Mesopic vision occurs at intermediate levels of luminance, where rods and cones are believed to work together. By definition, sensitivity has a reciprocal relationship to threshold. That is, This results in the threshold curve being inverted, as on the right side of the figure , and allows one to speak in terms of sensitivity, which is a more meaningful term to most individuals. The region of maximal sensitivity is 555 nm for cone vision and 510 nm for rod vision. Visual stimulation at scotopic levels does not produce color vision anywhere along the spectrum.
Color vision is possible only with light levels of sufficient magnitude to activate the cone system.
When only the rods are functioning, all wavelengths are seen as a series of lighter or darker grays. Weak light is visible, but hue is absent. They are shown in the figure below. The relative sensitivity curves are used for photometric problems to represent the response of a standard human observer. The Purkinje effect is a shift in the maximum sensitivity of the eye from photopic to scotopic vision.