New equations for determining ocular deviations produced by spectacle corrections

The relation between prismatic deviations at the spectacle plane and corresponding ocular deviations is demonstrated and expressed in the form of an equation. In addition, basic equations for the ocular prismatic deviations produced by spectacle lenses placed before the eyes are developed. These equations are a sequel to previous studies demonstrating that the current use of Prentice's rule for finding the prismatic effect produced by realistic spectacle lenses is not suitable for present day spectacle corrections. One set of equations is based on tangents of angles of ocular rotation, whereas another set of equations expresses ocular deviations in degrees. The former is appropriate for clinical purposes; the latter is suitable when greater accuracy is desired.

[Clinical picture and treatment of diplopia]

A total of 203 patients with stable binocular diplopia were observed. Examinations included coordimetry, analysis of double images, and investigation of the doubling field in the gaze field. All examinations were performed using an original method with a serial computer. The treatment included therapy of the underlying disease, choice of correcting prisms, exercises aimed at extension of separate vision field, and operation on the oculomotor muscles. Three main forms of diplopia were distinguished: 1) muscular; 2) sensory; and 3) mixed. The first form includes diplopias in paresis of n.III and n.IV, n.VI, late development of convergent squint, orbital injuries, endocrine ophthalmopathy, and myasthenia. The second form includes diplopias developing after delayed operations for convergent squint, and the third form are diplopias after operations for detachment of the retina and implantation of intraocular lenses. The treatment led to complete neutralization of doubling in 18.7% cases, neutralization in the main part of the gaze field in 53.7%, the same with prism correction in 12.8% cases; in 14.8% patients adaptation to diplopia was attained after suppression of the second image. Diplopia in late squint is prognostically the most favorable, and the following conditions are listed in the order of more or less good prognosis: paresis of n.VI, n.IV, orbital injuries, endocrine ophthalmopathy, and artiphakia. The least favorable prognoses are for patients after delayed operation for squint and detachment of the retina. A new operation, plastic repair of the external m. rectus, was used in 96 patients with pareses of n.VI; no compensation was attained in only 2 patients.

Optical Fourier filtering for whole lens assessment of progressive power lenses

Four binary filter designs for use in an optical Fourier filtering set-up were evaluated when taking quantitative measurements and when qualitatively mapping the power variation of progressive power lenses (PPLs). The binary filters tested were concentric ring, linear grating, grid and "chevron" designs. The chevron filter was considered best for quantitative measurements since it permitted a vernier acuity task to be used for measuring the fringe spacing, significantly reducing errors, and it also gave information on the polarity of the lens power. The linear grating filter was considered best for qualitatively evaluating the power variation. Optical Fourier filtering and a Nidek automatic focimeter were then used to measure the powers in the distance and near portions of five PPLs of differing design. Mean measurement error was 0.04 D with a maximum value of 0.13 D. Good qualitative agreement was found between the iso-cylinder plots provided by the manufacturer and the Fourier filter fringe patterns for the PPLs indicating that optical Fourier filtering provides the ability to map the power distribution across the entire lens aperture without the need for multiple point measurements. Arguments are presented that demonstrate that it should be possible to derive both iso-sphere and iso-cylinder plots from the binary filter patterns.

The depth, contour and angle of the glazing grooves of spectacle frames: don't blame the glazer?

The groove of a spectacle frame is responsible for lens retention. It also tends to fill with unsightly dirt which cannot be readily removed by our patients. The results of this investigation would suggest that both loose spectacle lenses and dirt accumulation may often be due to poor quality control in the frame industry. The data also suggests that unwanted stress may be present in both frames and lenses due to non-optimal groove shape.

[Effectiveness of eyeglasses for protection against ultraviolet rays]

PURPOSE

The relationship between eyeglass size and protection of the eye surface from the effects of solar ultraviolet (UV) rays was investigated.

