Night vision

Night vision is visual perception under low-light conditions, supported by adaptations that increase sensitivity to scarce photons at the cost of fine detail and color.

Rod cell: A retinal photoreceptor specialized for high sensitivity in dim light and largely absent from color vision. Rods provide most human vision after light levels fall.
Retina: The light-sensitive tissue lining the back of the eye, containing photoreceptors and neural circuits. Night vision begins with retinal detection and processing of dim light.
Nocturnality: An animal activity pattern in which most activity occurs at night. Night vision is one adaptation that helps nocturnal animals find food and avoid danger.
Color vision: Visual perception that distinguishes wavelengths of light as colors. Rod-dominated night vision sacrifices the color discrimination supplied by cones.
Rhodopsin: A light-sensitive pigment in rod cells, formed from opsin and retinal. Its activation lets rods respond to individual photons.
Cone cell: A retinal photoreceptor that supports color vision and high acuity in brighter conditions. Cones lose effectiveness sooner than rods as illumination declines.
Tapetum lucidum: A reflective layer behind the retina in many vertebrates that sends light through photoreceptors again. It boosts dim-light sensitivity in many nocturnal animals and causes eye shine.
Scotopic luminous efficiency function: A standardized curve describing the relative sensitivity of human rod vision to different wavelengths. Its blue-green peak explains why red objects appear especially dark at night.
Dark adaptation: The gradual increase in visual sensitivity after exposure to bright light ends. Rod sensitivity recovers over minutes as rhodopsin regenerates.
Visual acuity: The ability to distinguish fine spatial detail, commonly measured with standardized optotypes. Dim-light sensitivity comes with reduced sharpness because rods are less densely arranged centrally.

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