Abstract:
An image-recording device for a printing form ( 29 ), including an array of light sources ( 12 ) and a downstream microoptics ( 14 ), which generates a virtual intermediate image ( 18 ) of the light sources ( 12 ), which is distinguished by the microoptics ( 14 ) having a downstream optical system ( 10 ) of a convex mirror ( 26 ) and of a concave mirror ( 24 ) having a common center of curvature, a combination of the Offner type, which produces a real image ( 28 ) of the virtual intermediate images ( 18 ). By employing a monolithic structure ( 40 ) of the optical system ( 10 ) of a convex mirror ( 26 ) and of a concave mirror ( 24 ), a more compact, space-saving design is able to achieved. The image-recording device according to the present invention may be utilized to special advantage for a printing form ( 29 ) in a plate-exposure unit or in a print unit of a printing press.

Description:
[0001]    Priority to German Patent Application No. 101 15 875.0, filed Mar. 30, 2001 and hereby incorporated by reference herein, is claimed.  
         BACKGROUND INFORMATION  
         [0002]    The present invention is directed to an image-recording device for a printing form, including an array of light sources and a downstream microoptics which generates a virtual image of the light sources.  
           [0003]    The use of light source arrays in rows or in matrix form for recording images on printing forms, whether in a printing-form exposure unit or in a direct-imaging print unit, places high demands on the imaging optics to be used. Typically, the light source arrays are made up of a specific number of diode lasers, preferably of single-mode lasers, which are mounted at a defined distance from one another, usually spaced apart at substantially the same intervals on a semiconductor substrate, and which share a common exit plane that is precisely defined over the crystallographic plane of fracture. The light-emission cones of these light sources or diode lasers open at different widths in the two planes of symmetry which are substantially orthogonal to one another. From this, the necessity arises of an imaging optics which, on the one hand, reduces, preferably minimizes this asymmetry by using a preferably small number of subassemblies, and, on the other hand, renders possible a global imaging of the array of emitters that is as free of aberrations as possible.  
           [0004]    From the related art, one knows of a number of optical imaging systems, which are specially implemented for imaging diode laser arrays to form images on a light-sensitive medium. For example, from U.S. Pat. No. 4,428,647, a semiconductor laser array is known, each of whose individual lasers is assigned an adjacent lens between the laser array and the objective lens. The purpose of these lenses is to change the angle of divergence of the light beams emerging from the surface of the laser array, such that the light is collected as efficiently as possible by the objective lenses and is focused at a light sensitive medium. The optical power of these lenses is selected such that, for each laser, a virtual intermediate image is formed behind the emitting surface, whose spacings correspond approximately to the spacings of the emitted light beams, the emitter&#39;s intermediate image being magnified.  
           [0005]    EP 0 694 408 B 1 describes, for example, how a microoptics is able to reduce the divergence of the emerging light by using axially symmetric optical elements.  
           [0006]    The often exceptionally large difference in the lateral field dimensions of a light source array of this kind, for example 10×0.001 mm 2 , therefore requires a specific microscopic and macroscopic image formation. A use of spherical optics for these dimensions can only succeed by employing a relatively large and costly optical design. A disadvantage encountered when using a spherical macrooptics is the variable image quality as a function of the distance to the optical axis. Even the use of cylinder lenses and cylinder lens arrays has, to date, not produced the consistent quality desired for an imaging of a light source array, particularly in the form of a diode laser array.  
           [0007]    From U.S. Pat. No. 3,748,015, one knows of an optical system for forming an image of an object with unit magnification and high resolution, which includes an arrangement of a convex and concave spherical mirror, whose centers of curvature coincide at one point. This mirror arrangement produces at least three reflection points within the system and two conjugate regions set apart from the optical axis, at unit magnification in a plane which contains the center of curvature, the optical axis of the system being orthogonal to this plane in the center of curvature. Such a combination of mirrors is free of spherical aberration, coma and distortion, and, when the algebraic sum of the powers or refractive powers of the mirror reflecting surfaces utilized is zero, the image produced is free from third order astigmatism and field curvature. An optical system of this kind is referred to as an optical system of the Offner type.  
           [0008]    U.S. Pat. No. 5,592,444, for example, describes a method and a corresponding device for writing and reading data to an optical storage medium, simultaneously in a plurality of tracks. The imaging optics described in this document for a plurality of individually controllable diode lasers includes, in this context, a system of spherical mirrors of the above-described Offner type, thus a combination of spherical concave and convex mirrors having a common center of curvature. However, no virtual, in particular no magnified intermediate image is produced by the divergence-reducing micro optics.  
