WO2006125718A1 - Active matrix video image display device with correction for luminance non-uniformities - Google Patents

Active matrix video image display device with correction for luminance non-uniformities Download PDF

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Publication number
WO2006125718A1
WO2006125718A1 PCT/EP2006/062041 EP2006062041W WO2006125718A1 WO 2006125718 A1 WO2006125718 A1 WO 2006125718A1 EP 2006062041 W EP2006062041 W EP 2006062041W WO 2006125718 A1 WO2006125718 A1 WO 2006125718A1
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WIPO (PCT)
Prior art keywords
cells
calibration
image
modulator
current
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PCT/EP2006/062041
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French (fr)
Inventor
Sylvain Thiebaud
Jean-Paul Dagois
Philippe Le Roy
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Thomson Licensing
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Publication of WO2006125718A1 publication Critical patent/WO2006125718A1/en

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    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
    • G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
    • G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
    • G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
    • G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
    • G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
    • G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
    • G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/04—Structural and physical details of display devices
    • G09G2300/0404—Matrix technologies
    • G09G2300/0417—Special arrangements specific to the use of low carrier mobility technology
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2300/00—Aspects of the constitution of display devices
    • G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
    • G09G2300/0809—Several active elements per pixel in active matrix panels
    • G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0233—Improving the luminance or brightness uniformity across the screen
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/0285—Improving the quality of display appearance using tables for spatial correction of display data
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/02—Improving the quality of display appearance
    • G09G2320/029—Improving the quality of display appearance by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/04—Maintaining the quality of display appearance
    • G09G2320/043—Preventing or counteracting the effects of ageing
    • G—PHYSICS
    • G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
    • G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
    • G09G2320/00—Control of display operating conditions
    • G09G2320/06—Adjustment of display parameters
    • G09G2320/0693—Calibration of display systems

Definitions

  • the present invention relates to a device for displaying video images composed of pixels, to each of which is assigned an image data value, comprising a plurality of light-emitting cells organized in rows and in columns and each connected in series with a current modulator across the terminals of a power supply generator, each row and each column comprising at least two cells, and image processing means for generating, at the control of each current modulator, a control voltage from the images to be displayed, which control voltage is a function of the data value of the image to be displayed by the cell in series with the considered modulator.
  • the invention may be more particularly applied to OLED (Organic Light-Emitting Diode) displays in which the light-emitting cells are organic light-emitting diodes.
  • OLED Organic Light-Emitting Diode
  • the modulators and also the control switches are generally TFTs (Thin Film Transistors).
  • the control means formed by the modulators, the control switches and the storage capacitors form the active matrix of the display.
  • the transistors or modulators of the various light-emitting cells possess electrical characteristics, for example trigger threshold voltages, that are different, luminance defects are observed in the display of the images.
  • the reason for this is that, if the modulators of these cells have different trigger threshold voltages, the same control voltage will not result in the same amount of light being emitted from one cell to another of the display, which is often the case when the active matrix is fabricated from a polycrystalline semiconductor substrate obtained by recrystallization of an amorphous semiconductor.
  • the recrystallization process is effected by an excimer laser.
  • Excimer lasers are high-power pulsed lasers that emit in the ultraviolet, a wavelength range where the absorption of silicon is very high. These lasers allow amorphous silicon to be crystallized very efficiently, in order to thus form polycrystalline silicon in the course of an extremely rapid process of fusion-solidification lasting of the order of a hundred nanoseconds. During this process, the surface of the amorphous silicon substrate is scanned in a given direction, generally vertical or horizontal, by a laser beam. The wavelength, typically 308 nm, produced by the excimer laser is absorbed by the amorphous silicon, deposited as a thin film, avoiding an excessive elevation of the temperature of the glass substrate.
  • the process of crystallization of the amorphous silicon into polysilicon takes place during the cooling phase of the silicon.
  • This recrystallization process is widely employed for the fabrication of active matrices since it allows glass substrates to be used at a lower cost than silica glass.
