Visibility of Rectangular Targets as a Function of Length and Width

Abstract

The human threshold contrast of circular targets has been thoroughly studied by Blackwell and his data has been used in a number of visibility prediction models. These models either assumed the difference in visibility between circular and non-circular targets to be negligible or they used some form of a target shape conversion method. The present research is an effort to quantify the effects of the target aspect-ratio (shorter dimension to longer dimension) on the human threshold contrast. Threshold contrast data from two published target detection studies were combined with data from a small laboratory experiment conducted at Ohio University using a regression model (adjusted R2=0.991). The results clearly suggest that small aspect-ratios should be used if, for a given target area, the rectangular target visibility must be maximized. Applications of this research may include retro-reflective conspicuity markings used for improved nighttime visibility of pedestrians and vehicles or reflectorized posts of crossbucks at passive railroad crossings.