Abstract:
Apparatus for exposing an image recording medium, the apparatus comprising a radiation source; a switch comprising an input arranged Lo receive radiation from the radiation source, and a plurality of imaging outputs, wherein the switch selectively routes the radiation received at the input to a selected one of the imaging outputs; and a device for directing the radiation from each imaging output onto the image recording medium to expose the image recording medium.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for exposing an image recording medium, such as a thermal printing plate. 
     DESCRIPTION OF THE PRIOR ART 
     FIG. 1 is a side view of a conventional single beam internal drum imagesetter. A laser  1  generates a laser beam  2  which is directed onto an angled reflective surface  3  of a spinning mirror  4 . The spinning mirror  4  is rotated by a motor  5  which is mounted on a carriage (not shown). The carriage (not shown) is driven parallel to the axis of a drum  7  by rotation of a lead screw  6 . Items  3 - 6  are housed inside the drum  7 . One or more image recording plates (not shown) are mounted on the inner surface of the drum  7 . To expose the image recording plates on the drum  7 , the motor  5  moves along the axis of the drum  7 , and rotates the spinning mirror  4  about the axis of the drum  7  whereby the reflected laser beam  8  exposes a series of circumferential scan lines. 
     As can be seen in FIG. 2, which is an end view of the apparatus shown in FIG. 1, during the lower 80° of its revolution, the reflected laser beam  8  is blocked by the carriage  136 . This creates a shadow area  9  which prevents the scanner from exposing a full 360° of the drum  7  and reduces the speed and efficiency of the system. The angle of the area outside the shadow area  9  is conventionally known as the “drum angle”. 
     A known way of improving on the efficiency and scanning time of the system of FIG. 1 is to add a second spinner and a second laser as illustrated in FIG.  3 . 
     FIG. 3 illustrates the lower half  10  of a cylindrical drum. A first mirror  11  and a second mirror  12  are mounted at 180° to each other on a common shaft  13  which is rotated by a motor (not shown). A first laser  14  is directed at the spinning mirror  11 , and a second laser  15  is directed at the spinning mirror  12 . The distance between the reflective surfaces of the spinning mirrors  11 , 12  is equal to half the length of the drum. The laser  14  directs image radiation to the mirror  11  during one half cycle to expose a line on the upper half of the drum. The laser  15 t directs image radiation to the mirror  12  during the next half cycle to expose another line on the upper half of the drum. The process continues until the right-hand spinner  12  has exposed the right-hand upper quarter of the drum, and the left-hand spinner  11  has exposed the left-hand upper quarter of the drum. Therefore the entire upper half of the drum can be exposed in half the time when compared with the system of FIG.  1 . In addition the overall efficiency is increased since the lower half of the drum (which includes the shadow area  9 ) is not exposed. 
     A problem associated with the system of FIG. 3 is that two lasers  14 , 15  are required. The cost of lasers can be very high. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is provided apparatus for exposing an image recording medium, the apparatus comprising a radiation source; a switch comprising an input arranged to receive radiation from the radiation source, and a plurality of imaging outputs, wherein the switch selectively routes the radiation received at the input to a selected one of the imaging outputs; and means for directing the radiation from each imaging output onto the image recording medium to expose the image recording medium. 
     In accordance with a second aspect of the present invention, there is provided a method of exposing an image recording medium, the method comprising generating radiation in a radiation source; inputting the radiation to a switch having a plurality of imaging outputs; routing the radiation during a first period to one or more selected ones of the imaging outputs; routing the radiation during a second period to one or more different selected ones of the imaging outputs; and exposing the image recording medium with radiation, from the or each selected imaging output. 
     The present invention provides a routing device which enables a single radiation source to be used in a scanner of the type illustrated in FIG.  3 . This results in a much simplified system with reduced cost. 
     The radiation which exposes the image recording medium is generally encoded with image information to expose a desired pattern of pixels. The radiation may be encoded downstream of the routing device, for instance with an acousto-optic modulator. Preferably however the radiation which is input to the routing device is already encoded, for instance by suitable control of the radiation source. Typically the radiation source inputs radiation in the form of a series of pulses to the routing device. This enables pixels to be exposed on the image recording medium with short, high power pulses, resulting in low thermal leakage. 
