Revisions to tolerances in cylinder axis and in progressive addition lens power in ANSI Z80.1-2005

BACKGROUND

ANSI Z80.1 (American National Standards Institute) is a voluntary standard that provides tolerance guidelines for ophthalmic lens parameters. The recently approved ANSI Z80.1-2005 changes tolerances for cylinder axis for all ophthalmic lenses and for distance power for progressive addition lenses (PALs).

METHODS

The ANSI Z80.1 committee analyzed the amount of unwanted residual cylinder created by rotating a cylinder axis away from its desired orientation for a range of cylinder powers. The limits of axis orientation error that keep the residual unwanted cylinder error within the tolerances for cylinder power were determined. For PALs, a tolerance for distance power error was sought that considers the currently followed standards for front surface power error and the tooling steps used for the rear surface.

RESULTS

Tolerances for cylinder axis error were changed to +/-14 degrees for 0.25 diopter (D) cylinders and to +/-7 degrees for 0.50 D cylinders. Tolerance for error in back vertex power of PALs was changed to +/-0.16 D for both sphere and cylinder.

CONCLUSION

Tolerances for cylinder axis for low cylinders have been made in ANSI Z80.1-2005 to be consistent with tolerances in cylinder power. Changes in tolerance for power errors in PALs have been made to recognize fabrication challenges.

Sequential Tilt and Tilted Power of Thin Lenses

PURPOSE

The purposes of this article were, for any sequence of tilts applied to a thin lens, to calculate an equivalent turn and single tilt, to show how to use the equivalent turn and tilt to calculate the tilted power of the lens, and, given the desired tilted power, to calculate the power of the untilted lens necessary to compensate for the effects of tilt. The untilted lens may be stigmatic (spherical) or astigmatic (spherocylindrical).

METHODS

The analysis makes use of rotation matrices to represent rotation in space and previous work in third-order optics on oblique central refraction.

RESULTS

Equations are presented for calculating the combination of turn and tilt that is equivalent to any sequence of tilts. They are specialized for the particular case of combinations of faceform and pantoscopic tilts and allow the decomposition of an arbitrary tilt into a combination of turn and pantoscopic and faceform tilts. The equations also lead to a procedure for calculating or compensating for the tilted power of a sequentially tilted thin lens.

CONCLUSIONS

Previous work on the effect of tilt on thin lenses has been generalized to handle combinations of arbitrary tilts.

The Effect of Multiple Antireflective Coatings and Center Thickness on Resistance of Polycarbonate Spectacle Lenses to Penetration by Pointed Missiles

PURPOSE

Previous work has shown that the impact resistance to blunt missiles is affected by coatings applied to either CR-39 or polycarbonate lenses. We investigated the effects of multiple antireflection (minimum angle of resolution [MAR]) coatings on the resistance of polycarbonate lenses to puncture on impact by sharp, high-speed missiles.

METHODS

Four groups of surfaced plano polycarbonate lenses were investigated. Two groups had a scratch-resistant (SR) coating applied to both surfaces. One of these groups had a 2-mm center thickness and the other had a 3-mm center thickness. The other two groups of 2-mm and 3-mm thick lenses had a MAR coating applied over the SR coating. The lenses were impacted by a missile consisting of an industrial sewing machine needle mounted in a cylindrical aluminum carrier.

RESULTS

The sharp missiles were able to pierce the lenses at speeds between 29.6 m/s and 46.2 m/s. Impact resistance was lowest for the thinner lenses and lenses with a MAR coating. The effect of the MAR and lens thickness was subadditive.

CONCLUSIONS

We have confirmed previous observations that polycarbonate lenses are more susceptible to penetration by sharp, high-speed missiles than blunt missiles. We have also found that reducing lens center thickness and applying a MAR coating further reduces the penetration resistance. Therefore, the use of 2-mm center thickness and MAR-coated polycarbonate lenses should be discouraged for industrial eye protectors where sharp missile hazards are possible.

Protection from ultraviolet radiation by spectacle lenses available in India: a comparative study

BACKGROUND

Exposure to ultraviolet radiation (UVR) can result in several ocular ailments. We studied the UV absorption (UV-A and -B bands) in 20 lenses (11 hard resin and 9 glass) commonly available in India as uncut lenses for the purpose of prescription as well as non-prescription wear.

