Abstract:
An autofocus module for a microscope-based system includes at least two light sources, each of which generates a light beam for focusing. An optical directing device is provided that directs a respective portion of each light beam onto an incoupling means, which couples each of the light beams into the illuminating light beam of the microscope-based system and directs the light beams onto a specimen. A first and a second detector receive the light beams of the first and second light source reflected from the surface of the specimen, and ascertain the intensities on the first and second detector in time-multiplexed fashion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to German patent application 102 04 367.1, which is hereby incorporated by reference herein. 
   BACKGROUND 
   The invention concerns an autofocus module for a microscope-based system having an objective that defines an optical axis in which an illuminating light beam, which is perpendicular to a surface of a specimen onto which focusing occurs, propagates. 
   The invention further concerns a microscope system having an objective that defines an optical axis which is perpendicular to the surface of a specimen, and having a stage on which the specimen rests and which is displaceable in the direction of the optical axis. 
   In addition, the invention concerns an autofocus method for a microscope-based system which encompasses at least one objective that defines an optical axis of the microscope-based system. 
   German patent document DE 32 19 503 describes an apparatus for automatic focusing on specimens to be viewed in optical devices. After reflection at the surface of a specimen and reflection at a splitter mirror, the reflected measured light beam passes through a pinhole. A portion of the measured light beam is reflected out by means of a fully reflective surface and, after passing through a slit diaphragm, is directed onto a differential diode. In the focused state, the focus is located between the two diodes. Upon defocusing, the measurement spot migrates onto one of the two diodes, which are connected to corresponding control means. Optical or mechanical means of the microscope are adjusted by the control means to as to bring the measurement spot once again between the two diodes and thus re-establish the focal position. 
   European Patent Application EP-A-0 124 241 describes a microscope having an automatic focusing device. The microscope encompasses a storage device for saving the data for the objectives that are used in the microscope. Also provided is a control device that monitors and regulates the various microscope functions. The tasks of the control device also include moving the focusing stage. A CCD element, which receives an image from the respectively selected objective and, together with a calculation unit, ascertains the image sharpness from the optimum contrast, is provided as an image acquisition device. The objective data of the objective currently being used must be taken into account in ascertaining the optimum degree of sharpness. These data are, as already mentioned above, stored in a memory. 
   German Unexamined Application DE 41 33 788 describes a method for autofocusing of microscopes and an autofocus system for microscopes. The image of a specimen or of a pattern superimposed onto the specimen is conveyed to two regions on a detector or to two different detectors, such that in the focused position, one image occurs in front of one detector, and one image behind the other detector. The image sharpness states on the detectors are converted into electronic signals whose difference is used to focus the objective. The distances of the image or of the respective pattern from the respective detectors are adjustable. Deliberate offset settings as well as “IR offset” correction settings can be implemented. 
   In the context of automatic focusing in microscopes in the semiconductor industry, it is problematic that transitions from highly reflective regions to less-reflective regions cause an autofocus system to make incorrect settings. These transitions are referred to as “edges.” The influence of these edges on focusing using an autofocus system is referred to in the description below as the “edge effect.” 
   SUMMARY OF THE INVENTION 
   An object of the present invention is accordingly to provide an autofocus module for a microscope-based system that, irrespective of edge effects, ensures reliable focusing on a specimen to be examined. 
   According to an embodiment of the present invention
         at least two light sources are provided, each of which generates a light beam for focusing;   an optical means is provided that directs a respective portion of each light beam onto an incoupling means which couples each of the light beams into the illuminating light beam of the microscope-based system and directs it onto the specimen; and   at least a first and a second detector are provided,   whereby respective light beams of said at least two light sources are reflected from the surface of said specimen and are directed onto said first and said second detector.       

