Abstract:
An optical wedge compensates for a vertically drifting laser beam in response to ambient temperature fluctuations. A slit sensor is formed onto a surface of the wedge to separate a single light pulse from a plurality of light pulses. A reflective filter layer is applied to a side opposite the slit sensor so an incident laser beam is attenuated thereby precluding damage to the slit sensor. The reflective filter coating further filters out unwanted light energy. The wedge is shaped so as to maintain a position of a focal point of a laser beam on the slit sensor and to prevent reflected light from damaging a laser source.

Description:
BACKGROUND OF THE INVENTION 
     The invention generally relates to the field of imaging printing plates on a platesetter or imaging film on an imagesetter. 
     The process of transferring text and/or graphic information from electronic form to visual form on an imagable medium is called imaging. The information can be transferred to an imagable media using light such as produced by a laser beam or beams. The imagable media may be a printing plate or film that is sensitive to the wavelength of the laser beam(s) used to accomplish imaging. 
     The laser beam used to image a printing plate or film is often modulated with pulses. The laser beam must be calibrated to assure proper exposure of the imagable media is achieved. 
     SUMMARY OF THE INVENTION 
     A laser beam used to image a printing plate and/or film media is focused to a sharp point. This focal point is aligned on or with the surface of the media to be imaged. If the focal point of the laser beam falls short of the media surface, or is located beyond the media surface, the resulting image placed on the media is distorted. Consequently, proper positioning of the focal point of the laser beam or beams is crucial to producing a distortion free image on the media. 
     Preparation for imaging a printing plate or film includes calibration of the laser beam or beams. Various parameters of a laser beam (hereafter laser beam means a single beam OR may mean a plurality of laser beams) must be measured at precisely the focal point of the laser beam. This is often accomplished with a photo-detector or other light sensor. Often a single laser source or a plurality of laser sources are configured to provide a single line of laser light that is coupled to a modulating device such as a grating light valve (GLV). The GLV effectively creates a large number of individual light beams from the single laser light line. This is accomplished by manipulating small components inside the GLV so each individual light beam is turned on, and then off resulting in a pulse of light. Consequently, the original single line of laser light supplied to the GLV is transformed into a plurality of light pulses forming a train of pulses. 
     Characteristics of each pulse must be measured. Since each pulse must be measured, a slit sensor can be employed to separate or isolate a single pulse from the rest of the pulses. The slit sensor must be placed at precisely the focal point of the laser beam so as to measure the characteristics of the portion of the laser beam that actually strikes the surface of the imageble media. The slit sensor reflects all other pulses except a single pulse. This reflected light energy must not be directed back toward the laser source (or GLV) producing the pulsed laser beam as the laser source or GLV may be damaged. Tilting the slit sensor results in an offset between the focal point of the laser beam and the actual slit introducing an error in the measurement. The problem is exacerbated by vertical movement of the laser beam as various components expand or contract due to temperature fluctuations. 
     What is needed is a device that allows a single pulse to be isolated from a plurality of pulses at the focal plane of a laser beam without damaging the laser source or GLV. Further, the device must compensate for a vertically drifting laser beam so the focal point of the laser beam remains precisely aligned on the portion of the device that separates the pulses. 
     An object of the invention herein is to maintain a position of the focal point of a laser beam onto a slit aperture as said laser beams drifts vertically. 
     A further object of the invention herein is to attenuate the amplitude of the laser beam before said laser beam illuminates said slit aperture. 
     Another object of the invention herein is to redirect light away from a laser beam source that is reflected by a surface of the invention. 
     An object of the invention herein is to separate a single pulse of light from a plurality of light pulses modulated onto said laser beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description may be further understood with reference to the accompanying drawings in which: 
     FIG. 1 is an external drum imaging machine with various components showing the need for the invention herein. 
     FIG. 2 shows how a laser beam drifts as the components of FIG. 1 expand or contract and placement of a light detector forming a plane of focus. 
     FIG. 3 shows a slit sensor placed at a focal plane of the laser beam of FIG.  2 . 
     FIG. 4 is a front view of the slit sensor and laser beam of FIG.  3 . 
     FIGS. 5 a - 5   e  show laser pulses and the function of the slit sensor of FIG.  4 . 
     FIG. 6 shows the deviation of a portion of a slit sensor from the focal plane of FIG. 3 as the slit sensor is tilted. 
     FIG. 7 shows how a portion of the invention herein provides variable shifting of a focal point of the laser beam of FIG.  6 . 
     FIG. 8 shows how the optical wedge portion of the invention is formed from a slab of optical material. 
     FIGS. 9 a - 9   c  are alternate embodiments of the optical wedge portion of the invention herein. 
