Vision, Driving Exposure, and Collisions in Bioptic Drivers.

SIGNIFICANCE: Lack of knowledge regarding the mileage driven by drivers with low vision who use bioptic telescopes could obscure the relationship between vision and road safety. This study provides data suggesting that worse vision is correlated with less mileage driven but more collisions per mile in bioptic drivers. PURPOSE: The purpose of this study was to determine whether vision or demographic factors predict mileage driven in bioptic drivers and per-mile motor vehicle collision rate and also to compare the collision rate of bioptic drivers with previous estimates for the general population. METHODS: Driver data were collected retrospectively from clinic records. Collision data were collected from the Ohio Bureau of Motor Vehicles database. Subjects were also asked to estimate their yearly mileage. Regression models were used to investigate relationships between vision and collision rates. RESULTS: Seventy-three licensed Ohio bioptic drivers (36 male) were included. Mean ± standard deviation age was 51 ± 16 years. Mean logMAR visual acuity was 0.67 (approximately 20/100). Mean log contrast sensitivity was 1.57. Mean reported annual mileage was 9746. Age, sex, and previous (nonbioptic) driving experience were not associated with mileage. LogMAR visual acuity was inversely related to mileage (P = .02), and contrast sensitivity (P = .01) and horizontal visual field (P = .02) were directly associated with mileage. Visual acuity (P = .02) and visual field (P = .005), but not contrast sensitivity (P = .19), were associated with number of collisions. CONCLUSIONS: Visual acuity, visual field, and contrast sensitivity were associated with driving exposure in bioptic drivers (with drivers with poorer vision reporting lower annual mileage), and poorer visual acuity and visual field were associated with more collisions. The per-mile collision rate for bioptic drivers was within the range of that previously reported for fully sighted drivers, although higher than would be expected for fully sighted drivers of similar age distribution.

Design, fabrication, and testing of a Shack-Hartmann sensor with an automatic registration feature.

In this research, design, construction, and testing of an innovative Shack-Hartmann sensor are described. As the most critical component, a polymer microlens array is injection molded and mounted on a board-level CMOS camera such that the focal plane of the microlens array is on the camera's image plane. To allow for automatic registration of the spots of the measured area, a diffusing surface was created at the center of the lens array in the same diamond machining process in an uninterrupted operation. This unique diffusing surface does not generate an image spot. The no-spot feature functions as the reference in the measurement on the camera's image plane. Using this unique feature, large global tip-tilt error can be detected and eliminated. In this research, both experiments and simulation have shown that the Shack-Hartmann sensor built using low cost components is capable of precision wavefront detection. This research also demonstrated that automatic registration based on the diffusing surface is simple and reliable.

Previous Driving Experience, but Not Vision, Is Associated With Motor Vehicle Collision Rate in Bioptic Drivers.

PURPOSE: Bioptic telescopic spectacles (BTS) consist of a small telescope (or telescopes) mounted high in a pair of spectacle lenses. More than 40 states allow for some form of bioptic driving licensure for people with decreased central vision. The purpose of this study was to determine significant associations among previous driving experience, vision, and motor vehicle collisions (MVCs) for bioptic drivers in Ohio. METHODS: We conducted a retrospective study of patients who received a vision examination and subsequently obtained bioptic licensure. We obtained driving records from the Ohio Bureau of Motor Vehicles in order to determine MVC involvement. Relationships among vision measures, age, sex, previous experience, and MVCs were investigated using time-to-event analysis and the Cox proportional hazards regression model. RESULTS: We identified 237 bioptic drivers (65% male). Age at initial exam ranged from 16 to 81 years, and mean visual acuity was approximately 20/120. The number of MVCs per driver ranged from 0 to 11, with 124 (52%) drivers having had at least one MVC. Visual acuity and contrast sensitivity were not significant predictors of MVC. Drivers without previous driving experience were significantly more likely to have been involved in an MVC (P < 0.001), and this association remained significant after adjusting for age and sex (P = 0.01). The rate of MVC per year decreased steadily over a 10-year period for drivers without previous experience. CONCLUSIONS: Previous nonbioptic driving experience, but not visual acuity or contrast sensitivity, was associated with yearly MVC rate in bioptic drivers.

