Resolving power to narrow and extended light sources and its influence on legibility of the optotypes

The resolving power of the human visual system has been found to depend on the sizes of the light sources. The sources used in this experiment were two rectangular white fields separated by a vertical black bar. The minimal angle of resolution measured on five subjects decreased by about 10-20 times while the light sources were extended up to the effective range of spatial summation. For larger sources no significant changes of spatial resolution were observed. The experimental data revealed biphasic dependence of the minimal angle of resolution on the sizes of the light sources and permitted estimation of the effective range of spatial summation for 3-9 min arc. This range encompasses the angular diameters of typical optotypes recognized by normal observers (visual acuity greater than 1, optotypes smaller than 5 min arc), therefore we show how the spatial arrangement of the lines and strokes of optotypes determines their legibility.