Note 1 to entry: The wavelength in a medium is equal to the wavelength in vacuo divided by the refractive index of the medium. In photometry, unless otherwise stated, values of wavelength are generally those in air. The refractive index of standard air (for spectroscopy: T = 15 °C, p = 101 325 Pa) lies between 1,000 27 and 1,000 29 for visible radiation.

Note 2 to entry: In some specific applications, such as fluorescence measurements, the symbol μ is used for the wavelengths of excitation radiation.

Note 3 to entry: λ = v / ν, where λ is the wavelength in a medium, v is the phase velocity in that medium and ν is the frequency.

Note 4 to entry: The wavelength is expressed in metres (m) or decimal submultiples of metre.

Note 5 to entry: In optical radiation, the units nanometre (nm) and micrometre (μm) are generally used.

Note 6 to entry: This entry was numbered 845-01-14 in IEC 60050-845:1987.