Mars Letter Contrast Sensitivity Test

Frequently Asked Questions

How long does the Mars test take to administer and score? About a minute per eye. Once you hold the chart in front of them, patients immediately comprehend the task and spontaneously read the letters. The test continues until two consecutive errors have been made. The CS score is the value of the final correct letter, minus 0.04 for each incorrect letter prior to the two final consecutive errors.

Do you have a chart that can guide me in interpretation of Mars test scores? Yes, here is a chart you can download. It contains approximate norms for test scores, gleaned from the literature and from the experience of regular users of the test, including Drs. Eleanor E. Faye and Bruce Rosenthal.

When patients get to the lowest contrast letters they can read, should I encourage them to guess? It is important to encourage your patients to guess even after they say they can no longer read letters, so as to minimize the impact of nonvisual factors, such as personality. For example, patients who tend to be cautious will be reluctant to read letters that appear very faint to them. Without encouraging them to guess, such patients' scores might be lower than would be expected purely on visual factors. Note that encouraging patients to guess is good practice for acuity testing as well!

Do you recommend scoring procedures other than those described in the user manual? We absolutely do not recommend scoring the test in any way other than that described in the test's User Manual. The scoring procedure is designed for simplicity and accuracy. The scoring algorithm gives credit "letter-by-letter", and is extremely simple to use. We also do not recommend accepting "C" for "O" (or vice versa or any other ad hoc modifications), because such procedural changes alter the guessing probability from 1/10 in a complicated way that can also complicate score interpretation. Note that because Cs and Os are relatively infrequent, scoring will be affected only when the contrast threshold is close to one of these particular letters. Accepting C for O and vice versa as correct responses, makes Os and Cs on the chart easier than the other Sloan letters because for these letters alone, you now have two chances in ten to get it correct on the basis of guessing. It's well known that there are small differences in identifiability even between Sloan letters (which were originally designed for equal legibility), but that is probably true for any optotype symbol set. Another important reason to stick with the User Manual scoring procedure is to insure that test scores are comparable from clinic to clinic, research lab to research lab.

How does the Mars Letter Contrast Sensitivity Test differ from the Pelli-Robson Contrast Sensitivity Chart? The Pelli-Robson uses triplets of letters at each contrast, and these decline in 0.15 log unit steps. The Mars test declines 0.04 log units in contrast from letter to letter. The scoring of the tests is different, as well. These differences result in the Mars test's increase in accuracy of 28%. There are major physical differences as well. The Pelli-Robson is a large wall chart intended for viewing at 100 cm. The Mars test is handheld, and intended for 50 cm viewing distance. You can read more about technical differences between the tests here.

What are the similarities between the Mars test and the Pelli-Robson? Both tests have 8 lines of 6 letters declining in contrast across and down the chart. Both cover roughly the same contrast sensitivity range. Both use the common Sloan letter optotypes used in the ETDRS acuity charts and many other tests, and both measure peak contrast sensitivity. Because the letter stimuli are virtually identical in the two tests, and because of design features of the scoring system, the results you get with the Mars test can be compared to normative values collected with the Pelli-Robson.

What principles guided the design of the Mars Letter Contrast Sensitivity Test? Read about them in a paper recently published in Investigative Ophthalmology and Visual Science. You can download a copy of the paper here.

I am preparing a PowerPoint presentation (or book chapter) about contrast sensitivity testing and would like to include some images of the Mars Letter Contrast Sensitivity Test. Where can I get some? Look here.