   The present invention provides a microscope system that, irrespective of edge effects, ensures reliable focusing on a specimen to be examined. 
   The present invention provides a microscope system which is characterized in that there is connected to the microscope system an autofocus module that contains at least two light sources, each of which generates a light beam for focusing; that an optical means is provided that transfers a respective portion of each light beam into the optical axis of the microscope system and directs it onto the specimen; and that at least a first and a second detector are provided, each of which, via the optical means, directs a respective light beam of the first and second light source, reflected from the surface of the specimen to be examined, onto the first and the second detector. 
   It is an object of the present invention to provide an autofocus method for a microscope-based system that ascertains the optimum focal position rapidly and reliably and irrespective of edge effects. 
   The present invention provides an autofocus method including the following steps:
         displacing a specimen in the direction of the optical axis and around a region that contains the optimum focal position;   generating, in each of at least two light sources, a light beam for focusing, the at least two light sources being operated alternately with one another;   directing a portion of the first and the second light beam onto the surface of the specimen by means of an optical means;   receiving on a first detector the light of the first light beam reflected from the surface of the specimen, and receiving on a second detector the light of the second light beam reflected from the surface of the specimen; and   determining the optimum focal position from the measured intensities on the first and second detectors.       

   The use of at least two light sources, each of which emits a light beam for focusing, is advantageous because the use of two light beams restores the symmetry of the system that was lost by the use of only a portion of the light for focusing in each case. The two light sources each emit a light beam for focusing, these two light sources being operated alternately with one another. As a rule, alternating operation of the two light sources is determined, e.g., by their pulse duration. The result is that when the stage with the specimen is displaced along the optical axis, the focus moves on the surface of the specimen in two mutually opposite directions. The consequence of this is that information is obtained from two foci approximately simultaneously. During displacement of the specimen, intensities are ascertained alternately at the first and at the second detector, and the intensities thus ascertained are stored as intensity profiles in a memory of the control computer or compared to sample profiles from the memory for the optimum focal position. 
   The optical means is embodied as a prism, and arranged in such a way that the light of the first light source is directed into a first half of the illuminating light beam, and the light of the second light source into a second half of the illuminating light beam. The optical means can each be constructed from two mirror-coated elements, arranged at right angles to one another. The mirror-coated elements are inclined at a 45° angle to the respectively incident light beams for focusing. 
   It is advantageous if the first and the second light source, the first and the second detector, the optical means, and further beam deflection means are arranged in a housing that can be connected to the microscope-based system. Since it is important in terms of measurement accuracy for the first and second halves of the light beam for focusing each to be coupled in exactly halved fashion into the illuminating light beam of the microscope-based system, arrangement of the various optical components in a housing is important in order to prevent any misalignment of the device. Laser diodes that generate the light beam for focusing are used as light sources. In addition, the laser diodes can easily be operated in pulsed fashion. 
   The housing of the autofocus module can be connected to the microscope-based system via a flange. Microscope-based systems comprise a dichroic beam splitter that couples the respective light beam for focusing into the microscope-based system, and couples the light beam reflected from the surface of the specimen back into the autofocus module. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is elaborated upon below based on exemplary embodiments, with reference to the drawings, in which: 
       FIG. 1   a  depicts a prior art autofocus system in which the specimen is located below the focus; 
       FIG. 1   b  is a detail view of the region marked with a dashed line in  FIG. 1   a , to illustrate the movement of the light spot on the specimen; 
       FIG. 2   a  shows an example of illumination of a microscopic preparation using the system of  FIG. 1 , no regions of differing reflectance being present on the preparation; 
       FIG. 2   b  depicts focus determination based on the intensity of the light reflected from the specimen and the migration of the center point of the intensities; 
       FIG. 3   a  shows an example of illumination of a microscopic preparation using the system of  FIG. 1 , the direction of travel of the focus spot being perpendicular to an edge on the preparation; 
       FIG. 3   b  depicts focus determination based on the intensity of the light reflected from the specimen and the migration of the center point of the intensities; 
       FIG. 4   a  shows an example of illumination of a microscopic preparation using the system of  FIG. 1 , the direction of travel of the focus spot being parallel to an edge on the preparation; 
       FIG. 4   b  depicts focus determination based on the intensity of the light reflected from the specimen and the migration of the center point of the intensities; 
       FIG. 5  schematically shows a configuration of the autofocus system according to the present invention with an alternating pupil splitting system, depicting the path of the focusing light beam emitted by the first laser; 
       FIG. 6  schematically shows a configuration of the autofocus system according to the present invention with an alternating pupil splitting system, depicting the path of the focusing light emitted by the second laser; 
       FIG. 7  shows an autofocus module, having the autofocus system of the invention, which can be attached to a microscope-based system; 
       FIG. 8   a  shows the shape and motion of the focus spot on the surface of the specimen, and evaluation thereof by means of two PSDs; 
       FIG. 8   b  shows the shape and motion of the focus spot close to the optimum focal position on the surface of the specimen, and evaluation thereof by means of two PSDs; and 
       FIG. 8   c  shows the shape and motion of the focus spot at the optimum focal position on the surface of the specimen, and evaluation thereof by means of two PSDs. 
   

