Abstract:
The imaging debris produced by imaging a media with a higher power laser is prevented from accumulating on an optical imaging element by establishing a substantially non-turbulent fluid flow across the optical element. The non-turbulent flow forms a barrier between the optical element and the imaging environment such that debris is not able to reach a surface of the optical element during an imaging operation.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of applying images to imaging media. Specifically, the invention relates to methods and apparatus for preventing the build-up of debris on optical components in imaging systems. 
     BACKGROUND OF THE INVENTION 
     Laser imaging is well known. In a typical laser imaging process a laser-sensitive media is mounted onto the surface of an imaging cylinder and an image is imparted onto the media using a focussed write laser.  FIG. 1  shows a prior art external drum imaging system  1  having an imaging head  2  directing a laser beam or beams  18  towards a media  4 . The media  4  is rotated on a drum  5  while the imaging head  2  is translated along the drum by a lead screw  6 , thus scanning or writing a series of bands or a helical pattern around the drum. 
     Alternatively, the scanning motion can be generated using a flatbed imaging system where the media is held on a platen and relative motion is generated in two orthogonal axes between the media surface and the imaging beam. The imaging systems described are used in dedicated devices for imaging many different kinds of media including lithographic plates, flexographic plates, screens for screen printing, gravure cylinders as well as layers for flat panel displays, printed circuit boards and the like. Furthermore, the imaging system could be incorporated directly on a printing press for imaging plates in situ. Since such systems are well known in the art they will not be further discussed herein. 
     During imaging, the interaction of the laser and the media causes a physical and/or chemical change to the imaged areas of the media. In the process of imaging, matter may be expelled from the laser sensitive media. The expulsion of matter from the media is referred to as ablation. The matter expelled may include solids, liquids, gases, or plasma, or a combination thereof, more commonly referred to by the terms “smoke” or “particulate debris”. Ablative media are imaged by selectively dislodging or evaporating material from a layer of the media to form an image. While ablative media by nature produce ablation debris, media traditionally regarded as non-ablative can also produce some smoke fumes and/or particle debris, particularly when imaged by high power lasers such debris can also be termed “ablation debris”. 
     Ablation debris presents several difficulties, which may hamper the imaging process. A first problem is that the debris may obstruct the laser beam thus affecting the imaging of the media. Ablation debris can also resettle onto the media; this is known as redeposit. Redeposit is a particularly critical problem in imaging laser sensitive media, because redeposit can cause imaging artifacts that may be visible on the final product. Once redeposit has occurred it is difficult to remove without damaging the imaged media. A third problem associated with ablation debris is related to its tendency to accumulate in the sensitive areas of the imaging lasers and other areas of the imaging device. Accumulation of ablation debris can cause severe degradation and/or damage to the components in the imaging system, particularly the laser optics. For example, if a layer of debris collects on a lens, it may drastically affect the lens&#39; optical performance. Furthermore, the danger of ablation debris is not limited to optical degradation, since some media have partially conductive material compositions. Ablation debris from such materials can cause failures in electrical and electronic systems if it is released into the machine environment during imaging. 
     The escapement and subsequent accumulation of debris over a long time represents a maintenance cost related to cleaning the affected components. The issue is particularly relevant to the problem of build-up on the optical surfaces, since these components are delicate and difficult to clean and may require the dispatch of a specially trained service person to perform maintenance. Furthermore, as customers become increasingly demanding in respect of image quality, the tolerance for even slight degradation due to ablation debris on the optical elements is substantially reduced. If the time between cleaning of the optical components can be extended this represents a significant reduction in downtime and maintenance cost for the imaging system. 
     There is a need for better methods and apparatus for reducing the accumulation of ablation debris on optical elements in imaging devices. 
     SUMMARY OF THE INVENTION 
     An apparatus for maintaining the cleanliness of an optical element used in imaging a laser beam onto a sensitive media provides a substantially non-turbulent flow of fluid past the optical element. Fluid from a fluid source is channelled through a flow collimator that transforms a turbulent flow into a substantially non-turbulent flow. The non-turbulent flow is directed across the surface of the optical element, thus forming a barrier to debris accumulating on the optical surface. Advantageously, flow collimation is achieved by partitioning an orifice into a number of individual flow channels. Alternatively, the flow can be collimated by allowing it to flow along a straight passage, with optionally tapered walls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate non-limiting embodiments of the invention: 
         FIG. 1  is a depiction of a prior art external drum imaging system; 
         FIG. 2-A  is a top view of an imaging head showing the path of debris in an imaging operation; 
         FIG. 2-B  is a depiction of an embodiment of the present invention; 
         FIG. 3  is a depiction of an alternative embodiment of the present invention; and, 
         FIG. 4  is a graph showing the results of a test performed using the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     This invention is described in relation to an apparatus associated with an imaging head. The apparatus generates a substantially non-turbulent flow to form a barrier between an outermost optical element and the environment. The substantially non-turbulent flow is particularly effective since it will not draw debris particles toward the optical elements, as would a turbulent flow. The term “debris” or “imaging debris” is used herein to refer to gaseous and/or particulate matter generated in the imaging of a laser sensitive medium. 
