Abstract:
The present invention uses an open loop feedback technique to control emissive pixels of a printhead of a printer. The open loop feedback technique involves integrating the light intensity of the emissive pixel over a predetermined period of time, averaging the integrated value, comparing the averaged value to a threshold value, and adjusting the input voltage to the OLED of the pixel based on the comparison.

Description:
RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/660,725, filed Mar. 11th, 2005, which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to printing technology, and specifically to controlling the printhead of a printer using feedback control techniques. 
   BACKGROUND OF THE INVENTION 
   A printhead is a part of a computer printer that contains the printing elements. The printing elements include light emitting elements such as lasers that are used to write information such as graphic images and alphabetic text to a drum coated with a light sensitive material such as a selenium compound. The drum acquires a charge proportional to the intensity of the light. The charges on the drum replicate a desired image. The drum is then rotated through a toner application system, which coats the drum with the toner. The thickness of the coat of the toner is controlled by the charge on the drum. The drum continues to rotate and transfers the toner to a blank sheet of paper. 
   Alternatively, the light emitting elements can be used to directly write an image to a light sensitive medium such as photographic paper.  FIG. 1  illustrates how the printhead  10  is used to write a two-dimensional image. The drum  20  or paper  20  moves with respect to the printhead  10 , which is held stationary as the paper  20  or drum  20  moves past the printhead  10 . Data is fed to the printhead  10  for each line of the image. The size of the image dots written to the drum  20  or paper  20  depends on the velocity of the drum  20  or paper  20 . For example, if the printhead  10  holds the line data for one millisecond and the paper  20  moves at the velocity of 10 cm/second the image dot is 0.1 millimeters long. 
   After the first line is written, the data in the printhead  10  is replaced by the image data for the second line. Since this takes some time, the paper  20  has moved causing a separation from the first image line on the drum  20  or paper  20 . The second line is written to the drum  20  or paper  20  when the next line of data is sent to the printhead  10 . This process continues until the completed image has been written to the drum  20  or paper  20 . 
   A new organic light emitting diodes (OLED) technology, which replaces the laser with an OLED as the light emitting elements, is simpler, faster and superior in resolution to the laser technology. However, the lack of manufacturing uniformity and differential color aging of the OLED over the lifetime of the products that implement the OLED are hindering the commercialization of the OLED technology. 
   Nuelight Corporation, the assignee of the present application, has several pending provisional and non-provisional patent applications that relate to improving the use of light emitting elements, for example, OLED, to illuminate displays such as the LCD displays. See, for example, U.S. patent application Ser. No. 10/872,344 entitled Method and Apparatus for Controlling an Active Matrix Display and U.S. patent application Ser. No. 10/872,268 entitled Controlled Passive Display Apparatus and Method for Controlling and Making a Passive Display. Those patent applications relate to the use of feedback systems to control the emissions of the display pixels. 
   The techniques of the present invention relate to improving the use of light emitting elements, for example, OLED, in printhead applications. The light emitting elements serve different purposes in the printheads than in the displays. In the displays, for example, in the liquid crystal displays (LCD), millions of light emitting elements are arranged in two-dimensional arrays to illuminate the display pixels. In printheads, on the other hand, the light emitting elements are arranged in a linear array to write information to a drum or a photographic paper via emissive pixels. 
