MEASURING RESOLUTION IN THE CONTRAST DOMAIN - THE SMALL LETTER CONTRAST TEST

Background. Recent evidence suggests that small letter contrast sensit ivity (CS) is more sensitive than visual acuity (VA) to defocus, lumin ance, binocular enhancement, and visual differences among pilot traine es. it would be valuable to make this test available for general use. We developed a hard copy (letter chart) version called the Small tette r Contrast Test (SLCT) and evaluated its sensitivity and reliability i n comparison to standard vision tests. Methods. The SLCT has 14 lines of letters with 10 letters per line. The letters are of constant size (20/25 or 4/5 at 4 m), but vary in contrast by line in 0.1 log steps ( 0.01 log units per letter). Normal room illumination is used. The SLCT was evaluated in 16 subjects under various conditions (spherical and astigmatic blur, low luminance, 2 eyes vs. 1 eye) to determine test se nsitivity and reliability, and in patients with clinical conditions. S cores were compared to those obtained with standard tests of VA (Baile y-Lovie) and CS (Pelli-Robson). Results. SLCT scores were similar to p revious measures, and retest reliability was one line. The SLCT was mo re sensitive than VA to spherical and astigmatic blur, low luminance, and vision with two eyes vs. one eye. Greater sensitivity of the SLCT endured despite correction for variability. The SLCT was more sensitiv e than standard tests to visual loss from early cataract, keratoconus, corneal infiltrates, edema, and amblyopia. Conclusions. The SLCT is a sensitive, adjunctive test, which complements existing measures of VA . it can reveal subtle visual deficits that may be undetected by stand ard clinical techniques. The SLCT should prove useful for monitoring v ision in refractive surgery, corneal and macular edema, optic neuritis , and for selection and evaluation of candidates for occupations requi ring unique visual abilities like aviation.