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Contrast sensitivity is a measurement of your ability to discern subtle differences between two objects. It's a critical part of your visual acuity, but for years, doctors didn't measure it during routine eye exams. (Learn more)
That changed in 1988 when British researchers developed a quick test to measure visual acuity. The test was named after them, and it's now considered the gold standard in contrast sensitivity measurement.
During a Pelli-Robson test, you'll look at letters printed in black ink on a white screen. The ink will grow progressively lighter. Your score is a measurement of how faint the letters are before you can no longer read them. (Learn more)
The test could be helpful for almost anyone, but some conditions can impact your ability to discern contrast. If you have one of these diseases or they run in your family, this test is a vital part of your eye exam routine. (Learn more)
There's no need to prepare for this test, as it's noninvasive and over quickly. But you will want to be as honest as you can, so your doctor has accurate information about your eye health. (Learn more)
What Is Contrast Sensitivity?
Contrast sensitivity is your ability to separate one object from another even if they're the same size, shape, and color. This ability can keep you safe in everyday situations that demand sharp, crisp vision.
For example, an evening walk to your car could become hazardous without contrast sensitivity. You might not spot tripping hazards, including:
- Loose sidewalk tiles
In low light, all of these items might be the same color. They might also be the same size and shape. Depth sets them apart, so they can pop from the background. If you can't see them, you could trip and fall. You might also feel incredibly anxious, as you can't see the dangers that surround you.
Researchers say people can score well on tests of distance vision but have poor contrast sensitivity.
In addition to night walking, you might also struggle with:
- Traffic signs
- Facial recognition
Anything that involves the ability to pick an item out of its background will be difficult for you. Without treatment, it's an issue that's likely to worsen over time.
Without a Pelli-Robson test, your doctor might be unable to explain why you can't see as well as you once did. But with scores in hand, your treatment path can become clear. Then, you can work with your doctor to correct the underlying problem so you can see clearly.
How the Test Works
During a standard vision exam, your doctor uses letters and numbers to measure your ability to see objects near to and far from your eyes. The Pelli-Robson test also uses letters, but its goal is slightly different.
Before 1987, doctors used the results of standard visual acuity tests to guess at contrast sensitivity. In a study published in Clinical Vision Sciences, Pelli, Robson, and Wilkins proposed a new way of capturing that measurement.
Their test involves a series of letters printed with black ink on white paper. There are six letters in each line, and as they march down the page, they grow progressively lighter. Each line has the same ink level, but the line below it is slightly lighter.
Doctors ask their patients to read out the letters they can see. Once people can no longer read two letters in a line, the test is complete. The score is based on how faint the letters are when people can no longer read them.
Doctors don't have to guess about contrast sensitivity with this test. A person with a healthy eye has a score of 2.0. A rating of 1.5 or less is consistent with poor vision, researchers say.
There are at-home versions of the test, including some you can take on your computer. Unfortunately, research suggests that viewing the test on a computer screen leads to skewed results. It's best to take this test at a doctor's office with a printed scorecard to get accurate measurements.
What Diseases Play a Role?
Contrast sensitivity issues are often sparked by disease. A low score on this test could be the first sign your doctor has that your eye health isn't optimal and needs help. That's why speaking up about poor vision and asking for testing is so critical.
The American Optometric Association says low contrast sensitivity can be caused by:
- Glaucoma. Someone with this eye disease may have good visual acuity, but contrast details may be hard to discern. There may be differences in ability between one eye and another too.
- Diabetes. This disease can damage the eye's blood vessels, and that can lead to poor contrast sensitivity.
- Cataracts. Clouding of the eye's lens can make plucking items from their backgrounds very difficult.
- Age. Growing older isn't exactly a disease, but it can change the way your eyes work. Some of those shifts will impair your contrast sensitivity.
A low score could prompt your doctor to perform additional tests. A pressure test could detect glaucoma, for example, while a blood test could highlight the presence of diabetes. These conditions are treatable, especially if caught early, so the Pelli-Robson test could help to save your vision.
If you've already had eye surgery, the Pelli-Robson test could be used to address your recovery. For example, if you've had surgery to replace a lens clouded by cataracts, your doctor might compare your score before surgery to the results after surgery. If the numbers aren't moving in the right direction, your doctor can make an adjustment to help you see clearly.
How to Prepare for the Test
The Pelli-Robson test is completely noninvasive. There are no needles, knives, or pins to worry about. It's also a very quick test, so it should be over before you know it. Preparing for it is mainly a mental exercise.
It's tempting to attempt to perform in front of our doctors. We want them to be impressed by our performance. In tests like the Pelli-Robson test, that tendency can backfire.
Your scores are dependent on your experience during the test. You tell your doctor what you can see, and the scores move accordingly. If you lie, either by guessing at things you can't see or claiming you can't pick out the letters you can, you're depriving your doctor of critical information.
Be as honest as you can throughout the testing period. Don't make anything up, and don't downplay anything. Work closely with your doctor, and together, you can find a way to make your vision better than it ever was before.
The Design of a New Letter Chart for Measuring Contrast Sensitivity. (1988). Clinical Vision Sciences.
Quality of Vision in Refractive and Cataract Surgery, Indirect Measurers: Review Article. (December 2013). Arquivos Brasileiros de Oftalmologia.
How Accurate is an LCD Screen Version of the Pelli-Robson Test? (August 2018). International Ophthalmology.