What is spectral radiance?

Spectral radiance is a measure of the amount of radiant energy emitted per unit time, per unit solid angle, per unit area, and per unit wavelength. It is expressed in watts per steradian, per square meter, and per nanometer (W·sr−1·m−2·nm−1).

The spectral radiance of a material depends on its temperature and its reflectivity or emissivity. Materials that are good absorbers and good emitters of radiation, also known as black bodies, have a characteristic spectral radiance distribution called the Planck distribution. This distribution peaks at a particular wavelength that depends on the temperature of the material, and it falls off rapidly at higher and lower wavelengths.

The spectral radiance of a material can be measured using a spectrometer, which separates the radiation into its constituent wavelengths and measures the intensity of each wavelength. Spectral radiance measurements are used in a variety of applications, such as remote sensing, thermal imaging, and radiometry.