Biomedical subjects

Jim Schwiegerling

Publications and source records attributed to Jim Schwiegerling.

10 recordsLinked to original sources

PubMed2006

Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications.

Presbyopia is an age-related loss of accommodation of the human eye that manifests itself as inability to shift focus from distant to near objects. Assuming no refractive error, presbyopes have clear vision of distant objects; they require reading glasses for viewing near objects. Area-divided bifocal lenses are one example of a treatment for this problem. However, the field of view is limited in such eyeglasses, requiring the user to gaze down to accomplish near-vision tasks and in some cases causing dizziness and discomfort. Here, we report on previously undescribed switchable, flat, liquid-crystal diffractive lenses that can adaptively change their focusing power. The operation of these spectacle lenses is based on electrical control of the refractive index of a 5-mum-thick layer of nematic liquid crystal using a circular array of photolithographically defined transparent electrodes. It operates with high transmission, low voltage (<2 Vrms), fast response (<1 sec), diffraction efficiency > 90%, small aberrations, and a power-failure-safe configuration. These results represent significant advance in state-of-the-art liquid-crystal diffractive lenses for vision care and other applications. They have the potential of revolutionizing the field of presbyopia correction when combined with automatic adjustable focusing power.

Accommodation, Ocular↗

PubMed2006

Blue-light-absorbing lenses and their effect on scotopic vision.

PURPOSE: To correct assumptions about material transmission and provide a more realistic assessment of the optical properties of intraocular lenses (IOLs) made from AcrySof Natural material (Alcon). SETTING: University of Arizona Department of Ophthalmology, Tucson, Arizona, USA. METHODS: The transmission characteristics of conventional and AcrySof Natural IOL materials immersed in a balanced salt solution were examined to determine the differences in scotopic vision. The removal of the crystalline lens during cataract surgery and its effect on scotopic vision were also analyzed. RESULTS: When only considering the IOL properties, the AcrySof Natural lens decreased light entering the eye under scotopic by 14.6%, much lower than previously reported. Furthermore, when removal of the crystalline lens was taken into consideration, the AcrySof Natural material actually increased the amount of light entering the eye by 52% under these same conditions. CONCLUSIONS: Previous discrepancies in the transmission characteristics of IOL materials have led to an underestimation of the performance of the AcrySof Natural material under scotopic conditions. Accounting for both transmission and the crystalline lens demonstrates an increase in light levels entering the eye with the AcrySof Natural material when compared to young phakic subjects.

Absorption↗

PubMed2006

Recent developments in pseudophakic dysphotopsia.

PURPOSE OF REVIEW: Photic phenomena associated with intraocular lenses can degrade visual performance following intraocular lens implantation. Postoperative dysphotopsia introduces glare, halos, starbursts and shadows in a small number of patients. Understanding the optical mechanisms behind the introduction of these artifacts can lead to improved lens design and a reduction in the deleterious effects of stray light. This review looks at the improvement efforts of recent years to illustrate the systematic hunt for lens problems. RECENT FINDINGS: Improvements in edge designs have diminished the effects of positive dysphotopsia. However, negative dysphotopsia remains poorly understood and a variety of lens designs and materials can cause negative dysphotopsia. In other efforts, a testing procedure has been developed to improve understanding of the visual percept of a patient suffering dysphotopsia. This test should enlighten practitioners to the deficits their patients face and provide clues to the root causes of the problems. SUMMARY: Intraocular lenses can introduce stray light artifacts into the eye. These artifacts manifest themselves as glare, halos, starbursts and shadows. While positive dysphotopsia (glare, halos and starbursts) has been largely attributed to edge effects of the implant, negative dysphotopsia remains somewhat mysterious and appears to be more related to the patient's anatomical structure than to specific lens designs or materials.

Glare↗

PubMed2006

A Shack-Hartmann-based autorefractor.

PURPOSE: Autorefractors are typically based on either the optometer or the Scheiner principles, or a combination of the two techniques. These devices have dominated the market for objective assessment of refractive error for >30 years. The purpose of this investigation is to test a Shack-Hartmann-based system as an alternative to these systems. METHODS: Fourteen subjects with varying levels of refractive error were measured with a Topcon autorefractor and a Shack-Hartmann-based autorefractor. Fourier transform techniques were used to extract sphere, cylinder, and axis information from the Shack-Hartmann images, avoiding the need for image processing. The deviation of the refractive error from a subjective refraction was used as a means of comparing the two devices. RESULTS: The two devices performed similarly on this group of subjects. The mean difference in refraction between the two devices was nearly zero, suggesting that the likelihood and magnitude of errors for the two devices are equivalent. CONCLUSIONS: The Shack-Hartmann-based autorefractor shows promise as an alternative to conventional optometer or Scheiner-based technologies. However, issues with extending the myopic range of the device still need to be resolved.

Diagnostic Techniques, Ophthalmological↗

PubMed2006

Extracting wavefront error from Shack-Hartmann images using spatial demodulation.

