Contrast is a measure of the relative distribution of the lighter and darker parts of a visual stimulus. Contrast sensitivity relates to the visibility of objects varying in size, contrast, and orientation, and the ability of the human eye to detect these differences. Contrast sensitivity is measured as contrast sensitivity acuity.
The Real-Life Visual World
Snellen acuity is in many ways an optical construct, a carefully controlled measure of black and white vision that purposefully eliminates some of the variables present in the real visual world. It is a doctor’s office which may fail to adequately approximate visual performance when light and dark, shades of gray, and, color define the visual experience outside the boundaries of the phoropter or the refracting lane.
Contrast is part of daily life, and it is contrast sensitivity acuity that is relevant in living and functioning in the real-life visual world where background and lighting conditions affect contrast and may profoundly influence vision and vision-related quality of life. Ultimately, it is this real-life vision that is important to the individual who requires vision correction.
Defining Contrast Sensitivity
Contrast is a measure of the relative distribution of the lighter and darker parts of a visual stimulus. Put more simply, contrast sensitivity acuity measures how well the eye can discriminate various shades of gray.
Contrast sensitivity function is defined by the Michaelson formula, which relates the magnitude of the difference in light intensity between light and dark areas to the overall luminance of a stimulus. Contrast sensitivity function can be measured clinically by using chart-based instruments such as the Peli-Robson and Regan low contrast acuity charts.