   DETAILED DESCRIPTION 
   An autofocus system of the existing art is depicted in  FIG. 1   a , those parts of the microscope system not necessary for elucidation of the composition and manner of operation of the autofocus system being omitted.  FIG. 1   a  describes the focal position in which a specimen  4  is located below focus  4   a . The region around specimen  4  is marked in  FIG. 1   a  with a dashed circle  5 , and this region is depicted in detail in  FIG. 1   b ; in  FIGS. 1   a  and  1   b , identical reference characters are used for identical features. A light source  6  emits a light beam  8  that is split by a pupil splitting system  10 . Light source  6  can be embodied as a laser. Light beam  8  strikes a beam splitter  11  and is thereby coupled into optical axis  13  defined by a tube lens  12  and an objective  14 . A reflected light beam  15  (or light beam bundle) emerges from specimen  4 , which is located below focus  4   a , undeflected by beam splitter  11 . The deviation of reflected light beam  15  from optical axis  13  can be determined in an intermediate image plane  16 . Specimen  4  lies on a stage (not depicted) that can be brought into a different focal position by displacement in Z direction Z. This modification of the focal position is converted into a change in X position X of reflected light beam  15  in intermediate image plane  16 . 
     FIG. 2   a  shows the shape of a focus spot  4   a  on specimen  4 . The direction of travel of focus spot  4   a  on specimen  4  is depicted by an arrow P. Focus spot  4   a  is constituted by zero-order focus spot  9   0 , a negative-first-order focus spot  9   −1 , and a positive-first-order focus spot  9   +1 . No regions having differing reflectances are present on specimen  4 ; only one high-reflectance region  100  is provided. In general, specimen  4  to be examined is arranged on a specimen stage. The optimum focal position is established by generating a relative motion between the specimen stage and objective  14  of the microscope-based system. This relative motion is performed over a small region around the optimum focal position. As a result of the relative motion, focus spot  4   a  moves over the surface of specimen  4  in the manner indicated by arrow P. In  FIG. 2   b , the intensity recorded in intermediate image plane  16  is plotted as a function of focal position. The pixel number of the individual photosensitive elements of the CCD element is plotted on abscissa  17  in  FIG. 2   b . The intensity of the light reflected from the surface of the sample is plotted, in arbitrary units, on ordinate  18 . The curve that yields the optimum focal position is shown as a dashed line. In each curve, the center point of the intensity reflected from the surface of the specimen is depicted as a solid circle. If the focus setting is above or below the optimum focal position, it is clearly evident that the width of the intensity peak decreases as the optimum focal position is approached. The direction of motion of the center point is unequivocal, and in the case discussed here runs from left to right. The optimum focal position can thus be ascertained unequivocally from the center point profile and the shape of the intensity peak. 
     FIG. 3   a  shows the shape of a focus spot  4   a  on specimen  4 . The direction of travel of focus spot  4   a  on specimen  4  is depicted by an arrow P. Focus spot  4   a  is constituted by zero-order focus spot  9   0 , a negative-first-order focus spot  9   −1 , and a positive-first-order focus spot  9   +1 . A high-reflectance region  100  and a low-reflectance region  102  are provided on specimen  4 . The regions are separated by an edge  104  that, in this example, extends perpendicular to the direction of travel of focus spot  4   a . As already mentioned in the description of  FIG. 2   a , the optimum focal position is established by means of a relative motion between the specimen stage and the objective of the microscope-based system. As a result of the relative motion, focus spot  4   a  moves over the surface of specimen  4  in the manner indicated by arrow P. At edge  104  the reflectance changes, thereby changing the light quantity reflected into intermediate image plane  16 . The pixel number of the individual photosensitive elements of the CCD element is plotted on abscissa  17  in  FIG. 3   b . The intensity of the light reflected from the surface of the sample is plotted, in arbitrary units, on ordinate  18 . The curve that yields the optimum focal position is shown as a dashed line. In each curve, the center point of the intensity