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.
0 Comments