METHODS

Using a UV-visible spectrophotometer (Thermo-Spectronic, UV-1 model, Thermo Electron Corporation, USA), the percentage transmittance was scanned between 190 and 400 nm, from which the results at 280, 320, 340, 360, 380 and 400 nm were analysed.

RESULTS

At 360 nm (UV-A), 75% of the lenses studied failed to offer 95% protection as recommended by the United States Food and Drug Administration (US FDA). At 280 nm (UV-B), 35% of the lenses failed to offer 99% protection against UV-B. Hard resin lenses showed a higher degree of variation in protection from UV-A.

CONCLUSIONS

Most of the lenses studied failed to meet the US FDA recommendations in terms of protection from UV-A. Glass lenses afforded better protection up to 320 nm compared with hard resin lenses. As the claims of lens manufacturers regarding UVR protection fall short of international standards, certification of UVR protection for different lenses should be introduced in India.

The effect of relative distance enlargement on visual acuity in the visually impaired

BACKGROUND

Prescribing magnification is typically based on distance or near visual acuity. This presumes a constant minimum angle of visual resolution with working distance and therefore enlargement of an object moved to a shorter working distance (relative distance enlargement). This study examines this premise in a visually impaired population.

METHODS

Distance letter visual acuity was measured prospectively for 380 low vision patients (distance visual acuity between 0.3 and 2.1 logMAR) over the age of 57 years, along with near word visual acuity at an appropriate distance for near lens additions from +4 D to +20 D. Demographic information, the disease causing low vision, contrast sensitivity, visual field and psychological status were also recorded.

RESULTS

Distance letter acuity was significantly related to (r = 0.84) but on average 0.1 +/- 0.2 logMAR better (1 +/- 2 lines on a logMAR chart) than near word acuity at 25 cm with a +4 D lens addition. In 39.8 per cent of patients, near word acuity was more than 0.1 logMAR worse than distance letter acuity. In 11.0 per cent of subjects, near visual acuity was more than 0.1 logMAR better than distance letter acuity. The group with near word acuity worse than distance letter acuity also had lower contrast sensitivity. The group with near word acuity better than distance letter acuity was less likely to have age-related macular degeneration. Smaller print size could be read by reducing working distance (achieved by using higher near lens additions) in 86.1 per cent, although not by as much as predicted by geometric progression in 14.5 per cent.

DISCUSSION

Although distance letter and near word acuity are highly related, they are on average 1 logMAR line different and this varies significantly between individuals. Near word acuity did not increase linearly with relative distance enlargement in approximately one in seven visually impaired, suggesting that the measurement of visual resolution over a range of working distances will assist appropriate prescribing of magnification aids.

[Analysis of anthropometry on head and eye for stipulating of children's spectacle frames]

OBJECTIVE

To investigate the normal value of six parameters of head and eye in Chinese children, including interorbit distance (IOD), interpupillary distance (IPD), superior otobasion-ectocochion distance (obs-ec), intersuperior otobasion distance (obs-obs), exophthalmos and nasal basis angle, for stipulating the sizes of children spectacle frames.

METHODS

The value of these six parameters in 10 171 children ages 5 to 17 years old were measured with caliper in four cities and the results were analyzed.

RESULTS

The IOD, IPD, obs-ec and obs-obs were increased and positively correlated with the age. The differences of these parameters among different ages were statistically significant (F = 0.97, P < 0.01), especially in IPD before 15, IOD before 13, obs-ec and obs-obs before 12 and exophthalmos before 12 years old. The fastest growth age in IPD, IOD, exophthalmos, obs-ec and obs-obs was 10, 11, 5, 7 and 11 years old, respectively. The growth period of IPD, IOD, exophthalmos, obs-ec and obs-obs was 5 - 15, 5 - 13, 5 - 8, 5 - 12 and 5 - 15 years old. The difference of these six parameters among children in 4 cities were statistically significant (t = 3.1, P < 0.01).

CONCLUSIONS

The most active growth period of IPD, IOD, obs-obs, obs-ec and exophthalmos lies between 5-15 years old in Chinese children 5-17 years old. Although the differences of these six parameters between male and female and among children in four cities are statistically significant, practically they are not important for stipulating the sizes of children's spectacle frames.