   The challenges associated with the application of the light emitting elements to the displays and the printheads are different. The displays are inherently restrictive in the amount of area the feedback sensor circuitry can occupy because each pixel is surrounded by other pixels, and therefore, a feedback sensor must be included inside a pixel area. The printheads, on the other hand, use linear arrays in which a pixel is not surrounded by other pixels and so the feedback sensor can be mounted outside the pixel, for example, above or below the pixel. The techniques of the present invention relate to using the emission of light emitting elements of a printhead as feedback signals to control the light emitting elements. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a technique for controlling an emissive pixel of an array of emissive pixels of a printhead of a printer using an open feedback loop. A light emitting element of the emissive pixel is optically coupled to a sensor. Several values of an output parameter such as the intensity of the light emitted by the light emitting element are measured over time by the sensor and converted to measurable parameter values such as voltage values. The measurable parameter values are integrated and then averaged. The averaged value is compared to a threshold value and the result is used to adjust an input parameter for the light emitting element, such as the voltage signal provided as an input to the light emitting element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
       FIG. 1  illustrates an exemplary embodiment of a printer including a printhead and an image-recording medium; 
       FIG. 2  illustrates an exemplary embodiment of a printhead implemented in a passive matrix configuration; 
       FIG. 3  illustrates an exemplary embodiment of a printhead implemented in an active matrix configuration; 
       FIG. 4  illustrates another exemplary embodiment of a printhead implemented in an active matrix configuration; 
       FIG. 5  illustrates an exemplary flow chart of a method of the present invention; 
       FIG. 6  illustrates an exemplary embodiment of a printhead implemented in a passive matrix configuration having an interrupted loop feedback control; 
       FIG. 7  illustrates an un-exploded view of an exemplary embodiment of a printhead implementing protective optical shields around a light emitter coupled to an optical sensor; 
       FIG. 8  illustrates an un-exploded view of an exemplary embodiment of a printhead implementing protective optical shields around a light emitter coupled to an optical sensor; 
       FIG. 9  illustrates an un-exploded view of another exemplary embodiment of a printhead implementing protective optical shields around a light emitter coupled to an optical sensor; 
       FIG. 10  illustrates an exemplary embodiment of a printhead implemented in an active matrix configuration having an interrupted loop feedback control; 
       FIG. 11  illustrates an exemplary embodiment of a printhead implemented in a passive matrix configuration having an open loop feedback control; 
       FIG. 12  illustrates an exemplary embodiment of a printhead implemented in an active matrix configuration having an open loop feedback control; 
       FIG. 13  illustrates an exemplary embodiment of a printhead implemented in a Chip On Glass (COG) configuration having an open loop feedback control; 
       FIG. 14  illustrates an exemplary Chip On Glass (COG) topology; 
       FIG. 15  illustrates an exemplary embodiment of a printhead in which an emissive pixel includes multiple light emitting elements; 
       FIG. 16  illustrates an exemplary embodiment of a printhead implemented in an active matrix configuration having an open loop feedback control in which an emissive pixel includes multiple light emitting elements; and 
       FIG. 17  illustrates an exemplary embodiment of a printhead having a page wide array of emissive pixels. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   This present invention relates to the use of optical feedback to control and maintain pixel brightness and uniformity over time in a printhead  10 . As shown in  FIG. 2 , a linear array of optical sensing elements  30  are deposed in a one-to-one correspondence adjacent to the linear array of light emitting elements  40 . The emission data read by the optical sensors  30  is fed back to the control circuitry  50  that regulates the emission levels of the light emitting elements  40 . 
   The present invention can be implemented with either passive matrix controlled pixels as shown in  FIG. 2  or with active matrix controlled pixels as shown in  FIG. 3 . An advantage of the active matrix pixel control, in which the drive circuitry  80  that drives the light emitting elements  40  is located on the printhead substrate thin film  60 , is the reduction of input/output (IO) lines to the printhead  10 . An alternative to active matrix circuitry is the use of chip on glass (COG) technology for pixel and optical sensor control and feedback as shown in  FIGS. 13 and 14 . 
   Referring to  FIGS. 2 and 3 , if the printhead  10  is printing to a light sensitive drum  20  that will pick up toner, the light emitting materials  40  are selected to emit the optimum wavelength required by the sensitive drum. If, however, the printhead  10  is used to expose a photographic color medium  20 , there must be three light emitting linear arrays, for example, a red array, a green array, and a blue array, or alternatively the printhead  10  may contain a linear array of light emitters  40  in which for example, every third light emitter is red, every third is green and every third is blue. 
   Since the light emitters  40  are deposed in a single row (linear array) there is no need to insert either pixel drive circuitry  80  or sensor circuitry  30  within the pixel area itself, but both circuitries  80  and  30  may be located adjacent to the light emitting elements  40  and in an array of circuits extending along side in thin film form, as illustrated in  FIG. 3 . Alternatively, both circuitries  80  and  30  can be off the printhead substrate  60  employing multiple-line flexible connectors and a Chip On Glass (COG) form leading to a printed circuit board containing feedback and pixel control functions. If a high-speed thin film semiconductor is employed, all the drive circuitry  80  may be located on the printhead  10  thereby minimizing input/output leads to the printhead  10 . 
