BACKGROUND: Color vision testing is a well accepted tool in the detection and classification of both congenital and acquired deficiencies. While most practitioners are aware of the importance of proper test illumination, the specialized lamps required are typically either unavailable (eg: Macbeth lamp), costly or inconvenient for routine testing, resulting in the frequent and inappropriate use of incandescent illumination. This leads to inappropriate clues when testing color deficient persons. Barker (1997) introduced a gel 6500o Kelvin (Illuminant C) color correction filter spectacle to be worn during color testing with incandescent light. When testing with color albums, such as the Ishihara plates, the "C-Daylight" glasses are used in much the same way as polaroid glasses are used with a stereo test.
CASE REPORT(S). 5 color deficient patients were examined (2 protonopic, 2 deuteranopic and 1 strongly deuteranomalous, verified by Nagel anomaloscope) using the Ishihara and Farnsworth-Munsell 100-hue tests illuminated with a Macbeth lamp, a 60W bulb with C-Daylight glasses (Gulden Ophthalmics) and with only a 60W bulb. Under the Macbeth lamp and using the C-Daylight glasses, responses were as would be expected based on the Nagel diagnosis. However, the 60W incandescent illumination lead to certain characteristic diagnostic errors: Protanopic patients showed a deutan response on Ishihara and 100-hue testing. The 3 Deutan patients demonstrated many normal responses on the Ishihara plates with a shift of the 100-hue axis toward the tritan axis.
CONCLUSIONS. As a matter of convenience and the limited availability of Illuminant C sources, the use of inappropriate color test illumination is a common practice in eye offices. This leads to inappropriate visual clues that may enable a patient to pass or perform better on a given test. In addition, the clockwise axis shift seen with incandescent light of the 100-hue test color confusion axis translates to a mis-classification of protan as deutan and shifts the performance of deutan patients toward the tritan axis, with corresponding errors on the Ishihara plates.