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Encyclopedia > Luminance

Luminance (also called luminosity) 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 luminosity is candela per square metre (cd/m2). Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. ... The word density or dense has a variety of senses in the physical, mathematical, and quantitative sciences. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... A solid angle is the three dimensional analog of the ordinary angle. ... Cover of brochure The International System of Units. ... The candela (symbol: cd, Latin for candle) is one of the seven SI base units. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...


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. Diagram of diffuse reflection Diffuse reflection is the reflection of light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... This article or section does not cite its references or sources. ... Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. ... The human eye The pupil is the central transparent area (showing as black). ... This article is about the unit of measure. ...


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. In physics, invariants are usually quantities conserved (unchanged) by the symmetries of the physical system. ... See also list of optical topics. ... Illuminance is the total luminous flux incident per unit area. ...


Definition

Luminance is defined by

L = frac{d^2 F}{dA,d{Omega} cos theta}

where

L is the luminance (cd/m2),
F is the luminous flux or luminous power (lm),
θ is the angle between the surface normal and the specified direction,
A is the area of the source (m2), and
Ω is the solid angle (sr).

The candela (symbol: cd, Latin for candle) is one of the seven SI base units. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... This article is about angles in geometry. ... A surface normal, or just normal to a flat surface is a three-dimensional vector which is perpendicular to that surface. ... Area is a physical quantity expressing the size of a part of a surface. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... A solid angle is the three dimensional analog of the ordinary angle. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ...

See also

SI photometry units

edit Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. ... If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the amount of light an observer sees, the surface luminance, is the same regardless of their angle of view. ... Diagram of diffuse reflection Diffuse reflection is the reflection of light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles. ... Cover of brochure The International System of Units. ...

Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lm·s units are sometimes called Talbots
Luminous flux F lumen (= cd·sr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd
Luminance Lv candela / square metre cd/m2 also called luminosity
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy lumens / watt lm/W ratio of luminous flux to radiant flux, maximum possible is 683

  Results from FactBites:
 
More details about Luminance in Histograms (1429 words)
The luminance signal in your TV set uses the same formula, in fact, that was its origin.
So, gray tone range in Luminance values is used by the histogram to map, or to tell us about the actual distribution of, the range of light and dark tones in our color images.
Photographs generally have more nearly all luminance values represented, where both of these samples were intentionally limited, to be able to better show the details better.
Photographing the Moon (2472 words)
The luminance of the moon is the sum of the light from the sky and the light from the moon, and the amount of light in the sky defines three different situations.
Measuring the luminance of the moon with a handheld meter is difficult; because the moon has an angular diameter of approximately 1/2°, even a 1° spot meter won’t suffice.
At an altitude of 5°, the luminance of the moon is 561 cd/m
  More results at FactBites »

 

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