     In a first embodiment shown in  FIG. 2-A , an imaging head  2  has optical element  12  and window  13  through which an imaging beam  18  is directed at a media  14  producing imaging debris  25 . The imaging beam  18 , through its interaction with the media  14 , creates debris  25 . It is desired to prevent debris  25  from being deposited onto window  13 . While it is common to use a laser source for producing the imaging beam, it is also possible to use non-laser sources in some instances. The way in which the imaging beam is generated is not important to this invention. Imaging head  2  is typically at least partially sealed due to the sensitive nature of the optical components therein. Commonly only a window  13  (which is itself an “optical element”) is directly exposed to the outside environment. Alternatively if a window is not used a final optical element  12  may be exposed to the outside environment. It should be noted that the exposed optical element could be any one of a number of commonly known optical elements or combinations thereof, including but not limited to lenses, mirrors, prisms, windows or window domes and crystal elements. Regardless of the exact nature of the exposed optical element, it will be at least somewhat sensitive to contamination by debris. 
     The exposed optical element may be flat or may be curved. However, the curvature of the exposed optical element should not be too great. If the exposed optical element is too sharply curved then it will cause turbulence in the fluid flowing past it. As discussed below, turbulence is undesirable. 
     A fluid flow indicated by arrowed lines  30  is established in close proximity to the exposed optical element to form a barrier between debris  25  and the surface of the exposed optical element (in this case, window  13 ). The fluid most commonly used is air, supplied from either a blower or a clean pressurised air source such as a compressor or any other source of pressurised air. If factory air is used, a filter should be included to remove particulate and other contaminants commonly present in factory compressed air supplies. The fluid could also comprise a specialized fluid like clean nitrogen, a mixture of several gaseous fluids or any ionized gas. 
     A vacuum sink  17  is optionally provided. Vacuum sink  17  is generally operative to remove debris from the vicinity of the laser beam  18 . Vacuum sink  17  is connected to a vacuum source (not shown). The sink functions to collect the debris  25  and will typically direct most of the debris in the direction of lines  38  towards the sink  17 . Vacuum sink  17  is advantageously situated and oriented such that it draws the fluid flow in a similar direction to the flow  30 . Vacuum sink  17  may also stabilize flow  30  and promote non-turbulent flow of the gas in flow  30 . 
     In the illustrated embodiment, a smooth protrusion  19  is located between vacuum sink  17  and window  13 . Fluid flow  30  is directed across window  13  toward protrusion  19 . Fluid flow  30  flows around protrusion  19  to reach vacuum sink  17 . Protrusion  19  is useful for maintaining flow  30  separate from the flow of gases which carry debris  25  from plate  14  toward vacuum sink  17 . 
     Preferably, fluid flow  30  is generated such that it is substantially non-turbulent. Fluid flow  30  may be, but is not necessarily, laminar flow. A turbulent fluid flow will tend to mix with debris-contaminated fluid drawing in the particulate and fumes that may deposit on the window  13 . In extreme cases where a turbulent flow is used the situation can actually be worse than if no flow at all is used, since particles that may not normally have deposited on window  13 , may be gathered by the turbulent flow and deposited on window  13 . 
     Referring now to  FIG. 2B  a flow-collimating element  32 , connected to a fluid source  36 , is used to promote a substantially non-turbulent flow across exposed optical element  13 . In this embodiment the collimation (which may also be called laminarization) is achieved by partitioning the cross section of the flow orifice into a plurality of small passages, each extending in the direction of the fluid flow so that there is sufficient interaction of the passage with the fluid to make that portion of the flow substantially non-turbulent at the exit to the orifice. The flow contributions from the plurality of passages combine to form a substantially non-turbulent flow across the outer optical element. 
