The results of interference registration of human eye movements are presented. The semiconductor laser autodyne has been used for the diagnostics of the saccadic eye movements. The frequency and amplitude of the eye tremor was measured.

Laser-generated ultrasound has found a number of niche applications in non-destructive testing and evaluation and there is now a growing trend to examine potential applications for materials characterization in medicine. Conventional ultrasound techniques for measuring various important dimensions within the eye are in extensive use. However, one problem remains outstanding, which is that the dimensions of the cornea, anterior chamber and lens can be measured using a high frequency, high resolution transducer, but the dimensions of the overall eyeball (i.e., cornea to retina) have to be measured with a lower frequency transducer in order to achieve the necessary penetration. We have conducted a number of in vitro studies using bovine eyes to determine whether the use of laser induced ultrasound would be able to overcome the aforementioned problem. The results of these measurements will be presented, together with a discussion of the many difficulties that remain to be overcome. In addition, our studies involve the potential use of laser ultrasound to quantify the degree of cataract formation, both primary and secondary. This paper will also consider the work accomplished to data in this area.

The pre-corneal tear layer provides an important optical as well as physiological function. Hence, the ability to assess the quality and behavior of tears, especially in dry-eye, marginal dry-eye and contact lens patients is important. While a large range of clinical tests exists for assessing tear quality, many are subjective and show poor repeatability. Since the tear meniscus is dependent on factors such as volume, viscosity and 'wettability,' we attempted to develop an optical system for imaging the tear meniscus. A basic prototype to test feasibility has been constructed. An algorithm for computing the profile of the meniscus was also developed. The methodology is described together with some examples of tear menisci imaged and analyzed.

Oftenly the evaluation of a cornea is made using a Slit Lamp where the aspect of the cells is qualitatively observed, there are elaborated apparels and consequently much more expensive that accomplish a quantitative analysis of the endothelium cells. To overcome the limitations of subjective analysis, manipulation difficulties and high cost, we developed a system coupled to the Slit Lamp (that magnifies 290X the cells) exhibiting the image in a computer monitor and a software dedicated for identification and counting the endothelium cells to at a low cost standardizing the diagnosis for the donated corneas.

We have developed an automatic optical system attached to the Slit Lamp in order to provide automatic keratometry at low cost. The system consists of projecting a light ring as a target at the patient's cornea and the further analysis of the deformation of the target in order to obtain the radius of curvature as well as the axis of the associated astigmatism. The reflected image of the target is displayed in a PC's monitor and a dedicated developed software performs the analysis of the image, that provides the corneal keratometry. Also, the projection of two additional targets provide information about two other regions of the cornea. The clinician is able to understand the behavior of the cornea's curvature radius in these regions by the comparison of the graphical analysis of regular astigmatism that is generated by the software. Irregular astigmatism's measurements are possible in the developed system. The system is easy to use and it has a friendly software interface for the user. Measurements in volunteer patients have been made and the results that were obtained in the system, for the standard target ring are in good agreement with commercial automatic and manual systems, presenting a correlation coefficient of 0,99347 and 0,97637, respectively regarding the radius of curvature and 0,96841 and 0,9568, respectively, regarding the axis. The system's precision is 0,005 mm for the curvature radius and 2 degrees for the axis component.

Er:YAG laser system generating a radiation in a free-running, long-pulse mode regime with an output energy up to 610 mJ and a wavelength in a mid-infrared region (2.94 micrometer) was used for ophthalmic micro-surgery. As a delivery system a cyclic olefin polymer-coated silver hollow glass waveguide was used. The delivery system was ended either by a focusing optics (CaF₂ lens) or by a special sapphire tip, for a non-contact and contact treatment, respectively. The corneal ablation and perforation of a sclera tissue were examined for the energies from 5 mJ up to 280 mJ and from 1 to 10 pulses in the case of non-contact and contact preparation. The threshold energy for the sclera ablation was measured for the non- contact procedure to be 122 mJ (24.3 J/cm²) in the case when 1 pulse was used. For the contact preparation this threshold energy was 270 J (34.4 J/cm²) for 5 pulses used. The iris contact ablation threshold energy was measured to be 10 mJ (1.3 J/cm²) for 1 pulse. The results show the usability of the 2.94 micrometer radiation in the ophthalmic surgery.

