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1. A transparent material with one or both surfaces having a concave or convex curve; acts upon electromagnetic energy to cause convergence or divergence of light rays. 2. [NA] The transparent biconvex cellular refractive structure lying between the iris and the vitreous humor, consisting of a soft outer part (cortex) with a denser part (nucleus), and surrounded by a basement membrane (capsule); the anterior surface has a cuboidal epithelium, and at the equator the cells elongate to become lens fibers.crystalline l; [L. a lentil]
achromatic l. a compound l. made of two or more l.'s having different indices of refraction, so correlated as to minimize chromatic aberration.
acoustic l. in ultrasonography, a l. used to focus or diverge a sound beam; may be simulated by electronic manipulation of signals.
aplanatic l. a l. designed to correct spherical aberration and coma (q.v.).periscopic meniscus;
apochromatic l. a compound l. designed to correct both spherical and chromatic aberrations.
aspheric l. a l. with a paraboloidal surface that eliminates spherical aberration.
astigmatic l. cylindrical l
biconcave l. a l. that is concave on two opposing surfaces.concavoconcave l., double concave l;
biconvex l. a l. with both surfaces convex.convexoconvex l., double convex l;
bifocal l. a l. used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general; the reading addition may be cemented to the l., fused to the front surface, or ground in one-piece form; other bifocal l.'s are the flat-top Franklin type, or blended invisible.
cataract l. any l. prescribed for aphakia.
compound l. an optical system of two or more lenses.
concave l. a diverging minus power lens.minus l;
concavoconcave l. biconcave l
concavoconvex l. a converging meniscus l. that is concave on one surface and convex on the opposite surface.
contact l. a l. that fits over the cornea and sclera or cornea only; used to correct refractive errors.
convex l. a converging l.plus l;
convexoconcave l. a minus power l. having one surface convex and the opposite surface concave, with the latter having the greater curvature.
convexoconvex l. biconvex l
corneal l. contact l. of plastic without scleral portions.
crystalline l. lens (2)
cylindrical l. (cyl., C) a l. in which one of the surfaces is curved in one meridian and less curved in the opposite meridian; e.g., a teaspoon or a football.astigmatic l;
decentered l. a l. so mounted that the visual axis does not pass through the axis of the l.
dislocation of l. ectopia lentis
double concave l. biconcave l
double convex l. biconvex l
eye l. the upper of the two planoconvex l.'s of Huygens' ocular.ocular l;
field l. the lower of the two planoconvex l.'s of Huygens' ocular.
Fresnel l. a l. with a surface consisting of a concentric series of zones that duplicate the power of a l. or prism but with less thickness.lighthouse l;
immersion l. an objective (for a microscope) constructed in such a manner that the lower l. may be moved downward into direct contact with a fluid which is placed on the object being examined; by using a fluid with a refractive index closely similar to that of glass, the loss of light is minimized.
lighthouse l. Fresnel l
meniscus l. a l. having a spherical concave curve on one side and a spherical convex curve on the other.meniscus (1);
minus l. concave l
multifocal l. a l. with segments providing two or more powers; commonly, a trifocal l.
ocular l. eye l
omnifocal l. a l. for near and distant vision in which the reading portion is a continuously variable curve.
orthoscopic l. a spectacle l. corrected for distortion and curvature of the periphery.
periscopic l. a lens with 1.25 D base curve.
photochromic l. a light-sensitive spectacle l. that reduces light transmission in sunlight and increases transmission in reduced light.
planoconcave l. a l. that is flat on one side and concave on the other.
planoconvex l. a l. that is flat on one side and convex on the other.
plus l. convex l
safety l. a l. that meets government specifications of impact resistance; the increased impact resistance required for safety l.'s is obtained by tempering, by an ion-exchange process, or by using laminated or plastic lenses.
slab-off l. a spectacle l. with a base-up prism below; used in unequal myopia to equalize image displacement when reading.
spherical l. (S, sph.) a l. in which all refracting surfaces are spherical.
spherocylindrical l. a combined spherical and cylindrical l., one surface being spherical, the other cylindrical.spherocylinder;
toric l. a lens in which both meridians are curved but not to the same degree.
trial l.'s a series of cylindrical and spherical l.'s used in testing vision.
trifocal l. a l. with segments of three focal powers: distant, intermediate, and near.
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