Abstract:
A printhead ( 10 ) forms an image onto a photosensitive medium ( 22 ) by exposing pixels in a succession of exposures. The printhead ( 10 ) has a housing with a first position for an illumination array ( 20 ) of LED emitters ( 16 ), a second position for a lens array of lens elements ( 28 ) and a light-guiding array ( 26 ) of uniformizer elements ( 34 ), arranged within a corresponding array of cavities extended between the first and second positions ( 32, 46 ). For each pixel exposed on the photosensitive medium ( 22 ), a single LED light emitter ( 16 ) in the illumination array ( 20 ) provides light into a single corresponding uniformizer element ( 34 ) in the light-guiding array ( 26 ), which directs light to a single corresponding lens assembly ( 38 ) of the lens array ( 28 ).

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to printing apparatus and more particularly relates to a printhead having multiple channels using light emitting diode light sources.  
       BACKGROUND OF THE INVENTION  
       [0002]     Light emitting diode (LED) light sources offer a range of advantages over conventional types of illumination for printing apparatus. Among salient advantages of LEDs are low energy requirements, long life, relatively low cost, component durability and resistance to shock and vibration, and very good color performance and power output levels. LED arrays provide a compact packaging arrangement that makes these light sources particularly attractive for use in high-resolution printing applications.  
         [0003]     LED arrays have been widely used in electrophotographic printing. For example, a typical non-contact LED array image printer is disclosed in U.S. Pat. No. 4,837,589, which discloses an LED array mounted on a substrate bearing an interface control circuit that receives image data through a ribbon cable. The LED array is imaged by a lens onto an exposure plane on a platen parallel to the direction of scanning. A photosensitive medium is driven in registration in forward and reverse directions biased against the exposure platen which defines the exposure plane. Other examples in which an LED array is used within a scanned printing arrangement include U.S. Pat. No. 4,837,587, which discloses a printing apparatus that employs a bank of LEDs and commonly-assigned U.S. Pat. No. 6,163,332.  
         [0004]     LED arrays have been used to provide exposure energy applied to an intermediate drum or platen in some types of toner-based electrophotographic systems. Typical electrophotographic system designs employing an LED array use selfoc lenses or other lens array structures to provide 1:1 imaging, with minimal distance between the LED source and the imaged medium. Efficient capture of the LED light energy is not important with these devices; in general, only a portion of the light emitted from the LED array is needed for electrophotographic imaging.  
         [0005]     However, although LED arrays have provided suitable exposure energy for toner-based electrophotographic systems, output characteristics of the emitted LED light constrain these devices from use with photosensitive film media. Among the limited uses of LED arrays proposed for photofinishing is the printing of metadata characters on the edge of a piece of film outside the image area, as disclosed in U.S. Pat. No. 6,429,924.  
         [0006]     Thus, although there are a number of features of LED arrays that make these components attractive for use in printing images onto photosensitive media, there are still significant barriers to the use of LED arrays in photofinishing and related imaging applications. LEDs behave as extended Lambertian sources, radiating light over a broad range of angles, making it difficult to effectively capture much of their radiated power. Using LED sources for high-resolution imaging applications with photosensitive film and other media requires optics having very high numerical aperture. Additional difficulty is presented by the spatial energy profile of the LED itself. Referring to  FIG. 1   a , there is shown a spatial energy profile curve  12  for a typical LED emitter. An ideal energy profile curve  14  for photographic quality printing is represented in phantom, with the smooth appearance shown. As is apparent, spatial energy profile curve  12  contains both high- energy density points, or “hot spots”, and nearby areas of very low energy density. The jagged, irregular exposure energy profile of spatial energy profile curve  12  is thus not well-suited to the characteristics of a photosensitive medium such as photographic film. Moreover, hot spot distribution can vary from one LED to the next.  
         [0007]     A top view of the light-providing surface of the emissive area of a single LED emitter  16  in a typical configuration is shown in  FIG. 1   b . As is intuitively clear, the shape of the emissive area of LED emitter  16  is related to its spatial energy profile curve  12 . A typical hot spot  18  is shown in phantom in the view of  FIG. 1   b.    
