Changes

Luminance is invariant in [[geometric]] [[optics]]. 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 at most equal to the input. As an example, if you form a demagnified image with a [[lens]], 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.

Luminance is invariant in [[geometric]] [[optics]]. 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 at most equal to the input. As an example, if you form a demagnified image with a [[lens]], 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.

==Definition==

==Definition==

Luminance is defined by

Luminance is defined by