The True Daylight Illuminator (TDI): a less expensive source of illumination for color vision screening

This experiment was designed to examine the feasibility of using the True Daylight Illuminator (TDI) as an illumination option for conducting color vision screening with the Ishihara Test(s). The MacBeth Easel Lamp was designed to provide proper illumination for performing color vision screening with a variety of pseudoisochromatic (PIC) plate tests including the Ishihara. However, over the years, the MacBeth Lamp has become so expensive that many smaller programs cannot afford to purchase one. This problem has promoted the use of alternate light sources that have had a deleterious effect on test results and in some instances contributed to job discrimination. Recently the MacBeth Division of the Kollmorgan Corporation discontinued the manufacture of the MacBeth Lamp due to lack of demand. Thus, it is important to find an inexpensive illumination option for conducting color vision screening. Two groups of subjects were used to compare test results for the 24-plate edition of the Ishihara Test using both the MacBeth Lamp and the True Daylight Illuminator. The first group contained 45 subjects with inherited color defective vision. The second group was made up of 30 color normals. The Nagel anomaloscope was used to confirm the color vision status of the subjects in both groups. Statistics to test the significance of differences between group means clearly demonstrated that there were no significant differences between the mean error scores of the two groups for these two pieces of equipment. Thus, it was concluded that the TDI can be used in lieu of the MacBeth Easel Lamp for screening color vision with the Ishihara test.