The detection of small luminance differences between objects and their backgrounds is an important tool for navigating in the visual environment. Many visual system disorders cause patients to complain that their vision is cloudy or blurred, or that the borders of objects are blurred, even though they retain 20/20 Snellen acuity. These patients may suffer from losses in contrast sensitivity that cannot be detected using standard clinical testing. A more appropriate measure for these patients is their ability to detect small changes in luminance within a pattern. One widely used experimental measure of this ability is the contrast sensitivity function.