Inference of nested variance components in a longitudinal myopia intervention trial

This paper was motivated by a double-blind randomized clinical trial of myopia intervention. In addition to the primary goal of comparing treatment effects, we are concerned with the modelling of correlation that may come from two possible sources, one among the longitudinal observations and the other between measurements taken from both eyes per subject. The data are nested repeated measurements. We suggest three models for analysis. Each one expresses the correlation differently in various covariance structures. We articulate their differences and describe the implementations in estimation using commercial statistical software. The computer output can be further utilized to perform model selection with Schwarz criterion. Simulation studies are conducted to evaluate the performance under each model. Data of the myopia intervention trial are reanalysed with these models for illustration. The results indicate that atropine is more effective in reducing the progression rate, the rates are homogeneous across subjects, and, among the suggested models, the one with independent random effects of two eyes fits best. We conclude that model selection is a crucial step before making inference with estimates; otherwise the correlation may be attributed incorrectly to a different mechanism. The same conclusion applies to other variance components as well.

Impact resistance of common spectacle and safety lenses to airgun and rimfire projectiles

PURPOSE

To evaluate the penetration resistance of common spectacle and safety lenses to high-velocity projectiles and to examine the current recommendations regarding the use of such lenses.

METHODS

The penetration threshold of glass, high-index, and polycarbonate spectacle and safety lenses was determined by firing BBs, pellets, and 0.22-caliber projectiles. The mass, diameter, velocity, and energy of each projectile were measured, and the center thickness, mass, and refractive power of each lens were recorded. The penetration threshold for each lens type was calculated, and a comparison of impact resistance was made.

RESULTS

The maximum velocity of the BB was measured at 221 m/s; pellets at 210 m/s; CB.22 rimfire projectiles at 204 m/s; and 0.22 rimfire projectiles at 290 m/s. The BB penetration thresholds were as follows: glass lenses = 84.0 m/s (2.4 J) and high-index lenses = 107.7 m/s (4.09 J). Polycarbonate spectacle and safety lenses were not penetrated by BBs regardless of their velocity; however, these lenses were penetrated by CB.22 rimfire projectiles (89.2 J). Review of the penetrated lenses showed that center thickness and refractive power was comparable between the various groups of projectiles.

CONCLUSIONS

Polycarbonate offers a vastly superior degree of penetration resistance compared with other commonly used lens materials. The current recommendations regarding the use of polycarbonate in prescription and protective lenses, as endorsed by the American Academy of Ophthalmology and the American Academy of Pediatrics, must be reevaluated.

Identifying sources of verification errors in progressive addition lenses

BACKGROUND

Occasional disagreement over spectacle lens prescription accuracy between the optical laboratory and the practitioner is to be expected, especially with a progressive addition lens (PAL). The PAL continues to evolve in design, yet retains some unique, inherent problems. The purpose of this study was to determine if the source of these problems results from the lens design, laboratory error, or verification procedures.

METHODS

Six Varilux Comfort Orma Supra progressive addition lenses were surfaced to plano distance power, and then verified using manual and automatic lensmeters. The same lenses were then re-surfaced for various sphere and cylinder powers, and verified in the same manner. The results of each trial were spherocylindrically averaged, and compared to the desired result. The lenses were also verified by several incorrect methods to investigate positional tolerance during verification and possible verification errors.

RESULTS

The averages for the lenses surfaced to the "prescribed" power were not far from the expected power by all measurement techniques when measured at the central DRP location. Positional errors of as little as 2 mm from the DRP center, however, can lead to erroneous results.

CONCLUSION

Unless correct verification procedures are carefully followed, progressive lenses can be mistakenly identified as being in error when they are not.

Sunglasses and sunglass standards

Sunglasses and sunglass standards are reviewed from the point of view of the history of sunglasses and the development of sunglass standards. The need for eye protection from solar radiation is discussed and the provisions of the various national sunglass standards are discussed in relation to that need.

Head and eye movements in visual search using night vision goggles

BACKGROUND

Night-vision goggles (NVGs) provide only a restricted field of view and have other characteristics that may affect the head and eye movements used in visual search.

METHODS

We measured head scan patterns, the magnitude and duration of gaze saccades, and fixation duration as subjects searched computer generated imagery either with or without NVGs. Subjects searched for either a large (6 degrees) target on a low-detail background (high conspicuity condition) or a small (2 degrees) target on a high-detail background (low conspicuity condition).

