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The application of lasers in neurosurgery is at present limited to a few special techniques like laser-induced interstitial thermotherapy (LITT) or percutaneous laser disc decompression (PLDD). Some neurosurgeons use high-power cw lasers for tissue coagulation to avoid bleeding. One reason for the small spreading of laser applications is the missing control of heat transfer into healthy tissue. As heat diffusion takes place symmetrically in all directions, exact coagulation is restricted to tumours with simple geometries.

We investigated refractive corneal surgery in vivo and in vitro by intrastromal photodisruption using a compact ultrafast femtosecond laser system. Ultrashort-pulsed lasers operating in the femtosecond time regime are associated with significantly smaller and deterministic threshold energies for photodisruption, as well as reduced shock waves and smaller cavitation bubbles than the nanosecond or picosecond lasers. Our reliable all-solid-state laser system was specifically designed for real world medical applications. By scanning the 5 micron focus spot of the laser below the corneal surface, the overlapping small ablation volumes of single pulses resulted in contiguous tissue cutting and vaporization. Pulse energies were typically in the order of a few microjoules. Combination of different scanning patterns enabled us to perform corneal flap cutting, femtosecond-LASIK, and femtosecond intrastromal keratectomy in porcine, rabbit, and primate eyes. The cuts proved to be highly precise and possessed superior dissection and surface quality. Preliminary studies show consistent refractive changes in the in vivo studies. We conclude that the technology is capable to perform a variety of corneal refractive procedures at high precision, offering advantages over current mechanical and laser devices and enabling entirely new approaches for refractive surgery.

Time-resolved video photographs have been taken in order to investigate plasma induced surface ablation of soft tissue by IR picosecond laser pulses with energies of about 1 mJ. The emission and propagation of shock waves in the irradiated tissue as well as in the surrounding air environment was studied. The pressure amplitudes of the shock transients were determined from the measured shock velocities. A decay of the pressure amplitude below 100 MPa was observed within a distance of about 200 micrometers from the center of laser induced optical breakdown. The dynamics of the ablation crater and the ejection of the ablation fragments was studied on a larger time scale. The maximum expansion of the ablation crater was measured to be about 200 (Mu) m at temporal delays of 4-5 microsecond(s) after the impact of the laser pulse. Furthermore, generation and propagation of a surface deformation wave was observed. Thus, we present a detailed and consistent description of all phenomena occurring during plasma-mediated surface ablation of soft tissue.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The dependence of the fluence at the threshold of laser- induced optical breakdown on the laser pulse duration has been investigated experimentally and theoretically for human cornea, human enamel, and bovine brain tissue. For the experiments in the range from 100 fs to 200 ps, we used a femtosecond dye laser system and a picosecond Nd:YLF laser system. We observed a significant decrease of the fluence at the threshold when reducing the pulse duration. The measured dependence on the pulse duration is in good agrement with our model.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In Ophthalmology the application of ultrashort laserpulses is already well established for special intraocular surgeries like posterior capsulotomy or periphere iridotomy (see e. g. [SP85]). Also the application of picosecond pulses for intrastromal ablation in refractive surgery is under investigation. Recently the process of plasma-induced or photodisruptive laser-tissue interaction is also discussed with respect to other medical applications, e. g. lithotripsy, angioplasty and the resection of brain tumours in neurosurgery [Fis+94]. As the pulse duration is much shorter than the thermal diffusion time, tissue ablation without thermal side-effects (carbonisation and coagulation zones) is possible. On the other hand, the expansion of the microplasma leads to the generation of a shock wave, a high pressure transient moving radially outword with an initial velocity exceeding the sound velocity. Histological examinations do not reveal any dammage of adjacent tissue, but with these methods possible effects on a subcellular level cannot be examined.

