Abstract:
A device configured to read an exposed imaging plate, comprises a light-source that generates read-out light. A deflection unit directs the read-out light in a scanning movement over the imaging plate. The deflection unit comprises a micromirror that deflects impinging read-out light towards the imaging plate. The micromirror swivels about a first swivel axis and about a different second swivel axis. The micromirror oscillates with a first frequency about the first swivel axis and simultaneously with a different second frequency about the second swivel axis. A detector unit detects fluorescent light which is emitted from the imaging plate at locations where the read-out light impinges.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of International Patent Application PCT/EP2012/004017, which was filed on Sep. 26, 2012 and claims benefit of German patent application Ser. No. 102011119049.3 filed Nov. 22, 2011. The full disclosure of these earlier applications is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a device and a method for reading an exposed imaging plate. 
         [0004]    2. Description of Related Art 
         [0005]    In X-ray technology, particularly in dental X-ray technology, imaging plates are used nowadays for the purpose of recording X-ray images. These imaging plates include a phosphor material which has been embedded in a transparent matrix. As a result, so-called storage centres arise, which can be brought into excited metastable states by incident X-ray light. If such an imaging plate is exposed in an X-ray apparatus, for example for the purpose of recording the dentition of a patient, the imaging plate contains a latent X-ray image in the form of excited and non-excited storage centres. 
         [0006]    For the purpose of reading the imaging plates, the latter are scanned with read-out light, point by point, in a scanner, as a result of which the metastable states of the excited storage centres are brought into a state that relaxes rapidly, emitting fluorescent light. This fluorescent light can be registered with the aid of a detector unit, so that the X-ray image can be made visible with appropriate evaluating electronics. 
         [0007]    Conventional scanners, such as a drum scanner for example, conduct the imaging plate along a cylindrical surface across a read-out gap. In the interior of the cylindrical surface a rotary mirror has been provided by way of deflection unit, which generates a circumferential read-out beam. The latter falls through the read-out gap onto the imaging plate and reads the latter in pointwise manner. Meanwhile the imaging plate is conducted past the read-out gap by a mechanical drive, so that the entire surface of the imaging plate is registered. 
         [0008]    With such a drum scanner, particularly in the dental field in which mostly small-format imaging plates come into operation, it is disadvantageous that the imaging plate has been arranged only along a small region of the periphery of the cylinder. For a large proportion of the read-out time the read-out beam therefore circulates within a region where no imaging plate is present, so that, on average, the read-out beam actually impinges on the imaging plate only during approximately 10% of the read-out time. This results in unnecessarily long read-out times. 
       SUMMARY OF THE INVENTION 
       [0009]    It is therefore an object of the invention to provide a device and a method with which the read-out efficiency has been improved. 
         [0010]    With regard to the device, this object is achieved by a device that is configured to read an exposed imaging plate, comprises a light-source that generates read-out light. A deflection unit directs the read-out light in a scanning movement over the imaging plate. The deflection unit comprises a micromirror that deflects impinging read-out light towards the imaging plate. The micromirror swivels about a first swivel axis and about a different second swivel axis. The micromirror oscillates with a first frequency about the first swivel axis and simultaneously with a different second frequency about the second swivel axis. A detector unit detects fluorescent light which is emitted from the imaging plate at locations where the read-out light impinges. 
         [0011]    It has been recognised that with a controllable mirror that is capable of being swivelled back and forth, the read-out light can be guided in such a way that most of the time it falls onto the imaging plate. As a result, the time that is needed overall for reading an imaging plate is reduced. 
         [0012]    The micromirror may, in particular, take the form of a MEMS micromirror (MEMS: microelectromechanical system). Characteristic of MEMS technology is the integration of mechanical elements, actuators and electronics on a common substrate, with manufacture being undertaken in a manner similar to that in the case of processors and memory chips, and including the application of layers of material and subsequent selective etching. MEMS micromirrors are very reliable and, by reason of their low mass, react very quickly to drive signals, so that high deflection speeds can be obtained. 
         [0013]    By virtue of a deflection about two axes, a two-dimensional imaging plate can be scanned even without relative motion between the deflection unit and the imaging plate. The two axes in this case have preferentially been arranged parallel to the edges of the mostly rectangular imaging plate. 