   The optical sensor data reader  65  interface the sensor  30  to the control circuitry  50 . The optical sensor data reader  65  also coverts the light intensity measured by the sensor  30  into a measurable parameter, for example, a voltage value. The geometric relationship shown between the reader  65  and the control circuitry  50  is exemplary and many other geometric relationships between the two  50 ,  65  are possible. For example, in one embodiment, both the reader  65  and the control circuitry  50  may be located on the same side of the light emitters  40 . 
   In one embodiment, as illustrated in  FIG. 4 , an enhanced optical coupling of the optical sensors  30  with the light emitting elements  40  is accomplished by having an extended section of the pixel light emitting element  45  outside the pixel area to overlap the optical sensors. The present invention uses a luminance feedback to stabilize and make uniform the linear arrayed light emitting elements  40  in a printhead  10 . The light emitting elements  40  are used to write an image to light sensitive materials  20  including photographic media  20  and materials designed to pick up toner inks  20  for transfer to non-optically sensitive materials such as paper stock, transparencies and others. 
   Feedback systems are typically sorted into three broad classes: closed loop, open loop, and interrupted loop feedback systems. The closed loop is a system in which a change is detected in the output of a system and directly fed back to the input, which causes another output, which is again fed back to the input. An oscillator is an example of a closed loop system. If there is enough damping in an oscillating system the system will eventually settle to a constant output value. The exact value and the time it takes to settle are dependent on the loop parameters. 
   The open loop system does not feed back output values directly to the system input. Rather an output value is measured, evaluated and the result of the evaluation is used to make a decision on changing the input at a point in the future. The interrupted loop starts with a varying input and as the output varies it is measured and compared to a reference. When the output matches the reference the input is interrupted and input value held; thus, the output is fixed at a desired value determined by the reference. This is a fast and highly accurate method to achieve a desired output. The present invention uses both the interrupted loop and the open loop systems. 
   A method of the open loop feedback system of the present invention is now described with reference to the flow chart of  FIG. 5 .  FIG. 5  illustrates the functionality of the image data controller  100 . Image data  102  is fed to the gray level block (GL)  104 , which converts the image data to gray levels. The number of gray levels depends on the number of bits used to define the gray level. For example, a 1 bit gray level has two levels—on or off. An 8-bit gray level has 0 to 255 levels of gray. The image data is a serial data stream of analog pixel values (voltages). An analog pixel voltage enters block GL  104  and a digital number representing the gray level corresponding to the analog voltage exits. 
   The digital gray level value enters block GL Correction  106  and may or may not be changed depending on the information inputted from block Correction Storage  108 . The gray level value (changed or unchanged) exits the GL Correction block  106  and enters the Line Buffer (LB 1 ) block  110 , which collects pixel values until one line of pixels is collected, at which point the total line of pixel values is down loaded to the Printhead Linear Array block  112 . 
   The values of the down loaded pixels determine the luminance levels of the light emitters in the printhead. The value of the luminance over the time the printhead is on is collected and read to the Sensor Data (SB 1 ) buffer block  114 . The sensor data is sent to the Comparator block  116 , which compares the sensor data to calibration (reference) data sent to the Comparator block  116  from the Calibration LUT (look-up table) block  118 . The two pieces of data are subtracted and the resulting value is sent to the Correction Storage block  108 . The values stored in the Correction Storage block  108  are gray levels or portions of gray levels that will be added or subtracted from the initial gray level determined from the incoming image data and converted to a gray level in the GL block  104 . 
     FIG. 5  illustrates an open loop system. The advantage of this system is that the luminance data is collected during a time interval, which will tend to cancel out random noise generated in the optical signal plus the optical signal will be amplified by a factor determined by dividing the measurement time into the integration time. For example, if the time interval (integration time) is 800 microseconds and the measurement time is 8 microseconds the amplification is 100 times or 20 dB. Various embodiments of the present invention are now described in detail with references to  FIGS. 6–16 . 
   In one embodiment of the present invention, an interrupted loop feedback control is implemented in a printhead  10  having a passive matrix configuration. Referring to  FIG. 6 , the printhead substrate  60 , which may be glass in the case of a down-emitter OLED (organic light emitting diode), or of an opaque material in the case of an up-emitter. The terms “down-emitter” and “up-emitter” are familiar terms used in the OLED display industry signifying whether or not the light emitted by the OLED materials passes down through the substrate or up and away from the substrate. Both systems are in common use in the industry, but present developments favor the up-emitter, because thin film circuitry does not interfere with the light path. 
