This book provides a faithful and robust simulation of the optical and visual performances of the human eye for axial vision of distant objects in a variety of visual conditions. The author moves from intrinsically theoretical aspects (the optical and neurophysical models of the eye) to include a great number of experimental measurements from the scientific literature, in order to adapt the model parameters to the observed phenomenology and validate the predictivity power of the models themselves. The results are very satisfactory in terms of quantitative and qualitative adherence of model predictions to field measurements.
Resulting from the author's investigations over the last decade, the book material is largely original, and the most relevant achievement can be found in the capacity to evaluate visual acuity for a range of visual conditions, such as variations in pupil size, refractive error, and ambient illumination.
Thanks to the general organization of the book, chapters and paragraphs with high level mathematical and physical optics content can be safely skipped without compromising the overall comprehension. To this end, a brief summary is provided at the end of each chapter, making this book appropriate for readers with greatly varying degrees of technical knowledge.
Many different types of intraocular lenses (IOL) are currently available for implantation, both as crystalline lens replacements and as phakic refractive elements. Their optical design is increasingly sophisticated, including aspherical surface profiles and multi-zone multifocal structures, however a quantitative and comparative characterization of their imaging properties is lacking. Also a qualitative visualization of their properties would be very useful for patients in the lens choice process. To this end an experimental eye model has been developed to allow for simulated in-vivo testing of IOLs. The model cornea is made of PMMA with a dioptric power of 43 D, and it has an aspherical profile designed to minimize spherical aberration across the visible spectrum. The eye model has a variable iris and a mechanical support to accomodate IOLs, immersed in physiological solution. The eye length is variable and the retina is replaced by a glass plate. The image formed on this "retina" is optically conjugated to a CCD camera, with a suitable magnification in order to mimic the human fovea resolution, and displayed onto a monitor. With such an opto-mechanical eye model, two types of images have been used to characterize IOLs: letter charts and variable contrast gratings, in order to directly simulate human visual acuity and contrast sensitivity.
Recent technological advancements have opened new perspectives to photorefractive surgery: the target is going to broaden from the simple correction of defocusing errors in ametropic eyes to the compensation of high order optical aberrations on an individual basis, even in emmetropic subjects. In this way, optimization of visual performance may result in `supervision', namely a visual acuity equal to or better than logMAR equals -0.3 (Snellen: 20/10). Assuming that all monochromatic aberrations can be completely nulled through corneal sculpting, several factors remain to limit the ultimately achievable visual acuity: diffraction, photoreceptor spacing, internal noise, chromatic aberration. All these factors have been included in a numerical model based on first principles and best fitting of experimental data, providing estimates on visual acuity and contrast sensitivity. In an aberration-free human eye, the maximum achievable visual acuity is logMAR equals -0.24 (Snellen: 20/11.5) at 2.7 mm pupil size. Diffraction turns out to be the limiting factor up to 2 mm pupil size, chromatic aberration from 2 to 5 mm, and internal noise dominates above 5 mm. The maximum visual acuity gain attainable with a `super-vision' procedure appears thus restricted to only 2 lines above the `mean' logMAR equals 0 level (20/20). Chromatic aberration requires thorough consideration in all photo-refractive procedures.
Human visual performances (visual acuity, contrast sensitivity) are commonly measured under heterogeneous ambient luminance conditions, generally referred to as 'mesopic,' without any control of the subject's pupil size. Actually, the optical performances of the eye are strongly dependent on the pupil aperture, due to the increasing effect of optical aberrations, mainly spherical aberration, which progressively degrade vision. We have developed and tested an apparatus intended to measure visual acuity and contrast sensitivity of emmetropic or ametropic subjects while continuously measuring the pupil size, which is allowed to be varied by changing the background luminance. The apparatus is mounted onto a helmet for indirect ophthalmoscopy and is driven by a personal computer, which generates characters and gratings of variable size and orientation. An infrared pupillometer keeps trace of the pupil dimension every second. The apparatus is fully described and the preliminary tests on emmetropic and ametropic subjects are reported. The measurement system is particularly suited for assessing visual performances in professional categories where keen vision is to be associated with scotopic conditions (e.g. car drivers, aircraft pilots) and as pre/post examination for all types of refractive surgery.