We quantitatively evaluated a technique for combining multiple videokeratograph views of different areas of cornea. To achieve this we first simulated target reflection from analytic descriptions of various shapes believed to mimic common corneal topographies. The splicing algorithm used the simulated reflections to achieve a good quality estimation of the shapes. Actual imagery was then acquired of manufactured models of the same shapes and the splicing algorithm was found to achieve a less perfect estimation. The cause was thought mainly to be image blur due to defocus. To investigate this, blur was introduced into the reflection simulation, and the results of the splicing algorithm compared to those found from the actual imagery.

We present experimental results of investigation of the optical properties of the human eye sclera controlled by administration of osmotically active chemical, such as glucose solution with various concentrations. Administration of chemical agent induces diffusion of matter and as a result equalization of the refractive indices of collagen and ground material. Results of experimental study of influence of osmotical liquid (glucose solution) on reflectance and transmittance spectra of human sclera are presented. In vitro reflectance and transmittance spectra of the human sclera samples were investigated by commercially available spectrophotometer CARY-2415. The significant increasing of the transmittance and decreasing of the reflectance of human sclera samples under action of osmotical solutions were demonstrated. Results of our study show that the degree of the sclera samples clearing is increased with increasing of the chemical agent concentration in solution. The diffusion coefficients of glucose solution with various concentrations within scleral tissue was estimated.

It is concluded that there is no angular dependence of back scattering of light from a nuclear cataract. This implicates that reading one meridian, e.g. the 90 degree, is enough for measurement of nuclear cataract with slit lamp microscopy with Scheimpflug projection. Altogether, ten patients with nuclear cataract were measured with the NIDEK EAS-1000 system in 6 meridians. There was a slight angular dependence fitting a second order polynomial with a maximum at 90 degrees. In total, three different concentrations of standard scattering solution were measured with the same system at the same meridians. Also for these measurements, there was a slight angular dependence fitting a second order polynomial with the maximum at 90 degrees. Thus, there is a system related factor that causes a slight change of intensity of back scattered light as function of meridian.

A growing cataract can be detected at the molecular level using the technique of dynamic light scattering (DLS). However, the success of this method in clinical use depends upon the precise control of the scattering volume inside a patient's eye and especially during patient's repeat visits. This is important because the scattering volume (cross-over region between the scattered light and incident light) inside the eye in a high-quality DLS set-up is very small (few microns in dimension). This precise control holds the key for success in the longitudinal studies of cataract and during anti-cataract drug screening. We have circumvented these problems by fabricating a new DLS fiber optic probe with a working distance of 40 mm and by mounting it inside a cone of a corneal analyzer. This analyzer is frequently used in mapping the corneal topography during PRK (photorefractive keratectomy) and LASIK (laser in situ keratomileusis) procedures in shaping of the cornea to correct myopia. This new instrument and some preliminary clinical tests on one of us (RRA) showing the data reproducibility are described.

Increased corneal light scatter or 'haze' has been associated with excimer laser photorefractive surgery of the cornea. The increased scatter can affect visual performance; however, topical steroid treatment post surgery substantially reduces the post PRK scatter. For the treatment and monitoring of the scattering characteristics of the cornea, various methods have been developed to objectively measure the magnitude of the scatter. These methods generally can measure scatter associated with clinically observable levels of haze. For patients with moderate to low PRK corrections receiving steroid treatment, measurement becomes fairly difficult as the haze clinical rating is non observable. The goal of this development was to realize an objective, non-invasive physical measurement that could produce a significant reading for any level including the background present in a normal cornea. As back-scatter is the only readily accessible observable, the instrument is based on this measurement. To achieve this end required the use of a confocal method to bias out the background light that would normally confound conventional methods. A number of subjects with nominal refractive errors in an Air Force study have undergone PRK surgery. A measurable increase in corneal scatter has been observed in these subjects whereas clinical ratings of the haze were noted as level zero. Other favorable aspects of this back-scatter based instrument include an optical capability to perform what is equivalent to an optical A-scan of the anterior chamber. Lens scatter can also be measured.