         [0014]    In one embodiment, the mirror is capable of being swivelled about the two swivel axes continuously between two end positions. By virtue of a continuous swivelling capacity along the swivel axes, the micromirror is able to scan an uninterrupted scan line on the imaging plate. 
         [0015]    A MEMS micromirror is usually provided with a cardanic suspension, the suspension points of which take the form of solid joints. As a result, the micromirror with the solid joints forms a system capable of oscillating, which exhibits a natural frequency for each degree of freedom. If the micromirror is driven at, or close to, natural frequency, then oscillation amplitudes are obtained that are as large as possible with as little expenditure of energy as possible. 
         [0016]    The mirror may have different natural frequencies about the two swivel axes. In this way, differing oscillation frequencies in differing scanning directions can be obtained with low expenditure of energy. 
         [0017]    A predetermined pattern may be scanned on the imaging plate with the read-out light, whereby the pattern may cover the entire imaging plate with uniform density. By virtue of a uniform density in the course of the scanning of the imaging plate, the signal-to-noise ratio and the resolution of the X-ray image that has been read are approximately constant over the entire imaging plate. 
         [0018]    If a control unit drives the mirror in such a way that a Lissajous figure on the imaging plate is scanned with the read-out light, then a complete scanning of the entire imaging plate may be obtained. In particular, the micromirror and the imaging plate in this case may have been arranged relative to one another in such a way that the Lissajous figure extends beyond the edges of the imaging plate. The part of the pattern projecting beyond one of the edges preferentially corresponds to between 5% and 15% of the dimension of the imaging plate perpendicular to this edge. 
         [0019]    In the case of the Lissajous figures the control unit may be able to drive the mirror about the two swivel axes at frequencies that correspond to a large, preferentially variable, preferentially integral, multiple of a base frequency, in particular to 0.5 times the desired number of lines or the number of columns of the image, and/or that differ slightly from one another from such an integral multiple, in particular by approximately 10% of the lower frequency. The slight difference may also amount to only approximately 2% of the lower frequency. 
         [0020]    For example, frequencies of 30 Hz and 40 Hz may be used, in which case the base frequency would then be 10 Hz, and a ratio of 3:4 results. If the two frequencies or frequency multiples of the Lissajous figure differ slightly, then a moving Lissajous figure is scanned. ‘Moving’ here means a Lissajous figure changing in a manner similar to a beat. As a result, the entire imaging plate can be scanned. 
         [0021]    Another method for scanning the entire imaging plate consists in using frequencies being large integral multiples of one another, in particular 0.5 times the desired number of lines or number of columns of the image. ‘Large’ in this connection means, in particular, that the two frequencies at which the mirror is driven differ by more than a factor of more than 250, preferentially more than 500. 
         [0022]    An evaluating unit may be connected to the detector unit, which in the course of evaluating takes account of the fact that, by virtue of the oscillatory motion of the mirror, various points of the imaging plate are scanned variably often and/or for variably long times, in particular by means of a position dependent correction factor. 
         [0023]    In the course of the scanning of Lissajous figures and some other patterns, some points are scanned repeatedly, and other points once only. As a result, storage centres that have remained in the excited state are read out additionally in the course of the later scanning. The diminution, increasing after each further scanning of a point, of the excited storage centres can, for example, be taken into account by a weighted averaging. However, the variable scanning of various points may result in a locally variable signal-to-noise ratio. For this reason, the evaluating unit may, in the course of evaluating, have recourse to a correction table that was created on the basis of calibration measurements. 
         [0024]    The Lissajous figures can be simulated mathematically, and the registered intensities can be corrected accordingly. Better, however, is a calibration on the basis of calibrating imaging plates, for example a uniformly exposed imaging plate. Once this has been read, the intensities obtained are converted into correction values which are saved in the correction table. 
         [0025]    The device may include a sync detector, preferentially a photodiode, with which the read-out light can be detected at a predetermined scanning location in order to synchronise the evaluating unit with the motion of the mirror. In this way, the registration of the detector signal can be synchronised by the evaluating unit, as needed, with the scanning motion of the read-out light beam. 