  • the polycrystalline silicon or polysilicon is composed of grains of single-crystal silicon with different orientations that are separated from one another by disordered regions called grain boundaries. These boundaries tend to capture and immobilize free carriers in the polysilicon and increase the resistivity of the substrate in these regions, which leads to the disparities in electrical characteristics between the transistors subsequently fabricated from such a substrate.
  • the document WO 01/95301 proposes a display device comprising a correction circuit allowing these differences in luminance to be eliminated by correcting, pixel by pixel, the control voltage of the current modulators and a uniform luminance over the whole of the display to be obtained.
  • the role of this correction circuit is to determine the control voltage to be applied to the control of each modulator so that the corresponding cell delivers the desired level of luminance.
  • This circuit notably comprises means for measuring, during a display calibration phase, the current flowing through each light-emitting cell when a given control voltage is applied to the control of its modulator. The current could be measured for several values of control voltage.
  • the voltage to be applied to the modulator of the cell is then determined. These characteristics are then stored in a look-up table. During the normal operation phase of the display device, the information stored in the look-up table is used by the device's image processing means in order to generate, at the control of each modulator, the control voltage needed for the corresponding light-emitting cell to deliver the required level of luminance.
  • the current flowing through each cell is measured by a current detector disposed within the active matrix or in the column drivers of the array of cells. The installation of such a correction circuit therefore requires the layout of the active matrix or of the drivers of the display to be modified. Furthermore, this measurement is not reliable because the measured current (current from a single cell) is very low.
  • the invention provides a device that allows all or part of these drawbacks to be overcome.
  • the measured current is the total current flowing through a group of light-emitting cells placed along a line perpendicular to the direction of displacement of the laser beam used for the crystallization of the semiconductor substrate.
  • each row of cells of the display represents a group of cells according to the invention and in the case of a substrate scanned horizontally (the beam moves in a horizontal direction with respect to the substrate), each column of light- emitting cells represents a group of cells according to the invention.
  • the light-emitting cells of the display are turned on cell group by cell group.
  • the measurement of the current flowing through a group of cells then amounts to measuring the current output from the power supply generator for the display cells.
  • the total current measurement can thus be carried out by a single circuit placed in series with the power supply generator for the display cells.
  • a subject of the invention is therefore a device for displaying video images composed of pixels, to each of which is assigned an image data value, comprising:
  • each row and each column comprising at least two cells
  • - image processing means for generating, at the control of each current modulator, a control voltage from said images to be displayed, which control voltage is a function of the data value of the image to be displayed by the cell in series with said modulator.
  • the device also comprises:
  • each calibration image comprising image data activating, via said image processing means, the cells disposed along a rectilinear axis belonging to said calibration image with a non-zero calibration control voltage, each cell of the device being activated only once and with the same image data value during said sequence,
  • the axis along which the cells are activated during a calibration image is an axis that is perpendicular to the direction of displacement of said laser beam with respect to the substrate.
  • each calibration image comprises image data values for activating the cells belonging to the same row of cells. If the substrate of the active matrix has been horizontally scanned by the excimer laser, each calibration image comprises image data values for activating the cells belonging to the same column of cells.
  • the invention also relates to a method for displaying images composed of pixels by means of the device according to the invention in which, for each image to be displayed, a data control voltage that is a function of the image data value is generated at the control of each modulator for the cell in series with said modulator. This method comprises initial steps consisting in:
  • each image data control voltage associated with a pixel of this image is corrected using the correction value determined for the cell corresponding to said pixel.
  • FIG. 1 is a schematic representation of an OLED display according to the invention.
  • FIG. 1 illustrates the flow of data within the display in Figure
  • FIG. 4 illustrates the flow of data within the display in Figure 2 during an operational phase of the latter.
  • the invention will be more particularly described with regard to disparities between rows of organic light-emitting cells without this implying any limitation of the invention to this orientation of the disparities and to this type of light-emitting cells.
  • the display device or display according to the invention comprises an array 1 of light-emitting diodes D n , p disposed in rows and columns. For simplicity, only four light-emitting diodes
  • the current flowing in each light-emitting diode is controlled by a current modulator M n , p .
