Luminance is aphotometric measure of theluminous intensity perunit area oflight travelling in a given direction.^[1] It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a givensolid angle.

The procedure for conversion from spectralradiance to luminance is standardized by theCIE andISO.^[2]

Brightness is theterm for thesubjective impression of theobjective luminance measurement standard (seeObjectivity (science) § Objectivity in measurement for the importance of this contrast).

TheSI unit for luminance iscandela per square metre (cd/m²). A non-SI term for the same unit is thenit. The unit in theCentimetre–gram–second system of units (CGS) (which predated the SI system) is thestilb, 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. Luminance levels indicate how muchluminous power could be detected by thehuman eye looking at a particular surface from a particularangle of view. Luminance is thus an indicator of howbright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye'spupil.

Luminance is used in the video industry to characterize the brightness of displays. A typical computer display emits between50 and 300 cd/m². The sun has a luminance of about1.6×10⁹ cd/m² at noon.^[3]

Luminance isinvariant ingeometric optics.^[4] 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 isat 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 theilluminance 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.

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.^[5]

The IEC 60825 series gives guidance on safety relating to exposure of the eye to lasers, which are high luminance sources. The IEC 62471 series gives guidance for evaluating the photobiological safety of lamps and lamp systems including luminaires. Specifically it specifies the exposure limits, reference measurement technique and classification scheme for the evaluation and control of photobiological hazards from all electrically powered incoherent broadband sources of optical radiation, including LEDs but excluding lasers, in the wavelength range from200 nm through3000 nm. This standard was prepared as Standard CIE S 009:2002 by the International Commission on Illumination.

Aluminance meter is a device used inphotometry that can measure the luminance in a particular direction and with a particularsolid angle. The simplest devices measure the luminance in a single direction while imaging luminance meters measure luminance in a way similar to the way adigital camera records color images.^[6]

The luminance of a specified point of a light source, in a specified direction, is defined by themixed partial derivative$${\displaystyle L_{\mathrm{v}}=\frac{{\mathrm{d}}^2{\mathrm{\Phi }}_{\mathrm{v}}}{\mathrm{d}\mathrm{\Sigma }\mathrm{d}{\mathrm{\Omega }}_{\mathrm{\Sigma }}\cos {\theta }_{\mathrm{\Sigma }}}}$$ where

• L_v is the luminance (cd/m²);
• d²Φ_v is theluminous flux (lm) leaving the areadΣ in any direction contained inside the solid angledΩ_Σ;
• dΣ is aninfinitesimal area (m²) of the source containing the specified point;
• dΩ_Σ is an infinitesimalsolid angle (sr) containing the specified direction; and
• θ_Σ is theangle between thenormaln_Σ to the surfacedΣ and the specified direction.^[7]

If light travels through a lossless medium, the luminance does not change along a givenlight ray. As the ray crosses an arbitrary surfaceS, the luminance is given by$${\displaystyle L_{\mathrm{v}}=\frac{{\mathrm{d}}^2{\mathrm{\Phi }}_{\mathrm{v}}}{\mathrm{d}S\mathrm{d}{\mathrm{\Omega }}_S\cos {\theta }_S}}$$ where

• dS is the infinitesimal area ofS seen from the source inside the solid angledΩ_Σ;
• dΩ_S is the infinitesimal solid anglesubtended bydΣ as seen fromdS; and
• θ_S is the angle between the normaln_S todS and the direction of the light.

More generally, the luminance along a light ray can be defined as$${\displaystyle L_{\mathrm{v}}=n^2\frac{\mathrm{d}{\mathrm{\Phi }}_{\mathrm{v}}}{\mathrm{d}G}}$$ where

• dG is theetendue of an infinitesimally narrow beam containing the specified ray;
• dΦ_v is the luminous flux carried by this beam; and
• n is theindex of refraction of the medium.

The luminance of a reflecting surface is related to theilluminance it receives:$${\displaystyle {\int }_{{\mathrm{\Omega }}_{\mathrm{\Sigma }}}{L}_{\text{v}}\mathrm{d}{\mathrm{\Omega }}_{\mathrm{\Sigma }}\cos {\theta }_{\mathrm{\Sigma }}=M_{\text{v}}=E_{\text{v}}R,}$$ where the integral covers all the directions of emissionΩ_Σ,

• M_v is the surface'sluminous exitance;
• E_v is the received illuminance; and
• R is thereflectance.

In the case of a perfectlydiffuse reflector (also called aLambertian reflector), the luminance is isotropic, perLambert's cosine law. Then the relationship is simply$${\displaystyle L_{\text{v}}=\frac{E_{\text{v}}R}{\pi }.}$$ 

A variety of units have been used for luminance, besides the candela per square metre. Luminance is essentially the same assurface brightness, the term used in astronomy. This is measured with a logarithmic scale, magnitudes per square arcsecond (MPSAS).

• Relative luminance
• Orders of magnitude (luminance)
• Diffuse reflection
• Etendue
• Exposure value § EV as a measure of luminance and illuminance
• Lambertian reflectance
• Lightness (color)
• Luma, the representation of luminance in a video monitor
• Lumen (unit)
• Radiance, radiometric quantity analogous to luminance
• Brightness, the subjective impression of luminance
• Glare (vision)

• A Kodak guide toEstimating Luminance and Illuminance using a camera's exposure meter. Also available inPDF form.
• Autodesk Design AcademyMeasuring Light Levels