The ability of the eye to adjust to different levels of brightness (luminance) is known as adaptation. The process is performed by retinal sensory receptors and the pupil, which contracts or dilates according to the level of brightness.

The iris regulates the luminous flux like the aperture of a camera and at the same time improves the depth of field. In very bright light, the circular muscle of the iris contracts. Its central opening – the pupil – becomes smaller and less light reaches the retina. In low light, the pupil dilates to increase the incident luminous flux.

More light = better visual performance

The adaptive process and thus the time it takes the eye to adapt depend on the luminance at the beginning and end of any change in brightness. Our eyes can adapt from dark to light within seconds. Adapting to darkness takes considerably longer. When we leave a brightly lit workroom at night, our eyes need around 30 minutes to adapt to the darkness outdoors.

Where marked differences in brightness need to be processed in too short a space of time, vision is impaired. Our eyes must be given an opportunity to adapt to the change (adaptation zone).