PURPOSE: To determine whether the spatial demodulation processing of Shack-Hartmann images is suitable for extracting wavefront gradients for ocular wavefront sensors. METHODS: We developed a custom software program to implement the spatial demodulation technique. To test the algorithm's performance, we generated simulated spot images and obtained an eye examination image. We generated a collection of simulated aberrated spot images corresponding to: astigmatic wavefront (-5.00 -2.00 x 17), highly aberrated defocus (+/-20.00 diopters [D]), high-resolution defocus (-0.01 D), and third-order aberrations (trefoil and coma). The eye examination image and its measured Zernike coefficients were obtained from a Shack-Hartmann ocular aberrations system. We evaluated the output from the algorithm in terms of comparing the results to the known Zernike coefficients (for the simulated images) or the previously measured Zernike coefficients (for the eye examination image). RESULTS: The spatial demodulation algorithm was able to correctly recover the aberrations to better than 1/100 (0.01) D for the simulated spot images. The processing of the eye examination image yielded results within approximately 1/4 (0.25) D to the values provided by the Shack-Hartmann system. CONCLUSIONS: From the set of simulated images and the eye examination image used to test the spatial demodulation technique, it appears that the method is suitable for application in ocular wavefront aberrations Shack-Hartmann systems. The method appears capable of accurately processing high levels of aberrations (+/-20.00 D) as well as providing high resolution as evidenced by finding the -0.01 D defocus. The method may be especially well suited for processing highly aberrated wavefronts.

Algorithms↗

PubMed2004

Gaussian weighting of ocular wave-front measurements.

The measurement of ocular wave-front error gives insight into the optical performance of the eye and possibly a means for assessing visual performance. The visual system responds not only to the quality of the optical image formed on the retina but also to the processing that occurs in the retina and the brain. To develop a metric of visual performance based on wave-front error measurements, these latter processes must somehow be incorporated. In representing the wave-front error in terms of Zernike polynomials, it appears that terms with lower angular frequency have a greater deleterious effect on visual performance than higher-angular-frequency terms. A technique for weighting the pupil function of the eye with a Gaussian filter is demonstrated. It is further demonstrated that the variance of the Gaussian-weighted wave-front error is well correlated with visual performance.

Journal Article↗

PubMed2002

Scaling Zernike expansion coefficients to different pupil sizes.

Recent developments in technologies to correct aberrations in the eye have fostered extensive research in wave-front sensing of the eye, resulting in many reports of Zernike expansions of wave-front errors of the eye. For different reports of Zernike expansions, to be compared, the same pupil diameter is required. Since no standard pupil size has been established for reporting these results, a technique for converting Zernike expansion coefficients from one pupil size to another is needed. This investigation derives relationships between the Zernike expansion coefficients for two different pupil sizes.

Humans↗

PubMedDate not supplied

Higher order aberrations in normal, dilated, intraocular lens, and laser in situ keratomileusis corneas.

PURPOSE: To compare repeated measures of Zernike polynomial higher-order aberrations in 29 normal, 13 dilated normal, 11 intraocular lens (IOL), 11 laser in situ keratomileusis (LASIK), and one refractive keratectomy (RK)/IOL subject. METHODS: At least three Shack-Hartmann images were obtained from each subject, and higher order (uncorrectable by spectacles) Zernike representation was determined. For each subject, confidence intervals for each Zernike coefficient were determined as a function of pupil size. Significant (P<.05) coefficients were averaged within groups, and group means were compared to normal subjects. RESULTS: No differences were seen between the normal and dilated groups at P=.05. The patients with prior LASIK and IOL surgery showed statistically significant elevation of 4th order spherical aberration and total wavefront variance for pupil sizes greater than 5 mm, compared to normals. CONCLUSION: Both IOL and LASIK surgery elevate spherical aberration and wavefront variance, with increasing magnitude of effect with increasing pupil size, although pupillary dilation alone did not produce statistically different changes, as compared to normal subjects. These findings demonstrate that IOL implantation can produce more net aberrations than LASIK, and demonstrate a new opportunity to optimize surgical results.

Cornea↗

PubMedDate not supplied

Wavefront and topography: keratome-induced corneal changes demonstrate that both are needed for custom ablation.

PURPOSE: To examine the effects of laser in situ keratomileusis (LASIK) flap incision and healing on the shape of the cornea and the wavefront error of the eye. METHODS: Four weeks prior to bilateral LASIK, study subjects had a flap cut using a keratome in one eye. The fellow eye remained untouched as a control. Corneal topography and wavefront errors were measured at 1 day, 1, and 4 weeks after the flap was created. After 4 weeks, the flap was lifted and LASIK was performed. The control eye also had LASIK at this time. RESULTS: Differences in corneal shape and wavefront error consistent with a mild hyperopic shift were seen as a result of the keratome incision. CONCLUSION: Cutting the flap in LASIK causes subtle changes to corneal shape and the optics of the eye that may affect customized treatments. Additional work is needed to quantify these changes so that their effect can be incorporated into future treatments.

Cornea↗

PubMedDate not supplied

Modal reconstruction methods with Zernike polynomials.

PURPOSE: To compare the advantages and disadvantages of different techniques for fitting Zernike polynomials to surfaces. METHODS: Two different methods, Orthogonal Projection and Gram-Schmidt orthogonalization, are compared in terms of speed and performance at fitting a complex object. RESULTS: Orthogonal Projection provides an extremely rapid fitting of a surface, but leaves residual high frequency noise. The Gram-Schmidt technique provides a more accurate fit of the original object, but consumes much more computing time. Orthogonal Projection, and its associated noise, may be tolerated in classifying corneal topography. CONCLUSIONS: Orthogonal Projection has a distinct advantage in calculation time over other methods for fitting surfaces. This advantage may be exploited in cases where accurate surface fitting is not necessary, but only general features need to be extracted for classification. If fit accuracy is needed, then slower fitting techniques, such as Gram-Schmidt, should be used.

Cornea↗