anomaloscope

An anomaloscope is an optical instrument used to diagnose and quantify color vision deficiencies by measuring how a person matches colors under controlled illumination. It is the primary device for performing color matching experiments, particularly for assessing red–green color vision abnormalities.

In its most common form, a bipartite field presents two halves of light that differ in spectral composition; the subject adjusts one or both halves so that the two halves appear identical in hue and brightness. The point at which a match is achieved reflects the individual's spectral sensitivity; normals have a red-green match at a single setting, whereas those with protan or deutan defects require altered mixing or cannot reach a match across the available range.

The best-known model of anomaloscope is the Nagel-type instrument, and other variants exist. Interpreting results yields classifications such as protanomaly, deuteranomaly, tritanomaly, or dichromacy, and can be used to grade severity.

Applications include clinical assessment in ophthalmology and optometry, personnel screening in occupations requiring accurate color discrimination (e.g., pilots, electricians), and research on color processing. Limitations include dependence on stable lighting, observer adaptation, and the need for standardized procedures; results complement other color vision tests rather than replace them.