   In the case of the printhead, light is not interfered with in either case since the thin film circuitry and sensing elements are not under the light emitting elements as illustrated in  FIG. 3 , which shows the light emitting elements  40  running linearly down the center of the printhead substrate  60  with the pixel driver circuitry  80  in the upper third of the substrate  60  and the optical sensor array  30  in the bottom third of the substrate  60 . 
   The substrate  60  can be fabricated by using techniques well known in the semiconductor industry including material deposition processes including but not limited to evaporation, sputtering and plasma enhanced chemical vapor deposition; etching processes including but not limited to wet chemical etching, reactive ion etching and sputter etching; and photolithographic processes. 
   It is understood that the light emitting elements  40  may be formed from a number of light emitting materials including but not limited to organic light emitting diode materials such as Kodak&#39;s small molecule material, the polymer OLED materials, and phosphorescent OLED materials introduced by Universal Display Corporation. Other light emitting materials include electroluminescent materials and inorganic materials such as the indium phosphides used in the well-known red LEDs. 
   This embodiment shown in  FIG. 6  is referred to as a passive matrix because all the light emitter drive  80  circuitry is off the printhead substrate  60  and in an integrated circuit (IC)  120  or on a printed circuit board (PCB)  120 . The only circuit components on the printhead substrate  60  are the light emitting elements  40  and the optical sensors  30 . The interrupt feedback loop embodiment of  FIG. 6  operates by generating image data in the form of a serial analog voltage signal that enters the Image Data Controller  100 , which then sends gray level voltages to the line buffer LB 1 . These gray level voltages are sent to pin P 1  of voltage comparator VC 1 . There is one P 1  and VC 1  for each light emitter  40  in the printhead linear array. The first light emitter  40  is labeled 1 st  pixel and the second light emitter  40  is labeled 2 nd  pixel and so on until the last light emitter  40 , which is labeled nth pixel. There may be any number of light emitters  40  in the linear array depending on the dots per inch and the total length of the array. 
   Initially there is no voltage on pin P 4  of amplifier A 1  and therefore when the gray level voltage is applied from line buffer LB 1  to pin P 1  of VC 1 , there is no voltage on pin P 2  of VC 1 . VC 1  is designed so that when pin P 1  has a higher voltage than pin P 2 , the output of VC 1  pin P 3  is on the positive voltage rail, which, for example, may be +15 volts. Therefore, a positive 15 volts is applied to all the gates of transistors T 1  in the IC chip or PCB. Simultaneously voltage generator Vdd applies a voltage, for example, 10 volts to the drains all the T 2   s  and sensors S 1  and ramp generator RG 1  begins to ramp up voltage to the drains of all the T 1   s.    
   It is understood that sensor S 1  may be formed from any optically sensitive material including but not limited to amorphous silicon, poly-silicon, cadmium selenide, cadmium sulfide, and tellurium sulfide to name a few. The ramp voltage is transferred to the gates of all the T 2   s  and the capacitors Cs, because of the plus 15 volts on the gates of the T 1   s . As the ramp voltage increases, T 2  begins to force current through light emitting element, D 1  causing the emission of light to illuminate sensor S 1 . The current generated by S 1  can be fine tuned by the voltage placed on dark shield DS 1  (which acts as a gate element to the sensor). 
   Due to the optical current flowing from sensor S 1  through resister RI to ground, the voltage on pin P 4  begins to increase causing the output voltage from A 1  to be placed on pin P 2  of voltage comparator VC 1 . The gain of A 1  is designed to amplify the voltage from the optical current so as to be compatible with the gray level voltage on pin P 1  of VC 1 . As the ramp voltage further increases, the resulting increased optical current increases the voltage on pin P 4 , and thus, the voltage on pin P 2  of VC 1 . At some point in the voltage ramp the luminance of D 1  is high enough that the voltage from the optical current causes the voltage on pin P 2  to exceed the voltage on P 1 , at which point the output voltage on pin P 3  of VC 1  switches to the negative rail placing, for example, −5 volts on the gate of T 1 , thus, locking the ramp voltage on capacitor Cs and the gate of T 2 . 
