Abstract:
Device for detecting information which is contained in a phosphor layer, with a light source for emitting stimulation light incident upon the phosphor layer along a stimulation line is suitable for stimulating emission light in the phosphor layer, and a detector for detecting the emission light which is stimulated in the phosphor layer. To ensure the highest possible quality of the detected image, an elongated concave mirror is provided, for focusing the stimulation light emitted by the light source onto the phosphor layer. The light source is projected onto the phosphor layer at a scale of 1:M, where M takes a value between 0.5 and 2.

Description:
FIELD OF THE INVENTION 
   The invention concerns a device for detecting information which is contained in a phosphor layer. 
   Devices for detecting information which is contained in a phosphor layer are used in the field of computer radiography (CR), particularly for medical purposes. X-ray images are recorded in a phosphor layer, X-ray radiation which passes through an object, for instance a patient, being stored as a latent image in the phosphor layer. To read out the latent image, the phosphor layer is irradiated with stimulation light and thus stimulated to emit emission light, which corresponds to the latent image which is stored in the phosphor layer. The emission light is detected by an optical detector and converted into electrical signals, which are processed further as required, and presented on a monitor or an appropriate output device, e.g. a printer. 
   From the prior art, devices in which the stimulation light from a light source is focused on a linear area of the phosphor layer using cylinder lenses are known. In the case of such devices, the stimulation line, on which the stimulation light from the light source is focused, can usually not be delimited as sharply as required. However, for high image quality of the image to be read out, the sharpest possible delimitation of the stimulation line is required. 
   SUMMARY OF THE INVENTION 
   The present invention provides a device for detecting information which is contained in a phosphor layer, and with which the highest possible image quality is ensured. This is achieved using an elongated, particularly cylindrical, concave mirror for focusing the stimulation light which the light source emits onto the phosphor layer. The light source is projected onto the phosphor layer at a scale of 1:M, where M takes a value between approximately 0.5 and 2. 
   By using an elongated concave mirror, projection errors which result in unsharp projection of the light source onto the phosphor layer are greatly reduced compared with cylinder lenses. In particular, in this way projection errors because of spherical aberration are completely eliminated, and projection errors because of astigmatism and coma are greatly reduced. According to the invention, this great reduction of projection errors is achieved by the projection scale, i.e., the ratio of the image size to the object size, taking a value between 2:1 (M=0.5, enlargement) and 1:2 (M=2, reduction). Through the invention, a high degree of sharpness in the projection of the light source onto the phosphor layer is achieved. This ensures high quality of the image which is read out from the phosphor layer. 
   Preferably, the light source is projected onto the phosphor layer at a scale of 1:1. With this scale, projection errors are particularly small, and thus the image quality is particularly high. 
   In another embodiment, it is provided that the light source extends parallel to the stimulation line. In this way, even greater sharpness of the stimulation line is easily achieved. 
   Advantageously, the concave mirror is arranged parallel to the stimulation line. In this way, the sharpness of the image can easily be further increased. 
   In another preferred version of the invention, it is provided that the concave mirror is in the form of a cylinder mirror, which in cross-section has the form of an arc of a circle. In alternative variants of this embodiment, the cylinder mirror is in the form of an arc of an ellipse, or aspherical, in cross-section. With all the above-mentioned cross-section forms of the cylinder mirror, compared with correspondingly formed cylinder lenses, sharper projection of the light source onto the phosphor layer, and thus higher image quality, are achieved. 
   In another embodiment, it is provided that the optical axis in the cross-section of the concave mirror, which has, in particular, a cross-section in the form of a circular arc, cuts the concave mirror in a vertex, and that the light source is tilted by a tilt angle around the vertex against the optical axis of the concave mirror. By tilting the radiation sources against the optical axis of the concave mirror, the stimulation line, on which the stimulation light from the light source is focused, is tilted in the opposite direction against the optical axis of the concave mirror. Thus, by an appropriate choice of tilt angle, the distance of the light source from the stimulation line can be adapted to structural requirements. 
