The “what number do you see” eye test is called the Ishihara color test, and it uses circles filled with colored dots that hide a number inside. People with normal color vision see one number, people with color blindness see a different number or no number at all. The test has been the standard screening for color vision deficiency since 1918, when Japanese ophthalmologist Shinobu Ishihara developed it for military use.
How the Test Works
Each plate in the Ishihara test is a circle packed with dots of varying sizes. Hidden within those dots is a number made up of dots in a contrasting color. The trick is that the contrast only works if your color-sensing cells are functioning normally. If you have a color vision deficiency, the number blends into the background, becomes invisible, or transforms into a completely different number.
The full clinical version has 38 plates, but the concise edition used in most screenings has 14 red-green test plates plus a demonstration plate. You’re shown each plate at a distance of about 20 to 40 inches under natural daylight, and you get roughly five seconds to identify the number. A passing score is 12 out of 14 plates correct, a threshold that catches color deficiency with 97% sensitivity and 100% specificity when using printed plates under proper lighting.
What Each Plate Reveals
The test is cleverly designed with several types of plates that do different jobs. Some plates are visible to everyone, some disappear for color-deficient viewers, and some actually flip into a different number depending on your type of color vision.
Plate 1 shows the number 12, and virtually everyone can read it regardless of their color vision. It’s the demonstration plate, used to confirm you understand the task. After that, the real screening begins:
- Plates that change numbers: A person with normal vision sees 8 on Plate 2, but someone with red-green deficiency sees 3. Normal vision reads 29 on Plate 3, while color-deficient vision reads 70. Plate 4 shows 5 to normal eyes but 2 to deficient ones. Plate 5 flips from 3 to 5, Plate 6 from 15 to 17, and Plate 7 from 74 to 21.
- Plates that vanish: Plates 8 through 13 show numbers like 6, 45, 5, 7, 16, and 73 to people with normal color vision. People with red-green deficiency see nothing at all.
- Hidden digit plates: Plates 14 and 15 work in reverse. People with normal vision see no number, but those with red-green deficiency can see 5 and 45. These plates exploit the fact that color-deficient individuals rely more heavily on a different set of color receptors (the ones sensitive to blue light) to distinguish the dots, making patterns visible that normal-sighted people miss entirely.
Why You See the Wrong Number
Color vision depends on three types of cone cells in your retina, each tuned to a different range of light wavelengths: red, green, or blue. When one type is missing or underperforming, your brain can’t separate certain colors from each other. The two most common forms of color blindness both affect the red-green spectrum. Protanopia means the red-sensitive cones aren’t working properly, making it hard to distinguish blue-green from red-green. Deuteranopia involves the green-sensitive cones and causes difficulty separating red-purple from green-purple.
The Ishihara plates are designed so that the number and the background sit on colors that look identical to someone missing red or green cone function. It’s like trying to read a message written in a color you literally cannot perceive. The “transformation” plates go a step further: they embed two different numbers in the same image using color contrasts that are visible to different types of vision, so the number you read reveals which cones are working.
What the Test Does Not Catch
The Ishihara test screens specifically for red-green color deficiency, which accounts for the vast majority of color blindness cases. It does not detect blue-yellow deficiency, a rarer condition called tritanopia. If you pass the Ishihara test but still feel like you struggle with certain colors, a different test may be needed.
For more detailed assessment, eye care professionals sometimes use the Farnsworth-Munsell 100-Hue test, which asks you to arrange colored caps in order. This test measures the severity of your deficiency and can help with vocational guidance for careers where accurate color perception matters, like electrical work, piloting, or graphic design. However, it’s not a perfect substitute for the Ishihara screening. About 50% of people with mild color anomalies score within the normal range on the Farnsworth-Munsell test, so both types of tests are often used together.
Taking the Test Online
Plenty of websites and apps offer versions of the Ishihara test, and many are reasonably accurate. Research comparing screen-based Ishihara tests to printed plates found that smartphone displays achieved 96% sensitivity and 94.7% specificity, while computer monitors came in at 94.4% sensitivity and 82.4% specificity. The gap mostly comes down to screen calibration.
Your results can be thrown off by several factors. Blue light filters or “night mode” settings shift the color balance of your screen and can make normal vision look deficient. Screen brightness matters too: a dim or washed-out display changes the contrast between the dots. The original test was designed for viewing under diffused natural daylight on printed plates, so any screen introduces some variability. If you get a concerning result online, it’s worth confirming with a printed test in a clinical setting before drawing conclusions.
What Your Results Mean
If you read 12 or more of the 14 screening plates correctly, your red-green color vision is normal. Missing three or more plates suggests a color vision deficiency. The specific numbers you see on the transformation plates can help distinguish between protanopia and deuteranopia, though a full clinical workup is usually needed to pin down the exact type and severity.
Color blindness affects roughly 8% of men and 0.5% of women of Northern European descent, with lower rates in other populations. It’s overwhelmingly genetic and present from birth, though it can occasionally develop later due to eye disease, certain medications, or aging. There is no cure, but most people with red-green deficiency adapt well. Specialty tinted lenses can enhance color contrast for some individuals, and awareness of your specific deficiency helps you develop practical workarounds for tasks like reading color-coded charts or choosing clothing.