The spectral sensitivity of human vision has been measured in the near infra-red, in two areas of the dark adapted eye: the central fovea (cones) to 1000 mμ, and a peripheral area, in which the responses are primarily caused by rods, to 1050 mμ. In both cases the estimates of spectral sensitivity are based upon determinations of the visual thresholds for radiation passing through a series of infra-red filters. By successive approximation, sensitivity functions were chosen which were consistent with the observed thresholds.
The spectral sensitivity of the fovea determined in this way is consistent with previous measurements of Goodeve on the unfixated eye. At wave-lengths beyond 800 mμ the periphery becomes appreciably more sensitive than the fovea. This tendency increases at longer wave-lengths, so that at the longest wave-lengths studied, the radiation appeared colorless at the threshold and stimulated only rods.
Lengthening the exposure time increases the sensitivity of the peripheral retina relative to the fovea. Our measurements involved exposures of 1 second and fields subtending a visual angle of 1 degree. With shorter exposures or smaller fields the fovea is favored, so that under such circumstances the fovea may become more sensitive than the periphery well into the infra-red.
At 1050 mμ the sensitivity of the peripheral retina is only 3×10−13 times its maximum value at 505 mμ. A computation shows that by 1150 or 1200 mμ radiation should be more readily felt as heat by the skin than seen as light by the eye.
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