Purpose: To improve the applicability of the Gullstrand's eye model for retinal image evaluation and prediction of photo-refractive surgery outcome.
Methods. The Gullstrand's eye model has been modified to allow for variable asphericity of all the refracting surfaces in the model. Formation of the retinal image is studied by means of numerical ray-tracing and by evaluation of modulation transfer function and point/line-spread-function.
Results. By comparison of the model line-spread functions with experimental ones taken from the literature, best-fitting values of the asphericity parameters have been derived. The model visual performances are evaluated and compared with other eye models. Application of the model to photo-refractive surgery reveals that proper aspheric curvature in the mid periphery is essential to minimize spherical aberration and guarantee ultimate visual performances in the emmetropic eye, at the limit allowed by diffraction and photoreceptor density in the fovea.
Conclusions: The proposed eye model can be useful in vision research as well as in practical applications like the design of ocular lenses and the design of improved algorithms for photorefactive surgery.
The behavior of stress waves induced by Argon-Fluoride laser ablation of the cornea in the typical operative conditions of clinical laser keratectomy have been studied experimentally and analyzed in porcine eyes and in an artificial eye model. Laser-induced stresses with pressure peaks as high as 100 bar were measured in the anterior chamber of irradiated eyes. Analysis of stress wave propagation the eye evidenced diffraction effects modifying the temporal shape of the stress transient with the formation of a rarefaction phase. Besides, significant pressure enhancements caused by focusing of the stress wave front were observed to occur when the spot diameter exceeded 3 mm. For the maximum laser spot diameter of 6.5 mm, diffraction and focusing effects produced at the acoustic focus in the eye compression peaks of 250 and negative peaks of -90 bar, respectively. Implications to clinical procedures, as possible damages due to tissue stretching and cavitation formation are discussed.
Frances Westall, Pier Giorgio Gobbi, Giovanni Mazzoti, Dane Gerneke, Robert Stark, Tanya Dobrek, Wolfgang Heckl, Everett Gibson, David McKay, Carlton Allen, Andrew Steele, Kathy Thomas-Keprta
A variety of microscope techniques have been used to study surficial phenomena on the fracture surfaces of the Martian meteorite ALH84001. The aim of the investigation was to determine the most useful microscopy methods in the search for morphological signs of biogenic activity. Emphasis was placed on scanning electron microscopy (SEM) using secondary, backscatter and cathodoluminescence modes combined with observation of samples at a variety of accelerating voltages. High resolution SEM imaging was compared with atomic force microscopy. These techniques revealed a number of structures of possible abiotic and biotic origin: (1) a large, fibrous-looking carbonaceous structure, (2) fine, flaky films coating pyroxene surfaces, (3) finely granular calcium carbonate deposit is associated with the fine film, and (4) lacy-structured, mineralized polymers on the pyroxene surface. Another sample contains further evidence of water-lain deposits in a cracked, iron oxide coat on a fracture surface.
We have evaluated the refractive evolution in patients treated with yhyperopic PRK to assess the extent of the initial overcorrection and the time constant of regression. To this end, the time history of the refractive error (i.e. the difference between achieved and intended refractive correction) has been fitted by means of an exponential statistical model, giving information characterizing the surgical procedure with a direct clinical meaning. Both hyperopic and myopic PRk procedures have been analyzed by this method. The analysis of the fitting model parameters shows that hyperopic PRK patients exhibit a definitely higher initial overcorrection than myopic ones, and a regression time constant which is much longer. A common mechanism is proposed to be responsible for the refractive outcomes in hyperopic treatments and in myopic patients exhibiting significant central islands. The interpretation is in terms of superhydration of the central cornea, and is based on a simple physical model evaluating the amount of centripetal compression in the apical cornea.
