Visual Sensitivity

The range from scotopic (rod) absolute threshold to a light level that can possibly cause damage covers a luminance range of about 14 log units. This is a range of 1: 100,000,000,000,000. At the lower end of this range the visual system trades color perception and good visual acuity for very high sensitivity to low light levels.

Photopic (cone) threshold is almost 4 log units above rod threshold. Approximately the next 2 log units is called the mesopic range and it is here that both the rods and cones contribute to vision. The reader will notice that just inside the photopic range rod saturation begins. Rod saturation refers to the rods output not increasing as luminance increases. They are already responding as vigorously as they can.

The beginning of color vision occurs in the mesopic range because of cone stimulation. Color vision reaches its best in the photopic region as does visual acuity. However, as luminance continues to increase, to very high levels, visual performance deteriorates. When the light energy is high enough it can cause retinal damage.

From this description it would appear that the sensitivity differences between rods and cones explains the entire 14 log unit range of visual sensitivity. The time course of receptor sensitivity increasing in darkness and the role of the photopigments is understood. There is more to the story. Although we do not yet fully understand the whole story, more of it can be found in more on sensitivity.