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Contrast sensitivity is a component of human vision which involves the ability to distinguish between differing levels of luminance. Luminance is a measure of brightness, and when people have low contrast sensitivity, it can impair their vision. They may have trouble seeing street lights, tracking objects at night, reading, or watching movies and television. There are a number of conditions which can lead to low sensitivity, including macular degeneration and diabetes. Aging is also a factor, as people naturally lose contrast sensitivity with age, starting at around age 20 when it tends to peak.
Physicians can use tests to determine a patient's level of contrast sensitivity. It is important to note that patients can have 20/20 vision and still have very low contrast sensitivity which impacts their functional vision. A doctor may administer a test in response to patient complaints or abnormalities observed during routine eye exams and testing. Some ophthalmologists administer such tests automatically to all their patients as part of a physical exam.
In a contrast sensitivity test, a patient will be shown a series of charts which show black, white, and gray lines and dots on varying backgrounds. These charts are viewed with corrective devices such as glasses or contacts in place. Most people have the highest level of contrast sensitivity with medium sized objects and high levels of contrast. As objects grow larger and smaller at low contrast levels, it becomes harder to discern them. People with high levels of contrast sensitivity may be able to discern most of the objects on the chart while people who have low contrast sensitivity will have difficulty.
A common test for contrast sensitivity involves a chart which shows lines of varying widths along a horizontal axis and adjusts contrast along the vertical axis. While all of the lines are of the same length, they will appear longer in the middle of the chart because the width facilitates ease of discernment even when there is not a great deal of contrast.
In addition to being important for visual health, sensitivity to contrast is an important concern for graphic designers. Designers need to think about how readable images will be, and they must design with the limitations of the human eye in mind. On the Internet, for example, an important concern with colored text and backgrounds involves using widely varying luminance in order to ensure that users will be able to read the text comfortably.