         [0008]     In addition to Lambertian characteristics and irregular spatial energy profile, other practical problems have constrained the usability of LED arrays in printing to photosensitive media. For example, there are limits to the number of LED emitters  16  that can be packaged into an array for a single printhead. For a photofinishing application, for example, packaging requirements make it difficult to provide a printhead that is sized to provide a complete 8×10 or 14×17 inch print in a single exposure.  
         [0009]     A major challenge in forming high-resolution images on photosensitive media is achieving precision placement of exposed dots or pixels. As is well known in the electronic imaging arts, imaging artifacts on a printed medium are most easily perceived when they occur within a certain range of spatial frequencies. Even a slight variation in distance between adjacent LED channels of a fraction of a percent of the correct spacing can result in banding or streaking, objectionable in a printed image. Therefore, a scanning optical printhead used for this purpose must be designed to within sub-micron tolerances in order to obtain acceptable levels of precision.  
         [0010]     Thus, because of a variety of performance and packaging problems and because of the need for high precision dot or pixel placement, LED die arrays have been overlooked or dismissed as being unsuitable for high-resolution printing to photosensitive media. However, there is a long-felt need to develop low-cost printing apparatus that take advantage of the cost and efficiency of LED die arrays and the image quality capabilities of photosensitive media.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of the present invention to provide a printhead using an LED die array and a printing apparatus using the printhead. With this object in mind, the present invention provides a multichannel printhead for forming an image onto a photosensitive medium by exposing pixels in a succession of exposures, the printhead comprising: 
        (a) an illumination array of LED light sources fitted into a housing at a first position;     (b) a lens array comprising a plurality of lenses fitted into the housing at a second position; and     (c) a light-guiding array of unifornizer elements, arranged within a corresponding array of cavities formed within the housing and extended between the first position and the second position;     wherein, for each pixel exposed on the photosensitive medium: 
            a single LED light source in the illumination array provides light into a single corresponding uniformizer element in the light-guiding array which directs light to a corresponding lens of the lens array.    
               
 
         [0017]     It is a feature of the present invention that it provides a printhead for scanning a photosensitive medium to expose an image thereon, the image formed as a series of scanned lines of pixels. The printhead of the present invention is designed to provide precision accuracy of dot placement, with LEDs, lenses, and uniformizers fabricated to very close tolerances.  
         [0018]     It is an advantage of the present invention that it provides a low-cost printhead solution for exposure of photosensitive media.  
         [0019]     It is a further advantage of the present invention that it adapts LED illumination to the characteristics needed for exposure of a photosensitive medium. The printhead of the present invention thereby takes advantage of the long life, relatively low cost, small size and low energy requirements of LED die array illumination.  
         [0020]     It is yet a further advantage of the present invention that it provides a relatively low-cost lens array having a high numerical aperture for directing light energy from each LED die array onto a photosensitive medium.  
         [0021]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a 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  
       [0022]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0023]      FIG. 1   a  is a graph showing typical spatial profile of illumination intensity from a miniature LED;  
         [0024]      FIG. 1   b  is a plane top view of a single miniature LED, such as would generate light having the overall spatial profile of  FIG. 1   a;    
         [0025]      FIG. 2  is a plane top view of a printhead according to the present invention;  
         [0026]      FIG. 3  is a side view of a printhead according to the present invention;  
         [0027]      FIG. 4  is an exploded, perspective view showing key assemblies of a printhead according to the present invention;  
         [0028]      FIG. 5  is a plan view of an LED die array used in the printhead of the present invention;  
         [0029]      FIG. 6  is a cutaway view of a uniformizer in the printhead of the present invention;  
         [0030]      FIG. 7  is a ray diagram showing an arrangement of lenslets used in a preferred embodiment of the present invention;  
         [0031]      FIG. 8  is a perspective view showing the arrangement of a printing apparatus using the printhead according to the present invention; and  
         [0032]      FIG. 9  is a perspective view showing an alternate arrangement of a printing apparatus using the printhead according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0034]     Referring to  FIG. 2 , there is shown a printhead  10  using an LED die array  20  for forming an image onto a photosensitive medium  22 . Printhead  10  mounts, onto a single base  24 , LED die array  20 , a uniformizer array  26  and a lenslet array  28 . A signal interface  30  routes the electronic signal for energizing each LED emitter  16  in LED die array  20 . Connections to signal interface from driver circuitry (not shown) carry signals to energize individual LED emitters  16  in LED die array  20 .  