  • a storage capacitor C n , p and a control switch S n , p are provided for controlling the current modulator M n , p .
  • the current modulator M n , p and the control switch S n , p are thin-film transistors operating as MOS transistors.
  • a power supply generator 2 is provided for supplying current to the diodes D n , p when the associated modulators M n , p are turned on. It is connected between a terminal common to all the light-emitting diodes and a terminal common to all the current modulators M n , p .
  • the display device also comprises an array of column electrodes Xp and an array of row electrodes Y n .
  • Each row electrode Y n connects together the controls of the control switches S n , p of the n-th row of light- emitting diodes.
  • Each column electrode X p connects together one of the terminals of the control switches S n , p of the p-th column of diodes.
  • a row driver 3 and a column driver 4 are provided for controlling the voltage respectively applied to the rows and the columns of the device. The row driver 3 controls the state of the control switches S n , p and the column driver 4 controls the voltage applied to the control of the current modulators M n , p .
  • the display device also comprises a video input circuit 5 for receiving the video signal to be displayed and an OLED interface circuit 6 for processing the received video data and generating, as a function of this data, the control voltages to be applied, via the drivers 3 and 4, to the controls of the modulators M n , p .
  • the display device is completed by:
  • a calibration image generator 7 for successively displaying a white line over each of the rows of cells of the display; - a multiplexer 8 with two inputs receiving at a first input the video signal coming from the video input circuit 5 and at a second input the video signal coming from the calibration image generator 7; the multiplexer delivers the video signal coming from the calibration image generator 7 at its output during the calibration phase of the device and the video signal coming from the input circuit 5 during the normal operation phase of the device;
  • a current measurement circuit 9 for measuring, during the calibration phase of the display, the current flowing through the cells of a row of cells of the display; since the rows of cells are turned on one after the other during the calibration phase, the current to be measured corresponds to the current output from the power supply generator 2; the current measurement circuit is therefore placed in series with said power supply generator 2; for each row of cells (i.e. for each calibration image), it delivers information that is representative of the measured current;
  • correction value determination circuit 10 for determining one correction value per row of cells from the measurement information from the circuit 9; the same correction value will be applied to all the cells of the row; this correction value is for example a multiplying coefficient or a linear function or a second degree function;
  • a memory 11 for storing the correction values determined by the circuit 10 in the form of a look-up table during the calibration phase; only one correction value is saved for each row of cells;
  • this row correction circuit 12 for modifying the video levels of the signal coming from the video input circuit by applying to them, during the normal operation phase, the correction values saved in the memory 11 ; this row correction circuit is placed between the output of the multiplexer 8 and the OLED interface circuit 6.
  • the device thus completed comprises two operational phases: a calibration phase and a normal operation phase.
  • the flow of the data exchanged by the various elements of the display in Figure 2 during the calibration phase is shown in Figure 3.
  • the generator 7 delivers a sequence of calibration images designed to successively and separately turn on the rows of cells of the display. This sequence is transmitted via the multiplexer 8 and the correction circuit 12 to the OLED interface circuit 6.
  • the correction circuit 12 is transparent during this phase, i.e. in that it does not modify the video levels of the video signal supplied by the multiplexer 8.
  • This sequence of video images is used by the OLED interface circuit 6 and the drivers 3 and 4 for successively and separately turning on the rows of cells of the display.
  • a current measurement is performed for each of the rows (i.e. for each of the calibration images) by means of the circuit 9.
  • the circuit 10 determines a correction value for each row of cells from the current measurement relating to it. This correction value is saved in the memory 11.
  • the correction value for the rows is for example a value calculated with respect to the row having the lowest current measurement.
  • the luminance level of the rows is thus aligned with that of the row of lowest luminance.
  • the video level used to power the cells that are lit in the calibration images is for example the maximum displayable video level (255 for an 8-bit coding).