RESULTS

All subjects displayed head-scan patterns that were qualitatively similar to those reported in the literature. Although both head-scan speed and amplitude were higher for the NVG condition as compared with the no-NVG condition, the difference was not statistically significant. Head-scan speed did vary significantly with target conspicuity, however. Gaze saccade amplitude varied with target conspicuity but only when NVGs were used. Fixation duration did not vary with either NVG use or target conspicuity. A two-parameter (exponent and scaling parameter) power function was fitted to the amplitude-duration data. The power-function exponents varied from about 0.30 to 0.44, but there was a concommitant variation in the scaling parameter, and the result was no significant difference in the form of the power functions fitted to the data.

CONCLUSIONS

NVG use did not significantly affect any of the individual head or eye movement variables involved in searching the computer-generated imagery studied here. However, the decrease in gaze-saccade amplitude with increased target conspicuity when NVGs were used is evidence of the sensitivity of the head and eye movement measurement techniques used here, and suggests that all available measures of response efficiency be considered when evaluating NVGs using complex stimuli.

Free surface nickel in CE-marked and non-CE-marked spectacle frames

Three hundred and ninety-five new frames were tested for evidence of nickel release using dimethylglyoxime and ammonia. For areas of CE-marked frames in direct contact with the skin a considerable improvement was found over the previous such survey by the authors, carried out before the widespread introduction of CE marking. However, free nickel was found in 6.5% of CE-marked metal frames (excluding titanium and titanium alloy) in such areas and a majority of half joints. Some titanium alloy frames also tested positive for nickel in areas in contact with the skin.

Comparing optical systems, and the concept of the converter system

The purpose of this article was to present a complete and general method for comparing the first-order optical character of optical systems. The method provides a common basis for quantifying the difference between systems of all kinds including thin lenses, ophthalmic prisms, eyes before and after accommodation, eyes before and after refractive surgery, etc. Systems may be astigmatic or stigmatic, coaxial or noncoaxial. In special cases, the method reduces to being equivalent in essence to ostensibly incommensurate comparisons implicit or explicit in current optometric and ophthalmological usage (difference in power for refractions, corneas, and thin lenses, difference in prismatic power for prisms, ratio of magnifications for afocal telescopes, etc.). The method uses the concept of a converter system that when placed in front of or behind one system, converts its first-order optical character to the equivalent of a second system. Equations are presented for the ray transferences of the anterior and posterior converter systems for pair-wise comparisons in general. For any two systems, the transferences of the converter systems always exist and are unique. Numerical examples are presented; they illustrate converter systems that may be thin in special cases but thick otherwise. The transference of a converter system embodies and quantifies the optical difference between systems or characterizes the change from one state of a system (presurgical or preaccommodative, for example) to another (postsurgical or postaccommodative). The method provides a rational and uniform methodology for research and clinical applications in many areas of optometry and ophthalmology.

Resistance of plastic ophthalmic lenses: the effect of base curve on different materials during static load testing

PURPOSE

This study was designated to evaluate, through a static load test, the influence of lens base curve on the fracture resistance of three common plastic materials.

METHODS

A JJ Lloyd load cell machine was used to test the fracture resistance of -4.00 D spherical lenses. The samples had a nominal center thickness of 2.0 mm and a base curve distributed in one of five groups (+0.50, +2.50, +4.50, +6.50, and +8.50 D). The lenses were manufactured in CR39, polycarbonate, and TL16, a high refractive index plastic (n = 1.599).

RESULTS

The lens base curve influenced fracture resistance for all materials. For these materials, resistance increased as the base curve varied from +0.50 to +8.50 D. The resistance of CR39, TL16, and polycarbonate lenses was found to be linearly dependent on lens base curve. The effect is stronger for polycarbonate. Fracture resistance was higher for TL16 than for CR39, and polycarbonate was much more resistant to breakage than the two other materials.

CONCLUSIONS

For a given power, the fracture resistance of an ophthalmic lens is reduced when its base curve has a low value. Consequently, the flattening of ophthalmic lenses for cosmetic purposes is not recommended as far as fracture resistance is concerned.

