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

Bibliography

(1) - BOYD (R.W.) - Radiometry and the detection of optical radiation. - Wiley, New York (1983).
(2) - BURRUS (J.) & al - La photométrie en éclairage. - Sté Édition Lux, Paris (1991).
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