  • each row of cells may be calibrated by using several calibration images per row of cells, each of said images activating the cells of the row with a different control voltage. For example, during a first calibration image, a control voltage corresponding to a 255 level is applied to the cells of the row then, during a second calibration image, a control voltage corresponding to a 128 level is applied. The correction value is subsequently defined from the current measurements performed for these two calibration images.
  • the device of the invention allows the luminance defects generated by the excimer laser to be corrected whatever its displacement direction relative to the substrate of the active matrix.
  • the device in Figure 2 only corrects the variations in electrical characteristics in the direction of displacement of the laser, i.e. the variations between rows in the case of a laser beam moving vertically.
  • the laser beam is not always homogeneous and, over a given single row of cells, there may be variations in luminance between sections of the row, each row having identical row sections.
  • a first calibration is performed over the rows of cells of the display screen, then a second calibration is performed over the columns of cells of the display screen, the video levels of the images used for said second calibration being corrected by the correction values obtained after the first calibration (the correction circuit 12 is no longer transparent during this second calibration).
  • the correction values obtained at the end of the first calibration are replaced in the memory 11 by those obtained at the end of the second calibration.
  • the flow of data exchanged by the various elements of the display in Figure 2, during the normal operation phase of the device, is shown in Figure 4.
  • the multiplexer 8 transmits the signal coming from the video input circuit 5 to the correction circuit 12.
  • the correction circuit 12 modifies the video signal received with the correction values stored in the memory 11.
  • the video signal thus corrected is then transmitted to the OLED interface circuit 12 which generates the appropriate control voltages for obtaining an image without disparities in luminance.
  • this display is very simple since the calibration image generator 7, the multiplexer 8, the correction circuit 12 and the programmable memory 11 can very easily be implanted onto the control board of the active matrix.
  • this can be implemented by a shunt resistor associated with an analogue/digital converter. This device can thus be made without having to modify the architecture of the active matrix or of the row or column drivers.
  • the current measurements are more reliable since the currents measured are higher than in the prior art.
  • the number of correction values recorded in the memory is reduced since only one correction value is calculated per row of cells.
  • the present invention has been described with reference to an OLED active matrix display. It goes without saying that it can equally be applied to any type of active matrix display.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Computer Hardware Design (AREA)
  • General Physics & Mathematics (AREA)
  • Theoretical Computer Science (AREA)
  • Control Of Indicators Other Than Cathode Ray Tubes (AREA)
  • Electroluminescent Light Sources (AREA)
  • Control Of El Displays (AREA)

Abstract

The present invention relates to an active matrix image display device that automatically corrects the luminance non-uniformities generated by the defects of the active matrix. Such defects are principally generated by the recrystallization process of the semiconductor substrate of the active matrix. Conventionally, in order to fabricate the active matrix, a low-cost amorphous semiconductor is employed that needs to be recrystallized by an excimer laser. During this process, the substrate is scanned by a laser beam and it has been observed that the defects are linked to the displacement of the laser relative to the substrate and that the transistors of the matrix placed along an axis perpendicular to the displacement axis of the laser have identical electrical characteristics. According to the invention, it is proposed that the cells belonging to the same row of cells (in the case of a vertical laser scanning) or to the same column of cells (in the case of a horizontal laser scanning) be corrected in an identical fashion.

Description

ACTIVE MATRIX VIDEO IMAGE DISPLAY DEVICE WITH CORRECTION FOR LUMINANCE NON-UNIFORMITIES
The present invention relates to a device for displaying video images composed of pixels, to each of which is assigned an image data value, comprising a plurality of light-emitting cells organized in rows and in columns and each connected in series with a current modulator across the terminals of a power supply generator, each row and each column comprising at least two cells, and image processing means for generating, at the control of each current modulator, a control voltage from the images to be displayed, which control voltage is a function of the data value of the image to be displayed by the cell in series with the considered modulator.
The invention may be more particularly applied to OLED (Organic Light-Emitting Diode) displays in which the light-emitting cells are organic light-emitting diodes.
