Modulation thresholds have been measured for sine-wave gratings at various spatial frequencies. At low frequencies, the threshold is determined in part by the gradient between adjacent bright and dark bars. This is related to the effect of blurring the border between the bright and dark halves of a bipartite field; the results obtained with a sine-wave grating have been compared to the results obtained with a single border. An attempt has been made to isolate the roles played by optical blur and physiological spread of excitation and inhibition.
GLENN A. FRY, "Visibility of Sine-Wave Gratings," J. Opt. Soc. Am. 59, 610-617 (1969)