Radiometric quantities and units
From ColorWiki
Radiometric quantities are quantities related to electromagnetic radiation.
- Radiant Energy is energy carried from any electromagnetic field.
- Radiant Flux is radiant energy per unit time (also called Radiant Power); it is considered the fundamental radiometric unit.
- Radiant Exitance, or Radiant Emittance, is radiant flux emitted from an extended source per unit area.
- It is denoted by Me
- Its SI unit is: watt per square meter (W/m2)
- Irradiance is radiant flux incident on a surface unit area.
- It is denoted by Ee
- Its SI unit is: watt per square meter (W/m2)
- Radiant Intensity is radiant flux emitted from a point source per unit solid angle
- Radiance is radiant flux emitted from an extended source per unit solid angle and per unit projected source area.
- It is denoted by Le
- Its SI unit is: watt per steradian and square meter (W/(sr m2))
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:
- it is conserved in the course of its propagation through optical systems, minus absorption losses;
- it is distance-independent;
- 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.
Bibliography
- G. Wyszecki, W.S. Stiles: Color Science: Concepts and Methods, Quantitative Data and Formulae Wiley 1982 (II ed.)
- CIE: International Lighting Vocabulary 1970 (III ed.)