The Farnsworth D-15 test asks you to arrange 15 colored caps in order, and passing means placing them in a sequence with fewer than two major crossover errors on the scoring diagram. If you have normal or near-normal color vision, you’ll pass without difficulty. If you have a color vision deficiency, the test is specifically designed to detect that, and there is no reliable trick or shortcut to beat it.
That said, understanding how the test works, what counts as a pass, and what factors influence your performance can help you do your best on test day.
How the Test Works
You sit at a table with 15 removable colored caps and one fixed reference cap (called the pilot cap). The examiner places all 15 caps color-side up and mixes them randomly. Your job is to pick the cap closest in color to the reference cap and place it next to it, then pick the cap closest to that one, and continue building a sequence until all 15 are arranged in order.
The caps form a continuous loop of hues around the color spectrum. A person with normal color vision sees smooth, obvious transitions from one cap to the next. Someone with a color vision deficiency will confuse certain caps that look identical or nearly identical to them, placing them out of order.
Once you finish, the examiner flips the tray over. Numbers on the back of each cap reveal the order you chose, which gets plotted on a circular scoring diagram.
What Counts as a Pass
On the scoring diagram, your arrangement is drawn as lines connecting each cap to the next one you placed. A perfect arrangement creates a smooth circle. Errors show up as lines that cut across the center of the circle, called crossovers.
A pass requires fewer than two crossover lines that cut across the center and run parallel to one of the three color-deficiency axes (protan, deutan, or tritan). One minor crossover is generally tolerated. Two or more crossovers running parallel to the same axis is a failure, indicating a specific type of color deficiency. People with dichromatic vision (the more severe form of color blindness) typically produce six to twelve crossovers, forming an unmistakable pattern on the scoring sheet.
The Time Limit Matters
Research has shown that people with color vision deficiencies can sometimes improve their scores by taking a very long time, carefully comparing caps under different angles and lighting. To counter this, a growing number of examiners enforce a two-minute time limit. A 2024 study found that a two-minute cap produced results that matched each subject’s true color vision status as measured by more precise instruments. At ten minutes, some practiced subjects with genuine deficiencies managed to arrange caps correctly through trial and error.
If your examiner enforces the two-minute limit, you won’t have time to second-guess. Work at a steady pace, trust your first impression of each color match, and move on.
Lighting Can Help or Hurt You
The D-15 test is highly sensitive to the lighting in the room. The standard calls for daylight-equivalent lighting at a color temperature around 6,500 Kelvin with high color rendering (a color accuracy index above 90). Illumination should fall between 200 and 1,000 lux, with an optimal level around 270 lux for detecting deficiencies.
What this means practically: warm indoor lighting, dim rooms, or fluorescent bulbs with poor color rendering can make the caps harder to distinguish, even for people with normal vision. If the testing room seems unusually dark or has yellowish lighting, it’s reasonable to mention this to the examiner. Proper lighting is part of the test protocol, and poor lighting can produce inaccurate results in either direction.
Color-Correcting Lenses Don’t Work
If you’ve seen ads for color-correcting glasses or contact lenses (like the X-Chrom lens), you might wonder whether wearing them during the test would help. The short answer: they don’t meaningfully improve D-15 scores, and they’re often explicitly banned.
An FAA study tested color-deficient subjects with and without X-Chrom lenses. The number of crossover errors barely changed. Subjects with deutan-type deficiency averaged 5.3 crossovers without the lens and 3.7 with it, still well above the passing threshold. The D-15 uses colors from all parts of the spectrum simultaneously, so a tinted lens that shifts your perception of some hues just creates new confusions with others.
The FAA specifically states that color vision correcting lenses are not acceptable as a means for correcting a pilot’s color vision deficiencies. Other agencies and employers with color vision requirements typically follow the same policy.
Practical Tips for Test Day
While you can’t change your underlying color vision, you can avoid factors that make your performance worse than it should be.
- Don’t touch the colored surfaces. Handle caps by their edges. Oils and fingerprints degrade the pigments over time, and smudged caps are harder to distinguish. A well-maintained test set works in your favor.
- Sit at a comfortable viewing distance. The standard distance is about 50 centimeters (roughly arm’s length). Leaning too close or too far away changes how you perceive the colors.
- Avoid eye fatigue beforehand. Prolonged screen time, bright sunlight, or wearing tinted sunglasses shortly before the test can temporarily affect color perception. Give your eyes time to adapt to normal indoor lighting.
- Work from first impressions. Place each cap based on your initial sense of which color is closest. Overthinking and rearranging wastes time and doesn’t improve accuracy for people with normal vision.
- Look for smooth transitions. After placing all caps, quickly scan the row. If any cap seems to “jump” in color compared to its neighbors, consider swapping it. People with normal color vision can usually spot an out-of-place cap this way.
What the D-15 Does and Doesn’t Detect
The D-15 is a screening test, not a precision instrument. It’s designed to separate people with moderate to severe color vision deficiency from those with normal or mildly reduced color discrimination. If you have mild anomalous trichromacy (the most common and least severe form of color deficiency), you will likely pass the D-15, because your color confusions aren’t large enough to produce crossover errors on this test.
A more sensitive test, the Farnsworth-Munsell 100 Hue test, uses 85 caps and can detect subtle deficiencies the D-15 misses. There’s also a desaturated version of the D-15 that uses paler, washed-out colors to catch milder deficiencies. If you passed the standard D-15 but were told you have a mild deficiency on another test, this explains the discrepancy.
If You’re Taking It for an FAA Medical
As of January 2025, the FAA overhauled its color vision testing requirements for pilots. Color vision screening is now a one-time test rather than something repeated at every medical exam. The FAA uses approved computer-based screening tests as the primary method, not the D-15 panel.
If you’re a first-time applicant who fails all approved screening tests, you’ll receive a third-class medical certificate with a limitation restricting you to daytime visual flight rules only. Upgrading to first or second class requires an appeal to the Federal Air Surgeon. Pilots who previously passed any FAA-approved color vision test don’t need to retest.
If your employer or licensing body uses the D-15 specifically, the pass/fail criteria described above apply. The test is common in military screening, maritime licensing, rail transportation, and some electrical or industrial trades where color-coded wiring or signals are safety-critical.