Deuteranopia

The green-sensitive cone is missing entirely. Red and green stop being separate colors, but brightness is barely touched, which is why it so often goes unnoticed for years.

What Is Happening in the Eye

Normal color vision compares the output of three cone types. Deuteranopia removes the middle one, the M cone, which peaks around 534 nm. With only L and S cones left, the retina has two numbers to work with instead of three, and every color that differed only in the L-versus-M comparison collapses onto its neighbour.

The gene for the M pigment sits on the X chromosome, so this is inherited, present from birth, identical in both eyes, and stable for life. It affects roughly 1.2% of men and 0.01% of women of Northern European descent.

What It Actually Looks Like

Not a world without green. A world where the red-to-green stretch of the spectrum has been folded into a single band of yellows, beiges and khakis, with blue and yellow left largely intact.

• Red and green traffic lights differ by position and a little by brightness, not by color.
• Ripe and unripe fruit look the same. So does cooked and raw meat.
• Pink reads as grey or pale blue. Purple reads as blue, because the red half of it is gone.
• Green foliage against brown bark is close to one color in two shades.
• Red text on a black background stays perfectly readable, unlike in protanopia.

You can see this directly rather than imagining it: the simulator renders any photo through the deuteranopic projection.

Deuteranopia versus Deuteranomaly

These get used interchangeably and should not be. Deuteranopia means the M cone is absent, a dichromat, two working cone types. Deuteranomaly means the M cone is present but tuned too close to the L cone, so the two overlap and the comparison between them is weak rather than gone.

Deuteranomaly is far more common, about 5% of men, making it the single most common form of color vision deficiency by a wide margin, and it spans a huge range. A mildly deuteranomalous person may pass most plates and never suspect anything; a strongly deuteranomalous one performs close to a deuteranope.

A plate test can tell you the axis is deutan. It is not reliable at telling you which of the two you have, and it cannot grade anomaly severity. That takes an anomaloscope, which is a clinic instrument.

What It Affects in Practice

Day to day, less than people expect. Deuteranopes drive, cook, dress and work without much trouble, because the world is full of redundant cues, position, shape, label, brightness, and a lifetime of using them becomes invisible skill.

Where it bites is anywhere color is the only signal: red/green status indicators, heat maps, wiring by color code, resistor bands, mismatched clothing, and charts whose legend is a row of colored squares. It also closes some occupational doors, commercial piloting, some military roles, parts of the electrical trades and marine and rail work all run color vision standards, and those are tested against printed plates or a lantern test, not this site.

Can It Be Corrected?

No. The pigment is not there, and no lens can invent it. The tinted glasses marketed for color blindness cut a slice of the spectrum where the L and M responses overlap, which widens the gap between colors that used to look alike. For deuteranomaly that can be genuinely useful, because there is a weak signal to amplify. For true deuteranopia there is no M cone signal to work with, and the benefit is far smaller, the scene mostly just looks tinted.

Testing for It

Our color blind test puts four plates behind the deutan axis, built so the figure and background differ only along the L-M comparison. If those four are the ones you miss while the protan and tritan plates come out clean, the result points here.

For anything with consequences attached, a job requirement, a licence, a child's school assessment, get it done properly. An optometrist has printed plates under standard illumination and an anomaloscope, and a screen you have never calibrated is not a substitute for either.

See also: protanopia, tritanopia, and all six types compared.