M-unit (print size)

The M-unit is a standardized unit used to specify the size of printed text or optotypes for visual acuity testing independent of testing distance. By definition, the detail of 1M print subtends a visual angle of 1 minute of arc (1/60 of a degree) at a viewing distance of 1 meter. The overall 1M letter or optotype subtends 5 minutes of arc at this same 1 meter viewing distance. Likewise, the detail of 2M print subtends 2 minutes of arc at 1 meter and the letter subtends 1o minutes of arc at 1 meter.