BACKGROUND OF THE INVENTION 
     This invention relates to a printhead for a printing engine, such as a xerographic printing engine, having printing elements arranged in a plurality of arrays and, more particularly, to a printhead with separately energizable parallel arrays of light emitting elements positioned for illumination of a common region of image space. 
     Xerographic print engines are constructed, typically, with a drum of photosensitive material providing a photoreceptor surface for receipt of a latent image, the drum being operated in conjunction with a developer that converts the latent image to a printable image by use of electrostatic charges for securing toner particles to the photoreceptor surface at the latent image. The latent image is produced by a printhead having sources of light, such as a single line of light-emitting diodes (LEDs) serving as points of an object to be imaged, and an elongated optical focussing element which focuses the line of LEDs upon the photoreceptor surface to produce the latent image. 
     Due to the construction of printheads with a single line of LEDs, a faulty diode introduces a noticeable pattern in the printed image outputted by the print engine, which pattern manifests itself as a streak or line which is disturbing to a person viewing the printed image. Furthermore, it is recognized that inputted data to the engine, from which data the latent image is created, may be for a relatively low or a relatively high resolution image, yet the engine is capable of printing only at the higher value of resolution. 
     SUMMARY OF THE INVENTION 
     The aforementioned disadvantages are overcome and other benefits are provided by a printhead constructed of plural rows of light-emitting print elements in accordance with a first aspect of the invention, and a xerographic print engine operative with the printhead in accordance with a further aspect of the invention, wherein, in the printhead, the plural rows are located side by side within an object plane of a focussing element capable of concurrently focussing the light from the plural rows of printing elements to generate a row of image points in a latent image on a photoreceptor of the engine. 
     The print engine comprises a photoreceptor with an image receiving surface, and a developer for converting a latent image produced on the receiving surface to a printable image to be transferred from the photoreceptor to a print medium. The printhead directs light to the photoreceptor to produce the aforementioned latent image, and a printing controller imparts relative motion between the photoreceptor and the print medium to print the printable image on the medium. The print controller includes imaging electronics for applying imaging data to the printhead for generation of the latent image. 
     The printhead generates a set of points of the latent image, the latent image being composed of rows of the image points. The printhead is constructed with a substrate extending in a direction parallel to a row of the latent image, and includes an arrangement of light-emitting printing elements disposed in plural arrays on the substrate. The plural arrays of the printing elements extend in a direction parallel to the row of the latent image. A first of the plural arrays is located alongside a second of the plural arrays. Also included in the printhead is driver circuitry that connects with the imaging electronics, is disposed on the substrate on both sides of the arrangement of printing elements, and drives individual ones of the printing elements in accordance with commands from the imaging electronics to emit light for imprinting points of the latent image on the image receiving surface. 
     The printhead includes, furthermore, an optical element of elongated shape for focussing light of the printing elements to form the row of the latent image. The focussing is accomplished concurrently for individual ones of the printing elements located in each of the first and the second arrays of the printing elements. In a preferred embodiment of the invention, each of the first and the second arrays comprises a single row of the printing elements. 
     In the driver circuitry of the printhead, a first portion of the driver circuitry comprises an arrangement of plural rows of printing-element drivers and plural rows of wire-bonding pads. The plural rows of printing-element drivers are interconnected to respective ones of the printing elements of the first array of printing elements via respective pads of the plural rows of wire-bonding pads, wherein the arrangement of plural rows of printing-element drivers and plural rows of wire-bonding pads reduces a spacing of the printing elements for improved resolution of the latent image. 
     In accordance with various embodiments of the invention, the pitch of the printing elements in the first array of printing elements may be equal to the pitch of the printing elements in the second array of printing elements, and the imaging electronics may activate the printing elements of the first and the second arrays in checkerboard fashion, or in random fashion. The checkerboard or random modes of operation serve to break up any unwanted pattern in the latent and printable images resulting from a defective print element and, thereby, counteract an observer&#39;s perception of a streak or line imperfection in the image. Alternatively, the imaging electronics may activate the printing elements of the first and the second arrays in a mode of reduced intensity of light emitted from the printing elements while directing the printing elements of the second array to print the same data as is printed by the printing elements of the first array to compensate for the reduced intensity of the emitted light, thereby to extend the lifetime of the printing elements. In addition, the imaging electronics may activate the printing elements of the first array while reserving activation of the printing elements of the second array for a backup mode of operation in the event of a failure of operation of a printing element of the first array. 
