In its general sense, the word radiometry refers to techniques for measuring radiation of any kind, from radio waves to the particles emitted by radioactive bodies. Only radiation whose propagation can be represented by the laws of geometrical optics is considered here: this means only electromagnetic radiation whose wavelengthsλ are large compared to interatomic distances in condensed media (of the order of 5 pm), and small compared to the aperture diameter of instruments (" 1 cm), so 1 nm < λ < 1 mm approximately.
The energy carried by electromagnetic radiation is quantified. To the wavelength λ measured in vacuum corresponds the frequency ν = c / λ, c being the speed of light in vacuum (c = 2.997 924 58 × 108 m/s); the quantum of energy is :u = hν = hc / λ
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Radiometric and photometric quantities