Mid infrared lasers are considered a potential interesting approach for safer and tractionless removal of pathological tissue in vitreoretinal surgery. They include the Erbium:YAG laser emitting at 2.94 micrometers and the semiconductor diode laser at 1.93 micrometer. Due to their capability of inducing photothermal incision and ablation effects, procedures like precise tissue cutting, tissue removal and coagulation can be achieved. Examples of various steps of surgery performed with such laser sources are shown, including retinotomies, retinectomies, and removal of vitreous, epiretinal and subretinal membranes. Advantages and drawbacks of each wavelength are physically and clinically discussed in relation to the surgical procedure type (contact, noncontact), the target tissue and the absorbance of the vitreous substitute adopted (hydrated fluids, silicone oils, perfluorocarbons). According to our three years experience with these cutting lasers, it can be said that: (1) they require special knowledge and the frequent presence of a technician, (2) they surely represent a useful tool in vitreoretinal surgery, but not yet able to entirely replace conventional surgical instrumentation, (3) even if some surgical steps are better performed with such lasers, cost and complexity of use limit their effective diffusion into the clinical practice.
Pulsed laser ablation of corneal tissue at the typical operative fluences of Argon-Fluoride photorefractive keratectomy produces stress waves propagating into the eye. Temporal and spatial evolution of these pressure pulses during propagation were measured directly in porcine eyes with a needle hydrophone. Beside diffraction effects that give rise to a bipolar shape of the pressure pulse with a positive compression peak followed by a marked rarefaction phase, we found a significant focusing of the stress wavefront caused by the spherical shape of the cornea, that increased for larger irradiation spot diameters.
Laser welding of corneal incisions was performed using two different diode laser wavelengths. Tissue fusion was attempted both with direct absorption of radiation at 1950 nm radiation and with ICG dye-enhanced technique at 810 nm. Thirty deep (not full thickness) corneal incisions of 15 fresh-enucleated porcine eyes were treated. Semiconductor diode lasers emitting in continuous wave at 1950 nm and at 810 nm were used. Both were coupled to an optic fiber handpiece. Wound samples were explanted on day 0 after treatment for histological evaluation. The group of corneal wounds treated with the ICG-enhanced technique revealed tissue welding in 70% of treated wounds. Using 1950 nm tissue fusion was observed in 50% of treated wounds. Macroscopic evaluation revealed a pronounced thermal damage of the epithelium in the samples treated with 1950 nm radiation. Wound closures obtained using 1950 nm were characterized by superficial welding of corneal layers. (Abstract truncated.)
In this work we studied the propagation of the acoustic wave generated during laser keratectomy with a clinical ArF laser source. In vitro measurements of the temporal pressure profiles induced inside the ocular bulb were performed on porcine eyes with a fast risetime needle hydrophone introduced into the eyeball. Peak pressure values as high as 90 bars have been measured at a laser fluence of 180 mJ/cm2, depending on the diameter of the irradiation spot and on the distance from the surface of the cornea.
A surgical laser instrument is presented, based on a semiconductor diode emitting in the spectral region around 2 micrometer, in continuous mode. At this wavelength tissue absorption is via histological water, and the interaction is purely thermal. A portable laser station was built, emitting up to 250 mW of optical power at 1.94 micrometers, out of the tip of a low- hydroxil fused-silica optical fiber of 200 micrometer diameter. Experiments have been performed both in vitro with enucleated porcine eyes and in vivo, on rabbits and rats. In vitreo-retinal surgery, retinectomies and incisions of epiretinal fibrotic membranes have been achieved. At reduced output power the laser radiation has been applied to the welding of tissues, in particular cornea, sclera and skin, with interesting results. This new laser source, characterized by ergonomic advantages like compactness, portability, long lifetime, reduced maintenance, is potentially attractive for a number of microsurgical procedures like micro- incisions, shallow coagulations, and welding of hydrated biological membranes.
Congenital or post-operative high degree astigmatism is currently corrected surgically by corneal arcuate incisions. Due to differences in manual skill in producing corneal incisions with precise depth, width, and length the final refractive results of these surgical techniques may be unpredictable. The aim of the study was to evaluate the feasibility of excimer laser in producing corneal arcuate excisions on rabbit eyes.
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