In the mid IR region, where water acts as the main chromophore in soft tissue, pulsed erbium lasers emit radiation with sufficient energy for photoablative tissue processing. In this study, the ablation characteristics of these lasers were analyzed in order to evaluate their potential for clinical application in corneal and intraocular surgery. Histopathological examination of thermal side-effects was performed on porcine corneas. Ablation rates were investigated by on-line depth profiling in free-running and in q-switched mode. Structure and intensity of laser induced acoustic transients were analyzed by piezo-electric PVDF-films.

Treatment of peripheral post-mortem human corneas with the Holmium laser in a ring pattern resulted in opaque spots. One pair of treated eyes was immediately processed for light and electron microscopy and three other treated eyes were preserved for 4 days in medium in order to compare direct and short-term effects of the Holmium laser. Cross as well as frontal light microscopical sections of all eyes revealed interconnecting bands between the spots. At the ultrastructural level the anterior corneal tissue within these spots was characterized by coagulation of cells and collagen and shoed either a dramatic distorting effect on the epithelium in the eyes processed immediately or a single layer of flattened multi-nucleolated epithelial cells having more than one nucleolus per nucleus in the eyes stored in medium. Furthermore, the spots showed disturbed Bowman's layer, destroyed keratocytes and collagen fibrils which were either coagulated or organized chaotically. The interconnecting bands contained alternating normal and coagulated collagen fibers. The rest of the cornea outside the spots had a normal appearance. In corneas stored in medium, both keratocytes and epithelial cells over the entire cornea exhibited accumulations of cytoplasmic fibrils and glycogen particles. These phenomena were not observed in non-preserved corneas, suggesting that the differences are due to preservation and not due to the laser treatment. It is concluded that morphological changes occur mainly in the treated peripheral cornea whereas the central untreated cornea remains unaffected,indicating that the Holmium laser is a reliable instrument to treat hypermetropic patients.

The aim of this study was to evaluate the implication of the mechanical factor 'decentration' on the visual outcome after PRK. 100 eyes of 70 patients were included. The mean decentration was 0.27 mm +/- 0.18. Decentration was less than 0.5 mm in 84 percent of the cases. The importance of the decentration was investigated by the statistical correlation of decentration from the pupilcenter and the visual outcome. We did not find any statistical significant association for decentrations less than 1 mm. Our conclusion is that decentration, if less than 1 mm, does not play an important role in the final visual outcome after PRK.

Laser thermokeratoplasty (LTK) is a procedure in refractive surgery to correct hyperopia and astigmatism. The rise in refractive power of the eye is achieved by thermally induced shrinkage of collagen in the corneal periphery leading to an increase of the central corneal curvature. Biomechanical effects, which play an important role in LTK, were investigated in order to simplify the transfer of data from animal experiments to the situation in the human eye and to get a basis for improving the understanding of the influence of physical parameters involved in the surgical procedure. A 2D axially symmetric biomechanical LTK model of the cornea was developed using the finite element method. In this model the cornea is represented as a partial sphere under pressure fixed at the limbus. The coagulations were modeled by shrinking a corneal volume. The analysis showed the influence of the material stiffness, the corneal geometry, the amount of shrinkage and the ring diameter of an annular coagulation on the resulting corneal curvature. The model allows to analyze the influence of some biomechanical and of LTK parameters on the resulting corneal curvature, which is only possible with great effort or not at all by experiments. THe strong dependence of the results on Young's modulus points out that it would be useful to determine the patient's individual mechanical properties of the cornea before applying LTK.

Retinal photocoagulation was observed with repetitive short laser pulses of a Nd:YAG laser. The number of pulses applied was 10 and 500. Threshold curves for angiographic and ophthalmoscopical visibility as well as for bleeding were established. No macroscopic disruptive effects were observed, however macroscopic visible bubble formation occurred regularly at energy levels 2 times above ophthalmoscopical visibility. Bubble formation as primary interaction mechanism due to local overheating of single melanin granules is postulated.

