Abstract:
The present invention provides techniques to calibrate the emissive pixels used in printers and displays. The emissive pixels are arranged in a linear array or in a two dimensional array. For the transparent substrate on which the emissive pixels are formed, the light emitted by a pixel is measured by attaching one or more optical sensors, either directly or via optical fibers, to the transparent surfaces of the transparent substrate. That measurement is compared to a reference value and corrections are accordingly made to the emissive pixels. In case of a printer, the emissive pixels can be tested for their luminescent strengths in the period following the printing of a page while the next page to be printed is being positioned.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/663,838, filed Mar. 14th, 2005, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to emissive pixels used in printheads and displays, and specifically to monitoring and calibrating the emissive pixels.  
       BACKGROUND OF THE INVENTION  
       [0003]     Emissive pixels are used in printers and displays. In the displays, the emissive pixels are arranged in two-dimensional arrays. There can be several million emissive pixels in a television or a computer display depending on the size of the display. The resolution of a display is defined in terms of the number of pixels per square inch of the display. The higher the resolution, the better the picture shown on the display. An emissive pixel is typically turned on and off by using a voltage source. In addition to turning the pixel on and off, the voltage source is typically also used to control the gray scale of the pixel.  
         [0004]     A gray scale is a scale of achromatic colors having several equal gradations ranging from white to black. At a given gray scale, the emissive pixel is designed to illuminate at a certain predetermined brightness level depending on the design criteria that was used to design the particular display. As the display ages, there is often a decline in an emissive pixel&#39;s luminescent strength such that it becomes progressive dim. Non-uniformities in the fabrication of the emissive pixels during manufacturing also adversely affect the luminescent qualities of the emissive pixels. This problem is amplified in the displays that use the new organic light emitting diodes (OLED) to illuminate the emissive pixels, and that is inhibiting the commercialization of the OLED technology.  
         [0005]     The aging problem associated with the display pixels is also applicable to the printhead pixels. Presently, the printer technology only uses linear arrays of emissive pixels. Applicants are not aware of any prior art in the area of printer technology that discloses a two-dimensional array of emissive pixels. The published Japanese Patent Application No. 2000-349576 (P2000-349576) to Hiromasa Sugano (“Sugano Publication”) seems to disclose a printhead having a two dimensional array of emissive pixels in  FIGS. 1, 4  and  7  at the first glance, but that disclosure is ridden with enablement problems and discloses a system that is unworkable.  
         [0006]      FIGS. 1 and 7  of the Sugano Publication show an optical head  51  having a two dimensional array of picture elements  100  arranged in rows and columns. Each picture element pixel  100  of a column of picture elements  100  is shown connected to the detector circuit  130  via the same input line  131  to the detector circuit  130 .  FIG. 4  of the Sugano Publication shows that the line  131  is connected to the anode of the photodiode PD and the capacitor Cs of each picture element  100 . The current discharged by the capacitor Cs into the line  131  is purportedly detected by the detector circuit  130  to determine the light emitted by the photo emitter diode LD. Sugano Publication at ¶27.  
         [0007]     That arrangement is unworkable because the capacitors Cs for all the picture elements  100  of a column discharge in the same line  131  and the detector circuit  130  cannot separate the discharges from the various capacitors Cs. Also, the discharging of the capacitor Cs is problematic because the line  131  is not connected to the ground. Furthermore, the Sugano Publication does not disclose if the paper must be momentarily stopped so that the two dimensional array of pixels can be flashed and the image data emitted by the emissive pixels for forming the image on the paper. If the Sugano Publication intends to flash the two-dimensional array of emissive pixels while the paper is moving, the details of how that would be accomplished are not disclosed.  
         [0008]     There is a need in the art to stabilize the light emissions of the emissive pixels throughout the life span of the printers and the displays.  
