Green cone deficiency
Short answer
Deutan color blindness affects the green-sensitive cone and makes some reds, greens, browns, and oranges harder to separate. It is the most common form; severity varies.
In detail
Deutan color blindness is the most common form of color vision deficiency, affecting roughly 6% of men. It comes from the eye's green M-cone being shifted along the spectrum (deuteranomaly) or absent altogether (deuteranopia), which collapses the separation between reds and greens.
Unlike protan deficiency, deutan vision keeps normal brightness, so reds stay bright — they just stop looking distinctly red. Most deutans have the mild anomalous form and describe confusion between green, brown, olive, and orange rather than any sense of missing color.
Prevalence figures are approximate and vary between populations; they are highest in people of Northern European descent.
| Color pair | Why it collapses |
|---|---|
| Green and brown | Both sit in the middle of the collapsed red-green range |
| Green and orange | Without M-cone separation, warm and mid tones converge |
| Olive and khaki and mustard | Small hue steps in a range that is already compressed |
| Blue and purple | Purple loses its red component and reads as plain blue |
| Pink and grey | Pale pinks lose the red signal that would mark them out |
| Red and black in low light | Dark reds have little signal left to separate them |
Deutan is the family name for color vision deficiency involving the M-cone, the photoreceptor that responds most strongly to medium-wavelength light — what we loosely call green. There are two members: deuteranomaly, where the M-cone pigment is shifted toward the red end of the spectrum, and deuteranopia, where the M-cone is missing and vision runs on two cone types instead of three.
Because the L (red) and M (green) cones already overlap heavily in normal vision, shifting the M pigment even a little collapses much of the remaining difference between them. That small genetic change is why deutan deficiency is the most common form of color blindness in the world, and why it varies so widely in degree: the size of the shift determines how much separation survives.
Deutan deficiency is inherited on the X chromosome, present from birth, and stable for life. It does not progress, and it does not affect sharpness, night vision, or field of view.
Not grey, and not a world with colors missing. Deutans see a full, saturated world — the range is simply narrower in one direction. Blues and yellows are unaffected and often look vivid. What compresses is the stretch from red through orange and yellow to green, which becomes a single family of warm tones rather than a series of distinct hues.
The everyday reports are consistent. Traffic lights are read by position because green against a bright sky is uncertain. Ripe and unripe fruit look the same on the tree. Red text on a dark slide vanishes. Heat maps and status dashboards that use red for bad and green for good become unreadable. Matching clothes involves either asking someone or keeping a system.
The one thing deutans do not report is dimness. Reds look bright — just not red. That distinguishes deutan from protan, where the red cone also carries much of the brightness signal, so deep reds look dark or nearly black.
Deuteranomaly is by far the more common of the two and by far the more variable. Someone with a very mild form may pass a plate test, hold a job with a color vision standard, and go a lifetime without knowing. Someone with a strong form performs on plate tests much like a dichromat. The label is the same; the experience is not.
Deuteranopia is the dichromatic end: no M-cone at all, two functioning cone types, and no possibility of separating red from green by hue. It is stable and, unlike the anomalous form, not a matter of degree.
Screening tests separate normal from deficient reliably. Sorting mild deuteranomaly from mild protanomaly is a different and harder question, which is why arrangement tests like the Farnsworth D-15 exist and why an online result should be read as an estimate.
This is the group filter lenses are designed around. By cutting a narrow band of the spectrum where the L and M signals overlap most, the lens exaggerates whatever difference remains between them. For a mild-to-moderate deutan in bright daylight, that can make previously confusable colors look genuinely more separated — an effect people often describe as colors looking more distinct rather than looking correct.
The honest limits matter as much as the effect. The lenses do not restore normal color vision, do not work well for deuteranopia where there is no M-cone signal left to exaggerate, dim the overall scene, and are excluded from every occupational color vision standard. Indoors and at night the benefit largely disappears.
Anyone considering them should confirm their type and degree first. Mild deuteranomaly is the best case; strong deuteranopia is the weakest.
A plate test tells you whether a red-green deficiency is present; an arrangement test is what separates the two families. Start with the color blind test, then run the Farnsworth D-15 to see your confusion axis.
Mild deutan and mild protan sit close together — read any type from an uncalibrated screen as an estimate. See protan vs deutan for the full comparison.
Protan vs deutan
The two red-green families side by side, and how to tell which is yours.
Red-green color blindness
The whole family: all four forms, prevalence, and testing.
Farnsworth D-15
The arrangement test that reveals your confusion axis.
Do color blind glasses work?
What filter lenses do for deutans specifically, and where they fall short.
Last reviewed July 26, 2026. Informational only and regularly reviewed — not a substitute for advice from an eye-care professional.