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Luminance is a photometric
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
 measure of the luminous intensity
Luminous intensity

In Photometry , luminous intensity is a measure of the wavelength-weighted Power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye....
 per unit area of light travelling 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
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
. The SI
Si

Si, si, or SI may refer to :...
 unit for luminance is candela
Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
 per square metre
Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one 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
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....
 surfaces.






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Luminance is a photometric
Photometry (optics)

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of radiant energy in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by a luminosity function that models human b...
 measure of the luminous intensity
Luminous intensity

In Photometry , luminous intensity is a measure of the wavelength-weighted Power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye....
 per unit area of light travelling 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
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
. The SI
Si

Si, si, or SI may refer to :...
 unit for luminance is candela
Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
 per square metre
Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one 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
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....
 surfaces. The luminance indicates how much luminous power
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 will be perceived by an eye
Eye

Eyes are Organ that detect light, and send signals along the optic nerve to the visual system and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system....
 looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright
Brightness

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target....
 the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye's pupil
Pupil

The pupil is the sphere that is located in the center of the Iris of the eye and that controls the amount of light that enters the eye. It appears black because most of the light entering the pupil is absorbed by the biological tissue inside the eye....
. 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
Nit (unit)

The candela per square metre is the SI unit of measurement of luminance; nit is a non-SI name also used for this unit. It is often used to quote the brightness of computer displays, which typically have luminances of 50 to 300 nits ....
". A typical computer display emits between 50 and 300 nits.

Luminance is invariant
Invariant (physics)

In mathematics and theoretical physics, an invariant is a property of a system which remains unchanged under some Transformation .The gravitational field of the Sun is invariant under a change of time ....
 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
Illuminance

In photometry , illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception....
 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

where is the luminance (cd
Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function ....
/m2
Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one metre....
), is the luminous flux
Luminous flux

In photometry , luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
 or luminous power (lm
Lumen (unit)

The lumen is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light....
), is the angle
Angle

In geometry and trigonometry, an angle is the figure formed by two Ray sharing a common endpoint, called the vertex of the angle . The magnitude of the angle is the "amount of rotation" that separates the two rays, and can be measured by considering the length of circular arc swept out when one ray is rotated about the vertex to coincide...
 between the surface normal
Surface normal

A surface normal, or simply normal, to a Flatness is a vector which is perpendicular to that surface. A normal to a non-flat surface at a Point P on the surface is a vector perpendicular to the Tangent space to that surface at P....
 and the specified direction, is the area
Area

Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....
 of the surface (m2
Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one metre....
), and is the solid angle
Solid angle

The solid angle, O, is the angle in three-dimensional space that an object subtends at a point. It is a measure of how big that object appears to an observer looking from that point....
 (sr
Steradian

The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a Plane ....
).

See also

  • Diffuse reflection
    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....
  • Etendue
    Etendue

    Etendue or ?tendue is a property of an optics, which characterizes how "spread out" the light is in area and angle. The ?tendue can be defined in several equivalent ways....
  • Illuminance
    Illuminance

    In photometry , illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception....
  • Lambert
    Lambert (unit)

    The lambert is a unit of luminance named for Johann Heinrich Lambert , a German mathematician, physicist and astronomer. A related unit of luminance, the foot-lambert, is used in the lighting, cinema and flight simulation industries....
  • Lambertian reflectance
    Lambertian reflectance

    If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view....
  • Lightness
    Lightness (color)

    Lightness is a property of a color, or a dimension of a color space, that is defined in a way to reflect the subjective brightness perception of a color for humans....
    , property of a color
  • Luma
    Luma (video)

    As applied to video signals, luma represents the brightness in an image . Luma is typically paired with Chrominance. Luma represents the achromatic image without any color, while the chroma components represent the color information....
    , the representation of luminance in a video monitor
  • Radiance
    Radiance

    Radiance and spectral radiance are radiometry 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....
  • Exposure value
    Exposure value

    In photography, exposure value denotes all combinations ofcamera shutter speed and relative aperture that give the sameexposure . The concept was developed in Germany in the 1950s...


External links

  • A Kodak guide to using a camera's exposure meter. Also available in .