     In a preferred embodiment the radiation source comprises an optical amplifier having a pump energy source. The average power of the optical amplifier can then be conveniently adjusted by adjusting the power input by the pump energy source. The pump energy source may input electrical pump energy into the amplifier, but preferably the pump energy source comprises a radiation source such as an array of laser diodes. 
     The radiation source may be operated in a continuous wave mode as illustrated schematically in FIG. 4. A power source (not shown) provides a power signal on input line  16 . When switch  17  is closed the laser cavity  18  outputs a laser beam  19 . A problem with continuous wave mode is that the laser beam  19  cannot have a power any greater than the power on input line  16 . This is a particular problem in thermal printing imagesetters where high laser power may be required. 
     Therefore preferably the radiation source is operated in pulsed mode, as illustrated schematically in FIG.  5 . In this case a power source provides a power signal on input line  20  which is input continuously to the laser cavity  21 . The laser cavity  21  stores the energy from input line  20  until switch  22  is closed to release the energy in the form of a high power pulsed laser beam  23 . As a result, the power of the pulsed laser beam  23  can be higher than the power on input line  20 . This enables pixels to be exposed on the image recording medium with short, high power pulses, resulting in low thermal leakage. 
     An example of a suitable radiation source is shown in FIG.  6 . FIG. 6 illustrates a fibre amplifier of the type described in WO95/10868. The fibre amplifier comprises a fibre  30  having a Erbium-Ytterbium doped single-mode inner core  31  and a multi-mode concentric outer core  32 . A single mode seed laser  33  directs an encoded laser beam  34  into the inner core  31 . Pump radiation is provided by a pump source  35  (an array of multi-mode laser diodes) which is coupled, transversely with respect to the optical axis of the fibre  30 , to the outer core  32 . The method of coupling the pump source  35  to the fibre  30  is described in detail in WO96/20519. Pump radiation from the pump source  35  propagates through the outer core  32  and couples to the amplifying inner core  31 , and pumps the active material in the inner core  31 . Thus the fibre optic amplifier provides a highly amplified encoded output beam  36  at the wavelength of the encoded laser beam  34 . 
     The fibre optic amplifier illustrated in FIG. 6 is primarily designed for use in telecommunications in which the encoded input laser beam  34  will not be off for a significant length of time. If the seed laser  33  is off for an extended period, the fibre  30  continues to accumulate energy from the pump source  35 , and as a result the fibre  30  will go into spontaneous emission. This problem is common to all pulsed laser sources and as a result pulsed laser sources are generally not used in imaging applications where the laser may be off for an extended period of time. 
     In order to solve this problem, the apparatus preferably further comprising an energy dump; and means for directing the radiation from the radiation source either to the energy dump or to the image recording medium. This solves the spontaneous emission problem by providing an energy dump which is utilised to prevent excessive build up of energy in the radiation source. 
     The means for directing the radiation to the energy dump or the image recording means may comprise a switch. However it may be difficult for a conventional switch to operate at the switching frequency required. Therefore preferably the radiation source comprises a data radiation source and a dump radiation source which generate encoded radiation at respective different wavelengths, and an optical amplifier which amplifies the encoded radiation; and wherein the means for directing the radiation either to the energy dump or to the image recording medium comprises a filter which directs the amplified radiation to the image recording medium or to the energy dump in accordance with the wavelength of the amplified radiation. In this case the apparatus typically further comprises means for encoding the radiation from the dump radiation source whereby radiation is only generated by the dump radiation source when radiation is not being generated by the data radiation source. This increases efficiency and further reduces the risk of spontaneous emission. 
     The switch typically comprises an electro-optic switch, such as an integrated optic switch. Any suitable radiation source may be used, such as a continuous wave laser or a pulsed laser (for instance the laser of FIG.  6 ). 
     The radiation may be transmitted through air to the image recording medium, but preferably the means for directing the radiation from each imaging output onto the image recording medium comprises a plurality of fibre-optic cables, each coupled to a respective one of the imaging outputs. This arrangement improves coupling efficiency, reduces alignment problems, and makes the apparatus safer by confining the imaging radiation beams (which may have dangerously high power). Preferably the radiation source comprises a fibre laser which provides an output suitable for coupling to the fibre-optic cables. 