reflected from the surface of the specimen is depicted as a solid circle. If the focus setting is above or below the optimum focal position, it is clearly evident that the symmetry of the intensity peak, and its height, increase as the optimum focal position is approached. The direction of motion of the center point runs initially from left to right, but reverses after the optimum focal position and runs from right to left. From the plurality of curves obtained, it is not possible to ascertain unequivocally the one for the optimum focus. Pairs of curves exist that have the same center point location but do not coincide with the optimum focal position. The determination of focal position is therefore not unequivocal in the case of an edge  104  perpendicular to the direction of travel of the focus spot. 
     FIG. 4   a  shows the case in which focus spot  4   a  moves parallel to edge  104 , which is defined by the boundary between a high-reflectance region  100  and a low-reflectance region  102 . The direction of travel of focus spot  4   a  on specimen  4  is again indicated by arrow P. The focus spot moves along edge  104  during focusing, so that the light quantity reflected into the intermediate image plane changes. In  FIG. 4   b , the intensity recorded in the intermediate image plane is plotted as a function of focal position. The graphical depiction of the recorded intensity may be compared to the depiction in  FIG. 2   b . The curve for the optimum focal position is once again depicted as a dashed line. It is clearly evident that the difference between the curve shape for the optimum focal position and the curves above and/or below the optimum focal position is not as clear as in  FIG. 3   b . The direction of motion of the center point—which is unequivocal and which, as already explained in  FIG. 2   b , runs from left to right—nevertheless allows an unequivocal statement. It is thus possible, similarly to the situation in  FIG. 2   b , to ascertain the optimum focal position unequivocally from the center point profile and the shape of the intensity peak. 
   The autofocus system according to the present invention is depicted schematically in FIG.  5  and FIG.  6 . In contrast to the focus system depicted in  FIG. 1   a , here destination of the focal position is possible reliably and also irrespectively of the location of edges (transition from a high-reflectance region to a low-reflectance region) on specimen  4 . Autofocus system  2  comprises a first and a second light source  20  and  21 , both of which are, e.g., embodied as lasers. First and second light sources  20  and  21  are operated alternately at short time intervals during the relative motion between the specimen stage and objective. First and second light sources  20  and  21  thus transmit short flashes or pulses of light onto the surface of specimen  4 .  FIG. 5  depicts the situation in which first light source  20  is emitting its light flash as a divergent light beam  22  that is parallelized by an optical system  23 . Parallel light beam  24  strikes an optical deflection means  25  in such a way that only one half  24   a  of the light beam is deflected, and the other half continues to propagate and is unused. Light beam half  24   a  continues to propagate in parallel fashion, passes uninfluenced through a first beam splitter  26 , and strikes a second deflection means  27 . Light beam  24   a  is imaged by an imaging optical system  29  onto an intermediate image plane  28 , and from there the light beam is imaged by a further optical system  30  onto the surface of the specimen (not depicted). To eliminate undesirable reflected light, an aperture can additionally be provided in intermediate image plane  28 . Parallel light beam  31  reflected from the specimen, after passing through further optical system  30  and imaging optical system  29 , strikes a third deflection means  32 . Reflected light beam  31  deflected by third deflection means  32  passes uninfluenced through a second beam splitter  33  and is imaged by an optical system  34  onto a first light-sensitive detector  35 . First light-sensitive detector  35  can comprise, for example, a position-sensitive detector (PSD), a CCD array, or an area sensor having a defined number of sensor elements. In a preferred embodiment, first light-sensitive detector  35  is embodied as a PSD. 