         [0026]    A control unit may be provided, with which periodic pulses can be generated which excite the mirror to oscillate. In this connection the periodicity, the pulse width, the pulse shape and/or the pulse height may be variable, in order to influence the excited oscillation amplitude and/or oscillation frequency of the mirror, as a result of which differing patterns can be generated. 
         [0027]    But a control unit may also have been provided, with which the mirror can be driven with a sawtooth voltage or with a delta voltage. As a result, the imaging plate can be read uniformly. 
         [0028]    Several mirrors may also have been arranged in a mirror array. In this case, differing mirrors may be assigned to differing regions of the imaging plate. For example, with two biaxial mirrors two Lissajous figures arranged side by side can be generated in succession, in order to read larger imaging plates completely. 
         [0029]    The imaging plate and the deflection unit may be capable of moving relative to one another, the imaging plate preferentially being conducted past the deflection unit. In this way, large-format imaging plates can be read. In the case where use is made of an only uniaxial mirror, the relative motion is necessary in order to be able to read the imaging plate in two dimensions. 
         [0030]    A control unit may have been provided, with which the mirror is capable of being driven in stepwise manner, so that the imaging plate can be read out, pixel by pixel, in a matrix. ‘Stepwise’ in this connection means that the mirror comes to a state of rest after any change in the position of tilt. This enables the read-out with a constant signal-to-noise ratio. 
         [0031]    A maximal read-out range may have been defined, in which the imaging plate is capable of being scanned solely by deflecting the read-out light. Furthermore, the device may include an erasing appliance with which erasing light can be directed onto the imaging plate after the reading of the imaging plate in an erasing region, the erasing region on the imaging plate being at least as large as the read-out region. If the read-out region and the erasing region coincide, the imaging plate can be erased at the same location at which it is read, so that no drive means for conveying in the direction of an erasing appliance are necessary. 
         [0032]    According to a further embodiment, the read-out light may deflected by the deflection unit may generate an impingement spot on the imaging plate, and the erasing appliance includes a switching element with which the impingement spot can be enlarged for the purpose of erasing the imaging plate, whereby the read-out light is used as erasing light. 
         [0033]    If, for the purpose of erasing the imaging plate, use is made of components that are needed anyway for the purpose of reading, additional components need hardly be incorporated. An enlargement of the impingement spot (“beam footprint”) on the imaging plate can be obtained, for example, by means of an actuator for displacing or curving the mirror. But use may also be made, for example, of a displaceable condenser lens on the laser, or of a diffusing screen that is retractable into the beam path. 
         [0034]    The erasing appliance may include an intensity-controlling appliance with which, for the purpose of erasing the imaging plate, the intensity of the read-out light can be increased. In this way, the same light-source can be used for erasing that is also utilised for reading, whereby, by virtue of the increase in the intensity, a complete erasure of possibly still excited storage centres is obtained. 
         [0035]    Alternatively, the erasing appliance may include a light-source for generating erasing light, the erasing light being directed onto the controllable mirror, so that with the aid of the deflection unit the erasing light can be directed sequentially onto the imaging plate for the purpose of erasing the imaging plate. Special erasing light can also be used in combination with an enlargement of the impingement spot. 
         [0036]    In this case a feed element may have been arranged in the beam path upstream of the controllable mirror, with which, simultaneously or alternately, the read-out light and the erasing light can be directed onto the controllable mirror. 
         [0037]    The erasing appliance may include several sources of erasing light, in particular light-emitting diodes, which have been arranged around the imaging plate. This is a structurally very simple configuration of an erasing appliance. Preferentially, the sources of erasing light have in this case been arranged in such a way that the erasing light falls onto the entire imaging plate. 
         [0038]    The sources of erasing light may have been arranged on a side of the imaging plate from which the read-out light falls onto the imaging plate. In this way, both transparent and non-transparent imaging plates can be erased. 
         [0039]    The erasing light may have a broader-band spectrum than the read-out light, as a result of which a higher efficiency of erasure is achieved. 
         [0040]    The deflection unit may have been arranged in such a manner that with it the read-out light can be directed onto a front surface of the imaging plate. The detector unit for fluorescent light has in this case been arranged in such a manner that with it the fluorescent light emerging from a rear surface of the imaging plate can be detected. With such an arrangement, transparent imaging plates can be read, whereby, depending on the arrangement, the fluorescent light is able to reach the detector unit directly or indirectly via a reflector. 
