How the test works

The main test follows the Cambridge Colour Test, used in clinical research. You will see a “C” made of dots on a background of dots. The brightness of the dots varies at random, so colour is the only clue to finding the C.

For each of the three cone types, the colour of the C moves along its confusion line: the colour direction that type of colour blindness cannot tell apart. A staircase procedure lowers the difference when you answer correctly and raises it when you miss, until it finds your threshold. Compared with each other, the three thresholds tell the type; how high they are tells the strength.

The diagram is a magnified part of the CIE u′v′ colour space around the screen’s white. The three lines are the confusion lines: along P, protans confuse reds and greens; along D, deutans do; along T, tritans confuse blues and yellows. Dashed outline: the normal limit. Hatched area: an example of deuteranomaly, stretched along line D.

A second, optional flicker test measures how bright red looks to you compared with green. It is the most reliable way to separate protan from deutan.

Duration: 12–18 minutes in total
Trials: about 90–130 answers
Unit: ×10⁻⁴ u′v′ (distance in the CIE 1976 diagram)

Why the conditions matter

Turn off Night Shift, True Tone, blue-light filters, reading mode or “warm colours”. They distort the tritan axis in particular.
Screen brightness high and fixed (turn off auto-brightness).
Browser zoom at 100% and a window large enough to see the whole circle.
Dim room, no direct light on the screen. Stay in the dim light for a couple of minutes before starting.
Sit facing the screen at about 60–70 cm. On laptops the viewing angle changes colours: look at it straight on.
If you wear prescription glasses, keep them on. Remove tinted or “colour-blind” lenses (EnChroma and similar).

Calibrate your screen’s gamma

Every screen turns colours into light a little differently. This step measures yours, so the test shows exactly the intended colours. It takes about a minute.

Each colour has two squares side by side. The left one is made of very fine alternating coloured and black lines: it is the reference, and up close it may just look a little darker or striped. The right one is solid and you adjust it. Step back from the screen or squint, then move the slider until the two squares look like a single rectangle of the same brightness.

The thresholds and what they mean

A threshold up to 100 on the protan and deutan axes, or up to 150 on the tritan axis, is normal. Up to 150 and 230 it is borderline; above that it is raised. Near 1100 (1000 for tritan) that type of cone no longer helps tell colours apart: that is dichromacy.

Values in ×10⁻⁴ u′v′ units: the smallest colour difference you recognised 70% of the time. “≥” means you reached the maximum the screen can show.

Minimum-flicker photometry

A disc alternates between red and green 15 times a second. When the two colours look equally bright, the flicker almost disappears. People with a protan deficiency see red as darker and must turn it up a lot; people with a deutan deficiency turn it down a little. You will make 4 adjustments, each starting from a different point.

Simulator

The filter uses the physiological model by Machado, Oliveira and Fernandes (2009), applied in linear RGB. A person with normal vision looking at the filtered part sees roughly what you see.

Limits

A screen is not calibrated like an instrument: colours, brightness and viewing angle change from one device to another, and the calibration only corrects gamma. Certification (driving licence, work, exams) requires an eye doctor’s anomaloscope. A deficiency that appears in adulthood should always be checked.

Congenital tritan deficiencies are rare (about 1 in 10,000). Before drawing conclusions, check that Night Shift and blue-light filters are off and repeat the test. An acquired tritan deficiency can have medical causes and is worth a check-up.

Sources

Regan B. C., Reffin J. P., Mollon J. D. (1994). Luminance noise and the rapid determination of discrimination ellipses in colour deficiency. Vision Research, 34(10).
Smith V. C., Pokorny J. (1975). Spectral sensitivity of the foveal cone photopigments between 400 and 500 nm. Vision Research, 15(2).
Machado G. M., Oliveira M. M., Fernandes L. A. F. (2009). A physiologically-based model for simulation of color vision deficiency. IEEE Transactions on Visualization and Computer Graphics, 15(6).
CIE (1976). Spazio colore uniforme CIE 1976 UCS (u′, v′).

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