     The apparatus may be used in a conventional imagesetter. However it is particularly suited to a thermal imagesetter in which the radiation source generates radiation of a wavelength and power suitable for exposure of a thermal imaging plate. Suitable wavelengths are in the infra-red region. Typically the image recording medium has a media sensitivity of 50-200 mJcm −2 . Typically the average power delivered by the radiation source at the image recording medium is 2-10W (in the case where the image recording medium is exposed uniformly). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A number of examples of the present invention will now be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a side view of a conventional single mirror imagesetter; 
     FIG. 2 is an end view of the imagesetter of FIG. 1; 
     FIG. 3 is a side view of a double mirror imagesetter; 
     FIG. 4 is a schematic illustration of a continuous wave laser; 
     FIG. 5 is a schematic illustration of a pulsed laser; 
     FIG. 6 is a schematic illustration of a pulsed laser of the type described in WO95/10868 and WO96/20519; 
     FIG. 7 is a schematic side view of a double mirror imagesetter incorporating an example of apparatus according to the present invention; 
     FIG. 8 illustrates the surface of the drum shown in FIG. 7; 
     FIG. 9 is an example of the radiation source and control means of FIG. 7; and 
     FIG. 10 illustrates an encoding scheme for the system of FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to FIG. 7, an internal drum thermal imagesetter comprises a drum  50  carrying one or more thermal imaging plates (not shown) on its inner surface. Two spinning mirrors  51 , 52  are mounted at 180° to each other on a common shaft  45  which is rotated by a motor  46  on a carriage (not shown) which is driven by a lead screw  47 . An encoder  48  encodes the angular position of the shaft  45  to provide a series of pulses which are frequency multiplied by a desired factor to generate a clock signal  49  at a desired frequency (typically 20-120 MHz). A laser is schematically indicated at  53 , and has a pair of imaging outputs  54 , 55 . Radiation from the imaging output  54  is input to a first fibre optic cable  56  which is fixed at its far end to a first lens  57  which is fixed in relation to the spinning mirror  52 . Radiation from the imaging output  55  is input to a second fibre optic cable  58  which is fixed at its far end to a second lens  59  which is fixed in relation to the spinning mirror  51 . Control means schematically indicated at  159  controls the laser  53  such that encoded radiation is selectively directed to a selected one of the spinning mirrors  51 , 52 . 
     FIG. 8 is a flattened representation of the outer surface of the drum  50 . The shadow area  9  lies between 140° and 220° and the upper half of the drum lies between 270° and 90°. Four thermal imaging plates  60 - 63  are mounted on the upper half of the drum. The left-hand mirror  51  exposes plates  60  and  61  (in the upper left quarter  64  of the drum) with cyan and magenta image separations, and the right-hand mirror  52  exposes plates  62  and  63  (in the upper right quarter  65  of the drum) with yellow and black image separations. 
     FIG. 9 illustrates an example of the radiation source  53  and control means  159  indicated schematically in FIG.  7 . The radiation source  53  comprises an optical fibre laser amplifier of the type illustrated in FIG. 6 (like reference numerals being used for like components) and described in WO95/105868 and WO96/20519. A suitable radiation source is the IRE-Polus YLPM-Series Pulsed Yterrbium Doped Fibre laser. 
     A single data laser  80  directs an encoded beam into the inner core  31  of the fibre  30  under control of microprocessor  78 . Dump laser  81  directs an encoded beam at a different wavelength into the core  31 . The beam from dump laser  81  is encoded by microprocessor  78  such that the laser  81  is only on when the data laser  80  is off. 
     Wavelength filter  82  directs amplified signal from the data laser  80  to a switch  83 , and amplified signal from the dump laser to energy dump  72 . Microprocessor  78  operates the switch  83  such that radiation is directed along fibre-optic  56  for the first half of a revolution of the shaft  45 , (FIG. 7) and along fibre-optic  58  for the second half. Suitable switches  83  are the SM-TOS1.3.M.250 or SM-TOS1.5-M-250 single-mode thermo-optic switch modules distributed by Photonic Integration Research, Inc. 
     One example the data laser  80  emits radiation at 1010 nm, and the dump laser  81  emits radiation at 1020 nm. The pair of imaging outputs  54 , 55  (output  1  and output  2 ) are coupled to the fibre-optic cables  56 , 58 . The seed lasers  80 , 81  are low power single mode lasers. 
