Editing Luminance (section)

==Description==
Luminance is often used to characterize emission or reflection from flat, [[diffuse reflection|diffuse]] surfaces. Luminance levels indicate how much [[luminous flux|luminous power]] could be detected by the [[human eye]] looking at a particular surface from a particular [[angle of view]]. Luminance is thus an indicator of how [[brightness|bright]] the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye's [[pupil]].

Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between {{val|50|and|300|u=cd/m<sup>2</sup>}}. The sun has a luminance of about {{val|1.6|e=9|u=cd/m<sup>2</sup>}} at noon.<ref>{{cite web |url=http://www.schorsch.com/kbase/glossary/luminance.html |title=Luminance |work=Lighting Design Glossary |access-date=Apr 13, 2009}}</ref>

Luminance is [[Invariant (physics)|invariant]] in [[geometric optics]].<ref>{{cite book |title=Handbook of Optical Systems |volume=5, Metrology of Optical Components and Systems |first1=Bernd |last1=Dörband |first2=Herbert |last2=Gross |first3=Henriette |last3=Müller |page=326 |publisher=[[Wiley (publisher)|Wiley]] |year=2012 |isbn=978-3-527-40381-3 |editor-first=Herbert |editor-last=Gross}}</ref> This means that for an ideal optical system, the luminance at the output is the same as the input luminance.

For real, passive optical systems, the output luminance is {{em|at most}} equal to the input. As an example, if one uses a lens to form an image that is smaller than the source object, the luminous power is concentrated into a smaller area, meaning that the [[illuminance]] is higher at the image. The light at the image plane, however, fills a larger solid angle so the luminance comes out to be the same assuming there is no loss at the lens. The image can never be "brighter" than the source.