Light adaptation: night vision goggle effect on cockpit instrument reading time

BACKGROUND

Light adaptation to the intensified image provided by a night vision device may handicap pilots who have set cockpit instrument luminance too low.

METHODS

Under conditions simulating night flying, subjects adapted to an NVG image at 3 or 10 footlamberts (fL), then used a joystick to indicate the position of the horizon in an ADI illuminated by NVIS-compatible light at luminances 2 to 3.5 log units lower than the NVG image.

RESULTS

Response times increased no more than a few tenths of a second when the decrease in luminance was only 2 log units. Greater decreases produced correspondingly longer delays in response, reaching as much as 5.5 s for subjects in their twenties and 8-15 s for older subjects.

CONCLUSIONS

While a decrease of more than 2 log units is not likely to occur under most operational conditions, it is certainly possible, and pilots should be aware that significant risk can be incurred by setting cockpit instruments to luminance levels below 0.03 fL.

Verifying and evaluating progressive addition lenses in clinical practice

BACKGROUND

Despite the fact that more than 50% of multifocal lenses dispensed in the United States are progressive addition lenses, adequate methods for clinical verification of these lenses have been lacking. Using automated lens meter techniques, the author describes a simplified method for verification of these complex lenses.

METHODS

Thirty pairs of progressive lenses were measured in a modified method using a Humphrey 330 Lens Analyzer. Fifteen pairs were "premium-quality" progressive lenses: fifteen pairs were "non-premium-quality" progressives. Five criteria were assessed on each lens: Distance Zone Width (DZW). Intermediate Zone Width (IZW), Near Zone Width (NZW), Drop Distance (DD), and Maximum Astigmatic Distortion (MAD).

RESULTS

"Premium-quality" progressive lenses failed to demonstrate clear-cut superiority over "non-premium-quality" progressive lenses in the five specified criteria. Individual measurements indicate considerable product inconsistency affected every brand tested.

CONCLUSIONS

Premium- and non-premium-quality progressive lenses demonstrated similar performance characteristics in this study. Zone size variation in these lenses was found to be considerable, a characteristic that seemed to cut across brand lines. The AO Compact lens seemed to demonstrate a shorter drop distance than other lenses, which does enhance its suitability for use with small frames. A comparison of the Essilor Natural PAL to the Younger Image lens showed little difference in the categories measured, although peripheral distortions seemed closer to the reading zone in the image. A comparison of the MAD of lenses in this study to lenses tested in 1986 indicates a considerable improvement has been made in that important characteristic.

Deflectometric measurement of mechanical spectacle lens deformation

The warpage of a set of six hard resin spherical lenses was measured when they were compressed by means of the tangential force exerted by a metal frame. The technique employed to measure the lenses improves over previous work both because greater precision is achieved, and because radii of curvature are measured in two perpendicular meridians. In this work, a representative sample of lenses has been used in order to test the way in which deformation depends on lens power and base. It is found that the induced warpage is enough to move the tested lenses between different base families, spoiling the initial design of the manufacturer.

The impact of variable tint optics (Transitions Gray) on vision-related quality of life in normal volunteers: a randomized clinical comparison in a warm climate

PURPOSE

A randomized crossover comparison of Transitions Gray variable tint optics (VTO) vs clear and fixed-tint lenses was undertaken to evaluate the impact of VTO on vision-related quality of life (VRQOL) in a warm climate.

METHODS

Fifty-nine patients were randomized to one of four lens crossover groups: Transitions-->clear; clear-->Transitions; Transitions-->fixed-tint; fixed-tint-->Transitions. Each lens was worn for 30 days. VRQOL was measured using a newly developed and validated questionnaire instrument-the Transitions VRQOL. Changes in visual acuity were assessed by functional exam.

RESULTS

Overall, Transitions was associated with the greatest improvement in VRQOL relative to clear and fixed-tint lenses without compromise in acuity. Transitions proved statistically superior to clear lenses, most notably in vision comfort both indoors and outdoors. Seventy percent of all patients selected Transitions as their primary lens at the end of the study.

CONCLUSIONS

Transitions brand VTO offer patients significant and clinically meaningful improvements in VRQOL superior to clear lenses. VRQOL assessments provide clinicians with valuable information above and beyond visual acuity to help optimize lens product selection and enhance patient satisfaction.