METHODS

Solar UV rays irradiating the eye surface were measured on a mannequin which modeled the standard facial bone structure of a Japanese female. UV sensor chips (photo-sensitivity: 260-400 nm) were attached to the ocular surface of the lid fissure. UV measurement was done from 12:00 to 15:00 on a sunny day in March. UV intensity was measured under the following conditions: 1) with or without eyeglasses, 2) wearing sunglasses with side protectors, and 3) wearing a cap with a 7 cm brim. Eyeglasses of four frame sizes (width: 48-57 mm) were put on the mannequin. All lenses were made of plastic and coated so as to be impervious to rays shorter than 400 nm. The refractive power was 0 diopters. At the same time, UV irradiation intensity from all directions (excluding from the earth direction) was measured using a polyhedron type UV sensor with 25 sensor chips.

RESULTS

Except for eyeglasses with the smallest frame size, eyeglasses effectively reduced UV exposure to sunlight from the upper front direction. However, protection against rays from the upper temporal direction was extremely poor. Sunlight from the upper back was reflected by the posterior surface of the eyeglasses and reached the eye surface.

CONCLUSION

The efficacy of eyeglasses against UV depends on their size. The shape of the eyeglasses and reflection from the posterior lens surface are also of great importance. Small eyeglasses do not offer ideal UV protection for the Japanese face shape.

Impact resistance of ceramic brackets according to ophthalmic lenses standards

The overall resistance to accidental blows of the many ceramic brackets that are sold today has not been explored. Facing a similar diversity, the eyeglasses industry has chosen to standardize the testing of lenses by subjecting them to the drop of a steel ball. By slightly modifying this test, 10 brands of ceramic brackets were examined. In most cases, the findings coincided with those found by other authors when duplicating debonding. Thus, polycrystalline ceramics with bulkier structures and glazed surfaces were found to be more resistant to impact than the monocrystalline brackets, the loftier real "twins," and the less dense attachments. Protruding tie wings and bases were liabilities, and domed configurations seemed to deflect the blows. Bulkier "single" designs alone did not offer a guarantee of impact resistance when not accompanied by an appropriate microstructure and a smooth surface. The ceramic brackets most resistant to impact were found to be 20/20 by American Orthodontics and Fascination by Dentaurum. Medium resistance was displayed by Lumina by Ormco, Allure III and Allure by GAC, Transcend 2000 and Transcend by Unitek/3M; the last was not as good as the other four. The least resistant were Illusion by Ortho-Organizers, Intrigue by Lancer Orthodontics, and Starfire TMB by "A"-Co. Probably because of its real twin design, the last bracket lends itself to the highest probability for accidental breakage. Although resistance to impact and accidental debonding is desirable from the point of view of treatment, the advantage should be weighted against the chance of enamel fracture. Indeed a weak bracket attached with a soft adhesive may be preferable when the chance of an increased exposure to accidental blows is probable. In such cases, the ceramic may take the brunt of the force, instead of the tooth.

Comparison between anti-reflection-coated and uncoated spectacle lenses for presbyopic highway patrol troopers

BACKGROUND

The use of anti-reflection coatings (AR) for spectacle lenses is steadily increasing. These ultra-thin coatings reduce reflections in lenses, increasing light transmittance about 8% (from 91% to 99%). Patient reports of crisper detail and brighter environment indicate that this perceived increase in visual function would probably be of benefit to many, including highway patrolmen who must make daily judgments on the basis of fine visual detail.

METHODS

Nineteen presbyopic Missouri State Highway Patrolmen (MSHP) participated in this crossover study. Subjects randomly wore Varilux Comfort progressive addition lenses in CR-39 that were uncoated or AR-coated. The AR coating was Crizal. Subjects wore each lens for 1 month, then compared the lenses directly for 1 week. Data were collected for lens scoring (both general and job specific) and lens preferences.

RESULTS

The AR-coated lens was found to be significantly better in most categories. Reduction in glare and reflections during the day, night, indoors, and in the patrol vehicle--as well as improvement in overall night vision--were highly significant. Eighty-nine percent of the subjects chose the Crizal-coated lenses as their overall preferred lens.

