An anomaloscope is the clinical gold-standard instrument for red-green color vision: you look at a split circle and turn two knobs — a red-to-green mixture and a yellow brightness — until both halves match (a 'Rayleigh match'). Where you match, and how wide a range of mixtures you accept, tells protan (red-weak) from deutan (green-weak) and reveals dichromacy. This free online version is an interactive demonstration of that match, not a test: a real anomaloscope mixes single-wavelength red, green, and yellow light, and a screen cannot — its 'yellow' is itself made of red and green pixels, so at the match point both halves emit identical light and every observer matches at the same place. That means a screen cannot read protan vs deutan from it. Use it to understand how the instrument works; for an actual screening take a plate or arrangement test, and for a diagnosis see an eye-care professional. The Rayleigh match is red-green only and never assesses blue-yellow (tritan).
Rayleigh match · interactive demonstration
Anomaloscope (Rayleigh Match)
Adjust a color mixture to match the reference field. This screen demonstration cannot replace a clinical anomaloscope examination.
How the anomaloscope test works
1. Match the two halves
The circle is split: one half is a fixed yellow, the other a red-green mixture. Slide the mixture from red to green, and adjust the yellow's brightness, until the two halves look like one solid color. There's no clock — match by color, not by which half looks brighter.
2. Repeat the match
You make the match a few times, each starting from a different mixture — the same repeated-matching the examiner does on the real instrument. It's a hands-on way to feel how a Rayleigh match is found.
3. What it shows (and can't)
On a real anomaloscope, where you match and how wide a range you accept would classify protan vs deutan and reveal dichromacy. On a screen it can't: the reference yellow is itself red+green, so at the match both halves are physically identical light and every eye matches at the same place — there's nothing to separate the types. So this is a demonstration, not a result.
What this demonstration can and cannot do
It's the real match. You perform an actual Rayleigh match — the principle behind the clinical gold standard for red-green vision — and get a feel for how it's found.
It can't diagnose you. On a screen the yellow is itself red + green, so at the match both halves are identical light and every eye lands in the same spot — nothing separates protan from deutan.
Red-green only. The Rayleigh match never touches blue-yellow (tritan). For that, use the HRR or Farnsworth D-15 test.
Want a real result? Plate and arrangement tests do work on a screen — take the color blind test — and for a diagnosis, see an eye-care professional.
This is a free screening and self-assessment tool, not a medical diagnosis. Results are affected by your screen, its color settings, and room lighting.
Anomaloscope & Rayleigh match — frequently asked questions
- What is an anomaloscope?
- An anomaloscope is an optical instrument that is the clinical gold standard for diagnosing and grading red-green color vision deficiency. The best-known model is the Nagel anomaloscope. The observer looks at a circular field split in two and adjusts a red-green mixture in one half and a yellow brightness in the other until both halves match exactly — the Rayleigh match. The mixture ratio at which they match, and the range of ratios they accept, quantify the type and degree of red-green deficiency.
- What is a Rayleigh match?
- A Rayleigh match is the color match an anomaloscope is built around: matching a mixture of red (about 670 nm) and green (about 546 nm) light to a fixed yellow (about 589 nm) by adjusting the red-green ratio and the yellow's brightness. Lord Rayleigh described it in 1881. People with normal color vision match in a narrow range; people with anomalous trichromacy need an unusual amount of red (protanomaly) or green (deuteranomaly); dichromats can match the yellow to almost any red-green ratio by changing brightness alone.
- How does the anomaloscope tell protan from deutan?
- By the direction the match shifts. A protan (red-weak) observer perceives red as dim and needs to add more red to the mixture to match the yellow, and sets the yellow dimmer. A deutan (green-weak) observer needs to add more green. The amount of the shift is summarized as the anomaly quotient — around 1 for normal vision, below 1 for protan-leaning matches and above 1 for deutan-leaning. A complete dichromat accepts the whole mixture scale. This separation depends on the instrument's pure single-wavelength lights, which is exactly why it cannot be reproduced on a screen.
- Can an anomaloscope test be done online?
- Not as a real test — only as a demonstration. The instrument works by metamerism: its single-wavelength yellow (589 nm) and its red+green mixture are physically different light that happens to look identical, and different cone pigments break that coincidence at different mixtures, which is what separates protan from deutan. A computer display has no single-wavelength primaries, and its 'yellow' is itself produced from the red and green pixels — so at the match point both halves emit identical light and every observer, color-normal or not, matches at the same place. There is no metamer to break, so an online version cannot classify your color vision. It can faithfully demonstrate the matching task and explain the principle, which is what this page does.
- Does the anomaloscope test for blue-yellow (tritan) color blindness?
- No. The Rayleigh match only involves red, green, and yellow, so it assesses the red-green axis only and is blind to blue-yellow (tritan) deficiency. To screen for tritan as well, use a plate test such as the HRR, or an arrangement test such as the Farnsworth D-15, which cover the blue-yellow axis.
- If I can't get a result here, how should I test my color vision online?
- Use a pseudo-isochromatic plate test or a color-arrangement test — those genuinely work on a screen for screening, because they don't depend on single-wavelength light. Try our color blind test, the Ishihara test, the HRR test, or the Farnsworth D-15. They reliably screen for a deficiency; treat the exact type and severity as estimates, and confirm anything important with an eye-care professional, who can use a real anomaloscope.