The essential parts of a colorimeter are:
In addition, there may be:
Changeable optics filters are used in the colorimeter to select the wavelength of light which the solute absorbs the most, in order to maximize accuracy. The usual wavelength range is from 400 to 700 nanometres (nm). If it is necessary to operate in the ultraviolet range (below 400 nm) then some modifications to the colorimeter are needed. In modern colorimeters the filament lamp and filters may be replaced by several light-emitting diodes of different colors.
In a manual colorimeter the cuvettes are inserted and removed by hand. An automated colorimeter (as used in an AutoAnalyzer) is fitted with a flowcell through which solution flows continuously.
The output from a colorimeter may be displayed by an analogue or digital meter and may be shown as transmittance (a linear scale from 0-100%) or as absorbance (a logarithmic scale from zero to infinity). The useful range of the absorbance scale is from 0-2 but it is desirable to keep within the range 0-1 because, above 1, the results become unreliable due to scattering of light.
A tristimulus colorimeter—colloquially shortened to colorimeter—takes a limited number of wideband spectral energy readings (~3-7) along the visible spectrum by using filtered photodetectors; e.g. silicon photodiodes.
Originally, three glass filters whose transmittance spectra mimicked the CIE color matching functions (shown on the right) were employed. A filter bank may be used to decompose the individual color matching functions if more accuracy is desired.
A camera or colorimeter is said to be colorimetric if it satisfies the Luther condition (also called the "Maxwell-Ives criterion"), reducing observer metamerism color errors, if the product of the spectral responsivity of the photoreceptor and the spectral transmittance of the filters is a linear combination of the CMFs.
The quality of a colorimeter may be assessed using the means in CIE publication 179:2007.
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