Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction in other words in other words it describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.

The concept of luminance is widely used in fields such as physics, photography, chemistry, engineering, astronomy and any other fields. 

Especially it is a very important property in photography as it describes the amount of light that will be used in the photograph formation. The input luminance of an ideal optical system is equal to the output luminance of the system. In a practical situation, the output luminance is always smaller than the input luminance. The image cannot be more bright than the source itself.

Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and 300 cd/m2. The sun has a luminance of about 1.6×109 cd/m2 at noon.

Luminance is often mistaken for luminosity and illuminance due to the similarity of the words, but these two are totally different from each other.

A luminance meter is a device used in photometry that can measure the luminance in a particular direction and with a particular solid angle. The simplest devices measure the luminance in a single direction while imaging luminance meters measure luminance in a way similar to the way a digital camera records color images.