X-Chrom lens

The X-Chrom lens is a deep red contact lens worn on one eye to help people with red-green color vision deficiency tell colors apart.^1,2 It works through brightness rather than color. The red filter darkens greens far more than reds, so the two eyes see the same object at different brightness, and the wearer learns to use that difference as a cue.³

The X-Chrom lens does not correct color vision. It raises scores on pseudoisochromatic plates, but lantern and signal-light tests, the Farnsworth-Munsell 100-hue test and the anomaloscope do not improve.^2,4 The unequal brightness between the eyes can also distort the apparent path of moving objects (a Pulfrich effect).² Stereoacuity changed little in one study and fell in three of five wearers in another, smaller one.^2,4