Vision, training hours, and road testing results in bioptic drivers.

PURPOSE: Bioptic telescopic spectacles can be used by people with central visual acuity that does not meet the state standards to obtain an unrestricted driver's license. The purpose of this study was to examine the relationships among visual and demographic factors, training hours, and the results of road testing for bioptic drivers. METHODS: A retrospective study of patients who received an initial daylight bioptic examination at the Ohio State University and subsequently received a bioptic license was conducted. Data were collected on vision including visual acuity, contrast sensitivity, and visual field. Hours of driver training and results of Highway Patrol road testing were extracted from records. Relationships among vision, training hours, and road testing were analyzed. RESULTS: Ninety-seven patients who completed a vision examination between 2004 and 2008 and received daylight licensure with bioptic telescopic spectacles were included. Results of the first Highway Patrol road test were available for 74 patients. The median (interquartile range) hours of training before road testing was 21 (17) hours (range, 9 to 75 hours). Candidates without previous licensure were younger (p < 0.001) and had more documented training (p < 0.001). Lack of previous licensure and more training were significantly associated with having failed a portion of the Highway Patrol test and points deducted on the road test. CONCLUSIONS: New bioptic drivers without previous nonbioptic driving experience required more training and performed more poorly on road testing for licensure than those who had previous nonbioptic licensure. No visual factor was predictive of road testing results after adjustment for previous experience. The hours of training received remained predictive of road testing outcome even with adjustment for previous experience. These results suggest that previous experience and trainer assessments should be investigated as potential predictors of road safety in bioptic drivers in future studies.

Fabrication of microinjection-molded miniature freeform Alvarez lenses.

Microinjection molding is a mass production method to fabricate affordable optical components. However, the intense nature of this process often results in part deformation and uneven refractive index distribution. These two factors limit the precision of replicated optics. In order to understand the influences of injection molding on freeform optical devices, in this study, finite element method (FEM) was employed to investigate the miniature microinjection-molded Alvarez lenses. In addition, an innovative metrology setup was proposed to evaluate the optical wavefront patterns in the molded lenses using an interferometer-based wavefront measurement system. This measurement setup utilized an optical matching liquid to reduce or eliminate the lenses' surface power such that the wavefront pattern with large deviation from the freeform lenses can be measured by a regular wavefront setup. The FEM simulation results were also used to explain the differences between the nominal and experimentally measured wavefront patterns of the microinjection-molded Alvarez lenses. In summary, the proposed method combining simulation and wavefront measurements is shown to be an effective approach for studying injection molding of freeform optics.

Propagation of aberrated wavefronts using a ray transfer matrix.

A ray transfer matrix is used to calculate the propagation of aberrated wavefronts across a homogeneous refractive index. The wavefront is represented by local surface normals, i.e., by a ray bundle, and the propagation is accomplished by transferring those rays across the space. Wavefront shape is generated from the slopes and positions of the collection of rays. Calculation methods are developed for the paraxial case, for higher-order expansions, and for the exact tangent case. A numerical example is used to compare results between an analytical method and the methods developed here.

Simulation and measurement of optical aberrations of injection molded progressive addition lenses.

Injection molding is an important mass-production tool in the optical industry. In this research our aim is to develop a process of combining ultraprecision diamond turning and injection molding to create a unique low-cost manufacturing process for progressive addition lenses (PALs). In industry, it is a well-known fact that refractive index variation and geometric deformation of injection molded lenses due to the rheological properties of polymers will distort their optical performance. To address this problem, we developed a method for determining the optical aberrations of the injection molded PALs. This method involves reconstructing the wavefront pattern in the presence of uneven refractive index distribution and surface warpage using a finite element method. In addition to numerical modeling, a measurement system based on a Shack-Hartmann wavefront sensor was used to verify the modeling results. The measured spherocylindrical powers and aberrations of the PALs were in good agreement with the model. Consequently, the optical aberrations of injection molded PALs were successfully predicted by finite element modeling. In summary, it was demonstrated in this study that numerically based optimization for PAL manufacturing is feasible.

Comparison of progressive addition lenses by direct measurement of surface shape.

