Nagel anomaloscope

Historical Significance: The anomaloscope is an instrument used to test for color blindness. It is able to detect whether a person is a protanope, deuteranope, protanomal or deuteranomal. It presents a circle of light, the lower half of which is yellow spectral light, the brightness of which can be varied using one knob to vary the width of one of 3 entrance slits. The other half is a mixture of red and green light generated by two entrance slits before the prism system. The second knob varies the proportions of red and green so that a yellow can be found to match the colour of the yellow half circle. An exact match can be made by adjusting the brightness of the yellow with the first knob. The colour vision normal observer makes a particular match that varies little among a population of people with normal colour vision. Protanomals and deuteranomals make anomalous matches finding a match which to the normal observer appears too red (protanomals) or too green (deuteranomals). Protanopes and deuteranopes can match all mixtures of red and green to the yellow since they have no red-yellow-green discrimination. Protanopes are distinguished because they lack a red cone receptor and have reduced red light sensitivity, so when matching reds to the yellow they have to dim the yellow. The apparatus was invented by the German ophthalmologist and physiologist Willibald A. Nagel (1870-1911) who named it "anomaloskop" in 1907. Nagel, W.A. (1907) Zwei Apparate fur die Augenarzliche Funktionsprufung. Adaptometer und kleines Spektralphotometer (Anomaloskop). Zeitschrift fur Augenheilkunde, 17, 201-22.