The primary process responsible for dark adaptation in the human eye is the conversion of retinal to rhodopsin in rods.

- When we move from a brightly lit environment to a dark one, our eyes need to adjust to the lower levels of light so that we can see better.
- This adaptation mainly involves the rods, which are photoreceptor cells specialized for vision in dim light.

- Rhodopsin, also known as visual purple, is a photopigment found in the rods.
- It is composed of a protein called opsin and a light-sensitive molecule called retinal (derived from vitamin A).
- When exposed to light, rhodopsin breaks down into opsin and retinal, initiating the phototransduction process that allows us to perceive light.
- In darkness, the retinal is converted back into rhodopsin, restoring the rods’ sensitivity to light.
- This regeneration of rhodopsin increases the rods' ability to detect low-light stimuli, enabling improved vision in the dark.

- Although cones are responsible for color vision and function better in bright light, their photopigments (such as iodopsin) do not play a significant role in dark adaptation.
- Additionally, while the dilation of the pupil does increase the amount of light entering the eye, it is only part of the initial adjustment and less critical compared to the biochemical regeneration of rhodopsin.

Key points:
- Dark adaptation primarily occurs due to rhodopsin regeneration in rods.
- Conversion of retinal back to rhodopsin increases rod sensitivity.
- Cones and their pigments are less involved in dark adaptation.
- Pupil dilation assists but is not the main mechanism.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Vision and Eye, Chapter 53