PURPOSE: To compare the optical properties of five state-of-the-art progressive addition lenses (PALs) by direct physical measurement of surface shape. METHODS: Five contemporary freeform PALs (Varilux Comfort Enhanced, Varilux Physio Enhanced, Hoya Lifestyle, Shamir Autograph, and Zeiss Individual) with plano distance power and a +2.00-diopter add were measured with a coordinate measuring machine. The front and back surface heights were physically measured, and the optical properties of each surface, and their combination, were calculated with custom MATLAB routines. Surface shape was described as the sum of Zernike polynomials. Progressive addition lenses were represented as contour plots of spherical equivalent power, cylindrical power, and higher order aberrations (HOAs). Maximum power rate, minimum 1.00-DC corridor width, percentage of lens area with less than 1.00 DC, and root mean square of HOAs were also compared. RESULTS: Comfort Enhanced and Physio Enhanced have freeform front surfaces, Shamir Autograph and Zeiss Individual have freeform back surfaces, and Hoya Lifestyle has freeform properties on both surfaces. However, the overall optical properties are similar, regardless of the lens design. The maximum power rate is between 0.08 and 0.12 diopters per millimeter and the minimum corridor width is between 8 and 11 mm. For a 40-mm lens diameter, the percentage of lens area with less than 1.00 DC is between 64 and 76%. The third-order Zernike terms are the dominant high-order terms in HOAs (78 to 93% of overall shape variance). Higher order aberrations are higher along the corridor area and around the near zone. The maximum root mean square of HOAs based on a 4.5-mm pupil size around the corridor area is between 0.05 and 0.06 µm. CONCLUSIONS: This nonoptical method using a coordinate measuring machine can be used to evaluate a PAL by surface height measurements, with the optical properties directly related to its front and back surface designs.

Comparison of three techniques in measuring progressive addition lenses.

PURPOSE: To measure progressive addition lenses (PALs) by three techniques and to compare the differences across techniques. METHODS: Five contemporary PALs (Varilux Comfort Enhanced, Varilux Physio Enhanced, Hoya Lifestyle, Shamir Autograph, and Zeiss individual) with plano distance power and a +2.00 diopters (D) add were evaluated under the condition of lateral displacement of the lens (no rotation and no tilt) using three methods. A Hartmann-Shack wavefront sensor (HSWFS) on a custom-built optical bench was used to capture and measure wavefront aberrations. A Rotlex Class Plus lens analyzer operating as a moiré interferometer was used to measure spherical and cylindrical powers. A coordinate measuring machine (CMM) was used to measure front and back surfaces of PALs and converted to desired optical properties. The data were analyzed with MATLAB programs. Contour plots of spherical equivalent power, cylindrical power, and higher-order aberrations (HOAs) in all PALs were generated to compare their differences. RESULTS: The differences in spherical equivalent and cylinder at distance, near, and progressive corridor areas among the HSWFS, Rotlex, and CMM methods were close to zero in all five PALs. The maximum differences are approximately 0.50 D and located below the near power zone and the edge areas of the lens when comparing the HSWFS and CMM with the Rotlex. HOAs measured both by the HSWFS and CMM were highest in the corridor area and the area surrounding the near zone in all PALs. The HOAs measured by the CMM were lower than those from the HSWFS by 0.02 to 0.04 µm. CONCLUSIONS: The three measurement methods are comparable for measuring spherical and cylindrical power across PALs. The non-optical method, CMM, can be used to evaluate the optical properties of a PAL by measuring front and back surface height measurements, although its estimates of HOAs are lower than those from the HSWFS.

Evaluation of single pixel step sizes in visual acuity assessment.

