Patent Publication Number: US-6700598-B1

Title: Digital imaging system employing non-coherent light source

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Provisional Application No. 60/199,531 filed Apr. 25, 2000 for “Digital Imaging System Employing Non-Coherent Light Source” by F. Hull. 
    
    
     INCORPORATION BY REFERENCE 
     The aforementioned Provisional Application No. 60/199,532, U.S. application Ser. No. 09/505,017 for “Digital Imaging System Utilizing Modulated Broadband Light” filed Feb. 16, 2000 by F. Hull, and U.S. application Ser. No. 09/484,405 for “Digital Imaging System Utilizing Modulated Broadband Light” filed Jan. 14, 2000 by F. Hull are hereby incorporated by reference in their entirety and made a part of this application. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a high resolution digital imaging device employing a non-coherent light source that is selectively modulated to create the individual pixels of the image to be recorded. The invention is particularly useful in the graphic arts industry for exposing a variety of light-sensitizable media such as printing plates, proofing materials, relief plates and the like. 
     In the graphic arts industry, high resolution images are formed by exposing a light-sensitizable medium such as a printing plate with an appropriate light pattern. Traditionally, the printing plates were covered with a patterned film and exposed by broadband light to create the desired image on the plate. The broadband light was utilized at low energy levels, such that the film intermediary was required to properly expose the plate. In this use the low level of energy strikes the entire area to be exposed for a fairly long time, 10 seconds to as much as 20 minutes, depending on the consumable. 
     More recently, methods that do not utilize film intermediaries have been developed. Such methods include utilizing digital laser imaging at much higher levels of energy. However, such laser devices employed with these methods are quite expensive due to the relatively high costs of lasers and of the special plates that operate with the laser imaging device, and the laser devices also operate at relatively low speeds. Attempts have also been made to implement printing devices capable of utilizing a broadband, non-coherent light source, with addressing of image pixels being accomplished by reflective spatial light modulators. 
     These devices have proved to be impracticable, in part due to the inability of the reflective spatial light modulators to withstand the required exposure to intense ultra-violet light, which rapidly breaks down the movable micro-mirrors of ferro-electric liquid of the modulators. The alternative is to employ a very fast and expensive media. There are a few varieties of modulators such as prism-type electro-optics, piezoelectric Kerr-cell and bi-morph piezoelectric combs that are capable of withstanding such energy. These modulators, however, are all based upon a line array as opposed to an area array. There are several patents, including U.S. Pat. No. 5,033,814 issued to Brown et al., that suggest the concept of using a DC short arc lamp, condensers and reflectors to illuminate a round bundle of fibers at one end, while the opposite end of the bundle of fibers are assembled in a straight line. There are several problems with this approach including light transmission efficiency, polishing of fibers and expense. 
     Additionally, the ultra-violet (actinic) radiation required to expose such graphic arts media has a wavelength in the range of 330 to 430 nano-meters, which further increases costs and reduces the overall efficiency when compared to the Brown et al. patent, which was not designed to deliver large amounts of power at these wavelengths. 
     It would therefore be a significant improvement in the art to provide a durable, high resolution digital imaging system utilizing a non-coherent light source, operable at high speeds with conventional printing plates. The requirements of such a light would include but not be limited to, the following: a stable illumination without an appreciable flicker; a nearly instant “on”, minimizing a warm-up period; a high ratio of actinic radiation to total radiation; minimizing a “ripple” component, caused by an AC power supply, interfering with modulation; evenly distributing illumination intensity over a specific area; providing an output spot nearly matching a modulator shape to eliminate the need for large fiber arrays; and providing a smaller and more uniform divergence angle in the illumination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a light source assembly of the present invention. 
     FIG. 2 is a diagram of a non-coherent light source of the present invention. 
     FIG. 3 is a block diagram of exemplary circuitry for driving the non-coherent light source of the present invention. 
     FIG. 4 is an exemplary diagram of the light source assembly used in a first embodiment of the present invention. 
     FIG. 5 is a exemplary diagram of the light source assembly used in a second embodiment of the present invention. 
     FIG. 6 is a perspective view of a bi-morph comb array of the second embodiment of the present invention. 
     FIG. 7 is a perspective view of a bi-morph comb array of the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A non-coherent light source assembly of the present invention is indicated generally at  10  in FIG.  1 . The light source assembly  10  comprises a glass ellipsoidal trough reflector  12  and a light source  14 . Light source  14  comprises a non-coherent, ultra-violet transmissive fused silica cooling tube  16 , capillary lamp  18 , and mercury plasma  20  within capillary lamp  18 . Each component of light source  14  is coaxially aligned and positioned proximate to glass ellipsoidal trough reflector  12  or similar apparatus for focusing non-coherent light in a selected direction. Preferably, an inside surface  22  of glass ellipsoidal trough reflector  12  is coated with an ultra-violet reflective dichroic coating, which reflects only actinic radiation. Thus, only actinic radiation is focused by glass ellipsoidal trough reflector  12 , while unwanted white light and infrared radiation pass there-through. 