We investigated the transmission, scattering and reflection of plasmas produced in water by Nd:YAG laser pulses of 6 ns and 30 ps duration. The transmission measurements comprise a large energy range at a wavelength of 1064 nm and various focusing angles between 1.7 degrees and 22 degrees. This parameter range covers the parameters used for intraocular microsurgery, but also allows to asses the influence of self-focusing on plasma shielding, which is only relevant at small focusing angles. We found that most of the laser light is either absorbed or transmitted; scattering and reflection amount to only a few percent of the incident laser energy. The transmission is considerably higher for ps pulses than for ns pulses, regardless of the focusing angle. The plasma transmission increases with decreasing focusing angle. Self- focussing, which occurs at focusing angles below 2, leads to a further increase of transmission. The efficacy of plasma- mediated intraocular laser surgery is higher with 6-ns pulses than with 30-ps pulses, because with the ns pulses nearly 50 percent of the laser pulse energy is absorbed already at threshold, whereas it is only 8 percent with the ps-pulses. The small fractional energy deposition with ps pulses together with a low energy threshold for breakdown can, however, be useful for the generation of very fine tissue effects. Structures beyond the laser focus are 2-6 times more effectively shielded from laser radiation by plasmas generated with ns pulses than by ps plasmas. The transmitted energy at equal normalized energy E/E_th is, nevertheless, always by more than a factor of 8 less than for ps-pulses because of their lower energy threshold for plasma formation.

A new method for corneal structure investigation is reported. The light reflected form rabbit cornea is recorded by a video-camera so that the images of epithelium and endothelium are obtained. Because of the great magnification factor due to the corneal curvature, the characteristics of the cells can be directly visualized. The theoretical evaluation of the optical magnification fits reasonably well the experimental results for collimated laser beams; therefore in the future it will be possible to determine with a good accuracy the corneal cell dimensions. Corneal diagnosis based on this method is discussed.

In this present study a pump-and-probe imaging set up was arranged to image and analyze the evolution of pressure waves induced by ArF ablation of the cornea, during their propagation into the eyeball. In vitro experiments simulating the effects of clinical PRK have been performed by using an artificial model of the human eyeball, composed of a cell filled with hyaluronic acid gel with a sample of freshly excised bovine cornea placed on the gel surface. LAser irradiation was provided at a fluence of 180 mJ/cm². Irradiation spot diameters were varied in the range 2.0-5.0 mm. Images of the traveling acoustic waves evidenced diffraction effects, related to the diameter of laser spots on the corneal surface.

Vascular effect of photodynamic therapy has been studied in patients with corneal neovascularized transplant in 10 cases. THe injection of photoheme intravenously were made with subsequent irradiation by light of argon-pumped dye laser with light density of 150-300 mW/cm² for 10-15 minutes. Energy density consisted 150-300 J/cm². In all the cases at the time of irradiation the aggregated blood flow was appeared followed by blood flow stasis. In post- operative period the vessels disintegrated into separate fragments which disappeared completely after 10-15 days. Taking into account the data of light microscope, the disappearance of the vessels took place as a result of the vascular endothelium lysis along the vascular walls. The vessel alteration study presented in this paper, may also serve to specify the mechanism of photodynamic destruction of neovascularized stroma of tumor.

This research is devoted to a problem we do not know enough about--ergophthalmological estimation of ophthalmologist's labor activity. Taking into account the fact that professional activity of ophthalmologists is connected with using different optical devices, the most of which creates high levels of brightness at the retina of investigators, main attention is sparing to this aspect. In a special experiment, 3 young women who are occupied with work in a ophthalmological consulting room, were examined for 9 days. Examination was performed by means of ergophthalmological methods of complex visual system evaluation. Developing visual fatigue, weakness of accommodation and retinal function disorder and also some weakening oculomotor muscles were revealed.

Retinal function can be evaluated with the scanning laser ophthalmoscope (SLO). the main advantage is a precise localization of the psychophysical stimulus on the retina. Four alternative forced choice (4AFC) and parameter estimation by sequential testing (PEST) are classic adaptive algorithms that have been optimized for use with the SLO, and combined with strategies to correct for small eye movements. Efficient calibration procedures are essential for quantitative microperimetry. These techniques measure precisely visual acuity and retinal sensitivity at distinct locations on the retina. A combined 632 nm and IR Maxwellian view illumination provides a maximal transmittance through the ocular media and has a animal interference with xanthophyll or hemoglobin. Future modifications of the instrument include the possibility of binocular evaluation, Maxwellian view control, fundus tracking using normalized gray-scale correlation, and microphotocoagulation. The techniques are useful in low vision rehabilitation and the application of laser to the retina.

