A pseudoisochromatic plate test for tritan contrast measurement. The C test is a spiral bound book with 10 test plates that provides a very repeatable ordinal score (the C score) for both eyes within 90 seconds in most subjects. It is more sensitive than the Ishihara test to changes in optic nerve function, and in retinal vascular disease a low C score has prognostic significance. The C score is normal in amblyopia, which is useful in cases of unexplained vision loss.
Clinical colour vision testing is not complete without measuring blue / yellow contrast.
The normal C score for observers without significant nuclear cataract (LOCS III nuclear colour < 4) is 9 or 10. In the absence of significant cataract a score of less than 9 is highly suggestive of pathology.