         [0035]     Referring to the side view of  FIG. 3 , there is shown the arrangement of LED die array  20 , fitted into a slot  32 , uniformizer array  26  (shown in phantom) formed as a series of grooves, and lenslet array  28  as fitted onto a seat  46  of base  24 . This arrangement provides a single, robust package for printhead  10 .  
         [0036]     Referring to the exploded view of  FIG. 4 , there are shown key components of printhead  10 . Each LED emitter  16  in LED die array  20  has a corresponding uniformizer  34  for uniformizing the light output of LED emitter  16  and providing this uniformized light to a lens assembly  38  in lenslet array  28 . A cover  36  is provided to complete the assembly of printhead  10 . Cover  36  is also used to form uniformizer array  26  as is described subsequently.  
         [0037]     In a preferred embodiment, printhead  10  is fabricated on base  24  that is, in turn, fabricated using precision assembly techniques, such as Silicon Optical Bench (SiOB) methods, widely used for photonic components packaging, for example.  
         [0000]     LED Die Array  20   
         [0038]     Referring to  FIG. 5 , there is shown a plan view of LED die array  20 . Here, LED emitters  16  are spaced apart by a center-to-center pitch P. In a preferred embodiment, center-to-center pitch P is typically about 325 μm and LED emitters  16  have dimensions of about 260×315 μm and emit light at  450  nm (nominal). LED die arrays  20  providing components and layout of this type are available from various sources, including AXT Inc., Fremont, Calif., for example. Not shown in  FIG. 5  are supporting wire trace connections made to individual LED emitters  16 .  
         [0000]     Uniformizer Array  26   
         [0039]     Referring back to  FIG. 2 , uniformizer array  26  is used both to direct light from each LED emitter  16  to its corresponding lens assembly  38  in lenslet array  28  and to smooth out the energy profile of LED emitters  16 , with a single uniformizer  34  used for each LED emitter  16 . Referring to  FIG. 6 , there is shown a cross-sectional view of a pair of uniformizers  34  according to one embodiment of the present invention. In this embodiment, uniformizer  34  is formed by applying a reflective coating to surfaces of sides  40  on both base  24  and cover  36 . In this four-sided embodiment, angles A are preferably about 90 degrees, so that the overall cross-sectional shape of uniformizer  34  is square. Other cross-sectional shapes are possible, including hexagonal shapes, where cover  36  and base  24  would each have three sides  40 , for example. With this arrangement, uniformizer  34  is essentially formed using a hollow cavity with reflective sides  40 . The hollow cavity is formed when cover  36  and base  24  are joined, allowing straightforward fabrication and allowing the working length of uniformizer  34  to be optimized to meet performance requirements. As a general principle, the greater the length of uniformizer  34 , the more uniform is the light output. A number of alternative types of uniformizer components could be employed, including optical fibers, for example. Any of a number of different types of reflective coating could be applied to sides  40  of uniformizer  34 . Cover  36  and/or base  24  could alternately be formed from a reflective material, eliminating the need for any reflective coating.  
