Abstract:
Image scanning apparatus comprising a profiling surface; a feed mechanism for feeding a flexible radiation sensitive record medium across the profiling surface; a scanning system for scanning a modulated radiation beam across the record medium to expose the record medium; and a pressurizer operable simultaneously with the feed mechanism to apply a pressure difference across the record medium as it is fed across the profiling surface whereby the record medium engages the profiling surface and conforms with the shape of the profiling surface.

Description:
FIELD OF THE INVENTION 
     The present invention relates to image scanning apparatus and in particular to such apparatus comprising a profiling surface; a feed mechanism for feeding a flexible radiation sensitive record medium across the profiling surface; and a scanning system for scanning a modulated radiation beam across the record medium to expose the record medium. 
     DESCRIPTION OF THE PRIOR ART 
     A conventional internal drum imagesetter is illustrated in the schematic end view of FIG. 5. A drum  50  has a semi-cylindrical internal profiling surface  51 . A film  52  is mounted on the surface by attaching one end of the film to a loading carriage  53  which traverses round the drum. After the film has been loaded, it is exposed by a scanning radiation beam  54 . 
     The imagesetter of FIG. 5 suffers from the problem that the film  52  will not conform precisely with the profile of the surface  51 . Therefore the loading carriage will load more than the required length of film into the imagesetter. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is provided image scanning apparatus comprising a profiling surface; a feed mechanism for feeding a flexible radiation sensitive record medium across the profiling surface; a scanning system for scanning a modulated radiation beam across the record medium to expose the record medium; and a pressurizer operable simultaneously with the feed mechanism to generate a pressure difference between opposed sides of the record medium as it is fed across the profiling surface whereby the record medium engages the profiling surface and conforms with the shape of the profiling surface. 
     In accordance with a second aspect of the present invention there is provided a method of loading a flexible radiation sensitive record medium into an image scanning apparatus, the method comprising feeding a flexible radiation sensitive record medium across a profiling surface; generating a pressure difference between opposed sides of the record medium as it is fed across the profiling surface whereby the record medium engages the profiling surface and conforms with the shape of the profiling surface; and scanning a modulated radiation beam across the record medium to expose the record medium. 
     By applying a pressure difference to the record medium during the loading operation we ensure that the record medium conforms to the profiling surface during loading, and as a result the correct length of record medium is loaded into the apparatus. 
     The conventional imagesetter of FIG. 5 also suffers from the problem of dust particles—ie. dust particles can fall directly onto the film  52  and will either adhere to the film or fall down to the lower region  55  of the imagesetter. Dust particles can also collect on the surface  51  when a film is not present. The loading arrangement of the present invention enables the profiling surface to be oriented in alternative ways to reduce the problems causes by dust particles. For example the profiling surface can be oriented such that the normal to the profiling surface does not point directly upwards at any point. In addition it will be noted that the conventional imagesetter of FIG. 5 is oriented such that the normal to the surface  51  points upwards in the range of angles 0°-90°, and 270°-360° (with gravity g pointing directly downwards at 180°). As a result the surface  51  provides a reaction force to the gravitational force of the film  52  at all points (except at the extreme edges). In contrast, the normal to the profiling surface in the present invention can point in any direction, including downwards, ie. in the range of angles 90°-270° (in which the surface provides no reaction force and the record medium is supported by the pressure difference). 
     In other words, compared to the orientation of FIG. 5, the profiling surface can be oriented on its side (or even upside down) to prevent dust from falling or collecting on the profiling surface or the record medium. Similarly, a flat-bed scanner with a planar surface can be oriented with its planar profiling surface at an angle, or even upside down. 
     The pressurizer may generate a vacuum on one side of the record medium. However preferably the pressurizer increases the pressure on one side of the record medium. 
     Typically the pressurizer comprises a pressure chamber defined by a plurality of walls including the profiling surface, and means for increasing the pressure in the pressure chamber. The use of a pressure chamber ensures a relatively uniform pressure and also reduces the power requirements. 
     Typically the pressure chamber has one or more openings (e.g., slots) adjacent the profiling surface. This enables gas to exit from the pressure chamber in a controlled manner. By positioning the opening(s) adjacent the profiling surface we ensure that any gas flow acts to force the record medium against the profiling surface. 
