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...
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Units
Propagation. Optical systems. Secondary sources
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