3. Units

Three systems of units are used:

• two physical systems that can be used throughout the spectral range:

  • energy: the unit of light quantity is the joule ;

  • photonics: the unit of light quantity is dimensionless (a number of photons);

• a psychophysical system of luminous units, usable only in the visible range, whose fundamental unit is the candela, the intensity, in a given direction, of a source emitting monochromatic radiation of 1/683 watt per steradian at a frequency of 540 × 10 ¹² Hz (wavelength in vacuum very close to 555 nm).

  Table 1 summarizes the quantities...