Contrast is the difference in brightness that separates an object from its background. It is the difference in brightness and/or color that makes an object (either real or an image) distinguishable from other objects and from its surroundings.
Contrast sensitivity is a person’s ability to see these differences. The better able one is to detect objects of low contrast, the higher one’s contrast sensitivity.
In seeing everyday objects, our eyes collect several bits of information using various visual channels. These visual channels collect information about size, shape, contrast, color and motion. Each channel collects and feeds this information individually to our eye/brain system. The Snellen letter chart tests only one of these visual channels.; it measures one’s ability to see well-defined black letters on a white background, which is a very high contrast environment. While most objects we see are larger than the letters on a letter chart, they are much lower in contrast.
“I’m 20/25 but I can’t see well a lot of the time”
Many people with normal or near-normal high-contrast acuity have low contrast sensitivity. Low contrast is common among the elderly, as it is a common characteristic of the aging eye.
Low contrast sensitivity often goes un-diagnosed, in part because a contrast sensitivity test is often not administered as part of a routine eye exam.
When contrast sensitivity is not measured, those who are experiencing vision difficulties become confused and uncertain about what they are experiencing. People with glaucoma, for example, often complain of “poor” vision despite testing normal or near-normal for visual acuity. And without the understanding of how their contrast vision has changed, they are not able to become aware of the challenges to their safety and daily living that reduced contrast sensitivity brings, such as night driving or moving through a dimly lit room.
Without testing it can be difficult for someone to distinguish between vision challenges from reduced acuity (blurring) and reduced contrast sensitivity.
Acuity and contrast sensitivity
Contrast and visual acuity are not independent. Reduced contrast sensitivity may well co-exist with normal or near-normal high-contrast acuity (which is tested for in an arbitrarily high contrast setting of black letters on a white background).
Raising a red flag
Optical problems that affect the eye’s ability to deliver a clear image of the outside world to the retina include:
- lack of correction or under-correction of refractive errors (often correctable with glasses or contact lenses)
- a cataract, a clouding of the lens in the eye (usually correctable with surgery).
- retinal disorders such as macular degeneration and glaucoma (requiring medical treatment)
Reduced contrast sensitivity is associated with diseases of the organs of vision, both optical problems such as cataracts and refractive error and diseases of the outer retina such as Macular Degeneration and of the inner retina such as glaucoma and optic neuropathy.
Like visual acuity, contrast sensitivity loss is non-specific; test results do not in themselves lead to a diagnosis. But measuring and documenting contrast sensitivity can play an important role in the early detection of certain visual system conditions, especially those that leave high contrast visual acuity unaffected such as optic neuropathies, glaucoma, and diabetes. Well before visual acuity is affected, retinal disorders may cause deficits in contrast sensitivity that will show up in testing, indicating a need for a more comprehensive professional examination.
Contrast sensitivity in the real world
In everyday life you encounter high contrast situations, low contrast situations and everything in between. So loss of contrast sensitivity can have a greater impact on the activities of daily living than the loss of visual acuity alone.
Even in normal lighting situations, reduced contrast sensitivity affects how and what you see.
Loss of contrast sensitivity – either from aging or eye conditions – can negatively affect performance of activities of daily living. You may find it challenging to walk down poorly lit stairs.
You may have difficulty reading a restaurant menu or the newspaper or the label on a medication bottle, seeing road signs or pedestrians while driving at dusk, and even recognizing the faces of people you know in dimly lit settings.
When driving along a poorly maintained road at night, you may wish that the side and center markings were re-striped to enhance the contrast.
As contrast sensitivity changes, you may find that you see well at one moment and poorly the next. Lighting situations have a strong effect on the vision of the those with low contrast sensitivity. Low light situations – outdoors at dusk or in fog or glare, indoors in poorly lit places – can make you feel you are not seeing well.
Reduced contrast sensitivity not only increases inconveniences such as difficulty reading, it also increases your risks of hurting yourself and/or others. Low contrast sensitivity can increase your risk of not seeing a curb or step and falling or not seeing cars and pedestrians when driving in low contrast situations such as dusk or night. .
Becoming aware of one’s reduced contrast sensitivity through testing is the critical first step to learning to adapt to and make adjustments for low contrast sensitivity. (See Testing below) When you have identified low contrast sensitivity as a cause of vision difficulties, you can adapt strategies to optimize your vision.
Strategies for those with low contrast sensitivity may include focusing awareness on those situations that are most challenging such as:
- driving at night, low contrast hazards such as curbs and steps, dimly lit rooms….
- improving illumination. While magnification is helpful for visual acuity loss, better lighting creates a higher contrast lighting and brings out objects that are not visible in low light. You might be unable to see white rice on a white plate on a white tablecloth until the lighting is increased. Better lighting is key to improving one’s ability to read.
- enhancing the contrast of your environment. If your stairs are hard to see, you can mark them with tape of a contrasting color. Writing with a wide-tip dark black pen can make writing easier. For suggestions see Helping yourself
- adopting glasses with specially designed color tints (often yellow or amber) that often improve contrast.
Measuring contrast sensitivity for monitoring vision changes
The MeyeSight Visual Fitness Test – contrast vision
Contrast sensitivity is the second test in the MeyeSight Visual Fitness test suite. After the first part, in which the size of the symbols changes, in the second test the contrast of the symbol changes. As you identify the direction of the symbol, the next symbol is displayed with one step less contrast.
Contrast sensitivity testing can also effectively measure vision changes over weeks, months, or years. This could be especially beneficial in monitoring progression (or regression) of the disease process of conditions such as cataracts, age-related macular degeneration, diabetic retinopathy and glaucoma.
Contrast sensitivity testing can demonstrate whether medical treatments are effective or whether cataract surgery has resulted in improved contrast sensitivity.