Anomaloscope

ANOMALOSCOPE (from Greek anomalia-irregularity and scopein-to look), an instrument used for detecting various deviations in color vision. The device is based on color-blind individuals creating 'equations' between different spectral tones. For comparison, two light fields are used, one of which is illuminated by monochromatic yellow light (λ=589 mμ) and the other field is illuminated by two monochromatic lights—red (λ=670 mμ) and green (λ=535 mμ). A normal eye can create an equation between these two fields only at one strictly determined ratio between the mixed spectral tones in one field, and at one strictly determined brightness of the yellow spectral tone in the other field—the Rayleigh equation. Conversely, color-blind individuals, by varying the brightness of the monochromatic yellow light, can create an equation with it at any ratio of red and green light in the second field—from purely red to purely green. In this case, red-blind individuals (protanopes) equate the red tone to a very dark yellow tone, while green-blind individuals (deuteranopes) equate it to a light yellow tone. The anomaloscope proves especially useful in determining those forms of color vision where there is not yet complete color blindness, but there is a weakening of perception of any particular color—color vision anomaly. Such patients can no longer create equations between pure monochromatic spectral tones, but when performing the Rayleigh equation, they use a different ratio of red and green tones than people with normal color vision.