Rabin Cone Contrast Test

What the cone contrast test measures

Passing scores: the cut-off depends on who is testing

The same instrument and the same score can mean “deficient” to one organization and “certified” to another, because the pass mark is set by the regulator rather than by the test.

The full FAA picture — all three approved computer tests, the day-VFR limitation if you fail, and the grandfathering rules — is on the FAA color vision test page.

What is the Rabin Cone Contrast Test?

The cone contrast test asks a different question from a plate test. Instead of "can you read the number hidden in these dots?", it asks "how faint can this color get before you stop seeing the letter?" — and it asks that question once for each of your three cone classes. The letters are computed so that only one cone class can detect them: an L-cone letter is invisible to M and S cones, and so on. Letter and background are matched for brightness using the CIE standard observer model, so there is no luminance edge to fall back on. If that cone class is working, you read the letter; if it is not, you see a blank field.

Because contrast is stepped down until you fail, the output is a threshold — a measured quantity — rather than a tally of plates missed. Jeff Rabin introduced the method in 1996 while at the Aerospace Ophthalmology Branch of the US Air Force School of Aerospace Medicine, published the validation in Investigative Ophthalmology & Visual Science in 2011, and it reached the US Air Force commercially in 2012. Rabin is now Professor of Optometry and Assistant Dean for Research at the University of the Incarnate Word Rosenberg School of Optometry, and remains a part-time senior scientist with USAFSAM.

How is the cone contrast test scored?

Thresholds are converted to a 100-point scale, with each correctly identified optotype worth five points, and you get one score per cone class per eye — for example L 95, M 90, S 85. Higher is better: a high score means you needed very little color contrast to see the letter. The tested contrast range runs from about 27.5% down to 1% for the L and M cones, and from about 173% down to 7% for the S cones, which is why S-cone scores are read on their own terms rather than compared directly with the red-green ones.

The pass mark is set by whoever is doing the testing, not by the test itself, and this is the single most common source of confusion. US Air Force criteria treat a score below 75 as a color vision deficiency. The FAA's requirement for pilots and air traffic control specialists is lower — 55 or higher on each of red, green and blue, each eye tested separately. An applicant can therefore be "deficient" by the Air Force's occupational standard and still meet the FAA's certification standard on the very same instrument and the very same score.

Can you take the Rabin cone contrast test online?

No. There is no official free or online version of the RCCT, and this is firmer than the usual "it would be unreliable" caveat. The score only means anything because the display is color-calibrated: the whole method depends on producing a letter that is invisible to two cone classes and matched in luminance to its background, which an uncalibrated phone or laptop cannot be trusted to do. The commercial implementation, Konan Medical's ColorDx CCT HD, ships as calibrated hardware — co-developed with the Air Force's Operational Based Vision Assessment laboratory and FDA-listed for that reason.

The regulatory side is blunter still. The FAA's instrument list states that testing must be performed in person and that "virtual testing, downloaded, or printed versions of color vision tests are prohibited." So no website can give you an RCCT result that counts — including this one. Anything advertising "the Rabin cone contrast test online, free" is a lookalike, and treating its number as your CCT score will only mislead you before an appointment.

Why you cannot practice your way past a cone contrast test

This is the part worth understanding if a medical is coming up. Rabin's validation study ran 1,446 applicants for pilot training through the CCT, the anomaloscope, and pseudo-isochromatic plates. Against the anomaloscope, the CCT detected and correctly categorized 100% of the deficiencies it found — 40 deutan, 7 protan, 2 tritan — with specificity of 100% for the L and M cone tests and 99.8% for S. The plate tests, on the same applicants, detected only 80%: one in five deficiencies was missed. The paper attributes that gap in part to applicants' prior experience with the plates and pre-test preparation.

