Ishihara vs D-15 vs Cambridge: which color blindness test tells you what

Why there are so many colour vision tests

A colour vision test has to answer one of several different questions. Is colour vision normal or not? If not, which type of defect is it? How strong is it? Is it good enough for a specific job? Each classic test was designed for one or two of these questions, and that is why eye clinics, schools, the military and transport regulators use different ones.

Ishihara plates: the fast red-green screen

The Ishihara test, created by the Japanese ophthalmologist Shinobu Ishihara, is the best-known colour vision test in the world. Each plate is a circle of dots of different sizes. Some of the dots form a number, and the colours are chosen along red-green confusion lines, so a typical eye reads the number while a red-green deficient eye sees a different number or none at all.

• Good at: quickly screening for red-green (protan and deutan) defects.
• Not designed for: blue-yellow (tritan) defects or grading severity.
• Practical note: it needs the person to read digits, which rules out small children.

Plates of this kind are called pseudo-isochromatic: the figure and the background look the same colour to a deficient eye but not to a typical one. Many national tables follow the same principle, for example the Rabkin polychromatic tables widely used in Russia.

HRR plates: symbols, type and degree

The Hardy-Rand-Rittler (HRR) test also uses dotted plates, but the figures are simple symbols (a circle, a cross and a triangle), and the plates come at several colour strengths. Because some plates sit on the blue-yellow axis, HRR can screen for tritan defects too. Counting which strengths you can still see gives a grade: mild, medium or strong.

Arrangement tests: D-15, D-15d and the 100 Hue

Arrangement tests ask you to put coloured caps in order so that each cap is closest in colour to the previous one.

• Farnsworth D-15 has 15 caps around the hue circle. People with typical vision and mild defects order them correctly. Moderate and strong defects make you jump across the circle, and the direction of those crossings lines up with the protan, deutan or tritan axis. Its strength is telling the type of a significant defect.
• Lanthony desaturated D-15 (D-15d) uses pale versions of the colours. It is harder, so it catches milder defects that pass the standard D-15.
• Farnsworth-Munsell 100 Hue uses 85 caps in four trays with very small steps between them. The result is a total error score (TES) that measures fine colour discrimination, and the error pattern can show an axis. It takes longer and is used in research, in occupations that need very fine colour judgement and to follow acquired changes.

The Cambridge Colour Test: computerised thresholds

The Cambridge Colour Test, described by Regan, Reffin and Mollon in 1994, runs on a calibrated monitor. You look for the gap in a Landolt C (a ring with an opening) made of dots that vary randomly in size and brightness, so the ring can only be seen by its colour. An adaptive staircase moves the colour of the C closer to the background after each correct answer and further after each mistake, separately along the protan, deutan and tritan confusion lines.

The result is a threshold for each axis, reported in CIE u′v′ chromaticity units. That gives a number for how strong a defect is along each line, including tritan, and it is sensitive to mild and acquired changes. The catch is equipment: the clinical version needs a precisely calibrated display.

City University test and lanterns

The City University test shows a central colour surrounded by four others; you pick the one that looks closest. The distractors sit on protan, deutan and tritan lines, so wrong picks point to the type. Lantern tests such as the Farnsworth lantern show pairs of small red, green and white lights, like signal lights at a distance. They do not classify the defect; they answer a practical question used in some aviation, maritime and rail screening: can this person name signal colours reliably?

Comparison at a glance

Test	Finds	Type	Degree	Tritan
Ishihara-type plates	Red-green screening	Limited	No	No
HRR plates	Screening and grading	Yes	Yes	Yes
Farnsworth D-15	Moderate and strong defects	Yes	Pass/fail	Yes
Lanthony D-15d	Milder defects	Yes	Pass/fail	Yes
100 Hue	Fine discrimination	Pattern	Score (TES)	Yes
Cambridge Colour Test	Thresholds on three axes	Yes	Yes	Yes
City University	Type of defect	Yes	Rough	Yes
Lantern	Signal colour naming	No	Practical	No

In a clinic, the anomaloscope remains the reference for classifying red-green defects, and the final answer usually comes from a combination of tests rather than one.

Which test should you take?

• Just curious, or a quick check: an Ishihara-type plate test.
• Want to know the type and how strong it is: HRR-type plates plus an arrangement test.
• Suspect blue-yellow problems, or your colour vision has changed: see an eye doctor; a tritan-sensitive test such as HRR, D-15d or a Cambridge-style threshold test can help, but a change needs a medical check.
• Need a result for a job or licence: use the exact test your regulator requires, done by an approved examiner. Home results do not count.

All of them on one phone

Trichrome brings screening versions of all eight methods above into one iPhone app: Ishihara-type number plates (free), HRR-type symbol plates, shapes for kids, D-15, D-15d, 100 Hue, a Cambridge-style gap ring with adaptive staircases in u′v′, a City University-style closest colour test and signal lights. The stimuli are generated by the app along colour confusion lines rather than copied from printed books. A phone screen is not a calibrated instrument, so results are screening only and should be confirmed by an eye-care professional.