Measuring Contrast Sensitivity Using the M&S Smart System II versus the Pelli-Robson Chart

Contrast sensitivity (CS) refers to the ability of the visual system to detect differences in luminance (i.e., brightness) between an object and its background. Assessment of CS provides valuable information in the early detection and monitoring of ocular diseases, as well as evaluating the impact of therapy. The most widely used clinical spatial CS test is the Pelli-Robson chart (Clement Clarke International, Essex, UK). Several factors may influence the CS threshold measured. First, although the recommended luminance is 85 candelas/m2 (range, 60–120), maintaining consistent luminance across the entire chart can be difficult. Overhead lighting in most examination rooms illuminates preferentially the top portion of the chart, and decreases nonuniformly toward the lower portion. In addition, patients tested in different examination rooms with different light fixtures may exhibit some variation in threshold measurement. Second, the Pelli-Robson chart fades over time with exposure, with a manufacturer-recommended expiry of 7 years. Variation and inaccuracy may occur when comparing measurements using charts of different ages. Third, the chart has only 2 versions with different triplets of optotypes. Patients may recall letters with frequent use, especially those letters that are found around their threshold.