   Each T 1  in the array will be turned off at a time determined by the gray level voltage that was placed on pin P 1  of VC 1 . It is understood that the number of gray levels is purely arbitrary and can range from two to thousands of levels depending on the application. The actual gray level voltage depends on the calibration of the sensor and the driver circuitry for the light-emitting element. Therefore, calibration data is taken for each driver  80  and sensor circuit  30 . This is optional depending on the uniformity of the semiconductor processes and the optical response of the optical sensor S 1 . The calibration data is stored in the Image Data Controller  100  and is used to modify the image data entering the Image Data Controller. There are many methods known in the art to do this; therefore, the details of how this is done are left to the printhead system designer. 
   As circuits age and/or the light emitters  80  age, the brightness caused by a particular voltage placed on the gate of T 2  decrease. This may be caused by the light emitter becoming less efficient or by the circuit parameters of T 2  drifting over time. In either case, the ramp voltage will continue to increase the voltage on the gate of T 2  until the emission of D 1  is high enough to cause the output of VC 1  pin P 3  to switch to the negative rail, and thus, switching off T 1  and locking the ramp voltage on the gate of T 2  and capacitor Cs. Therefore, as the circuit and light emitter age, the voltage on the gate of T 2  increases keeping the light emission at the correct level for the desired gray level. 
   If fine levels of gray are required, cross talk between adjacent light emitters and optical sensors can become a problem; therefore means can be provided to reduce optical cross talk.  FIG. 7  shows the apparatus for minimizing optical cross talk by the use of dark shields  130 , 135  to block both ambient light noise and noise from adjacent light emitters  40 .  FIG. 8  is an exploded view for clarity. In a transparent substrate such as those used by down-emitter systems, light can travel from a light emitter  40  over to the adjacent optical sensor  30  in the substrate glass or other transparent medium. 
   A dark shield  130 , 135  constructed of opaque material such as a metal is deposed on the glass and under the optical sensor  30 . This shield is designated in the drawing as the Bottom Dark Shield  135 . To protect the sensor from light from the top of the light emitter/optical sensor stack a Top Dark Shield  130  is deposed. Optionally, one or the other or both can be used depending on the circumstances. These dark shields  130 , 135  may be used in any of the embodiments described herein.  FIG. 9  shows the dark shields  130 , 135  may be continuous strips of opaque material running the length of the linear array of optical sensors  30 . 
     FIG. 10  shows the active matrix embodiment of the interrupted loop feedback system. In this embodiment, some of the pixel drive circuitry  80  is deposed on the printhead substrate  60 . The circuitry is constructed using thin film semiconductor technology well known in the industry. The semiconductor materials may be any suitable semiconductor, including but not limited to amorphous silicon, poly-silicon, or cadmium selenide naming a few. The figure shows that the data transfer TFT T 1 , storage capacitor Cs and TFT T 2  have been deposed on the printhead substrate  60 . 
   It is understood that any amount of the attendant circuitry may be deposed onto the printhead substrate  60  depending on the speed of the semiconductor material used. For example, if high quality poly-silicon is used the speed is high enough to depose thin film circuitry on the printhead that includes the high speed line buffer LB 1  and the operational comparators and amplifiers, VC 1  and A 1 . The operation of this embodiment of  FIG. 10  is an interrupted loop and is identical to the embodiment discussed above with reference to  FIG. 6 . The advantage of this embodiment is the reduction of input/output lead to the printhead  10 . The cost, on the other hand, may be higher due to the requirement for high-speed thin film materials and the added yield loss due to the added circuit complexity. 
     FIG. 11  shows an example of a circuit schematic for a photon integration open loop feedback system. On the printhead substrate  60  are deposed the linear array of light emitting elements D 1  from the first pixel to the nth pixel. Deposed adjacent to the light emitting elements D 1  are optical sensors S 1 . The dark shields are designated DS 1  and are connected to line L 3  which is driven by voltage generator VG 1 . The use of the voltage placed on DS 1  has been explained above with reference to  FIG. 6 . Shorting across S 1  is capacitor C 2 . One side of both S 1  and C 2  are connected to ground as is the cathode of D 1 . 
   This is a passive matrix because there are no active devices deposed on the printhead substrate  60 . It could be argued that dark shield DS 1  causes optical sensor S 1  to be an active device, but the distinction between active and passive has traditionally been determined by where the pixel driving circuit is placed-either on the substrate  60  locally with the pixel (active) or off the glass and out of the active area of the display (passive). 
