Colorimetry is "the science and technology used to quantify and describe physically the humancolor perception".[1]It is similar tospectrophotometry, but is distinguished by its interest in reducing spectra to the physical correlates of color perception, most often theCIE 1931 XYZ color space tristimulus values and related quantities.[2]
TheDuboscq colorimeter was invented byJules Duboscq in 1870.[3]
Colorimetric equipment is similar to that used in spectrophotometry. Some related equipment is also mentioned for completeness.
Indigital imaging, colorimeters are tristimulus devices used forcolor calibration. Accuratecolor profiles ensure consistency throughout the imaging workflow, from acquisition to output.
The absolutespectral power distribution of a light source can be measured with aspectroradiometer, which works by optically collecting the light, then passing it through amonochromator before reading it in narrow bands of wavelength.
Reflected color can be measured using aspectrophotometer (also calledspectroreflectometer orreflectometer), which takes measurements in the visible region (and a little beyond) of a given color sample. If the custom of taking readings at 10nanometer increments is followed, thevisible light range of 400–700 nm will yield 31 readings. These readings are typically used to draw the sample'sspectral reflectance curve (how much it reflects, as a function of wavelength)—the most accurate data that can be provided regarding its characteristics.
The readings by themselves are typically not as useful as theirtristimulus values, which can be converted intochromaticity co-ordinates and manipulated throughcolor space transformations. For this purpose, aspectrocolorimeter may be used. A spectrocolorimeter is simply a spectrophotometer that can estimate tristimulus values bynumerical integration (of thecolor matching functions'inner product with the illuminant's spectral power distribution).[6] One benefit of spectrocolorimeters over tristimulus colorimeters is that they do not have optical filters, which are subject to manufacturing variance, and have a fixed spectral transmittance curve—until they age.[7] On the other hand, tristimulus colorimeters are purpose-built, cheaper, and easier to use.[8]
TheCIE (International Commission on Illumination) recommends using measurement intervals under 5 nm, even for smooth spectra.[5] Sparser measurements fail to accurately characterize spiky emission spectra, such as that of the red phosphor of a CRT display, depicted aside.
Photographers andcinematographers use information provided by these meters to decide whatcolor balancing should be done to make different light sources appear to have the same color temperature. If the user enters the reference color temperature, the meter can calculate themired difference between the measurement and the reference, enabling the user to choose a correctivecolor gel orphotographic filter with the closest mired factor.[9]
Internally the meter is typically asilicon photodiode tristimulus colorimeter.[9] Thecorrelated color temperature can be calculated from the tristimulus values by first calculating thechromaticity co-ordinates in theCIE 1960 color space, then finding the closest point on thePlanckian locus.
The process recommended by the CIE for computing the tristimulus values is to use 1 nm interval or 5 nm interval if the spectral function is smooth
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