An experimental determination was made of absolute visual thresholds in the human fovea. An apparatus permitting independent variation of area and exposure time, and having a fixation device that minimally affects the adaptation level of the region under investigation was used. Seven stimulus areas, ranging from 1 minute to 1 degree in diameter were presented to 4 subjects at durations of 1.1, 3.8, 10.0, and 48.0 milliseconds. The results show that all area-threshold functions depart from Ricco’s law. The functions which were obtained depart from linearity, while the degree of curvature varies among subjects. As exposure time is increased there is a regular related decrease in thresholds for all stimuli. This decrease in threshold is in good agreement with the Bunson-Roscoe law, which states that the product of time and intensity is a constant.
BEVERLY M. HILLMANN, "Relationship between Stimulus Size and Threshold Intensity in the Fovea Measured at Four Exposure Times," J. Opt. Soc. Am. 48, 422-428 (1958)