Definition
Radiance is defined by
where
- L is the observed or measured radiance (W·m−2·sr−1), in the direction θ,
- d is the differential operator,
- Φ is the total radiant flux or power (W) emitted
- θ is the angle between the surface normal and the specified direction,
- A is the area of the surface (m2), and
- is the solid angle (sr) subtended by the observation or measurement.
- The approximation only holds for small A and Ω where cos θ is approximately constant.
In general, L is a function of viewing angle through the cos θ term in the denominator as well as the θ, and potentially azimuth angle, dependence of . For the special case of a Lambertian source, L is constant such that is proportional to cos θ.
When calculating the radiance emitted by a source, A refers to an area on the surface of the source, and Ω to the solid angle into which the light is emitted. When calculating radiance at a detector, A refers to an area on the surface of the detector and Ω to the solid angle subtended by the source as viewed from that detector. When radiance is conserved, as discussed above, the radiance emitted by a source is the same as that received by a detector observing it.
The spectral radiance (radiance per unit wavelength) is written Lλ and the radiance per unit frequency is written Lν.
Read more about this topic: Radiance
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