Thesis (Ph.D.)--University of Nebraska-Lincoln, 2006. / Title from title screen (site viewed on Nov. 13, 2006). PDF text: xiii, 188 p. : ill. ; 11.13Mb. UMI publication number: AAT 3217532. Includes bibliographical references. Also available in microfilm and microfiche format.

Phoropters are the most common instrument used to detect refractive errors. During a refractive exam, lenses are flipped in front of the patient who looks at the eye chart and tries to read the symbols. The procedure is fully dependent on the cooperation of the patient to read the eye chart, provides only a subjective measurement of visual acuity, and can at best provide a rough estimate of the patient's vision. Phoropters are difficult to use for mass screenings requiring a skilled examiner, and it is hard to screen young children and the elderly etc. We have developed a simplified, lightweight automatic phoropter that can measure the optical error of the eye objectively without requiring the patient's input. The automatic holographic adaptive phoropter is based on a Shack-Hartmann wave front sensor and three computer-controlled fluidic lenses. The fluidic lens system is designed to be able to provide power and astigmatic corrections over a large range of corrections without the need for verbal feedback from the patient in less than 20 seconds.

The launch of silicone hydrogel contact lenses has led to a rise in the incidence of mechanically-related clinical complications, which is thought to be due to the increased stiffness of these materials compared to conventional hydrogel lens materials. The mechanical characteristics of hydrogel contact lenses have traditionally been investigated using tensile testing which investigated the bulk material characteristics. This thesis presents a study intended to establish a repeatable method for local mechanical measurement of hydrogel contact lenses using nanoindentation. Hydrogel materials in phosphate buffered saline were indented using a Hysitron Triboindenter mounted on a Veeco Explorer AFM using Triboscope software (version 3.5a) with a specially constructed wet cell. A model hydrogel (poly(HEMA-MMA)) was used to validate the methodology and investigate a the effect of controlled change in specimen thickness. A range of commercially available hydrogel contact lenses were then characterised (including conventional and silicone hydrogel lenses) using the same method. Two different analytical techniques were employed to determine the mechanical properties data; elastic analysis and a time-dependent viscoelastic analytical technique.A strong influence of specimen thickness on apparent mechanical properties was seen with the elastic analysis and an empirical relationship was derived to correct for this which was found to be appropriate for all contact lens specimens studied and reported in the thesis. The viscoelastic analysis results were more complex and exhibited a less clear influence of specimen thickness. However, as this is a very simple approximation as contact lenses are suspected to be poroelastic rather than viscoelastic this work could not be fully resolved in the scope of this thesis. For all contact lenses analysed, nanoindentation produced data similar to that found with conventional tensile testing, however, there was evidence for a slight dependence of elastic properties across the lens that does not correlate with sample thickness. This thesis shows the development of a way of accounting for the variation of thickness of a range of contact lenses, and demonstrated that traditional analysis is accurate enough to determine local differences in modulus across contact lenses. The viscoelastic analysis may be more appropriate for hydrogels, however, it produced irregularities that will require further work to fully resolve.

Printed lens antennas offer the possibility of realizing shaped beam patterns using no more complexity than is required for pencil beam patterns. Shaped beam patterns can be obtained by appropriately determining the complex transmission coefficient required for each cell (or element) of the printed lens, taking into account the varying feed field over the input surface of the lens. Certain ranges of transmission coefficient amplitude and phase are undesirable (eg. too low an amplitude implies a large reflection at the lens input surface). It would be preferable to constrain the range of values that the transmission coefficient can take as an integral part of the lens synthesis procedure, and thus the transmission coefficient itself needs to be the synthesis variable. In this thesis a synthesis technique for doing this is developed based on the method of generalized projections, modified to “operate” in the space of transmission coefficients. This makes it possible to immediately perceive what influence constraints on the actual transmission coefficients have on the possible radiation pattern performance. In addition, an approach that allows one to constrain the transmission coefficient to values that must be selected from an available database of transmission coefficients is incorporated into the synthesis technique.

