Colour Vision & Colour Blindness: The LMS Model Explained

Three cones, one trick of overlap

Human colour vision is trichromatic: the retina has three types of cone cell, each with a photopigment sensitive across a broad but different range of wavelengths, conventionally labelled by their peak sensitivity — S (short, ~420 nm, "blue"), M (medium, ~534 nm, "green") and L (long, ~564 nm, "red"). None of these curves is narrow; they overlap heavily, especially M and L, which peak only about 30 nm apart. Colour perception isn't three independent channels reporting wavelength — it's the brain comparing the relative response across all three, which is why a single wavelength and a mixture of wavelengths can look identical (metamerism) as long as they produce the same S:M:L response ratio.

Downstream of the cones, the visual system recodes these three signals into opponent channels: a red-green axis (roughly L minus M), a blue-yellow axis (roughly S minus L+M), and a black-white luminance channel. This opponent-process stage, not the raw cone responses, is what most colour-blindness effects and colour-contrast illusions actually operate on.

What actually goes missing in colour blindness

The common inherited forms are anomalies or absences of the L or M cone photopigment, both encoded on the X chromosome (which is why red-green colour blindness affects far more men than women). Protanopia is a complete absence of L-cone function; deuteranopia is a complete absence of M-cone function. Both collapse the red-green opponent axis, since it depends on comparing L and M, but they aren't identical — protanopia also dims perceived brightness in the red end of the spectrum because L cones normally contribute to luminance, while deuteranopia does not.

Tritanopia, absence of S-cone function, is rare and not sex-linked; it collapses the blue-yellow axis instead, so blues and yellows become confused while red-green discrimination stays largely intact. Achromatopsia, true absence or near-absence of all cone function (relying on rods alone), is very rare and eliminates colour vision almost entirely, leaving only luminance-based, low-acuity vision.

condition        cone affected     axis collapsed        typical confusion
protanopia       L absent          red-green              red ↔ dark green/brown
deuteranopia     M absent          red-green              red ↔ green (brighter than protan)
tritanopia       S absent          blue-yellow            blue ↔ green, yellow ↔ violet
achromatopsia    all cones         both                   greyscale vision only

Simulating it: LMS transform, not just a colour filter

A perceptually honest colour-blindness simulator doesn't just desaturate or hue-shift an image; it works in LMS space. First the image's RGB values are converted to an estimate of actual L, M and S cone excitation using a standard transform (such as the Hunt-Pointer-Estevez matrix). Then, for the deficiency being simulated, the missing cone's response is estimated from the other two — for example, under a common protanopia model, the L response is replaced by a linear combination of M and S. Finally the modified LMS triple is converted back to RGB for display. This reproduces genuine confusion lines — the actual sets of colours a dichromat cannot distinguish — much more faithfully than a naive hue rotation, which tends to either under- or over-state the confusion.

Why Ishihara plates work the way they do

The classic Ishihara test embeds a numeral in a field of coloured dots, where the numeral's dots and the background dots are chosen to differ mainly along the red-green confusion axis while being matched in brightness and saturation as closely as possible. A trichromat sees the number easily because their L and M signals differ. A protanope or deuteranope, missing that axis, sees the numeral and background as nearly the same perceived colour and the number vanishes into visual noise — the test is essentially a direct, deliberately low-tech probe of exactly the LMS-to-opponent-channel pathway described above.

Frequently asked questions

Try it live

Everything above runs in your browser — open Colour Vision Simulator and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.