Plasma mediated ablations of calf brain tissue and corneal tissue have been performed with ultrashort pulses provided by different laser systems: A 30 ps Nd:YLF amplified laser with up to 1.5 mJ of pulse energy and a powerful 180 fs Ti:Sapphire oscillator/regenerative amplifier laser system capable of pulse energies of up to 300 (mu) J. With the femtosecond pulses the threshold in energy density needed for initiating the ablation process was found to be significantly lower than with longer pulse durations. Furthermore, the amount of ablated material is by a factor of two greater with the femtosecond pulses compared to the results obtained with the picosecond pulses. Histological examinations did not show any thermal or structural damage in adjacent tissue with pulses in the pico- and femtosecond range as it could be found after irradiation with longer pulses. The morphology of the excisions is of a very high quality.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-vitro ablation experiments of calf brain tissue with ultrashort lasers are reported. Excisions have been performed by the mechanism of plasma-mediated ablation with focused picosecond laser pulses. Results achieved with a 35 ps oscillator/regenerative amplifier Nd:YLF laser at 1053 nm and its second and fourth harmonic at 526 and 263 nm, respectively, are presented. Preliminary experiments with 19 ps laser pulses at wavelengths near 3 micrometers generated by optical parametric amplification are reported. Here, during the ablation with low photon energies no plasma was induced. Furthermore, the spectrum of laser-induced plasma sparks on the surface of fresh calf brain tissue was recorded. Atomic line widths were measured and used to determine the electron density of the plasma. With energy densities of about 100 J/cm2 values between 2 (DOT) 1017 and 5 (DOT) 1018 cm-3 were found.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In order to predict corneal curvature change after intrastromal surgery three finite element models are developed and compared. The models differ essentially in the underlying assumptions about the mechanical behavior of the cornea. The first model assumes an isotropic corneal structure with a nonlinear stress/strain behavior, thus providing a lower limit for the corneal curvature change. Contrary model two assumes transverse isotropy combined with a linear stress/strain behavior, in order to provide an upper limit for corneal curvature change. Although differing in their structural assumptions, model one and two consider the cornea as being a solid structure. A third model, based on a multilayered thin shell theory, shows that this might not be sufficient. In addition the calculations are run for two different ablation patterns. The performed analyses are extremely useful in choosing ablation patterns and dimensions for intrastromal refractive surgery. But as the model predictions of corneal curvature change differ in about 4 dpt for a given pattern, more subtle modifications and a better knowledge of the mechanical parameters of the cornea are necessary in order to obtain applicability of model calculations to predict the actual outcome of a performed surgery.

A Hartmann-Shack wave-front sensor is used to measure the wave aberrations of the human eye by sensing the wave front emerging from the eye produced by the retinal reflection of a focused light spot on the fovea. Since the test involves the measurements of the local slopes of the wave front, the actual wave front is reconstructed by the use of wave-front estimation with Zernike polynomials. From the estimated Zernike coefficients of the tested wave front the aberrations of the eye are evaluated. It is shown that with this method, using a Hartmann-Shack wave-front sensor, one can obtain a fast, precise, and objective measurement of the aberrations of the eye.

A new method for the fast measurement of the Z—transferfunction in the Fovea Centralis of the human eyeusing a confocal laser scanning device is presented. The tested eye is illuminated by a collimated helium—neonlaser beam and therefore focuses the light onto the retina. Reflected light is detected using confocal techniques, i.e. a pinhole in the detection unit assures that only light originating from the focal plane is sampled. Thefocus of the laser beam is scanned in one dimension along the visual axis of the eye through the retina by adding a slide defocus to the incoming beam. The Z—transferfunction can be obtained directly by measuringthe intensity of the reflected light for different focal planes. A fixation target is offered to the eye at infinity tostabilize the accommodation as well as the direct viewing. Different pinholes in the detection unit are testedto investigate signal to noise ratio and depth resolution. Furthermore, the wave aberrations of the tested eyesand the laser scanning system itself are measured using a Hartmann—Shack wavefront sensor. Once the waveaberrations are known the intensity distribution in the plane of the detection pinhole can be calculated usingdiffraction theory. Thus, the intensity which passes through a pinhole of a certain diameter on the optical axisof the system can be obtained and the Z—transferfunction can be simulated. A comparison of the measuredand simulated Z—transferfunctions for different sizes of the detection pinhole has been performed and showssubstantial similarity.

Changes in parapapillary retina height may be an early indicator of glaucomatous tissue damage. The subject of this study is the development of an automatic computer analysis for the height variations of the parapapillary retina obtained from simultaneously taken pairs of stereophotographs of optic nerve heads. The exact evaluation of stereo photographs is, so far, processed manually in the USA (Takamoto, Boston) in a timeconsuming procedure. An automatic analysis can be considered as an essential improvement in glaucoma diagnosis.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Glaucomatous diseases are one of the main reasons for blindness in modern indu- stry nations. In its chronical form this disease leads to a reduction of the nerve fi-ber layer of the retina. Therefore it is a point of interest to measure the thickness of the nerve fiber layer in the human eye in vivo. Because of the birefringent abili- ties of the nerve fiber layer an ellipsometric method can be used to measure its thickness. The next sections will describe a method based on standard ellipsometry, but using electro-optical devices instead of mechanical ones to measure the nerve fiber layer thickness faster and with less errors then it is done by mechanical ellip- sometry. 1. Introduction An eye disease that is relatively wide spreaded in modern industry nations is the chronic Glaucoma that is caused by an abnormal high eye pressure. The average va- lue for the eye pressure lies by about 20 mbar and fluctuates about S mbar during the day. Glaucomatous diseases decrease the blood flow through the retina and da