     In yet another embodiment of the invention, the pitch of the printing elements in the first array of printing elements is greater than the pitch of the printing elements in the second array of printing elements, and the imaging electronics activates the printing elements of the first array or the printing elements of the second array to produce, respectively, a first latent image or a second latent image on said photoreceptor, wherein a resolution of the first latent image is higher than a resolution of the second latent image. In this way, the resolution of the latent image may be adjusted to match the resolution of the imaging data provided by the imaging electronics so as to avoid unnecessary usage of the printing elements in situations of low resolution data, thereby to extend the lifetimes of the printing elements. 
     Typically, each of the printing elements comprises a light-emitting diode (LED), such as GaAsP or AlGaAs, which, in combination with an epoxy or ceramic or electrically insulated substrate, provides for improved temperature stability. Printing by the print engine may be done in black and white, or in color. In the practice of the invention, it is understood that the term “light” such as that radiated by the LED is not limited to radiation in the visible spectrum, but includes light of longer wavelength, such as infrared, and light of shorter wavelength, such as ultraviolet, in the event that the photochemistry of the photoreceptor is operative in the infrared or ultraviolet portions of the electromagnetic spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein: 
         FIG. 1  shows a simplified diagrammatic view of a xerographic printing engine incorporating features of the invention; 
         FIG. 2  shows a stylized view of a printhead of the engine of  FIG. 1 , the printhead incorporating features of the invention, the view being partially exploded by displacement of an optical focussing element to show light-emitting printing elements; 
         FIG. 3  shows diagrammatically focal plane of the optical element of  FIG. 2 ; 
         FIG. 4  is a stylized fragmentary view of the optical element of  FIG. 2 ; 
         FIG. 5  is a stylized fragmentary view of LEDs and their driver circuitry for the printhead of  FIG. 1 , and wherein a first array and a second array of the LEDs are disposed on a single die; 
         FIG. 6  shows a portion of the first and the second arrays of the LEDs of  FIG. 5  in accordance with a further embodiment of the invention wherein the first and the second arrays are disposed on separate dies; 
         FIG. 7  shows a portion of the first and the second arrays of the LEDs of  FIG. 5  in accordance with a further embodiment of the invention wherein the LEDs of each of the first and the second arrays are provided in line arrays of differing pitch to provide for a printing of images with different values of resolution, the two arrays being disposed on a single die; 
         FIGS. 8 ,  9  and  10  are diagrammatic representations showing the energization of LEDs of the first and the second arrays of the printhead of  FIG. 1  during a succession of print lines for the cases, respectively, of checkerboard printing, double (over) printing, and random printing; and 
         FIG. 11  is a block diagram showing details of the imaging circuitry of FIG.  1 . 
     
    
    
     Identically labeled elements appearing in different ones of the figures refer to the same element but may not be referenced in the description for all figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , a xerographic printing engine  20  comprises a photoreceptor  22  in the form of the cylindrical drum with an outer image receiving surface  24  of photosensitive material, and a printhead  26 . The printhead  26  has an elongated shape, in the form of a bar, and includes printing elements in the form of sources of light. In a preferred embodiment of the invention, the sources of light are provided by an assembly  28  of LEDs which radiates light through an optical focusing element in the form of an elongated group of fibers of a lens  30  to produce a latent image on the receiving surface  24 . The LED assembly  28  is mounted on a substrate  32  which also carries LED driver circuitry  34 , wherein heat produced by the driver circuitry  34  and the LED assembly  28  is dissipated by a heat sink  36  disposed on a backside of the substrate  32  opposite the LED assembly  28 . Also included in the printhead  26  is a frame  38  which holds the lens  30  adjacent to, but with a small spacing from, the LED assembly  28 , and supports the printhead  26  relative to the photoreceptor  22  to maintain a desired spacing between the lens  30  and the image receiving surface  24 . Also included within the engine  20  is an image developer  40  comprising a developer roll  42  and a toner dispenser  44  wherein, upon rotation of the photoreceptor  22 , the developer roll  42  rotates to transfer particles of the toner from the dispenser  44  to the image receiving surface  24 . Electrostatic charges defining the latent image on the image receiving surface  24  secure the toner particles to the image receiving surface  24 , thereby to convert the latent image to a printable image. 
