Retina Sensitivity-Dark and Light Adaptation

When retinal decreases in the retina,it is formed from Vitamin A,and when increases it is changed into Vitamin A.This interconverstion helps in long term adaptation of the retina to different light intensities.

In vitamin A deficiency dark adaptation is impaired,leading to "night blindness"in which light vision is defective.

Dark Adaptation:
When one passes from daylight into dark place,vision is at first very imperfect,but gradually it improves by increasing the sensitivity to light.This is called dark adaptation and is due to re-synthesis of receptor photopigments,together with change of vitamin aA into retinal.

Sensitivity in dark adaptation increases in two phases:
-Initial rapid but slight increase,which is completed within few minutes.
-Later slow but marked increased,which is completed within 20 minutes up to 1 hour.

When dark adaptation is complete,the retinal sensitivity to light increases more than 10 000 times.

Light Adaptation:
When one passes to light after prolonged darkness,dazzling occurs at first,but rapidly the sensitivity to light decreases which is called light adaptation.It is due to dissociation of photopigments into retinal and opsins and change of retinal in vitamin A.It needs 5 minutes to be complete.It can be prevented by wearing red glasses in the bright light.