The phase retardation of light induced by the birefringent parts of the human retina in vivo is measured with an electro-optical ellipsometer using the principle of confocal imaging. A scanning unit allows to examine an area of 25 degrees by 12.5 degrees on the retina with a resolution of 256 by 128 points with both, incident and exit beam transmitting the cornea at a fixed position. Due to this fixed beam position the Muellermatrix of the cornea can be calculated using light which is specularly reflected on blood vessels lying above the nerve fiber layer of the retina. The obtained images show a homogenous radial distribution of the retinal retardation around the fovea without the appearance of the so called Haidinger brushes. The areas with the thickest nerve fiber layers, the arcuate bundles, appear in their typical arc and were measured quantitatively. In addition, an alternative method for the compensation of corneal birefringence is evaluated by focusing the light beam onto the surface of the lens. Hereby, the measured area in the center of the cornea is 3 X 0.75 mm².

Three nonlinear (beta) -barium borate (BBO) crystals were used to generate the fourth and fifth harmonic of a Nd:YLF picosecond laser with a fundamental wavelength of 1053 nm. The UV laser pulses were used to study the surface ablation of bovine soft tissue. Whereas the general study was concentrated on brain tissue, first results were also obtained with cornea tissue. The ablation efficiency as a function of pulse energy and the ablation threshold were determined for both UV wavelengths. Histology and SEM- microscopy of the performed laser lesions are presented. Furthermore, time-resolved flash photographs we recorded by illuminating the ablation area with green picosecond laser pulses and detecting the probe beam with a CCD chip. The photographs yielded information about the physical properties of the ablation process. The emission of a shock wave into the air environment as well as the ejection of dissected ablation fragments was observed for pulse energies down to 10 (mu) J. Both UV wavelengths can be combined with an ophthalmic application system, which comprises an xyz- scanning unit for realizing precalculated ablation patterns and a z-tracker for detecting the current surface position of the cornea. The information of the z-tracker can be used to compensate the axial eye-movement with the xyz-scanning unit.

An imaging spectrometer, developed at the University of Jena, enables locally resolved spectral measurements at the fundus of the human eye. From the measured profile of a blood vessel the value of vessel diameter may be detected and the weakening of intensity is a result of wavelength- dependent absorption and scattering in blood. Although extinction spectra of hemolyzed oxygenated and reduced hemoglobin are known very exactly, in vivo measurements of oxygen saturation at streaming whole blood in single vessels are difficult. The extension of Lambert-Beer's law by a scattering part is used as an attempt for the calculation of the oxygen saturation from the locally resolved reflectance spectra of the fundus. Measurements of vessel diameter according to the principle of optical centroid are insensitive to variations of real illumination. This principle results in exact values if there are at least three observations in each edge of vessel profile. Practical measurements of oxygen saturation over vessel cross section and of vessel diameter are given.

Multispectral images can provide useful information for objective diagnosis, control of the effect of therapy and for a patient-specific optimization of therapy regime in ophthalmology. Laser scanner systems have the advantage of a high radiation power also in case of small spectral bandwidth. Additionally, the flying spot principle reduces the irradiation of the patient. Commercial laser scanner ophthalmoscopes (LSO) are developed till now only for qualitative, visual interpretation. Maximal four fixed wavelengths are available with a stabilized radiation power. Using the spectral properties of fundus pigments like xanthophyll, rhodopsin or of pathological alterations, e.g. hard exudates, its optical density or local distribution can be determined in this way before and after therapy. As also three wavelengths can be chosen which are best suited for determination of oxygen saturation (OS) in the blood, the validity of the 3-(lambda) -method for 2D calculation of OS can be tested. These investigations are first steps in functional diagnosis of the metabolism in the human ocular fundus.

Many ocular diseases of the human fundus could be quantified from the visible pathological features. Fundus images are usually recorded on a photographic film using a fundus camera that employs very high levels of illumination. Scanning laser ophthalmoscopy is a new method of imaging the fundus that offers the unique capability of differentiating various pathological conditions using very low light levels and has the confocal capability to collect light from different layers. The scanning laser ophthalmoscope (SLO) uses a narrow beam of laser to illuminate the fundus. Only a single point is illuminated at a time and the fundus is scanned in a raster pattern and the reflected light is detected y a photodiode. Patients with pathological conditions such as cataract, exudates, macular and optic disc drusen were imaged using lasers ranging from 488 nm to 830 nm. Images were analyzed using Visilog image processing software on a SUN IPX Sparc station. Image processing to quantify the area of exudates macular drusen and optic disc drusen showed better accuracy when compared to registered digitized fundus photographs. Quantitative analysis of contrast of retinal vessels in fundus images of cataract patients demonstrated significantly higher contrast for the SLO at all wavelengths tested. Development of a color SLO using multiple lasers and tomographic imaging suing very small confocal apertures are in progress and preliminary results will be presented.