           [0009]    However, the use of an image-recording device for a printing form in a printing-form exposure unit or in a print unit in a printing press requires additional measures. Since, on the one hand, machines of this kind have a very limited assembly space, and, on the other hand, little can be altered on the design or on the configuration of the printing-form exposure unit or on the print unit to implement an image-recording device, it is necessary to reduce the required assembly space. In addition, an imaging optics on a printing press or a printing-form exposure unit is subject to shocks or vibrations, so that it should have as few parts as possible that require relative adjustment. For that reason, known related-art optical systems cannot simply be transferred for use on a printing-form exposure unit or within a print unit of a printing press.  
         SUMMARY OF THE INVENTION  
         [0010]    An object of the present invention is, therefore, to devise an imaging optics for an array of light sources, which will reduce the divergence of the emitted light in simple fashion and render possible an image formation having few aberrations. It is additionally or alternatively intended to realize an imaging optics for an image-recording device for a printing form which will require the least possible amount of overall space and as few as possible parts, and therefore, as few as possible degrees of freedom in the adjustment.  
           [0011]    The present invention provides an image-recording device for a printing form ( 29 ), including an array of light sources ( 12 ) and a downstream microoptics ( 14 ) which generates a virtual intermediate image ( 18 ) of the light sources ( 12 ). Arranged downstream from the microoptics is an optical system ( 10 ), which includes at least one sector of a convex mirror ( 26 ) and one sector of a concave mirror ( 24 ) having a common center of curvature, which produces a real image ( 28 ).  
           [0012]    The image-recording device according to the present invention for a printing form, having an array of light sources and a downstream microoptics which produces a virtual image of the light sources, is distinguished by the microoptics having the downstream optical system, which includes at least one concave mirror sector and one convex mirror sector having a common center of curvature, the algebraic sum of the powers of the refractive powers preferably being zero, in other words, a macrooptics or combination of the Offner type, which produces a real image of the virtual intermediate image. In the following, a convex and concave mirror arrangement is also discussed in simplified terms, although, here as well, at least one mirror may only have one sector that defines a surface that is simply as well as non-simply cohesive, in a specific subspace angular range of maximally 4π. In this context, one specific embodiment provides, in reality, that the centers of curvature of the concave mirror and of the convex mirror need not coincide with complete precision in order to obtain the desired properties of the Offner-type optical system, exactly enough, for use in an image-recording device according to the present invention.  
           [0013]    Using a small number of optically refractive surfaces, in the image-recording device of the present invention, each light source of the array is adapted via a virtual intermediate image to the microscopic requirements, thus, in particular, to the divergence. A downstream macroscopic imaging, utilizing known properties of an Offner-type optical system, thus a combination of at least one convex mirror sector and one concave mirror sector having a common center of curvature, enables points to be advantageously imaged along a line that essentially runs in a circle. The optical system, which, as macrooptics, is positioned downstream from the microoptics, of the image-recording device of the present invention is designed such that the virtual intermediate image points of the light sources, which are essentially arranged in one row, are spaced at a smaller distance to this circular line. In other words: the image-recording device of the present invention makes it possible for the emission from a multiplicity of light sources, in particular from diode lasers, to be constantly corrected using a small number of optical elements. By combining cylindrical lenses, one achieves a micro-optical symmetrization, simultaneously accompanied by magnification, using a virtual intermediate image of each light source and a, to the greatest degree, aberration-free imaging of these virtual intermediate images into a real image, by way of a downstream optical system of a convex mirror and a concave mirror, to create an image-recording device for a printing form having especially beneficial image-forming properties.  
           [0014]    To facilitate adaptation of the divergence of the emitted light, the microoptics preferably has an aspherical design. These may be, for example, cylindrical lenses or a combination of anamorphotic prisms. The downstream, macroscopic, optical system of a convex and a concave mirror has at least one circular segment of rotationally symmetric optics, to whose assigned object circle, the essentially straight-line projection of the row of virtual, intermediate image points exhibits a spacing that is kept small, the object circle being situated within one of the two conjugate regions of the optical system of a convex and concave mirror. Thus, using the optical system of the Offner type, the essentially straight-line row of virtual, intermediate image points may be produced as real images, with unit magnification, in the second conjugate region. Especially advantageous in this context is the absence of aberration in the optical system of a convex and a concave mirror.  