PURPOSE: Adaptive thresholding procedures (e.g., ZEST) benefit from testing near the current estimate of threshold. Therefore, small increments may improve precision of these techniques. We evaluated the validity of visual acuity assessment when letters are not constrained to a multiple of 5 pixels high. METHODS: Snellen letters were presented over a ±5 pixel range spanning the final threshold estimate. Sizes that were a multiple of 5 pixels (e.g., 20 and 25) were presented 160 times, and other sizes were presented 40 times. This resulted in 320 letter presentations at multiples of 5 and at non-multiples of 5. Post hoc ZEST procedures were used to compute acuity threshold estimates based on testing at those two sets of sizes. Two subjects participated: subject 1 for the right and left eyes and subject 2 in two sessions with binocular viewing. Psychometric functions were fit to the data and were used to model simulated observers. Simulations were run for each data set, and the number of presentations at each size matched the number presented to the subject. Ninety percent limits for proportion correct at each letter size were determined from the simulation data, and these limits were used as criteria for identifying outliers in the subject's data. RESULTS: No significant difference in acuity thresholds were found when letters were rendered in multiples of 5 pixel heights vs. letters without this constraint. Empirical data fell within the bounds defined by simulations, with no idiosyncratic results found for either set of letter sizes. CONCLUSIONS: Letter sizes that do not conform to a multiple of 5 pixel letter height yield valid acuity scores. This facilitates adaptive psychometric procedures for acuity testing, such as ZEST and QUEST, in which letter sizes are selected to be as close to the current estimate of threshold size as possible.

Whole-surface characterization of progressive addition lenses.

PURPOSE: To characterize and compare progressive addition lens (PAL) surfaces in terms of the Zernike polynomials. METHODS: We examine the spherocylindrical characteristics of PALs by performing a physical surface height measurement of three lenses. Surface shape is described as the sum of Zernike polynomials. These data were used to analyze and compare each surface, using the second derivatives of surface height. We compare PALs to each other and to a prototypical progressive surface, i.e., the Alvarez lens surface. RESULTS: The high order freeform surfaces of PALs produce a gradient of spherical power across the lens aperture and also a profile of astigmatic power with high order aberration. Each lens is similar to the Alvarez surface, but each has characteristics that differentiate it from the others. Each high order Zernike term contributes sphere and/or astigmatism, and aberration, to the surface. CONCLUSIONS: This approach to PAL analysis, in which the entire lens surface is defined and expressed as the sum of Zernike polynomial terms, is different from most previous approaches in which a lens is described as an array of subapertures. It is potentially an efficient and useful method for defining and designing PAL surfaces.

Development of a battery of functional tests for low vision.

PURPOSE: We describe the development and evaluation of a battery of tests of functional visual performance of everyday tasks intended to be suitable for assessment of low vision patients. METHODS: The functional test battery comprises-Reading rate: reading aloud 20 unrelated words for each of four print sizes (8, 4, 2, & 1 M); Telephone book: finding a name and reading the telephone number; Medicine bottle label: reading the name and dosing; Utility bill: reading the due date and amount due; Cooking instructions: reading cooking time on a food package; Coin sorting: making a specified amount from coins placed on a table; Playing card recognition: identifying denomination and suit; and Face recognition: identifying expressions of printed, life-size faces at 1 and 3 m. All tests were timed except face and playing card recognition. Fourteen normally sighted and 24 low vision subjects were assessed with the functional test battery. Visual acuity, contrast sensitivity, and quality of life (National Eye Institute Visual Function Questionnaire 25 [NEI-VFQ 25]) were measured and the functional tests repeated. Subsequently, 23 low vision patients participated in a pilot randomized clinical trial with half receiving low vision rehabilitation and half a delayed intervention. The functional tests were administered at enrollment and 3 months later. RESULTS: Normally sighted subjects could perform all tasks but the proportion of trials performed correctly by the low vision subjects ranged from 35% for face recognition at 3 m, to 95% for the playing card identification. On average, low vision subjects performed three times slower than the normally sighted subjects. Timed tasks with a visual search component showed poorer repeatability. In the pilot clinical trial, low vision rehabilitation produced the greatest improvement for the medicine bottle and cooking instruction tasks. CONCLUSIONS: Performance of patients on these functional tests has been assessed. Some appear responsive to low vision rehabilitation.

Automated decision tree classification of corneal shape.