This sections of the nuclear part of the bovine lenses were investigated. Angular intensity distributions as a function of temperature was measured. For older lenses the phase separation is not observed. For relatively young lenses the intensity with increasing temperature decreases to a minimum value. This value of the intensity depends on the age of the lens. In the region where the intensity decreases the scattering occurs from elements responsible for the phase separation as well as from structural elements. For very young lenses the minimum value of the intensity tends to zero. Correlation distances for aggregates as well s for distances between the aggregates are calculated by application of the random density fluctuation theory. The spinodal temperature is determined from the temperature dependence of the scattered intensity.

There is a problem in photorefractive eye microsurgery of mapping the refraction distribution in the eye, caused by distortions of the cornea shape, lens aberrations, and non- homogeneities of the components of eye media. We investigated shearing interferometer and Shack-Hartmann sensor for obtaining the refraction map without procedure of scanning. Like in adaptive optics, Shack-Hartmann sensor uses a microlens matrix that transforms wave front of a retinal single point source into plurality of sections giving point projections ont eh photodetector matrix. Displacement of these points from zero positions determines local wave front distortions, corresponding to refraction distribution. Shearing interferometry needs two sets of 2D data go at two orthogonal beam offsets. Shack-Hartmann sensor gets the entire information at one single measurement. Multiplexed aberrationless hologram is used for avoiding the errors caused by microlenses positioning. Sensitivity better than 0.1 diopter of refraction non- homogeneity measurement can be reached and is limited by several factors, signal-to-noise ratio being the principal.

The tear film fulfills several important functions in the eye. Apart of its physiologic functions like maintaining a moist environment for the epithelial cells of the cornea and conjunctiva, bacterial properties, transporting metabolic products etc., this film causes that the corneal surface has the optical quality. This smooth surface allows to apply interferometry for measurements. The paper presents tear's layer distribution on the soft contact lens and the cornea in comparison. Tv frame speed registration in the Twyman- Green interferometer was used to observe an unstable biomedical objects like the eye. The proposed method has the advantage of being noncontact and applies the low energy laser beam in interferometric set-up. This provides non- invasive testing of human cornea in vivo and enables observation the kinetics of its tear layer deterioration. The evaluation of non-invasive tear breakup time is possible by use of proposed setup. Further analysis of recorded interferograms helps to examine the matter of the breakup process and can be used for detection of the 'dry eye' symptoms.

Optical coherence tomography (OCT) has become one of the most promising imaging techniques in biology and medicine. Microscopic techniques using rapid image acquisition by scanning beam techniques and large-sample macroscopic techniques using scanning-stages have been developed. Basic to all OCT imaging schemes described so far is the use of low coherence techniques to detect the depth position of light remitting sites in the object. Either time-domain OCT imaging or Fourier-domain OCT imaging can be used. Furthermore, OCT techniques have been developed for metrologic application s like the measurement of the refractive index of tissue and the velocity profile of fluids in tubes. This paper reviews OCT imaging techniques used in experimental and clinical applications and discusses arrangement s to improve longitudinal and transversal resolution.

Partial coherence interferometry, a new optical non-invasive technique, that has been developed for high precision measurements of intraocular distances, has been extended to a 2D imaging instrument, called scanning partial coherence interferometer. Since the improved version of this interferometric technique is capable of performing measurements of intraocular distances at arbitrary angles between vision axis and measurement direction, or along preselected linear or circular scans in equidistant measurement points, tomographic and topographic images can be obtained, containing information about the contour and the thickness profile of different retinal structures, e.g., the retinal thickness, the retinal nerve fiber layer thickness or the thickness of choroidal layers. We report on first preliminary clinical results of measurements of the retinal thickness on healthy subjects. Furthermore the papillary retinal nerve fiber layer thickness distribution of glaucoma patients was obtained and compared with measurements performed on healthy eyes. These results were documented and correlated with topographic maps of the optic disc obtained with a Heidelberg Retina Tomograph and with visual field testing performed by automatic perimetry.

We report on the possibility to reduce the coherence length and therefore improve the longitudinal resolution of partial coherence interferometry by an incoherent superposition of two spectrally displaced superluminescent diodes. If the object under investigation is dispersive, this resolution is limited due to a broadening of the detected signals. We demonstrate this group dispersion effect in water. In addition, two spectrally displaced diodes can also be used to measure the group refractive index and the group dispersion of ocular components. FIrst results of in vivo measurements in the cornea, the lens and along the axial length of human eyes are reported.

