What is Contrast Sensitivity?
Contrast sensitivity is the ability to distinguish between an object and the background behind it. This differs from visual acuity, which measures your vision's clarity at a distance. You can have excellent visual acuity but reduced contrast sensitivity and vice versa.
Contrast sensitivity also allows you to distinguish between the foreground and background. It's an important aspect of your visual function, especially in low-light situations. For instance, foggy and high-glare situations require good contrast sensitivity for clear vision.
Decreased contrast sensitivity can be a symptom of a refractive error and other eye conditions, such as:
Why is Good Contrast Sensitivity Important?
Contrast sensitivity is an essential measure of visual function. It’s necessary to have good contrast sensitivity for different reasons, such as:
Good contrast sensitivity helps ensure safety when driving in low-vision conditions, such as:
- Low light
Contrast sensitivity helps you see road signs, pedestrians, curves, and the difference between the street and sidewalks. A contrast sensitivity deficit significantly increases your chance of getting in an accident.
If you have difficulty reading small print, good contrast sensitivity will help you read faster and more easily. This is useful when reading important information.
For instance, good contrast sensitivity allows you to distinguish words on documents easily. It reduces any errors or confusion when reading text messages or emails.
Poor contrast sensitivity makes it harder to detect objects around you. You might miss things like people walking behind you, cars approaching from the side, or animals crossing the road.
You could be injured if you fail to notice these dangers. For example, if you cannot detect obstacles in front of you, you could walk into them and get hurt.
What Situations Require Contrast Sensitivity?
Common situations that require contrast sensitivity include:
- Driving at night, in the rain, or in fog
- Locating objects against a similarly colored background
- Reading material with poor contrast, such as a newspaper
- Stepping off curbs or steps
- Distinguishing facial features of others
Symptoms of Reduced Contrast Sensitivity
It’s essential to have contrast sensitivity in many low-contrast situations.
People with reduced contrast sensitivity may experience:
- Poor vision while driving at night
- Difficulty seeing curbs and steps
- Eye strain from watching TV or reading
- Images that appear washed out
- Inability to identify the food on their plate
Contrast Sensitivity vs. Visual Acuity
You may confuse contrast sensitivity with visual acuity. Both terms refer to how well you can see details. However, contrast sensitivity measures two variables, size and contrast, while acuity measures only size.
Testing for contrast sensitivity and visual acuity is also different. The test measures how well you can distinguish between light and dark. Your doctor will use a chart where the characters gradually fade from black to gray.
Visual acuity is measured when you read the eye chart during an exam. This is considered a high-contrast test (black letters on a white background).
How is Contrast Sensitivity Measured?
Contrast sensitivity is measured in units called log contrast sensitivity units. The lower the score, the poorer the contrast sensitivity:8
- A score of 2.0 indicates normal contrast sensitivity
- A score of 1.52 to 1.76 for individuals over 60 years of age
- A score of 1.72 to 1.92 for individuals younger than 60 years
How Do You Measure Contrast Sensitivity?
There are two main ways to check and measure contrast sensitivity. These include:
Pelli-Robson Contrast Sensitivity Chart
The most common way to check for a contrast sensitivity problem is using a Pelli-Robson contrast sensitivity chart.
The chart features horizontal lines of uppercase letters of the same size. This vision test is usually performed while wearing your eyeglasses or contact lenses (if needed).
As you go from left to right, the contrast of each letter will decrease. The person starts from the top of the chart and reads each row until they can no longer see any letters against the white background.
The Mars Letter contrast sensitivity test uses a similar chart as the Pelli-Robson test, except that the chart is smaller and viewed closer.
A more sophisticated way of taking contrast sensitivity measurements uses sine-wave grating targets.
Sine-wave gratings look like fuzzy lines of alternating light and dark shades of grey. Sometimes the lines are thicker, and sometimes they are thinner. The person views multiple gratings on a computer screen or a wall chart.
This information tells the doctor how well you can see under low, medium, and high-contrast situations.
Your doctor can then plot a graph of these results to determine your contrast sensitivity function (CSF). CSF is more informative than visual acuity in determining an individual's spatial vision abilities.
Factors That Affect Contrast Sensitivity
Many factors contribute to the loss of spatial contrast sensitivity, including:
Cataracts are a common age-related eye condition. They occur when the natural lens inside your eye becomes cloudy and yellow.
Many people with cataracts have good visual acuity but poor-quality vision due to decreased contrast sensitivity. Cataract symptoms worsen when you experience glare, such as oncoming car headlights when driving at night.
Glaucoma is a progressive eye disease that damages your optic nerves.
Your optic nerves transmit visual signals to your brain, including information on color, brightness, and contrast. This is why damage to your optic nerves can affect contrast sensitivity.
Besides glaucoma, other diseases that can damage your optic nerves include:
- Multiple sclerosis
- Poor blood flow to the optic nerves
The macula is a small area in the retina, which is the tissue lining the back of your eye. Your macula is responsible for seeing fine details, such as reading text in a book.
Macular degeneration is an age-related disease that damages the macula and causes vision loss. Patients may lose visual acuity, color vision, and contrast sensitivity. However, most people retain normal peripheral vision.
Diabetic retinopathy involves damage and leaking of the blood vessels in the retina. This condition can cause a significant loss of contrast sensitivity. Diabetic people without retinopathy may also have some decrease in contrast sensitivity.
Any eye injury, especially one that damages your optic nerve, can potentially affect your contrast sensitivity. Injuries are often caused by trauma, such as being hit in the head.
Laser Eye Surgery (LASIK and PRK)
However, studies show that customized laser eye surgeries (such as wavefront-guided or wavefront-optimized LASIK and PRK) deliver better contrast sensitivity results versus conventional procedures.
How to Improve Contrast Sensitivity
Here are some ways to improve contrast sensitivity:
Treating an underlying disease that causes poor contrast sensitivity can improve your vision.
Professional treatments may include:
- Cataract surgery
- Medicated eye drops that lower your eye pressure, such as brimonidine
- Eye injections, such as aflibercept
Besides treating the underlying disease, wearing corrective lenses can improve contrast sensitivity. Schedule an eye exam to ensure you wear the correct eyeglass prescription.
Wearing glasses with high-quality lenses can make a big difference. Some may even see better in low-light conditions when wearing lenses that include an anti-reflective coating.
Yellow or copper-tinted lenses enhance contrast and work in low-light environments. They increase sharpness perception during the day since they reduce glare.
One study showed that young participants respond significantly more rapidly to hazards when wearing a yellow filter during a driving hazard perception task.8
However, wearing yellow filter lenses while driving at night is not recommended because they provide no additional benefits.
Contrast sensitivity refers to how well you can perceive objects against their backgrounds. It affects your ability to drive safely, read, and perform other tasks. Poor contrast sensitivity can lead to serious accidents.
To improve contrast sensitivity, schedule an appointment with your optometrist. They will perform tests to determine whether there is a problem. Wearing corrective eyewear and yellow-tinted glasses can also help.
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