         [0041]    A supporting plate that is transparent to the fluorescent light may have been provided, against which the imaging plate bears. In this way, the imaging plate can be supported on its rear side, without the fluorescent light emerging there being lost for the measurement. The supporting plate in this case may act as an optical filter that blocks read-out light and lets fluorescent light pass. By reason of the filter action of the supporting plate, only fluorescent light then emerges downstream of the supporting plate. 
         [0042]    The supporting plate may have a cylindrically curved supporting surface for the imaging plate. As a result, at least in one read-out direction a perpendicular incidence of the read-out light onto the imaging plate can be ensured. In this case a clamping element may have been provided, with which the imaging plate is pressed against the curved supporting surface. A clamping element co-operating with the supporting surface, such as a clamping bracket for example, is a structurally simple arrangement in order to bring the imaging plate into the cylindrical shape. 
         [0043]    The supporting plate may also have been constituted by an entrance window of the detector unit. Given appropriate adaptation of the read-out region, in this way the imaging plate may bear directly against the entrance window of the detector unit and be registered. Particularly in combination with the filter action, a particularly simple structure of the device arises in this way. 
         [0044]    The entrance window in this case may be at least as large as the imaging plate to be read, and may, in particular, have the basic shape thereof. As a result, the entire imaging plate can be registered without the latter having to be displaced. The size of the entrance window in this case may be adapted to the customary standard sizes (size 0, 1, 2 etc.) of the imaging plates. 
         [0045]    A supporting frame and a clamping element may have been provided, between which the imaging plate is held, said frame and element having been, in particular, configured and arranged in such a manner that a cylindrical curvature is imposed on the imaging plate. 
         [0046]    Instead of an uninterrupted supporting plate, only a supporting frame may have been provided, which carries the imaging plate only in marginal regions, in which case both a fully circumferential frame and a frame that is present only intermittently are conceivable. As a result, in the beam path of the fluorescent light there is still less material that could partially absorb or reflect the fluorescent light. The supporting frame may bear against the front side or against the rear side of the imaging plate, in which case the clamping element then undertakes the correspondingly complementary function. 
         [0047]    With respect to the method, the aforementioned object is achieved by a method having the following steps:
   a) generating read-out light;   b) directing the read-out light on a micromirror;   c) directing the read-out light in a scanning movement over the imaging plate, wherein the micromirror oscillates with a first frequency about a first swivel axis and simultaneously with a second frequency, which is distinct from the first frequency, about a second swivel axis;   d) detecting fluorescent light which is emitted from the imaging plate at locations where the read-out light impinges.   
 
         [0052]    A control unit is able to drive the micromirror about the two swivel axes at frequencies that correspond to a large, preferentially variable, preferentially integral, multiple of a base frequency, in particular to 0.5 times the desired number of lines or the number of columns of the image, and/or that differ slightly from one another by such an integral multiple, in particular by 10% of the lower frequency. As a result, patterns are generated that scan the imaging plate completely. In particular, the sequential scanning of the imaging plate can be effected along a Lissajous figure. 
         [0053]    As a result, the micromirror can be driven with a drive signal, the frequency of which may be at least approximately equal to a natural frequency of the micromirror. 
         [0054]    The imaging plate may be erased after the scanning, while the imaging plate remains at the location of the scanning. For the purpose of erasing the imaging plate, the intensity of the read-out light can be increased, and then the imaging plate can be scanned with the aid of the deflection unit. For the purpose of erasing the imaging plate, an impingement spot of the read-out light falling onto the imaging plate, or of an erasing light falling onto the imaging plate, can be enlarged. 