     FIG. 10 is a perspective view of an embodiment of the invention herein. 
     FIG. 11 is an alternate view of the invention of FIG. 11 showing all 3 layers of the invention. 
    
    
     The drawings are shown for illustrative purposes only, and are not to scale. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention herein is employed on an imaging machine  5  generally shown in FIG.  1 . Imaging machine  5  may be of the external drum type in which an imageble media is supported on the outside surface of a drum  10 . However, the invention is not limited to machines of the external drum type, and may be used with machines employing internal drum configurations (not shown) and/or with flatbed imagers. 
     Imaging machine  5  employs a laser source  40  producing at least one laser beam  45  which is separated into a plurality of laser beams and modulated to form pulses of light. The modulating apparatus is not shown, and only a single laser beam  45  will be used to describe the invention herein for simplicity. It is understood that the plurality of laser light pulses described infra may each represent one of the plurality of light beams produced by said GLV or the equivalent thereof as described supra. 
     Laser source  40  is moveably mounted on optical mounting apparatus generally shown at  25 . Apparatus  25  may include a carriage assembly  30  operative to move laser source  40  along a direction indicated by arrow  35 . Drum  10  is supported by frame  15  on a base  20  also used to support optical mounting apparatus  25 . Laser beam  45  is directed toward drum  10  such that a focal point of laser beam  45  is located at the surface of an imageable media (not shown) mounted on drum  10 . A focal plane  50  is shown in FIG. 1 representing the imageable surface of an imageable medium such as a printing plate or film. 
     Various components including  10 ,  15 ,  20 ,  25 , and  30  of imaging machine  5  expand and/or contract in response to ambient room temperature fluctuations causing laser beam  45  to move vertically as shown by arrow  55  relative to drum  10 . The vertical movement is best represented in FIG.  2 . Laser beam  45  moves vertically up and/or down as shown by  45 A,  45 B, and  45 C. Laser beam  45 A,  45 B and  45 C has a focal point shown by  60 A,  60 B, and  60 C respectively. Focal points  60 A,  60 B, and  60 C are shown aligned with, or positioned on, focal plane  50  that represents the surface of an imageable medium mounted on drum  10  as described supra. It is also understood that laser beam  45 A,  45 B and  45 C are pulsed, forming a light pulse as is described and shown infra. 
     Various parameters of laser beam  45  are measured using a light sensor  65  having a receiving area  70 . The light sensor may be a diode photo-detector, photo-transistor, photo-multiplier tube or any other type of light sensor. 
     Laser light beam  45  is sampled using a device called a slit sensor  75  which is positioned at the focal plane  50 , in front of light sensor  65  as shown in FIG.  3 . The slit sensor  75  and light sensor  65  are placed slightly away from an end of drum  10  (not shown). This is because it is impossible to place slit sensor  75  and light sensor  65  at focal plane  50 , and in front of drum  50  due to volume requirements. 
     Light beam  45  is scanned across slit sensor  75  as shown in FIG. 4 by arrow  95 . Laser lines  90 A,  90 B, and  90 C correspond to laser beam  45 A,  45 B and  45 C respectively and are another view of said beam as seen from A—A in FIG.  3 . The actual slit  85  in slit sensor  75  is shown in FIG.  4 . The shape of slit  85  may be other than what is shown depending on specific requirements. 
     Laser beam  45 A,  45 B and  45 C is modulated to form a plurality of pulses  100   a,    100   b,    100   c,    100   d,  and  100   e  shown in FIG. 5 a . If all of said pulses  100   a,    100   b,    100   c,    100   d,  and  100   e  were to illuminate light sensor  65  at once, the sensor would integrate the all pulses forming a complex waveform  105  similar to the waveform shown in FIG. 5 b . Slit sensor  75  is used to isolate or separate a single pulse  100   d  from the rest of the pulses as shown in FIGS. 5 c,    5   d,  and  5   e . Slit  85  is designed to have a width W such that only a single pulse is allowed to pass while blocking all other pulses. As light beam  45  is moved across slit sensor  75  as shown in FIGS. 4 and 5 d,  each individual pulse is allowed to pass through slit  85 , one at a time. This allows light sensor  65  to accurately measure each pulse separately without interference from any of the other pulses. 