     By way of example, a latent image  46  is shown on the image receiving surface  24  as an array of dots  48  produced by activation of various LEDs of the assembly  28  wherein the dots  48  are shown located on lines which are parallel to a rotational axis  50  of the photoreceptor  22 . Further lines of dots  48  in the latent image  46  are imprinted by the printhead  26  during further increments of rotation of the photoreceptor  22  about the axis  50 . After conversion of the latent image  46  to a printable image by the developer  40 , the printable image is transferred to a suitable medium, such as a sheet of paper  52 . The paper  52  is carried by paper transport rolls  54  and  56  past a region of contact of the paper  52  with the image receiving surface  24  during rotation of the photoreceptor  22 . The resulting output image  58  imprinted on the paper  52  is shown in the figure to have the same form as the latent image  46 . A paper transport drive  60  rotates the rolls  54  and  56  to translate the paper  52  (indicated by an arrow) past the photoreceptor  22 . The photoreceptor  22  is rotated (indicated by a curved arrow) by a photoreceptor drive  62 . Synchronism between operation of the paper transport drive  60  and the photoreceptor drive  62  is maintained electrically by connection of these drives to imaging circuitry  64 . The imaging circuitry  64 , in addition to providing the synchronization, also stores data of an image to be printed by the engine  20 , and transmits command signals to the LED driver circuitry  34  for activation of the LEDs of the LED assembly  28  to produce the latent image. 
       FIG. 2  also shows the foregoing components of the printhead  26 , namely, the LED assembly  28 , the lens  30 , the substrate  32 , the LED driver circuitry  34  and the heat sink  36 . The driver circuitry  34  is located on both sides of the LED assembly  28  to facilitate connection of electric leads between the driver circuitry  34  and the numerous LEDs of the assembly  28 . Also shown are signal buses  66  located on both sides of the LED assembly  28  and supported by the substrate  32  for carrying signals from the imaging circuitry  64  ( FIG. 1 ) to drivers of the driver circuitry  34  disposed on both sides of the LED assembly  28 . Electric leads  68 , in the form of small wires, are shown connecting between the buses  66  and the driver circuitry  34  as well as between the driver circuitry  34  and the LED assembly  28 . An object plane  70  of the lens  30  is indicated in front of the surface of the lens  30  which faces the LED assembly  28 . Due to the exploded view of  FIG. 2 , the object plane  70  appears at a considerable distance from the LED assembly  28 , however, the true position of the lens  30  is much closer to the LED assembly  28  than that shown in  FIG. 2  so that the object plane  70  is at the emitting surface of the LED assembly  28 . An image plane  72  is similarly formed in front of the opposite surface of the lens  30  and, upon emplacement of the printhead  26  in its position relative to the photoreceptor  22  as shown in  FIG. 1 , lies at the image receiving surface  24 . 
     The foregoing relationship of the object plane  70  and the image plane  72  relative to the lens  30  is indicated diagrammatically also in  FIG. 3 , wherein the object plane  70  is located at the LED assembly  28  and the image plane  72  is located at the surface of the photoreceptor  22 . Also indicated in  FIG. 3  is an input cone  74  of light propagating from the LED assembly  28  to the lens  30  wherein the width of the cone  74  at the object plane  68  is wide enough to encompass two rows of LEDs as will be described further with reference to  FIG. 5. A  corresponding output cone  76  of light propagates from the lens  30  to the photoreceptor  22 , enabling the light of two rows of the LEDs to the imaged upon the photoreceptor  22 . 
     The lens  30 , in the preferred embodiment of the invention, is constructed in a well-known form available commercially under the name of a SELFOC gradient index lens, as shown in the fragmentary view of  FIG. 4 , wherein one or more optical fibers  78 , constructed as gradient index fibers, are held between two opposed sidewalls  80 . The fibers  78  extend in the direction of light propagation between the object plane  70  and the image plane  72  of  FIG. 3 , and are indicated also in phantom view in FIG.  2 . 