         [0055]    The imaging plate may, in addition, be brought into a cylindrical shape prior to the scanning. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0056]    The invention will be elucidated below on the basis of embodiments with reference to the drawings. Shown therein are: 
           [0057]      FIG. 1  a simplified perspective representation of a scanner for reading imaging plates in accordance with a first embodiment; 
           [0058]      FIG. 2  a perspective representation of a micromirror that is used in the scanner; 
           [0059]      FIG. 3  a top view of an imaging plate that is being read along a scan path according to a first mode; 
           [0060]      FIG. 4  a top view of an imaging plate that is being read along a scan path according to another mode; 
           [0061]      FIG. 5  a simplified perspective representation of a scanner according to another embodiment; 
           [0062]      FIG. 6  a simplified perspective representation of a scanner for transparent imaging plates; 
           [0063]      FIG. 7  a simplified perspective representation of a scanner for transparent imaging plates according to an embodiment in which another erasing appliance and another detector unit are used; 
           [0064]      FIG. 8  a simplified perspective representation of a scanner for transparent imaging plates according to a further embodiment with modified erasing appliance; 
           [0065]      FIG. 9  a simplified perspective representation of a scanner for transparent imaging plates according to an embodiment in which the imaging plate is carried by a supporting frame. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     1. First Group of Embodiments 
       [0066]      FIG. 1  shows a scanner, denoted overall by  10 , for reading an imaging plate  12  which in the form of metastable storage centres excited by X-ray radiation bears a latent X-ray image. 
         [0067]    The scanner  10  exhibits a supporting surface  14  for the imaging plate  12 . For the purpose of fixing the imaging plate  12 , the supporting surface  14  in this case has been provided with a plurality of suction bores  16  which can have a vacuum applied to them via a vacuum source which is not shown, so that the imaging plate  12 , which is generally flexible, is able to conform to the supporting surface  14  in flat manner. 
         [0068]    The scanner  10  further includes a laser  18  by way of source of read-out light, which generates a read-out light beam  20  having a wavelength in the red spectral region, with which the excited storage centres of the imaging plate  12  can be excited to fluoresce, so that said storage centres emit fluorescent light  22  which is typically in the blue spectral region. 
         [0069]    The laser  18  has been arranged in such a way that it directs the read-out light beam  20  onto a controllable micromirror  24  which has been arranged on a deflection unit  26 . This micromirror  24 , which has been constructed as a MEMS component, is shown in  FIG. 2 . Thanks to its cardanic solid-joint suspension  28 , it is capable of swivelling about two axes  30  and  32  and can be swivelled continuously about the two axes  30 ,  32  with the aid of capacitive actuators  33   a ,  33   b ,  33   c ,  33   d  acting on its underside and with the aid of assigned control circuits of the deflection unit  26 , which are not shown. 
         [0070]    In order to obtain, at least in one direction, a perpendicular incidence of the read-out light beam  20  onto the imaging plate  12 , the supporting surface  14  in the embodiment shown exhibits a partly cylindrically curved shape which extends parallel to the swivel axis  30  of the micromirror  24 . However, the supporting surface may also have been constructed to be completely flat. In addition, an f-theta lens system  35  can be provided as needed, which adapts the angle of incidence of the read-out light beam  20  so as to correspond to the shape of the supporting surface  14 . 
         [0071]    The laser  18 , the supporting surface  14  with the imaging plate  12  and also the deflection unit  26  with the micromirror  24  have been geometrically spaced from one another and arranged relative to one another in the scanner  10  in such a way that with the aid of the micromirror  24  the read-out light beam  20  is able to scan at least the entire surface area of the imaging plate  12 . If the scanner  10  enables the reading of variably large imaging plates  12 , then, of course, the largest imaging plate  12  predetermines the total surface area to be scanned. 
         [0072]    If it is accepted that, for example, marginal regions of the imaging plate  12  are not read, then the various components may, however, also be arranged in such a manner that by swivelling the micromirror  24  only a partial region of the imaging plate can be scanned. 
         [0073]    The scanner  10  further includes a reflector  34 , indicated in dotted manner in the drawing, which encloses the entire measuring space around the imaging plate  12  in light-tight manner, so that the fluorescent light  22  emanating from the imaging plate  12  is finally reflected to a photodetector  36 . In order to prevent scattered read-out light  20  from also reaching the photodetector  36 , both the reflector  34  and an input window of the photodetector  36  may have been provided with a dichroic filter material which blocks or absorbs the read-out light  20  and is transparent to the fluorescent light  22 . 
