Abstract:
The present invention relates to a system for optically adjusting a computer-to-plate (CTP) imaging head which includes an a light source ( 104 ), optical fiber ( 124 ) for transmitting light emitted from the light source; an imaging lens ( 204 ) for focusing light emitted from a distal end of the optical fiber ( 128 ) on a substrate; wherein a portion of light striking the substrate is reflected back to the distal end of the optical fiber; a fiber optic coupler ( 116 ) in the optical fiber which transmits the reflected light to a light detector ( 112 ); wherein the light detector measures intensity of the reflected light; and a control unit configured to adjust the imaging head according to the intensity of the reflected light.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. Patent Application Ser. No. US2006615025A filed Dec. 22, 2006, and entitled DIRECT ENGRAVING OF FLEXOGRAPHIC PRINTING PLATES, by Siman Tov et al., the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     This present invention relates to a confocal fiber optic sensor configured to operate within imaging heads of computer-to-plate (CTP) machines. 
     BACKGROUND OF THE INVENTION 
     Optical heads for imaging emit a plurality of light spots on a light-sensitive medium, the optical imaging head may be configured from an array of pigtailed laser diodes. Each laser diode is optically coupled to a proximal tip of a multi-mode optical fiber. The distal tips of the optical fibers are supported in a linear array by opto mechanical means, and imaged onto a printing plate. 
     Different types of plates may be used in CTP machines. For example, for direct engraving of a flexography plate high power lasers are used. To achieve a good ablation of the imaging media, the optical head should be provided with features such as autofocus, thermal lens compensating and plate sensitivity sensor. 
     Such features become feasible by the present invention which offers a confocal fiber optic sensor configured to operate within the imaging heads of CTP machines. 
     The confocal technique is well known in the literature. The fundamental principles and advantages of confocal microscopy are described in U.S. Pat. No. 3,013,467 (Minsky et al.). 
     U.S. Pat. Nos. 6,310,710 and 6,466,352 describe a rotating reading and writing scan system that incorporates a rotatable confocal microscopy. The rotatable configuration is the essence of those inventions, which also utilize the well known confocal microscopy. The novelty of these patents is in the new rotatable configuration. The present invention addresses different features in the realm of CTP devices, and more specifically in the field of three dimensional direct engraving of flexographic plates. The application utilizes the confocal principle as well, but beyond that it also offers new configurations which are adapted to achieve numerous features required for CTP devices. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention is a system for optically adjusting a computer-to-plate (CTP) imaging head which includes an optical fiber for transmitting light emitted from the light source; an imaging lens for focusing light emitted from a distal end of the optical fiber on a substrate; wherein a portion of light striking the substrate is reflected back to the distal end of the optical fiber; a fiber optic coupler in the optical fiber which transmits the reflected light to a light detector; wherein the light detector measures intensity of the reflected light; and a control unit configured to adjust the imaging head according to the intensity of the reflected light. 
     The object of the present invention is to provide a confocal fiber optic sensor for imaging heads used for various types of CTP printing machines. The sensor provides new features that are essential in order to receive a good quality of the image on a plate. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a confocal sensor adjusted within an optical head; 
         FIG. 2  is an illustration of a confocal sensor adjusted aside to an optical head; 
         FIG. 3  is an illustration of confocal sensor used to measure the profile of a flexographic plate surface; 
         FIG. 4  is an illustration of a confocal sensor used to detect the edge of a plate; 
         FIG. 5A  is an illustration of two adjacent confocal sensors, utilizing optical fibers with different core diameters, operating simultaneously for expanding the dynamic range; 
         FIG. 5B  is an illustration of a response function showing the relation between the back reflected signal as a function of distance to target; 
         FIG. 6  is an illustration of two adjacent confocal sensors, utilizing imaging lenses with different NAs, operating simultaneously for expanding the dynamic range; and 
         FIG. 7  is an illustration showing detection of plate separation from a rotating drum. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a confocal sensor  100 . The confocal sensor  100  is integrated with a plurality of fiber coupled laser sources  120  into a mechanical assembly  152 . 