     When laser beam  45  strikes slit sensor  75 , a portion of the light energy  80  is reflected as shown in FIG.  3 . If the reflected light energy  80  enters source  40  (or GLV modulator not shown) damage can occur to source  40 . A solution is to tilt slit sensor  75  away from the vertical (either direction is suitable) as shown in FIG.  6 . This re-directs reflected light energy  80  away from laser source  40  (or GLV or other modulator) preventing damage to source  40 . However, focal point  60 A is no longer positioned at or on slit sensor  75  as shown by  110  in FIG. 6, while focal point  60 C remains positioned on slit sensor  75 . The result is a measurement of light beam  45  is performed at other than the desired focal point  60 A. The invention herein solves this problem by use of a wedge shape piece of material  115  having an index of refraction as shown in FIG.  7 . As is well known in the art of optics, material having an index of refraction different than air shifts or moves a focal point of a light beam. The magnitude of the shift is dependent upon the thickness of the material and the value of the index of refraction (in addition to other variables). The value of the index of refraction to be used with the invention herein is dependent upon the scenario in which the invention is to be used, and is a design choice. Optical wedge  115  is used to significantly move focal point  60 A of laser beam  45 A while at the same time, keeping focal point  60 C of laser beam  45 C at very near the same position. This is because laser beam  45 A must travel a longer distance through optic wedge  115  than laser beam  45 C does. Laser beam  45 A travels through the thick portion of the optic wedge  115 , and laser beam  45 C travels through the thinnest portion of optic wedge  115  as shown in FIG.  7 . 
     Optic wedge  115 , having an index of refraction greater than air in the preferred embodiment, is placed so the thinnest portion of the wedge is at the focal point  60 C of laser beam  45 C which represents the lowest vertical position laser beam  45  can drift to. As laser beam  45  drifts upward, more material is introduced into the optic pathway of laser beam  45  causing focal point  60  to shift by a larger amount. This variable shift in focal point position, proportional to the thickness of the optic wedge  115 , re-aligns the loci of focal points  60 A,  60 B, and  60 C for laser beams  45 A,  45 B, and  45 C respectively, onto tilted slit sensor  75 , which is formed on the backside of optic wedge  115  as shown in FIG.  7 . 
     Optic wedge  115  may be formed from a slab of optical glass  120  as shown in FIG.  8 . The first step is to calculate the desired tilt angle  125  of the front surface  150  shown in FIG. 9 c  in order to achieve the desired angle of reflection to re-direct reflected energy  80  away from source  40  or any other component. Secondly, the plane of focus within the glass  120  created by the index of refraction is calculated yielding an angle  130  relative to the front surface  150 . The optic wedge is formed using angle  130  and angle  125 . 
     Depending on the design requirements, alternate embodiments of the optical wedge are possible as shown in FIGS. 9 a,    9   b,  and  9   c . One variable is angle  165  shown in three configurations, perpendicular to an adjacent side, less than 90 degrees to an adjacent side or greater than 90 degrees to an adjacent side respectively. Further surfaces  150  and/or  155  may be angled differently than as shown. For example, surface  150  in FIG. 9 a  is shown angled downward. Surface  150  may be angled upward. 
     A perspective view of optic wedge  115  is shown in FIG. 10 having slit  85  formed on one side. A feature of the invention herein is the relatively sharp edge  160  formed by two sides of optic wedge  115 . This provides a very short optic pathway for laser beam  45 . This short optic path in optic wedge  115  serves to shift a focal point  60 C of laser beam  45 C a very small amount as described supra. Optic wedges known in the art of optics do not have sharp edge  160 , but are truncated. 
     Slit sensor  75  is formed on a side  155  of optic wedge  115  as shown in FIG. 11. A coating  145 , opaque at least to the wavelength of source  40 , is applied to a surface  155  of optic wedge  115  that faces detector  65 . An aperture  85 , preferably slit shaped, is formed into opaque coating  145  by etching away some of coating  145 , or alternatively, by selectively applying coating  145  on areas of surface  155  other than in an area designated to form aperature  85 . The purpose of coating  145  and aperture  85  is to allow only a single pulse at a time to illuminate detector  65  as described supra. Coating  145  may comprise gold, silver, chrome, aluminum, tin, titanium, tungsten or any other suitable material which blocks light from the source. 
     Another feature of the invention herein is a reflective filter layer  140  applied to front surface  150  of optic wedge  115 . Reflective filter layer  140  attenuates the amplitude of laser beam  45  sufficiently so laser beam  45  does not damage opaque coating  145 . Reflective filter layer  140  also filters out any stray or unwanted light that may ultimately illuminate detector  65  causing an erroneous measurement of laser beam  45 . The type and thickness of the material used for reflective filter layer  140  depends upon the wavelength and power level of source  40 , (in addition to other design variables) and is a design choice dependent upon the particular scenario. Many current, well known optic coatings are suitable for this application such as described and sold by Coherent Corporation. 
     Though the invention herein is described with reference to imaging machines in the graphic arts industry, the invention is not restricted to use in the graphics industry and may be used wherever a laser beam needs to have an optic parameter measured. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.