       SUMMARY OF THE INVENTION  
       [0009]     In one aspect of the present invention a device, such as a display or a printhead of a printer, is disclosed having a substrate having a transparent portion including one or more transparent surfaces. One or more arrays of emissive pixels are embedded in the substrate for emitting light. An optical sensor is externally coupled to a transparent surface of the substrate. The transparent portion of the substrate provides a path for a light emitted by an emissive pixel of the one or more arrays of emissive pixels to exit through the transparent surface. The optical sensor is optically coupled to the emissive pixel by means of the path. The optical sensor is configured to detect the light emitted by the emissive pixel that exists the transparent surface. The optical sensor can be embedded in a wall of a receptacle module designed for holding the substrate.  
         [0010]     In another aspect of the present invention, a method for a printhead of a printer having one or more arrays of emissive pixels is disclosed. Initially, a page is printed. Following, a plurality of emissive pixels of the one or more arrays is stopped from emitting light. Following, an emissive pixel of the plurality of emissive pixels is caused to emit light. Following, the light emitted by the emissive pixel s detected. Following, a measurable parameter for the detected light is calculated. Following, the measurable parameter for the detected light is compared with a threshold value. Finally, a result of the comparison is stored in a memory location.  
         [0011]     In yet another aspect of the present invention, a substrate for a display or a printhead of a printer is disclosed including a linear array of emissive pixels and an optical sensor strip that runs the length of the linear array of emissive pixels and overlaps with a plurality of the emissive pixels of the linear array. The optical sensor strip is optically coupled to the emissive pixels of the linear array of emissive pixels. The shortest distance between any emissive pixel of the plurality of pixels of the linear array and the optical sensor strip is the same for all the emissive pixels of the plurality of pixels of the linear array. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     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:  
         [0013]      FIG. 1  illustrates an exemplary flowchart of a method for calibrating emissive pixels;  
         [0014]      FIG. 2   a  and  FIG. 2   b  illustrate exemplary transparent substrates;  
         [0015]      FIG. 3  illustrates an exemplary substrate including an optical sensor strip optically coupled to emissive pixels;  
         [0016]      FIG. 4   a ,  FIG. 4   b ,  FIG. 4   c  and  FIG. 4   d  illustrate exemplary embodiments of optical sensors attached to transparent substrates;  
         [0017]      FIG. 5   a  and  FIG. 5   b  illustrate other exemplary embodiments of optical sensors attached to transparent substrates;  
         [0018]      FIG. 6   a  and  FIG. 6   b  illustrate exemplary embodiments of optical sensor receptacles having embedded optical sensors and cavities for holding transparent substrates;  
         [0019]      FIG. 7   a  and  FIG. 7   b  illustrate exemplary embodiments of optical sensors attached to transparent substrates by means of optical fibers;  
         [0020]      FIG. 8  illustrates an exemplary embodiment of an optical sensor receptacle having embedded optical fibers connected to an optical sensor and a cavity for holding a transparent substrate; and  
         [0021]      FIG. 9  illustrates an exemplary flowchart of a method for calibrating emissive pixels. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     According to one aspect of the present invention, the present invention uses luminance feedback from emissive pixels to stabilize and make uniform a linear array or a two dimensional array of emissive pixels deposed on a transparent substrate of a printhead or a display. A printhead is a device used to write an image to light sensitive materials including photographic media and photosensitive drums designed to pick up toner inks for transfer to non-optically sensitive materials such as paper stock, transparencies and others.  
         [0023]     Feedback systems are sorted into three broad classes: closed loop, open loop, and interrupted loop. 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.  
         [0024]     In one aspect of the present invention, luminescence feedback is implemented using the open loop technique. The open loop technique for a printhead is now described with reference to  FIG. 9 , which shows a flowchart of 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.  
         [0025]     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 .  
         [0026]     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 .  
         [0027]     The advantage of the open loop feedback system of  FIG. 9  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 40 microseconds and the measurement time is 8 microseconds the amplification is 5 times or 7 dB.  
         [0028]     The open loop method illustrated in  FIG. 9  is an exemplary one. Other methods are available such as the ones disclosed in the provisional application No. 60/660,725, filed by Applicants on Mar. 11th, 2005, which is incorporated herein by reference. The open loop techniques of the present invention can be implemented in passive, active, or COG (chip on glass) circuitry or in any combination thereof as is explained in the provisional application No. 60/660,725.  