In this work, the feasibility of optoacoustic tomography to act as diagnostic tool in laser cyclophotocoagulation was investigated. The experiments were performed with enucleated porcine and rabbit-eyes. The absorption of short laser pulses in the wavelength range between 750 and 1000 nm generated thermoelastic stress waves in the ciliary muscle and the underlying pigmented epithelial layers. These stress transients were detected at the surface of the sclera with piezoelectric transducers in order to analyze the structure. Depths of the ciliary body could be determined at different distances from the corneoscleral limbus. Furthermore, changes in the optoacoustic signal due to coagulation with a diode laser using typical therapeutical parameters could be observed.

With the increasing complexity of contact lens designs and the demand on visual performance required by wearers, the ability to assess the design and optical quality of a contact lens is an important capability. While more complex systems such as Hartmann-Shack devices exist, practitioners often call for a simple, quick and inexpensive system that can provide the above functions. A simple optical system based on modifying the knife-edge test has been developed for assessing design and optical quality. Incorporation of a Badal-type lens layout added lens power measurement capability. This system provides the practitioner with the ability to undertake rapid visual assessment of a contact lens's quality.

Purpose: To evaluate the effect of increased intraocular pressure (IOP) on the corneal shape of human cadaver eyes. Methods: Eight cadaver eyes, unsuitable for transplantation donated by the Florida Lions Eye Bank, were assessed for overall integrity. The epithelium was debrided, the eye placed into an artificial orbit, and a 30-gauge needle (connected to the IOP monitor) was inserted into the vitreous cavity. The IOP was altered as necessary by adjusting the height of the lactated ringers IV bottle. The surface contour was assessed at pressure levels: hypotony (2-4 mmHG), normal physiologic IOP (12-20 mmHg), hypertony (135-142 mmHg) and again at normal and low IOP. At each pressure level, two corneal topographic measurements (PAR CTS^TM Vision Systems Corp.) were captured and averaged. Keratometric analysis was completed to examine the dioptric effects of varying IOP. An elevation analysis was performed to determine the corneal locations which conformed to the pressure adjustments. Results: The keratometric and elevation (both 0 and 90 degree meridians) data revealed decreasing radii with increasing IOP however, variability precluded statistical significance. Both keratometric and elevation data displayed probable plastic deformation, as the radii deviated from the original measurement upon the return to 2-4 mmHg. The elevation analysis did suggest an astigmatic conformation to pressure fluctuations, as 90 degree meridian radii were greater than 0 degree meridian radii. Corneal deformation is minimal in the 2 to 140 mmHg range and the PAR system not sensitive enough to accurately determine changes in curvature. Conclusion: The cornea does not uniformly conform to IOP variation. Further investigations with IOP levels of up to 500 mmHg will provide more information with respect to changes in curvature as a function of IOP.

The surgical techniques proposed to restore accommodation past the onset of presbyopia can be categorized in two classes, those based of scleral expansion and those based on intraocular lens implantation and lens capsule refilling. This paper describes and discusses the different techniques and summarizes the results of clinical and experimental animal trials. Restoration of accommodation has been demonstrated by at least two different teams in non-human senile primates using modifications of the lens capsule refilling technique originally described by Julius Kessler in the late fifties. The advent of recent advances in microsurgery, devices, and injectable polymers are very promising and human clinical trials may soon be envisaged.

