Vision explained · 6 min read
The Ishihara test explained: what the plates show, and why online copies mislead
The Ishihara test is a book of dot plates that screens for red-green colour vision. What the plates show, how they are read, and why online copies mislead.
In short
The Ishihara test is a printed book of plates, each a circle of coloured dots with a number or a winding line hidden in it, first published in 1917 by the Japanese ophthalmologist Shinobu Ishihara. The colours are chosen so that the figure stands out for typical colour vision and blends into the background for many people with a red-green colour vision difference. It is a screening test for red-green differences, read in good light against the book's own key, and an eye-care professional uses it as a first step. Copies on a screen look like the same thing and are not: a screen changes the colours the plates depend on, which is why an online pass proves less than it seems.
Try a screen check: the colour vision check · plates made for this site, control plates for your screen, nothing saved.
What the plates are
Each plate is a circle filled with dots of different sizes. Some of the dots form a figure, usually a number, and the rest form the background. The dots vary in brightness as well as colour, so the figure cannot be picked out by brightness alone; only the difference in colour separates it from the background.
The figure and background colours are chosen in pairs that look clearly different to most people but alike to people whose colour vision works differently. A person with typical colour vision reads the number at a glance. A person with a red-green colour vision difference may see no number at all, or a different one.
The National Eye Institute describes the colour plate test in the same terms: a circle of coloured dots with a shape inside it that people with a colour vision deficiency have a hard time seeing. Different plates check for different kinds of colour vision deficiency.
How an eye-care professional uses it
The book is shown in good, daylight-like light, at reading distance, a few seconds per plate. You say what you see, or that you see nothing. The answers are compared with the key printed in the book, which says how many misses point to a red-green difference.
It is a screening step. If the result is not clear, the eye-care professional can use other tests, such as an anomaloscope, where you match two lights, or a hue test, where you arrange coloured blocks in order. Those tests can say more about the kind of difference than a plate book can.
The Ishihara book is for red-green differences: Birch (1997) calls it the most widely used screening test for red-green colour deficiency. It has no plates for blue-yellow differences, so those need other plates or tests.
Why copies on a screen mislead
The plates were made for printed inks seen in a particular kind of light. A photograph or scan of a page, shown on a phone or a monitor, is a different set of colours: the camera, the file and the screen each change them, and so do the screen's brightness, filters such as Night Shift, True Tone and blue-light modes, and the light in the room.
That matters in one direction more than the other. A shift in colour can make a hidden number easier to see, so a person with a red-green difference can pass a copied plate on a screen. A screen pass is the result to trust least.
This site shows none of the Ishihara plates, not even as an example.
The kinds of colour vision difference
Red-green differences are the most common kind. The National Eye Institute lists four: deuteranomaly, the most common, makes some greens look redder; protanomaly makes some reds look greener and less bright; and in protanopia and deuteranopia red and green cannot be told apart at all. Most of them are mild and do not get in the way of everyday life.
Blue-yellow differences are less common and make several colour combinations hard to tell apart, such as blue and green or yellow and pink. A complete absence of colour vision is rare.
Most colour vision differences are present from birth, and they are far more common in men. Some appear later, from an eye condition, an injury, a medicine or age, and a change in the colours you see is worth having checked.
The check on this site, and when to see someone
The colour vision check on this site is not the Ishihara test and uses none of its plates. Its plates were made for this site from published colour science, it adds blue-yellow plates and three control plates that everyone should read, and if two of the control plates do not come through it says your screen cannot show the check rather than naming a pattern. Its result is a pattern, never a score or a type.
It is still a screen check with a screen's limits, and one limit matters more than the rest: most colour vision differences are the milder, anomalous kinds, and a fixed set of screen plates catches only part of them. In a simulation of those milder kinds, our plates mostly stay readable. A pass therefore says least about the people most likely to have a difference.
If your work, a licence or a course depends on colour vision, if a plate felt hard, or if colours have changed for you, ask an eye-care professional for a plate test under proper light.
Questions people ask
Can I take the Ishihara test online?
You can look at copies of the plates online, but a screen changes the colours they depend on, so the result is not the same as the test. An online pass is the least reliable result, because a colour shift can make a hidden number easier to see.
Does the Ishihara test check for blue-yellow color blindness?
No. The Ishihara book has plates for red-green colour vision differences only. Blue-yellow differences need other plates or other tests.
What happens if I fail an online color blind test?
Nothing is decided by it. Turn off screen filters, raise the brightness and try again in ordinary light, and if the pattern stays, ask an eye-care professional for a plate test under proper light.
Is color blindness more common in men?
Yes. The National Eye Institute says about one in twelve men have a colour vision deficiency, and that men have a much higher risk than women.
Sources
- Wikipedia's article on the Ishihara test says Shinobu Ishihara first published it in 1917 and that it detects red–green colour deficiencies.
- Birch (1997), in Ophthalmic and Physiological Optics 17(5):403–408, describes the Ishihara test as the most widely used screening test for red-green colour deficiency.
- The National Eye Institute describes the color plate test as the most common color vision test: a circle of colored dots with a shape inside it, such as a number, that people with color vision deficiency have a hard time seeing.
- The National Eye Institute says that if a color plate test is not clear, an eye doctor may use other tests, such as an anomaloscope or a hue test that asks you to arrange colored blocks in order.
- The National Eye Institute says red-green color vision deficiency is the most common type, and that a less common blue-yellow type makes several color combinations hard to tell apart.
- The National Eye Institute says about 1 in 12 men have color vision deficiency, and that men have a much higher risk than women.
- The National Eye Institute advises talking with your eye doctor about getting checked if you think you may have color vision deficiency.
- The National Eye Institute says color vision deficiency can also come from an injury to the retina, the optic nerve or the brain, from certain medicines and health problems, and that color vision may get worse with age, often because of cataracts.
Text on this page is original to MyTestAtlas. It explains published standards and definitions; it is not psychological, medical or admissions advice.