     In  FIG. 5 , the fragmentary view of the printhead  26  shows the substrate  32  with the heat sink  36  on a backside thereof, and the LED assembly  28  connected by the leads  68  to the driver circuitry  34  which, in turn, are connected by still further leads  68  to the signal buses  66  for receipt of signals from the imaging circuitry  64 . The LED assembly  28  comprises a first (or primary) array  82  of LEDs  84  arranged in a single line or row extending parallel to the buses  66 . Each LED in a line of the LEDs  84  prints a corresponding pixel of the image being printed. The LED assembly  28  further comprises a second (or secondary) array  86  of LEDs  84  arranged in a single line or row extending parallel to the buses  66 . In this embodiment of the invention, the LEDs  84  of both the first array  82  and the second array  86  are constructed on a single die  88 . Also included on the die  88  are pads  90  and  92  to facilitate securing of the leads  68  whereby, for each LED  84 , the corresponding lead  68  makes electrical connection with a pad  90  or  92  which, in turn, connects by a conductor  94  to the LED  84 . Each of the pads  90 ,  92  is a bonding pad for wire bonding of the wires of the leads  68 . In a preferred embodiment of the invention, the LEDs  84  comprise GaAsP or AlGaAs, and the substrate  32  comprises epoxy or ceramic or an electrically insulated metallic layer for temperature stabilization from heat generated in the LEDs  84  and in the driver circuitry  34 . 
     In accordance with a feature of the invention, a closer spacing of the LEDs  84  in each of the respective array  82  and  86  is attained by staggering the positions of the pads  90  and  92  such that the pads  90  are arranged along an inner row of the pads closer to the LEDs  84  than the pads  92  which are arranged along an outer row of the pads further from the LEDs  84 . By virtue of the reduced spacing among the LEDs  84 , the printhead  26  is able to provide a higher resolution image. The LED driver circuitry  34 , on each side of the LED assembly  28 , is composed of a set of driver chips  96  arranged side-by-side in a row parallel to the buses  66 . Connection of the driver chips  96  to respective ones of the buses  66  is facilitated by use of relay pads  98  whereby a lead  68  connects between a driver chip  96  and a relay pad  98  and wherein a further lead  68  makes connection from the relay pad  98  to the corresponding bus  66 . As is apparent from  FIG. 5 , the arrangement of the connection of a bus  66  and its associated driver chips  96  for the first array  82  is symmetric to the arrangement of the connection of the other bus  66  and its associated driver chips  96  for the second array  86 . Thereby, the imaging circuitry  64  is able to provide independent control for the LEDs  84  of the first array  82  and the LEDs  84  of the second array  86 . 
     In accordance with a further feature of the invention, the row of LEDs  84  in the first array  82 , while being spaced apart from the row of the LEDs  84  of the second array  86 , have a sufficiently small spacing to enable both rows of the LEDs of the assembly  28  to fall within the acceptance angle of the lens  30  (represented by the input cone  74  of  FIG. 3 ) for directing their light upon the photoreceptor  22 . This permits the imaging circuitry  64  to operate the printhead  26  in conjunction with the photoreceptor drive  62  ( FIG. 1 ) to print two rows of dots  48  for one position of the photoreceptor  22  prior to advancing the photoreceptor  22  for a subsequent imprinting of two rows of dots  48 . Alternatively, if overprinting is desired, or if only one of the arrays  82  and  86  is to be employed, the imaging circuitry  64  directs rotation of the photoreceptor  22  to advance at only one row of dots  48  at a time. By way of example in a use of the printing engine  20 , it may be desirable to employ the first array  82  alone for a printing process, and to rely on the second array  86  as a backup array in the event of a detection of failure in one of more of the LEDs  84  of the first array  82 . Alternatively, by way of further example, it may be desired to use some of the LEDs  84  of the first array  82  and some of the LEDs  84  of the second array  86  in a printing process so as to increase the lifetime of the LEDs  84 . These optional modes in the utilization of the printing engine  20 , as well as other optional modes, will be described in further detail below. 