   To initialize the circuit, voltage, 10 volts for example, is applied to P 1  of CA 1 . CA 1  is a charge amplifier and when 10 volts is applied to pin P 1   10  volts appears on pin P 2  and charges the line connecting pin P 2  to the drain of TFT T 3 . To complete the initialization the Image Data Controller  100  sends a voltage to the gate of TFT T 3 , which charges C 2  to 10 volts. In operation the Image controller  100  (see above for details of the Image Controller  100 ) sends pixel data voltages to line buffer LB 1 . These data voltages in analog form are down loaded to the TFT T 1   s  in all the pixels in the linear array of light emitting elements. The Image Data Controller  100  then sends a gate voltage to all the TFT T 1   s  which causes the data voltages to transfer to the gates of all the TFT T 2   s  and the storage capacitor C 1   s.    
   After the address time, TFTs T 1  are turned off by the Image Data Controller  100  removing voltage from the gates of TFTs T 1 . Storage capacitor, C 1  then maintains the voltage on the gates of TFTs T 2  for the design on-time of the pixel. Consequently TFT T 2  is turned on and current is forced through light emitting elements D 1 ; therefore, causing light emitting elements D 1  to emit light which impinges on optical sensors S 1 . The 10 volt charge placed on capacitor C 2  is drained to ground through optical sensor S 1 . The rate at which C 2  is drained depends on the level and time duration of the light emitted by D 1 . Therefore the amount of charge drained over the illumination time interval is a measure of the photo emission level (photon flux) from D 1 . 
   After the design on-time for the pixels the pixels are turned off by sending 0 Volts (or grounding the drains of TFTs T 1 ) to capacitor C 1 , and thus, removing the gate voltages on TFTs T 2  in the linear array. During the ensuing dark period before the next line of data voltages is downloaded (this is analogous to the horizontal retrace time in the display industry) the Image Data Controller  100  sends a voltage to the gates of TFTs T 3  causing charge amplifier CA 1  to recharge to 10 volts capacitor C 2 . The amount of charge required to recharge C 2  to the 10 Volts is drained from charge amplifier capacitor C 3  causing a voltage to appear on pins P 3  of charge amplifiers CA 1 . The level of the voltage on pin P 3  depends on the amount of charge and the ratio of C 2  to C 3 . The voltage on pin P 3  is collected in Sensor Data Buffer SB 1   114  where it is sent to the Image Data Controller  100  to be processed and compared to calibration voltages and the results are stored to be used in later image frames to modify the initial gray level data. See the functional description of the open feedback loop system above with reference to  FIG. 5 . 
     FIG. 12  shows an open loop feedback control embodiment of the present invention, in which the circuitry deposed onto the substrate  60  of the printhead  10  is contained in the solid line box designated at the Printhead Linear Array. The circuitry enclosed within the dashed line box of the printhead substrate  60  may be in the form of integrated circuits (ICs) or simply on a printed circuit board (PCBs).  FIG. 12  illustrates an active matrix circuit, because the driving circuitry  80  of the light emitting elements  40  is embodied in TFTs T 1  and T 2 , which are deposed on the same substrate  60  as the light emitting elements  40 . 
   It is understood that  FIG. 12  is exemplary and that circuit designers versed in the art will be able to construct various circuits that perform the functions of the invention. It is also understood that the term active matrix can refer to any additional circuitry deposed onto the printhead substrate. Therefore, all attendant circuitry including the line buffers can be deposed onto the printhead circuitry depending on the speed of the semiconductor materials. The active matrix configuration has been described above with reference to  FIG. 10 . The operations of the embodiments described with references to  FIGS. 11 and 12  are identical. The advantage of open loop feedback systems is their better noise immunity and the amplification factor as explained above. 
     FIG. 13  shows the open loop configuration schematic where the linear array of light emitting  40  and sensing elements  30  are deposed on the printhead substrate  60 . Also shown deposed also on the printhead substrate  60  are IC chips (integrated chips)  140  using the chip on glass (COG) technology. The COG technology is well known and in present use in the industry. The topology of a COG IC chip is shown in  FIG. 14 . The IC chips  140  include all the drive circuitry  80  including the line buffer LB 1  and the sensor data buffer SB 1 . This configuration is the same as the active matrix circuitry having all the drive circuitry  80  including the buffers deposed in thin film on the substrate  60 . But instead of the thin film technology, Chip On Glass (COG) technology is used. The preference of one embodiment over the other depends on speed and cost requirements. It is understood that the COG technology can be used with any of the embodiments described herein and with any amount of active matrix circuitry. 
