The three most important physical factors in the visibility of objects are size of visual angle of the detail to be discriminated, difference in coefficient of reflection between object and background and intensity of illumination. These factors are, of course, interacting in their effect on the eye. That is, changes in size will produce different changes in visibility with different relations of object to background and at different intensities of illumination. A visual angle of 1 minute, for example, represents a low visibility at a low intensity of illumination or with a small difference in coefficient of reflection between object and background, and a comparatively high visibility at a high illumination or with a large difference in coefficient of reflection between object and background. The visibilities for two values of visual angle may sustain one ratio at one intensity of illumination and relation to background and a
FERREE CE, RAND G. SIZE OF OBJECTS IN RELATION TO THEIR VISIBILITY AND TO THE RATING OF VISION. Arch Ophthalmol. 1930;4(1):37–72. doi:10.1001/archopht.1930.00810090045005
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