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Lensometry is a basic skill required by all ophthalmic medical personnel (OMP). But with the advent of automated lensometry, many OMPs do not routinely evaluate glasses and contact lenses with the manual lensometer.
Various lensometry techniques are used to measure sphere power, cylinder power and axis, prism strength and base position, contact lens power, and bifocal or multifocal powers of a lens, In addition, lensometry helps locate the optical center position on single vision and multifocal lenses.
The article on basic manual lensometry, in the July/August issue of the Journal of Ophthalmic Nursing & Technology (Vol. 19, No. 4, pp. 184-- 191), provided readers with a self-study guide for measuring the power of single vision glasses and evaluating distance and near powers of bifocal lenses.
This article on advanced manual lensometry will provide information about measuring progressive bifocals, multifocal and specialty lenses, contact lens powers, prism power and base position, as well as marking the optical centers on the lens.
Evaluating Multifocal and Occupational Multifocal Lenses
Multifocal lenses are prescribed to correct refractive conditions at various focal distances. Standard bifocals are used for presbyopes needing refractive assistance at distance and specific near positions. In bifocal lenses, the reading add is placed at the bottom of the lens. Either a change in the reading segment's radius of curvature or a fused bifocal segment creates the plus power required.
Bifocals constructed of the same lens material throughout can be identified by the raised convex lens surface of the reading add. In contrast, fused bifocals are evident by the appearance of the reading add segment, but usually not a change in the surface curvature between the distance lens and bifocal segment. Standard bifocal lenses incorporate different types of add segments. These can include round-top, flat-top, ribbon, round, Ultex, blended, and executive configurations. Lensometry evaluation is performed on these lens types using the standard techniques described in the article on basic manual lensometry in the previous issue.
Multifocal lenses, which deviate from the standard bifocal configurations, include specialty and occupational lenses. These lenses may correct for specific intermediate and near fixation needs, with or without including the distance requirements. Multifocal corrections, prescribed only for a combination of intermediate and near fixation, are appropriate for individuals doing extended periods...