           [0015]    To reduce the overall space required for the image-recording device of the present invention, the optical path is advantageously folded at least once within the optical system of one convex and one concave mirror. Therefore, at least one path-folding surface is beneficially provided in the optical system situated downstream from the microoptics, whether it be upstream and/or downstream from the reflective surfaces of the optical system of a convex and a concave mirror. This yields a compact optical path through the imaging optics of the image-recording device of the present invention, so that it is possible to reduce the overall required space for an implementation within a printing-form exposure unit or a print unit. Moreover, at least one part of the optical system of a convex mirror and of a concave mirror may be fashioned quite advantageously as a single component, thus monolithically from a suitable material having a refractive index that differs from the ambient environment, for example from a glass or a another transparent material. The individual component, i.e., the monolith may then have partially internally reflecting surfaces, which, for example, form the concave and convex reflective surfaces, respectively, of the optical system of a convex and a concave mirror. These internal surfaces are also described as the active internal surfaces of the monolith. Provided at the monolith are at least one entrance window and one exit window for the light emitted by at least one light source, the windows preferably having an antireflection coating in the form of an interference filter. In one advantageous further embodiment, other optical elements, such as prisms or path-folding surfaces may be assigned to the monolithic structure for purposes of beam deflection.  
           [0016]    An image-recording device according to the present invention may be utilized to special advantage in a printing-form exposure unit or in a print unit. A printing press in accordance with the present invention, which includes one feeder, at least one print unit, and a delivery unit, has the distinguishing feature of having at least one print unit equipped with an image-recording device according to the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    Other advantages, beneficial specific embodiments, and further refinements of the present invention are presented on the basis of the subsequent figures and descriptions thereof. In detail, the figures show:  
         [0018]    [0018]FIG. 1 a schematic representation of a configuration of optical elements in one specific embodiment of the image-recording device according to the present invention for a printing form;  
         [0019]    [0019]FIG. 2 a schematic representation of a configuration of optical elements in an alternative specific embodiment of the image-recording device according to the present invention, including additional beam-profile filters;  
         [0020]    [0020]FIG. 3 a schematic representation illustrating the position of the focal line of the optical system of the convex mirror and concave mirror with respect to the row of virtual image points of the array of light sources;  
         [0021]    [0021]FIG. 4 a schematic representation of a monolithically constructed optical system of a convex mirror and a concave mirror;  
         [0022]    [0022]FIG. 5 a schematic representation of a monolithically designed, alternative optical system of a convex mirror and a concave mirror, utilizing two path folds;  
         [0023]    [0023]FIG. 6 a schematic representation of a symmetric, monolithically designed, alternative optical system of a convex mirror and a concave mirror, including additional path-folding elements in the form of prisms; and  
         [0024]    [0024]FIG. 7 a schematic representation of a monolithically designed, alternative optical system of a convex mirror and a concave mirror, including a convex sphere and a prism for coupling in the light to be imaged. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 1 shows a schematic representation of a configuration of optical elements in one specific embodiment of the image-recording device according to the present invention for a printing form. The image-recording device of the present invention has a light source  12 , including an assigned microoptics  14 , and a downstream optical system  10 . Divergent light  16  emitted by light source  12  is imaged by microoptics  14  onto a virtual image  18 . Through downstream optical system  10 , light beams  20 , emanating from virtual intermediate image  18  via various optical elements, are transformed into a real image point  28 . In this specific embodiment, optical system  10  has, first of all, a deflecting element  22  and, configured along optical axis  23  and rotationally symmetric thereto, a pair of mirrors, concave mirror  24  and convex mirror  26 , having a common center of curvature  25  along optical axis  23 . This pair, made up of concave mirror  24  and convex mirror  26 , images points in one object region onto points in an image region. These regions are conjugate to one another. The symmetry of the optical path through optical system  10  is broken by additional deflecting element  22 , so that, as a conjugate point, virtual intermediate image  18  is assigned to image point  28 , and not conjugate point  27  without a deflecting element in printing form plane  29 . The optical path length between virtual intermediate image  18  and concave mirror  24  is, however, equal to the optical length between concave mirror  24  and image point  28  in printing form plane  29 .  