     The power of the pump laser diodes  35  can be selected in accordance with the desired power to be delivered on the film. The required power is determined by the media sensitivity (typically 50-200 mJcm −2 ), drum angle (typically 209 degrees), resolution (typically 48-144 lines/mm), film height (typically 930 mm), film width (typically 1130 mm) spinner speed (typically 30,000 RPM), and optics efficiency (typically 90%). As a result the power of the pump diodes is typically selected to give an output power of 3-10W. In the example of FIG. 9, the pump diodes  35  deliver 8W. 
     A first data store  90  contains binary image data to be recorded as a pattern of pixels on the upper left quarter of the drum  50  via first imaging output  54  (output  1 ). A second data store  91  contains binary image data to be recorded as a pattern of pixels on the upper right quarter of the drum via second imaging output  55  (output  2 ). The microprocessor  78  reads out the data from the stores  90 , 91  in response to the clock signal  49  from encoder  48 . The microprocessor  78  controls the lasers  80 , 81  and switch  83  as described in the example of FIG.  10 . 
     FIG. 10 illustrates the radiation output by imaging output  54  (output  1 ), imaging output  55  (output  2 ) and dump output  72 . The binary image data read out from data stores  90  (data  1 ) and  91  (data  2 ) are also shown, along with the clock signal  49  which has a clock period of 20 ns. 
     For the first halt revolution of shaft  45  (to tile l,it of line  110 ), mirror  52  (output  1 ) exposes a line on the upper right quarter  65  of the drum (FIG.  8 ). Only part of the line is illustrated in FIG.  10 . For the second half revolution of shaft  45  (to the right of line  110 ), mirror  51  (output  2 ) exposes a line on the upper left quarter  64  of the drum. 
     The microprocessor  78  controls the seed laser  80 , 81  such that a radiation pulse is output by the amplifier on each positive clock step. If data  1  is high, then a radiation pulse is output on the first output  54  to expose a single pixel. If data  2  is high, then a radiation pulse is output on the second output  55  to exposes a single pixel. If neither data lines are high, then a radiation pulse is output to energy dump  72 . Therefore the dump laser  81  is encoded as NOT(DATA 1  OR DATA 2 ). In the encoding scheme of FIG. 10 it can be seen that data  1  and data  2  are never high at the same time. 
     For example, at the first positive clock step  100 , neither data  1  nor data  2  are high. Therefore the microprocessor  78  causes the dump laser  81  to emit a 2 ns pulse which is, amplified to generate a 2 ns amplified radiation pulse  101  to be output to the energy dump  72 . 
     After a short time lag  140  (exaggerated in FIG. 10 for illustrative purposes) after the positive clock pulse  100 , the microprocessor receives a pulse  103  from store  90 . 
     Hence at the second positive clock step  102 , data  1  is high and the microprocessor  78  causes the data laser  80  to emit a 2 ns pulse which causes an amplified 2 ns radiation pulse  104  to be emitted from output  54 . 
     The duration of the pulses emitted by the seed lasers  80 , 81  can be adjusted by an RS 232 command before running an image. The pulse duration can be set equal to the clock period of 20 ns, resulting in a continuous wave mode in which the pulses  101 , 104  are not temporally separated, and in which radiation is continuously input to the filter  82 . However preferably the pulse duration is set to less than the 20 ns clock period (for instance 2 ns as shown in FIG.  10 ), resulting in a pulsed mode in which the pulses are temporally separated (in the example of FIG. 10 by 18 ns) and in which radiation is input as a series of pulses to the filter  82 . The total energy deposited over a 20 ns clock cycle is the same in both continuous and pulsed mode, and is set by the power of the pump diodes  35  (in this case 8W*20 ns=0.16 microjoules). However it is preferable to deposit this energy in a short time (eg. 1 or 2 ns) since this results in less thermal leakage. In addition the energy deposited on the film convolves less across the film when the pulse duration is short. 
     The imaging beam is switched between the fibre-optic cables  56 , 58  at  110  by a switching signal  111  from microprocessor  78 . 
     Other laser wavelengths and/or clock frequencies may be required for the system of FIG. 9 if dictated by the performance of the switch  83 .