Vision
From the retina and S/M/L cones to metamerism, adaptation and the standard observer: why human perception became the basis of RGB and modern colorimetry.
The eye does not record a spectrum like a laboratory instrument. It converts light into a limited number of receptor signals and the brain compares them with each other and with the surround. The same physical stimulus can therefore be perceived differently under different conditions.
Quick reference
Overlapping spectral sensitivities; not literal B/G/R detectors.
Three independent quantities can model a large class of colour matches.
The modern standard describes use for visual fields of roughly 1–4°.
Intended for larger visual fields.
Conceptual shape only: the key idea is overlap, not literal RGB filters.
Retinal photoreceptors include rods and cones. Rods are especially important at low light levels and provide very little colour discrimination, while cones dominate photopic vision and form the basis of colour perception. Display and video colorimetry largely assumes viewing conditions where cone-mediated photopic vision is active.
The three cone classes have overlapping spectral sensitivity curves. S cones are biased toward shorter wavelengths, M toward medium wavelengths and L toward longer wavelengths, but each class responds across a broad range. Renaming S/M/L literally as B/G/R is therefore misleading.
For a large class of colour stimuli, visual matching can be represented using three independent quantities. This makes tristimulus colorimetry, RGB systems and CIE XYZ possible. Importantly, mathematical tristimulus coordinates are a perceptual matching model, not direct readings from the three cone classes.
Two different spectra can produce the same or nearly the same tristimulus response and appear as the same colour. This allows a three-primary display to mimic a huge range of real-world colours. But a metameric match can fail for another observer or another illuminant, leading to metameric failure in difficult cases involving fabrics, LED lighting and cameras.
Neural processing builds opponent and luminance relationships: roughly red–green, blue–yellow and light–dark components. Perceptual hue, brightness and colourfulness therefore cannot be read directly from three cone responses. This is one reason perceptual colour models are more complex than plain RGB.
A white sheet of paper can look white under warm indoor lighting and under daylight even though the spectrum reaching the eye changes substantially. The visual system partially compensates for the illuminant and redefines neutrality. Technical chromatic-adaptation transforms attempt to model part of this behaviour mathematically.
The same image patch can appear lighter, darker, warmer or cooler depending on neighbouring regions and overall adaptation. A reference monitor alone therefore does not solve the whole problem; the viewing environment is part of the image-evaluation system.
Real people differ slightly in colour sensitivity. To make measurements reproducible, the CIE standardised colour-matching functions. The CIE 1931 2° Standard Colorimetric Observer is used for smaller visual fields, while the supplementary CIE 1964 10° observer represents larger fields. This is not a 'perfect human'; it is an agreed measurement model.
Scopes provide objective information about the signal, while vision judges the perceptual result. But eyes adapt, fatigue and depend on the surround. Serious grading therefore combines a calibrated display, controlled viewing environment, scopes, reference images and regular breaks rather than relying on any one of them alone.
When an image 'suddenly looks normal' after ten minutes, that does not necessarily mean you fixed it — your visual system may have adapted to it. This is why reference viewing and scopes matter.
Technical definitions are anchored to standards bodies and primary specifications wherever that distinction matters.