The paper reviews two techniques being developed for ex-vivo testing of crystalline lens substitutes. The techniques may be used to test polymers developed for the Restore Accommodation (RA) project. Harvested rabbit and human lenses were used in preliminary tests to determine the feasibility of topographical analysis of the crystalline lens and ex-vivo cell culturing of the lens capsule. Lens capsules were cultured to test formation of posterior capsule opacification (PCO) in the presence of RA polymers. Feasibility of analyzing lens shape with the PAR^TM Sys Corneal Topography System was determined using whole lenses. Preliminary results indicate that topography and cell culturing of lens capsules may be used to evaluate RA polymers.

One glaucoma challenge is the treatment of leaking trabeculectomy blebs. Simple methods such as patching, autologous blood injection, compression sutures or cyanoacrylate glue application often fail. Because the conjunctiva is thin and ischemic, it often can't be sutured together so major surgery is required to excise the thin tissue and advance healthy conjunctiva. We report the preliminary results of Tisseel and Tisseel treated with two wavelengths from Vanderbilt's free electron laser placed on leaking trabeculectomy bleb holes in Dutch belted rabbits. The holes were healed at one week in the sutured group and in the 7.7 micrometer FEL-treated Tisseel group. One hole was healed in the cyanoacrylate glue-treated group. Holes remained in the other treatment groups. Tisseel irradiated with 7.7 micrometer energy from the free electron laser may promote healing of trabeculectomy bleb holes.

The new surgical UV ophthalmic laser system Medilex based on the KrCl (223 nm) excimer laser for refractive surgery was created. The comparative analysis of using the UV ophthalmic laser systems Medilex based on the ArF (193 nm) and the KrCl (223 nm) excimer lasers for the correction of refractive errors was performed. The system with the radiation wavelength of 223 nanometer of the KrCl excimer laser for refractive surgery was shown to have several medical and technical advantages over the system with the traditionally used radiation wavelength of 193 nanometer of the ArF excimer laser. In addition the use of the wavelength of 223 nanometer extends functional features of the system, allowing to make not only standard for this type systems surgical and therapeutic procedures but also to treat such ocular diseases as the glaucoma and herpetic keratities. For the UV ophthalmic laser systems Medilex three variations of the beam delivery system including special rotating masks and different beam homogenize systems were developed. All created beam delivery systems are able to make the correction of myopia, hyperopia, astigmatism and myopic or hyperopic astigmatism and may be used for therapeutic procedures. The results of the initial treatments of refractive error corrections using the UV ophthalmic laser systems Medilex for both photorefractive keratectomy (PRK) and LASIK procedures are presented.

The Feasibility of refractive surgery with ultrashort laser pulses in the fs-regime was studied. The Experiments were performed with an Erbium fiberlaser oscillator, which was amplified by means of chirped pulse amplification in a Titanium-Sapphire regenerative amplifier. With this system pulse durations of 180 fs and pulse energies of up to some hundred (mu) J were possible. The photodisruption was performed in corneal tissue in order to create corneal incisions and lamellars. Surface quality and thermal damages of the cuts inside the tissue were studied with the help of micromorphological analysis, such as scanning electron microscopy (SEM) and histological sections. High level precision along with minimal thermal damage could be observed. Saline solution as a model substance was used for studying side effects as bubble formation and pressure transients. The pressure measurements were performed using broadband acoustic transducers. Gas bubbles were analyzed with respect to formation, dynamics, interaction and composition of different gas constituents with the help of flash photography and gas chromatography. The results demonstrate the minimal invasive treatment of corneal stroma by fs-photodisruption. Studies of secondary effects indicate, that additional processes have to be considered.

The efficacy and predictability of Erbium (Er):YAG laser ablation of vitreous tissue and saline solution were determined. Both substances were ablated in vitro with an Er:YAG laser (pulse duration: 500 microsecond(s) ) connected to a sapphire fiber with the tip of the fiber placed in air. Pulse energies ranged between 1.0 and 20.8 mJ (vitreous) and 1.0 and 21.2 mJ (saline solution). Pulse repetition rates between 10 and 200 Hz were used. The net ablation rates were determined by weight measurement before and after laser treatment. Untreated control samples were used simultaneously to correct for evaporation. Net ablation rates were considered as the difference in the amounts of weight loss measured with and without laser treatment. Reproducible and constant ablation rates were found for both vitreous and saline solution in each of 3 consecutive measurements at constant laser parameters. Ablation rates did not significantly differ between both substances (p equals 0.25). The net ablation rates in both materials were found to increase linearly with increasing pulse frequency and nonlinearly with increasing pulse energy. The present studies suggest directions for the development of instrumentation for enhanced removal of vitreous and other tissues as well. These directions include the use of higher hertz (> 200) instruments and appropriately designed endo- probe geometries.

