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Positive Relative Accommodation
Positive relative accommodation (PRA) in biology, is a measure of the maximum ability to stimulate eye accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist during an eye examination using a phoropter. After the patient's distance correction is established, she or he is instructed to view small letters on a card 40 cm from the eyes. The examiner adds lenses in −0.25 diopter increments until the patient first reports that they become blurry. The total value of the lenses added to reach this point is the PRA value. High PRA values (>= 3.50 diopters) are considered to be diagnostic of disorders involving accommodative excess. Those with accommodative insufficiency typically have PRA values below −1.50 diopters. See also * Accommodation in fish *Adaptation (eye) *Amplitude of accommodation *Convergence insufficiency * Mandelbaum Effect *Negative relative accommodation Negative ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary information encoded in genes, which can be transmitted to future generations. Another major theme is evolution, which explains the unity and diversity of life. Energy processing is also important to life as it allows organisms to move, grow, and reproduce. Finally, all organisms are able to regulate their own internal environments. Biologists are able to study life at multiple levels of organization, from the molecular biology of a cell to the anatomy and physiology of plants and animals, and evolution of populations.Based on definition from: Hence, there are multiple subdisciplines within biology, each defined by the nature of their research questions and the tools that they use. Like other scientists, biologists use the sc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Diopter
A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curved mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ..., which is a physical quantity equal to the Multiplicative inverse, reciprocal of the focal length, expressed in metres. For example, a 3-dioptre lens brings parallel ray (optics), rays of light to focus at metre. A flat window has an optical power of zero dioptres, as it does not cause light to converge or diverge. Dioptres are also sometimes used for other reciprocals of distance, particularly Radius of curvature (optics), radii of curvature and the vergence (optics), vergence o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Convergence Insufficiency
Convergence insufficiency is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence. Symptoms The symptoms and signs associated with convergence insufficiency are related to prolonged, visually demanding, near-centered tasks. They may include, but are not limited to, diplopia (double vision), asthenopia (eye strain), transient blurred vision, difficulty sustaining near-visual function, abnormal fatigue, headache, and abnormal postural adaptation, among others. In some cases, difficulty with making eye contact have been noted as a complaint amongst those affected. Note that some Internet resources confuse convergence and divergence dysfunction, reversing them. Diagnosis Diagnosis of convergence insufficiency is made by an eye care professional skilled in binocular vision dysfunctions, such as an orthoptist, to rule out any organic disease. Convergence insufficiency is chara ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Amplitude Of Accommodation
The amplitude of accommodation is the maximum potential increase in optical power that an eye can achieve in adjusting its focus. It refers to a certain range of object distances for which the retinal image is as sharply focussed as possible. Amplitude of accommodation is measured during routine eye-examination. The closest that a normal eye can focus is typically about 10 cm for a child or young adult. Accommodation then decreases gradually with age, effectively finishing just after age fifty. The average amplitude of accommodation, in diopters, for a patient of a given age was estimated by Hofstetter in 1950 to be 18.5 - (0.30 * patient age in years) with the minimum amplitude of accommodation as 15 - (0.25 * age in years), and the maximum as 25 - (0.40 * age in years). However, Hofstetter's work was based on data from two early surveysDonders F. On the anomalies of the accommodation and refraction of the eye. (W. Moore Trans.) UK: The New Sydenham Society, 1864. p 207-2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Adaptation (eye)
In visual physiology, adaptation is the ability of the retina of the eye to adjust to various levels of light. Natural night vision, or scotopic vision, is the ability to see under low-light conditions. In humans, rod cells are exclusively responsible for night vision as cone cells are only able to function at higher illumination levels. Night vision is of lower quality than day vision because it is limited in resolution and colors cannot be discerned; only shades of gray are seen. In order for humans to transition from day to night vision they must undergo a dark adaptation period of up to two hours in which each eye adjusts from a high to a low luminescence "setting", increasing sensitivity hugely, by many orders of magnitude. This adaptation period is different between rod and cone cells and results from the regeneration of photopigments to increase retinal sensitivity. Light adaptation, in contrast, works very quickly, within seconds. Efficiency The human eye can functio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Accommodation In Fish
Vision is an important sensory system for most species of fish. Fish eyes are similar to the eyes of terrestrial vertebrates like birds and mammals, but have a more spherical lens. Birds and mammals (including humans) normally adjust focus by changing the shape of their lens, but fish normally adjust focus by moving the lens closer to or further from the retina. Fish retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some are sensitive to polarised light. Among jawless fishes, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots. The ancestors of modern hagfish, thought to be the protovertebrate, were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators, and where it is advantageous to have a convex eye-spot, which gathers more light than a flat or concave one. Fish vision shows evolutionary adapta ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Accommodative Insufficiency
Accommodative insufficiency (AI) involves the inability of the eye to focus properly on an object. Accommodation is the adjustment of the curvature of the lens to focus on objects near and far. In this condition, amplitude of accommodation of a person is lesser compared to physiological limits for his age. AI is generally considered separate from presbyopia, but mechanically both conditions represent a difficulty engaging the near vision system (accommodation) to see near objects clearly. Presbyopia is physiological insufficiency of accommodation due to age related changes in lens (decreased elasticity and increased hardness) and ciliary muscle power. AI is commonly present in people with convergence insufficiency. Categories Accommodative insufficiency is further categorised into different categories. Ill-sustained accommodation Ill-sustained accommodation is a type of accommodative insufficiency in which, range of accommodation will be normal, but after excessive near work acc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Human Eye
The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows humans to use visual information for various purposes including seeing things, keeping balance, and maintaining circadian rhythm. The eye can be considered as a living optical device. It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye (the sclera) and one of its inner layers (the pigmented choroid) keeping the eye essentially light tight except on the eye's optic axis. In order, along the optic axis, the optical components consist of a first lens (the cornea—the clear part of the eye) that accomplishes most of the focussing of light from the outside world; then an aperture (the pupil) in a diaphragm (the iris—the coloured part of the eye) that controls the amount of light entering the interior of the eye; then another lens (the crystalline lens) that accomplishes the remaining focussing of light into ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies. In this, distances vary for individuals from the far point—the maximum distance from the eye for which a clear image of an object can be seen, to the near point—the minimum distance for a clear image. Accommodation usually acts like a reflex, including part of the accommodation-vergence reflex, but it can also be consciously controlled. Mammals, birds and reptiles vary their eyes' optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 dioptres in the mean). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles. The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye. This dramatic change in focal power of the eye of approximately 15 dioptres (the reciprocal of focal length in metres) ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Centimeter
330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter. A centimetre (international spelling) or centimeter (American spelling) (SI symbol cm) is a Units of measurement, unit of length in the International System of Units (SI), equal to one hundredth of a metre, ''centi'' being the SI prefix for a factor of . The centimetre was the base unit of length in the now deprecated centimetre–gram–second (CGS) system of units. Though for many physical quantities, SI prefixes for factors of 103—like ''milli-'' and ''kilo-''—are often preferred by technicians, the centimetre remains a practical unit of length for many everyday measurements. A centimetre is approximately the width of the fingernail of an average adult person. Equivalence to other units of length : One millilitre is defined as one cubic centimetre, under the SI system of units. Other uses In ad ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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