Correction of myopia with the ArF-excimer laser (PRK) sometimes leads to a so called 'central island' formation on the anterior corneal surface. The attenuation of the laser beam by the ablation plume might be one reason for this phenomenon. The attenuation properties of the ablation plume were investigated by a probe beam parallel to the surface of the tissue probe. By varying the laser parameters (fluence, repetition rate, spot size) and the target tissue (cornea, PMMA) the attenuation of the probe beam was measured time and spatial resolved. As a result of this study, a significant influence of the removal process due to scattering and absorption within the ablation plume can be assumed as a function of repetition rate, spot size and air flow on the tissue surface.

Laser sclerostomy ab externo with flashlamp excited mid-IR laser systems emitting in the 2-3 micrometer spectral range is in phase II clinical trials. Although acutely high success rates were achieved, the restenosis rate after several months is about 40%. Laser pulses of several hundreds of microseconds, known to induce thermo-mechanical explosive evaporation were used for this procedure. We investigated the ablation dynamics in tissue and the cavitation bubble dynamics in water by means of an Er:YAG laser system to estimate the extent of mechanical damage zones in the sclera and in the anterior chamber, which may contribute to the clinical failure. We found substantial mechanical tissue deformation during the ablation process caused by the cavitation effects. Stress waves up to several bar generated by explosive evaporization were measured. The fast mechanical stretching and collapsing of the scleral tissue induced by cavitation resulted in tissue dissection as could be proved by flash photography and histology. The observed high restenosis might be a result of a subsequent enhanced wound healing process. Early fistula occlusions due to iris adherences, observed in about 20% of the clinical cases may be attributed to intraocular trauma induced by vapor bubble expansion through the anterior chamber after scleral perforation. An automatic feedback system minimizing adverse effects by steering and terminating the laser process during scleral fistulization is demonstrated. Moreover, a new approach in laser sclerostomy ab externo is presented using a cw-IR laser diode system emitting at the 1.94 micrometer mid-IR water absorption peak. This system was used in vitro and showed smaller damage zones compared to the pulsed laser radiation.

Photorefractive keratectomy (PRK) is usually performed by an excimer laser at 193 nm wavelength. Ablatio of corneal tissue is, however, not only possible in the UV region of the optical spectrum but also in the IR where water is an excellent absorber. Therefore, an Er:YAG laser was used at 2.94 micrometer wavelength as an alternative laser light source to perform in vitro studies of corneal ablation and also first clinical experiments to correct myopia of patients with blind eyes.

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 double-pass method is applied to determine the optical image quality in contact lens (CLs) wearers. This is an objective optical non-invasive technique that permits us to test 'in vivo' the optical performance of CLs wearing eyes. The retinal image quality was measured for three subjects wearing two types of monofocal CLs. The results show the effectiveness of the technique to determine the optical performance of the CL and the CL-eye system. The importance of the ocular astigmatism on the MTF is also discussed. In eyes presenting corneal astigmatism, the best results are obtained when wearing RGP CLs. When the astigmatism is corrected, slightly better results were obtained with RGP lenses.

We have developed a new technique for the objective determination of the cone spacing in the living human fovea, and obtained direct measurements of the distance between cones at retinal eccentricities ranging from 0 to 1 deg. The method is similar to stellar speckle interferometry, and consists of recording a series of short-exposure images of small foveal patches, illuminated by a laser spot. Each individual image presents a speckle pattern, correlated with the topography of the cone mosaic, and contains spatial frequency information up to the diffraction limit of the optical system of the eye. The cone spacing is measured in the spatial domain, as the reciprocal of the radius of the ring present in the average power spectrum. The results obtained are in close agreement with estimates based on microscopy of excised fovea, with psychophysical measurements, and with recent results obtained with another high resolution imaging technique outside the center of the fovea.

We propose a new version of the double pass apparatus with unequal entrance and exit pupil to measure the complete optical transfer function in the human eye. This system overcomes the limitation of the double pass technique with equal pupil sizes where phase information and odd asymmetries are lost in the double pass images (Artal, Marcos, Navarro and Williams, J.Opt.Soc.Am.A., 1995). By using the asymmetric double pass apparatus with different entrance and exit pupils, one of them small enough (usually the entrance pupil) to consider the eye diffraction limited, the complete optical transfer function of the eye, including the phase transfer function, and the shape of the point spread function are determined.

