Scene:
Normal vision
Simulated view
Human colour vision is trichromatic: the retina contains three types of cone photoreceptors — S (short-wavelength, peak ~420 nm), M (medium, ~530 nm), and L (long, ~560 nm) — whose differential responses encode colour information. The brain then converts these LMS signals into opponent channels: a red-green channel (L−M), a blue-yellow channel (S−(L+M)), and a luminance channel (L+M). This opponent-process theory, proposed by Ewald Hering in 1878, explains why we cannot perceive "reddish green" or "yellowish blue" — those are opposing signals that cancel.
Select a colour-vision deficiency type (Protanopia — missing L cones, Deuteranopia — missing M cones, Tritanopia — missing S cones, or Achromatopsia — total colour blindness) to see a side-by-side simulation of how the chosen test pattern appears to someone with that condition. The Brettel 1997 dichromat model projects each pixel's LMS coordinates onto the confusion plane for the missing cone type and converts back to sRGB. Switch between Colour Wheel, Ishihara-style plate, Spectrum Strip, and Traffic Lights scenes.
What causes colour blindness?
Most colour vision deficiencies (CVDs) are caused by mutations in the genes encoding the opsin proteins of the L or M cones, which are located on the X chromosome. This is why CVD is far more common in males (~8%) than females (~0.5%) — males have only one X chromosome, so a single faulty copy is enough. Tritanopia (S-cone deficiency) is autosomal and rare (about 1 in 35 000), whilst deuteranopia (missing M cones) affects about 1 in 12 males.
What is a metamer and why does it cause confusion for colour-blind people?
A metamer is a pair of physically different light spectra that produce identical cone responses — and therefore look identical — to a given observer. Normal trichromats have a 3D colour space and relatively few metamers. Dichromats (missing one cone type) have a 2D colour space, so vastly more pairs of spectra are metameric. Red and green may produce the same LMS response in a protanope, making them indistinguishable — a potentially dangerous confusion in traffic or food-sorting contexts.
How does the Ishihara test work?
Ishihara plates embed a numeral (or path) in dots of one colour family surrounded by dots of a different, confusion-inducing colour. A person with normal colour vision reads the numeral easily; a red-green colour-blind person sees only a uniform field of dots because the numeral's dots are metamers with the background for their visual system. The simulator generates a stylised "5" in orange dots on a green background to illustrate this principle.
Both are red-green colour blindness, but they arise from different missing cones. Protanopes lack L (long-wavelength, red-sensitive) cones; deuteranopes lack M (medium-wavelength, green-sensitive) cones. Both groups confuse reds with greens, but protanopes also see reds as darker than normal people do (reduced luminance sensitivity at long wavelengths), whilst deuteranopes do not. Deuteranopia is about 10 times more common than protanopia.
Yes — dichromats still experience colour, just with fewer distinctions. A protanope or deuteranope can still distinguish blues from yellows, and light from dark; they simply cannot distinguish red from green (they may appear as similar shades of yellowish-brown or grey). Only achromatopsia (complete colour blindness, affecting ~1 in 33 000 people) results in a fully monochromatic world with no hue perception at all.
The Brettel model converts each pixel's sRGB values to LMS cone space using the Hunt-Pointer-Estévez matrix. For a dichromat missing one cone type, it projects the LMS triplet onto a 2D plane through the "white point" (equal LMS) and the "neutral axis" of the confusion colours. The projected point is then converted back to sRGB. This produces a perceptually accurate simulation of how a dichromat sees the image, validated against forced-choice psychophysical data.
Key principles include: never use colour as the only channel for conveying information (also use shape, texture, or labels); avoid red-green combinations for critical distinctions (e.g., use blue-orange instead); provide sufficient luminance contrast between foreground and background (WCAG 2.1 requires a minimum ratio of 4.5:1); and test designs using a CVD simulator like this one. The Spectrum Strip and Traffic Lights scenes in this simulator show common real-world accessibility challenges.
Colour confusion lines (or "copunctal lines") are lines in CIE chromaticity space along which a dichromat cannot distinguish colours. All colours on the same line appear identical to a protanope, deuteranope, or tritanope. For protanopes and deuteranopes, the confusion lines converge at a point (the "copunctal point") near 0.75, 0.25 and 1.08, −0.08 respectively. Two colours that lie on the same confusion line will look the same regardless of their intensity difference.
Deuteranopes lack M cones, so they cannot distinguish stimuli that differ primarily in M-cone excitation. The reds, greens, and browns in the colour wheel all produce similar LMS responses after the M channel is collapsed, appearing as varying shades of yellow and blue. The distinctive "collapsed" colour wheel you see in the deuteranopia simulation is a key diagnostic feature used in clinical colour vision tests.
Some research suggests that certain forms of colour blindness may improve discrimination of camouflage or subtle luminance textures that colour-normal observers are distracted from by chromatic differences. In some military and hunting contexts, colour-blind individuals have reportedly spotted camouflaged targets that normals missed. However, this effect is modest and context-specific; most daily-life and professional tasks are not advantaged by reduced colour discrimination.