         [0029]      FIG. 1  shows a flow chart of a method for implementing an emissive feedback technique in a printhead that does not have a one-to-one correspondence between the light emitting elements and optical sensors. According to block  12 , in step  1 , a first page is printed using the printhead. The printhead may include a linear array or a two dimensional array of emissive pixels. The linear arrays are well know in the industry and are used regularly with laser printers. In block  14 , after the first page is printed, step  2  includes the positioning of the next page to be printed. The amount of time to do this depends on the pages per minute to be printed and the length of the page.  
         [0030]     For example, if 30 10-inch pages are to be printed per minute the rate of page travel is 300 in/min or 5 in/sec. If the distance from the last printed line on the first page to the first printed line on the next page is 1 inch, then the time to position the next page is 200 ms or 200,000 microseconds. According to blocks  16  and  18 , step  3  includes printing the next page and step  4  includes repeating steps  1 ,  2  and  3  until the printing job is completed. Step  2  is the critical step in the emissive feedback operation. Step  2  is subdivided into steps  2   a - 2   f  as illustrated in blocks  20 - 30 .  
         [0031]     According to block  20 , in step  2   a , all light emitting elements are set to no emission (dark). According to block  22 , in step  2   b , a first light-emitting element in a printhead array is turned on to the highest design luminance (this is an example and any luminance level can be used). According to block  24 , in step  2   c , the luminance of the first light-emitting element is detected and converted into a measurable parameter such as a voltage reading. A read circuitry can be used to perform step  2   c , for example, the read circuitry disclosed in the provisional application No. 60/660,725.  
         [0032]     According to block  26 , in step  2   d , the measurable parameter value, for example, the voltage reading, measured in step  2   c  is compared with e reference value. The reference value can be stored in a table is a memory and correspond to the desired luminance of the light emitting element for a given set of circumstances, for example, for certain environmental conditions. According to block  28 , in step  2   e , a result of the comparison made in step  2   d  is stored in a memory location. According to block  30 , in step  2   f , steps  2   a - 2   e  are repeated for more light emitting elements of the one or more arrays of light emitting elements of the printhead. The number of the light emitting elements that can be tested for calibration according to step  2   f  depends on the time required to position the next page in block  14 .  
         [0033]     In order to produce the circuitry or the present invention, for example, the substrate, various techniques well known in the semiconductor industry are used 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.  
         [0034]     A printhead or a display substrate may be transparent in the case of a down-emitter OLED (organic light emitting diode) or made of an opaque material in the case of an up-emitter OLED. Terms “down-emitter” and “up-emitter” are familiar terms used in the OLED 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.  
         [0035]      FIG. 3  shows the light emitting elements running linearly down the center of the printhead substrate  40 , or a display substrate  40 , with the pixel driver circuitry  44  in the upper third of the printhead and a single optical sensor  58  in the bottom third of the printhead body. The drive circuitry  44  shown in the diagram is a COG. (chip on glass) IC. Alternately, the drive circuitry  44  may be fabricated in thin film. Alternately, the drive circuitry  44  may be located off the substrate  40  on a companion printed circuit board or as an IC chip mounted on flex circuitry attached to the printhead substrate  40 . Alternately, any combination of COG, thin film or off glass circuitry may be used.  
         [0036]     It is understood that the light emitting elements  46 ,  48 ,  50  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 the Universal Display Corporation. Other light emitting materials including electroluminescent materials and inorganic materials such as the indium phosphides used in the well known red LEDs may also be used.  
         [0037]     The optical sensor  58  is comprised of an optically sensitive material, such as amorphous silicon, poly-silicon or any other material that changes electrical properties under changing levels of illumination. In the embodiment shown in  FIG. 3 , a single sensor strip  58  runs the length of the linear light emitting element array  46 ,  48 ,  50 . A projection of light emitting material  52  from each light-emitting element  46 ,  48 ,  50  overlaps a portion of the optical sensor strip  58 . During the read step (step  2   c  in  FIG. 1 ), the light emitting elements  46 ,  48 ,  50  are turned on one at a time beginning with the first light emitting element  46  and ending with the nth light-emitting element  50 . In the printing example given above, the time to position the next page is 200,000 microseconds.  