     The profiling surface may be planar but in a preferred embodiment the profiling surface is curved, eg semi-cylindrical. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An example of a system incorporating apparatus according to the present invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 shows the main components of an image processing system; 
     FIG. 2 is a cross-sectional view of an internal drum imagesetter; 
     FIG. 3 is a plan view of the baffle assembly and a film being loaded with the drum omitted; 
     FIG. 4 is a side view of the baffle assembly with the drum omitted; and 
     FIG. 5 is a schematic end view of a conventional internal drum imagesetter. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In the image processing system of FIG. 1, an original image  1  (such as a continuous tone color print or transparency) is scanned into an input scanner  2  which generates a set of greyscale image files  3 . The image files  3  are input to an imagesetter  4 . The imagesetter  4  converts the greyscale image files  3  into bit map form and prints a set of film separations  5  in accordance with the calculated bit maps. 
     The separations  5  are mounted on a film processor  6  which generates a set of printing plates  7 . The printing plates  7  are then mounted on a printer  8  which produces a color print  9 . 
     The imagesetter  4  is illustrated in detail in FIGS. 2-4. The imagesetter  4  is an “internal drum” type imagesetter with a drum  60  with cylindrical inner profiling surface  10  and a support surface  61  for supporting the drum  60  on a table in the orientation shown. 
     A film sheet  12  is fed into an input slot  13  from a storage cassette (not shown) by a pair of feed rollers  14 , 15 . The film sheet  12  is then fed across the surface  10  (as indicated at  26  in FIG. 3) until it reaches an output slot  16 . A baffle assembly  17  is mounted on a carriage (not shown) which is mounted on a friction drive system (also not shown) such as a lead screw extending along the length of the drum  60 . Also mounted to the carriage is a spinner  18  which directs a radiation beam  19  through a slit  21  in the baffle  17  (shown in FIGS. 3 and 4 only) to a focus point on the film  12 . As the spinner  18  rotates as indicated at  20 , the beam  19  traverses the film in a circumferential direction. At the same time the carriage is driven along the length of the imagesetter as indicated at  25  in FIGS. 3 and 4, causing the beam  19  to expose a helical series of scanlines on the film  12 . The beam  19  is modulated with image information in a conventional manner. 
     The baffle assembly  17  performs two functions. Firstly the baffle has a pair of black vanes  30 , 31  which extend close to the film  12  and act to enclose the radiation beam in the slit  21 . The vanes  30 , 31  absorb light reflected from the film  12  and thus prevent pre-exposure of the film  12  in regions outside the focus spot of the radiation beam  19 . Secondly the baffle assists feeding of the film into the imagesetter as discussed below. 
     A pair of fans  22 , 23  are mounted to the baffle in the region of the input slot  13 . Each fan comprises six angled fan blades, one of which is indicated at  27  in FIG.  3 . As the fans  22 , 23  rotate, they each draw air from outside the imagesetter and into a respective pressure chamber  28 , 29 . The pressure chamber  28  is defined by four walls, namely the profiling surface  10 , the vane  31 , a side wall  32  and a front wall  33  (with a curved profile as shown in FIG.  2 ). Similarly the pressure chamber  29  is defined by four walls, namely the profiling surface  10 , the vane  30 , a side wall  34  and a front wall  35  (which has the same profile as the front wall  33  shown in FIG.  2 ). A 5 mm gap  36  is provided between the vanes  30 , 31  and the profiling surface  10  and between the side walls  32 , 34  and the profiling surface  10 . In addition twelve to fifteen parallel vacuum grooves are provided in the profiling surface  10  running in the feed direction of the film. The base  62  of one of the grooves is shown in dotted line in FIG.  2 . The vacuum grooves are approximately 1 mm wide and rectangular in cross-section. A vacuum is applied to the grooves via a vacuum port  63  leading to a vacuum source (not shown). Therefore the pressure chambers  28 , 29  are each sealed apart from a pair of 5 mm slots on each side and the vacuum grooves in the surface  10 . When the fans  22 , 23  rotate, they each draw air into their respective pressure chamber  28 , 29 . The air exits through the 5 mm slots at a lower rate, so initially the pressure in the chambers  28 , 29  rises. Eventually an equilibrium situation is reached in which the pressure in the chambers  28 , 29  has risen to approximately 25 Pa and air exits from the 5 mm slots at the same rate as it is being drawn in by the fans  22 , 23 . 