     FIG. 2 is a diagram illustrating the construction of an exemplary non-coherent light source  14  in more detail. Capillary lamp  18  is a 5000 Watt high pressure plasma capillary light source. Capillary lamp  18  is water-cooled, as is known in the art, with a water inlet and water outlet provided as shown in FIG.  2 . However, it would also be within the scope of the present invention to air-cool capillary lamp  18 . The dimension of the capillary lamp  18  is approximately 150 mm in length and 7 mm in diameter. Plasma capillary  20  within capillary lamp  18  is about 125 mm in length and 0.1 mm in diameter. This style of illumination is referred to as a wall stabilized arc. Stabilization is created by the extremely high temperature gradient between capillary wall  24  on outside surface  26  and the plasma within a middle portion  28  of capillary  20 . 
     In contrast to spherical short arc lamps used in the prior art, the plasma is only a few millimeters in length, primarily spherical and is stabilized by cathodes. The wall stabilized plasma does not emit electrons from the cathodes as the short arc lamp but rather from a mercury pool located at each end. The capillary  20  can be run on high voltage, low current DC to avoid the severe ignition electro-motive pulse of short arc lamps and to remove an AC ripple. High accuracy in the trough reflector  12  combined with a small diameter make it possible to achieve a very tight focus line of approximately about 2 mm at a focal point or fold mirror position. 
     The actinic output (330-430 nm wavelength) of light source  14  is approximately 7% of the total power of the capillary lamp  18 , or about 350 watts. The combination of mirrored cooling tube  16  and ellipsoidal trough reflector  12  collects nearly 70% of the output power and focuses the power within a tight 2 mm by 125 mm stripe of light. This configuration provides a total energy density (power divided by stripe area) in the actinic 330-430 nm wavelength of about 1 W/mm 2 . 
     FIG. 3 is a block diagram of exemplary circuitry for driving non-coherent light source  14 . Power, preferably in the form of an AC 220 volt, 40 ampere signal, is supplied by AC mains, and input to AC/DC converter  36 . AC/DC converter  36  rectifies the AC input signal and transforms the signal to the appropriate level for operating light source  14 . The transformed and rectified signal is passed on to DC filter  38  and DC regulator  40 , which operate to smooth the signal from a rectified sine wave to a more nearly constant DC value. This operation reduces any “ripple” effects or resultant beat frequency patterns in the signal, which would otherwise experience fluctuations that could affect the performance of the light source, which desirably produces an output having a constant intensity based on a constant input signal thereto. 
     FIG. 4 is an exemplary diagram of a first embodiment of a digital imaging system of the present invention. Light source assembly  10  is used in conjunction with modulating system  42  and output system  44 . The modulating  42  system comprises parabolic reflector  46 , polarizer  48 , aperture  50 , modulator array  52 , driving circuit  54  and encoder  56 . The output system  44  comprises focusing light  58 , focusing lens  60 , light-sensitive medium  62 , drum  64  and output fiber assembly  70 . 
     In operation of the first embodiment of the present invention, actinic radiation from light source assembly  10  is focused onto parabolic reflector  46 , which is coated to reflect light only in the wavelengths of interest, such as in the 330-430 nanometer (nm) range. Light reflected by parabolic reflector  46  is convergent in two planes, and is directed through optional polarizer  48 , which passes only light waves that arc aligned with the polarization angle of polarizer  48 . The optionally polarized light impinges on aperture  50 , which serves to trim the polarized light to match the input dimensions of modulator array  52 , with the non-coherent light preferably being shaped to slightly over-flood the aperture  50 . Thus, polarized light passes through aperture  50  as a beam having a width equal to the length of modulator array  52  and having a height equal to the total height of the modulators in modulator array  52 , and the divergence of the light is low, preferably less than about 1.5 degrees. The light is thereby apportioned into the plurality of individually controllable modulators contained in modulator array  52 . Alternatively, it is within the scope of this invention to include a light-guide (not shown), as is known in the art, to position the modulator  52  further from the light source assembly  10  in the instances where temperature and heat from the light assembly  10  adversely affects modulator  52 . 
     Digital information  66  in the form of a binary image file is utilized by modulator driving circuit  54  to control modulator array  52  and thereby create a selected output light pattern for imaging. The position of drum  64  is provided to modulator driving circuit  54  by encoder  56  in order to synchronize the release of data to modulator array  52  with the position of drum  64 , so that the modulation pattern is correlated to the imaging row on light-sensitive medium  62  mounted to drum  64 . Output fiber assembly  70  is connected to receive and further transmit the light output from modulator array  52 . Each modulator in modulator array  52  refracts or bends the light according to the control signal applied thereto by modulator driving circuit  54 . For a first state of the modulator control signal, the light is refracted by an individual modulator at such an angle that the light is not transmitted through fiber assembly  70 , but instead passes through the fiber wall and is absorbed and dissipated by the cladding and/or jacket of the fiber therein. For a second state of the modulator control signal, the light is merely passed through the individual modulator, with no more than negligible refraction, and is emitted from an end of fiber assembly  70 . 