         [0038]     If the linear array in the printhead has 5,000 light emitting elements, the time to read each element is 40 microseconds. When the first light-emitting element  46  is turned on, the photon emission from the light emitting element projection  52  overlapping a portion of the single optical sensor  58  couples light into the sensor  58 , which sends an optical signal proportional to the level of light emission to the read circuitry (not shown). There are many types of read circuitry available to designers skilled in electrical engineering; and several such circuits were described in the provisional application No. 60/660,725.  
         [0039]     As described above with reference to,  FIG. 9  the optical data read from the optical sensor  58  for the first light-emitting element  46  is compared to the calibrated data and the difference (if there is a difference) is stored in the Correction Storage Block  108 . One by one, data from each light-emitting element  46 ,  48 ,  50  in the one or more arrays on the printhead is read, compared to the calibrated data and the differences stored in the Correction Storage Block  108  until all 5,000 light-emitting elements  50  are read. This operation takes 200 ms to complete, and thus, occurs during the time it takes to position the next page for printing.  
         [0040]     It is understood that the reading of all light emitting elements in  200  ms is an example and only a portion of the light emitting elements may be read during the positioning of the next page with the balance of the light emitting elements being read during following positioning of pages. For example, half the light emitting elements could be read per page change, or 1/10 th  of the light emitting elements could be read per page change. Any number of light emitting elements could be read per page change, which would extend the reading and updating of light emitting elements over many pages. It is conceivable that only one light-emitting element  50  may be read and that it would take 5000 pages to complete the light emitting element update. At 30 pages per minute this would only be an on time of the printhead of 166 minutes, or less than three hours. While this is a long a period between updates for the present materials, more stable materials may be developed in the future that may not need an update for at least 166 minutes.  
         [0041]     In the above example, one thin film optical sensor  58  was used to read all the light emitting elements in the printhead linear array. Alternatively, groups of light emitting elements could be read by one thin film optical sensor per group. For example, ten thin film optical sensors could be used to read five groups of fifty light-emitting elements, allowing data to be read in parallel for ten light-emitting diodes at a time. The groups of light emitting elements could range from 2 to 2500, for example, and anywhere in between.  
         [0042]      FIGS. 2   a  and  2   b  show that light inserted into the transparent printhead substrate  40  by an emissive pixel  46  can exit the glass substrate  40  through the edges of the glass  42 .  FIG. 2   a  shows that the light emitted from the edges  42  located nearest the illuminated light emitting element  46  is the most intense (arrow size indicating light emission intensity) and that the least intensity is on the edges  42  located most remotely from the illuminated light emitting element  46 .  FIG. 2   b  shows the nth light-emitting element  50  is illuminated and inserting light into the glass substrate  40 , and therefore, the intensity of the light emission from the edges  42  is the reverse of that shown in  FIG. 2   a . One of ordinary skill in the art will appreciate that the substrate  40  shown in  FIGS. 2   a  and  2   b  can be a printhead substrate or a display substrate.  
         [0043]      FIG. 4   a  shows that if an optical coupling material  62  is in contact with the top surface  42  of the printhead or the display substrate  40 , light inserted into the substrate by a light-emitting element  46  can be extracted through the optical coupling material  62 . The physics of manipulating the path that light follows using varying refractive index materials is well known in the industry and is made use of particularly in the fiber optics area.  FIG. 4   b  shows an optical sensor  64  attached to the optical coupling material  62 . The optical coupling material  62  may be a UV cured epoxy, which adheres to both the glass substrate  40  and the optical detector  64 .  
         [0044]     Alternately, any adhesive material that serves to both extract light from the substrate  40  and can adhere the substrate  40  to the optical sensor  64  may be used. The optical sensor  64  may be selected from many types of optically active materials including but not limited to silicon diodes, germanium diodes, cesium compounds, selenium compounds, and materials used to make solar cells naming a few. Alternately, thin film optically active materials can be deposed on the surface of the transparent substrate  40 .  
