darkadapted

Darkadapted describes eyes or organisms that have adjusted to darkness, relying primarily on scotopic vision. It is achieved through a process called dark adaptation, during which photoreceptors regenerate visual pigments and the retina adjusts its neural processing and pupil size to improve sensitivity in low light.

During dark adaptation, cones contribute first, then rods take over as rhodopsin regenerates in rod photoreceptors. Rods reach peak sensitivity after about 20 to 40 minutes, though some improvement can continue for over an hour in some individuals. The resulting dark-adapted state allows much lower light levels to be detected, albeit at the expense of color discrimination and fine detail.

Factors affecting the speed and extent of dark adaptation include age, exposure history, and ocular media clarity. Conditions such as cataracts, vitamin A deficiency, retinitis pigmentosa, diabetic retinopathy, or certain medications can blunt or delay dark adaptation. Bright lighting or recent exposure to intense light can reset the process, delaying return to a dark-adapted state.

Measurement of dark adaptation uses specialized tools called dark adaptometers or similar psychophysical tests, recording thresholds for light detection over time and producing an adaptation curve. These curves illustrate the transition from cone-dominated (photopic) sensitivity to rod-dominated (scotopic) sensitivity and the Purkinje shift, where rod vision is relatively more sensitive to blue-green wavelengths.

Applications include night navigation, aviation, astronomy, and safety in low-light environments. The term darkadapted is most commonly encountered in research and clinical contexts describing the state of vision after sustained darkness.