An instrument that measures colour directly as three numbers through filters that imitate the eye.
It uses three or more filtered sensors approximating the CIE colour-matching functions to report tristimulus values. Display calibrators are usually colorimeters.
Because it records only three numbers, a colorimeter cannot tell two metameric samples apart, and its accuracy depends on how well its filters match the standard observer for the light being measured.
A screen-calibration puck is a colorimeter tuned for display spectra.
Cheaper and faster than a spectrophotometer, and good for repeated measurements of similar things.
A colorimeter's quality is judged by how closely its filtered sensors follow the CIE colour-matching functions, since any mismatch becomes an error that varies with the spectrum being measured. For that reason a display colorimeter needs a correction suited to the type of screen, and is checked against a spectroradiometer when it matters. Output is tristimulus values or chromaticity and luminance, for one illuminant in the case of surface instruments.
| Sample and light | X | Y | Z |
|---|---|---|---|
| Sample A, Daylight, CIE D65 | 2.92 | 3.24 | 4.34 |
| Sample B, Daylight, CIE D65 | 2.92 | 3.24 | 4.34 |
| Sample A, Narrow-band tri-phosphor fluorescent, CIE FL11 | 3.01 | 3.18 | 2.61 |
| Sample B, Narrow-band tri-phosphor fluorescent, CIE FL11 | 2.53 | 2.52 | 2.42 |
Under D65 the two samples give the same X, Y and Z to two decimal places, so a colorimeter reports them as one colour. Their spectra are not the same: at 500 nm sample A reflects 3.7% and sample B 5.6%. Under the fluorescent tube the tristimulus values separate, X reading 3.01 for one and 2.53 for the other. Only an instrument that recorded the spectra could have predicted that.
How this was calculated: Sample A is a modelled reflectance for Gunmetal #2a3439; sample B adds a spectral ripple constructed to have zero X, Y and Z under D65. Tristimulus values use the CIE 1931 2° observer at 10 nm.
An older display colorimeter is used to calibrate an OLED or quantum-dot screen.
Its corrections were made for the spectra of earlier backlights. On narrow-band primaries the filter mismatch can become a larger error, and the calibration lands on a white that is visibly off.
A colorimeter is used to approve a colour match between a plastic and a painted part.
It can confirm that the two agree under its own light and cannot reveal that they are a metameric pair. The match fails later under shop or street lighting.