Absolute threshold

In neuroscience and psychophysics, an absolute threshold was originally defined as the lowest level of a stimulus – light, sound, touch, etc. – that an organism could detect. Under the influence of signal detection theory, absolute threshold has been redefined as the level at which a stimulus will be detected a specified percentage (often 50%) of the time. The absolute threshold can be influenced by several different factors, such as the subject's motivations and expectations, cognitive processes, and whether the subject is adapted to the stimulus.
The absolute threshold can be compared to the difference threshold, which is the measure of how different two stimuli must be for the subject to notice that they are not the same.

A landmark 1942 experiment by Hecht, Shlaer, and Pirenne assessed the absolute threshold for vision. They tried to measure the minimum number of photons the human eye can detect 60% of the time, using the following controls:

The researchers found that the emission of only 5-14 photons could elicit visual experience. However, only about half of these entered the retina, due to reflection (from the cornea), absorption, and other factors relating to transmittance of the ocular media. The researchers estimated that 5 to 14 of the estimated 500 rods in the test area would each absorb one photon, with a 4% chance that one rod would absorb two photons.

A 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 x 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 x 10⁻¹⁴ watts / steradian-cm². Interestingly, this base sensitivity varied only about 0.03 log steps between monocular (one-eyed) or binocular (two eyed) vision.

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