Abstract:
An optical scanner assembly exposes an image on photosensitive media positioned on the internal surface of a drum platen. The assembly includes: a laser assembly for producing a laser beam representative of the image to be exposed on photosensitive media; a semi-circular flexible lens curved to the shape of the photosensitive media positioned on the internal surface of the drum platen, the lens having a plano-convex cylinder lens having a convex side facing the media; a laser beam scanner positioned between the laser assembly and the lens to scan the laser beam through the lens across the media in an image-wide pattern; and a baffle located between the scanner and the lens for extinguishing laser beam reflections from the convex side of the lens.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to internal drum scanner assemblies and laser imaging systems incorporating such scanner assemblies. In particular, the present invention relates to a baffle for extinguishing second surface reflections in an optical scanner assembly having a lens located between the scanner and photosensitive media. 
     BACKGROUND OF THE INVENTION 
     Laser imaging systems are commonly used to produce photographic images from digital image data generated by magnetic resonance (MR), computed tomography (CT) or other types of medical image scanners. Systems of this type typically include a continuous tone laser imager for exposing the image on photosensitive film, a film processor for developing the film, and control subsystems for coordinating the operation of the laser imager and the film processor. 
     The digital image data is a sequence of digital image values representative of the scanned image. Image processing electronics within the control subsystem processes the image data values to generate a sequence of digital laser drive values (i.e., exposure values), which are input to a laser scanner. The laser scanner is responsive to the digital laser drive values for scanning across the photosensitive film in a raster pattern for exposing the image on the film. 
     The continuous-tone images used in the medical imaging field have very stringent image-quality requirements. A laser imager printing onto transparency film exposes an image in a raster format, the line spacing of which must be controlled to better than one micrometer. In addition, the image must be uniformly exposed such that the observer cannot notice any artifacts. In the case of medical imaging, the observers are professional image analysts (e.g., radiologists). 
     Film exposure systems are used to provide exposure of the image on photosensitive film. Known film exposure systems include a linear translation system and a laser or optical scanning system. The laser scanning system includes a laser scanner with unique optical configurations (i.e., lenses and mirrors) for exposure of the image onto the film. The linear translation system provides for movement of the laser scanning system in a direction perpendicular to the scanning direction, such that a full image may be scanned on a piece of photosensitive film. 
     U.S. Pat. No. 5,883,658, issued Mar. 16, 1999, inventors Schubert et al. discloses an optical scanner assembly for exposing an image on a photosensitive media. The media is positioned on the internal surface of a drum platen. The optical scanner assembly includes a laser assembly for producing a laser beam representative of the image to be exposed on the photosensitive media, a scanner, and a long, flexible lens curved to the semi-circular shape of the media positioned on the internal surface of the drum platen, the lens being positioned between the scanner and the film. This scanner operates to scan the laser beam across the media in an image-wide pattern. The lens is plano-convex with the convex side facing the film. The lens is tilted from a perpendicular position relative to the laser beam axis. 
     It has been found that laser beam reflections from the highly divergent surfaces of the convex lens propagate backwards to the media on the opposite side causing undesirable exposures and creating visually objectionable image artifacts in the media. There is thus a need to eliminate these undesirable reflections. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a solution to the problems discussed above. 
     According to a feature of the present invention, there is provided an optical scanner assembly for exposing an image on photosensitive media positioned on the internal surface of a drum platen, the assembly comprising: a laser assembly for producing a laser beam representative of the image to be exposed on photosensitive media; a laser beam scanner positioned between said laser assembly and said media to scan said laser beam through the lens across the media in an image-wide pattern; and a baffle located between said scanner and said media for extinguishing laser beam reflections from said lens. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention has the following advantages. 
     1. Laser beam reflections in an internal drum laser scanner assembly are extinguished to eliminate undesirable exposures and visually objectionable image artifacts in exposed media. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic elevational view of a laser imaging system including the present invention. 
     FIG. 2 is a perspective view of an exemplary media exposure assembly incorporating the present invention. 
     FIG. 3 is a perspective view of an optical scanner assembly including the present invention. 
     FIGS. 4,  5  and  6  are diagrammatic views useful in explaining the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is an elevational diagram illustrating an exemplary embodiment of a laser imaging system  30  suitable for use in the medical imaging industry including a film exposure assembly having a laser scanning assembly in accordance with the present invention. The imaging system  30  includes a film supply mechanism  32 , a film exposure assembly  34 , a film processing station  36 , a film receiving area  38 , and a film transport system  40 . The film supply mechanism  32 , film exposure assembly  34 , film processing station  36 , and film transport system  40  are all located within an imaging system housing  42 . 
     Photosensitive film is stored within the film supply mechanism  32 . The film transport system  40  allows the photosensitive film to be moved between the film exposure assembly  34 , film processing station  36 , and the film receiving area  38 . The film transport system  40  may include a roller system (not shown) to aid in transporting the film along a film transport path, indicated by dashed line  44 . The direction of film transport along film transport path  44  is indicated by arrows  46 . In particular, the film supply mechanism  32  includes a mechanism for feeding a piece of film along film transport path  44  into the film exposure assembly  34  for exposing the desired image on the photosensitive film using a laser or optical scanner assembly. After exposure of the desired image on the photosensitive film, the photosensitive film is moved along the film transport path  44  to the film processing station  36 . The film processing station  36  develops the image on the photosensitive film. After film development, the photosensitive film is transported to the film receiving area  38 . 
