Explain the Dark Adaptation of Vitamin A?

In the early stages of VAD, the individual cannot see objects in dim light. This' phenomenon is used as a criterion for assessment in the dark adaptation test. The subject is either kept in a dark room for some time and asked to identify an object which is dimly illuminated, the intensity of light being increased till the subject is able to see the object (or) he is exposed to bright light for some time, by which the visual purple is bleached. The time required for the rhodopsin to be regenerated is measured by the ability to see a dimly illuminated object. Longer time taken to identify the objects is indicative of variable stages of vitamin A deficiency.