     Alternatively, where the divergence of the light is sufficiently consistent, such that the fibers are not required to integrate the power distribution and divergence angles of the light on the light-sensitizable medium  62 , fiber assembly  70  may be omitted and a stop plate employed instead, with each modulator being configured so that light is either passed from the modulator or refracted to impinge upon the stop plate. In this manner, light beams are switched “on” (passed through the modulator) or “off” (refracted by the modulator so as to be absorbed and/or dissipated at the output fiber) to create a modulation pattern for imaging on printing plate  62  mounted on drum  64 . Final focusing lens  60  is provided to focus the modulated light output from fiber assembly  70  onto light-sensitizable medium  62  for proper exposure thereof. 
     Focusing lens  60  may be of the relay type, the telecentric type, or the anamorphic type, depending on the imaging requirements as is known in the art. By rotating drum  64  a full revolution, and advancing modulator array  52  horizontally to cover the entire width of medium  62  (with at least one revolution of drum  64  for each position of modulator array  52 ), the entire expanse of medium  62  can be imaged quickly, and with high resolution. In one embodiment, the horizontal advancement of modulator array  52  can be controlled such that there is partial overlap between horizontal positions, so that the intensity of the output from modulator array  52  can be reduced while still achieving full exposure of medium  62 . 
     FIG. 5 is an exemplary diagram of a second embodiment of the present invention. Light source assembly  10  is used in conjunction with a bi-morph comb array  72  and focusing lens  74 . Bi-morph comb array  72  is comprised of plurality of bi-morph fingers  76 , as is well known in the art, each of which comprises two sheets of piezoelectric polymers with opposite polarities glued together, forming a bending element, or bi-morph. FIG. 6 shows bi-morph comb array  74  in more detail. When a voltage is applied to the bi-morph finger  76 , one of the layers elongates, whereas the other contracts, causing the finger  76  to fold or bend. When a voltage with reverse polarity is applied, the bi-morph finger is folded in the opposite direction. Two bi-morph combs,  78  and  79 , comprise the array  72  and are positioned at an approximate right angle in relation to each other. Each bi-morph comb,  78  or  79 , comprises a group of bi-morph fingers  76  positioned longitudinally adjacent to one another, as is illustrated in FIGS. 6 and 7. The two bi-morphs combs,  78  and  79 , of array  72  are positioned in relation to each other such that proximate ends  80  of each finger  76  of one group are abuttably adjacent to a corresponding finger  76  in the other group, with each finger  76  being positioned somewhat off-set from the adjacent finger  76  of the adjacent comb. 
     Referring now to FIGS. 6 and 7, bi-morph comb array  72  is positioned such that the proximate ends  80  of each bi-morph comb,  78  and  79 , are positioned at or near a focal point of imager  10 . Thus, in the second embodiment of the present invention, the fingers  76  of bi-morph comb array  72  intersect rays emitted from imager  10 . Digital information  82  (FIG. 5) in the form of a binary image file is utilized by circuit  84  (FIG. 5) to control the voltages applied to bi-morph comb array  72 . The fingers  76  provide for the obstruction or transmission of the rays to an output system  86  (FIG.  5 ), such as an output screen or a bundle of fiber optics. 
     In either aforementioned embodiments, non-coherent light source  14  must satisfy a number of requirements for imager  10  to perform effectively. Light source  14  must provide constant and stable illumination, since any flickering could potentially cause an error in the pixel pattern imaged by the device, creating an improperly exposed pixel due to light source flicker when a “light” (exposed) pixel is desired. Light source  14  also must have a high energy density and provide light having a high degree of collimation. High energy density is required to achieve sufficient power in the light source output at the wavelengths of interest (330-430 nm) within a specified area. so that a light-sensitizable material is effectively exposed by the imaging system. 
     Finally, a light source  14  must provide light with an intensity sufficient to expose a light-sensitizable medium for imaging, which requires about 0.035 Watts per beam for typical offset printing plates based on a specification of 0.3 Joules per square centimeter at an exposing rate of 30 square centimeters per second. For an exemplary embodiment utilizing an array of 256 modulators to define a line of 256 pixels, about 9 Watts of power are required at the output of the imaging system in the wavelengths of interest (330-430 nm). The specification for typical flexographic printing plates is similar, with 3-5 Joules per square centimeter being required to expose the plate at a rate of 6-7 square centimeters per second. This specification yields a power requirement of about 0.08 Watts per beam, or about 20 Watts of total power provided to a 256-modulator array. 
     It should be understood that although the present invention has been described with reference to a standard rotating drum imaging system and a bi-morph comb array, the principles of the present invention are equally applicable to other types of imaging systems, such as systems that linearly translate the pre-sensitized medium and systems that employs a reciprocating, scanning imaging head to traverse and expose the medium. Other potential applications of the invention in a number of imaging applications will be apparent to those skilled in the art, and are within the province of the present invention. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.