The modulators and also the control switches are generally TFTs (Thin Film Transistors). The control means formed by the modulators, the control switches and the storage capacitors form the active matrix of the display. When the transistors or modulators of the various light-emitting cells possess electrical characteristics, for example trigger threshold voltages, that are different, luminance defects are observed in the display of the images. The reason for this is that, if the modulators of these cells have different trigger threshold voltages, the same control voltage will not result in the same amount of light being emitted from one cell to another of the display, which is often the case when the active matrix is fabricated from a polycrystalline semiconductor substrate obtained by recrystallization of an amorphous semiconductor. The recrystallization process is effected by an excimer laser. Excimer lasers are high-power pulsed lasers that emit in the ultraviolet, a wavelength range where the absorption of silicon is very high. These lasers allow amorphous silicon to be crystallized very efficiently, in order to thus form polycrystalline silicon in the course of an extremely rapid process of fusion-solidification lasting of the order of a hundred nanoseconds. During this process, the surface of the amorphous silicon substrate is scanned in a given direction, generally vertical or horizontal, by a laser beam. The wavelength, typically 308 nm, produced by the excimer laser is absorbed by the amorphous silicon, deposited as a thin film, avoiding an excessive elevation of the temperature of the glass substrate. The process of crystallization of the amorphous silicon into polysilicon takes place during the cooling phase of the silicon. This recrystallization process is widely employed for the fabrication of active matrices since it allows glass substrates to be used at a lower cost than silica glass. After the crystallization phase, the polycrystalline silicon or polysilicon is composed of grains of single-crystal silicon with different orientations that are separated from one another by disordered regions called grain boundaries. These boundaries tend to capture and immobilize free carriers in the polysilicon and increase the resistivity of the substrate in these regions, which leads to the disparities in electrical characteristics between the transistors subsequently fabricated from such a substrate. These disparities are seen as luminance differences between pixels as shown in Figure 1 in the case of the display of a white image on an OLED screen having an active matrix fabricated from a vertically-scanned substrate, which has led to the disparities in luminance between (horizontal) rows of the screen.
In order to overcome this defect, the document WO 01/95301 proposes a display device comprising a correction circuit allowing these differences in luminance to be eliminated by correcting, pixel by pixel, the control voltage of the current modulators and a uniform luminance over the whole of the display to be obtained. The role of this correction circuit is to determine the control voltage to be applied to the control of each modulator so that the corresponding cell delivers the desired level of luminance. This circuit notably comprises means for measuring, during a display calibration phase, the current flowing through each light-emitting cell when a given control voltage is applied to the control of its modulator. The current could be measured for several values of control voltage. Since the light emitted by the cell is directly proportional to the current flowing through it, it is then possible to deduce from this or these current measurements the voltage to be applied to the modulator of the cell in order to obtain a given level of luminance. These measurements are thus used to determine the current-voltage characteristics of each modulator. These characteristics are then stored in a look-up table. During the normal operation phase of the display device, the information stored in the look-up table is used by the device's image processing means in order to generate, at the control of each modulator, the control voltage needed for the corresponding light-emitting cell to deliver the required level of luminance. In this prior art device, the current flowing through each cell is measured by a current detector disposed within the active matrix or in the column drivers of the array of cells. The installation of such a correction circuit therefore requires the layout of the active matrix or of the drivers of the display to be modified. Furthermore, this measurement is not reliable because the measured current (current from a single cell) is very low.
The invention provides a device that allows all or part of these drawbacks to be overcome.
According to the invention, the measured current is the total current flowing through a group of light-emitting cells placed along a line perpendicular to the direction of displacement of the laser beam used for the crystallization of the semiconductor substrate. Indeed, it has been observed that all the transistors of the active matrix placed on the same line (or rectilinear axis) perpendicular to the direction of displacement of the laser beam have identical electrical characteristics. The transistors placed along this line therefore allow, for equal control voltages, equal currents to flow. A measurement of the total current flowing through these cells is therefore sufficient in order to know the current-voltage characteristics of each of the current modulators of the group of cells. In the case of a substrate scanned vertically by a laser beam (the beam moves in a vertical direction with respect to the substrate), each row of cells of the display represents a group of cells according to the invention and in the case of a substrate scanned horizontally (the beam moves in a horizontal direction with respect to the substrate), each column of light- emitting cells represents a group of cells according to the invention.
