The legibility of displayed letters depends upon their size, or more accurately, their subtended visual angle at any viewing distance. Current design standards recommend letter heights in the range from 0.003 to 0.007 rad (10 to 24 mill of arc) for good viewing conditions, with 0.0015 rad (5 min) considered a lower limit based on normal visual acuity. A field study involving some 2000 measures for over 300 printed displays found a mean letter height of 0.0019 rad (7 min) at the limit of legibility, with over 90% legibility at 0.003 rad and virtually 100% at 0.007 radians.
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