The lens design is a fundamental but challenging problem, while modern lens design processes still follow the classic aberration optimization theory and need preliminary designs and experienced optical engineers to control the optimization process constantly. In this thesis, we develop a differentiable ray-tracing model and apply it to automatic lens design. Our method can do ray-tracing and render images with high accuracy, with the power to use the back-propagated gradient to optimize optical parameters. Different from traditional optical design, we propose to use the rendered images as the training criteria. The rendering loss shows superior results in optimizing lenses while also making the task easier. To remove the requirements of preliminary design and constant operations in conventional lens design, we propose a curriculum learning method that starts from a small aperture and field-of-view(FoV), gradually increases the design difficulty, and dynamically adjusts attention regions of rendered images. The proposed curriculum strategies empower us to optimize complex lenses from flat surfaces automatically. Given an existing lens design and setting all surfaces flat, our method can entirely recover the original design. Even with only design targets, our method can automatically generate starting points with flat surfaces and optimize to get a design with superior optical performance. The proposed method is applied to both spheric and aspheric lenses, both camera and cellphone lenses, showing a robust ability to optimize different types of lenses. In addition, we overcome the memory problem in differentiable rendering by splitting the differentiable rendering model into two sub-processes, which allows us to work with megapixel sensors and downstream imaging processing algorithms.

The aim of this work is to characterize the microstructural and mechanical behaviors of Additive Manufactured (AM) Inconel 718 and investigate the feasibility of using this material to produce thermal management devices. This was done in two parts: a literature review of AM phenomena that effect heat transfer rates and impede or benefit production using these methods, and a study that characterized Laser Engineered Net Shaping (LENS) Inconel 718 while investigating and manipulating the thermal history. The literature review provides evidence that while there are still many unsolved issues, it’s quickly becoming possible to use AM to produce thermal management devices that will exceed current limitations. The study showed that although LENS Inconel 718 exhibits nonstandard material behaviors, evidence indicate that it’s possible to control these behaviors to influence desired results. Overall, it is believed that the use of AM in producing thermal management devices will be inevitable and beneficial.

Transcription growth factor beta (TGFβ) mediated epithelial to mesenchymal transition (EMT) of lens epithelial cells (LEC) is known to cause posterior capsular opacification (PCO). In this work, I have focused on the TGFβ-induced EMT pathway governed by the cellular actin cytoskeleton dynamics. This study is the first to report the involvement of transcription co-factor myocardin related transcription factor-A (MRTF-A) in TGFβ-induced EMT in the lens. Using rat lens epithelial explants, I have conclusively established that in LECs, TGFβ induces nuclear migration of MRTF-A leading to induction of αSMA expression. Furthermore, I have manipulated the intracellular translocation of MRTF-A indirectly using actin binding drugs and established that inhibiting nuclear migration of MRTF-A reduces αSMA production by the cells. In addition, direct manipulation of MRTF-A using adenoviral vectors carrying modified gene constructs show that presence of functional MRTF-A construct in the nucleus is necessary to trigger αSMA expression by causing EMT. In order to understand the involvement of matrix metalloproteinase -9 (MMP-9) in this specific pathway, explants were treated with an MMP2/9 inhibitor and rhMMP9. I have established that rhMMP-9 does not significantly affect the intracellular migration of MRTF-A. Nevertheless, gene expression studies showed that MMP-9 induces the expression of MRTF-A. Taken together, I believe MMP-9 functions through a feedback mechanism controlling MRTF-A expression in the cell. However, the presence of MMP-9 is necessary but not sufficient for induction in MRTF-A nuclear translocation. Overall, the work presented in this thesis demonstrates for the first time the presence of MRTF-A in LECs and successfully shows that the intracellular translocation of MRTF-A as an integral part of TGFβ induced EMT. Therefore, in the future, MRTF-A may be used as a successful target molecule to inhibit in order to prevent EMT leading to PCO. / Thesis / Doctor of Philosophy (PhD)

