The CN Lantern Test and Different Viewing Distances.

Significance: This research shows that some color-vision-defective patients could identify railway signal lights correctly if they are working in the yard where sighting distances for signal lights are shorter.

Purpose: When interpreting railway signal lights, sighting distance can vary depending on the employee's location and job requirements. Individuals with a color-vision-defect may pass railroad employment color vision testing for positions with shorter sighting distances, despite failing to qualify for positions with longer sighting distances. The CN Lantern (CNLan) simulates railway signal lights. We evaluated performance and repeatability on CNLan at different viewing distances in color-normal and color-deficient individuals.

Methods: Fifty-six subjects with normal color vision and 63 subjects with a red-green color-vision-defect participated. The CNLan test was performed at 4.6-, 2.3-, 1.15-, and 0.57-m viewing distance. The test was repeated after 10 days.

Results: All individuals with normal color vision passed the CNLan at all distances at both visits without errors. For the group with a color-vision-defect, the pass rate increased from 12% at 4.6 m to 62% at 0.57 m. The repeatability of the CNLan between visits for the color-vision-defective group was very good with AC1 agreement values greater than 0.85.

Conclusions: An increase in retinal illumination was likely responsible for the improved performance as the test distance was decreased. Typical sighting distances in railway yards correspond to the 0.57-m test distance in our study. The results of this study suggest that 62% of the individuals with a red-green color-vision-defect may correctly identify colored signal lights in a railway yard where sighting distances are less than 100 m.