     FIG. 6  depicts the situation in which second light source  21  is in operation, i.e., is emitting a light beam  36  as a light flash. As already mentioned above, first and second light sources  20  and  21  are operated in time-multiplexed fashion. Second light source  21  emits divergent light beam  36 , which is parallelized by an optical system  37 . Parallel light beam  38  strikes a fourth deflection means  40  in such a way that only one half  38   a  of the light beam is deflected, and the other half continues to propagate and is unused. From fourth deflection means  40 , parallel light beam  38  strikes second beam splitter  33  and is directed by it onto third deflection means  32 . Light beam  38   a  is imaged by imaging optical system  29  onto intermediate image plane  28 , and from there light beam  38   a  is imaged by further optical system  30  onto the surface of the specimen (not depicted). Parallel light beam  41  reflected from the specimen, after passing through imaging optical system  30  and further optical system  29 , strikes second deflection means  27 . Light beam  41 , reflected from the specimen and deflected by second deflection means  27 , is directed by first beam splitter  26  onto an optical system  43  and imaged by the latter onto a second light-sensitive detector  45 . This light-sensitive detector  45  is equipped in the same way as first light-sensitive detector  35 . Second detector  45  is also embodied as a PSD. 
     FIG. 7  depicts an autofocus module  200  having the autofocus system of the invention, as depicted in  FIGS. 5 and 6 , that is attached to a microscope-based system  1  which is merely indicated here. Autofocus module  200  encompasses a housing  202  and a mounting element  204 , joined to housing  202 , that ends in a flange  206 . Autofocus module  200  can be attached via flange  206  to microscope-based system  1 . In the exemplary embodiment depicted here, a further deflection means  203  that deflects the measured light for incoupling is provided in mounting element  204 . Incoupling without this deflection is also conceivable. Microscope-based system  1  defines a beam path  1   b . Provided in beam path  1   b  is a dichroic beam splitter  205  that couples the measured light for focus adjustment into and out of beam path  1   b  of microscope-based system  1 . In housing  202  of autofocus module  200 , the optical elements are immovably arranged and thus aligned with respect to one another. A first laser diode  208  generates a first focusing beam  208   a  that is directed, via an optical system  210  and a beam splitter  212 , onto first deflection means  214 ; by way of further optical systems  216 , focusing beam  208   a  travels to a deflection means that couples focusing beam  208   a  into microscope-based system  1 . The incoupling of focusing light beam  208   a  is such that it is approximately exactly half of the illuminating light beam of microscope-based system  1 . Focusing light beam  208   a  returning from the surface of the specimen strikes a second deflection means  218  and is directed via multiple optical means  220  onto a first detector  222 . A second laser diode  224  generates a second focusing light beam  224   a  that is directed via the various optical means  220  onto second deflection means  218 . Focusing beam  224   a  is coupled into microscope-based system  1  in a manner corresponding to that for first focusing beam  208   a . Second focusing beam  224   a  returning from the specimen travels via further optical means  226  to a second detector  228 . 
     FIGS. 8   a  through  8   c  show the shape and motion of the focus spot on the surface of specimen  4 , and evaluation thereof by means of a first and a second PSD  50  and  52 . With the apparatus described in  FIG. 7 , the images of a first and a second focus spot  4   a   1  and  4   a   2  are generated on the surface of specimen  4 . In  FIG. 8   a , the upper part depicts the direction of travel of focus spots  4   a   1 ,  4   a   2  that results when illumination occurs alternately using first and second light sources  20  and  21 . For illumination with first light source  20 , the direction of travel of focus spot  4   a   1  is depicted by an arrow P 20 . For illumination with second light source  21 , the direction of travel of focus spot  4   a   2  is depicted by an arrow P 21 . This labeling is also used in  FIGS. 8   b  and  8   c . In the situation depicted in  FIG. 8   a , the optimum focus position is still quite distant, and focus spots  4   a   1  and  4   a   2  are reproduced on the surface of specimen  4  as semicircles. Focus spots  4   a   1  and  4   a   2  are also correspondingly imaged onto first and second PSD  50  and  52 . First focus spot  4   a   1  is imaged onto first PSD  50  at lower intensity, since it derives from reflection