MultiBit is an app to perform the vision test, an evolution of the standard eye chart. This method, unlike the eye chart, also handles more special vision problems.
The patients can carry out the vision test themselves. Test results are automatically sent to the doctor.
Everybody is familiar with the visual acuity chart with its test letters of different sizes. This is the most common of all vision tests. The test has many advantages, not the least when fitting spectacles, but it also has several disadvantages. One disadvantage is the need for an examiner, who can check and record the patient's responses. An even greater disadvantage is that the acuity test is fairly insensitive to many non-optical causes of visual impairment, for example, age-related macula degeneration (AMD).
There are two reasons why acuity tests don't work very well with diseases of the visual system. One reason is that the test time isn't standardized, which affects the results. The MultiBit test uses a fixed and very short presentation time.
The second reason why acuity tests don't work very well with disease is that letters (and digits) may be legible even in the presence of visual impairment- they have a large ’margin of error‘. As shown in the figure, substantial parts of letters (or digits) can be removed without affecting legibility. For eyes with normal vision it actually suffices to outline the test symbol with a small number of dots. For eyes with abnormal vision, larger numbers of dots are needed. This is the principle of the MultiBit test: find the smallest number of dots required for recognition of the symbol.
Clearly, the size of the test dots must be important. Ideally, their size should match the size of the light receptors in the retina.
These have a diameter of a few thousands of a millimeter. By using similarly sized test dots, the test becomes sensitive to damage to single photoreceptors.
The combination of minuscule test dots and brief exposure times is named "rarebit" testing. This test principle was developed at the Sahlgren's University Hospital in Gothenburg, Sweden, in the 1990's and was made public in 2002. Initially, small numbers of rarebits were used but it was soon found useful to combine several rarebits to form digits. This test principle is called "multiple rarebits" or MultiBit for short. In contrast to the different sizes of symbols used in acuity charts, the MultiBit test uses a fixed symbol size and varies the number of rarebits. MultiBit uses digits rather than letters to keep the number of alternatives at a reasonable level.
The test task is the same as that of the ordinary acuity test: the patient calls out what he or she can see. The results are recorded in different ways, by the examiner in case of the ordinary test, and by the patient in the MultiBit test. In both cases, the final evaluation is made by the caregiver.
More in-depth information about the test principle can be found here