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Standard colorimetric observers.
A standard observer is an empirical model of human color vision expressed as three spectral sensitivity functions x-bar(lambda), y-bar(lambda), z-bar(lambda), collectively the color matching functions (CMFs). Integrating a spectral power distribution against these functions yields CIE XYZ tristimulus values. Because the functions differ between observers, the XYZ values for the same spectrum differ between observers and the two spaces are not interchangeable.
§Which observer to use
Every standard RGB color space (sRGB, Rec. 709, Rec. 2020, Display P3,
ACES, ProPhoto) is defined relative to Cie1931. The ICC profile
connection space is Cie1931. Use Cie1931 for display and imaging
pipelines.
Cie1964 is recommended by CIE 015:2018 for surface color evaluation
of object colors subtending more than roughly 4 degrees of visual field:
textiles, paints, plastics, printed matter viewed at close range.
Industrial spectrophotometers routinely report results under both observers.
ISO 105-J01 (textile color fastness) mandates Cie1964.
§Converting between observers
There is no general lossless conversion between observer spaces at the tristimulus level. The only correct path is to return to the original spectral data and integrate against the target observer’s CMFs. Once a spectrum has been collapsed to XYZ, the information needed to reconstruct it is gone. Observer adaptation transforms (approximate 3x3 matrices) appear in some literature but they do not preserve metamerism and are not standardized. This crate does not provide them.
Structs§
- Cie1931
- CIE 1931 2-degree standard observer (CIE 015:2018).
- Cie1964
- CIE 1964 10-degree supplementary standard observer (CIE 015:2018).
Traits§
- Standard
Observer - Marker for a CIE standard colorimetric observer.