CONCLUSIONS

The results of this study appear to demonstrate a noticeable subjective preference for AR-coated lenses when used by a group engaged in a visually demanding occupation.

The effect of relative spectacle magnification on aniseikonia

PURPOSE

The purpose of this article is to study the effects of modifying relative spectacle magnification to determine what effect this has on aniseikonia, binocularity, and visual comfort.

METHODS

A prospective analysis of 34 aniseikonic patients was undertaken. The etiology of aniseikonia varied from physiologically occurring to induced. Aniseikonic screening included manifest refraction, keratometry, axial length, Randot stereoacuity, associated phoria, and Keystone space eikonometry. A modified pair of spectacles was fabricated on the basis of magnification principles for iseikonic lenses. Each patient was also given a control pair of conventional spectacles. A 4-week trial period was allowed for each pair of spectacles, pertinent examination measurements were repeated, and a patient survey was administered. Data were analyzed by t-test and chi-square.

RESULTS

Modifying relative spectacle magnification reduced mean aniseikonic error by 1.06% (P < 0.0001). A difference was found between the control and modified spectacles for subjective reports of visual comfort, performance, and eye-strain (P < 0.05). There was no difference between the two groups for stereoacuity or cosmetic appearance of lenses. At the conclusion of the study, 93% of patients preferred the modified lenses in direct comparison.

CONCLUSIONS

Our results confirm that modification of lens designs to equalize relative spectacle magnification reduces aniseikonia and improves subjective comfort and performance of anisometropic spectacles.

The leaching of nickel from new and used metal spectacle frames

Fifty "used" and 373 new spectacle frames were tested for evidence of nickel release using dimethylglyoxime and ammonia. Nearly all the "used" frames (96%) tested positive. A lower proportion (24.5%) of new frames showed evidence of nickel leaching. It is suggested that this is evidence of non-conformance with the current British Standard although all our suppliers believed that their frames conformed with BS 6625 (1985).

Facial measurements for frame design

Anthropometric data for the purpose of spectacle frame design are scarce in the literature. Definitions of facial features to be measured with existing systems of facial measurement are often not specific enough for frame design and manufacturing. Currently, for individual frame design, experienced personnel collect data with facial rules or instruments. A new measuring system is proposed, making use of a template in the form of a spectacle frame. Upon fitting the template onto a subject, most of the measuring references can be defined. Such a system can be administered by lesser-trained personnel and can be used for researches covering a larger population.

Ophthalmic anthropometry for Hong Kong Chinese adults

A database of ophthalmic anthropometry is established which is useful for the design of spectacles for southeast Asian people. Statistical analysis reveals significant variations in the facial dimensions between male and female subjects. Results of the study are given separately for each sex and for the combined population. Hong Kong Chinese are found to have larger frontal angles, bigger angles of splay, and a less prominent bridge in the nose. Spectacle frames should be designed according to the targeted gender population, and to accommodate the characteristics of Asian faces.

Efficacy and durability of ultraviolet tints in CR-39 ophthalmic lenses

BACKGROUND

Ocular protection from solar ultraviolet (UV) radiation has been emphasized in recent years as a result of the thinning of the ozone layer in the atmosphere. The purpose of this study was to evaluate the absorptive properties of UV tints in CR-39 lenses.

METHODS

We used a spectrophotometer to measure the UV transmittance of three groups of UV tinted CR-39 lenses, including (1) lenses tinted by local optical laboratories: (2) lenses tinted by us, using commercially available dyes: and (3) stock UV lenses that have UV absorptive molecules throughout the lens. We also tested the durability of these tints to daily washing/drying by measuring their UV transmittance characteristics at 3, 6, and 12 months.

RESULTS

All the tested lenses absorbed all of the UV-B and at least 99% of UV-A. The durability of these UV tints when exposed to daily washing/drying was excellent: all lenses continued to absorb all of the UV-B and at least 99% of UV-A after 1 year.