         [0074]    For the purpose of controlling the read-out process, the scanner  10  includes a control unit  38  and an evaluating unit  40  with a correction memory  42 , which here have been shown as parts of integrated instrumental electronics  44 , but may also have been implemented as control software on a separate PC. For the purpose of operation, the control unit  38  and the evaluating unit  40  have been connected to a display-and-operating unit  46  with which working parameters can be established and the image  48  which has been read from the imaging plate  12  can be displayed. 
         [0075]    The scanner  10  operates as follows: 
         [0076]    By swivelling of the micromirror  24  which is controllable about the two swivel axes  30 ,  32 , the imaging plate  12  is scanned sequentially in pointwise manner with the read-out light beam  20 . In the process, the intensity of the emitted fluorescent light  22  is registered with the aid of the photodetector  36  and the evaluating unit  40  and is prepared for display. 
         [0077]    The control unit  38  drives the micromirror  24  in such a manner that the latter executes oscillations about its two swivel axes  30 ,  32 . As a result of the superposition of the two oscillations, the read-out light beam  20  scans the imaging plate  12  along a Lissajous  FIG. 50  by way of scan pattern. By virtue of the two oscillation frequencies and the relationship thereof to one another, the shape of the Lissajous  FIG. 50  is determined, so that other Lissajous  FIG. 50  are generated by other frequencies. In the embodiment shown in  FIG. 1  the Lissajous  FIG. 50  exhibits four oscillations in the longitudinal direction and three oscillations in the transverse direction before the scan pattern repeats. 
         [0078]    For the purpose of synchronising the evaluating unit  40  with the motion of the read-out light beam  20 , one or more photodiodes  51  may have been arranged in the supporting surface  14  alongside the imaging plate  14 . When the read-out light beam  20  roams over these photodiodes  51 , the location of which is known, the evaluating unit  40  receives corresponding synchronisation signals. 
         [0079]    In order to obtain a high read-out efficiency, the micromirror  24  is driven at oscillation frequencies that are close to the natural frequencies about the two axes  30 ,  32 . The natural frequencies in this case are established, inter alia, by the mass of the micromirror  24  and by the angular spring constants of the cardanic solid-joint suspension  28 . 
         [0080]    In order to scan all regions of the imaging plate  12 , on the one hand very large frequency ratios, such as 200:1 for example, can now be selected, so that the scanning, as indicated in  FIG. 3 , corresponds virtually to a line-by-line scanning, in the course of which a slow motion of the read-out light beam  20  occurs along one direction, whereas in the other direction several panning motions follow one another rapidly. In the process, the controllable micromirror  24  can be driven with an oscillation amplitude at which the read-out light beam  20  runs beyond the edges of the imaging plate  12 . In this way, the regions of the reversal-points, in which the read-out light beam  20  lingers for a relatively long time, are displaced into regions outside the imaging plate  12 , as a result of which an even more uniform scan pattern arises on the imaging plate  12 . 
         [0081]    However, the frequency ratios of the two oscillations can also easily be detuned in relation to one another, for example in a ratio 3:4.05, so that the lines of the Lissajous  FIG. 50  are easily displaced upon each sweep of the figure and in this way a moving Lissajous  FIG. 50  is generated which gradually scans all regions of the imaging plate  12 . This procedure is indicated in  FIG. 4 , in which the initial regions of the first, second and third sweeps of the Lissajous  FIG. 50  have been indicated respectively by  52 ,  54  and  56 . 
         [0082]    Since with this method the imaging plate  12  is scanned repeatedly at many points, for example at the points of intersection of the Lissajous  FIG. 50 , and, in the process, excited storage centres possibly still remaining additionally emit fluorescent light  22 , the evaluating unit  40  has to take this into account appropriately in the course of evaluating the photodetector signal and in the course of building up the image. 
         [0083]    To this end, for the purpose of calibration a completely uniformly exposed imaging plate  12  may be used which is scanned with the Lissajous  FIG. 50  provided for the purpose of read-out. In the process, the signals received from the photodetector  36  are added up, whereby, by reason of the multiple scanning of the points of intersection, brighter summed intensities arise in the image  48  at these points. The calibration image  48  obtained in this way is converted into correction values which are saved in the correction memory  42  of the evaluating unit  40 . If subsequently an imaging plate  12  is read that contains actual image information, the ascertained intensities are corrected so as to correspond to the correction values contained in the correction memory  42 . 