Furthermore, in order to obviate the need for modifying the layout of the active matrix or of the drivers of the display device, the light-emitting cells of the display are turned on cell group by cell group. The measurement of the current flowing through a group of cells then amounts to measuring the current output from the power supply generator for the display cells. The total current measurement can thus be carried out by a single circuit placed in series with the power supply generator for the display cells.
A subject of the invention is therefore a device for displaying video images composed of pixels, to each of which is assigned an image data value, comprising:
- a plurality of light-emitting cells organized in rows and in columns and each connected in series with a current modulator across the terminals of a power supply generator, each row and each column comprising at least two cells,
- image processing means for generating, at the control of each current modulator, a control voltage from said images to be displayed, which control voltage is a function of the data value of the image to be displayed by the cell in series with said modulator.
According to the invention, the device also comprises:
- means for supplying at least one sequence of calibration images to the image processing means, each calibration image comprising image data activating, via said image processing means, the cells disposed along a rectilinear axis belonging to said calibration image with a non-zero calibration control voltage, each cell of the device being activated only once and with the same image data value during said sequence,
- means for measuring the current delivered by said power supply generator for each of said calibration images, and
- means for determining, from each of said current measurements, a correction value for the cells disposed along the axis associated with the calibration image in question, said image processing means being designed to correct the image data values of each video image pixel to be displayed as a function of said correction value determined for the corresponding cells. The axis along which the cells are activated during a calibration image is an axis that is perpendicular to the direction of displacement of said laser beam with respect to the substrate.
Thus, if the substrate of the active matrix has been vertically scanned by the excimer laser, each calibration image comprises image data values for activating the cells belonging to the same row of cells. If the substrate of the active matrix has been horizontally scanned by the excimer laser, each calibration image comprises image data values for activating the cells belonging to the same column of cells. The invention also relates to a method for displaying images composed of pixels by means of the device according to the invention in which, for each image to be displayed, a data control voltage that is a function of the image data value is generated at the control of each modulator for the cell in series with said modulator. This method comprises initial steps consisting in:
- using the means for supplying at least one sequence of calibration images and the image processing means simultaneously applying a non-zero calibration control voltage to the control of the modulator of the cells disposed along a rectilinear axis belonging to each calibration image, a calibration control voltage being applied only once to all of the cells during said sequence, - during the application of a calibration control voltage to the modulators of the cells disposed along the same rectilinear axis, measuring the current output from said power supply generator, using said current measurement means,
- using the means for determining the correction values and from the measurement of said current for each calibration image, deducing a correction value for the cells disposed along the rectilinear axis belonging to the calibration image in question, the same correction value being assigned to the whole of the cells disposed along said axis.
Subsequently, for each image to be displayed, each image data control voltage associated with a pixel of this image is corrected using the correction value determined for the cell corresponding to said pixel.
The invention will be better understood upon reading the description that will follow, presented by way of non-limiting example, and with reference to the appended drawings among which: - Figure 1 shows an OLED screen displaying an image without correction for the non-uniformities of the electrical characteristics of the transistors of the active matrix;
- Figure 2 is a schematic representation of an OLED display according to the invention; - Figure 3 illustrates the flow of data within the display in Figure
2 during a calibration phase of the latter; and
- Figure 4 illustrates the flow of data within the display in Figure 2 during an operational phase of the latter.
The invention will be more particularly described with regard to disparities between rows of organic light-emitting cells without this implying any limitation of the invention to this orientation of the disparities and to this type of light-emitting cells.
With reference to Figure 2, the display device or display according to the invention comprises an array 1 of light-emitting diodes Dn,p disposed in rows and columns. For simplicity, only four light-emitting diodes
(n=1 or 2, p=1 or 2) have been shown in the figure. The current flowing in each light-emitting diode is controlled by a current modulator Mn,p. A storage capacitor Cn,p and a control switch Sn,p are provided for controlling the current modulator Mn,p. The current modulator Mn,p and the control switch Sn,p are thin-film transistors operating as MOS transistors.