CONCLUSIONS

These data suggest that UV tinted CR-39 lenses provide protection against UV radiation that meets the ANSI Z80.3-1996 Standard for non-prescription sunglasses and fashion eyewear. Furthermore, normal daily washing/drying for 1 year does not cause a significant decrease in the protective effect of the UV tint.

OPTX 20/20 and Press-On Optics bifocal segments: an evaluation

BACKGROUND

OPTX 20/20 and Press-On Optics removable bifocal segments were evaluated in three areas: optical quality, effects on vision, and stability under simulated environmental conditions.

METHODS

To evaluate optical quality, sphere, cylinder, and spherical equivalent powers were measured in center and peripheral portions of 10 segments, with powers ranging from +1.00 to +2.50 D.

RESULTS

Twelve of 20 OPTX 20/20 segments and six of 20 Press-On Optics +1.00 and +1.50 D segments failed to pass ANSI Z80.1 standards because of unwanted cylinder powers. With regard to vision, +2.00 D OPTX 20/20 segments provided a 1 Snellen line advantage in near acuity, as compared with the Press-On Optics segments. contrast sensitivity for low- and mid-spatial frequencies was not significantly different for segments of the two types.

CONCLUSION

For higher powers, the optical characteristics of the OPTX 20/20 segments are better and provide better visual acuity. For lower powers, distortion and small amounts of unwanted cylinder reduce the optical quality of the OPTX 20/20 segments. Both segment types retained adhesion to CR-39 carriers equally well and should be satisfactory for their designated purposes of providing temporary and removable adds for occasional use.

Power vectors: an application of Fourier analysis to the description and statistical analysis of refractive error

The description of sphero-cylinder lenses is approached from the viewpoint of Fourier analysis of the power profile. It is shown that the familiar sine-squared law leads naturally to a Fourier series representation with exactly three Fourier coefficients, representing the natural parameters of a thin lens. The constant term corresponds to the mean spherical equivalent (MSE) power, whereas the amplitude and phase of the harmonic correspond to the power and axis of a Jackson cross-cylinder (JCC) lens, respectively. Expressing the Fourier series in rectangular form leads to the representation of an arbitrary sphero-cylinder lens as the sum of a spherical lens and two cross-cylinders, one at axis 0 degree and the other at axis 45 degrees. The power of these three component lenses may be interpreted as (x,y,z) coordinates of a vector representation of the power profile. Advantages of this power vector representation of a sphero-cylinder lens for numerical and graphical analysis of optometric data are described for problems involving lens combinations, comparison of different lenses, and the statistical distribution of refractive errors.

The variation in the threads of the dowel screws of spectacle frames

British Standards specify a very small number of screw sizes as being acceptable for use in spectacle frames. We have measured the screw sizes of 50 frames, selected at random from the stock of Glasgow Caledonian University Clinic and have shown that very few of the screws used conform to these standards.

Reflections on antireflection films

BACKGROUND

The development of antireflection films 50 years ago was based on interference of light waves. Antireflection films have broad ophthalmic applications by increasing transmittance and decreasing reflection as a function of wavelength and polarization. Design considerations of thin films are enormous, as are the rigors of successful film production.

METHODS

A computer search of optometric and ophthalmologic databases produced very few pertinent references. Discussions with optical engineers led to extensive publications in optical engineering journals and texts.

RESULTS

Literature search and interviews with optical engineers provided the historical development, design, and production considerations of optical antireflection coatings.

CONCLUSIONS

Ophthalmic thin-film antireflection coatings have a long but unknown development. Many design and production parameters must be considered in producing effective, long-lived ophthalmic coatings, which do offer significant improvement in optical performance.

Shatter resistance of spectacle lenses

OBJECTIVE

To evaluate the relative strength and shatter resistance of spectacle lenses currently used in sunglasses and dress, sports, and industrial eyewear.

