a) Field of the Invention
The present invention is directed to an optical scanning system for medical applications, preferably in ophthalmology. The optical scanning system can be used for a number of diagnostic and therapeutic applications.
b) Description of the Related Art
Applications in the field of diagnostics include retina scanners, OCT systems and systems for mapping the cornea. Retina scanning is an image-generating process for high-resolution representation of the retina for displaying physiology and clinical images. OCT (optical coherence tomography) systems are used for three-dimensional representation of the transparent optical media and make it possible to observe a wide variety of cross sections of the eye, e.g., the cornea, anterior chamber, or eye lens, and in particular also enable an exact measurement of eye length. The measurement principle employed by OCT systems is based on scanning an interferometric beam path over the entire pupil surface for length measurement in transparent media. The spatial structure of the eye can be reproduced from the multitude of local distance data at every location on the pupil. A spatial image of the cornea geometry is determined with topography devices for measuring the cornea in order to prepare for surgical procedures, e.g., LASIK, LASEK, PRK or the like, on the cornea.
However, scanning systems also have therapeutic uses for various eye disorders. The majority of applications are concerned with refractive surgery for correcting defective vision in the human eye. These procedures include, in particular, LASIK, PRK and LASEK. A specific change in the curvature of the cornea is brought about by means of laser radiation to compensate for defective vision of the eye. The procedures mentioned above make use of a therapy laser beam which is guided in a scanning manner over the pupil surface to be corrected.
In most of the optical scanning systems known from the prior art, the visual axis of the eye must be determined and/or maintained. For this purpose, a fixation object is presented to the eye and the patient gazes at this fixation object during the treatment or diagnosis, so that the eye is fixated. An image with a pivot or a small light point can be used as a fixation object. Since the human eye moves the pivot into the center of sharpest vision (fovea), the visual axis of the eye is directed to the optical axis of the external diagnostic or therapeutic system.
FIG. 1 shows by way of example the basic construction of an optical system known from the prior art for scanning the cornea.
The generated measurement beam or therapy beam 1 is deflected corresponding to the desired scan field depending on the quantity of scan directions by at least one moving reflector 2 and strikes a first focusing optical scanner arrangement 3 which generates an intermediate image 4 of the scan field. By means of collimating scanner optics 5, the scan field is imaged to infinity and directed to the objective 7 at a stationary dichroic deflecting mirror 6. This objective 7 serves to focus the measurement beam or therapy beam 1 on the desired imaging plane 8 (in this case, the cornea) in the eye 9. A fixation object 11 is focused on the patient's retina in order to fixate the eye to be examined and/or treated. The fixation object 11 is imaged by imaging optics 13 in the mirror plane of the dichroic deflecting mirror 6 as an intermediate image 12. The intermediate image 12 is imaged to infinity on the cornea by the objective 7 and is focused on the retina 10 through the optical action of the cornea and eye lens.
In contrast to the construction shown in FIG. 1, the fixation object can be coupled into the beam path through an additional dichroic beamsplitter or semitransparent mirror.
In the example shown in the drawing, the intermediate image of the fixation object lies on the surface of the dichroic deflecting mirror. Significant optical imaging errors (astigmatism) which limit the sharpness and point size of the fixation object occur when convergent or divergent beam bundles pass through the medium of the deflecting mirror. Owing to the extensive effect of the imaging errors, it is not possible to generate complicated fixation objects with fine structures on the retina.
The disadvantages of the solutions known from the prior art result from the complexity of the required components. Owing to the combination of the fixation beam path with the measurement beam path and therapy beam path, an intermediate imaging of the scan field is required which substantially increases the quantity of optical components and their requirements for corrective measures. This results in an enormous expenditure on development, manufacture and adjustment.
Another disadvantage which results from the intermediate imaging of the scan field is the unwanted occurrence of nonlinear optical effects like random optical breakthrough, phase modulation, or the like. This can lead to reduced reproducibility of the treatment results, particularly for therapy beams with high peak outputs.