       FIG. 6  shows an LED assembly  28 A having the same geometric arrangement of LEDs  84  and the pads  90 ,  92  with the respective leads  68  and conductors  94  in the assembly  28  as has been disclosed in FIG.  5 . However, in accordance with an alternative embodiment of the invention of  FIG. 6 , the LEDs  84  of the first array  82  are disposed on a first die  100  and the LEDs  84  of the second array  86  are disposed on a second die  102  separate from the first die  100 . The two assemblies  28  and  28 A are functionally equivalent in the operation of the engine  20 , however, one or the other on the assemblies  28  and  28 A may present a convenience in manufacture of the printhead  26 . 
       FIG. 7  shows and LED assembly  28 B of an alternative embodiment of the invention which differs from the LED assembly  28  of  FIG. 5  in that different arrangements of LEDs are employed in the first array  82  and in a second array  86 A of the assembly  28 B of FIG.  7 . The first array  82  comprises a line array of LEDs  84 , as was disclosed for the first array  82  of FIG.  5 . However, in  FIG. 7 , the second array  86 A comprises a line array of LEDs  104  having a lower pitch than the pitch of the LEDs  84  of the first array  82 . As can be seen in  FIG. 7 , the spacing, on centers, of the LEDs  104  is greater than the spacing, on centers, of the LEDs  84 . The LEDs  84  and  104  are shown disposed on a single die  88 A, however, if desired, the LEDs  84  and  104  can be provided on two separate dies analogous to the construction disclosed in FIG.  6 . In  FIG. 7 , the LEDs  104  are connected by conductors  106  to pads  108 , and via the leads  68  from the pads  108  to the LED driver circuitry  34 . Connection of the LEDs  84  via the pads  90  and  92  to the driver circuitry  34  is the same as has been disclosed above reference to  FIGS. 5 and 6 . The embodiment of  FIG. 7  is convenient for implementing an option in the operation of the engine  20  wherein the first array of LEDs can be employed for printing an image at a higher value of resolution and the second array of the LEDs can be employed for printing an image at a lower value of resolution. The applying of drive signals to the LEDs of the requisite one of the two arrays is accomplished by the imaging circuitry  64  (shown in FIG.  5 ). 
     In each of  FIGS. 8 ,  9  and  10 , there is a diagrammatic showing of the LEDs of the first array and of the second array wherein the LEDs of the first array and the LEDs of the second array are represented by different forms of hatching. Beneath the arrays of the LEDs, there are shown eight rows of markings imprinted on the photoreceptor  22  by the printhead  26  (FIG.  1 ). The arrangement of the markings is in rows and columns, the columns being numbered consecutively at the bottom of the figure, with 24 columns being shown by way of example. 
     For the checkerboard printing of  FIG. 8 , in any one row of the markings, the first mark is produced by activation of an LED from one of the arrays and the next mark is produced by activation of an LED of the other array. By way of example, with reference to the first row (shown at the bottom of  FIG. 8 ) the first mark is from an LED of the second array, the second mark is from an LED of the first array, with the sequence of markings continuing in alternating fashion. In the second row, the first mark is from an LED of the first array and the second mark is from an LED of the second array. The checkerboard printing mode reduces the utilization of the LEDs so as to extend their lifetimes, and also inhibits generation of a noticeable line or streak in an output image of the engine  20  due to a defective LED or its drive circuit. 
     For the double printing, also referred to as overprinting, of  FIG. 9 , a line of an image is printed by the LEDs of the first array, and then the photoreceptor  22  ( FIG. 1 ) is rotated by an incremental rotation corresponding to the spacing between lines of the image, whereupon the LEDs of the second array are activated to print markings upon the markings already imprinted at the corresponding locations by the LEDs of the first array. This printing mode has the benefit of hiding an empty space resulting in an image from a failure of an LED of one of the arrays to print. 
