Contrast sensitivity is the ability to difference slight changes in luminance between regions which are not separated by definite borders and is just as important as the ability to perceive sharp outlines of relatively small objects. Simply contrast sensitivity is the ability to differentiate relative darkness and brightness and the ability to see details, edges or borders of images. It generally express the quality of vision. It is a diagnostic procedure which is used to measure an individual’s ability to see objects or patterns against a background. This is used to assess visual function in people with visual impairments, such as those with cataracts, glaucoma, optic nerve disorders and other eye conditions.
These test are short time taking and non-invasive procedures that do not require the use of ocular medicine such as eye drops or other medications.
Contrast Sensitivity tests
There are so many tests available such as:
- The Pelli-Robson test
- Bailey-Lovie test
- Striped or sinusoidal tests
- Vision Contrast Test System (VCTS)
- Cambridge test
- Functional Acuity Contrast Test (F.A.C.T)
- MARS test
The Pelli-Robson contrast sensitivity test
The Pelli-Robson chart consists of letters of equal size, in sixteen group of 3 letters, with a reduce in brightness of 0.15 log units per three letter groups . It contain 6 letters in each row. The test is set one meter from the patient and the letter size is 4.9 × 4.9 cm and consists of eight rows of letters. The Pelli-Robson test is easy to use as it resembles the visual acuity measurement that most patients are familiar with, its fast and with good repeatability. The patient reads down the rows of letters until the lowest resolvable group of three is reached. The illumination of the chart is 85 cd/mm2
During the Pelli-Robson test, the patient is presented with a series of large, high-contrast letters on a white background, and is asked to read the letters as they are progressively made more difficult to see by reducing the contrast between the letters and the background. This test is typically performed in a well-lit room, and the patient is asked to wear their glasses or contact lenses if they normally use them.
Causes of Low Contrast Vision
Many ocular condition causes decrease in both visual acuity and contrast sensitivity but some of these conditions cause decrease in contrast while patient can have normal visual acuity
- Optic Neuritis
- Open Angle glaucoma
- Diabetic retinopathy
Factors affecting contrast sensitivity
- Refractive errors (Uncorrected)
- Age (decrease contrast sens. in increasing age)
- Contact lenses
- Refractive surgery (LASIK)
- Diabetic retinopathy.
- optic neruopathies
- pituitary adenoma.
- toxic chemicals
(All of these above mentioned conditions decrease the contrast sensitivity.)
Note: Only having visual acuity 6/6 or 20/20 doesn’t denotes for quality vision, for quality vision assessment it involves a large number of parameters, including:
- Visual Acuity
- Colour vision
- Visual field