PURPOSE: The volume and complexity of data produced during videokeratography examinations present a challenge of interpretation. As a consequence, results are often analyzed qualitatively by subjective pattern recognition or reduced to comparisons of summary indices. We describe the application of decision tree induction, an automated machine learning classification method, to discriminate between normal and keratoconic corneal shapes in an objective and quantitative way. We then compared this method with other known classification methods. METHODS: The corneal surface was modeled with a seventh-order Zernike polynomial for 132 normal eyes of 92 subjects and 112 eyes of 71 subjects diagnosed with keratoconus. A decision tree classifier was induced using the C4.5 algorithm, and its classification performance was compared with the modified Rabinowitz-McDonnell index, Schwiegerling's Z3 index (Z3), Keratoconus Prediction Index (KPI), KISA%, and Cone Location and Magnitude Index using recommended classification thresholds for each method. We also evaluated the area under the receiver operator characteristic (ROC) curve for each classification method. RESULTS: Our decision tree classifier performed equal to or better than the other classifiers tested: accuracy was 92% and the area under the ROC curve was 0.97. Our decision tree classifier reduced the information needed to distinguish between normal and keratoconus eyes using four of 36 Zernike polynomial coefficients. The four surface features selected as classification attributes by the decision tree method were inferior elevation, greater sagittal depth, oblique toricity, and trefoil. CONCLUSION: Automated decision tree classification of corneal shape through Zernike polynomials is an accurate quantitative method of classification that is interpretable and can be generated from any instrument platform capable of raw elevation data output. This method of pattern classification is extendable to other classification problems.

Design and fabrication of a freeform phase plate for high-order ocular aberration correction.

In recent years it has become possible to measure and in some instances to correct the high-order aberrations of human eyes. We have investigated the correction of wavefront error of human eyes by using phase plates designed to compensate for that error. The wavefront aberrations of the four eyes of two subjects were experimentally determined, and compensating phase plates were machined with an ultraprecision diamond-turning machine equipped with four independent axes. A slow-tool servo freeform trajectory was developed for the machine tool path. The machined phase-correction plates were measured and compared with the original design values to validate the process. The position of the phase-plate relative to the pupil is discussed. The practical utility of this mode of aberration correction was investigated with visual acuity testing. The results are consistent with the potential benefit of aberration correction but also underscore the critical positioning requirements of this mode of aberration correction. This process is described in detail from optical measurements, through machining process design and development, to final results.

Corneal asphericity and apical curvature in children: a cross-sectional and longitudinal evaluation.

PURPOSE: The contour of the human cornea is closely modeled by a conic section, which is fully described by asphericity (Q) and apical radius of curvature (r(o)). The relationship between corneal shape and other ocular dimensions in children, including anterior and vitreous chamber depths, axial length, and spherical equivalent refractive error, was investigated. METHODS: Corneal asphericity and r(o) were calculated by using corneal topography data on 643 children (72 myopes, 370 emmetropes, and 201 hyperopes), ages 6 to 15 years, who participated in the Orinda Longitudinal Study of Myopia (OLSM) during 1991. Measurements from a younger subset of these children, including 8 myopes, 92 emmetropes, and 75 hyperopes, ages 6 to 9 years in 1991, were compared to 1996 data for longitudinal analysis. RESULTS: Mean +/- SD Q of the 1991 study sample was -0.346 +/- 0.101, representing a prolate corneal shape. Almost all (99.7%) of the corneas examined were prolate. Corneal asphericity was less prolate among myopes than in emmetropes and hyperopes (P = 0.010). Less prolate corneas were related to deeper anterior chamber depths among emmetropes (r = 0.324, P < 0.0001) and hyperopes (r = 0.275, P < 0.0001), but not among myopes (r = 0.230, P = 0.0515). Flatter values of r(o) were related to longer vitreous chamber depth (r = 0.607, P < 0.0001) and axial length (r = 0.606, P < 0.0001) in all refractive error groups. Initial corneal shape was unrelated to change in refractive error over a 5-year period. CONCLUSIONS: Most corneas examined in this study were prolate in contour. Deeper anterior chamber depths were related to less prolate corneas among emmetropes and hyperopes, which is probably the result of mechanical influences on the peripheral cornea as the anterior chamber elongates during ocular growth. Longitudinal results suggest initial corneal shape is of little or no value in predicting refractive error progression.

Repeatability of corneal topography measurement in keratoconus with the TMS-1.

