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.
Temporal sensitivity refers to our response to timing of visual signals.
Consider a light that is pulsing, such as the blinking cursor on a computer screen. It is easy to discern that the light is blinking because of the low frequency, or rate at which it flashes. Imagine gradually increasing the frequency. We say that our visual system has fused the flicker. CFF depends on a variety of physical and psychological factors.