DEFINITIONS

LENSTECH STANDARD FOR READING GLASSES (LCO993)

Copyright 1993, 1998, 2004, all rights reserved, Lenstech LLC
 

3.0 Definitions

3.1 Aberration, Optical:  The failure of a lens to bring all rays from an object point to a desired image point.  When the rays are not brought to a point focus, image blur results.  When the point does not respond to the desired point, image distortion results.  Image distortion produces curvature in the image of a straight line.  Aberration may be inherent in the design of a lens or may result from errors in processing.

3.2 Astigmatism:  A condition where rays from the object point are not focused at a single image point by the lens.  In this case, the rays are focused as two line images at different distances from the lens and generally at right angles to each other. Astigmatism can be caused by a cylindrical-spherical surface or when the rays are incident on a spherical surface at an oblique angle.

3.3 Astigmatism, Marginal:  Astigmatism which occurs at the periphery of the lens when light is incident at an oblique angle to the curved optical surface.

3.4 Axis, Optical:  A straight line perpendicular to both surfaces of a lens and passing through the centers of curvature of the surfaces of a lens.  Along the path of the optical axis, a ray will pass through the lens without deviation.  It represents the part of the lens where the prism power is zero.

3.5 Center, Datum:  The point located on the datum line and midway between the vertical tangents to the maximum lens shape.

3.6 Center, Optical:  The point on the surface of a lens where the optical axis intersects the surface of the lens.

3.7 Datum Line:  The line midway between and parallel to the horizontal tangents to the maximum lens shape.   It is recognized that manufacturers may wish to specify a datum line other than the line midway between and parallel to the horizontal tangents of the maximum lens shape.   In this case the manufacturer must specify the vertical location of the datum line.

3.8 Diopter: A unit of measurement (plus or minus) which is used to define the refractive power of a lens.  It is equal to the reciprocal of the focal length, expressed in meters, measured from the back vertex of the lens at a wavelength of 589 nm.

3.9 Diopter, Prism:  A unit of measurement used to express the angle of deviation of a ray of light produced by a lens.  It is measured as the displacement of the ray, in centimeters, perpendicular to its line of incidence at a distance of one (1) meter.  Thus, one prism diopter deviates a beam of light one (1) centimeter at a distance of one (1) meter.

3.10 Distance Between Optical Centers:  The linear distance, usually expressed in millimeters, between optical centers of lenses mounted in a frame.

3.11 Distance, lnterpupillary: The distance between the centers of the pupils of the eyes.  Unless otherwise indicated, it refers to the distance when the eyes are fixed at infinity.

3.12 Distance, Near lnterpupillary:  The linear distance, usually expressed in millimeters, between centers of the pupils of the eyes when converged and focused on a near object.  It is usually several millimeters smaller than the interpupillary distance when the eyes are fixed at infinity.

3.13 Distortion, Optical:  An optical aberration that results in straight lines being imaged as curved lines.  "Barrel" distortion is produced by minus lenses and "pin-cushion" distortion is produced by plus lenses.  Both types of distortion are inherent in the lens design.  Distortion evidenced by waves or ripples in the image of a line viewed through a lens held remotely from the eye can result from local surface irregularities produced by improper processing.

3.14 Grayness:  A polishing defect caused by incomplete or incorrect polishing that results in fine pits in the surface of a lens.  Such a surface appears to be slightly hazy or slightly frosted.

3.1 5 Index of Refraction, n:  The ratio of the velocity of light of a given wavelength in air to that of the lens material. The ratio indicates the ability of the lens material to refract or bend a ray of light incident to its surface. The higher the index, the greater the refractive power. The value for the index is usually stated for the wavelength of the helium d-line (587.56 nm).

 For ophthalmic glass most commonly in use, n= 1.523.  For allyl resin, polycarbonate and polymethyl methacrylate commonly used for plastic lenses, typical values of n are 1.498, 1.586 and 1.450, respectively.

3.16 Lens, Corrected Curve:  A lens that has been designed to reduce marginal astigmatism  under conditions of normal intended use.  A group of such lenses covering a range of powers based on the same performance criteria is referred to as a corrected curve series.

3.17 Lensometer: A commercially available instrument used to measure the refractive, astigmatic and prismatic powers of a lens.  The optical center, axis location and prismatic powers at a given point on the lens may also be determined with this instrument.

3.18 Multi-Focal Segment Drop:  Vertical distance, in millimeters, between horizontal line throught the reference point and optical center of segment.

3.19 Multi-Focal Segment Inset:  Horizontal distance, in millimeters, between the vertical line through the reference point and optical center of segment.

3.20 Optical Centers Imbalance, Horizontal:  The difference in horizontal positions of the optical centers of lenses mounted in a frame. This difference produces an undesirable prismatic imbalance since the image in one eye is displaced horizontally with respect to the image in the other eye.

 Horizontal imbalance may be expressed as a linear distance in millimeters. It may also be expressed as a prismatic imbalance determined by measuring the prismatic power at the reference point of each mounted lens.  The difference in prismatic powers in the horizontal direction,  measured at the reference points, is known as a prismatic imbalance in the horizontal direction.

3.21 Optical Centers Imbalance, Vertical:   The difference in vertical positions of the optical centers of lenses mounted in a frame.  This difference produces an undesirable prismatic imbalance since the image in one eye is displaced vertically with respect to the image in the other eye.

 Vertical imbalance may be expressed as a linear distance in millimeters.  It may also be expressed as a prismatic imbalance determined by measuring the prismatic power at the reference point of each mounted lens.  The difference in prismatic powers in the vertical direction, measured at the reference points, is known as a prismatic imbalance in the vertical direction.

3.22 Power, Astigmatic:  The maximum difference in refractive power between any two meridians within a lens.

3.23 Power, Meridional:  The refractive or surface power of a lens measured in a specific meridian.

3.24 Power, Plus:  A positive power measured in diopters.  A positive power lens converges an incident beam of parallel rays. A positive power spherical lens is thicker at the center than at the edge and, with the absence of astigmatism, has the same refractive power in all meridians.

3.25 Power, Prismatic:  The ability of a lens to cause an angular deviation of a ray of light transmitted through it.  It is the angular deviation of a ray of light normal to the back surface of a lens and penetrating the front surface at a specified point. The amount of deviation is usually expressed in prism diopters and may be specified in terms of its vertical and horizontal components.

3.26 Power, Refractive:  The ability of a lens to produce a change in the convergence or divergence of a beam of light and is usually expressed in diopters.

3.27 Power, Spherical:  In a spherical lens it is the power of the lens expressed in diopters, and is essentially equal in all meridians.  It is equal to the average of the maximum and minimum meridional powers when astigmatism is present.

3.28 Power, Vertex:  The refractive power of a lens as measured along its optical axis and from the back (ocular) surface.

3.29 Reference Points:  Points on the datum line located equidistant from the vertical center of the frame.  The following are typical reference points:

Note: The above data is intended as a guide and may be extended to include additional intended interpupillary or near-interpupillary distances.

3.30 Segment:  A specified area of a lens having a different refractive power from the major portion. This may also refer to the actual piece of material added to the lens in the case of a fused or cemented lens.

3.31 Ultraviolet Attenuating Lens:  A lens which attenuates, or blocks, ultraviolet radiation in the wavelength regions of 290 to 315 nm (ultraviolet UVB) and 315 to 380 nm (ultraviolet UVA).

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