   Preferably, the distance d, along the optical axis in the cross-section of the concave mirror, of the light source and/or the stimulation line from the vertex of the concave mirror satisfies the relationship d=R cos 2  ω, where R designates the radius of curvature of the concave mirror and ω designates the tilt angle. In this way, projection errors which are caused by astigmatism and coma, and increase with increasing tilt angle ω, can be kept very small, so that high projection sharpness and thus image quality are ensured. 
   Another embodiment of the invention provides that the detector has multiple detector elements which are arranged parallel to the stimulation line. In this way, location-resolved detection of the emission light from the stimulation line is made possible. 
   In one embodiment of the device, the light source comprises multiple individual radiation sources which are arranged along a line and each emit stimulation light bundles, which are focused by the concave mirror onto the stimulation line, and at least partially superimpose themselves on the stimulation line. By superposition of the individual stimulation light bundles in the direction of the stimulation line, high intensity of the stimulation light is achieved, with simultaneous low intensity variations along the stimulation line. 
   Another version of the invention provides that the light source is in the form of a linear light source, which is in a form for emitting stimulation light along a continuous line. In this way, intensity variations are kept specially low. 
   In an alternative form, it is provided that the light source comprises a radiation source and a deflection device. The stimulation light which the radiation source emits is periodically deflected by a deflection device, and focused by the concave mirror onto a focal area on the phosphor layer, in such a way that the focal area periodically spreads over the phosphor layer along the stimulation line. In such systems also, projection errors which would result in lack of sharpness and expansion of the focal area and thus a reduction of image quality are significantly reduced by the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below on the basis of following figures: 
       FIG. 1  shows a first embodiment of the invention in side view; 
       FIG. 2  shows the first embodiment shown in  FIG. 1 , in plan view; 
       FIG. 3  shows a second embodiment of the invention; and 
       FIG. 4  shows a third embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a first embodiment of the invention in side view. A light source  10  emits stimulation light  12  in the form of a divergent stimulation light bundle  13  in the figure plane, and this stimulation light bundle  13  is reflected on an elongated concave mirror  14 , which runs vertically to the figure plane, and is focused on a stimulation line  16  which also runs vertically to the figure plane. 
   In this example, the light source  10  comprises individual radiation sources  11 , which are arranged along a line. This is illustrated in  FIG. 2 , which shows the first embodiment shown in  FIG. 1  in plan view. The radiation sources  11  of the light source  10 , which are arranged along a straight line, emit stimulation light  12 , which has, also in the plane of  FIG. 2 , the form of divergent stimulation light bundles  15 . The divergent stimulation light bundles  15  of the individual radiation sources  11  in the plane of  FIG. 2  are reflected on the concave mirror  14  and superimpose themselves on the stimulation line  16 . As can be seen in  FIG. 2 , the individual radiation sources  11  and the elongated concave mirror  14  are arranged parallel to the stimulation line  16 . The stimulation line  16  lies on a phosphor layer (not shown) from which a latent image which is stored in it is to be read. 
   In this embodiment, the elongated concave mirror  14  is in the form of a cylinder mirror, the cross-section of which is in the form of a circular arc. Such cylinder mirrors can be manufactured economically, and with the described devices they result in a great reduction of projection errors. 
   However, the cross-section of the cylinder mirror can also be aspherical, i.e., deviate from the shape of a circular arc. In this way, variations of intensity, sharpness and breadth along the stimulation line  16  can be additionally reduced. In particular, the cylinder mirror can have a cross-section in the form of an arc of an ellipse. A cylinder mirror in this form has two focal lines, the light source  10  being arranged in a first focal line and the stimulation line  16  running along a second focal line. In this case, for any projection scales, aberration-free focusing of the stimulation light  12 , and thus very sharp delimitation of the stimulation line  16 , are achieved. However, the cylinder mirror can also have the form of an arc of a parabola in cross-section. This is advantageous, for instance, if the stimulation light  12  which is emitted by the light source  10  has the form of a parallel light bundle or a light bundle with only a very small divergence. 
