Digit Plate Colour Screen

What a digit plate actually measures

A pseudoisochromatic plate is a field of coloured dots that vary randomly in size and brightness. Hidden inside the field is a figure — here, a digit — drawn in dots of a different hue but a deliberately similar lightness. The randomness is the whole trick. Because the dots differ in size and brightness in a way that carries no information, the figure cannot be found by edge detection or by brightness alone. The only reliable cue is the hue difference between figure and background.

That is the principle that makes a plate useful for colour-discrimination screening. Someone whose colour vision separates those two hues sees a number immediately. Someone whose red and green cone responses overlap more than usual sees an undifferentiated field of dots, because for them the figure and the background genuinely are close to the same colour. “Pseudoisochromatic” means exactly that: falsely equal-coloured, for some viewers and not others.

The method dates to Shinobu Ishihara, who published the first plate series in 1917. It became the standard first-line screen for red-green deficiency because it is fast, requires no equipment beyond the plates themselves, and needs almost no instruction. More than a century later it is still the test most people encounter first, in school medicals, pre-employment checks and routine eye examinations.

The activity on this page is an original digit-plate implementation built on the same pseudoisochromatic principle. It is not a reproduction of the published clinical plate series, and its results are not interchangeable with them.

Reading your result

The screen reports how many original plates you identified as expected. It does not determine a protan or deutan subtype, measure severity, or reproduce the scoring of a clinical Ishihara edition. Treat the bands below as guidance for this session only.

Most or all plates identified. Your responses matched the encoded figures on this device. That is reassuring, but it does not rule out a colour-vision difference and says nothing about blue-yellow discrimination.

A small number of differences. Recheck brightness, colour filters, glare, and whether each instruction was clear, then repeat once under better conditions. This activity cannot tell whether the differences came from colour perception or the testing setup.

Many differences. Several responses did not match the figures encoded in the plates. Display settings can still contribute, so this is not a diagnosis. If the pattern repeats or matters for work, study, or safety, an eye-care professional can use controlled tests.

Where a browser version falls short

Clinical plate books are printed to controlled tolerances and viewed under a standardised daylight-equivalent illuminant. None of that holds on a laptop in a living room. Four things routinely distort a browser result:

• Adaptive display features. Night Shift, True Tone, f.lux and blue-light filters all shift the screen’s white point, which is precisely the variable the plates depend on.
• Panel and calibration differences. Two uncalibrated monitors can render the same dot colours far enough apart to move a borderline result across a band boundary.
• Ambient light. Coloured or dim room lighting changes the eye’s adaptation state.
• Visual acuity. Uncorrected refractive error or media opacity can blur the dot boundaries enough to hide a figure from someone with entirely normal colour vision.

There is also a limit that no amount of calibration fixes: a browser activity cannot establish or grade a red-green deficiency in a way that transfers between tests. Clinical classification may use a validated plate series and, when appropriate, an anomaloscope — an instrument that asks you to match a spectral yellow by mixing red and green, and reads severity directly from the mixture you accept.

Who should take this, and what to do next

An educational screen can be useful if colour vision deficiency runs in your family, if a child is approaching an age where classroom colour coding matters, or if you are considering work in aviation, rail, marine, electrical or certain medical fields where colour standards apply. It can be a private first look, but it should not be the basis for a medical, occupational, or licensing decision.

Screening is not the right tool if your colour vision has changed recently. Inherited deficiency is present from birth and stable for life. A genuine change in adulthood points elsewhere — to the optic nerve, the retina, cataract, or a medication effect — and warrants prompt assessment by an optometrist or ophthalmologist rather than a browser test. The same applies to colour changes in one eye only, which inherited deficiency never causes.

If you want to understand your result in more depth, what colour blindness is covers the biology, types of colour blindness explains how protan, deutan and tritan differ, and the Ishihara test explained goes further into the plate method itself. Our test methodology sets out how these activities were built and what they can and cannot support.