     A convenient way to provide a plurality of passages is to insert a length of miniature honeycomb material into the passage near the desired exit. The honeycomb comprises a plurality of hexagonal tubes fabricated as a single piece and is available commercially as a structural material used in the aircraft industry. The tubes extend through the material making it very suitable for use a flow-through element. Honeycomb material with cell sizes as small as ⅛″ or even 1/32″ is commercially available. The length of honeycomb required in the direction of the flow is related to the diameter of the cell. The interaction length is preferably on the order of 5 times the cell diameter or more. For example, a length of 6 diameters may be used—giving an interaction length of about 5 mm for the 1/32″ cell size. The passages may be formed in any suitable manner including drilling, electroforming, or laser machining. 
     In an alternative embodiment, also shown in  FIG. 2-B , a fluid jet  39  is directed at or near the surface of media where the imaging beam impinges on the media. Jet  39  works in combination with the vacuum sink  17 , and directs debris away from the imaging area towards the vacuum orifice  35 . Although such a jet is effective in blowing away a significant portion of the debris it is not typically effective enough to keep an optical element clean for an extended period of time under normal imaging conditions. The non-turbulent flow across element  13  is still desired in order to extend the service time of the imaging system before the optics have to be replaced or cleaned. 
     Referring back to  FIG. 2A , to promote minimum turbulence in the fluid flow  30  exiting the flow collimator  32 , it is advantageous to distribute the flow evenly among the passages, such that there is a relatively constant fluid velocity at the point of recombination. One method of promoting even distribution of the fluid velocity among the collimator&#39;s passages is to incorporate a fluid reservoir or “plenum”  34  behind the flow collimator  32 . A fluid reservoir is a chamber of fluid where the pressure within the chamber is relatively uniform. In this case, if the collimator&#39;s passages are of similar shape and length, the velocity of the fluid within the collimator&#39;s passages will be substantially the same. 
     In an alternative embodiment shown in  FIG. 3 , an orifice  40  is fed via a long straight channel  42 . The length of the channel  42  in the direction of fluid flow is typically somewhat greater than at least one of the dimensions of the orifice itself. For example, if the orifice is a wide slit of dimensions 1 unit by 30 units, then the length of the straight channel upstream of the orifice is at least somewhat greater than 1 unit. By straight, it is meant that for at least portion of the channel just upstream of the orifice, the main direction of the channel is relatively unchanged. The channel  42  can be gently tapered as shown in  FIG. 3 , either inwards or outwards as the fluid travels towards the orifice. The degree of acceptable taper is such that there is little increase in turbulence due to the introduction of the taper. 
     In the embodiments described above the introduction of the substantially non-turbulent flow across a sensitive optical element was found to have a significant effect. 
     EXAMPLE 
     The apparatus shown in  FIG. 2-B  was installed on an imaging head manufactured by Creo Inc. of British Columbia, Canada. The imaging head has a 40W laser source and is able to image media at a high enough power density to generate ablation debris. The imaging head was mounted on a Trendsetter™ imaging engine manufactured by the same company and used to image ablative media while the flow rate was varied. The results are shown in the Graph of  FIG. 4 . The flow rate through the flow collimator is plotted on the x-axis while a measure of debris accumulation is plotted on the y-axis. The measure used for debris accumulation was to monitor the amount of light scattered from the window of the imaging head by directing an auxiliary laser beam onto the window and measuring the reflected light. When the window is clean, the light scattered is minimal, but as debris accumulates the scattered light increases in magnitude. The y-axis is plotted in arbitrary units while the units on the x-axis are cubic feet per hour (cu ft/h). 
     There are two plots on the Graph of  FIG. 4 , one with the vacuum sink turned on, and the other with the vacuum sink turned off. With the vacuum sink turned off, it can be seen that with no airflow the debris accumulation is relatively high while at a flow of between 40 and 60 cu ft/h the accumulation is substantially reduced. With the flow increased to over 100 cu ft/h, the debris accumulation is again increased. With the vacuum sink turned on the overall accumulations are substantially reduced, with a very low accumulation recorded at just under 40 cu ft/h. 
     In both cases, there is a distinct increase in debris accumulation when the flow rate is high. This is because the flow collimation works well at low and moderate flow rates but at high flow rates, turbulence is increased and particles are drawn into the flow and deposited on the window. 
     It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Those skilled in the art will appreciate that various modifications can be made to the embodiments discussed above without departing from the spirit of the present invention. For example, a means for ionizing the gas may optionally be provided in the fluid source and a shroud may optionally be provided around the exposed optical element.  FIG. 2A  shows schematically a means  100  for ionizing the gas. Additionally, the embodiments discussed above refer to a cylindrical imaging surface, but the invention should be understood to incorporate imaging processes on flat surfaces or other shapes of imaging surfaces.