Contrast sensitivity is the ability to distinguish between different levels of brightness, and low sensitivity can impair vision. Tests can determine a patient’s level of sensitivity, which can be affected by conditions like macular degeneration and diabetes, as well as aging. Graphic designers must consider contrast sensitivity when designing legible images.
Contrast sensitivity is a component of human vision that involves the ability to distinguish between different 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 that can lead to low sensitivity, including macular degeneration and diabetes. Aging is also a factor, as people naturally lose contrast sensitivity as they age, starting around the age of 20, when it tends to peak.
Doctors can use the 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 affects their functional vision. A doctor may administer a test in response to patient complaints or abnormalities seen during routine eye exams and tests. Some ophthalmologists administer such tests automatically to all of their patients as part of a physical exam.
In a contrast sensitivity test, a patient will be shown a series of graphs showing black, white, and gray lines and dots against different backgrounds. These graphics 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 get larger and smaller at low contrast levels, it becomes more difficult to tell them apart. People with high levels of contrast sensitivity may be able to discern most objects on paper, while people who have low contrast sensitivity will have difficulty.
A common test for contrast sensitivity involves a graph that shows lines of varying width along a horizontal axis and adjusts the contrast along the vertical axis. While all lines are the same length, they will appear longer in the center of the graph because the width facilitates ease of discernment even when there isn’t much contrast.
In addition to being important for visual health, contrast sensitivity is a major concern for graphic designers. Designers have to think about how legible the images will be, and they have to design with the limitations of the human eye in mind. On the Internet, for example, a major concern with colored text and backgrounds is the use of highly variable luminance to ensure that users are able to read the text comfortably.
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