and the greater the apparent area that can be seen the higher the luminance will be.
Luminance is a useful concept as it can be regarded as a measurable form of the sensation of brightness. Thus luminance is a measure to use when assessing peoples reactions to the lit environment. However the relationship between luminance and brightness is not that straight forward. The figure below illustrates the point.
The two grey squares in the diagram are the same colour and thus under uniform lighting they have the same luminance. However, the square with the dark surround looks brighter than the square on the left.