Fig 4b the plot of these values in terms of frequency shows how closely their relation fits the function expressing a rectangular hyperbola, having an equation:
,
f being the given frequency and /, its complementary. (80)
7. Standard Conditions for Pure Cone Vision
In order to secure reliable conditions for complete color vision even in the normal observer, it is necessary to restrict the stimulus to the retinal cones, excluding the rods, which yield only achromatic colors. Pure cone vision can be secured by satisfying the following requirements.
A. Choice of Observers.—Recent investigations by Abney indicate that a considerable number of individuals possess rods in the center of the retina as well as in the periphery, so that before relying upon the restriction of the stimulus to a central field, the observer should be tested for the Purkinje phenomenon in central vision.
B. Size of Field.—The normal retina possesses no rods in an area slightly greater than three degrees in diameter, surrounding the intersection of the line of sight with the retina (73, 10). Consequently, in the case of an observer known to be normal in this respect, a field of three degrees, with fixation on the center of the field, insures pure cone vision at all intensities.
C. Intensity.—With all observers and all field sizes, pure cone vision is obtainable at intensities above approximately one hundred photons, provided the eye has not previously been exposed for a considerable time to a much lower intensity or assuming a condition of equilibrium adaptation to the given intensity level, which should be reached within ten minutes (75; 34). One hundred photons represents an external stimulus surface brightness of one hundred candles per square meter, used with a pupillary opening of one square millimeter, or equivalent conditions as regards retinal illumination.
IV. Physical Standards