         [0026]    While in FIG. 1, the imaging of a light source  12  using microoptics  14  and a downstream optical system  10 , thus a macrooptics, is graphically shown to facilitate a better understanding of the image-recording device of the present invention, in a corresponding, preferred specific embodiment of the present invention, a plurality of light sources  12 , typically arranged in a row, is imaged by a microoptics  14 , preferably individually formed for each light source  12 , and by a macrooptics acting on the plurality of intermediate images  18 , in accordance with optical system  10  of a convex and a concave mirror.  
         [0027]    [0027]FIG. 2 shows a schematic representation of a configuration of optical elements in an alternative specific embodiment of the image-recording device according to the present invention for a printing form, including an additional beam-profile filter. In this context, the image-recording device of the present invention includes a light source  12 , microoptics  14 , an entrance window  32  into an encapsulation  33 , in which optical system  10  is situated, and an exit window  34 , printing form  29  being configured subsequently thereto. Here, optical system  10  includes a deflecting element  22 , a concave mirror  24 , a wavefront-correction element or beam-forming element  30 , a so-called beam-profile filter, preferably for transmitting the fundamental mode of light source  12 , for example having a Gaussian beam profile, and a concave mirror  26 . Optical system  10  is, thus, likewise that of a convex mirror and a concave mirror having conjugate regions, virtual intermediate image  18  being generated from divergent light  16  from light source  12  using microoptics  14  in the first conjugate region, and image point  28  in printing form plane  29  in the second conjugate region. By folding the optical path, as shown, using deflecting element  22 , whether it be, as shown here in FIG. 2, passing in front of convex mirror  26 , crossing optical path between convex mirror  26  and concave mirror  24 , or alternatively thereto, passing behind the convex mirror, it is possible to achieve an even more compact design.  
         [0028]    In a schematic representation, FIG. 3 elucidates the position of a focal line, i.e., selected points in a first conjugate region of the optical system of a convex and concave mirror with respect to the row of the virtual image points of the array of light sources. FIG. 3 shows a projection along optical axis  23  of concave mirror  24  and of convex mirror  26  of optical system  10 . The essentially circular focal line  36  represents the projection of the conjugate regions on concave mirror  24  for the case of a symmetrical path of rays selected here exemplarily. In other words: the object point and the image point of the optical system of a convex mirror and of a concave mirror lie essentially in phase opposition on a circular focal line  36 , thus  180  degrees out of phase about optical axis  23 . Focal line  36  essentially describes those points having an optimal advantageous transformation property, thus having minimal aberrations. The aim, at this point, is to approximate the row of virtual image points  38  of this focal line  36 . In so doing, it is unimportant in the context of the present invention which precise metrics or measure is selected to measure the distance of line  38  to circular segment  36 . As a measure, one may utilize, for example, the average distance of the light sources in projection  38  to optical axis  23 , thus the sum of the distances divided by the number of light sources. To achieve an advantageously aberration-minimized imaging through optical system  10 , the distance of the projection of the row of virtual image points  38  to the radius of focal line  36  is kept small or is adapted.  
         [0029]    In addition, it is clear that optical system  10  of a convex and a concave mirror should be designed such that the projection of focal line  36  exhibits a largest possible radius of curvature. In other words: considered locally, thus considered in the projection of light sources  38 , on the scale of the light sources&#39; image point distances which are maximally distant from one another, focal line  36  should have a flattest possible curve shape in comparison to the projection of the row of light sources  38 . Thus, the employed optical system  10  only needs to have at least one circular segment of a rotationally symmetric optics of a convex mirror and of a concave mirror.  
