Frequently Asked Questions (FAQ)

Less than a minute per eye. Once you hold the chart in front of them, patients immediately comprehend the task and spontaneously read the letters (or numerals). The test continues until two consecutive errors have been made. The CS score is the value at tthe final correct response, minus 0.04 for each incorrect response prior to the two final consecutive errors.

Keep in mind that because of binocular enhancement effects, expect an average of  0.15 higher scores under binocular testing conditions when the two eyes have similar contrast sensitivity.

It is very important to encourage your patients to guess even after they say they can no longer read the letters or numerals, 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 items 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!  

The Pelli-Robson uses triplets of letters at each contrast, and these decline in 0.15 log unit steps. The Mars test declines in much smaller contrast steps (0.04 log units) 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 40-50 cm viewing distance. You can read more about technical differences between the tests here.

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 will correspond closely (within about 0.01 log unit) to Pelli-Robson scores.  However, several different scoring variations are in use with the Pelli-Robson, and which one was used is not always known.  Because of that, and improvements in the scoring procedure developed for the Mars CS tests that result in 28% better accuracy, we believe that the Mars test scores provide a better standard.

Read about them in a paper published in Investigative Ophthalmology and Visual Science. You can download the paper, along with several independent evaluations of the test here.

Look on the Downloads page here under the section labeled “images.”

This is a placeholder tab content. It is important to have the necessary information in the block, but at this stage, it is just a placeholder to help you visualise how the content is displayed. Feel free to edit this with your actual content.Mars letters and numerals are large, subtending 2 degrees at a 50 cm test distance and 2.5 degrees at 40 cm.  These are equivalent to 20/480 (6/144) and 20/600 (6/180), respectively. The large size ensures that even patients with low vision can see them. If necessary, for patients with lower visual acuity than this, the test can be administered at closer distances as well.

Research has consistently shown that large letters are identified by the most sensitive of the visual system’s size-tuned spatial frequency channels. The Mars tests accordingly yield the sensitivity of the most sensitive channel, and therefore the peak sensitivity of the visual system as a whole. Mars values correspond most closely to grating contrast senstivity at low spatial frequencies, about 0.5-2.0 cycles/degree.