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. They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. The SI unit of radiance is watts per steradian per square metre (W·sr^-1·m^-2). In telecommunication and physics, radiometry is the science of radiation measurement. ... A solid angle is the three dimensional analog of the ordinary angle. ... 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. ... The watt (symbol: W) is the SI derived unit of power. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ...

Radiance characterizes total emission or reflection, while spectral radiance characterizes the light at a single wavelength or frequency. The radiance is equal to the sum (or integral) of all the spectral radiances from a surface. Spectral radiance has SI units W·sr^-1·m^-3 when measured per unit wavelength, and W·sr^-1·m^-2·Hz^-1 when measured per unit frequency interval.

Radiance is useful because it indicates how much of the power emitted by an emitting or reflecting surface will be received by an optical system looking at the surface from some angle of view. In this case, the solid angle of interest is the solid angle subtended by the optical system's aperture. Since the eye is an optical system, radiance and its cousin luminance are good indicators of how bright an object will appear. In optics, an aperture is something which restricts the diameter of the light path through one plane in an optical system. ... This article or section does not cite its references or sources. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ...

The radiance divided by the index of refraction squared is invariant in geometric optics. This means that for an ideal optical system in air, the radiance at the output is the same as the input radiance. This is sometimes called conservation of radiance. For real, passive, optical systems, the output radiance is at most equal to the input, unless the index of refraction changes. As an example, if you form a demagnified image with a lens, the optical power is concentrated into a smaller area, so the irradiance is higher at the image. The light at the image plane, however, fills a larger solid angle so the radiance comes out to be the same assuming there is no loss at the lens. In physics, invariants are usually quantities conserved (unchanged) by the symmetries of the physical system. ... See also list of optical topics. ... Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. ...

Definition

Radiance is defined by

where

the approximation holds for small A and Ω,

L is the radiance (W·m^-2·sr^-1),

Φ is the radiant flux or power (W),

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 spectral radiance (radiance per unit wavelength) is written L_λ and the radiance per unit frequency is written L_ν. The watt (symbol: W) is the SI derived unit of power. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... Luminous flux or luminous power is the measure of the perceived power of light. ... The watt (symbol: W) is the SI derived unit of power. ... 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

• Luminance
• Lambertian reflectance
• Etendue