from a low-reflectance region of the surface of specimen  4 . First PSD  50  determines position X 1  of the center point of first focus spot  4   a   1 . Second PSD  52  determines position X 2  of the center point of second focus spot  4   a   2 , which is generated by a high-reflectance region of the specimen. Position X 1  of the center point and position X 2  of the center point are at identical distances from centers  54  of first and second PSD  50  and  52 . On first PSD  50 , first focus spot  4   a   1  is located to the right of center  54 . On second PSD  52 , first focus spot  4   a   2  is located to the right of center  54 . But because the intensity determined by the PSD is not the same, it is unequivocally clear that the optimum focus has not yet been achieved. 
   In  FIG. 8   b , the upper part depicts the direction of travel of focus spots  4   a   1  and  4   a   2  that results when illumination occurs alternately using first and second light source  20  and  21 . In the situation depicted in  FIG. 8   b , the optimum focus position has almost been achieved and focus spot  4   a   1 ,  4   a   2  is constituted, as in  FIG. 2   a , by zero-order focus spot  9   0 , a negative-first-order focus spot  9   −1 , and a positive-first-order focus spot  9   +1 . Focus spots  4   a   1  and  4   a   2  are also correspondingly imaged onto first and second PSD  50  and  52 , respectively. First focus spot  4   a   1  is imaged with less intensity on first PSD  50 , because it derives mostly from reflection from a low-reflectance region of the surface of specimen  4 ; this is indicated by the cross-hatching of zero-order focus spot  9   0  and negative-first-order focus spot  9   −1 . First PSD  50  determines position X 1  of the center point of first focus spot  4   a   1 . Second PSD  52  determines position X 2  of the center point of second focus spot  4   a   2 , which is generated mostly by a high-reflectance region of the specimen. Position X 1  of the center point and position X 2  of the center point are at different distances from centers  54  of first and second PSD  50  and  52 , respectively. On first PSD  50 , the center point of first focus spot  4   a   1  is located on center  54 . On second PSD  52 , the center point of second focus spot  4   a   2  is located to the right of center  54 . Since the positions of the center points on first and second PSD  50  and  52  are not identical, the optimum focus has not yet been achieved. 
   In  FIG. 8   c , the upper part depicts the direction of travel of focus spots  4   a   1  and  4   a   2  that results when illumination occurs alternately using first and second light source  20  and  21 . In the situation depicted in  FIG. 8   c , the optimum focal position has been achieved, and focus spot  4   a   1 ,  4   a   2  is constituted, as in  FIG. 2   a , by zero-order focus spot  9   0 , a negative-first-order focus spot  9   −1 , and a positive-first-order focus spot  9   +1 . Focus spots  4   a   1  and  4   a   2  are also correspondingly imaged onto first and second PSD  50  and  52 , respectively. Focus spot  4   a   1  is imaged onto first PSD  50  in such a way that half of focus spot  4   a   1  derives from a low-reflectance region of the surface of specimen  4 , and the other half of focus spot  4   a   1  derives from a high-reflectance region of the surface of specimen  4 . The region of the focus spot on the PSD that results from low reflectance is marked with cross-hatching. First PSD  50  determines position X 1  of the center point of first focus spot  4   a   1 . Second PSD  52  determines position X 2  of the center point of second focus spot  4   a   2 . Position X 1  of the center point and position X 2  of the center point are at identical distances from center  54  of first and second PSD  50  and  52 . Position X 1  of the center point and position X 2  of the center point are each shifted by the same amount to the right of center  54 . The positions of the center points on first and second PSD  50  and  52  are quantitatively equal, so that with this measurement result from first and second PSD  50  and  52 , the optimum focus has been achieved. 
   The invention has been described with reference to exemplary embodiments. It is self-evident, however, that changes and modifications can be made without thereby leaving the range of protection of the claims below. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               REFERENCE NUMERAL LIST 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
                1 
               Microscope-based system 
             
