ANSI Z80.23 pdf free download
ANSI Z80.23 pdf free download.Corneal Topography and Tomography Systems一 Standard Terminology, Requirements.
3 Terminology 3.1 corneal apex: The location on the corneal surface, of a normal cornea, where the mean of the local principal curvatures is greatest. 3.2 corneal eccentricity (e): The eccentricity (e) of the ellipse that best fits the corneal me- ridian of interest (see 3.9). If the meridian is not specified, the corneal eccentricity is that of the flattest corneal meridian (see Table 1 and Annex A). 3.3 corneal meridian (θ): The curve created by the intersection of corneal surface and a plane that contains the CT axis. A meridian is identified by the angle, θ, that the plane creating it makes to the horizontal as described by ISO 8429. The value of 0, for a full meridian, takes val- ues from 0 to 180 degrees. 3.3.1 corneal semi-meridian: The portion of a full meridian extending from the CT axis toward the periphery in one direction. The value of θ for a semi-meridian takes values from 0 to 360 de- grees. 3.4 corneal shape factor (日): A value that specifies the asphericity and type (prolate or ob- late) of conic section that best fits a corneal meridian. Unless otherwise specified, it refers to the meridian with least curvature (flattest meridian) (see Table 1 and Annex A). E=1-p NOTE – The negative of E is defined by ISO 101 10-12, Part 12: Aspheric surfaces, as the conic constant designated by symbol K. The negative of E has also been called asphericity and given the symbol Q. 3.5 corneal topographer: An instrument or system that measures features of the corneal surface of living human eyes in a noninvasive manner by using reflected images from the precor- neal tear film.
3.9.4 normal curvature: The curvature at a surface location of the curve created by the inter- section of the surface with any plane containing the local surface normal. 3.9.4.1 mean curvature: The arithmetic average of the principal curvatures at a surface location. 3.9.4.2 principal curvature: The maximum or minimum normal curvature at a surface location. 3.10 eccentricity (e): A value descriptive of a conic section and the rate of curvature change away from the apex of the curve, i.e., how quickly the curvature flattens or steepens away from the apex of the surface (see table 1). Eccentricity ranges from zero to positive infinity for the group of conic sections: Circle (e=0); elipse(0<e<1); parabola (e=1); and hyperbola (e>1). In order to signify use of an oblate curve of the ellipse, e is sometimes given a negative sign that is not used in computations. Otherwise, use of the prolate curve of the ellipse is assumed. 3.11 elevation: The distance between the corneal surface and a defined reference surface, measured in a defined direction from a specified position. 3.11.1 axial elevation: The elevation as measured from a selected point on the corneal surface in a direction parallel to the CT axis. 3.11.2 normal elevation: The elevation as measured from a selected point on the corneal sur- face in a direction along the normal to the corneal surface at the point. 3.11.3 reference normal elevation: The elevation as measured from a selected point on the corneal surface in a direction along the normal to the reference surface. 3.12 keratometric constant: The value 337 .5 used to convert corneal curvature from inverse millimeters (mm 1) to keratometric diopters.ANSI Z80.23 pdf download.
