Other meanings of Night vision
Biology & Physiology
Night vision is the ability to see in low-light conditions, a capability that varies widely across the animal kingdom. It is achieved through a combination of physiological adaptations, including larger eyes, more sensitive photoreceptor cells, and reflective layers behind the retina. In humans, night vision is limited compared to many nocturnal animals, but it can be enhanced with technology such as image intensifiers and thermal imaging.
Night vision relies on the rod cells of the retina, which are highly sensitive to light but do not perceive color. In humans, rods contain the photopigment rhodopsin, which undergoes a chemical change when struck by photons, triggering a nerve signal. The sensitivity of rods is such that they can respond to a single photon, but their response is slow, and they are saturated in bright light, which is why it takes time to adapt to darkness.
Nocturnal animals often have enhanced night vision through adaptations such as a tapetum lucidum, a reflective layer behind the retina that reflects light back through the photoreceptors, increasing the chance of absorption. This layer is responsible for the eyeshine seen in cats and dogs. Some species, like owls, have large tubular eyes with a high density of rods, while others, like geckos, have evolved multifocal lenses to focus different wavelengths.
Human night vision is relatively poor compared to many animals. Under scotopic conditions (low light), humans rely on rods, but visual acuity drops to about 20/200, and color perception is lost. The fovea, responsible for sharp central vision, contains only cones, so in dim light, humans often use peripheral vision, where rods are more concentrated.
Night blindness (nyctalopia) is a condition where the eyes cannot adapt to low light, often due to vitamin A deficiency, which is essential for rhodopsin synthesis, or genetic disorders like retinitis pigmentosa. Adaptation to darkness takes about 30 minutes for full rod sensitivity, but exposure to bright light can reset this process.
Night vision devices (NVDs) enhance human vision in low light using image intensifier tubes that amplify ambient light, including near-infrared. These devices convert photons into electrons, accelerate them, and then strike a phosphor screen to produce a visible image. Generation 3 devices use gallium arsenide photocathodes for higher sensitivity.
Thermal imaging, another technology, detects infrared radiation emitted by objects, creating images based on temperature differences. Unlike image intensifiers, thermal cameras work in total darkness and can see through smoke and fog. These technologies are used in military, law enforcement, and wildlife observation, but they have limitations such as reduced field of view and the need for power.
Some animals have evolved remarkable night vision capabilities that go beyond simple rod sensitivity. For example, the nocturnal gecko has color vision in low light due to having three types of cones that are more sensitive than human cones. The elephantnose fish uses electric fields to navigate in murky waters, a form of 'active' sensing rather than visual.
In human history, night vision has been a subject of military interest since World War I, with early devices using infrared searchlights. The development of passive image intensifiers in the 1960s revolutionized night operations. Interestingly, some pilots have used 'scotopic vision' techniques, such as looking off-center to see dim objects, and the US Air Force has trained pilots to use 'night vision goggles' with specific scanning patterns to avoid 'empty field myopia'.
This article focuses on the biological and technological aspects of night vision, excluding the 2000 film of the same name.
Help improve the encyclopedia. Reports go straight to the site manager.