Abstract:
Method and device for determining one or more Z distances from an object surface to a reference plane, for example the primary plane of the primary objective of a microscope. By projecting an optical pattern onto an object, and subsequently detecting and computationally evaluating the object&#39;s reflection of this pattern by means of an image processing unit, it is possible to obtain relief-like imaging of the object and to identify the individual Z distances, irrespective of the object&#39;s contouring.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
   Federal Republic of Germany Priority Application 101 25 971.9, filed May 29, 2001 including the specification, drawings, claims and abstract, is incorporated herein by reference in its entirety. 
   FIELD OF THE INVENTION 
   The invention relates to a novel method for determining one or more Z distances from a surface to a reference plane, for example, the primary plane of the primary objective of an optical observation instrument, such as a (stereo) operation microscope, which provides spatial imaging of the object, as well as to a microscope for carrying out such a method. 
   BACKGROUND OF THE INVENTION 
   Determining the distance from an operation microscope to an object is very important, especially in surgery—for example in neurosurgical applications, since it is common for operations to be carried out with navigation systems and for the microscope to be used as a non-contact sensor. Once focusing has been carried out successfully via the defined measurement integral, for example, over a limited area around the focal point, a measured value can be sent to the navigation system, processed there and subsequently indexed in the data record of the object, which is generally the patient. In the currently known focusing systems, the working distance which is set once the image sharpness has been optimized is used for determining the working distance between a stereo microscope and the object surface. The evaluation of the image sharpness may be carried out in the entire field of view or in a restricted field of view, for example in a narrowly defined region around the centre of the field of view, normally a substantially point-like region with a diameter of approximately 1 to 2 mm. The focusing is normally carried out via the primary objective. 
   SUMMARY OF THE INVENTION 
   The inventor has found that the known systems are disadvantageous in respect to the following points:
         a) Methods of this type generally require a minimum degree of object contouring (edge sharpness at the borders) with a corresponding contrast. If this prerequisite is not satisfied, these methods normally fail.   b) In particular, anatomical preparations, for example brain tissue, sometimes show neither well-pronounced contours nor suitable texture with a minimum contrast.   c) The conditions, which are often sufficient, may become unsatisfactory owing to liquids wetting the object, for example washing liquids mixed with blood.   d) It is not possible to identify the Z distances over the entire field of view, or over fairly large regions of the object, i.e. relief-type recording or imaging of the object is not possible.   e) The required focusing via the primary objective may interfere with the user&#39;s work owing to changes in the image sharpness.       

   Different focusing for a further observer and/or a video system is not possible, i.e. it is not possible to involve a second observer with another focal plane. 
   It is an object of the invention to provide a method which permits accurate determination of object—operation microscope separations even under unfavourable conditions and/or over the entire image field, and which avoids the said disadvantages of known solutions. 
   This object is achieved by a method which produces an optical pattern by means of a pattern generator, projects this pattern onto the object via insertion into the primary beam path of the microscope, extracts the reflection on the object via the primary beam path and detects it by means of an observation device, for example a camera/CCD, computationally separates this detection into conjugate patterns and identifies the object relief—microscope distance from the topographical position of these conjugate patterns, as well as by a microscope for carrying out the said method. 
   It is based on the following effect: when the pattern is focused accurately onto the focal plane, the pattern appears as a single pattern; in the event of defocusing, two conjugate patterns are seen. 
   In a particular refinement, which can also be used independently of the pattern generator, separate optics, independent of the microscope&#39;s primary optics, are placed in front of the camera and the Z-identification is carried out using them. 
   The aforementioned use of a method involving pattern projection onto the object and subsequent detection for determining the focus, and optionally the implementation of further improvements which are referred to below, leads to the improvements listed below being achieved:
         owing to the inventive use of a separate light source (optical pattern), the object does not need to have a minimum degree of contouring;   contrast problems during the measurement—for example due to liquids—are eliminated;   it is possible to focus onto all parts of the object—in particular even ones which are in shadow or unilluminated;   since it has its own light source, the focus measurement is independent of the primary illumination, and in particular independent of the illumination beam&#39;s light-wave range which is used;   owing to the inventive use of optical patterns, it is possible to discriminate reliably against other light phenomena. In the case of large-area patterns, it is furthermore possible to identify the large-area relief of the object and therefore also to focus on different focal planes;   owing to the inventive use of separate optics for the camera, it is unnecessary to change the focusing of the primary optics in order to identify the focus; the user&#39;s work under the microscope is therefore not disturbed.       