DESIGN

Seven lenses that met the US American National Standards Institute (ANSI) Z80 standards for dress glasses (made of high-index plastic, allyl resin plastic, heat tempered glass, chemically tempered glass, and polycarbonate, and with center thickness ranging from 1 mm to 2.2 mm) and 4 lenses that met ANSI Z87 standards for industrial safety eyewear (allyl resin plastic, heat-tempered glass, chemically tempered glass, and polycarbonate, all with 3.0-mm center thickness) were tested for impact resistance to 5 projectiles (air gun pellets, golf balls, tennis balls, lacrosse balls, and baseballs).

MAIN OUTCOME MEASURES

Impact energy required to shatter spectacle lenses.

RESULTS

Based on 348 lens impacts, dress and industrial lenses made from glass, allyl resin plastic, and high-index plastic shattered at impact energies less than those expected to be encountered from the test projectiles during their routine use. Polycarbonate lenses demonstrated resistance to impact for all tested projectiles exceeding the impact potential expected during routine use.

CONCLUSIONS

Under the test conditions of this study, polycarbonate lenses demonstrated greater impact resistance than other commonly used spectacle lenses that conform to prevailing eyewear standards. These findings suggest that current ANSI Z80 and ANSI Z87 standards should be reevaluated.

Measurement of mechanical warpage in CR-39 lenses

The warpage of CR-39 ophthalmic spherical lenses of a variety of powers and compressed in the radial direction is measured as a function of the applied tangential force. The lenses are cut with circular shape and then mounted on a frame specially designed to measure the tangential force by means of a calibrated coil spring. The measurements are performed with a high resolution micrometer at different points on the lens surfaces. The warpage of the two surfaces of each lens is calculated and then a quantitative value of surface power variation is given. From these data, vertex power and oblique astigmatism variation are shown. Departure of the surfaces from sphere as a function of the applied tangential force is analysed.

U.S. Army sunglasses: issues and solutions

BACKGROUND

Some military aviators have reported difficulty discriminating the color of cockpit warning lights when wearing plastic sunglasses. This difficulty could be due to lenses that are too dark or are nonneutral and thus alter color vision. A study was performed to identify the origin of this problem, recommend solutions, and determine a sunglass transmittance that optimizes visual performance.

METHODS

Five pairs of plastic sunglasses (-4 to +4 D) were ordered from each of seven military optical laboratories (70 lenses total). Each laboratory was instructed to dye the lenses neutral gray with 21 percent transmittance. Light transmittance and color distortion were evaluated across laboratory and lens power. Spatial and color vision were assessed through a range of sunglass transmittances.

RESULTS

There was no systematic effect of lens power, but light transmittance and color distortion varied widely across laboratories (transmittance = 1 to 30 percent; p < 0.001).

CONCLUSIONS

Because light transmittance and color distortion were related inversely, it was believed that both factors could be corrected by accurate verification of transmittance, but commercial transmittance meters proved to be inaccurate. The high transmittance of deep red and infrared light through plastic lens dyes is read as visible light by transmittance meters, making readings too high. A filter was identified that provides accurate readings when used with transmittance meters. A sunglass transmittance of 23 percent resulted in minimal decrease in visual performance relative to normal clinical test conditions.

Thickness extrema at the edge of astigmatic lenses including those with straight, circular and elliptical edges

The problem of locating the points of maximum and minimum thickness at the edge of a lens, and of calculating the thickness at those points, is examined for lens powers and for edge shapes in general. The edge extremal problem, as the problem is called, is solved explicitly for general powers along straight cut edges. The extremal problem is also analysed for lenses with circular and elliptical edges but explicit solutions are obtained only for centrally cut lenses, that is, lenses with coincident optical and geometrical centres. For edges that are neither straight nor centrally cut ellipses (including circles), it appears that explicit solutions cannot be obtained for the edge extrema: one has to resort to numerical solution of implicit equations or to the calculation of thickness at sufficiently many points around the edge. For the centrally cut ellipse the edge extremal problem turns out to be the eigenvalue problem of linear algebra. In general the thickness extrema at the edge do not lie on the principal meridians of the lens, nor do they lie on meridians that are mutually perpendicular. With minor modification the results apply equally well to the edge extrema for sagitta of a surface.