   In the shown example, the elongated concave mirror  14  has straight surface lines which run vertically to the plane of  FIG. 1 . However, within the invention the elongated concave mirror  14  can also have a form in which the surface lines are curved. In this way a further degree of freedom in the form of the concave mirror  14 , and thus in the reduction of projection errors, for instance because of additional optical components in the beam path of the stimulation light bundle  13 , is obtained. 
   As  FIG. 1  shows, the light source  10  and stimulation line  16  are each tilted by a tilt angle ω relative to the optical axis  5  of the concave mirror  14 . The tilt is around the vertex S, where the optical axis  5  cuts the concave mirror  14 . By choosing the tilt angle ω, the distance between the light source  10  and stimulation line  16  can be adapted to structural requirements, e.g., a particular lighting and/or detection geometry. 
   By using an elongated concave mirror  14  for focusing the divergent stimulation light bundle  13  from the individual radiation sources  11 , a stimulation line  16  which is significantly more sharply delimited, and the breadth of which is more homogeneous, is obtained than is achieved with focusing of the stimulation light  12  using cylinder lenses, as is known from the prior art. This can be explained because in the focusing of the individual stimulation light bundles  13  of the radiation sources  11  using cylinder lenses, projection errors occur along the stimulation line  16  because of curvature of the individual focal lines, also called focus bow, resulting in variation of the sharpness and consequently of the breadth of the line. By using an elongated concave mirror, these projection errors are eliminated or at least greatly reduced, so that variation of the breadth of the stimulation line  16  in the linear direction is avoided or reduced. In this way, even with a light source  10  consisting of multiple individual radiation sources  11 , an evenly broad and sharply delimited stimulation line  16  is obtained on the phosphor layer. 
   In the first embodiment, which is shown in  FIGS. 1 and 2 , the light source  10  is projected onto the stimulation line  16  at the scale of 1:1. The scale here refers to projection or focusing in the plane of  FIG. 1 . By this 1:1 projection, projection errors which cause, in particular, lack of sharpness in the projection of the light source  10  onto the stimulation line  16 , are greatly reduced. In particular, in this way projection errors because of spherical aberration are avoided. However, the sharp delimitation (which is required for high image quality) of the stimulation line  16  is not only achieved with a projection scale of 1:1, but also with scales between 1:2 and 2:1. With projection scales other than 1:1, within the range from 1:2 to 2:1, projection errors occur because of spherical aberration, astigmatism and coma, but surprisingly these are so small that nevertheless very high image quality can be ensured. 
   To keep projection errors because of astigmatism and coma particularly low, the distance of the light source  10  and/or stimulation line  16  from the concave mirror  14  is chosen depending on the chosen tilt angle ω. In particular, the distance d, along the optical axis  5  in the cross-section of the concave mirror  14 , of the individual radiation sources  11  and/or the stimulation line  16  from the vertex S of the concave mirror  14  satisfies the equation d=R cos 2  ω, where R designates the radius of curvature of the concave mirror. In the case of the concave mirror  14 , which in the example of  FIG. 1 , as a cylinder mirror, has a cross-section in the form of an arc of a circle, the radius of curvature R corresponds to the distance of the circular arc from the centre P of this circle. 
   In the first embodiment of  FIGS. 1 and 2 , the light source  10  comprises multiple radiation sources  11 , which are arranged along a straight line. Alternatively, the light source  10  can also be in the form of a linear light source, which emits the stimulation light  12  along a continuous line. In this case, in contrast to a light source  10  consisting of multiple individual radiation sources  11 , breadth variations along the stimulation line  16  no longer occur. Projection errors because of spherical aberration and/or astigmatism are greatly reduced by the use according to the invention of an elongated, particularly cylindrical, concave mirror, so that significantly increased sharpness of the stimulation line is achieved. 
   In another alternative form, the light source  10  has only one radiation source  11 . The stimulation light bundle  13  from the radiation source  11  is periodically deflected by a deflection device, and focused by the elongated concave mirror  14  onto a dot-shaped or linear focal area on the phosphor layer. Because of the periodic deflection of the stimulation light bundle  13 , the focal area spreads over the phosphor layer along the stimulation line  16 . The emission light which is stimulated in each case is captured by a detector. Devices of this type are also called flying spot systems. In such systems, projection errors which would result in lack of sharpness and expansion of the focal area and thus to a reduction of image quality are significantly reduced by the invention. 