   Although the above text refers to an operation microscope, the invention is nevertheless not restricted thereto, but rather is available to other users of optical devices with range-finding and focusing instruments (e.g. projectors, video cameras and photographic cameras, etc.). 
   Moirée methods are not covered by the present invention. 
   The list of references and  FIG. 1  to  FIG. 8 , together with the subject matter described in the claims, are an integral part of the disclosure of this application. Further details and alternative embodiments can hence be found in the description of the figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The figures will be described coherently and comprehensively. The same reference numbers denote the same components, and reference numbers with different indices indicate functionally equivalent components. The figures conceptually show: 
       FIG. 1 : A symbolic overall structure of an optical observation instrument, for example a stereo operation microscope; 
       FIG. 2 : An example of a shutter for the (temporal) encoding of the pattern elements; 
       FIG. 3 : A schematic profile of an inserted point-like pattern; 
       FIG. 4   a:  A focused line pattern; 
       FIG. 4   b:  A conjugate pattern element of a defocused line pattern; 
       FIG. 5 : A pattern projection in various focal planes; 
       FIG. 6 : A control loop of the focusing instrument for the primary objective and of the focusing instrument for the camera; 
       FIG. 7 : An object surface and various distances z 1  to z 5  from the primary optics, which are to be detected; and 
       FIG. 8 : An object surface and various distances z 1  to z 5  from the primary optics, which are to be detected as well as a focal plane and two defocused planes. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  conceptually shows the overall structure of an optical observation instrument with a pattern generator  1 , optics  2  for the pattern generator, a camera  3 , for example a CCD, the associated optics  4 , a beam splitter  5  for the pattern generator  1 , a respective beam splitter  6  and  7  for the insertion and extraction of patterns in the left and right beam path  31 / 32 , zoom optics  8 ,  9  for the left and right beam path  31 / 32 , a primary objective  10 , as well as a shutter  11  for the (temporal) encoding of the pattern elements. Also shown are the object surface  12 , the optical axes, specifically  21  for the primary objective  10 , for the left and right beam path  22 / 23 , the optical axis  24  of the pattern generator  1 , as well as the optical axis  25  with the beam path of the camera  3 . 
     FIG. 2  schematically shows a shutter  11 , for example a rotating disc, with the object surface  12  and the inserted beam path  30  of the pattern generator  1  for the (temporal) encoding of the right/left perspective of the pattern, for the purpose of evaluation via a camera  3 . 
     FIG. 3  shows the profile of an inserted point-like pattern  52   a ,  52   b,  the primary objective  10 , the focused pattern  51 , the defocused pattern elements  51   a,    51   b  as well as the focal plane  13 , for example the object surface, and the defocused planes  13   a,    13   b.    
     FIG. 4   a  conceptually shows a focused line pattern  51 ; the pattern elements  51   a  and  51   b  lie precisely above one another.  FIG. 4   b  shows the conjugate pattern elements  51   a,    51   b  of a defocused line pattern. 
   In  FIG. 5 , the line pattern conceptually shown in  FIG. 4  is schematically represented with the focal plane  13 , the defocused planes  13   a,    13   b  as well as the focused pattern  51 /defocused patterns  51   a ,  51   b.    
     FIG. 6  schematically shows the control loop, comprising a camera  3 , an image processing unit  61  for calculating the effective focal point from the current focus parameters of the primary objective  10  and the camera objective  4 , a focusing instrument  62  for the primary objective  10  as well as a focusing instrument  63  for the optics  4  of the camera  3 . 
     FIG. 7  conceptually shows an object surface  12  and various distances z 1  to z 5  from the primary optics  10 , which are to be detected. 
     FIG. 8  shows, similarly to  FIG. 7 , an object surface  12 , various distances z 1  to z 5  from the primary optics  10 , which are to be detected, as well as the focal plane  13 , for example relating to z 2 , as well as the defocused planes  13   a,    13   b.    
   Function 
   In a pattern generator  1 , an optical pattern  51  is produced which is inserted, by means of beam splitters  6 ,  7 , into the left and/or the right beam path  31 ,  32  of an optical observation instrument, for example a stereo operation microscope. The pattern consists of at least two points, although it ideally consists of two-dimensional line or grid patterns. The pattern is projected onto the object  12  via the primary objective  10 . The reflection, or scattering, of the pattern from object  12  is in turn detected via the primary optics  10  and the beam splitters  6 ,  7 , optics  4  and a camera  3 , for example a CCD ( FIG. 1 ). 
   When the pattern is focused accurately onto the focal plane  13 , the pattern  51  appears as a single point, or a single line or a pattern ( FIGS. 3-5 ). In the event of defocusing, the pattern  51   a,    51   b  is seen double in the form of two conjugate points or conjugate lines or conjugate patterns ( FIGS. 3-5 ). 
   The recognition of the conjugate patterns  51   a,    51   b  is carried out via temporal and/or spatial and/or geometrical and/or spectral encoding of the pattern for the left and right beam path ( 31  and  32 ). On the basis of knowledge of the beam path, it is possible to calculate, from the two patterns, the distance z of one or more object points from the primary objective. With knowledge of the distances, direct focusing onto any desired object point f=z(x,y) is possible. Temporal encoding may be carried out, for example, by encoding the pattern  51  using a rotating shutter  11 . The conjugate signals are separated by subtracting the normal image signal from the overall signal, respectively with the pattern  51   a ,  51   b  of the right and the left beam path. In a system unit  61 , the relation of conjugate pattern elements is determined by suitable algorithms, and the primary objective—object distance z 1  to z 5  is computationally identified point-wise ( FIG. 7 ,  FIG. 8 ). 
   Once the produced and imaged pattern  51  has been two-dimensionally formed, then the surface structure of the object  12  can be computationally identified. This makes it possible to focus the focal plane  13  with respect to the structure of the object surface  12  or individual object points. 
   The focusing onto the defined focal plane  13  is carried out by means of an already well-known focusing instrument. 
   The selection of the object point to be defined may be carried out electronically—for example using the eye&#39;s line of sight, by analysing the movements of an instrument in the image field, using a joystick, a mouse pointer or the like—or it may be manually predefined. 
   In a particular refinement of the invention, separately focusable optics are placed in front of the camera  3 , so that focusing changes of the measurement system do not need to be carried out via the primary objective. This permits focusing, which is independent of the primary beam path, for a second observer ( FIG. 6 ) and/or focus determination which is independent of the primary beam path. In this case, the focus parameters of both optical systems are taken into account when calculating the object distances. 
   Overall, owing to the facility of producing a separate light pattern, the measurement system can be used virtually irrespective of the properties of the object. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               LIST and LEGEND 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               1 
               pattern generator 
             