Research on novel solar tracking methods using two polymer-based approaches was conducted. The first design for a solar tracking system with luminescent particles moving along polymer coatings is reported to provide the localized absorption of sunlight by phosphors which can follow the sun and hence provides a higher light intensity to be guided to a solar cell with higher efficiency. The second approach for realizing a polymer-based deformable lens on the surface of patterned ITO glass for light concentrating is reported. Both approaches rely on the application of spatially defined heating to a diluted polymer. These experiments were motivated by a) the well-known Rayleigh-Bénard convection flow within a fluid and b) Bénard-Marangoni flow effects concerning the movement of liquid along the surface of the fluid based on a surface tension gradient as a function of temperature. In the first project, luminescent particles (YAG:Ce) are placed on the surface of a polymer film cast from a hot melt glue stick with a low melting point. When a heating wire 0.5 mm away from the top surface of the polymer film sample is transferred across the polymer, the floating YAG:Ce particles on the polymer surface can be pushed forward. In order to understand the mechanism of the particle movement, a laser-based measuring method was developed to view the surface profile of the melting polymer in situ. The melted glue stick polymer is observed to form a valley-like surface cross section that is able to transport YAG:Ce powder particles much like a surfer is carried forward on a wave. In the second project, a 0.25 mm thick polystyrene polymer containing a toluene solvent is cast on an ITO coated glass substrate with ITO stripe widths of 2, 6, and 10 mm. The heating source comprised of the ITO stripe can produce spatially selective heat when electric current is applied to thermally deform the cast polymer/solvent layer (polystyrene) on the substrate surface. After the deformation is complete an LED light source is used to determine the light concentration properties of the thermally formed lenses. The ITO stripe surface temperature profile was measured with a thermocouple and modelled with COMSOL Multiphysics software. The vaporization weight loss of solvent was also determined. The optics of the LED light concentration was modelled by Optics Lab software. When the light emitted from the source passes through the thermally deformed polymer, focusing into two beams occurs in agreement with modelling results. / Thesis / Master of Science (MSc)

In order for additive-manufactured parts to become widely utilized and trusted in application, their mechanical properties must be characterized. This study investigates the fatigue behavior and failure mechanisms of Ti-6Al-4V specimens fabricated using Laser Engineered Net Shaping (LENS), an additive manufacturing (AM) process. Fully-reversed strain-controlled fatigue tests were conducted on Ti-6Al-4V specimens manufactured via LENS in their as-built and heat-treated conditions. Scanning Electron Microscopy (SEM) is used to examine the fracture surfaces to qualify the failure mechanism, crack initiation sites, and defects. Due to the relatively high localized heating and cooling rates experienced during deposition, fabricated parts possess anisotropic microstructures and different mechanical properties than those of their traditionally-manufactured wrought counterparts. Porosity promotes unpredictable fatigue behavior, as evidenced by data scatter. Pore shape, size, location, and number were found to impact the fatigue behavior of additive-manufactured parts.

Laser Engineered Net Shaping (LENS™) is a rapid-manufacturing procedure that involves complex thermal, mechanical, and metallurgical interactions. The finite element method (FEM) may be used to accurately model this process, allowing for optimized selection of input parameters, and, hence, the fabrication of components with improved thermo-mechanical properties. In this study the commercial FEM code SYSWELD® is used to predict the thermal histories and residual stresses generated in LENS™-produced thin plates of AISI 410 stainless steel built by varying the process parameters laser power and stage translation speed. The computational results are compared with experimental measurements for validation, and a parametric study is performed to determine how the thermo-mechanical properties vary with these parameters. Thermal calculations are also performed with the code ABAQUS® to evaluate its potential use as a modeling tool for the LENS™ process.