Scripps Institution of Oceanography

SIO Reference 61-10. Binocular visual thresholds for targets of several sizes and occupying various positions in the visual field have been measured at photopic adaptation luminance. Circular targets of positive contrast were presented for 0.33 second, chosen as a duration typical of the dwell times used in normal visual search procedures. Four young male observers with normal vision were used as subjects for the experiment. The data, based on approximately 80,000 observations, exhibit the dependence of the visual contrast threshold upon target size and position. The findings of the study differ, as anticipated, from other data which relate to the case of very brief (10 millisecond) target flashes, and are believed more suitable than the latter for the construction of visibility lobes used in the prediction of sighting ranges. The roles of various factors such as retinal neuroanatomy and eye-movement have been suggested, and intercomparison between studies has been attempted in an effort to evaluate these factors as determinants of visual performance.