         [0084]    For calibration purposes, mathematical methods are also conceivable in which the scanning path of the Lissajous  FIG. 50  is modelled and taken into account appropriately in the course of the summation of the photodetector signal. Also in this way, the effects of the multiple scanning or of the variable speeds of motion along the scan pattern can be compensated, and correction values can be ascertained which are saved in the correction memory  42 . However, with these purely mathematical methods special properties of the plate, for example what percentage of the excited storage centres still remains after a first read-out process, can only be taken into account if the type of imaging plate is specified by the operator or by an automatic recognition system. 
         [0085]      FIG. 5  shows a scanner  110  according to another embodiment, wherein structurally similar components bear reference symbols increased by 100. 
         [0086]    In the embodiment shown in  FIG. 5 , instead of the micromirror  24  a micromirror array  124  has been provided on the deflection unit  126 , in which the individual micromirrors  125  can be switched in binary manner between two positions of tilt. Micromirror arrays  124  of such a type are used in the field of consumer electronics in DLP projectors for generating projected images and are available on the market in large numbers at favourable cost. 
         [0087]    In order to illuminate the entire micromirror array  124 , the read-out light beam  120  emanating from the laser  118  is expanded via an expanding lens system  160 . By switching the individual micromirrors  125 , component beams  121  of the read-out light beam  120  can then be directed either onto the imaging plate  112  or onto a beam absorber  162  which absorbs the component beams  121 . For this purpose the beam absorber  162  may have been constructed to be strongly absorbing. 
         [0088]    The scanner  110  operates in such a way that in each instance only one of the micromirrors  125  of the micromirror array  124  has been set to “ON”, i.e. the associated component beam  121  thereof has been directed onto a point assigned to it on the imaging plate  112 . All other micromirrors  125  have at this time been set to “OFF”, i.e. the component beams  121  thereof are directed onto the beam absorber  162 . In this way, the imaging plate  112  can be scanned, point by point, by pointwise switching of the micromirrors  125  on and off. 
         [0089]    In a modification, the micromirror  24  may be capable of being swivelled about only one swivel axis  30 . In this case the supporting surface  14  can, as indicated in  FIG. 1  by the double-headed arrow A, be moved along one direction, preferentially axially along the cylindrical shape of the supporting surface  14 , in order to move the imaging plate  12  past the read-out line constituted by the swivelling read-out light beam  20 . But the supporting surface  14  can also be moved in the case where use is made of the biaxial micromirror  24  if particularly large-area imaging plates  12  have to be read. 
         [0090]    In a further modification, the micromirror  24  may also be driven in stepwise manner, so that the micromirror  24  can remain in individual intermediate positions of tilt, allowing a read-out, pixel by pixel, in a matrix. As a result, the read-out beam is able to dwell at each point of the imaging plate  12  for a desired read-out time, whereby each point can then also be read out for an equally long time. This has the result that each point of the registered image exhibits the same signal-to-noise ratio. 
       2. Second Group of Embodiments 
       [0091]      FIGS. 6 to 9  show embodiments with alternative arrangements of the detector unit, and various options for erasing the imaging plate after the latter has been read. Structurally similar components in these cases bear reference symbols increased respectively by  200 ,  300 ,  400  and  500  with respect to those used in  FIG. 1 . In order to simplify  FIGS. 6 to 9 , the instrumental electronics  44  have not been shown in these Figures. 
         [0092]      FIG. 6  shows a scanner  210  with which an imaging plate  212  that is transparent to fluorescent light can be read. 
         [0093]    The scanner  210  includes for this purpose a photodetector  236 , the entrance window of which is constituted by a filter plate  237  which blocks the read-out light  220  but lets the fluorescent light  222  pass. The photodetector  236  has furthermore been arranged in such a way that the surface of the filter plate  237  pointing outwards serves at the same time as supporting surface  214 , against which the imaging plate  212  bears. The imaging plate  212  is consequently arranged directly in front of the entrance window of the photodetector  236 . 
         [0094]    During the read-out process the read-out light  220  impinges from one side onto the imaging plate  212 . The fluorescent light  222  released in the process then emerges on the other side of the imaging plate  212 , in order to get from there through the filter plate  237  and into the photodetector  236  and to generate a signal therein. The filter action of the filter plate  237  prevents the read-out light  220  from also generating a signal in the photodetector  236 . 