This study aimed to gauge the effect of the patient's eye movement during Photo Refractive Keratectomy (PRK) on post- operative vision. A computer simulation of both the PRK procedure and the visual outcome has been performed. The PRK simulation incorporated the pattern of movement of the laser beam to perform a given correction, the beam characteristics, an initial corneal profile, and an eye movement scenario; and generated the corrected corneal profile. The regrowth of the epithelium was simulated by selecting the smoothing filter which, when applied to a corrected cornea with no patient eye movement, produced similar ray tracing results to the original corneal model. Ray tracing several objects, such as letters of various contrast and sizes was performed to assess the quality of the post-operative vision. Eye movement scenarios included no eye movement, constant decentration and normally distributed random eye movement of varying magnitudes. Random eye movement of even small amounts, such as 50 microns reduces the contrast sensitivity of the image. Constant decentration decenters the projected image on the retina, and in extreme cases can lead to astigmatism. Eye movements of the magnitude expected during laser refractive surgery have minimal effect on the final visual outcome.

A study was undertaken to assess the safety and efficacy of LASIK with the LADARVision laser by Autonomous Technologies, (Orlando, FL). The study included four subsets: Spherical myopia -- up to -11.00D, spherical hyperopia -- up to +6.00D. Both myopic and hyperopic astigmatism could be corrected, up to 6.00D of astigmatism. A total of 105 patients participated. Sixty-six patients were myopic and 39 were hyperopic. The mean (+/- SD) age was 42.8 +/- 9.3 years for myopia and 53.2 +/- 9.9 years for hyperopia. At 3 months postop. Sixty-one myopic eyes were available for evaluation. Uncorrected visual acuity was 20/20 in 70% of eyes and 20/40 in 92.9% of all eyes. The refractive outcome was within +/- 0.50D in 73.8% of eyes and within +/- 1.00D in 96.7 of eyes. Thirty-eight hyperopic eyes were available. Uncorrected visual acuity was 20/20 in 42.1% of eyes and 20/40 in 88% of all eyes. The refractive outcome was within +/- 0.50D in 57.9% of eyes and within +/- 1.00D in 86.8% of eyes. Complications were not sight threatening and were discussed in detail. Lasik with the LADARVision laser appears to be safe and effective.

We describe a bench top system for digital scanning laser ophthalmoscopy. This system is used for both regular patient screening and experimental imaging studies. The complete set- up is assembled from a number of modules (e.g. launching, detection, scanning, focusing unit) which may be altered readily to offer a high degree of flexibility in the imaging conditions. Both the launching and the detection unit can be used in a simultaneous, multiple channel configuration. This allows the acquisition of true color and false color images of the back of the eye. In particular, the use of infrared lines permits the investigation of deeper retinal structures. Digital image processing methods can then be used to generate differential images of frames taken with different wavelengths, such as red and infrared. Furthermore, the separate detection channels can also be used to obtain recordings in other imaging modes, such as tightly confocal, loosely confocal, indirect or polarization dependent contrast. We discuss the merits and problems of different imaging modes. Cases are presented where the differential imaging shows clear advantages over the standard (monochromatic) confocal method in the perceptibility of deeper laying structures (choriod).

