Clinical color vision testing
Short answer
The CAD test measures color sensitivity using a moving target in brightness noise. Official testing requires calibrated clinical equipment; an online screening cannot give a CAD score.
In detail
The CAD (Colour Assessment and Diagnosis) test is a clinical color vision test developed at City, University of London. It measures how much color signal you need to see a moving target hidden in brightness noise, and reports the result as a number in CAD units — so it grades severity instead of just pass/fail.
It is the test UK aviation uses to decide borderline cases. There is no official CAD test you can take online: it runs on calibrated equipment at approved centres. An online test can screen for the same deficiencies, but it cannot produce a CAD score.
The CAD test was developed at the Applied Vision Research Centre at City, University of London. On screen, a colored target moves diagonally across a background of small squares whose brightness flickers at random, and you press a key to say which direction it moved. Because the background is pure brightness noise, brightness gives nothing away — only a working color signal can reveal the target. The test then adjusts the color strength up and down to find your threshold: the weakest color you can still reliably see.
That design is what separates CAD from plate tests like Ishihara. A plate test asks a yes/no question — can you read the hidden number or not — and can be affected by lighting, printing, and memorized answers. CAD measures a continuous quantity on calibrated equipment, which is why regulators trust it to grade borderline cases rather than just fail them.
No. There is no official free or online version of the CAD test. The result is only meaningful because the display is color-calibrated, the viewing distance and lighting are controlled, and the software is licensed — none of which holds on a home screen. The test is administered at approved centres, mainly aviation medical examination centres and occupational-health providers.
Anything on the web calling itself "the CAD test online" is a lookalike at best. That includes this site: our tests use related ideas (brightness noise, thresholds), but they are screening tools, not the CAD test, and no online result — ours or anyone's — counts toward a licence.
Results are reported in CAD units, sometimes written as SN (standard normal) units. One CAD unit is the median threshold of people with normal color vision, so a score of 2 means you need twice the color signal a typical trichromat needs. Normal color vision generally falls below about 1.8 on the red-green axis; higher scores grade the deficiency from mild to severe, separately for the red-green and yellow-blue axes.
The number is what makes CAD useful for licensing. UK CAA guidance sets pass limits for pilot applicants in CAD units — commonly cited as 6 CAD units for deutan and 12 for protan applicants. The limits come from a simulated PAPI approach-light study at City, University of London: applicants inside them judged the safety-critical signal colors as reliably as normal trichromats, which is also why protans — who need a separate scale because red lights appear dimmer to them — get a numerically different limit. An applicant who fails Ishihara but scores inside those limits can still be certified.
The typical route is aviation: a pilot or air-traffic-control applicant misses plates on Ishihara at their initial medical, and CAD is the follow-up that decides the case properly. UK CAA and EASA both accept it, and versions of the protocol are used in other safety-critical fields, including rail and maritime research.
If that is your situation, the useful preparation is not practice — thresholds cannot be memorized — but information. Knowing before the appointment whether you are likely protan or deutan, and roughly how strong the deficiency is, tells you which side of the published limits you are likely to land on and removes most of the surprise.
A good online screening answers three questions the CAD appointment will also answer, minus the certified number: is a color vision deficiency likely present, is it protan or deutan (or tritan), and is it mild or strong. Our free test uses generated plates with brightness-noise masking — the same trick CAD uses to silence brightness cues — and an adaptive staircase to estimate your threshold per cone. On an uncalibrated screen the type and severity are estimates, and we label them that way.
That is genuinely useful before paying for a CAD appointment, and genuinely not a substitute for one. If a licence, a job medical, or any formal requirement is involved, the calibrated test at an approved centre is the only result that counts.
A CAD appointment will grade a deficiency precisely — but you can find out for free, in about two minutes, whether one is likely and which side it falls on. The color blind test estimates protan vs deutan and severity, and the Farnsworth D-15 shows your confusion axis.
Screening only — no online result, ours included, counts toward a licence or medical. See can you be a pilot if you are color blind for the certification routes.
Can you be a pilot if you are color blind?
FAA, CAA and EASA routes — where the CAD test fits in.
Jobs that require color vision
Which careers actually test color vision, and how strictly.
Rabin cone contrast test
The other threshold test on the FAA's list — scored per cone, out of 100.
Anomaloscope (Rayleigh match)
The other clinical reference test, demonstrated honestly in the browser.
Protan vs deutan
The two red-green families — and why aviation limits differ between them.
Last reviewed August 9, 2026. Informational only and regularly reviewed — not a substitute for advice from an eye-care professional.