             
                 
                1a 
               Beam path 
             
             
                 
                1b 
               Dichroic beam splitter 
             
             
                 
                2 
               Autofocus system 
             
             
                 
                4 
               Specimen 
             
             
                 
                4a 
               Focus spot 
             
             
                 
                4a 1   
               First focus spot 
             
             
                 
                4a 2   
               Second focus spot 
             
             
                 
                5 
               Dashed circle 
             
             
                 
                6 
               Light source 
             
             
                 
                8 
               Light beam 
             
             
                 
                9 0   
               Zero-order focus spot 
             
             
                 
                9 −1   
               Negative-first-order focus spot 
             
             
                 
                9 +1   
               Positive-first-order focus spot 
             
             
                 
                10 
               Pupil splitting system 
             
             
                 
                11 
               Beam splitter 
             
             
                 
                12 
               Tube lens 
             
             
                 
                13 
               Optical axis 
             
             
                 
                14 
               Objective 
             
             
                 
                15 
               Reflected light beam 
             
             
                 
                16 
               Intermediate image plane 
             
             
                 
                17 
               Abscissa 
             
             
                 
                18 
               Ordinate 
             
             
                 
                20 
               First light source 
             
             
                 
                21 
               Second light source 
             
             
                 
                22 
               Divergent light beam 
             
             
                 
                23 
               Optical system 
             
             
                 
                24 
               Parallel light beam 
             
             
                 
                24a 
               Light beam half 
             
             
                 
                25 
               First deflection means 
             
             
                 
                26 
               First beam splitter 
             
             
                 
                27 
               Second deflection means 
             
             
                 
                28 
               Intermediate image plane 
             
             
                 
                29 
               Imaging optical system 
             
             
                 
                30 
               Further optical system 
             
             
                 
                31 
               Reflected light beam 
             
             
                 
                32 
               Third deflection means 
             
             
                 
                33 
               Second beam splitter 
             
             
                 
                34 
               Optical system 
             
             
                 
                35 
               Light-sensitive detector 
             
             
                 
                36 
               Divergent light beam 
             
             
                 
                37 
               Optical system 
             
             
                 
                38 
               Light beam 
             
             
                 
                38a 
               Light beam half 
             
             
                 
                40 
               Fourth deflection means 
             
             
                 
                41 
               Parallel light beam 
             
             
                 
                43 
               Optical system 
             
             
                 
                45 
               Second light-sensitive detector 
             
             
                 
                49 
               Pupil 
             
             
                 
                50 
               First PSD 
             
             
                 
                52 
               Second PSD 
             
             
                 
                54 
               Center of PSD 
             
             
                 
                90 
               Control computer 
             
             
                 
               100 
               High-reflectance region 
             
             
                 
               102 
               Low-reflectance region 
             
             
                 
               200 
               Autofocus module 
             
             
                 
               202 
               Housing 
             
             
                 
               203 
               Deflection means 
             
             
                 
               204 
               Mounting element 
             
             
                 
               205 
               Dichroic beam splitter 
             
             
                 
               206 
               Flange 
             
             
                 
               208 
               First laser diode 
             
             
                 
               208a 
               First focusing beam 
             
             
                 
               210 
               Optical system 
             
             
                 
               212 
               Beam splitter 
             
             
                 
               214 
               First deflection means 
             
             
                 
               216 
               Further optical systems 
             
             
                 
               218 
               Second deflection means 
             
             
                 
               220 
               Multiple optical means 
             
             
                 
               222 
               First detector 
             
             
                 
               224 
               Second laser diode 
             
             
                 
               224a 
               Second focusing light beam 
             
             
                 
               226 
               Further optical means 
             
             
                 
               228 
               Second detector 
             
             
                 
               P 
               Arrow 
             
             
                 
               P 20   
               Arrow 
             
             
                 
               P 21   
               Arrow 
             
             
                 
               X 
               X position 
             
             
                 
               Z 
               Z direction 
             
             
                 
               X 1   
               Position of center point 
             
             
                 
               X 2   
               Position of center point