     The distal tip of the fiber coupled laser source  120  is imaged on substrate  148  by imaging lens  144  to form an image. 
     The sensor  100  comprised of a light source  104  coupled to optical fiber  124  and to fiber optic coupler  116 . Rays  136  emitted from the distal tip of optical fiber  128  are imaged on the surface of substrate  148 . The back reflected light  140  is coupled to the emitting optical fiber  128  and reaches detector  112  via coupler  116  and optical fiber  132 . The intensity measured by detector  112  is a function of the distance  160  to the printing plate. The position of the optical head  156  is adjusted according to the measured back reflected intensity levels, in order to achieve a predefined distance in respect to substrate  148 . The optical head  156  includes a joint structure of mechanical assembly  152  and imaging lens  144 . The focus can be achieved also by moving only the mechanical assembly  152 , while keeping the imaging lens  144  fixed in its original position. 
     This solution is advantageous due to the fact that the confocal sensor  100  which is used as an auto focus device is integrated in the optical imaging head. This configuration enables good compensation for defocusing that might be caused by thermal tensing or the movement of different elements within the lens. Thermal lensing or misalignment of different elements that construct the lens may be caused when a fraction of the power is absorbed by the different elements that construct the compound lens causing temperature gradients. 
     Confocal sensor  100  may be used to calibrate other range related sensors. Confocal sensor  100  and other types of range related sensor might coexist in certain configurations of a CTP device and may operate simultaneously. For example, such a calibration can be performed for an external laser range finder  168 . The external range finder  168  measures the range independent of the imaging lens  144 . The position of the optical head  156  is changed according to a signal received from the external laser range finder  168  in order to achieve a predefined distance in respect to substrate  148 . 
     The wavelength emitted by laser source  120  maybe dependent on its optical power. For example, for high power multi mode laser diodes that emit power of 10 Watts at 915 nm, a shift of, roughly, 1 nm per Watt is known. Since during operation the laser source  120  maybe operated at various optical powers, the confocal sensor  100  can be used to measure and compensate for defocusing that can be caused by the chromatic aberration of the imaging lens. 
       FIG. 2  shows confocal sensor  100 , used for auto focus purposes, operating independently and aside from the fiber optic mechanical assembly  152 , wherein both are parts of an imaging head. 
     Plate substrate  148  is often constructed of surface irregularities. The confocal sensor  100  is configured to scan the plate substrate  148  in advance of the actual imaging unit  200 . The readings of the irregular surface of plate substrate  148  are further communicated by feedback control unit  164  to imaging unit  200 . The imaging unit will adjust its position relative to substrate  148  irregularities prior to the actual imaging being performed. 
     Different types of flexography plates may be used for direct engraving. Furthermore, the sensitivity of these flexographic plates may change over time. Hence, there is a need to test the sensitivity of the plate prior to imaging. The following procedure is suggested. The lasers  120  are adjusted to a predefined power and a sample plate  148  is irradiated. The confocal fiber optic sensor is then used to measure the depth and shape of the imaged holes in plate substrate  148 . According to this measurement, the sensitivity of the plate substrate is calculated and the power of the lasers  120  is adjusted accordingly. 
     Light source  104  can be operated in two different modes. For example, if light source  104  is a multi-mode fiber-coupled laser diode, then in the first mode of operation, when operated at high power, it can be utilized to ablate and engrave the flexographic plate. In the second mode of operation, when the same light source  104  is operated at a relative low power, for example under the laser threshold, it can function as the light source of the confocal sensor. Operating light source  104  at a relative low power, under the laser threshold, may have some advantages. For example, in this mode of operation it will not activate the flexographic plate. Further more, the signal-to-noise ratio as measured by the detector  112  may be improved due to the shorter coherence length of the light emitted by  104 . 
     In another embodiment of this invention confocal sensor  100  is used for measuring the reflectivity of an irradiated object. The intensity of the back reflected light  140  may be calibrated first by irradiating an imaging object with a known reflectivity. Flexographic-imaging plates are characterized by a reflectivity value of about 10 percent. When a higher reflectivity is measured, it may indicate on a faulty plate or on a rotating drum with no plate attached to it. The confocal sensor  100  can be also used to scan and measure the reflectivity of the substrate in advance of the actual imaging unit. In situations where a high reflectivity is sensed, an interlock will be activated in order to prevent the diodes from emitting high powers. Such a procedure will save the diodes from high values of back-reflected light that can damage them. 