The effect of different wavelengths on corneal tissue ablation has been studied using the 1st, 2nd, and 4th harmonics of a picosecond Nd:YLF laser system. This laser system consists of an actively mode locked Nd:YLF oscillator generating 30 ps pulses and a regenerative amplifier. The laser beam was focussed onto fresh human donor or calf corneas. The corneas could be moved in 3D patterns using computer controlled stepping motors. The amount of the ablated tissue and the quality of the resulting surfaces were judged by light and scanning electron microscopy. The histology showed that the ablation depths of the IR and green laser pulses are very similar at a given energy density. SEMs revealed that UV light causes a denser packaging of the underlying collagen fibers, similar to the pseudo Bowman's membrane generated with excimer lasers. Further studies are planned concerning the investigation of the 5th harmonic at 211 nm.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In vitro ablation experiments of calf brain tissue using a picosecond Nd:YLF laser system are reported. The laser lesions were studied with special regard to the ablation rate and to possible injuries of tissue adjacent to the excisions. Histological examinations prove the excellent quality of the lesions, showing no signs of thermal damages or carbonization. Furthermore, the influence of higher photon energies on the ablation threshold was studied.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ultrashort laser pulses enable a process of tissue ablation that is of great interest for medical therapy. In contrast to long pulsed or cw lasers, where the applied energy leads to heating and vaporization of irradiated tissue, the ablation process in the ultrashort pulse range is initiated by a plasma formation in the laser focus, resulting in tissue removal.1 Using picosecond pulses this ablation mechanism is already well established in ophthalmology. The accuracy and the absence of undesired thermal side effects made the plasma-mediated ablation a promising technique in other medical fields, e.g., neurosurgery.

Three different laser systems have been investigated according to their possible application in dentistry: a free running and a Q-switched microsecond Ho:YAG laser, a free running microsecond Er:YAG laser and picosecond Nd:YLF laser system consisting of an actively mode locked oscillator and a regenerative amplifier. The experiments focused on the question if lasers can support or maybe replace ordinary drilling machines. For this purpose several cavities were generated with the lasers mentioned above. Their depth and quality were judged by light and electron microscopy. The results of the experiments showed that the picosecond Nd:YLF laser system has advantages compared to other lasers regarding their application in dentistry.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A laser system is described that offers manifold applications in ophthalmic surgery. The Nd:YLF solid-state laser emits at a wavelength of 1.053 micrometers . With pulse durations of 40 psec less energy is required to obtain optical breakdown compared to nanosecond lasers. The lower pulse energy allows a substantial confinement of the laser-tissue interaction. With a highly sophisticated delivery system eye movements can be tracked during surgery and a precise placement of the laser focus can be achieved. The high transmission coefficient of ocular structures for wavelengths around 1 micrometers enables the Nd:YLF laser to perform several internal eye surgeries such as cataract fragmentation, intrastromal refractive surgery, internal sclerostomy, or iridotomy. First clinical results of some of these procedures are summarized.

An intelligent surgical laser system, which can help the ophthalmologist to achieve higher precision and control during their procedures, has been developed by ISL as model CLS 4001. In addition to the laser and laser delivery system, the system is also equipped with a vision system (IPU), robotics motion control (MCU), and a tracking closed loop system (ETS) that tracks the eye in three dimensions (X, Y and Z). The initial patient setup is computer controlled with guidance from the vision system. The tracking system is automatically engaged when the target is in position. A multi-level tracking system is developed by integrating our vision and tracking systems which have been able to maintain our laser beam precisely on target. The capabilities of the automatic eye setup and the tracking in three dimensions provides for improved accuracy and measurement repeatability. The system is operated through the Surgical Control Unit (SCU). The SCU communicates with the IPU and the MCU through both ethernet and RS232. Various scanning pattern (i.e., line, curve, circle, spiral, etc.) can be selected with given parameters. When a warning is activated, a voice message is played that will normally require a panel touch acknowledgement. The reliability of the system is ensured in three levels: (1) hardware, (2) software real time monitoring, and (3) user. The system is currently under clinical validation.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In this paper an extension of the principle of the Hartmann-Shack wavefront sensor to the neasureirtent of the sphericity of a specular reflection surface is presented. The features of this widely usable system are discussed by means of the application of this approach to the modeling of surface aberrations of the cornea of the eye. The results of measurements performed with a first prototype on a set of stainless steel calibration spheres of different radii are shown.

