Estimating the power of a minus spectacle while worn by the patient

This paper presents a new method of estimating the power of a minus spectacle lens without removing the spectacle from the patient's face. The examiner shines a small light toward the spectacle and measures the separation between the images reflected from the front and rear surfaces of the lens. The study presents a relationship between separation of reflected images and the power of minus lenses from 0 to -12 D. The article also contains a theoretical explanation of the phenomenon.

Ultraviolet light, cataracts, and spectacle wear

Two groups of patients older than 60 years were examined for degree of development of nuclear sclerotic cataracts. The first group of patients had worn glass spectacles for a majority of their lives, while the patients in the second group had never worn spectacles or had worn only reading glasses. Subjects in the group with spectacles showed significantly less development of sclerotic nuclear lens changes. These findings may be due to the screening effect by spectacle glass of light in the ultraviolet range and suggest that uv light may be one of the risk factors in the development of nuclear sclerotic cataracts.

Eye protective techniques for bright light

There are two principal methods to protect the retina from light damage: filtration, and brightness reduction. Since short-wavelength light and ultraviolet radiation are the most hazardous, filters that strongly attenuate wavelengths less than 500 nm can be quite effective. As infrared radiation may enhance photochemical light damage of the retina, filtration of infrared radiation is also desirable. Finally, since the eye must focus incident light on the retina, the brightness (radiance) of a source plays an important role in determining the degree of hazard of a light source. Diffusing the light source can greatly reduce the risk of injury.

Aphakia: perceptual and refractive problems of spectacle correction

The perceptual variations due to removal of the crystalline lens and the employment of powerful plus spectacle lenses are described. These include refractive change, distortion, magnification, limitation of the visual field, peripheral vision, sensitivity of light and changes in the required convergence. The technique for satisfactory refraction of the aphakic eye is discussed and the elements leading to error such as vertex distance and pantoscopic tilt are considered. Other factors necessary for understanding the refraction such as the apparent accommodative range and the limitations affecting field charting are also covered. Method of securing binocularity via contact lens-spectacle combinations for monocular cases, and the need for rehabilitative procedures conclude this treatment.

Fresnel prism therapy for right hemianopia

Fresnel prisms placed on the spectacle lenses of a patient with right hemianopia provide an easier means for the patient to become aware of objects located in the blind field. Exact location of the prisms is important.

Impact resistance of variously mounted ophthalmic lenses

An experimental investigation was conducted to determine the frangibility of ophthalmic glass lenses loaded by the impact of steel spheres with diameters ranging from 6.35 to 28.6 mm when dropped from heights up to 1.905 m. These lenses were supported either simply, by a ring similar to that used in U.S. Food and Drug Administration (FDA) drop tests, or while inserted in three different commercially used frame configurations placed on a head form. Strains up to 4000 mu strain were recorded by gauges cemented to the lenses, and impact forces on the glass up to 10.2 kN were determined, with many lenses surviving. Plano lenses tested according to FDA standards experienced peak values of 2300 mu strain. The maximum force transmitted to the head form at the nose piece was found to be 91 N as determined by embedded crystal transducers. A series of special tests on circular plano lenses inserted in specially constructed rings and a circular eyewire indicated that eyewire tension is not a significant factor in impact resistance of lenses under both static and dynamic loading.

The properties of lenses used for the correction of aphakia

There is a wide variety of aphakic lenses available but remarkably little quantitative data to assist the optometrist in his choice of product. This paper presents the results of a survey of nearly all the available lens types. Quantitative data is given for the products of those manufacturers who cooperated. Data covers off axis acuity, magnification, distortion, thickness, weight and ultraviolet protection.

Terrien's marginal degeneration

Terrien's marginal degeneration (TMD) is a bilateral and progressive corneal disorder that causes corneal thinning, furrowing, neovascularization and high degrees of corneal astigmatism. Possible sequellae include ectasia and perforation of the affected corneal region. An asymptomatic patient presented to our clinic manifesting all of the classical signs of TMD except the high degree of corneal astigmatism. Excellent visual acuity was attained with the latest spectacle prescription.

Control of vertically polarized glare

Reflected glare often interferes with vision. Since such glare is usually polarized it can be controlled with polarizers. The use of polarized filters to eliminate vertically polarized glare from blackboards and glossy printed material is presented. Practical means for the construction of such filters are discussed.

Comparison of the acceptance of progressive addition multifocals with blended bifocals

An evaluation of the relative influence of the optical features of two forms of invisible addition lenses, one of progressive power form and one in blended bifocal form, is made upon a number of subjects. One group consisted of previous progressive lens wearers and served not only the above purpose but also as an analysis of patient preference after the initial novelty had worn off. The second group consisted of subjects unacquainted with either type presented in a double-masked procedure. Progressive forms of invisible lenses were overwhelmingly preferred over blended forms in both groups. The first group also showed a preponderant inclination for retention of progressive lenses after some span of wear even among subjects previously accustomed to the usual visible bifocal forms.