         [0095]    In order to erase an imaging plate  212  completely that has already been read, in the case of the scanner  210  a displaceable condenser lens  266 , an erasing-light source  268  and a feed element  270  have been provided by way of erasing appliance. With the feed element  270  both the read-out light  220  of the laser  218  and the erasing light  267  of the erasing-light source  268  feed into the beam path that leads to the deflection unit  226 . 
         [0096]    For the purpose of erasing the imaging plate  212 , the condenser lens  266  is displaced in such a way that the impingement spot of the read-out light  220  or of the erasing light  267  on the imaging plate  212  is enlarged. After this, the imaging plate  212  is scanned until such time as a sufficiently complete erasure of excited storage centres still remaining can be assumed. 
         [0097]    The use of broader-band erasing light  267  is not absolutely essential in this case. Accordingly, under certain circumstances it may suffice to increase the intensity of the read-out light  220  emitted by the laser  218  with the aid of the intensity-controlling unit  269 , and to carry out the erasure with this read-out light. In this case the erasing-light source  268  and the feed element  270  can be dispensed with. 
         [0098]      FIG. 7  shows a scanner  310  in which a separate erasing-light source  368  has likewise been provided. However, the light of the erasing-light source  368  is not fed into the same beam path as the read-out light  320  but is directed onto the micromirror  324  of the deflection unit  326  at an angle differing from that of the read-out light  320 . 
         [0099]    For the purpose of erasure, the micromirror  324  is then driven by the control unit  338 , taking a corresponding offset angle into account, in such a way that the erasing light  367  falling onto the micromirror  324  from a direction differing from that of the read-out light  320  is nevertheless directed onto the imaging plate  312 . 
         [0100]    Furthermore, the scanner  310  exhibits a detector unit  336  which exhibits a rectangular entrance window, the size and shape of which correspond roughly to those of the imaging plate  312 . 
         [0101]    The embodiment shown in  FIG. 8  shows a scanner  410  in which the filter plate  437  exhibits a cylindrically curved supporting surface  414  onto which the imaging plate  412  is placed with the aid of a clamping element  439 . The radius of curvature of the supporting surface  414  corresponds in this case to the spacing thereof from the micromirror  424 , so that in the transverse direction relative to the cylinder axis the read-out light  420  always falls perpendicularly onto the imaging plate  412 , regardless of the deflection angle. 
         [0102]    In the axial direction of the supporting surface  414  a perpendicular incidence on the imaging plate  412  can be generated via an f-theta lens system acting uniaxially. However, if a deterioration of the read-out quality in the axial direction, occurring by virtue of variable oblique incidence, is accepted, then an f-theta lens system can be dispensed with completely. 
         [0103]    For the purpose of erasing the imaging plate  412  completely, in this embodiment an LED strip  472  with erasing-light LEDs  474  has been provided by way of erasing appliance, which has been arranged circumferentially above the filter plate  437  in such a way that the uniformly spaced erasing-light LEDs  474  are able to direct their erasing light  467  onto the imaging plate  412 . 
         [0104]    Finally,  FIG. 9  shows an embodiment of a scanner  510  in which a cylindrically curved supporting frame  576  has been provided for a transparent imaging plate  512 . A clamping frame  578  co-operates with the supporting frame  576 , so that the imaging plate  512  introduced between the supporting frame  576  and the clamping frame  578  is kept cylindrically curved. 
         [0105]    In order to keep the scanner  510  as compact as possible, the latter further includes a reflector mirror  580  which here has been constructed in curved manner and which has been arranged with respect to the supporting frame  576  on the side situated opposite the micromirror  524 . The reflecting mirror  580  and the photodetector  536  have furthermore been arranged relative to one another in such a way that fluorescent light  522  emerging on the imaging plate  512  from the reverse side thereof falls onto the entrance window of the photodetector  536  after reflection on the reflector mirror  580 . By reason of the reflector mirror  580 , the position of the photodetector  536  can be chosen more freely, enabling a more compact structural shape of the scanner  510 . In addition, a photodetector  536  with a smaller entrance window can be used if the reflector mirror  580  has a focusing action.