         [0045]      FIG. 4   c  shows that multiple optical sensors can be coupled into the transparent substrate  40  of the printhead. Multiple sensors  64  can be used to add to the total reading of one light-emitting element  46 ,  48 ,  50  at a time. Signal to noise ratio can determine the speed at which the optical sensor  64  can be read. Therefore, multiple optical sensors  64  can be advantageously used to increase the signal to noise ratio, and thus, the maximum speed of recording the optical sensor data from one light-emitting element  46 ,  48 ,  50 . The optical sensors  64  are electrically wired to the substrate  40  (wires not shown). Cables (not shown) are subsequently attached to the printhead substrate  40  using technology well known in the industry. The cables conduct the optical readings to circuits for processing the information.  
         [0046]      FIG. 5   a  shows an optical sensor  64  attached to one edge  42  of the transparent printhead substrate  40 . The material used to attach the optical sensor  64  also extracts light from the edge  42 . The illuminated light emitter element  46  will generate a light component that exits the transparent substrate  40  through its six sides  42  and a judicious selection of the epoxy bonding material may increase the intensity of the light exiting to the optical sensor  64 . One optical sensor  64  can be used to read the light-emitting element  46 ,  48 ,  50 .  FIG. 5   b  shows another optical sensor  64  attached to the opposite side of the printhead transparent substrate  40  to increase and balance the optical signals from each light emitting element  46 ,  48 ,  50 . In  FIG. 4   d , ten optical sensors  64  have been attached to the six sides  42  of the print head&#39;s transparent substrate  40 , thereby increasing the signal to noise ratio by a factor of 10. The optical sensors  64  are electrically wired to a circuit board (not shown). One of ordinary skill in the art will appreciate that the substrate  40  shown in  FIGS. 4   a ,  4   b ,  4   c ,  4   d ,  5   a  and  5   b  can be a printhead substrate or a display substrate.  
         [0047]      FIG. 6   a  shows a printhead substrate  40  containing the printhead drive circuitry  44  and the linear array of light emitting elements  46 ,  48 ,  50  being inserted into a module  74  containing embedded optical sensors  64 .  FIG. 6   b  shows the printhead substrate  40  fully inserted into the optical sensor module  64 . The system is designed so that the printhead substrate  40  closely fits in the optical sensor-lined pocket  76 . Optical coupling epoxy may be used to maximize the light extracted from the printhead substrate  40 , which would also help hold the printhead substrate  40  in place.  
         [0048]     As described above, a full range of optical sensors  64  is known in the art. The optical sensors  64  can be electrically connected into circuitry carried by the optical sensor module  74  in much the same manner as printed circuit boards are constructed. That is, the optical sensor module  74  may itself be a printed circuit board with the optical sensors  64  embedded therein. An advantage of this embodiment is that the printhead need not have any optical sensors  64  attached to it or deposed on its surface. Therefore, any printhead having a transparent substrate  64  may be made uniform and maintained to light emission specification using this embodiment. One of ordinary skill in the art will appreciate that the substrate  40  shown in  FIGS. 6   a  and  6   b  can be a printhead substrate or a display substrate.  
         [0049]      FIG. 7   a  shows optical fibers  82  replacing the edge attached optical sensors  64 . Each optical fiber  82  conducts light from the printhead substrate  40  to an optical sensor  64 . Alternatively,  FIG. 7   b  shows all optical fibers  82  conducting light from the edges of the printhead substrate  40  to one optical sensor  64 . There is no electro-optical difference between the apparatuses of the  FIGS. 7   a  and  7   b  because adding electrical signals from  10  optical sensors, or alternately adding the light intensities from ten optical fibers provides approximately the same signal to noise ratio.  
         [0050]      FIG. 8  shows the use of optical fibers  82  embedded in the optical sensor module  74  having one optical sensor  64 . Alternately, multiple optical sensors  64  can be used in this embodiment and one or multiple optical fibers  82  can be connected to each optical sensor  64 . Also, an optical fiber  82  could also be attached through high refractive index material  62  to the top or bottom surface  42  of the transparent printhead substrate  40 . One of ordinary skill in the art will appreciate that the substrate  40  shown in  FIGS. 7   a ,  7   b  and  8  can be a printhead substrate or a display substrate.