You've probably noticed that when you enter a darkened area from a well lit area (e.g., going to a movie), that you have difficulty seeing at first, but after a while, you can see much more effectively. This change is called dark adaptation and refers to the process by which the eye becomes more sensitive to light under conditions of reduced illumination.
Research on dark adaptation helps us to better understand how rod and cone receptors work. As you may have already learned, cones are primarily responsible for resolving fine detail (acuity) and colour vision in good light, while rods allow us to see more effectively in dim lighting conditions. The recovery of sensitivity in dark adaptation involves the regeneration of photoreceptor photo-pigments as well as neural changes.
Rods and cones both play a role in dark adaptation. As you can see in the figure that follows below, cones adapt to dark conditions quickly over a short time, whereas rods adapt over a more extended time range. How do we know that rods and cones differ in the rate and extent of their adjustment to low light? One way is to measure cone sensitivity is to focus a small light source light on the cone-rich fovea. Similarly, we can measure changes in rod sensitivity by focusing light solely on rods, which are most densely packed well outside the fovea. Even better, we can measure rod sensitivity changes over time in rod monochromats who have a rare genetic abnormality in which the retina contains rods but few or no cones.
To watch the time course for the adaptation of cones and rods in dark conditions, click on the link below the graph. The animated version of the graph will follow adaptation over 5-minute intervals.
Some points to note about the dark adaptation function:
* Cone sensitivity adjustment lasts for about 4 to 5 minutes; rods increase their sensitivity for 20 to 30 minutes or so.
* At about 7 minutes rod sensitivity first matches that of cones, a point called the rod-cone break.
* The 30-minute time course for adaptation is about how long it takes for the Earth to shift from twilight to the darkness of the night.
* At the end of dark adaptation, the eye is about 100,000 times (i.e., 5 log units) more sensitive than it was at the beginning of the test, a level called dark adapted sensitivity.