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Luminance is a
photometricPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electroma ...
measure of the
luminous intensity In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical o ...
per unit area of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

light
travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given
solid angle In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

solid angle
.
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. It is not necessarily proportional to lumina ...

Brightness
is the
term
term
for the ''subjective'' impression of the ''objective'' luminance measurement standard (see for the importance of this contrast). The
SI unit The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system The metric system is a that succeeded the decimal ...

SI unit
for luminance is
candela per square metre The candela per square metre (symbol: cd/m2) is the derived SI unit of luminance Luminance is a Photometry (optics), photometric measure of the luminous intensity per units of measurement, unit area of light travelling in a given direction. It ...
(cd/m2), as defined by the International System of Units (SI is from the French ''Système international d'unités'') standard for the
modern metric system Modern may refer to: History *Modern history ** Early Modern period ** Modern age, Late Modern period *** 18th century *** 19th century *** 20th century ** Contemporary history * Moderns, a faction of Freemasonry that existed in the 18th century ...
. A non-SI term for the same unit is the nit. The unit in the Centimetre–gram–second system of units (CGS) (which predated the SI system) is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.


Description

Luminance is often used to characterize emission or reflection from flat,
diffuse 250px, Diffusion from a microscopic and macroscopic point of view. Initially, there are solution, solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the wh ...

diffuse
surfaces. Luminance levels indicate how much
luminous power up Integrating sphere used for measuring the luminous flux of a light source. In photometry, luminous flux or luminous power is the measure of the perceived power of light Light or visible light is electromagnetic radiation within the port ...

luminous power
could be detected by the
human eye The human eye is a sense organ A sense is a biological system A biological system is a complex biological network, network which connects several biologically relevant entities. Biological organization spans several scales and are determined ...

human eye
looking at a particular surface from a particular
angle of view The angle of view is the decisive variable for the visual perception of the size or projection of the size of an object. Angle of view and perception of size The perceived size of an object depends on the size of the image projected onto the ...

angle of view
. Luminance is thus an indicator of how
bright Bright may refer to: Common meanings *Bright, an adjective meaning giving off or reflecting illumination; see Brightness *Bright, an adjective meaning someone with intelligence People *Bright (surname) *Bright (given name) *Bright, the stage name ...

bright
the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye's
pupil The pupil is a black hole located in the center of the iris of the eye Eyes are organs of the visual system The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photore ...

pupil
. Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between 50 and . The sun has a luminance of about at noon. Luminance is invariant in
geometric optics Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space that are related ...
. 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 one uses a lens to form an image that is smaller than the source object, the luminous power is concentrated into a smaller area, meaning that the
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ob ...

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.


Health effects

Retinal damage can occur when the eye is exposed to high luminance. Damage can occur because of local heating of the retina. Photochemical effects can also cause damage, especially at short wavelengths.


Luminance meter

A luminance meter is a device used in
photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electroma ...
that can measure the luminance in a particular direction and with a particular
solid angle In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

solid angle
. The simplest devices measure the luminance in a single direction while imaging luminance meters measure luminance in a way similar to the way a
digital camera A digital camera is a camera A camera is an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behav ...

digital camera
records color images.


Mathematical definition

The luminance of a specified point of a light source, in a specified direction, is defined by the
derivative In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...

derivative
: L_\mathrm = \frac where * v is the luminance (
cd
cd
/ m2), * d2v is the
luminous flux In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ...

luminous flux
( lm) leaving the area d in any direction contained inside the solid angle dΣ, * d is an
infinitesimal In mathematics, infinitesimals or infinitesimal numbers are quantities that are closer to zero than any standard real number, but are not zero. They do not exist in the standard real number system, but do exist in many other number systems, such a ...
area ( m2) of the source containing the specified point, * dΣ is an infinitesimal
solid angle In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position o ...

solid angle
(
sr
sr
) containing the specified direction, * Σ is the
angle In Euclidean geometry Euclidean geometry is a mathematical system attributed to Alexandrian Greek mathematics , Greek mathematician Euclid, which he described in his textbook on geometry: the ''Euclid's Elements, Elements''. Euclid's method ...