PURPOSE: The purpose of this study was to report the test-retest variability of simulated indices derived from the TMS-1 topography instrument (Tomey Technology, Waltham, MA) in keratoconus subjects enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. METHODS: Four images were taken at an initial visit and at a repeat visit several weeks later. From these images, 17 indices were simulated from published formulas. Mixed-model analysis was used on test-retest data from the TMS-1 videokeratography instrument during the baseline year. This analysis yields estimates of within- and between-visit variability. RESULTS: Repeatability analysis revealed that within-visit standard errors were 1.0 to 5.9 times greater in keratoconus eyes than in normal controls when two images were analyzed from each visit. These values changed only slightly when more images were used. The ratio of between-visit standard errors of the indices were nearly equally greater than normal controls for (0.9-4.6 and 0.9-4.3) two images per eye and all images per eye, respectively. CONCLUSIONS: These results suggest that the repeatability of simulated indices derived from TMS-1 topography in keratoconus subjects is poorer than in normal controls.

Enhancing RGP contact lens performance: comparing back surface options.

BACKGROUND: In some cases, rigid gas-permeable (RGP) contact lenses may be the best--or only--means of refractive correction. High Dk RGP materials have markedly reduced hypoxia under those lenses. With aspheric lens back surface designs, post-lens circulation may be enhanced as well by maximizing the provision of nutrients and the clearance of metabolic by-products, toxins, and debris, while minimizing the risk of lens binding. METHODS: Performances of four aspheric back surface RGP designs were compared with a spherical optic zone and peripheral curve back surface reference design. Oxygen uptake rates were measured for each of 40 wearing combinations (five lens designs fitted to each of eight eyes) under non-blink and blink conditions for their effectiveness in reduction of post-lens hypoxia. Hypoxic reductions with blinking were scaled in exchange efficiency (EE) units, and used as relative indicators of post-lens tear exchange. RESULTS: Combined exchange efficiency index (EE) scores (averaged responses across all eight eyes) for each of the five lens back surface designs ranged from a best overall performance of +13.9 EE units down to only +2.5 EE units, with the spherical reference design averaging +8.9 EE units. Tear pump efficiency of each of the eight eyes (averaged responses across all five lens designs) ranged from a high of +12.3 EE units down to -9.8 EE units (i.e., worse than the non-blink condition of 0 EE units). Among the 40 eye-lens back surface design combinations studied here, the highest exchange efficiency score registered was +28.4 EE units, the lowest being -13.8 EE units. CONCLUSIONS: Aspheric lens back surface and/or peripheral curve designs were found to vary significantly in their post-lens exchange efficiency performances, but no "universal problem-solver" design was found among the five we investigated.

Between-eye asymmetry in keratoconus.

PURPOSE: To report baseline differences between eyes on key variables in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study cohort compared with a retrospectively assembled group of myopic contact lens wearers without ocular disease. METHODS: A total of 1,079 keratoconus patients who had not undergone a penetrating keratoplasty in either eye before their baseline visit were enrolled and examined at baseline. Records from 330 contact lens-wearing myopes were reviewed. Corneal curvature (keratometry), visual acuity, refractive error (manifest refraction), and corneal scarring were measured. RESULTS: The mean differences between keratoconic eyes are as follows (better eye-worse eye for each variable, separately). Flat keratometry: -3.59 +/-4.46 D and steep keratometry: -4.35 +/-4.41 D; high-contrast best-corrected visual acuity: 7.30 +/-6.83 letters; low-contrast best-corrected visual acuity: 8.53 +/-7.51 letters; high-contrast entrance visual acuity: 9.03 +/-8.40 letters; low-contrast entrance visual acuity: 9.43 +/-7.88 letters; spherical equivalent refractive error: 3.15 +/-3.84 D; and refractive cylinder power 1.55 +/-1.42 D. Twenty-one percent of the keratoconus patients had corneal scarring in only one eye. There is an association between patient-reported unilateral eye rubbing and greater asymmetry in corneal curvature, and between a history of unilateral eye trauma and greater asymmetry in corneal curvature and refractive error, with the rubbed/traumatized eye being the steeper eye most of the time. CONCLUSIONS: Keratoconus is asymmetric in the CLEK Study sample.