The contrast sensitivity calculator gives the Weber contrast, the Michelson contrast, the contrast ratio, the contrast sensitivity and its logarithm from the luminances of a target and its background and the threshold contrast of an observer.
How the calculation works
- Enter the luminance of the target and of the background.
- Enter the threshold contrast in percent.
- Read the contrasts, the sensitivity and the log value.
Example
A target of 100 cd/m² on a background of 20 cd/m² has a Weber contrast of 80 / 20 = 4.00 and a Michelson contrast of 80 / 120 = 66.7 %, a contrast ratio of 5.00. An observer with a threshold of 2.2 % has a contrast sensitivity of 100 / 2.2 = 45.5, a log contrast sensitivity of 1.66.
Why it matters
The calculator helps optometry students, vision scientists, display designers and accessibility specialists who work with contrast, and anyone who wants to link a measurement to the clinical scale.
Good to know
Clinical charts such as the Pelli-Robson chart use large letters of decreasing contrast, and the score is the lowest contrast level at which two out of three letters are read, expressed in log units. Other tests use sine-wave gratings of several spatial frequencies to give a contrast sensitivity function, which describes sensitivity across fine and coarse detail and is more informative than a single number. Contrast sensitivity decreases with age, with poor lighting and with glare, so the test conditions need to be standard. For display or sign design, accessibility guidelines use luminance ratios of at least 4.5 to 1 for text, which is far higher than the thresholds of a healthy eye, to allow for low vision and bad conditions.
Related tools
The hearing loss calculator treats another sensory measure, the visual acuity calculator converts acuity notations, the reading glasses calculator treats near vision and the keratometry calculator treats corneal power.