This paper presents the measurement of the wave aberrations of human eyes with a Hartmann-Shack Sensor (HSS), used in a system for active optical depth resolution improvement of the Laser Tomographic Scanner(LTS). A least-squares algorithm of modal wavefront estimation from the tested derivatives is described. The noise propagation of this algorithm is examined. And the experimental results of tested living eyes are presented.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Many problems in ophthalmology deal with the three-dimensional structure of various parts of the living human eye. A complete confocal laser scanning microscope for diagnosis of eye disorders and first clinical results are described. Geometric measurements of the cornea and topographic measurements of the retinal substructures are presented. In addition, laser scanning tomography can be used to assess quantitatively the morphometry of the retina of the human eye. The confocal imaging mode of the laser tomographic scanner (LTS) is applied to evaluate the thickness of the nerve fiber layer. The contrast of nerve fiber layer images can be enhanced by polarization-dependent imaging modes, such as Fourier ellipsometry (FE) and differential interference contrast (DIC) imaging. To improve the depth resolution, an active-optical focusing system is employed.

Degeneration of retinal pigment epithelial (RPE) cells severely impairs the visual function of retina photoreceptors. However, little is known about the events that trigger the death of RPE cells at the subcellular level. Two-photon excited autofluorescence (TPEF) imaging of RPE cells proves to be well suited to investigate both the morphological and the spectral characteristics of the human RPE cells. The dominant fluorophores of autofluorescence derive from lipofuscin (LF) granules that accumulate in the cytoplasm of the RPE cells with increasing age. Spectral TPEF imaging reveals the existence of abnormal LF granules with blue shifted autofluorescence in RPE cells of aging patients and brings new insights into the complicated composition of the LF granules. Based on a proposed two-photon laser scanning ophthalmoscope, TPEF imaging of the living retina may be valuable for diagnostic and pathological studies of age related eye diseases.

Nonlinear laser scanning microscopy is widely used for noninvasive imaging in cell biology and tissue physiology. However, multiphoton fluorescence imaging of dense, transparent connective tissue (e.g., cornea) is challenging since sophisticated labeling or slicing is necessary. High-resolution, high-contrast second harmonic generation (SHG) imaging of corneal tissue based on the intrinsic structure of collagen is discussed. The three-dimensional corneal ultrastructure in depths up to hundreds of microns can be probed noninvasively, without any staining or mechanical slicing. As an important application of second harmonic imaging in ophthalmology, the modification of corneal ultrastructure using femtosecond laser intrastromal ablation is systematically investigated to evaluate next-generation refractive surgical approaches.

Laser werden zunehmend sowohl als Analysegerat als auch als Werkzeug eingesetzt. Uber Entwicklungen in der Biologie und Medizin berichten wir in dieser und den kommenden funf Ausgaben.

For real-time analysis of light microscopic images by using structural features we have developed a special image processing system. The major components of the system are a parallel multi--microprocessor system, an image buffer, a stepping motor driver module for x-, y-, and z-movement of the microscope, a video processing module and a binary image module. The two latter modules are special hardware to generate structural parameters of the video image in real-time. The parameters are derived from the number and optical density of individual objects within each image line. An object is recognized if its optical density exceeds a predefined threshold and its size falls into a programmable interval. We have applied this system to objectively evaluate histological changes occuring in neoplasias treated preoperatively with chemotherapy. The result is an automated mapping of residual tumor lesions for each specimen. These maps are used to define chemotherapy effects. It should be emphasized that because of its flexibility, the system described can be used more generally to analyse structures of histological images.

A microprocessor controlled photo-thermoplastic device has been developed for the real-time recording of holographic filters in a hybrid digital-optical data processing system. It is possible to position the filters with an accuracy of 0,2 µm.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Es wurde eine objektunabhangige Rekonstruktionsmethode fur dreidimensionale lichtmikroskopische Bilder entwickelt. Durch inverse Filterung des Bildes mit der dreidimensionalen optischen Ubertragungsfunktion des Lichtmikroskops kann das Original weitgehend wieder hergestellt werden. Die Rekonstruktion von Testobjekten mit bekannter dreidimensionaler Struktur gewahrleistet die Zuverlassigkeit des Verfahrens.