     When equilibrium has been reached, the feed rollers  14 , 15  are turned on to feed the film  12  into the imagesetter. The pressure difference between the two opposed sides of the film  12  forces the film against the profiling surface  10 . Force is also applied to the film by the action of the air flowing through the 5 mm slots adjacent the surface  10 . 
     When the film  12  reaches the output slot  16  the feed rollers  14 , 15  are stopped, a vacuum is applied to the film  12  via the vacuum grooves, and the fans  22 , 23  are turned off to prevent vibration during exposure. The film  12  is then exposed, and after exposure the vacuum is turned off and the feed rollers  14 , 15  and fans  22 , 23  are turned on to feed the exposed length of film out of the image setter. As the exposed 900 mm length of film is fed out as indicated at  64 , it is guided by a pair of stainless steel guides  65 , 66  between a pair of output rollers  67 , 68 . The upper output roller  67  is mounted on a pivoting arm  69 . As the film is fed out of the imagesetter the arm  69  is held in its upper position (shown in dotted line). As the leading edge of the film reaches the roller  68 , the arm  69  is pivoted down as indicated at  70  to grip the film between the rollers  67 , 68 . The rollers  67 , 68  are then rotated to feed the film out of the imagesetter. The film rolls into a scroll  71 . When the exposed length of film has been unloaded, a cutter  39  (FIG. 2) cuts the film  12 . The feed rollers  14 , 15  are reversed to draw the unexposed film out of the imagesetter. The output rollers  67 , 68  are rotated until the rear cut edge of the film reaches an output slot  72  formed by a pair of stainless steel guides  73 , 74 . The rollers  67 , 68  are then reversed to feed the exposed length of film out of the slot  72  to the film processor  6  (as indicated at  75 ). 
     Alternatively the cutter  39  may be omitted and the film may be stored as a continuous length in a film cassette, such as the cassette described in EP-A-0856769. 
     The surface  10  is oriented in an unconventional way as shown in FIG.  2 . With the gravitational field vector g pointing directly downwards as shown in FIG. 2 it can be seen that the normal to the surface  10  (ie. a line extending away from the surface  10  to the centre of curvature  11  of the surface) does not point directly upwards at any point. As a result, dust particles falling on the film will not settle on the film. In addition the film is shielded from falling dust particles. However, because the profiling surface  10  does not provide any support to the film  12  in the upper region  40  of the imagesetter (in which the normal to the surface  10  points downwards), there is a risk of the film buckling and falling downwards away from the surface  10  in this region. The imagesetter is designed to avoid this problem in three ways. 
     Firstly, the fans  22 , 23  are mounted adjacent the upper region  40 . This results in a slightly higher pressure in the upper region  40  which ensures that sufficient upwards force is applied to the film as it passes round the top part of the profiling surface  10 . Secondly, the fans  22 , 23  are mounted adjacent the input slot  13 . This results in a slightly higher pressure adjacent the input slot  13  which ensures that the film is immediately forced to conform to the curved profile of the surface  10 . Thirdly, the baffle assembly  17  is shaped to provide increased force in the upper region as discussed below. 
     As shown in FIG. 3, the side walls  32 , 34  of the baffle  17  are stepped outwardly at  37 , 38 , resulting in an greater width of 500 mm in the upper region  40  compared to the width of 360 mm at the output slot  16 . The support surface  10  extends along a length of over 1130 mm so that the imagesetter can be used to expose a variety of film widths including 560 mm, 760 mm and 1130 mm. The greater area of the pressure chambers  28 , 29  in the upper region  40  results in a greater total force being applied to the film at this point, ensuring that the film  12  does not collapse downwards. 
     By mounting the fans  22 , 23  at the input end of the imagesetter, the general direction of the flow of air serves to assist in feeding the film into the imagesetter. 
     In the lower region  41  of the imagesetter the surface  10  provides a significant support force to the film. In addition the film has already been fully conformed to the a curved surface  10  and so it does not need to be forced against the surface at this point. Therefore the pressure chambers taper to an edge at  41  and no pressure is applied to the film in the lower region  41  of the imagesetter. 
     The front walls  33 , 35  are profiled as shown in FIG. 2 to provide a smooth taper in the pressure chambers  28 , 29 , from a region of maximum breadth  43  adjacent the fans  22 , 23 , to the edge  41 . This results in a smooth flow of air and ensures a relatively constant air pressure throughout the pressure chambers  28 , 29 .