Information AboutAstigmatism |
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In Optics , astigmatism is when an optical system has different Foci for Rays that propagate in two perpendicular Planes . If an optical system with astigmatism is used to form an image of a Cross , the vertical and horizontal lines will be in sharp focus at two different distances. There are two distinct forms of astigmatism. The first is a third-order Aberration , which occurs for objects (or parts of objects) away from the Optical Axis . This form of aberration occurs even when the optical system is perfectly symmetrical. This is often referred to as a "monochromatic aberration", because it occurs even for light of a single Wavelength . This terminology may be misleading, however, as the ''amount'' of aberration can vary strongly with wavelength in an optical system. The second form of astigmatism occurs when the optical system is not symmetric about the optical axis. This may be by design (as in the case of a Cylindrical lens), or due to manufacturing error in the surfaces of the components or misalignment of the components. In this case, astigmatism is observed even for rays from on-axis object points. This form of astigmatism is extremely important in Ophthalmology , since the human Eye often exhibits this aberration due to imperfections in the shape of the Cornea or the Lens . THIRD-ORDER ASTIGMATISM In the analysis of this form of astigmatism, it is most common to consider rays from a given point on the object, which propagate in two special planes. The first plane is the ''tangential plane''. This is the plane which includes both the object point being considered and the axis of symmetry. Rays that propagate in this plane are called Tangential Ray s. Planes that include the optical axis are ''meridional'' planes. It is common to simplify problems in radially-symmetric optical systems by choosing object points in the vertical ("y") plane only. This plane is then sometimes referred to as ''the'' meridional plane. The second special plane is the ''sagittal plane''. This is defined as the plane, Orthogonal to the tangential plane, which contains the object point being considered and intersects the optical axis at the Entrance Pupil of the optical system. This plane contains the Chief Ray , but does not contain the optic axis. It is therefore a ''skew'' plane, in other words not a meridional plane. Rays propagating in this plane are called Sagittal Ray s. In third-order astigmatism, the sagittal and transverse rays form Foci at different distances along the optic axis. These foci are called the ''sagittal focus'' and the ''transverse focus'', respectively. In the presence of astigmatism, an off-axis point on the object is not sharply imaged by the optical system. Instead, sharp ''lines'' are formed at the sagittal and transverse foci. The image at the tangential focus is a short line, oriented in the direction of the ''sagittal'' plane. The image at the sagittal focus is a short line, oriented in the ''tangential'' direction. In between these two foci, a round but "blurry" image is formed. This is called the ''medial focus'' or ''circle of least confusion''. This plane often represents the best compromise image location in a system with astigmatism. The amount of aberration due to astigmatism is proportional to the Square of the angle between the rays from the object and the optical axis of the system. With care, an optical system can be designed to reduce or eliminate astigmatism. Such systems are called Anastigmat s. ASTIGMATISM IN SYSTEMS THAT ARE NOT ROTATIONALLY SYMMETRIC If an optical system is not axisymmetric, either due to an error in the shape of the optical surfaces or due to misalignment of the components, astigmatism can occur even for on-axis object points. This effect is often used deliberately in complex optical systems, especially certain types of Telescope . In the analysis of these systems, it is common to consider tangential rays (as defined above), and rays in a meridional plane (a plane containing the optic axis) perpendicular to the tangential plane. This plane is called either the ''sagittal meridional plane'' or, confusingly, just the ''sagittal plane''. Ophthalmic astigmatism See Also: Astigmatism (eye) In Ophthalmology , the vertical and horizontal planes are identified as ''tangential'' and ''sagittal'' meridians, respectively. Ophthalmic astigmatism is a Refraction Error of the Eye in which there is a difference in degree of Refraction in different meridians. It is typically characterized by an aspherical, non-figure of revolution Cornea in which the corneal profile Slope and refractive power in one meridian is greater than that of the perpendicular axis. Astigmatism causes difficulties in seeing fine detail. In some cases vertical lines and objects such as walls may appear to the patient to be leaning over like the Tower Of Pisa . Astigmatism can be often corrected by Glasses with a Lens that has different Radii Of Curvature in different planes (a ''cylindrical'' lens), Contact Lens es, or Refractive Surgery . Astigmatism is quite common. Studies have shown that about one in three people suffer from it The prevalence of astigmatism increases with age [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15838729&query_hl=26 . Although a person may not notice mild astigmatism, higher amounts of astigmatism may cause blurry vision, , Asthenopia , Fatigue , or Headaches [http://www.medicinenet.com/astigmatism/article.htm {Link without Title} . There are a number of tests used by or Retinoscopy may provide an objective estimate of the eye's refractive error and the use of Jackson Cross Cylinder s in a Phoropter may be used to subjectively refine those measurements [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=3808608&dopt=Abstract An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to deterimine the astigmatic axis and power[http://www.quantumoptical.com/onlinecourses/nysso/brp/slide1.asp?courses=19 [http://www.nova.edu/hpd/otm/nbeo/refract1.htm]. Astigmatism due to misaligned or malformed lenses and mirrors Deliberate astigmatism in optical systems Compact Disc players use an astigmatic lens for focusing. When one axis is more in focus than the other, dot-like features on the disc project to oval shapes. The orientation of the oval indicates which axis is more in focus, and thus which direction the lens needs to move. A square arrangement of only four sensors can observe this bias and use it to bring the read lens to best focus, without being fooled by oblong pits or other features on the disc surface. SEE ALSO REFERENCES EXTERNAL LINKS |
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