         [0030]    [0030]FIG. 4 is a schematic representation of a monolithically designed specific embodiment of the optical system in the image-recording device according to the present invention. A monolithic design is employed to further reduce the size of the optical system of a convex and a concave mirror. Such a monolithic design is exemplified in FIG. 4 by a symmetric path of rays. Optical system  10  is symmetrical to axis  41 . Emanating from virtual intermediate image  18  of the light source (not shown here), together with microoptics, light beams  20  pass through an entrance window  32  into a monolith  40 , which is made exemplarily of a highly refractive glass or of a polymer that is transparent to the employed wavelength. The monolith has a concave surface  42 , which reflects light beams  20 , so that they impinge on an essentially plane reflecting surface  46  facing opposite concave surface  42 . From reflecting surface  46 , the beams are thrown at a convex surface  44 , emanating from there, symmetrically on the other side of axis of symmetry  41 , in turn, reflecting surface  46  and, subsequently, concave surface  42 , are hit by the light beams, until they exit the monolith through an exit window  34  and converge in an image point  28 , appropriately in the printing-form plane (not shown here). The monolithic design, as shown in this FIG. 4, utilizes the fact that, in an optical system of a convex and a concave mirror, it is above all those regions of the concave mirror, which are distant from the optical axis or axis of symmetry  41 , that are used for reflecting light beams from the first conjugate region to the convex mirror, and from the convex mirror into the second conjugate region. This makes it possible to introduce a reflecting surface  46 , so that concave surface  42  in the vicinity of the optical axis or axis of symmetry  41 , may be replaced by a convex surface  44 . The position and the curvature are, of course, determined by the conditions of an optical system of a convex mirror and a concave mirror. Convex surface  44  corresponds to a convex mirror at position  48 , upon which light beams  20  would impinge along optical paths  50 , if there were no reflecting surface  46 . While the sides of monolith  40 , off of which light beams  20  are to be reflected, are made as reflective as possible by suitable coatings, whether by a metal coating or interference filters, an antireflection coating, for example an interference filter, is provided for entrance window  32  and/or for exit window  34 , to achieve a strongest possible coupling of the light into and out of the monolith.  
         [0031]    [0031]FIG. 5 schematically depicts a monolithically designed, alternative optical system of a convex mirror and a concave mirror, utilizing two path folds. A light source  12  is transformed by microoptics  14  into a virtual intermediate image  18 .  
         [0032]    Light beams  20  emanating from this virtual intermediate image  18  enter into monolith  40  and are projected at a first deflecting surface  51  onto a concave surface  42 . Light beams  20  then impinge on a reflecting surface  46 , on a convex surface  44 , once more on reflecting surface  46  and on concave surface  42 , to then leave monolith  40  through an exit window  34  and converge in an image point  28 . A symmetrically designed alternative optical imaging of a convex mirror and of a concave mirror is schematically shown in FIG. 6, deflecting elements being additionally used in prismatic form. Light beams  20 , emanating from virtual intermediate image  18  from light source  12  (not shown here), enter into a prismatic deflecting element  54 , off of whose base they are reflected, to then attain monolith  40 . A symmetrical optical path is provided. Light beams  20  first impinge upon a concave surface  42 , a reflecting surface  46 , a convex surface, and once again on reflecting surface  46  and on concave surface  42 . Likewise provided subsequently thereto is a prismatic deflecting element  54 , off of whose base, light beams  20  are totally internally reflected. The light converges in an image point  28 .  
         [0033]    [0033]FIG. 7 is a schematic representation of another monolithically designed, alternative optical system of a convex mirror and a concave mirror, including an additional convex sphere and a prism for coupling in the light to be imaged. Light  20  from a virtual intermediate image  18  of a light source (not shown here), together with microoptics, enters into a prism  58  and, from there, into a convex sphere  56 . In its surface, a region is provided, through which light beams  20  are able to enter, in the most reflection-free possible manner, into monolith  40 . Light beams  20  are reflected off of the numerous internal surfaces of the monolith. These internal surfaces include facet  60 , a concave surface  42 , a reflecting surface  46 , and a convex surface  44 . The optical path of light  20  is indicated up to image point  28 . The light is able to leave monolith  40  through an exit window  34 . Typically, convex surface  44  is reflecting, so that light is reflected inside monolith  40 .  
         [0034]    The device for recording images in accordance with the present invention may provide images at a form cylinder in a print unit. A cylinder of this kind may constitute part of a printing press, for example as a substitute for the form cylinder in a print unit of the printing press in U.S. Pat. No. 6,318,264, which is hereby incorporated by reference herein.  
                                             Reference Numeral List                                    10   optical system           12   light source           14   micro-optics           16   divergent light           18   virtual intermediate image           20   light beam           22   deflecting element           23   optical axis           24   concave mirror           25   center of curvature           26   convex mirror           27   conjugate point without deflecting element           28   image point           29   printing-form plane           30   beam-forming element           32   entrance window           33   encapsulation           34   exit window           36   projection of the focal line           38   projection of the light sources           40   monolith           41   axis of symmetry           42   concave surface           44   convex surface           46   reflecting surface           48   position of the convex mirror           50   light beams without reflecting surface           51   first deflecting surface           54   prismatic deflecting element           56   convex sphere           58   prism           60   facet