Luminance
[1] 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.
The procedure for conversion from spectral radiance to luminance is standardized by the CIE and ISO.
The SI unit for luminance is candela per square metre (cd/m2).
Luminance is often used to characterize emission or reflection from flat, diffuse surfaces.
Luminance is used in the video industry to characterize the brightness of displays.
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 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.
Retinal damage can occur when the eye is exposed to high luminance.
[5] The IEC 60825 series gives guidance on safety relating to exposure of the eye to lasers, which are high luminance sources.
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 from 200 nm through 3000 nm.
where The luminance of a reflecting surface is related to the illuminance it receives:
where the integral covers all the directions of emission ΩΣ, In the case of a perfectly diffuse reflector (also called a Lambertian reflector), the luminance is isotropic, per Lambert's cosine law.
A variety of units have been used for luminance, besides the candela per square metre.
Luminance is essentially the same as surface brightness, the term used in astronomy.
This is measured with a logarithmic scale, magnitudes per square arcsecond (MPSAS).
