Dark Adaptation

DARK ADAPTATION

Definition

Dark adaption is the process by which the person is able to see the objects in dim light. If a person enters a dim-lighted room (darkroom) from a bright-lighted area, he is blind for some time, i.e. he cannot see any object. After sometime his eyes get adapted and he starts seeing the objects slowly. Maximum duration for dark adaptation is about 20 minutes.

Causes for Dark Adaptation

Dark adaptation is due to the following changes in eyeball:

1. Increased sensitivity of rods as a result of resynthesis of rhodopsin Time required for dark adaptation is partly determined by the time for resynthesis of rhodopsin. In bright light, most of the pigment molecules are bleached (broken down). But in dim light, it requires some time for regeneration of certain amount of rhodopsin, which is necessary for optimal rod function adaptation occurs in cones also.

2. Dilatation of pupil

Dilatation of pupil during dark adaptation allows more and more light to enter the eye. Radiologists, aircraft pilots and others, who need maximal visual sensitivity in dim light, wear red glass before entering dim-lighted area, because red light of spectrum stimulates the rods slightly while the cones are allowed to function well. Thus, the person wearing red goggles can see well in bright-lighted area and also can see the objects clearly, as soon as he enters the dim-lighted area.

Dark Adaptation Curve

Dart adaptation curve is the curve that demonstrates the relationship between threshold of light stimulus

(illumination) and time spent in dark.

Procedure

Experiment to obtain dark adaptation curve is done in a completely dark room. First, the subject is exposed to a bright light in order to bleach (breakdown) most of the photopigment in retina. The subject looks directly at a bright flashing light with a wavelength of 420 nm against dark background for about 5 to 7 minutes. Then the bright light is switched off and the subject is in dark. Now a small dim light (stimulus) is produced. Immediately, the absolute threshold (minimum strength of stimulus; minimum intensity of light stimulus) for detecting this dim light is determined by adjusting the intensity of light (illumination). Time interval between the switching off bright light and detection of dim light is noted. After a short time, absolute threshold is measured again and elapsed time is noted. This procedure is repeated for about 30 minutes.

When the experiment is completed, results are plotted and the dark adaptation curve is obtained.

Parts of dark adaptation curve

Dark adaptation curve is biphasic. First part of the curve represents threshold of photopic vision, which

indicates the cone adaptation. Second part of the curve represents threshold of scotopic vision, which

indicates the rod adaptation.

Cone adaptation

This first phase is rapid and it is completed in 8 to 10 minutes. During this period the threshold decreases

by 2 to 3 log units. That is the sensitivity of the eye in dark room increases by 1,000 times within 8 to 10

minutes. By this time, the cones get adapted.

Rod-cone break

After the first phase, there is a sudden change in slope of the curve and this point of curve is called rod-cone break. Rod-cone break represents the point where rod sensitivity begins to exceed cone sensitivity and the remaining part of the curve is determined by the continuing adaptation of rods. During this phase the threshold decreases further by 5 to 6 log units. That is the sensitivity of eye in dark room increases by 100,000 to 1,000,000 times within 20 to 30 minutes. By this time, rods get adapted completely.