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The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.
Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons. In higher organisms, the eye is a complex optical system which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system. Image-resolving eyes are present in molluscs, chordates and arthropods.
Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night vision compared to many animals such as cats, in part because the human eye lacks a tapetum lucidum, tissue behind the retina that reflects light back through the retina thus increasing the light available to the photoreceptors.
The macula (/ˈmakjʊlə/) or macula lutea is an oval-shaped pigmented area in the center of the retina of the human eye and in other animals. The macula in humans has a diameter of around 5.5 mm (0.22 in) and is subdivided into the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas.
Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal acuity using corrective lenses is still considered "naked".
Peripheral vision, or indirect vision, is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in the "corner of one's eye". The vast majority of the area in the visual field is included in the notion of peripheral vision. "Far peripheral" vision refers to the area at the edges of the visual field, "mid-peripheral" vision refers to medium eccentricities, and "near-peripheral", sometimes referred to as "para-central" vision, exists adjacent to the center of gaze.
The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.
Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.
Visual acuity (VA) commonly refers to the clarity of vision, but technically rates an examinee's ability to recognize small details with precision. Visual acuity is dependent on optical and neural factors, i.e. (1) the sharpness of the retinal image within the eye, (2) the health and functioning of the retina, and (3) the sensitivity of the interpretative faculty of the brain. The most commonly referred visual acuity is the far acuity, which describes the examinee's ability to recognize small details at a far distance, and is relevant to people with myopia; however, for people with hyperopia, the near acuity is used instead to describe the examinee's ability to recognize small details at a near distance.
The fovea centralis is a small, central pit composed of closely packed cones in the eye. It is located in the center of the macula lutea of the retina.
A deep-sky object (DSO) is any astronomical object that is not an individual star or Solar System object. The classification is used for the most part by amateur astronomers to denote visually observed faint naked eye and telescopic objects such as star clusters, nebulae and galaxies. This distinction is practical and technical, implying a variety of instruments and techniques appropriate to observation, and does not distinguish the nature of the object itself.
In astronomy, limiting magnitude is the faintest apparent magnitude of a celestial body that is detectable or detected by a given instrument.
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Entoptic phenomena are visual effects whose source is within the human eye itself.
The Purkinje effect is the tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of dark adaptation. In consequence, reds will appear darker relative to other colors as light levels decrease. The effect is named after the Czech anatomist Jan Evangelista Purkyně. While the effect is often described from the perspective of the human eye, it is well established in a number of animals under the same name to describe the general shifting of spectral sensitivity due to pooling of rod and cone output signals as a part of dark/light adaptation.
In the study of human visual perception, scotopic vision is the vision of the eye under low-light conditions. The term comes from Greek skotos, meaning "darkness", and -opia, meaning "a condition of sight". In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm (blue-green) and are insensitive to wavelengths longer than about 640 nm (red-orange). This condition is called the Purkinje effect.
In astronomy, magnitude is a unitless measure of the brightness of an object in a defined passband, often in the visible or infrared spectrum, but sometimes across all wavelengths. An imprecise but systematic determination of the magnitude of objects was introduced in ancient times by Hipparchus.
Fixation or visual fixation is the maintaining of the gaze on a single location. An animal can exhibit visual fixation if it possess a fovea in the anatomy of their eye. The fovea is typically located at the center of the retina and is the point of clearest vision. The species in which fixational eye movement has been verified thus far include humans, primates, cats, rabbits, turtles, salamanders, and owls. Regular eye movement alternates between saccades and visual fixations, the notable exception being in smooth pursuit, controlled by a different neural substrate that appears to have developed for hunting prey. The term "fixation" can either be used to refer to the point in time and space of focus or the act of fixating. Fixation, in the act of fixating, is the point between any two saccades, during which the eyes are relatively stationary and virtually all visual input occurs. In the absence of retinal jitter, a laboratory condition known as retinal stabilization, perceptions tend to rapidly fade away. To maintain visibility, the nervous system carries out a procedure called fixational eye movement, which continuously stimulates neurons in the early visual areas of the brain responding to transient stimuli. There are three categories of fixational eye movement: microsaccades, ocular drifts, and ocular microtremor. Although the existence of these movements has been known since the 1950s, only recently have their functions become clear.
Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups; a pigeon has been described as "two eyes with wings". Birds are theropod dinosaurs, and the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket. In addition to the two eyelids usually found in vertebrates, bird's eyes are protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.
Mammals normally have a pair of eyes. Although mammalian vision is not so excellent as bird vision, it is at least dichromatic for most of mammalian species, with certain families possessing a trichromatic color perception.
References
- ↑ Berman, Bob, “Tormented by open clusters”, Astronomy, March 2019, p. 12.
- ↑ Alexander, RG; Mintz, RJ; Custodio, PJ; Macknik, SL; Vaziri, A; Venkatakrishnan, A; Gindina, S; Martinez-Conde, S (2021). "Gaze mechanisms enabling the detection of faint stars in the night sky". European Journal of Neuroscience. 54 (4): 5357–5367. doi:10.1111/ejn.15335. PMC 8389526 . PMID 34160864.
- ↑ M41 possibly recorded by Aristotle
- ↑ Dyer, JL; Mittelman, MH (1998). "Evaluation of an unaided night vision instructional program for ground forces". Military Psychology. 10 (3): 159–172. doi:10.1207/s15327876mp1003_2.
- ↑ Azevedo, FS; Mann, MJ (2016). Embodied Cognition in Observational Amateur Astronomy. International Conference of the Learning Sciences. International Society of the Learning Sciences.
- ↑ Griffiths, M (2012). Planetary Nebulae and How to Observe Them. Springer Science & Business Media. p. 33.
- ↑ Wells-Gray, E. M.; Choi, S. S.; Bries, A.; Doble, N. (2016). "Variation in rod and cone density from the fovea to the mid-periphery in healthy human retinas using adaptive optics scanning laser ophthalmoscopy". Eye. 30 (8): 1135–1143. doi:10.1038/eye.2016.107. PMC 4985666 . PMID 27229708.
- ↑ Alexander, RG; Mintz, RJ; Custodio, PJ; Macknik, SL; Vaziri, A; Venkatakrishnan, A; Gindina, S; Martinez-Conde, S (2021). "Gaze mechanisms enabling the detection of faint stars in the night sky". European Journal of Neuroscience. 54 (4): 5357–5367. doi:10.1111/ejn.15335. PMC 8389526 . PMID 34160864.