Color blindness is most commonly tested using a book of dotted plates where you identify numbers hidden in colored circles. This quick screening takes under five minutes and catches the vast majority of color vision problems. But depending on why you’re being tested, your eye care provider may use more detailed methods to measure exactly how your color vision differs from normal and how severe the deficiency is.
About 8% of men and 0.5% of women have some form of color vision deficiency, with the most common type being difficulty distinguishing reds and greens. Several different tests exist, each designed for a slightly different purpose.
The Ishihara Plate Test
This is the test most people picture when they think of color blindness screening. You look at a series of circular plates covered in dots of varying sizes and colors. Within each plate, a number or path is formed by dots in a contrasting color. People with normal color vision see the number clearly, while people with a color deficiency either see a different number or no number at all.
The standard Ishihara book contains 14 plates. If you read 10 or more correctly, your color vision is considered normal. Seven or fewer correct readings indicates a deficiency. For a basic screening, only the first 11 plates are needed. The remaining plates help classify what type of deficiency you have.
The Ishihara test is designed specifically to detect red-green color deficiency, which accounts for the overwhelming majority of cases. It does not diagnose blue-yellow deficiency, a much rarer condition affecting roughly 0.008% of the population regardless of sex. Lighting matters during the test. Clinical settings use standardized daylight-simulating lamps with room lights turned off, because fluorescent or incandescent bulbs can shift how the colors appear and throw off results.
Tests for Children
Young children who can’t read numbers need a different approach. The Hardy-Rand-Rittler (HRR) test uses the same dotted-plate concept but replaces numbers with simple geometric shapes like circles, triangles, and crosses. Children point to or trace the shape they see. This makes screening possible as early as age four or five, which matters because undiagnosed color deficiency can affect how a child learns in school, particularly with color-coded materials.
The HRR test has an additional advantage: unlike the Ishihara plates, it can detect blue-yellow deficiency as well as red-green problems.
The Farnsworth-Munsell 100 Hue Test
When a screening test flags a possible deficiency, a more detailed assessment may follow. The Farnsworth-Munsell 100 Hue Test measures how precisely you can distinguish between similar shades of color across the entire spectrum.
You’re given four trays, each containing small colored caps (85 total movable caps plus fixed anchor caps at each end). Your job is to arrange the caps so they form a smooth, gradual color transition between the two anchor colors at the ends of the tray. A person with normal color vision will line the caps up in nearly perfect order. Someone with a deficiency will misplace caps in predictable zones that correspond to their specific type of color blindness.
Each misplaced cap generates an error score based on how far it is from its correct neighbors. These are added together into a total error score, where higher numbers mean more difficulty distinguishing colors. The pattern of errors, not just the total, reveals which colors give you the most trouble. This test takes considerably longer than plate screening, often 15 to 20 minutes, and is used when precision matters, such as in occupational assessments or monitoring changes in color vision over time.
The Anomaloscope
The gold standard for classifying color vision deficiency is the Nagel anomaloscope, a specialized optical instrument used primarily in research and clinical settings. You look through an eyepiece at a split circle. One half shows a fixed yellow light. The other half shows a mixture of red and green light that you control with a dial. Your task is to adjust the red-green mixture until both halves of the circle look identical.
A person with normal color vision needs a very specific ratio of red and green to match the yellow. Someone with a red-green deficiency will set the dial to a completely different position, and the exact position reveals the type and severity. People with a complete red-green deficiency (dichromats) will accept a very wide range of red-green mixtures as matching the yellow, because they simply can’t tell the difference. People with a partial deficiency (anomalous trichromats) accept a narrower but still abnormal range, and they’ll typically report that the mixture looks reddish or greenish even at settings that appear perfectly yellow to a normal observer.
The anomaloscope also distinguishes between the two main subtypes. A person missing red sensitivity adjusts the brightness control dramatically as the mixture changes, turning it way up near the green end and down near the red end. A person missing green sensitivity keeps the brightness roughly steady regardless of the mixture. This level of detail is rarely needed for a routine eye exam but becomes important for occupational certification or genetic research.
The CAD Test
The Colour Assessment and Diagnosis (CAD) test is a newer computer-based approach used increasingly in aviation and other safety-critical fields. You watch a screen filled with flickering gray pixels while a small colored square target moves across it. The flickering background eliminates brightness cues, so you can only detect the target if your color vision picks up the actual hue. The test gradually reduces the intensity of the color until you can no longer spot it, measuring your precise threshold for detecting color signals.
The CAD test can both diagnose the type of deficiency and quantify how severe it is on a numerical scale. It’s also highly repeatable, since the computer controls every variable and averages results from multiple presentations. Aviation regulators in several countries have adopted it because it provides a more standardized measurement than older tests that depend on the examiner’s setup and judgment.
Lantern Tests for Pilots and Mariners
Certain jobs require you to correctly identify colored signal lights rather than pass a theoretical color test. Lantern tests simulate this by showing you pairs of small colored lights (red, green, or white) at a distance and asking you to name the colors. The Farnsworth Lantern, developed by the U.S. Navy, is the most widely recognized version and is used by both the Navy and Air Force for aviation screening. It presents three series of paired lights, and you fail if you average more than one error across the second and third series.
The FAA accepts the Farnsworth Lantern as one of several approved methods for pilot medical certification. Other lantern tests have fallen out of use. The Edridge-Green Lantern was dropped from the FAA’s accepted list, and the agency could not even confirm that working examples still exist among aviation medical examiners.
Online Screening Tests
Digital versions of the Ishihara plates are widely available online, and a study comparing an online Ishihara test to the traditional printed book found 100% sensitivity and 100% specificity, meaning the online version caught every case the physical book did, with no false positives. That said, these results depend on your screen being reasonably well-calibrated. A monitor with unusual brightness, contrast, or color settings could distort the plates enough to produce inaccurate results.
Online tests are a reasonable first step if you’re curious about your color vision. They can reliably flag a deficiency, but they can’t replace a full clinical workup if you need a precise diagnosis or classification for occupational purposes. For that, you’ll need standardized lighting, calibrated test materials, and in some cases, specialized instruments like an anomaloscope or CAD test.
What Each Test Tells You
- Ishihara plates: Quickly screens for red-green deficiency. Yes or no, with basic type classification.
- HRR plates: Screens for both red-green and blue-yellow deficiency. Works for young children.
- Farnsworth-Munsell 100 Hue: Measures the severity of your deficiency across the full color spectrum.
- Anomaloscope: Precisely classifies the type and degree of red-green deficiency. The definitive diagnostic tool.
- CAD test: Quantifies your color detection threshold on a numerical scale. Used for occupational certification.
- Lantern tests: Determines whether you can correctly identify colored signal lights in real-world conditions.
Most people will only ever encounter the Ishihara plates, typically during a routine eye exam or a school vision screening. If the results suggest a deficiency, your eye care provider may follow up with one of the more detailed tests depending on your age, your occupation, and whether a precise diagnosis would change anything about how the deficiency is managed.