luminance n : the quality of being luminous; emitting or reflecting light; "its luminosity is measured relative to that of our sun" [syn: luminosity, brightness, brightness level, luminousness, light]

English

Noun

1. the quality of being luminous
2. the luminous flux emitted in a given direction divided by the product of the projected area of the source element perpendicular to the direction and the solid angle containing that direction, i.e. luminous intensity divided by unit area; measured in stilbs or apostilbs

Synonyms

• photometric brightness

See also

• brightness

Luminance is a photometric measure of the density of luminous intensity in 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 SI unit for luminance is candela per square metre (cd/m2). The CGS unit 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 eye looking 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 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

Luminance is defined by

L_v = \frac

where

L_v is the luminance (cd/m2),

F is the luminous flux or luminous power (lm),

\theta\, is the angle between the surface normal and the specified direction,

A is the area of the surface (m2), and

\Omega\, is the solid angle (sr).

See also

• Diffuse reflection
• Etendue
• Illuminance
• Lambert
• Lambertian reflectance
• Lightness, property of a color
• Radiance
• Exposure value

External links

• A Kodak guide to Estimating Luminance and Illuminance using a camera's exposure meter. Also available in PDF form.