     In another embodiment of this invention, a confocal sensor  100  is used to measure the profile of an engraved surface area  304  of a flexographic plate, this is depicted in  FIG. 3 . 
     In another embodiment of this invention as is depicted in  FIG. 4 , a confocal sensor  100  is used to detect the edge of a plate  404 . The attached plate material  404  is smaller than substrate  408 . There is a need before the actual imaging to find the exact position where plate material  404  starts.  FIG. 4  shows a confocal sensor  100  emitting light on a plate material  404  attached to a plate substrate  408 . Sensor  100  is used here for detecting the exact edge of plate  404 . 
     In another embodiment of this invention, the resolution and dynamic range of the fiber optic confocal sensor  100  can be controlled by adjusting its optics.  FIG. 5A  describes a method for expanding the dynamic range by using two adjacent confocal sensors  100 . The first uses a fiber optic guide  536  with a large core diameter and, therefore, provides relatively a large dynamic range. The second sensor  100  uses a fiber optic guide  540  with a smaller core diameter and, therefore, provides a relatively small dynamic range but with a better resolution. The usage of two confocal sensors  100  enables a measurement in both a wide dynamic range and a high resolution.  FIG. 5B  describes a schematic of a response function where the back reflected signal  504  is a function of distance  508  to target substrate  148 . The response function is measured by scanning the distance to target and by simultaneously reading the values measured by detectors  112 . The large dynamic range is indicated by function  512  derived from fiber optic  536  measurements whereas the higher resolution is indicated by function  516  derived from fiber  540  measurements. 
     The expansion of the dynamic range can be also achieved by using two different imaging lenses in parallel, where each of the two imaging lenses provide different numerical aperture (N.A) at its output.  FIG. 6  depicts two confocal sensors  100 , each having an imaging lens with a different numerical aperture. First sensor  100  emits light via imaging lens  612  characterized by a narrow numerical aperture, and adjusted in front of the optical fiber  604 . A second sensor  100  emits light via optical fiber  608  and imaging lens  616 , that provides a wider numerical aperture and adjusted in front of the optical fiber  608 . 
     In another embodiment of this invention, the confocal sensor  100  can be used to detect separation of plate  404  from drum  704  as is depicted in  FIG. 7 . In CTP devices plate  404  is attached to a rotating drum  704 . Drum  704  rotates in high rates. The centrifugal force created by the rotation of drum  704  combined with a possible malfunction of the CTP device holding the plate  404  attached to the drum  704  may cause the separation of plate  404  from drum  704 . The confocal fiber optic sensor  100  can sense the separation of the plate from the drum and stop the drum rotation. 
     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 scope of the invention. 
     PARTS LIST 
     
         
           100  confocal sensor (system for adjusting output power) 
           104  light source 
           112  light detector 
           116  fiber optic coupler 
           120  laser source 
           124  optical fiber connecting light source to coupler 
           128  optical fiber emitting light on substrate 
           132  optical fiber connecting coupler to detector 
           136  emitted rays to substrate 
           140  back reflected rays from substrate 
           144  imaging lens 
           148  substrate 
           152  mechanical assembly 
           156  optical head 
           160  distance from lens to printing plate 
           164  feedback control unit 
           168  external laser range finder 
           200  imaging unit 
           204  imaging lens 
           208  imaging lens 
           304  example of an ablated flexographic plate surface area 
           404  plate 
           408  plate substrate 
           504  back reflected signal axis 
           508  distance to target axis 
           512  first dynamic range 
           516  second dynamic range 
           536  thick optical fiber 
           540  thin optical fiber 
           604  optical fiber 
           608  optical fiber 
           612  imaging lens characterized by a narrow numerical aperture 
           616  imaging lens characterized by a wider numerical aperture 
           704  rotating drum