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Mathematically, for the spectral power distribution of a radiant exitance or irradiance one may write: =where M(λ) is the spectral irradiance (or exitance) of the light (SI units: W/m 2 = kg·m −1 ·s −3); Φ is the radiant flux of the source (SI unit: watt, W); A is the area over which the radiant flux is integrated (SI unit: square meter, m 2); and λ is the wavelength (SI unit: meter, m).
The reference spectrum in ASTM G177 is limited to the global irradiance in the ultraviolet (280–400 nm), and corresponds to "high-UV" conditions frequently encountered in arid and elevated sites, such as in the southwest USA. This spectrum is to be used as a reference for testing the degradation and durability of materials.
For example, if a solar simulator emits 17.8 percent of its total irradiance in the 400 nm–500 nm range, it would have a in that wavelength interval of 0.98. If a solar simulator achieves a spectral match ratio R S M {\displaystyle R_{SM}} between 0.75 and 1.25 for all wavelength intervals, it is considered to have class A spectral match.
The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a white point . If the profile is normalized , then the white point can equivalently be expressed as a pair of chromaticity coordinates .
Comparison of photometric and radiometric quantities. Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light.Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye.
A classic example is the automobile industry: the colorants used for interior fabrics, plastics and paints may be chosen to provide a good color match under a cool white fluorescent source, but the matches can disappear under different light sources (e.g. daylight or tungsten source). Furthermore, because of the differences in colorants ...
The detailed analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem. In practice it is hard to take the effects of noise and foreground sources into account.
If the instrument is designed to measure the spectrum on an absolute scale rather than a relative one, then it is typically called a spectrophotometer. The majority of spectrophotometers are used in spectral regions near the visible spectrum. A spectrometer that is calibrated for measurement of the incident optical power is called a ...