         [0000]     Lenslet Array  28   
         [0040]     Referring back to  FIG. 2 , lenslet array  28  directs the uniformized exposure light that is provided from uniformizers  34  onto photosensitive medium  22 . In order to obtain the high numerical aperture needed for collecting sufficient exposure energy with small lenses, some type of aspheric surface is generally required for lens assembly  38 . Manufacturability is a key concern when using miniature aspheric surfaces, such as those that would be required when using a single lens for lens assembly  38 . Fabrication techniques such as gray scale etching technology, used by MEMS Optical, Inc., Huntsville, Ala., allow highly accurate microlens designs, but have constraints on allowable sag. In the preferred embodiment, as shown in  FIG. 7 , lens assembly  38  is a compound lens, with lens elements  42  and  44  having aspheric surfaces that allow microlens fabrication using gray scale etching technology or alternate techniques such as deposition using shadow-mask lithography, as described in U.S. Pat. No. 5,882,468. Each lens assembly  38  is fabricated from two precision-aligned microlens arrays in this embodiment, one array providing lens element  42 , the other providing lens element  44 . For realistic fabrication using gray scale etching, lens elements  42  and  44  should have sag of less than 40 microns. To provide increased light-gathering capability with constrained surface sag, lens elements  42  and  44  are fabricated from material having a high refractive index, zinc sulfide (n=2.46) in a preferred embodiment. In general, a refractive index above 2.0 would be desirable. The combination of surface and material characteristics of the preferred embodiment provides an optical design with lenslets having a maximum sag of 40 microns or less. In a preferred embodiment, magnification of 0.5× is provided by lens assembly  38 .  
         [0041]     Significantly, each of the three major components of printhead  10  is fabricated using precision techniques, using tools such as lithographic masking, that provide highly accurate component dimensions. Fabricated in this manner, LED die array  20 , uniformizer array  26 , and lenslet array  28  can then be mated together with precision during assembly of printhead  10 . As a result, printhead  10  can be manufactured both inexpensively and to within very tight tolerances.  
         [0000]     Apparatus Using Printhead  10   
         [0042]     Referring to  FIG. 8 , there is shown one embodiment of a printing apparatus  50  using printhead  10  of the present invention. Image data is provided to a control logic processor  52  and then provided to printhead  10  for imaging onto photosensitive medium  22 . A media transport  56 , in communication with control logic processor  52 , translates photosensitive medium  22  in a scan direction M relative to printhead  10 . Media transport  56  may include one or more motors for driving one or more rotating drums, drive rollers, platens, or other mechanisms for moving photosensitive medium  22  in a controlled manner, as is well known in the printing arts. A head transport  58  is configured to move printhead  10  across photosensitive medium  22  in a scan direction H that is orthogonal to scan direction M of photosensitive medium  22 . Head transport  58  may use any of a number of mechanisms for providing printhead  10  movement, such as using a movable belt  54 , for example. Successive passes of printhead  10  across the surface of photosensitive medium  22  expose two-dimensional images onto photosensitive medium  22 .  
         [0043]     Referring to  FIG. 9 , there is shown an alternate embodiment of printing apparatus  50  using printhead  10  of the present invention. Here, media transport  56  moves photosensitive medium  22  past printhead  10  in the scan direction D, such as using a drum or other mechanism well known in the printing arts. Head transport  58  moves printhead  10  in scan direction C, orthogonal to direction D, using a lead screw  60  or other drive mechanism. With this arrangement, by moving printhead  10  continuously during imaging, printing apparatus  50  can expose a full, two-dimensional image onto photosensitive medium  22  as one continuous swath, imaged in a spiral pattern.  
         [0044]     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 as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, while LED die arrays are described as illumination sources for printhead  10 , other types of light source arrays could be used, with accompanying changes to system optics, as needed. Different types of LEDs and light emitting components are possible, including various types of Organic LEDs (OLEDs and PLEDs), and other components. All LED emitters  16  in LED die array  20  could have the same wavelength or an arrangement of LED emitters  16  having two or more wavelengths could be used, allowing scanning of printhead  10  to expose the same area of photosensitive medium  22  with a series of different wavelengths, to provide a full-color image, for example.  
         [0045]     Thus, what is provided is an apparatus and method for printing onto a photosensitive medium using an array of LED light sources.  
       Parts List  
       [0000]    
       
           10  printhead  
           12  spatial energy profile curve  
           14  ideal energy profile curve  
           16  LED emitter  
           18  hot spot  
           20  LED die array  
           22  photosensitive medium  
           24  base  
           26  uniformizer array  
           28  lenslet array  
           30  signal interface  
           32  slot  
           34  uniformizer  
           36  cover  
           38  lens assembly  
           40  side  
           42  lens element  
           44  lens element  
           46  seat  
           50  printing apparatus  
           52  control logic processor  
           54  belt  
           56  media transport  
           58  head transport  
           60  lead screw