The retinal thickness analyzer (RTA) is an ophthalmic imaging instrument for the measurement and mapping of the thickness of the retina. The RTA is based on the principle of optical triangulation. A laser beam, which is offset from the optical axis of the eye, is projected onto the retina so that it hits the retina at an angle relative to the optical axis. The light reflected back from this beam at the retina is imaged onto a CCD camera through an off-axis aperture opposite the offset of the incident beam. This cause the light reflected from different layers of the retina to be offset in the image. The size of the offset of light from a retina layer is a measure of the depth of the layer in the retina. Image processing algorithms, which is used to analyze the image for determination of the retinal thickness and structure. A curve fitting procedure based on a physical model of retinal light scattering is used. Thickness determination repeatability is 13 microns (1 sigma). For the registration of thickness measurements, conventional images of the retina are acquired in coincidence with the thickness measurements. Feature extraction algorithms are used to register these images relative to each other and to locate the thickness data on the retina.

Pharmacologic Vitreolysis is a new therapy intended to enzymatically liquefy vitreous and separate the posterior vitreous cortex from the retina. There are presently no non- invasive methods with which to quantify vitreous liquefaction. This study evaluated non-invasive Dynamic Light Scattering (DLS) to assess vitreous liquefaction in model vitreous solutions. Vitreous of 5 bovine eyes was measured by DLS. Methodology involved the addition 50 IU/ml of hyaluronidase and 5 mg/ml of collagenase to solutions of 0.1 mg/ml of hyaluronan and collagen with polystyrene beads (30 nm diameter), respectively. Solutions were incubated at 37 degrees Celsius for 24 hours and DLS measurements were made at various time intervals. Results showed that the diffusion coefficients and particle size distributions determined from DLS measurements in the 5 bovine vitreous specimens were consistent and reproducible. Adding enzyme to the model vitreous solutions increased diffusion coefficients in the collagen as well as the HA solutions. These results suggest that DLS is a useful non-invasive method to measure diffusion coefficients in both model solutions and bovine vitreous. This approach may provide a quantitative means to assess different pharmacologic vitreolysis agents in vitro, and may also serve as a clinical tool for monitoring the effects of pharmacologic vitreolysis in vivo.

Accurately measuring the oxygen saturation of blood within retinal arteries and veins has proven to be a deceptively difficult task. Despite the excellent optical accessibility of the vessels and a wide range of reported instrumentation, we are unaware of any measurement technique that has proven to be calibrated across wide ranges of vessel diameter and fundus pigmentation. We present an overview of our retinal oximetry technique, present the results of an in vitro calibration experiment, and present preliminary human data.

Several techniques for measuring the oxygen saturation of blood in retinal vessels have been reported. One interesting application of retinal vessel oximetry is the identification of occult blood loss in trauma victims. However, all the devices described to date are too bulky and cumbersome to be used in a trauma bay or in the field. We present a design for a handheld instrument that performs four-wavelength retinal vessel oximetry. This device is comparable in size and weight to a commercially available camcorder, and is suitable for use in the trauma bay. The compact size of this device could also extend its applications beyond traditional clinical settings, as it could be used by primary care physicians and home health care workers for the screening and monitoring of ophthalmic diseases. Principles of operation and preliminary data from the device will be described.

Photodynamic therapy (PDT) is a new treatment modality for exudative forms of age-related maculopathy. It can be combined with others types of selective or conventional laser therapy. Imaging and functional testing with the scanning laser ophthalmoscope (SLO) are important for detailed diagnostic information as well as for the interpretation of the long term outcome of different treatment strategies. For example, infrared imaging in a confocal mode superbly outlines areas of minimal edema due to slow leakage and switching of wavelengths enables simultaneous and repeated angiographic studies of the retina with the same instrument. Visual acuities are strongly influenced by background illuminance and binocular fixation patterns, and absolute but not incremental microperimetric thresholds measure correctly the functional status of the photoreceptor-pigment epithelium complex. The scanning laser ophthalmoscope has been adapted for use as a delivery system in microphotocoagulation and photodynamic therapy. A non- scanning external therapeutic laser source uses the same Maxwellian view entrance location into the eye as the SLO. Advantages include a non-contact delivery, fixation control, registration of treatment locations, and the possibility to spatially modulate the area being treated.

