Contrast Sensitivity Testing: How Does It Work?
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Contrast sensitivity testing measures how well the eyes can distinguish between finer and finer light increments compared to dark.
This testing is not usually part of a routine eye exam. The doctor may recommend it if someone has a specific visual complaint that could affect their visual contrast.
The most common contrast sensitivity test is the Pelli-Robson contrast sensitivity chart. This test is a chart with different capital letters organized in horizontal lines. The contrast decreases with each line. The person will move down the chart to determine the least level of contrast they can see.
A sine-wave grating test is a more sophisticated contrast sensitivity test. You will look at several parallel, fuzzy bars of dark and light. This test provides a more sensitive way to see how your eyes view contrast.
Once low contrast is diagnosed, the doctor will determine which type you are dealing with. This makes it possible to determine the cause and get started with a management plan.
What Is Contrast Sensitivity Testing?
Contrast sensitivity is your ability to tell the difference between similar shades. This ability allows you to see unmarked steps and curbs, gray cars on dark days, and the contours on a person’s face.
When someone has low contrast sensitivity, they may find it hard to drive at night or see people walking on dark streets. It can cause easier eye fatigue when you are watching television or reading. It also increases the risk for falls.
The Pelli-Robson test consists of rows of letters that become gradually less contrasted. They start a dark black and progress into a faint gray. The letters are on a white background.
The doctor will ask you to start at the top line and recite the letters from left to right. You will move down one line at a time. This allows your doctor to determine the lightest contrast you can successfully see.
This is a reliable and fast method to test a person’s contrast sensitivity in a clinical setting. Normal values on this test can be beneficial for evaluating people having refractive surgery or people with cataracts.
Sine-Wave Grating Test
The sine-wave grating test is more sophisticated than the Pelli-Robson test. It targets sine-wave gratings that include several parallel, fuzzy bars that are dark and light. They can vary in contrast and width from target to target.
This test can provide a more complete evaluation of your contrast sensitivity. Some of the tests include directing a bright light at your eyes to simulate a glare. This helps to determine how well you can see when there are headlights coming at you as you drive at night.
Contrast Sensitivity Function
To plot a person’s contrast sensitivity, doctors might recommend contrast sensitivity function. This uses contrast and spatial frequency as the contrast sensitivity measurements.
This may include sine-wave grating targets. However, this test will also include thicker bars that represent low spatial frequencies. For higher spatial frequencies, there will be thinner bars.
If the psychosocial method for testing contrast sensitivity is limited, doctors might consider using contrast sweep visual evoked responses. This test provides an objective method for measuring a person’s contrast sensitivity function. It uses a combination of spatial frequencies and contrast thresholds.
This plotting is similar to how doctors test the sensitivity of a person’s hearing by using variations in volume and pitch. They are plotting a curve that shows the lowest contrast you can see for every spatial frequency they test.
For people to be able to detect objects that have high spatial frequencies, they must have lower spatial frequencies with a significantly higher contrast.
Preparing for the Test
Once you start the contrast sensitivity part of the testing, you will usually wear your contact lenses or eyeglasses if you have them. If you have an eye disease, your doctor will typically test each eye individually. Any other specific preparation will vary per individual.
Dealing With the Results
Once you have the results of the test, this helps your doctor to determine if you have higher-order aberrations, a type of vision error. There are multiple higher-order aberrations, but spherical aberrations, coma, and trefoil are the ones doctors consider having clinical interest.
These aberrations are a type of distortion acquired by a light wavefront that has refractive component irregularities when it goes through an eye. These aberrations can cause issues, such as trouble seeing at night, halos, starburst patterns, glare, blurring, and double vision.
- Spherical aberrations: This type causes night myopia. The issue can cause reduced visual acuity when light is low. All people can experience this aberration, including those who have 20/20 vision. This aberration is correctable.
- Coma aberrations: This aberration type can occur due to lens problems or certain optical designs that cause off-axis point sources. For example, if you look at a star, it appears distorted. It will look more like a comet with a tail.
- Trefoil aberrations: This aberration type results in light smearing in three directions. Compared to a coma at a similar degree, the trefoil type usually causes less image quality degradation.
Low contrast sensitivity may occur with certain diseases of the eye, such as glaucoma, cataracts, or diabetic retinopathy.
Low contrast sensitivity may benefit from corrective lenses that have a yellow filter. This filter helps you to discern contrast better.
If you wear prescription eyeglasses and you have difficulty seeing in low-light conditions, lenses with an anti-reflective coating might help. Night vision and contrast sensitivity may improve with custom wavefront lenses.
Wavefront LASIK may help some people. It might improve contrast sensitivity and decrease higher-order aberrations. Following cataract surgery, certain intraocular lenses may improve contrast sensitivity and decrease higher-order aberrations.
The Path Forward
Issues with visual contrast can make it difficult to see the world at the right depth. The reason someone has issues seeing contrast vary, and not all of these issues can be effectively addressed.
Knowing you are having trouble with this type of vision can help doctors to determine the cause. Then, they can provide treatments to best preserve your ability to see varying levels of contrast.
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