     FIG. 3  shows a second embodiment of the invention. In this embodiment, the divergent stimulation light bundles  10  which are emitted from the light source  10  are first reflected on an elongated plane mirror  18 , before they meet the elongated concave mirror  14  and are focused by it along the stimulation line  16  onto the phosphor layer  21 , which is stabilised mechanically by a carrier layer  23 . By using the plane mirror  18 , compared with the first embodiment which is shown in  FIG. 1 , additional options for the spatial arrangement of the light source  10 , the concave mirror  14  and the phosphor layer  21  are obtained, making it possible to adapt the device according to the invention to structural requirements without causing additional projection errors. 
   Instead of the plane mirror  18 , a suitable optical prism can be used to deflect the stimulation light bundle  13 . It is also possible to use an elongated convex mirror or another elongated concave mirror to deflect the stimulation light bundle  13 . 
   In this example, the light source  10 , the concave mirror  14  and the stimulation line  16  on the phosphor layer  21  are arranged spatially relative to each other similarly to the first embodiment which is shown in  FIGS. 1 and 2 . In particular, the light source  10  and plane mirror  18  are arranged so that the light source  10  is also tilted by a tilt angle ω (virtual in this case) against the optical axis  5  of the concave mirror  14 . The distances of the light source  10  and phosphor layer  21  from the vertex S of the concave mirror  14  are also chosen so that 1:1 projection—in relation to the plane of FIG.  3 —takes place, and thus projection errors are reduced to a minimum. 
   The emission light  17  which is stimulated along the stimulation line  16  in the phosphor layer  21  is projected onto the detector  25  by projection optics  24 . The detector  25  is in the form of a linear detector, which has multiple individual detector elements which are arranged parallel to the stimulation line  16 . Also parallel to the linear detector  25 , run the projection optics  24  (which are preferably in the form of a linear micro-lens or gradient index lens array, particularly a self-focusing lens array). 
   For reading out the latent image which is stored in the phosphor layer  21 , the device which is combined into a read unit  30  is moved in a transport direction T over the phosphor layer  21 , which is successively stimulated and read out along the stimulation line  16 . Alternatively or additionally to the movement of the read unit  30 , the phosphor layer  21  on the carrier layer  23  can be moved relative to the read unit  30 . 
     FIG. 4  shows a third embodiment of the invention, with a specially compact construction. In this example, the cross-section of the concave mirror  14  is oriented substantially vertically to the phosphor layer  21 , so that the distance of the light source  10  from the phosphor layer  21  can be reduced. In this example, the divergent stimulation light bundles  13  from the light source  10  hit the concave mirror  14  directly and are focused by it onto the stimulation line  16 . A plane mirror  18  is inserted into the beam path between the concave mirror  14  and the phosphor layer  21 , and deflects the light bundle which is reflected by the concave mirror  14 , and is now convergent, onto the stimulation line  16 . In this embodiment, the plane mirror  18  opens up multiple possibilities for arranging the light source  10 , concave mirror  14  and phosphor layer  21  relative to each other, so that in particular a very compact arrangement of the individual components can be achieved. 
   In the embodiment of  FIG. 4  which is shown here, the phosphor layer  21  to be read out is on a carrier layer  23 , which is transparent to the stimulation light, so that the phosphor layer  21  can be stimulated from below. In the shown case, the emission light  17  is projected from above through projection optics  24  onto a detector  25 . Alternatively, however, the emission light  17  can be captured from below through the transparent carrier layer  23 . Regarding the projection optics  24  and detector  25 , the explanations about the second embodiment, which is shown in  FIG. 3 , apply correspondingly. Regarding the choice according to the invention of the elongated concave mirror  14 , of the angle of inclination ω and of the projection scale, the explanations about the embodiments which are described in  FIGS. 1 to 3  also apply correspondingly.