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.

Higher-order optical errors of the human eye are often responsible for a reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye involving optical structures. The presented wavefront analyzer bases on Tscherning's aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of 0.3 mm diameter holes which forms a bundle of thin parallel rays. These rays are focused by a lens in front of the eye that their intraocular focus point is located in a certain distance in front of the retina generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for the assessment of optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light CCD video camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave-front aberration is computed in the form of Zernike polynomials up to the 6th order.

Purpose: The aim of this work was to study the effect of currently used ablation profiles along with eccentric ablations on the increase of higher order aberrations observed after PRK. Material and Methods: The optical aberrations of 10 eyes were tested before and after PRK. Refractive surgery was performed using a ArF-excimer laser system. In all cases, the ablation zone was 6 mm or larger. The spherical equivalent of the correction was ranging from -2.5 D to -6.0 D. The measured wavefront error was compared to numerical simulations done with the reduced eye model and currently used ablation profiles as well as compared with experimental results obtained from ablation on PMMA balls. Results: The aberration measurements result in a considerable change of the spherical- and coma-like wavefront errors. This result was in good correlation with the numerical simulations and the experimental results. Furthermore, it has been derived that the major contribution on the induced higher order aberrations are a result of the small decentration (less than 1.0 mm) of the ablation zone. Conclusions: Higher order spherical- and coma-like aberrations after PRK are mainly determined by the decentration of the ablation zone during laser refractive surgery. However, future laser systems should use efficient eye-tracking systems and aspherical ablation profiles to overcome this problem.

Customized ablation aiming to optimize visual acuity in refractive surgery requires objective data on corneal surface, like the contrast sensitivity. Fast ray tracing, using the high resolution 3-D elevation data in conjunction with Snell's law describe the diffraction of the incident rays and the resulting image on a 'virtual retina.' A retroprojection leads to a 'surface quality map.' For objective contrast sensitivity measurement a sinus (or cos) wave of different frequencies is used for a calculated projection in analogy to the clinical contrast sensitivity charts. The projection on the individual cornea surface is analyzed for the Modular Transfer Function (MTF) and the Phase Shift Function (PSF) as a function of frequencies. PSF, not yet clinically used, is a parameter to determine even minimal corneal tilt. The resulting corneal aberration map (CAM) as described here and applied to a 4.5 D PRK (OZD equals 6.5 mm) reveals that the area of minimal aberration measures only 4.2 mm. The CAM can likewise be used to describe the 'quality' of a laser system's ablation pattern based upon the area of minimal optical aberrations. The CAM only describes surface aberration with high resolution, an advantage over wave front sensing which measures all accumulated optical aberrations including the changing ones of the lens during accommodation and the transient ones due to lens aging and early cataract formation.

Refractive gels, applied on the corneal surface in situ act as laser beam modulators. Depending upon their surface shape and absorbency, they can be used to reshape irregular surfaces or induce refractive effects. Three gels (Palm I Pallikaris, Greece; Refrax 33, Imperial, USA; Biomask, Maverick USA) have been investigated using Non Contact Photoacoustic Spectroscopy (NCPAS). Their use requires on line detection and monitoring of the ablation process, indicating when the gel is ablated and the corneal tissue is exposed to the incident laser pulse. NCPAS enables this on line control. With the gels in their present form, stable ablation rates where achieved only after 200 - 400 pulses, depending upon the gel. The typical frequency shift of the Fourier -- transformed time signal of NCPAS, observed for different target materials (gels, human, and porcine cornea) allowed the discrimination of the different gels as well as from human and porcine corneal tissue (epithelium and stroma). NCPAS discriminates and identifies even those gels with similar ablation rates, using the typical frequency shift (difference >= 1 kHz). Refractive gels in combination with a powerful on-line control such as NCPAS has a potential to be used as a low cost approach for reducing or minimizing the amount of surface bound corneal optical aberrations; it could thus bypass the complexity and costs of corneal reshaping systems, using topography or wavefront based customized treatment algorithms.