Optical prophylaxis for environmental myopia: an epidemiological assessment of short-term effects

The biologically plausible concept that relief of accommodation may reduce myopic progression, although not confirmed and not universally accepted, has nevertheless strongly influenced the practice of preventive optometry. Yet, no quantitative estimates of the strength of this postulated effect were found in the literature. From a cohort study of 120 young (18 to 20 years of age) male military students with low initial refractive error (less than or equal to +/- 1D, spherical equivalent), estimates were made of the short-term (5 months) effect of two forms of optical treatment to prevent school-related myopia. The cumulative incidence of clinically significant (greater than 0.25 D, spherical equivalent) myopic progression was less [standardized morbidity ratio (SMR) = 0.45] among those prescribed monofocal convex lenses (+1.25 D over far Rx) and prisms (2 delta base-in OU) and even more reduced (SMR = 0.32) among those prescribed bifocals (+1.5 D near adds) compared with those receiving sham treatment (no. 1 pink tint). Placebo-treated patients were more than twice [rate ratio (RR) = 2.2, 1.5] as likely as the monofocal group, and more than three times (RR = 3.2, 1.7) as likely as the bifocal patients, to suffer myopic progression during the period of follow-up. Etiologic fractions (EF, increase in myopic progression if treatment were eliminated) of 37 and 47% and preventive fractions (PF, decrease in myopic progression attributable to the treatment) of 27 and 32% were estimated for the monofocal and bifocal treatments, respectively. These findings suggest that while both modes of therapy are efficacious, the bifocal form is 20% (32/27 = 1.2) more preventive.

Effect of conicoid asphericity on the Tscherning ellipses of ophthalmic spectacle lenses

One of the criteria in ophthalmic spectacle lens design is the elimination of oblique astigmatism. For a range of equivalent powers, Seidel (primary or third-order) astigmatism can be eliminated, and the solutions of back- (or front-) surface power are commonly displayed graphically in the form of ellipses (Tscherning ellipses). The Tscherning ellipses apply only to lenses constructed from spherical surfaces. If one or both surfaces are made aspheric, the solutions for zero astigmatism are no longer in the form of ellipses. If one surface, usually the front surface, is made as a conicoid aspheric, the solutions for zero astigmatism can be presented graphically similarly to the Tscherning ellipses. For any given equivalent power, there are two or no solutions for spherical lenses. However, there is always one and up to three solutions for conicoid aspheric lenses.

Construction, specification, and mathematical description of aspheric surfaces

Rotationally symmetrical aspheric surfaces of spectacle lenses are constructed as either "zonal aspherics" or "continuous aspherics." Zonal aspherics consist of annular zones surrounding a central zone with each zone being nominally spherical with progressively lower surface power the farther the zone is from the surface vertex. Aspheric surfaces are often specified by the radial drop in surface power from the center to the edge of the lens (e.g., Welsh Four-drop), but for assessment purposes the surface shape must be specified more precisely. The formulas for the description of continuous aspherics can be manipulated into different forms. Mathematical descriptions are given or developed which will enable theoretical assessment of the performances of all lenses with rotationally symmetrical aspheric surfaces.

The role of sunlight in human cataract formation

This review summarizes and integrates new findings concerning the role of near-ultraviolet radiation, as is present in sunlight and common artificial light sources, in stimulating human and animal cataract formation. Epidemiological and basic research studies are summarized and critical statements concerning them are offered. Although certain questions still remain unanswered, the evidence that near-UV radiation does stimulate cataract formation is very strong. Avoidance of excessive exposure to near-ultraviolet light and the use of protective lenses that filter it out are suggested to prevent the enhancement of human cataract formation by near-UV light.

Use of light intensity reduction for amblyopia therapy

A form of amblyopia therapy is presented using light intensity reduction which provides a means of increasing the visual acuity in the amblyopic eye while preserving and promoting binocularity. In addition to the rationale and specific instructions for therapy, an illustrative case is presented of a 7-year-old boy with anisometropic amblyopia and eccentric fixation who was treated with light intensity reduction produced by Polaroid filters before his nonamblyopic eye. In this case acuity improved from 6/30 (20/100) to 6/7.8 (20/26), fixation improved from 2 degrees eccentric to unsteady central, and stereopsis increased from no measurable amount to 140 sec as measured by the Randot stereotest.

Therapy for early acquired noncomitant esotropia. A case report

A 7 year-old female demonstrated a noncomitant esotropia secondary to an early acquired lateral rectus paresis. Sensory fusion testing indicated that the patient was capable of maintaining single binocular vision only with a pronounced head turn. Prism and surgical therapy reduced the abnormal head posture and permitted fusion and stereopsis in the primary position.

A slide rule for calculating the power of an intraocular lens

We have designed a slide rule to calculate the emmetropizing and iseikonizing power of an intraocular lens (IOL). Like the dial biometer currently used for these calculations, the slide rule is inexpensive, easy to use, simple, and a compact pocket size. It can, however, also be used to calculate postoperative refractive error, corneal power when various keratometers are used, and conversion of refractive correction at the spectacle frame to that at the cornea, or vice versa. All the calculated data are within the accuracy suitable for clinical work.

Unwanted astigmatism in lenses with a concentric variation in sagittal power

A concentric variation in sagittal power, Fs, with zonal radius, y, has been used in some designs of varifocal and aphakic spectacle lenses. It is shown theoretically, using a paraxial approximation, that a gradient dFs/dy in sagittal power leads to the introduction of an amount y.dFs/dy of unwanted astigmatism. The validity of this result is illustrated experimentally.