A new fiber optic probe is developed to study different parts of the eye. The probe positioned in front of an eye, delivers a low power light from a laser diode into the eye and guides the light which is back scattered by different components (aqueous humor, lens, and vitreous humor) of the eye through a receiving optical fiber to a photo detector. The probe provides rapid determination of macromolecular diffusivities and their respective size distributions in the eye lens and the gel-like material in the vitreous humor. We report alpha-crystalline size distributions, as a function of penetration depth, inside the lens and hyaluronic acid molecular size distribution in the vitreous body. In a clinical setting, the probe can be mounted on a slit- lamp apparatus simply by using a H-ruby lens holder. The capability of detecting cataracts, both nuclear and peripheral, in their early stages of formation, in a non invasive and quantitative fashion, has the potential in patient monitoring and in developing and testing new drugs or diet therapies to 'dissolve' or slow down the cataract formation before surgery is necessary. The ability to detect biochemical and macromolecular changes in the vitreous structure can be very useful in identifying certain diseases of the posterior chamber, e.g., posterior vitreous detachment.

In-vivo dynamic light scattering measurements in the vitreous of the human eye are presented. The set-up employs single-mode optical fibers for the delivery of the excitation beam and for the collection of the scattered light. High sensitivity and good spatial resolution are achieved using a novel scattering geometry: the excitation beam is shaped into a hollow cone and the scattered light collected through its central part. The apparatus exhibits excellent safety characteristics: the optical power entering the patient's eye is 36 times less than the maximum laser power at the cornea recommended for intrapupil exposure by the ANSI standard. Following a description of the instrument, examples of in-vivo measurements in the vitreous are discussed in order to demonstrate the possible clinical use of this technique.

Ocular fundus reflectometry is known as a method for the determination of the optical density of pigments at the eye ground. This has been described for diagnostic investigations at single locations. The new technique of imaging spectroscopy enables the recording of one dimensional local distribution of spectra from the fundus which is illuminated confocal to the entrance slit of a spectrograph. A fundus reflectometer consisting of a Zeiss fundus camera, an imaging spectrograph, and an intensified CCD-camera are presented. The local resolved spectra gained by this apparatus are approximated by a mathematical model on the basis of the anatomy of the fundus as a structure of layers with different optical properties. Each spectrum is assumed to be described by a function of the absorption spectra of the pigments found in the retinal and choroidal tissue. Assuming the existence of parameters which are independent from the fundus location we have to approximate the measured local distribution of spectra by a system of coupled non-linear equations. By a least square fit the local distribution of the extinction of melanin, xantophyll and hemoglobin may be obtained as well as the extension of pathologic alterations at the fundus. The benefits of the method for clinical diagnostics are discussed at first measurements from physiological and pathological examples.

ICG videoangiography has widely demonstrated its importance as an investigation method in several ocular pathologies. By means of an optoelectronic system developed by the authors, many cases of age related macular degeneration and central serous coroidopathy affecting patients of different ages have been investigated. The analysis of the eye fundus infrared-light images obtained allows the clinician to draw medical conclusions otherwise not obtainable with traditional visible-light techniques. A comprehensive survey and discussion of the most significant cases are presented.

We present a high resolution, noninvasive method for measuring saccade profiles based on retinal images obtained with the scanning laser ophthalmoscope (SLO) and investigate the effect of SLO raster distortion on saccade measurements. The method uses the fact that images of horizontally moving objects made with the SLO show a systematic distortion: images of vertical edges show a tilt towards the horizontal. First, a mathematical framework is given for the quantitative determination of instantaneous saccade velocities from this tilt. Under the assumption that the SLO provides undistorted retinal images, it is shown that saccade profiles can be obtained in a simple way from a restricted number of successive digitized SLO video fields, which are calibrated with respect to the visual angle they cover. Second, SLO raster (and image) distortions that could affect saccade measurement are examined theoretically and by numerical methods. We show that for emmetropic eyes the retinal raster shape and size are virtually unaffected by changes in the axial and lateral positions of the beam-scan waist. Image distortions due to distortions of the SLO raster and due to the optical properties of the eye are found to be present but can be corrected by applying an empirically determined inverse distortion transform. The method is applied to sequences of successive digitized fundus images taken with a Rodenstock SLO. Measurements of retinal blood vessel tilts during saccades from these video fields were used to determine saccade profiles. Shapes of the saccade profiles measured using our method agree well with results obtained by others using different eye movement recording techniques.

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.

Photorefractive keratectomy (PRK) performed by means of the 193 nm excimer laser has stimulated considerable interest in the ophthalmic community because this new procedure has the potential to correct myopia, hyperopia, and astigmatism. The use of a laser beam to remove a controlled amount of tissue from the cornea implies that both the energy density of the laser beam and the target removed rate are accurately known. In addition, the required tissue ablation profile to achieve refractive correction must be predicted by optical calculations. This paper investigates: (1) Optical computations based on raytracing model to determine what anterior profile of cornea is needed postoperatively for ametropia. (2) Maximal depth of the removed corneal tissue against the ablation zone treated. And (3) the thickness of ablated corneal lenticule at any distance from the optical axis. Relationships between these data are well fitted by polynomial regressive curves in order to be useful as an algorithm in the computer-controlled delivery of the ArF laser beam.