The contrast sensitivity test measures visual function beyond simple clarity of sight. Standard eye charts assess fine details under ideal, high-contrast conditions. This specialized test evaluates the visual system’s ability to distinguish subtle differences in light and dark. This assessment provides eye care professionals with a more complete picture of a patient’s functional vision and visual processing capabilities.
Defining Contrast Sensitivity
Contrast sensitivity is the measure of the lowest contrast level required for the visual system to distinguish a target from its background. It quantifies the ability to perceive objects that blend into their surroundings, often referred to as “shades of gray.” This measurement is expressed as the inverse of the contrast threshold, which is the minimum difference in brightness an observer can detect.
Functional vision relies heavily on contrast sensitivity because the environment is rarely composed of perfect black-on-white edges. People with reduced contrast sensitivity may struggle with common tasks like identifying the pale gray curb on a cloudy day or reading a sign that is slightly faded. This ability to perceive objects blending into their surroundings is essential for real-world navigation.
Contrast Sensitivity Versus Visual Acuity
Visual acuity, commonly measured using the Snellen chart, assesses the sharpness of vision under maximum contrast conditions. This test requires identifying small, black letters printed on a bright white background. A patient achieving 20/20 visual acuity has excellent fine detail vision but only under this specific, high-contrast scenario.
Contrast sensitivity, by comparison, measures the ability to see objects that are fuzzy or low-contrast, not necessarily small. A person can maintain 20/20 vision but still have a significant reduction in contrast sensitivity, leading to poor functional vision. This condition often results in complaints of “washed-out” or foggy sight, especially in dim light or glare. Contrast sensitivity is a better predictor of the visual performance needed for daily activities like driving at dusk or navigating a staircase.
How the Test is Performed
Contrast sensitivity is typically measured using specialized charts or computer-based systems, such as the Pelli-Robson chart. Unlike a standard eye chart where the letter size decreases, the Pelli-Robson test uses large letters of a uniform size, often equivalent to a 20/60 optotype. The contrast of these letters progressively decreases as the patient reads down the chart.
The letters are arranged in groups of three, or “triplets,” with each triplet sharing the same level of contrast. The patient reads the highest-contrast letters first and continues until they can no longer correctly identify at least two of the three letters in a triplet. The chart must be placed at a set distance and illuminated uniformly for accurate results. Other methods, like the Vistech chart or computerized sine-wave grating tests, use parallel dark and light bars that vary in width and contrast. These tests require the patient to identify the orientation of the fuzzy bars, mapping out their full contrast sensitivity function.
Clinical Significance of Low Contrast Scores
A low contrast sensitivity score is often an early indicator of disease, sometimes appearing years before a noticeable drop in visual acuity. Conditions that affect the ocular media, such as cataracts, cause light to scatter, which directly degrades contrast sensitivity and makes night driving particularly challenging due to glare. Glaucoma and diabetic retinopathy can also cause a reduction in the ability to perceive low-contrast targets.
Low contrast sensitivity is also a significant finding in systemic neurological diseases, such as multiple sclerosis (MS). In MS, the loss of contrast sensitivity can occur even when Snellen acuity remains perfect. This selective loss is a sign of neural damage and serves as an auxiliary diagnostic tool, revealing visual involvement that standard tests miss. Functionally, a low score translates to difficulty in mobility, a higher risk of falls, and trouble reading, as it impairs the ability to discern object boundaries and depth in real-world environments.