     The random printing of  FIG. 10  is an alternative to the checkerboard printing of  FIG. 8  wherein, instead of implementing a specific pattern of alterations of excitation of the LEDs of the two arrays, as disclosed in  FIG. 8 , in  FIG. 10 , the selection of LEDs for activation in the two arrays is accomplished in random fashion. This printing mode is also useful in inhibiting generation of a noticeable line or streak in an output image of the engine  20  due to a defective LED or its drive circuit. Furthermore, since the LEDs are energized only part of the time, as compared to the full time printing of the double printing mode of  FIG. 9 , the random mode of  FIG. 10  extends the lifetime of the LEDs as compared to the double printing mode of FIG.  9 . 
     With reference to  FIG. 11 , the imaging circuitry  64  comprises a computer  110 , an address unit  112 , a memory  114 , an array selector  116 , a random number generator  118 , an LED selector  120  for the first array, and an LED selector  122  for the second array. In operation, data of an image to be printed is stored in the memory  114 . The data may have been obtained initially by the scanning of an object or by other means. In order to output the data for activation of the LEDs, the computer  110  addresses the memory  114  by use of the address unit  112 . In accordance with the addressing, the memory  114  outputs data of the respective pixels of the image to the array selector  116 , thereby to command the LEDs corresponding to the addressed pixels to emit light or to remain dark. Concurrently with the addressing of pixels of successive lines of an image stored in the memory  114 , the computer  110  outputs command signals to the photoreceptor drive  62  and to the paper transport drive  60  for advancing the photoreceptor  22  and the paper  52  to the requisite positions for printing the lines of the image. 
     The function of the array selector  116  is to steer the LED excitation signals to either the first array  82  or the second array  86  ( FIG. 5 ) of the LEDs  84 . Selection of either the primary array or the secondary array or of both arrays is commanded by the computer  110  based on the chosen mode of printing. In the event that the random mode of printing has been chosen, the signal outputted by the computer  110  is applied to the random number generator  118  for selecting the array wherein an LED is to be activated. By way of example, the random number generator  118  may operate modulo-2 for selecting one or the other of the arrays. 
     The function of each of the LED selectors  120  and  122  is to implement checkerboard printing. Each of the selectors  120  and  122  is able to select, within its array of LEDs, activation of only the odd numbered LEDs, or activation of only the even numbered LEDs, or activation of all of the LEDs. If the checkerboard printing mode is not desired, then the computer  110  commands the selectors  120  and  122  to pass the LED activation signals to all of the LEDs. If the checkerboard printing mode is desired, then the computer  110  commands one of the selectors  120 ,  122  to activate the odd numbered LEDs and the other of the selectors  120 ,  122  to activate the even numbered LEDs. 
     Each of the driver chips  96  in the LED driver circuitry  34  for the first array and for the second array includes a register  124  which receives the LED command signals from the memory  114  and a latch  126  which holds the command signals during operation of the LEDs  84 . As a further option in the operation of the printing engine  20 , in order to lengthen the lifetime of the LEDs  84 , both of the arrays  82  and  86  ( FIG. 5 ) can be operated concurrently but with the LEDs being operated at a lower level of energy output. The reduced energy output can be accomplished by reducing the interval of time during which an LED is radiating light. This is accomplished by the computer  110  by application of a strobe signal to the latch  126  in the LED driver circuitry  34  for each of the arrays, wherein the duration of the strobe signal controls the duration of the light pulse emitted by the LEDs. In the energy-saving mode, the duration of the strobe signal applied to the latch  126  is reduced from the normal duration of the strobe signal. This mode may be combined with the double printing mode of  FIG. 9  so that the photoreceptor  22  receives sufficient light energy for each of the markings of an individual print line. The total number of lines per page may be maintained the same as for printing by only the first array  82 . 
     It is to be understood that the above-described embodiments of the invention are illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.
A xerographic print engine employs a photoreceptor with an image receiving surface, a printhead for directing light to the photoreceptor to produce thereon a latent image, and a developer for converting the latent image to a printable image to be transferred from the photoreceptor to a print medium during a relative motion between the photoreceptor and the print medium. The printhead has light emitting diodes disposed in plural rows arranged alongside each other on a substrate which also supports driver circuitry connecting with imaging electronics for activating individual ones of the diodes. An optical element focuses light of the diodes onto a row of the latent image, the focussing being accomplished concurrently for individual ones of the diodes located in a plurality of the rows.