Tritanopia

The blue-sensitive cone is missing. Rare, spread evenly across men and women, and the one type where the result is worth taking to an optometrist, because unlike red-green deficiency, it is often something that happened rather than something you were born with.

What Is Happening in the Eye

Tritanopia removes the S cone, which peaks around 420 nm at the short-wavelength end. The comparison lost is S against the L and M cones together, the blue-yellow axis, leaving red-green discrimination intact.

The gene for the S pigment is on chromosome 7, not the X chromosome. Two consequences follow. It is not sex-linked, so men and women are affected about equally, at roughly 1 in 10,000 either way. And inheritance is autosomal dominant with incomplete penetrance, so it runs in families unpredictably, relatives carrying the same variant can be affected to very different degrees.

What It Actually Looks Like

• Blue and green become hard to separate. Teal is the worst case.
• Yellow and pink converge, and yellow can look washed-out white or grey.
• Violet reads as plain red, because the blue component is gone.
• Orange and red stay distinct, the red-green axis is untouched.
• The sky and the sea lose their difference from surrounding greens.

The simulator's tritanopia panel shows this: the reds and greens survive the projection almost unchanged while the blues collapse.

Inherited or Acquired, The Part That Matters

Congenital tritanopia is genuinely rare. Blue-yellow deficiency turning up in a test is much more often acquired, and the S cone pathway is the first thing to degrade under a long list of conditions:

• Glaucoma, which affects blue-yellow discrimination early.
• Diabetic retinopathy.
• Age-related macular degeneration.
• Optic neuritis and other optic nerve disease.
• Ordinary lens yellowing with age, which filters short wavelengths before they reach the retina.
• Some medications, including certain antimalarials and long-term ethambutol.

Three signs separate acquired from inherited: it changes over time, it can differ between your two eyes, and it often arrives alongside other visual symptoms. Inherited deficiency does none of those, it is fixed, identical in both eyes, and has been there since birth. If a blue-yellow result is new to you, book an optometrist. The underlying cause may be treatable, and several of the ones listed above are worth catching early.

Tritanopia versus Tritanomaly

Tritanopia is the S cone absent. Tritanomaly is the S cone present but mistuned, leaving partial blue-yellow discrimination. Tritanomaly is extremely rare as an inherited condition, rarer even than tritanopia, so a mild blue-yellow result is more likely to be an acquired change or a screen artefact than inherited tritanomaly.

Why Screen Tests Are Shakier Here

Blue-yellow plates are the hardest kind to run on a display you do not control. The blue primary is where monitors, color profiles, ambient light and night-mode filters diverge most, and a blue-light filter left on will produce a tritan-shaped result in someone with perfectly normal color vision.

So before trusting a tritan result: turn off night mode and blue-light filtering, set brightness high, and retake it. Our test includes control plates partly to catch exactly this, they are readable by every observer, so misreads on them mean the screen is interfering and the run is not scored.

Testing for It

Our color blind test runs four plates on the tritan axis alongside the red-green ones. If those are the plates you miss, the result points here, and given how much more often blue-yellow deficiency is acquired than inherited, that is the result most worth following up in person.

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