The Anomaloscope
The anomaloscope is considered the gold standard for diagnosing and grading red-green colour deficiency. It's precise enough to tell protanomaly from deuteranomaly and to measure exactly how strong the deficiency is.
How an anomaloscope works
You look into an eyepiece at a small split circle. The top half is a fixed yellow. The bottom half is a mixture of red and green light that you control with two dials. Your task is to make the two halves match, same colour, same brightness. This is called a Rayleigh match.
Someone with normal colour vision accepts only a narrow range of red-green mixtures as a match. Someone with a deficiency accepts a wider or shifted range:
- A protan observer needs much more red in the mix and turns the yellow down, because red looks dim to them.
- A deutan observer needs more green, with the yellow near normal brightness.
- A dichromat (protanopia or deuteranopia) will accept the entire range from pure red to pure green as matching yellow, a hallmark result.
What it diagnoses
Because the accepted match range and midpoint are measured numerically, the anomaloscope both classifies the type (protan vs deutan) and grades the severity. That's more than plate tests like Ishihara can do on their own, which is why the anomaloscope is the reference against which other colour-vision tests are validated.
Why it can't be done online
A true anomaloscope uses monochromatic (single-wavelength) light from precise sources, viewed through controlled optics. A phone or monitor emits broad-spectrum red, green and blue that can't reproduce those pure wavelengths, and no two screens are calibrated alike. Any "online anomaloscope" is at best a loose analogy.
What you can do at home is screen with Ishihara-style plates and try an arrangement test. If those flag a red-green deficiency and you need a precise classification, for a career, research or peace of mind, ask an optometrist or ophthalmologist whether they have an anomaloscope or can refer you to a clinic that does.
Anomaloscope vs other tests
- Ishihara plates: fast screening for red-green deficiency; widely available.
- Farnsworth D-15: grades severity and sorts protan/deutan/tritan by arrangement.
- Anomaloscope: the definitive red-green diagnosis and severity measure.
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