An alternative approach to measure intraocular optical distances in vivo is wavelength tuning interferometry. The principle of this technique is that every wavelength shift of a laser diode is associated with a phase shift of the waves reflected at the cornea and the retina. This results in a change of the interference order of the fringe system that will occur when the two wavefronts interfere. By Fourier transform of the intensity data, we cannot only measure the axial eye length but also other intraocular distances. In our approach we used an external cavity, single mode laser diode at a center wavelength of 780 nm which is tunable over 15 nm. First measurements were carried out in a model eye using the mode hop free tuning range of 9 nm, at the maximum tuning rate of 0.33 nm/s. The precision we obtained for the axial eye length was 0.04 mm. The large tuning range cannot be used of in vivo measurements because of the slow scanning rate of 0.33 nm/s. With this scanning rate the signal frequency corresponding to the axial eye length is on the order of 30 Hz. This is within the frequency range of the intensity modulations caused by fundus pulsations due to the heart beat and can therefore not be separated from them. The in vivo measurements have to be performed by tuning the wavelength of the laser with a piezoelectric transducer to achieve signal frequencies in the kHz range. Since the tuning range in this case in only 0.18 nm the resolution is about 50 times worse than that achieved in the model eye.

The method of spectral interferometry is used to perform 1D and 2D measurements of the thickness of glass lenses as well as the thickness of human corneas in vivo. Results of a computer-simulation and experiments are presented.

We have designed a laser complex which is a combination of a YAG:Er laser operating in the free-running mode, a radiation delivery system based on a chalcogenide fiber with uniquely high damage threshold, and a diagnostic optical coherent tomography (OCT) device for in situ monitoring of surface layers of the tissue under treatment. A flexible chalcogenide glass multimode fiber operates with a pulse energy of 150 mJ at a repetition rate of 3 Hz. Neither degradation nor laser damage were observed after transmission of 10⁴ laser pulses through the fiber delivery instrument. This complex was employed to study the effect of the radiation of a YAG:Er laser and some other mid-IR lasers on a cataract-suffered human lens. The pulse- to-pulse kinetics of the laser tissue treatment process with a scanning depth of several millimeters and spatial resolution of 15 micrometers was investigated.

Laser thermokeratoplasty (LTK) has been performed with a continuously emitting, tunable laser diode at 1.86 micrometers . A study on enucleated porcine eyes was conducted in order to demonstrate the feasibility of this laser source for LTK and to determine the useful irradiation modalities. Refractive changes achieved with different application systems and a standard coagulation pattern, consisting of 8 coagulation spots on a 6 mm ring, were measured. The most promising sets of parameters were carried out in a first animal study with mini pigs. Initial refractive changes up to 6 D could be achieved in vitro and in vivo with laser powers between 120 mW and 200 mW and irradiation times of several seconds. In conclusion, the mid-IR laser diode operated at a wavelength of 1.86 micrometers seems to be the optimal source for a clinical LTK system.

Corneal thermal lesions as typically applied in laser thermokeratoplasty were investigated with optical coherence tomography (OCT) and polarization microscopy. Both methods were used to compare the lesions with respect to the extent and degree of thermal denaturation. The coagulations wee applied on enucleated porcine eyes using a continuously emitting laser diode at a wavelength of 1.86 micrometers . The self developed OCT-system has an axial and lateral resolution of 15-20 micrometers and 26-48 micrometers , respectively, and was used to perform cross sections through the lesions. After OCT on the intact globe, the sections were fixed in formalin and embedded in paraffin. The sections were stained with sirius red and analyzed by polarization light microscopy. The comparison of the backscattered light of the OCT image with the changes in birefringence detected with the polarization microscopy technique found to be in good agreement. Up to four changes in birefringence detected with the polarization microscopy technique found to be in good agreement.Up to four different degrees of denaturation zones could be classified in both methods. OCT findings taken from a noninvasive method and correlated with histological experiences can be used to control the extent and degree of corneal lesions and monitor their healing response.

In the past, new techniques to measure axial eye length using partial coherent light or wavelength-tuned light have ben developed. They are based on the interference of the light reflected at the cornea and the retina of the eye. This method has then advantage of measuring independently of axial eye movements. However, the light is reflected from the retina and the cornea with different divergencies. These divergencies have to be matched to collect as much light as possible in order to achieve a sufficient interference signal. We have employed a diffractive optical element which focuses one part of the light onto the cornea while the other part of the light remains uninfluenced and is focused onto the retina by the optical system of the eye. As shown by measurements on model and on living eyes the signal-to- noise-ratio and the sensitivity of the dual beam partial coherence interferometry was improved.