angle
between the normal nΣ to the surface d and the specified direction. If light travels through a lossless medium, the luminance does not change along a given light ray. As the ray crosses an arbitrary surface ''S'', the luminance is given by : L_\mathrm = \frac where * d is the infinitesimal area of ''S'' seen from the source inside the solid angle dΣ, * dS is the infinitesimal solid angle
subtended In geometry, an angle is subtended by an arc (geometry), arc, line segment or any other section of a curve when its two ray (geometry), rays pass through the endpoints of that arc, line segment or curve section. Conversely, the arc, line segment ...

subtended
by d as seen from d, * S is the angle between the normal nS to d and the direction of the light. More generally, the luminance along a light ray can be defined as : L_\mathrm = n^2\frac where * d is the
etendueEtendue or étendue (; ) is a property of light in an optics, optical system, which characterizes how "spread out" the light is in area and angle. It corresponds to the beam parameter product (BPP) in Gaussian beam optics. From the source point of v ...

etendue
of an infinitesimally narrow beam containing the specified ray, * dv is the luminous flux carried by this beam, * is the
index of refraction In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or ...
of the medium.


Relation to Illuminance

The luminance of a reflecting surface is related to the
illuminance In photometryPhotometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical ob ...

illuminance
it receives: : \begin \int_ L_\mathrm \mathrm\Omega_\Sigma \cos \theta_\Sigma & = M_\mathrm \\ & = E_\mathrm R \end where the integral covers all the directions of emission , and * v is the surface's luminous exitance * v is the received illuminance, and * is the
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...

reflectance
. In the case of a perfectly
diffuse reflector Diffuse reflection is the reflection of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined ...
(also called a Lambertian reflector), the luminance is isotropic, per
Lambert's cosine law In optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics ...
. Then the relationship is simply : L_\mathrm = E_\mathrm R / \pi


Units

A variety of units have been used for luminance, besides the candela per square metre. One candela per square metre is equal to: *10−4 stilbs (the
CGS
CGS
unit of luminance) *π apostilbs *π×10−4 lamberts *0.292
foot-lambert A foot-lambert or footlambert (fL, sometimes fl or ft-L) is a unit of luminance in United States customary units and some other unit systems. A foot-lambert equals 1/π or 0.3183 candela per square foot, or 3.426 candela per square meter (the corres ...
s


See also

*
Relative luminance Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is ...
* Orders of magnitude (luminance) *
Diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflec ...

Diffuse reflection
*
EtendueEtendue or étendue (; ) is a property of light in an optics, optical system, which characterizes how "spread out" the light is in area and angle. It corresponds to the beam parameter product (BPP) in Gaussian beam optics. From the source point of v ...

Etendue
* *
Lambertian reflectance Lambertian reflectance is the property that defines an ideal "matte" or diffusely reflecting surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer's angle of view. More technically, the surf ...
*
Lightness (color) In colorimetry and color theory, lightness, also known as value or tone, is a representation of a color's brightness. It is one of the Color appearance model#Color appearance parameters, color appearance parameters of any color appearance model ...
* Luma, the representation of luminance in a video monitor *
Lumen (unit) The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time. Luminous flux differs from power (physics), power (radiant flux) in that radiant flux includes ...
*
Radiance In radiometry Radiometry is a set of techniques for measuring Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measurement are ...
, radiometric quantity analogous to luminance *
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. It is not necessarily proportional to lumina ...

Brightness
, the subjective impression of luminance *
Glare (vision) Glare is difficulty of seeing in the presence of bright light such as direct or reflected sunlight or artificial light such as car headlamps at night. Because of this, some cars include mirrors with automatic anti-glare functions and in buildings, ...


Table of SI light-related units

{{SI light units


References


External links

* A Kodak guide t
Estimating Luminance and Illuminance
using a camera's exposure meter. Also available i
PDF form
* Autodesk Design Academ
Measuring Light Levels
Photometry Physical quantities