Fig. 1. Estimated sensitivity of rod and cone systems for different wavelengths of light. These curves are based on the results of various psychophysical experiments. The reader ought note greater overall sensitivity of the rod system and the difference in the maximal sensitivity (lowest threshold) as a function of wavelength for the two systems. The rod system is most sensitive to wavelengths of approximately 500 nm. The cone system is most sensitive to wavelengths of about 560 nm. (Geldard F: The Human Senses. 2nd ed. New York: Wiley, 1972.)