Luminance

Luminance is the amount of light energy emitted or reflected from an object in a specific direction. Luminance is the only form of light we can see.

 

Luminance is measured in candela per square metre (cd/m2) or nit (these two terms being interchangeable).

Light energy reflected from an object

                                     Fig 6

1 nit is defined as the luminance of a purely white object being exposed to a light level of p lux.

 

Luminance is the unit which relates to the perceived brightness of a given object. Typical luminance values relevant to displays are between 100 and 10,000 nit.

 

As the light emitted from a display can be created by reflection or direct emitting (e.g. front projection being reflected and an LCD monitor being direct emitting) the relation between luminous flux, illumination and luminance is not as simple as might appear.

 

An easy way to illustrate this relationship is when looking at reflected light. The luminance will then be dependent on the amount of light striking the object combined with the reflection factor of the object.

 

If the object is completely dark and absorbent the reflection factor will be 0 and the object will be perceived as completely black no matter what the light level is. If the object is purely white, reflecting all colours evenly and evenly in all directions, the reflection factor is 1.

 

Scientifically, the standard reference material for this reflection factor of 1 is a surface of barium sulphate. Such a surface is also known as a Lambertian surface, which describes a surface which reflects light uniformly in a 180o hemisphere, no matter from which direction the light strikes it.

The Lambertian surface

                                   Fig 7

The relation is as follows: cd/m2= nit = Rf x  lux/p
 

Rf is the reflection factor, and in the case of projection screens it is known as the gain
 

Laptop computer screens typically have a luminance of 150-250 nit; sunlight readable screens have more than 1000 nit. Typical CRT monitors have between 50 and 125 nit.

 

An example: a projector with a light output of 1000 lumens is projecting an image with an area of 2m2. This results in an illumination, measured at the screen, of 500 lux. The surface has a reflection of 1 (gain = 1), this results in a luminance from the surface of, for example, 500 / p = 159.2 nit.

 

Light output of 1000 lumens

                             Fig 8

 

Illuminance  Contrast >

 

 

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