To study the retinal surface in the human eye in normal and diseased states we used laser scanning tomography. The confocal arrangement of the laser tomographic scanner permits examination of retinal topography in the axis perpendicular to the retinal surface. The eyes examined with the laser tomographic scanner included normal eyes, eyes with macular holes, impending macular holes, radiation retinopathy, macular edema, photocoagulation scars, subfoveal scars, and serous detachment of the fovea associated with subretinal neovascularization. The laser tomographic scanner is a new method that allows measurements of the topography of the internal limiting membrane in the macular area and may improve our understanding of the pathophysiologic characteristics and treatment of a variety of disorders of the macula.

In spite of the dramatic changes in feature size, die size and defect density, wafer inspection continues to be done manually. However, fast automatic wafer inspection will be a key element for efficient submicron VLSI-process technologies. A new two-step laser scanning procedure is described, resulting in processing speeds of up to 40 million pixels per second and spatial resolution down to 0.1 μm.

Die Korrelation stellt ein allgemeingultiges Konzept fur die Klassifizierung von Mustern dar. Mit analogen oder digitalen koharenten optischen Filterungsmethoden konnen Objektmuster erkannt werden. Im Falle des Spatial Filtering nach Van-der-Lugt [1] wird fur jedes Muster ein angepastes Filter benotigt. Daraus resultiert bei grosen Mustervielfalten fur die optischen Filterungsmethoden eine grose Filterzahl. Um diesen Nachteil zu vermeiden, wurden Methoden zur Erzeugung analoger und digitaler Komponentenfilter entwickelt, die einer Reihe von Objektmustern angepast sind. Dadurch wird der Merkmalraum in Unterraume mit vorgegebenem semantischen Inhalt aufgeteilt.

In adaptive optics, the compensating phase distribution can be generated by an expansion of the turbulence phase distortions into modes of a set of basis functions, i.e. Zernike-polynomials and Karhunen-Loeve-functions. By applying a modal control concept to a practical adaptive optical system, the electrodes of a membrane mirror can be controlled in parallel with greatly reduced cross-talk. The coefficients of the approximating functions, each of which corresponds to a certain mode of the mirror surface, are fed back to the actuators by a modal control matrix. The atmospheric turbulences are taken into account by extending the local differential operator in an "observer"-like structure.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

With optically generated component filters a binary recognition of 2-D objects is possible. The correlation signals of the filters are processed by a digital computer system to achieve a classification by signal intensity and shape. The computer also controlls the filtergeneration and carries out the recognition.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Laser scanning tomography can be used to quantitatively assess the morphometry of the retina of the human eye. In this paper, the confocal imaging mode of the Laser Tomographic Scanner (LTS) is applied to evaluate the thickness of the nerve fiber layer. The contrast of nerve fiber layer images can be enhanced by polarization dependent imaging modes, like Fourier - ellipsometry (F.E.) and differential interference contrast (DIC) imaging. To improve the depth resolution, an active-optical focusing system is employed.

Light propagation through the atmosphere is disturbed by atmospheric turbulence. This limits the high angular resolution in astronomical imaging. Active optics is a method to overcome this problem. It allows a real-time optimization of the resolving power. An active mirror was developed which consists of an electrostatically deformable membrane. The polymer foil is actuated by an electrode array with 63 hexagonal elements arranged in a ring structure. The sensitivity is 0,05 microns per volt. It works up to 4 kHz without resonances. The maximum local tilt of the membrane is 3 microns per 5 millimeter. For an atmospheric tilt compensation of the wavefront the mirror housing is in a gimbal mount. Piezo-electric actuators provide a total mirror angular movement up to 20 Hz and angular sensitivity of 77 arcsecs per kV. With this active mirror device and a multi-microprocessor control unit the stabilisation of the star-speckle pattern positions and the deconvolution of the speckle patterns are possible. Two control methods are in development and analyzed in comparison. One is working with a modified shearing interferometer as a wave-front sensor and a feedback with cross-talk compensation. The other is sensing the optical information in the image plane by a diode-array and computing the wave-front from its Fourier modulus. The compensating phase distribution is generated by an expansion of the turbulence phase distortions into modes of a set of basis functions, i.e. Zernike-polynomials and Karhunen-Loeve-functions. By applying a modal control concept to the adaptive optical system, the electrodes of a membrane mirror are controlled in parallel with compensated cross-talk. The coefficients of the approximating functions, each of which corresponds to a mode on the mirror surface, are fed back to the actuators by a modal control matrix. The system presented in this paper is designed for the 0,75 m RC-telescope with alt-az mount at the Landessternwarte in Heidelberg, F.R.G..© (1983) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.