A dioptre, or diopter, is a non-SI unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (i.e. 1/metres). For example, a 3 dioptre lens brings parallel rays of light to focus at 1/3 metre. Cover of brochure The International System of Units. ... Measurement is the determination of the size or magnitude of something. ... Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... A lens. ... A mirror is a surface with good specular reflection that is smooth enough to form an image. ... The focal point F and focal length f of a positive lens, a negative lens, a concave mirror, and a convex mirror. ... The metre, or meter, is a measure of length. ...

Quantifying a lens in terms of its optical power rather than its focal length is useful because when relatively thin lenses are placed close together their powers approximately add. Thus a thin 2 dioptre lens placed close to a thin 0.5 dioptre lens yields almost the same focal length as a 2.5 dioptre lens would have. This approximation enables an optometrist to prescribe corrective lenses as a simple correction to the eye's optical power, rather than doing a detailed analysis of the entire optical system (the eye and the lens). Optometrists are primary care practitioners for vision and ocular health concerns. ... A corrective lens is a lens worn on or before the eye, used to treat myopia, hypermetropia, presbyopia and astigmatism. ...

Since optical power is approximately additive, it can also be used to adjust a basic prescription for reading, e.g. an optometrist, having determined that a myopic person requires a basic correction of, say, -2 dioptres to restore normal distance vision, might then make a further prescription of 'add 1' for reading, to make up for lack of accommodation (ability to alter focus). This is the same as saying that +1 dioptre lenses are prescribed for reading. The Accommodation Reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). ...

The power of the eye's lens and cornea together is very high, about 40 dioptres, and the normal accommodation range of a young person is a further 15 to 20 dioptres. Most of the power comes from the curvature of the front face of the cornea, while the accommodation derives from the soft internal lens being squashed by muscles. As we get older, accommodation reduces, to about 10 dioptres at age 25 and around 1 dioptre or less at 50 and over, hence the almost universal need for older people to use reading glasses. Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eyes optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. ...

Convex lenses have positive dioptric value and are generally used to correct hyperopia (farsightedness) or to allow people with the limited accommodation of advancing age presbyopia to read at close range. Concave lenses have negative dioptric value and generally correct myopia (nearsightedness). Typical glasses for moderate myopia will have a power of -1.00 to -3.00 dioptres, while over the counter reading glasses will be rated at +1.00 to +3.00 dioptres. Optometrists usually measure refractive error using lenses graded in steps of 0.25 dioptres. Convex lens converging light rays A convex lens, or converging lens, is a lens that is curved outward (convex): the ends are narrow and the middle is wide. ... This page meets Wikipedias criteria for speedy deletion. ... Presbyopia is the eyes diminished power of accommodation that occurs with aging. ... concave lens diverging light rays A concave lens is a lens with inward-curving (concave) surfaces: the ends are wide, the middle is thin. ... Normal vision for an achromatopsic colour-blind person. ... See separate articles for over-the-counter trading and the medical condition Ornithine Transcarbamylase Deficiency. ... Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. ... Refraction error, also known as refractive error, is an error in the focusing of light by the eye and a frequent reason for reduced visual acuity. ...

The dioptre can also be used as a measurement of curvature equal to the reciprocal of the radius measured in metres. For example, a circle with a radius of 1/2 metre has a curvature of 2 dioptres. If the curvature of a surface of a lens is C and the index of refraction is n, the focusing power is ɸ=(n-1)C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them. This will give approximately the right result, as long as the thickness of the lens is much less than the radius of curvature of one of the surfaces. For a mirror the focusing power is ɸ=2C. Curvature is the amount by which a geometric object deviates from being flat. ... RADIUS (Remote Authentication Dial In User Service) is an AAA (authentication, authorization and accounting) protocol for applications such as network access or IP mobility. ... A circle, in Euclidean geometry, is the set of all points at a fixed distance, called the radius, from a fixed point, the centre. ...

See also

• Astigmatism
• Eyeglasses prescription
• Hyperopia
• Myopia
• Optics
• Optometry
• Refraction error