   The foregoing embodiments dealt only with solid pixels in a linear array.  FIG. 15  shows the solid light emitting elements  40  sub-divided  42 , 44 . Although each light-emitting element has been divided into two light emitters  42 , 44  the driving circuits  80  for both the light emitting elements  42 , 44  and the sensor read circuit  65  are not divided. That is one driver circuit  80  drives both the sub-pixels  42 , 44 . One optical sensor  30  is used by both sub-light emitting elements  42 , 44 . 
   The purpose of the sub-division is to provide redundancy. That is, light element D 1  is used unless D 1  is a failed light emitting element, in which case light element D 2  is used. Alternatively, D 1  and D 2  can be used simultaneously to provide an extra gray level bit. For example, an 8-bit gray level system includes 256 levels of gray. To increase the top gray level to the next gray level, i.e. the 357 th  level, an 8-bit system is inadequate and another bit is required. If the bit level is increased to 9 bits, greater power is used and the circuit complexity increases. The sub-divided light emitter elements  42 , 44  solves that problem by allowing D 1  to be used for the first 256 levels of gray and only when D 1  needs to be boosted to the 257 th  level, D 2  is turned on for the extra gray level. It is understood that the light-emitting element  40  can be divided into any number of sub-divisions to increase redundancy or gray levels. There can be three sub-divisions with each sub-division being a different primary color. Color mixing can be achieved by varying the time for which a sub-element  42 , 44  is on. 
     FIG. 16  shows an example of the circuit used to drive the sub-division system. Drive data is placed on the gate of TFTs T 2  in the same manner as explained above and the sensor data is read in the same manner as above. TFTs T 4  and T 5  are used to independently control the sub-divisions through gate lines LG 4  and LG 5  by using the Image Data Controller  100 . In the case of using sub-division for color mixing, the optical sensors  30  would also be sub-divided. 
     FIG. 17  shows a printhead  10  having a page wide array configuration of the emissive pixels. A plurality of emissive pixels  40  are shown arranged in rows and columns. Each row of the emissive pixels  40  is shown coupled to a line buffer LB 1 , LB 2  . . . or LBN. The line buffers LB 1 , LB 2  . . . and LBN are controlled by the Image Data Controller  100 . Each row of the emissive pixels are also shown coupled to the voltage generator Vdd. The page wide array configuration can be implemented in both the active matrix and the passive matrix configurations. Also, the page wide array configuration can be implemented in both the interrupted loop and the open loop feedback systems. In one embodiment, the pixels  40  include organic light emitting diodes that are arranged according to the top-emitting configuration. In one embodiment, the pixels  40  include organic light emitting diodes that are arranged according to the bottom-emitting configuration. 
   In an application of the embodiment of  FIG. 17 , the paper  20  is positioned to receive emissions from the page wide array of pixels  40 . The digital data for an image to be printed is loaded for all the pixels  40  by the Image Data Controller  100  through the line buffers LB 1 , LB 2  . . . LBN. The line buffers LB 1 , LB 2  . . . LBN may be loaded serially, i.e. one line buffer at a time. The line buffers LB 1 , LB 2  . . . LBN may also be loaded in parallel, i.e. simultaneously. After all the line buffers LB 1 , LB 2  . . . LBN are loaded with the digital image data, the voltage generator is turned on such that all the pixels  40  of the page wide array simultaneously emit light corresponding to the image data. In one embodiment, the paper  20  is momentarily held stationary when the voltage generator Vdd is turned on to simultaneously flash the pixels  40 . In one embodiment, the paper  20  continues to travel when the voltage generator Vdd is turned on to simultaneously flash the pixels  40 . In that embodiment, the speed of the paper travel must be slow enough and the flash time of the pixels  40  must be fast enough to allow the paper to properly receive and form the image. 
   Although preferred illustrative embodiments of the present invention are described above, it will be evident to one skilled in the art that various changes and modifications may be made without departing from the invention. The respective embodiments described above are concrete examples of the present invention; the present invention is not limited to these examples alone. The claims that follow are intended to cover all changes and modifications that fall within the true spirit and scope of the invention.