Radiometric quantities and units

From ColorWiki

Radiometric quantities are quantities related to electromagnetic radiation.

• Radiant Energy is energy carried from any electromagnetic field.
  • It is denoted by Q_e
  • Its SI unit is the joule (J)

• Radiant Flux is radiant energy per unit time (also called Radiant Power); it is considered the fundamental radiometric unit.
  • It is denoted by P_e
  • Its SI unit is the watt (W)

• Radiant Exitance, or Radiant Emittance, is radiant flux emitted from an extended source per unit area.
  • It is denoted by M_e
  • Its SI unit is: watt per square meter (W/m²)

• Irradiance is radiant flux incident on a surface unit area.
  • It is denoted by E_e
  • Its SI unit is: watt per square meter (W/m²)

• Radiant Intensity is radiant flux emitted from a point source per unit solid angle
  • It is denoted by I_e
  • Its SI unit is: watt per steradian (W/sr)

• Radiance is radiant flux emitted from an extended source per unit solid angle and per unit projected source area.
  • It is denoted by L_e
  • 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:

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.

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.)