anomaloscopy

Anomaloscopy is a psychophysical method used to diagnose and quantify color vision deficiencies, particularly anomalous trichromacy and dichromacy. It employs an anomaloscope, an optical instrument that presents a field combining two primary lights and a reference field of a fixed color and luminance. The observer's task is to adjust the relative intensities of the two primaries until the two halves appear identical.

Most common designs use red and green primaries, and tests such as the Rayleigh match require the observer to match a yellow reference by balancing the red-green mixture. Normal trichromats can achieve a precise match at a characteristic ratio, whereas protanopes, deuteranopes, and those with protanomaly or deuteranomaly deviate in predictable ways. The resulting match data are plotted on color-matching lines, from which type and severity of deficiency can be inferred.

Anomaloscopy remains a standard clinical tool in ophthalmology and occupational color-vision screening, valued for its quantitative output and ability to distinguish between different forms of color vision loss. Results can be influenced by luminance adaptation, detector settings, and subject instructions, so testing is typically performed under controlled conditions and interpreted against normative data. The term anomaloscopy refers to both the procedure and the instrument used in this form of color-vision testing.