Colorimetry

Colorimetry or Colourimetry can refer to:
 * the quantitative study of color perception. It is similar to spectrophotometry, but may be distinguished by its interest in reducing spectra to tristimulus values, from which the perception of color derives.
 * the determination of the spectral absorbance of a solution.

This article is mainly about the former case, which is not an exact science due to the limitations inherent in the system (metamerism being the most troublesome), the design of the measurement devices, the values used to estimate a given light source, etc. Colors that look the same seldom have the same spectral characteristics in any colorimetric system you employ, even assuming identical viewing conditions and identical observers with normal color vision.

Instruments
Colorimetric equipment is similar to that used in spectrophotometry. Some related equipment is also mentioned for completeness.
 * A tristimulus colorimeter measures the tristimulus values of a color.
 * A spectroradiometer measures the absolute spectral radiance (intensity) or irradiance of a light source.
 * A spectrophotometer measures the spectral reflectance, transmittance, or relative irradiance of a color sample.
 * A spectrocolorimeter is a spectrophotometer that can calculate tristimulus values.
 * A densitometer measures the degree of light passing through or reflected by a subject.
 * A color temperature meter measures the color temperature of an incident illuminant.



Absorption colorimeter
In physical chemistry, a colorimeter is a device used to test the concentration of a solution by measuring its absorbance of a specific wavelength of light. To use this device, different solutions must be made, and a control (usually a mixture of distilled water and another solution) is first filled into a cuvette and placed inside a colorimeter to calibrate the machine. Only after the device has been calibrated can you use it to find the densities and/or concentrations of the other solutions. You do this by repeating the calibration, except with cuvettes filled with the other solutions. The filter on a colorimeter must be set to red if the liquid is blue. The size of the filter initially chosen for the colorimeter is extremely important, as the wavelength of light that is transmitted by the colorimeter has to be same as that absorbed by the substance.

Tristimulus colorimeter
In digital imaging, colorimeters are tristimulus devices used for color calibration. Accurate color profiles ensure consistency throughout the imaging workflow, from acquisition to output.

Spectroradiometer
The absolute spectral power distribution of a light source can be measured with a spectroradiometer, which works by optically collecting the light, then passing it through a monochromator before reading it in narrow bands of wavelength.

Spectrophotometer
Reflected color can be measured using a spectrophotometer (also called spectroreflectometer or reflectometer), which takes measurements in the visible region (and a little beyond) of a given color sample. If the custom of taking readings at 10 nanometer increments is followed, the visible light range of 400-700nm will yield 31 readings. These readings are typically used to draw the sample's spectral 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 their tristimulus values, which can be converted into chromaticity co-ordinates and manipulated through color space transformations. For this purpose, a spectrocolorimeter may be used, although tristimulus colorimeters offer a cheaper alternative. A spectrocolorimeter is simply a spectrophotometer that can estimate tristimulus values by numerical integration (of the color matching functions' inner product with the illuminant's spectral power distribution). The CIE recommends using measurement intervals under 5nm, even for smooth spectra.

Color temperature meter
Photographers and cinematographers use information provided by these meters to decide what color correction 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 the mired difference between the measurement and the reference, enabling the user to choose a corrective color gel or photographic filter with the closest mired factor.

Internally, the meter is typically a silicon photodiode tristimulus colorimeter.