     FIG. 2 depicts a type of a film exposure assembly referred to as an “internal drum” scanner configuration. Arrow  1  points to the mechanical structure inside of which is a section of a cylinder known as the platen. The platen holds the media being scanned in a cylindrical shape. Arrow  2  points to a motorized slow scan assembly. This assembly consists of a drive motor, flywheel assembly, precision guide rails, and cables. The assembly is used to translate the optical scanner assembly along the media. Arrow  3  points to a motorized optical scanner assembly. This assembly angularly sweeps a laser beam, in a radial direction, nearly perpendicular to the axis of the platen, at a high rate of speed. This assembly is sometime referred to as the “fast scan” assembly. This invention is more closely associated with item number  3 , the optical scanner (fast scan) assembly, and more particularly, an internal drum scan engine in which the optical scan angle approaches or exceeds 180 degrees. 
     FIG. 3 shows a modification of an internal drum scanner assembly as described in U.S. Pat. No. 5,883,658, issued Mar. 16, 1999, inventors Schubert and Li. Laser diode  4  emits a laser beam that is collimated by collimator lens  5 . The laser beam  6  is directed towards the first beam shaping lens L 1 . Beam shaping lens L 1  is tilted slightly such that the reflected component  7  can be used to provide a feedback signal to feedback sensor  8 . The transmitted portion of the laser beam  6  is further directed towards beam shaping lens L 2 . Beam shaping lens L 2  is tilted slightly such that the reflected component  9  can be directed to an absorption surface  10  thereby preventing undesirable effects. The transmitted portion of the laser beam  6  is then directed toward a fold mirror M 1 . Fold mirror M 1  reflects the laser beam toward a rotating or oscillating scanner mirror  11 . The direction of the reflected laser beam is slightly inclined to the plane of scan in order to prevent the fold mirror M 1  from obstructing the scanned laser beam  6 . Scanner mirror  11  is moved in an angular fashion by scanner motor  12 . The scanned laser beam  6  is then directed towards flexible lens L 3 . Lens L 3  is shown having a plano-convex portion facing the media It will be understood that any curvature, perpendicular to the plane of seam either convex or concave on any lens surface, can cause undesirable reflections which are mitigated by the present invention. The reflected component  13  of the scanned laser beam  6  is directed upwards toward an absorption surface  14  (the gist of this disclosure). Lastly, the transmitted portion of the scanned laser beam continues on its path toward the media  15 . 
     As shown in FIG. 4, a flat blackened cover  16 , functions as a light trap, to extinguish certain reflections from flexible lens L 3 . This functionality does exist for first surface reflections from flexible lens L 3  because the laser beam  6  possesses a relatively large F-number, the first surface of flexible lens L 3  possesses little or very weak curvature and the laser beam  6  is sufficiently inclined relative to the plane of the scanner motor  12 . However, the light trap functionality does not exist for second surface reflections. 
     As shown in FIG. 5, the blackened cover  16  was found not to be effective at extinguishing reflections from the second surface of flexible lens L 3 , due to the highly divergent nature of the reflected laser beam  13  caused by the strong curvature of this convex second surface. Reflections from this second surface were found to propagate backward, over the top of scanner mirror  11 , and pass through the flexible lens L 3  at approximately 180 degrees opposite the location of their initial incidence on flexible lens L 3 . The second surface reflections continued propagating on their path toward the media  15  causing undesirable exposures and creating visually objectionable image artifacts. 
     In addition, (depending upon the reflectivity characteristics of the media  15 , the apertures of the scan optics, the inclination angle of the scanned laser beam  6 , etc.) the potential may exist for energy reflected from the media  15  to pass back through the scan optics, exit at approximately 180 degrees away, and strike the media causing undesirable exposures and creating visually objectionable image artifacts. 
     According to the present invention as shown in FIG. 6, the solution to the problem at hand was to create a light trap/barrier that would extinguish the second surface reflections and at the same time shadow the aperture of flexible lens L 3  from these unwanted reflections. There are a number of geometrical shapes that may by used to create a light trap/barrier for this problem, but the functionality remains the same, extinguishing the reflections and shadow the lens aperture. The geometry is primarily driven by ease of manufacture and mounting robustness. In the present embodiment, we have chosen a circular cylindrical section geometry for absorption surface  14 . A simple vertical wall could have also been located over the scanner mirror  11  to create the light trap/barrier. In our chosen geometry, the circular cross-section is concentric with the scanner motor  12  and the flexible lens L 3 . Given the inclination angle of the scanned beam  6 , the effectiveness of this light trap/barrier is increased and mechanical tolerances are loosened by keeping its radius small and close to the scan mirror  11 . For example, if the absorption surface  14  possessed a radius approaching the bend radius of the flexible lens L 3 , the size and position of the cylindrical section would expose a major portion of the flexible lens in order to allow room for the scanned beam to pass by without obstruction. By keeping this radius small, the length of the cylindrical section can be increased to completely shadow the aperture of the flexible lens without obstructing the scanned beam  6 . The radius of the light trap/barrier must not be made excessively small. There is a limit. One must keep the radius large enough to accommodate the width and direction of the reflected beam  13  in the plane perpendicular to the page. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       1 , 2 , 3  arrows 
       4  laser diode 
       5  collimator lens 
       6  laser beam 
       7  reflected component 
       8  feedback sensor 
       9  reflected component 
       10  absorption surface 
       11  scanner mirror 
       12  scanner motor 
       13  reflected component 
       14  absorption surface 
       15  media 
       30  imaging system 
       32  film supply mechanism 
       34  film exposure assembly 
       36  film processing station 
       38  film receiving area 
       40  film transport system 
       42  imaging system housing 
       44  film transport path 
       46  arrows 
     L 1  shaping lens 
     L 3  beam shaping lens 
     L 3  flexible lens 
     M 1  fold mirror