Definition
The anomaloscope (Nagel Anomaloscope) is an optical instrument used for quantifying red-green colour vision deficiencies. The instrument shows the colour assessor a split field: on one side a yellow reference field (589 nm), on the other side a mixture of red (670 nm) and green (546 nm). The colour assessor adjusts the mixture until the two halves appear identical (metameric).
Background
Persons with normal colour vision make this match at a specific red:green ratio. Persons with protanomaly or deuteranomaly use a significantly deviating ratio. The result — the Rayleigh match — indicates whether there is a deficiency and how severe it is.
The anomaloscope is the most accurate instrument for diagnosing the nature and severity of red-green colour vision deficiencies and is used in research contexts and clinical settings. In industrial colour assessment contexts (ASTM E1499-16) the instrument is deployed as a supplementary screening instrument alongside the FM100 and Ishihara test.
Distinction from other tests
- Ishihara: screening — is there a deficiency? (yes/no)
- FM100: discrimination — how fine are the hue differences you can see?
- Anomaloscope: diagnosis and quantification — what type of deficiency and how severe?