How the Color Blind Test Works

How the plates are built

Every plate is original artwork. For each figure, the dot colours are chosen so that the number and the background differ only along a confusion line: a direction in colour space that one type of colour vision cannot detect. The confusion lines come from a published model of colour vision deficiency (Machado, Oliveira and Fernandes, 2009).

Dot brightness is varied at random across both the number and the background, so the number can't be read from lightness alone. Two control plates use a lightness difference that every type of colour vision can see.

How your type and severity are calculated

For each plate we know, from simulation, how severe a protan, deutan or tritan deficiency has to be before the number fades below a just-noticeable difference. Faint plates disappear with a mild deficiency; strong plates only disappear when a cone type is missing.

When you finish, the calculator tries every type at every severity from 0% to 100% and counts how many of your answers each one would explain. The best fit gives your type and severity. If a normal-vision result explains your answers almost as well, you are shown a normal or borderline result instead, so a single slip doesn't label you colour blind.

What the percentage means

The percentage is a position on the simulation scale. 100% corresponds to dichromacy: one cone type behaves as if absent. Around 60–75% is a moderate anomalous trichromacy; lower values mean a milder shift. It is not the share of colours you can't see.

Limits of an online test

• Screens differ in colour, brightness and settings such as night mode or True Tone, which can make plates easier or harder.
• Mild deficiencies, below roughly 45% on our scale, may not be detected.
• The test does not diagnose acquired colour vision loss or its cause.
• Jobs and licences that require normal colour vision use in-person tests such as the Ishihara plates, the Farnsworth D-15 or an anomaloscope.

How this compares with the Ishihara test

The Ishihara test, first published in 1917, is a printed set of plates (38, 24 or 14 in different editions) viewed under standard daylight. It screens for red-green deficiency. Our test follows the same principle but uses its own plates, adds blue-yellow plates, and works out a type and severity automatically.