Contrast sensitivity test online: contrast vision chart
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What it measures
Contrast sensitivity is the ability to see an object that differs only slightly in lightness from its background. A visual acuity chart asks how small a letter can be at full contrast. A contrast chart keeps the letters large and asks how faint they can be.
The two are not interchangeable. A patient can read 10/10 on an acuity chart and still struggle with faces, kerbs and steps in dim light, because those tasks rely on contrast rather than fine detail. Contrast sensitivity often falls early in cataract, glaucoma, diabetic retinopathy, macular disease and optic neuropathy, sometimes before acuity changes.
The score is expressed in log contrast sensitivity (log CS), the logarithm of the inverse of the faintest contrast read. Black letters on white, about 100 % contrast, are log CS 0.00. Each step of 0.15 makes the letters about 30 % fainter. Log CS 1.50 is letters at about 3 % contrast.
How it is administered
The patient sits 1 metre from the screen, wearing distance correction, in normal room light. The screen brightness is set to maximum and left there, and glare on the screen is avoided.
One eye is tested at a time, the other covered, and both eyes can be tested together afterwards. The chart has seven rows of two groups of three letters. Each group is 0.15 log units fainter than the one before, from black at the top left to log CS 1.95 at the bottom right.
The patient reads the letters from the top, left to right, as far down as they can, guessing when unsure. The clinician taps the faintest group in which the patient read at least two of the three letters.
Reading the result
The log CS of that group is the score for the eye tested. The chart shows the logarithmic scale under every group, and the results list gives the score with the contrast of the letters, for each eye.
As a guide on the printed chart, adults usually score 1.65 or more. A score below 1.50 suggests reduced contrast sensitivity, and below 1.00 a marked loss that affects daily tasks. A difference of 0.30 between the eyes, or a fall of 0.30 since the last visit, is worth attention.
The report can be downloaded with the patient, the date, each eye's score, the chart with the scores ringed, the logarithmic contrast scale and space for notes.
When it is used
The test is used when a patient's complaints exceed what their acuity explains: glare, difficulty in dim light, trouble recognising faces. It is used to follow cataract before and after surgery, and in glaucoma, macular disease, diabetic retinopathy and optic neuritis, where contrast often changes before acuity. In low vision, it helps predict how a patient will cope with daily tasks and guides lighting and contrast enhancement.
Limitations
An ordinary 8-bit screen cannot display contrast below about 1 %, so this chart stops at log CS 1.95, where the printed Pelli-Robson chart reaches 2.25. Very good contrast sensitivity is therefore capped.
Screens differ in brightness, gamma and anti-glare coating, and ambient light changes what the patient sees. The contrast steps are calculated for a standard display. Compare results taken on the same screen, in the same room, rather than against printed-chart norms alone.
The viewing distance is not measured by the app. At distances other than 1 metre the letter size changes, which matters little for contrast but should be kept constant between visits.
The test is subjective and depends on the patient's cooperation and willingness to guess at faint letters. It does not replace a full examination, and is in beta: it has not been validated against the printed chart.
이 검사에 대한 질문
What does a contrast sensitivity test measure?
How faint a letter can be and still be read. Visual acuity measures the smallest letter at full contrast; contrast sensitivity measures the lowest contrast at a fixed, large size. The score is given in log contrast sensitivity (log CS): the higher the number, the fainter the letters the patient could read.
How is the contrast vision chart read?
At 1 metre, one eye at a time, with distance correction. The patient reads the letter groups from the top down. The score is the faintest group in which at least two of the three letters were read.
What is a normal contrast sensitivity score?
On the printed Pelli-Robson chart, adults usually score 1.65 log CS or more, and a score below 1.50 suggests reduced contrast sensitivity. Scores fall with age, and a screen-based chart is best compared with itself over time.