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A researcher whose work includes color appearance and color constancy.

A researcher whose work includes visual adaptation and color appearance.

L, M and S describe the relative spectral sensitivities of cone classes. Calling them red, green and blue confuses receptor sampling with the color appearance constructed from comparisons and context. The shorthand also wrongly suggests three narrow, nonoverlapping detectors or the primaries of a display.

First, each class has a broad sensitivity curve. A long-wavelength-sensitive cone responds to many wavelengths, and its sensitivity maximum is not a unique red stimulus. L- and M-cone sensitivities overlap extensively, while S cone cells are relatively more sensitive at shorter wavelengths. Light at one wavelength can therefore excite more than one class. Andrew Stockman and colleagues derived L- and M-cone sensitivity estimates using color-matching data and measurements from observers of known pigment genotype. Their analysis emphasizes broad overlap and the need for isolation procedures to measure one class without contamination from others.^[ref] Exact curves depend on pigment variant and optical filtering, so the letters denote relative classes rather than three invariant color labels.

Second, the principle of univariance prevents one cone from identifying hue. For a monochromatic stimulus, let mean absorption be $Q=Ia(\lambda )$. Whenever $a({\lambda }_1)$ and $a({\lambda }_2)$ are positive, intensities satisfying $I_2=I_{1}a({\lambda }_1)/a({\lambda }_2)$ give the same $Q$. The single receptor cannot distinguish those two inputs on absorption count alone. Calling that receptor a red detector would assign a color judgment to a signal that cannot even uniquely distinguish the two wavelengths. It remains true that spectral sensitivity constrains the comparison; univariance does not mean all receptor types are identical.

Third, perceptual color requires combinations. Comparing L and M signals and comparing S with L+M supplies chromatic information; summing appropriately weighted signals contributes to luminance-related processing. An L-cone increment can be part of several stimulus changes depending on what happens to M and S and on the adapted background. Neither an L response nor a raw L:M ratio by itself specifies a context-independent color word. The retinal and cortical opponent circuitry demonstrated by Dennis M. Dacey and Bevil R. Conway provides mechanisms for comparisons, rather than a relabelling of three cone outputs as three sensations.^[ref][ref]

Fourth, “blue” or “red” need not identify a single wavelength. Spectrally different lights can be metamers, and a purple appearance can arise from a mixture rather than a monochromatic wavelength. Conversely, the same physical spectrum can look different against another surround or after adaptation. Cone labels cannot replace the distinction between emitted or reflected spectrum, receptor catch, opponent representation and reported hue.

Fifth, the display analogy is backwards. An RGB screen emits mixtures from three selected primary spectra. The cone cells receive and integrate the resulting spectrum; their response curves are not identical to those primary spectra. If primary intensities are $p_k$ and cone sensitivities to each primary form matrix $B$, then $\mathbf{Q}=B\mathbf{p}$. Off-diagonal terms are generally nonzero because each primary can excite several cone classes. If $B$ is nonsingular, an intended catch change can be computed through $B^{-1}$ within the display's feasible gamut. It does not follow that every requested catch vector is realizable with nonnegative bounded primary intensities. Calibration and observer differences therefore matter in cone-isolating experiments.

Finally, color terms themselves vary across languages and tasks, whereas receptor class is a biological classification. Receptor genes, color matches, discrimination and linguistic categorization are related but distinct measurements. The labels L, M and S prevent an unwarranted assumption that one anatomical cell class directly instantiates one learned word or unique subjective hue.

A researcher whose work includes color appearance and long-term chromatic adjustment.