In other words, plates can be studied and a threshold cannot. The classic Ishihara set is a fixed, published series, so a determined applicant can learn it; the CCT measures how much color signal your retina needs, which does not change because you have seen the test before. The practical implication is not that preparation is pointless — it is that useful preparation is informational. Knowing beforehand whether a deficiency is likely, whether it looks protan or deutan, and roughly how strong it is tells you which side of the published limits you are likely to land on. It removes the surprise, which is the only thing preparation can honestly do here.

Who uses the cone contrast test, and where can you take one?

In the military, the CCT is the current color vision screen for the Air Force, Army, Navy and Coast Guard. In civil aviation, the FAA accepts the Rabin Cone Test as one of just three approved computer-based color vision tests, alongside the CAD test and the Waggoner CCVT; the RCCT is administered by an Aviation Medical Examiner as part of the medical, not booked separately. Outside aviation, the same instrument appears in optometry and ophthalmology practices under the ColorDx CCT HD name, where cone-specific scores are also used to follow acquired color vision changes over time.

If you are not going through a military or aviation medical, the realistic route is an eye-care practice that lists cone contrast or ColorDx testing, or a university optometry clinic. It is worth asking before booking: many practices screen with plates only, and a plate result will not give you the per-cone scores that make the CCT useful.

What can you check online instead?

A screening test on your own screen can answer three of the four questions a CCT appointment answers: whether a color vision deficiency is likely present, whether it falls on the red-green or blue-yellow axis, and roughly how strong it appears to be. What it cannot produce is the certified number, because that number is a property of calibrated hardware.

Our free color vision test uses the same family of idea — cone-isolating stimuli stepped down in contrast, with brightness cues suppressed — run as an adaptive staircase in the browser. On an uncalibrated display the type and degree it reports are estimates, and we label them as estimates rather than dressing them up as a score. That is genuinely useful for finding out where you stand before you pay for an appointment, and genuinely not a substitute for one when a license or a job is involved.

Rabin cone contrast test — frequently asked questions

Is there a free Rabin cone contrast test online?

No. The RCCT depends on a color-calibrated display to produce letters visible to only one cone class, so it cannot be delivered accurately through a web page, and the FAA separately prohibits downloaded, printed, and virtual color vision tests for certification. Free online tests — ours included — can screen for the same deficiencies and estimate the type, but they cannot produce a cone contrast score.

What is a passing score on the Rabin cone contrast test?

It depends on who is testing you. For FAA pilot and air traffic control certification the requirement is 55 or higher on each of red, green and blue, with each eye tested separately. US Air Force occupational criteria are stricter: a score below 75 is treated as a color vision deficiency. The instrument is the same; the cut-off is set by the regulator.

What is a normal cone contrast test score?

Scores run from 0 to 100 for each cone class, and normal color vision typically sits high on all three — the US Air Force treats scores below 75 as deficient. Because the S-cone test spans a much wider contrast range than the L and M tests, S scores are interpreted on their own scale rather than compared directly against the red-green ones.

How is the cone contrast test different from the Ishihara test?

Ishihara is a fixed set of printed plates that asks whether you can read a hidden figure, giving a pass/fail-style screen for red-green deficiency. The CCT measures a threshold — how much contrast each cone class needs — and reports a score per cone per eye, covering blue-yellow as well. In Rabin's study of 1,446 pilot applicants the CCT detected 100% of the deficiencies confirmed by anomaloscope while plate testing detected 80%, a gap the paper attributes partly to applicants having seen the plates before.

Can you practice or prepare for the cone contrast test?

Not in a way that changes the result. The test measures how much color contrast your cones need, which is not something memorization or practice alters — unlike a fixed plate series, which can be learned. What you can usefully do beforehand is find out your likely type and degree with a screening test, so the measured result is not a surprise.

Where can I take the Rabin cone contrast test?

For aviation, an FAA Aviation Medical Examiner administers it as part of the medical examination. In the military it is the standard color vision screen for the Air Force, Army, Navy and Coast Guard. Otherwise, look for an optometry or ophthalmology practice — often a university clinic — that lists cone contrast or ColorDx CCT HD testing, and confirm when booking, since many practices screen with plates only.