             
                 
               2 
               optics for (1) 
             
             
                 
               3 
               camera (e.g. CCD) 
             
             
                 
               4 
               optics for (3) 
             
             
                 
               5 
               beam splitter (1) 
             
             
                 
               6 
               beam splitter of right beam path 
             
             
                 
               7 
               beam splitter of left beam path 
             
             
                 
               8 
               zoom optics of left beam path 
             
             
                 
               9 
               zoom optics of right beam path 
             
             
                 
               10 
               primary objective 
             
             
                 
               11 
               shutter (rotating) 
             
             
                 
               12 
               object surface 
             
             
                 
               13 
               focal plane (object) 
             
             
                 
               13 a, b 
               positively and negatively defocused planes 
             
             
                 
               21 
               optical axis of (10) 
             
             
                 
               22 
               optical axis of left beam path 
             
             
                 
               23 
               optical axis of right beam path 
             
             
                 
               24 
               optical axis of (1) 
             
             
                 
               25 
               optical axis of (3) 
             
             
                 
               30 
               beam path of (1) 
             
             
                 
               31 
               left beam path 
             
             
                 
               32 
               right beam path 
             
             
                 
               33 
               beam path of camera/CCD 
             
             
                 
               51 
               focused pattern 
             
             
                 
               51 a, b 
               conjugate pattern elements (defocused patterns) 
             
             
                 
               52 a, b 
               inserted point-like pattern 
             
             
                 
               61 
               image processing unit 
             
             
                 
               62 
               focusing unit for (10) 
             
             
                 
               63 
               focusing unit for (3) 
             
             
                 
               Z1-Z5 
               distances between (10) and (12)