Visual acuity as a function of luminance contrast of Landolt rings and adaptation luminance

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The purpose of this study is to investigate the visual acuity as a function of the luminance contrast of the test objects and the adaptation luminance. The test objects are Landolt rings with the luminance contrast from 3 to 92%. The observers, all with normal vision, judge the break orientations of the Landolt rings by guessing. Visual experiments are carried out according to the standard method of the visual acuity test. Results obtained are as follows.1. The resolvable visual angle θ can be expressed as a function of the luminance contrast C (%) and the adaptation luminance L (cd/m^2), θ=(1.6293 C^<-0.3234>(0.8034 C^<-0.1876>+L^<1/4>)^4)/L (minute)For practical use, following equation is derived as a simple and convenient formula to calculate the resolvable visual angle.θ=(1.7 C^<-1/3>(0.85 C^<-1/5>+L^<1/4>)^4)/L (minute)2. As the luminance contrast of the test objects can not be discriminated below a certain adaptation luminance, the visual acuity is not defined and the above equations do not hold below this luminance. This threshold luminance is expressed as a fuuction of the luminance contrast; i.e., L=2.974 C^<-1.7234> (cd/m^2)3. The visual acuity in the vertical and the horizontal directions are slightly higher than those in the oblique ones irrespective of the luminance contrast of the Landolt rings.
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