second absolute threshold for vision involves the minimum photon flux (photons per second per unit area). In this case the light covers a wide field over an extended period of time instead of being concentrated on one spot on the retina in a short burst. Knowing the pupil diameter and the wavelength of the light, the result can be described in terms of luminance (~0.000001 candela per square meter or 10−6 cd/m2) or retinal illuminance (~0.00002 Trolands). By including estimates for the probability of an average photon being absorbed by an average rod cell, the threshold stimulation for the rods is approximately one photon absorption per second per 5000 rods. In terms of total absolute power sensitivity, Denton and Pirenne in the Journal of Physiology in 1954 found that for diffuse, extended sources i.e. a relatively large (~ 45-degree wide source, as viewed by the subject) frosted glass aperture, and a long (5 second) observation and decision time, the human eye could begin to reliably distinguish the lit from unlit glass at a power level of approximately 7.6 × 10−14 watts / steradian-cm2 at the eye for green (510 nm) light. This power level was dependent on the wavelength of the light used according to the usual luminosity curve. For white light, the absolute sensitivity found was 5.9 × 10−14 watts / steradian-cm2. This base sensitivity varied only about 0.03 log steps between monocular (one-eyed) or binocular (two eyed) vision. In 1972 Sakitt conducted an experiment that combined elements of signal detection and threshold theory. Two key elements of the study were a high tolerance for false positives and a multiple-choice option on deciding whether or not a light was seen. In the classic studies described above, the tolerance for false positives was so low that threshold was biased upward. Based on statistical analysis of a large number of trials, 6 photons each absorbed by one rod near-simultaneously looked "very bright," 5 photons looked "bright," 4 photons "a moderate light," 3 photons "a dim light." Two observers were able to see 2 photons as "slightly doubtful if a light was seen." One observer saw a single photon as "very doubtful if a light was seen." Zero photons were seen as "did not see anything." Hearing The absolute threshold of hearing is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no