The aim of this study was to develop and evaluate a clinical standard capable of generating ordinal measures of monocular tritan discrimination. A novel pseudoisochromatic plate test was developed, called the "C test." It contains 10 progressively desaturated plates arranged in perceptually distinct steps that provide a standard for analyzing the threshold for tritan discrimination. The most difficult plate that can be detected is the "C score." Relevant diagnostic findings including the C score, Logmar acuity, Pelli-Robson score, and LOCS III lens grading were prospectively recorded in 568 eyes. A total of 355 normal eyes were selected for statistical analysis. The correlations between C score, Logmar acuity, and Pelli-Robson score with LOCS III grading were analyzed with Spearman rank analysis. The strongest correlations were between the Logmar acuity with posterior subcapsular cataract (PSC) (0.68), the C score with nuclear colour (NC) (-0.58), and the Logmar acuity with NC (0.57). The lower normal limit for the C score was 9 in pseudophakes and phakic eyes with NC < 3, 7 for eyes with NC 5 3, but undetermined for eyes with NC > 3. A comparison of C score distribution between three distinct age ranges of pseudophakic eyes (n 5 136) showed no significant variation (P 5 .486). The Spearman rank correlation between C score and Logmar acuity was 0.028, and between C score and Pelli-Robson score was 0.012. The normal limit for the C score used under 800 Lux is 9 when NC < 3 and 7 when NC 5 3 in all age groups. K E Y W O R D Sacquired dyschromatopsias, C test, nuclear color, pseudoisochromatic plate test, tritan discrimination | I NT ROD UCTI ONClinical vision testing consists mainly of central achromatic resolution, peripheral luminance contrast, and red/green chromatic discrimination. These tests explore the parvocellular and magnocellular pathways but leave the koniocellular pathway largely undetermined. The Lanthony tritan album was an attempt to fill an apparent clinical need for a rapidly administered inexpensive plate test of tritan threshold, but it has serious limitations. These include a high false positive rate due to having only four choices per trial, a small range of only five plates, and inconsistent colorimetric properties between editions. 9Most colour-sorting tests use "foveal" stimuli because they are designed around the 28 field of the 1931 CIE standard observer; however, the central 0.358 of the human fovea contains no short wavelength sensitive (S) cones, and the 58 |