Luminance is a photometric measure of the density of
luminous intensityin a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SIunit for luminance is candelaper square metre(cd/m2). The CGSunit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.
Luminance is often used to characterize emission or reflection from flat, diffuse surfaces. The luminance indicates how much luminous power will be perceived by an
eyelooking at the surface from a particular angle of view. Luminance is thus an indicator of how 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. In this industry, one candela per square metre is commonly called a "nit". A typical computer display emits between 50 and 300 nits.
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 illuminanceis 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 defined by:
where: is the luminance (cd/m2),: is the
luminous fluxor luminous power (lm),: is the anglebetween the surface normaland the specified direction,: is the areaof the surface (m2), and: is the solid angle(sr).
* A Kodak guide to [http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.shtml Estimating Luminance and Illuminance] using a camera's exposure meter. Also available in [http://www.kodak.com/cluster/global/en/consumer/products/techInfo/am105/am105kic.pdf PDF form] .
Wikimedia Foundation. 2010.