The most common way to test for color blindness is with Ishihara color plates, a set of images with numbers hidden inside colored dots. If you can’t see certain numbers, it signals a red-green color deficiency. But Ishihara plates are just one of several tests, and they can’t detect every type. The right test depends on whether you’re screening at home, getting a clinical diagnosis, or meeting requirements for a job like aviation.
The Ishihara Color Plate Test
Developed in 1918 by Japanese ophthalmologist Ishihara Shinobu, this is the test most people picture when they think of color blindness screening. You look at a series of circular images made up of colored dots, each containing a number or pattern. People with normal color vision see one number, while people with red-green deficiency see a different number or nothing at all.
The Ishihara test is fast, reliable, and widely used in schools, military screening, and routine eye exams. Its main limitation is that it only detects red-green deficiencies, the most common type. It cannot diagnose blue-yellow color blindness or total color blindness. It also doesn’t measure how severe a deficiency is or distinguish precisely between the two subtypes of red-green deficiency (red-weak versus green-weak).
What Each Type of Color Blindness Looks Like
Color blindness isn’t one condition. The type you have determines which colors get confused, and different tests are better at catching different types.
People with red deficiency (protan type) tend to confuse black with shades of red, mix up dark brown with dark green or dark orange, and struggle to tell some blues from reds and purples. People with green deficiency (deutan type), the most common form overall, confuse mid-reds with mid-greens, bright greens with yellows, and blue-greens with grey. These two types account for the vast majority of color blindness and are what the Ishihara test is designed to catch.
Blue-yellow deficiency (tritan type) is much rarer. People with this type confuse light blues with greys, dark purples with black, and oranges with reds. Detecting it requires a different test entirely.
The HRR Test for Children
Young children who don’t know their numbers yet can’t take the Ishihara test. The Hardy-Rand-Rittler (HRR) test solves this by using geometric shapes instead: circles, triangles, and X marks hidden in colored dots across 24 plates. A child just needs to point at the shape they see.
The HRR test has another advantage: unlike Ishihara, it screens for blue-yellow deficiency in addition to red-green. The tradeoff is slightly lower sensitivity. A small number of people who pass the HRR will fail the Ishihara test, meaning the HRR can miss mild cases of red-green deficiency.
The Anomaloscope: The Gold Standard
If you need a precise diagnosis, not just a screening, the Nagel anomaloscope is considered the gold standard for red-green color deficiency. It works by asking you to match two colors on a split screen. On one side, you see a yellow light. On the other, you adjust a mix of red and green light until it looks like a match.
The amount of red you add to make the match reveals exactly what type of deficiency you have and how severe it is. People with green deficiency need less red than normal to match the yellow. People with red deficiency need more red. Someone who accepts every possible red-green mixture as a match has a complete (dichromatic) deficiency rather than a partial one. This level of precision is why the anomaloscope is the definitive diagnostic tool, though it’s mostly found in specialized clinics rather than a typical optometrist’s office.
Online Tests and Their Limits
Plenty of free color blindness tests exist online, and they can give you a rough idea of whether you have a deficiency. But they come with real limitations. Computer displays use only three primary colors to create every shade on screen, and the specific colors used in printed Ishihara plates can’t be perfectly reproduced by a monitor. Screen brightness, contrast settings, blue light filters, and ambient lighting all affect results.
If you take an online test, you can improve accuracy by setting your screen brightness to at least 75%, turning off any night mode or color filters, keeping the screen out of direct light, and setting your monitor’s color temperature to warm. Even with all that, a computer-based test won’t produce the same results as the original printed version. Online tests are useful as a first step, but they shouldn’t be treated as a diagnosis.
Why Lighting Matters for Any Test
Color vision testing is sensitive to lighting conditions in a way most people don’t realize. Clinical guidelines recommend testing under a light source with a color temperature close to 6,500 Kelvin (similar to natural daylight) and a color rendering index of 90 or above, meaning the light accurately displays a wide range of colors. Standard fluorescent or warm incandescent bulbs can shift how colors appear on test plates, potentially changing your results. This is one reason a formal test at an eye care provider’s office is more reliable than holding a book of plates under your kitchen light.
Color Vision Tests for Pilots and Other Jobs
Certain careers require you to pass a formal color vision test. Aviation is the most well-known example, and the standards are strict. As of January 2025, the FAA requires pilots to take an approved computer-based color vision test administered in person. Downloaded, printed, or virtual versions are not accepted.
The FAA currently approves three computerized tests:
- Colour Assessment and Diagnosis (CAD) test: Measures red-green and yellow-blue sensitivity separately. If you pass the fast screening version, no further testing is needed. If you fail, a full version determines whether you fall within acceptable thresholds, which vary depending on whether you’re diagnosed as having red or green deficiency.
- Rabin Cone Contrast Test (RCCT): Tests each eye separately and scores your ability to detect red, green, and blue. You need a score of 55 or higher for each color.
- Waggoner Computerized Color Vision Test: Starts with general red-green screening, then automatically moves to more specific sections if you don’t pass. It also includes a blue-yellow component.
These tests go well beyond simple screening. They quantify the degree of deficiency and determine whether it falls within safe limits for operating an aircraft. Similar requirements exist in military service, maritime work, electrical trades, and rail transportation, though each industry has its own approved tests and passing thresholds.
Which Test Should You Take
If you’re just curious whether you have color blindness, an online screening test is a reasonable starting point. For a reliable answer, an Ishihara test at an eye doctor’s office takes only a few minutes and catches the most common deficiencies. If you need to know the exact type and severity, or if you’ve failed a screening and want a definitive answer, ask about anomaloscope testing or a detailed computerized test. And if you’re pursuing a career that requires color vision certification, check your industry’s specific requirements, since only certain approved tests will count.