Visual Thresholds for Line-Shaped Targets

Measurements of absolute and contrast thresholds values 7° nasal from the fovea for 6500 and 5250A for line-shaped targets 2′ in width, length l between 2′–256′ and time of observation t between 0.02-1 seconds are presented.The absolute thresholds agree with what was to be expected from the two-quanta theory, For l > 32′ for green flashes and for l > 8′ for red ones, N¯60 percent∾l12 and for t > 0.1 second, N¯60 percent∾t12. When t < 0.1 second or l < 32′ for green and l < 8′ for red light, Bunsen-Roscoe’s and Ricco’s law, respectively, is valid. Also, predictions for the behavior of the contrast thresholds for line-shaped sources are made which are also based on the quanta theories for these visual functions. The dependence of the contrast threshold 〈ΔN¯60 percent〉Av on l for large l for line-shaped targets is equal to the dependence of 〈ΔN¯60 percent〉Av on t and corresponds with the dependence of 〈ΔN¯60 percent〉Av on t and on the diameter d of circular flashes in the way predicted by theory.