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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.
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 will 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.
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.
There is 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.
What Is Contrast Sensitivity?
Contrast sensitivity, or CS, is the ability of your eyes to distinguish between the foreground and the background. When you have normal CS in your vision, you can recognize objects and details in low-light conditions, high-glare conditions or foggy situations.
A healthy contrast sensitivity level lets you enjoy better vision in various aspects, such as having a normal visual field. Your ability to detect motion and determine distance is within normal levels, too.
Your eyes can adapt to poor lit-condition and darkness. You can also easily recognize patterns, symbols and signs. Generally, reduced contrast sensitivity means having poor vision acuity.
Why Contrast Sensitivity Matters
Contrast sensitivity is an important measure of your visual acuity. It impacts your personal safety and your ability to enjoy life if you can’t recognize objects that are similar in color with their backgrounds.
Having low CS may prevent you from driving at night to avoid endangering your life, those of your passengers and those of other drivers. If you can’t distinguish traffic lights, you run the risk of committing moving violations.
When driving with low CS on a poorly lit street, you may have difficulty spotting pedestrians. If you don’t get treated for this condition or its cause, you may have to make do without comforts of life like enjoying TV shows or reading your favorite newspaper column.
Navigating spaces with stairs or curbs is similarly risky when you have poor contrast sensitivity.
The fact that this problem has potential underlying causes makes diagnosis and treatment very important. Some of its possible root causes are linked to permanent vision loss, including glaucoma, age-related macular degeneration, and cataracts.
If an eye disorder is the cause of your low contrast sensitivity, you must get it treated first to improve your vision.
Symptoms of Reduced Contrast Sensitivity
Even before taking a medical test, you may experience vision problems associated with low contrast sensitivity. These issues can impact your day-to-day life.
Typically, you’ll be unable to pick out objects with similar color to their backgrounds or environment. Common symptoms of reduced contrast sensitivity include:
- Poor night vision, with multiple issues, including inability to work out distances, read traffic signs and drive safely
- Difficulty reading a newspaper
- Eye strain when watching TV
- Inability to recognize faces or facial features
- Difficulty seeing steps
- Inability to spot tripping hazards such as potholes, curbs or rocks
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 do not 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.
Scoring: What is Normal Contrast Sensitivity?
Normal contrast sensitivity is when you have a score of 2.0. It means that you can clearly recognize things that have a similar hue as their backdrop.
If you score less than 1.5, you have moderate reduction in contrast sensitivity. In that case, you have some level of visual impairment.
If you score less than 1.0, you have some type of visual disability.
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 is not optimal and needs help. That is 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 is not 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 are not 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 are depriving your doctor of critical information.
Be as honest as you can throughout the testing period. Do not make anything up, and do not downplay anything. Work closely with your doctor, and together, you can find a way to make your vision better than it ever was before.
- SPARCS and Pelli-Robson Contrast Sensitivity Testing in Normal Controls and Patients with Cataract. (May 2014). Eye.
- 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.
- SPARCS and Pelli–Robson contrast sensitivity testing in normal controls and patients with cataract. (January 2017). Eye.