Radiometric quantities and units

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Radiometric quantities are quantities related to electromagnetic radiation.

Each of these quantities can also be considered spectrally, i.e. wavelength by wavelength. In that case the wavelength unit has to be added to the measurement unit, e. g., if the wavelength is measured in nanometers (nm), the spectral irradiance unit is W/m2 nm, whereas if the wavelength is measured in meters (m), it is W/m3.

For the purpose of investigating light and color perception, the most important radiometric quantities are irradiance and radiance. Radiance is important because:

  1. it is conserved in the course of its propagation through optical systems, minus absorption losses;
  2. it is distance-independent;
  3. it is correlated to light collection from the human eye, from optical instruments (radiometer, light meters) from TV cameras and from cameras.

For each radiometric quantity there is a corresponding photometric quantity. This is the radiometric quantity evaluated according to the sensitivity of the human visual system.


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