- The quality or state of being radiant.
- Physics. The radiant energy emitted per unit time in a specified direction by a unit area of an emitting surface.
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The physical quantity that corresponds closely to the visual brightness of a surface. A simple radiometer for measuring the (average) radiance of an incident beam of optical radiation (light, including invisible infrared and ultraviolet radiation) consists of a cylindrical tube, with a hole in each end cap to define the beam cross section there, and with a photocell against one end to measure the total radiated power in the beam of all rays that reach it through both holes (see illustration). If A1 and A2 are the respective areas of the two holes, D is the length of the tube (distance between holes), and Φ is the radiant flux or power measured by the photocell, then the (average) radiance is approximately given by the equation
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A simple radiometer.
noun
Definition: brightness, luminescence
Antonyms: cloudiness, dark, dimness, dullness
n
Definition: happiness
Antonyms: dullness, gloominess, sadness
The rate of radiant emission per unit solid angle and per unit projected area of a source in a stated angular direction from the surface.
There is one thing which gives radiance to everything. It is the idea of something around the corner.
— Gilbert Chesterton (1874-1936)
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).
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. For this reason, radiance and luminance are both sometimes called "brightness". This usage is now discouraged — see Brightness for a discussion. The nonstandard usage of "brightness" for "radiance" persists in some fields, notably laser physics.
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.
Radiance is defined by

where
The spectral radiance (radiance per unit wavelength) is written Lλ and the radiance per unit frequency is written Lν.
Radiance is sometimes, confusingly, called intensity. This usage is not common in physics or optics, but is sometimes encountered in other fields (notably astrophysics and astronomy). Intensity has many other meanings in physics, with the most common being power per unit area.
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