A power supply generator 2 is provided for supplying current to the diodes Dn,p when the associated modulators Mn,p are turned on. It is connected between a terminal common to all the light-emitting diodes and a terminal common to all the current modulators Mn,p.
The display device also comprises an array of column electrodes Xp and an array of row electrodes Yn. Each row electrode Yn connects together the controls of the control switches Sn,p of the n-th row of light- emitting diodes. Each column electrode Xp connects together one of the terminals of the control switches Sn,p of the p-th column of diodes. A row driver 3 and a column driver 4 are provided for controlling the voltage respectively applied to the rows and the columns of the device. The row driver 3 controls the state of the control switches Sn,p and the column driver 4 controls the voltage applied to the control of the current modulators Mn,p.
The display device also comprises a video input circuit 5 for receiving the video signal to be displayed and an OLED interface circuit 6 for processing the received video data and generating, as a function of this data, the control voltages to be applied, via the drivers 3 and 4, to the controls of the modulators Mn,p.
According to the invention, the display device is completed by:
- a calibration image generator 7 for successively displaying a white line over each of the rows of cells of the display; - a multiplexer 8 with two inputs receiving at a first input the video signal coming from the video input circuit 5 and at a second input the video signal coming from the calibration image generator 7; the multiplexer delivers the video signal coming from the calibration image generator 7 at its output during the calibration phase of the device and the video signal coming from the input circuit 5 during the normal operation phase of the device;
- a current measurement circuit 9 for measuring, during the calibration phase of the display, the current flowing through the cells of a row of cells of the display; since the rows of cells are turned on one after the other during the calibration phase, the current to be measured corresponds to the current output from the power supply generator 2; the current measurement circuit is therefore placed in series with said power supply generator 2; for each row of cells (i.e. for each calibration image), it delivers information that is representative of the measured current;
- a correction value determination circuit 10 for determining one correction value per row of cells from the measurement information from the circuit 9; the same correction value will be applied to all the cells of the row; this correction value is for example a multiplying coefficient or a linear function or a second degree function;
- a memory 11 for storing the correction values determined by the circuit 10 in the form of a look-up table during the calibration phase; only one correction value is saved for each row of cells; and
- a row correction circuit 12 for modifying the video levels of the signal coming from the video input circuit by applying to them, during the normal operation phase, the correction values saved in the memory 11 ; this row correction circuit is placed between the output of the multiplexer 8 and the OLED interface circuit 6.
As indicated in the description of the device in Figure 2, the device thus completed comprises two operational phases: a calibration phase and a normal operation phase. The flow of the data exchanged by the various elements of the display in Figure 2 during the calibration phase is shown in Figure 3. During this phase, the generator 7 delivers a sequence of calibration images designed to successively and separately turn on the rows of cells of the display. This sequence is transmitted via the multiplexer 8 and the correction circuit 12 to the OLED interface circuit 6. The correction circuit 12 is transparent during this phase, i.e. in that it does not modify the video levels of the video signal supplied by the multiplexer 8. This sequence of video images is used by the OLED interface circuit 6 and the drivers 3 and 4 for successively and separately turning on the rows of cells of the display. A current measurement is performed for each of the rows (i.e. for each of the calibration images) by means of the circuit 9. The circuit 10 determines a correction value for each row of cells from the current measurement relating to it. This correction value is saved in the memory 11. The correction value for the rows is for example a value calculated with respect to the row having the lowest current measurement. The luminance level of the rows is thus aligned with that of the row of lowest luminance. The video level used to power the cells that are lit in the calibration images is for example the maximum displayable video level (255 for an 8-bit coding).
As a variant, each row of cells may be calibrated by using several calibration images per row of cells, each of said images activating the cells of the row with a different control voltage. For example, during a first calibration image, a control voltage corresponding to a 255 level is applied to the cells of the row then, during a second calibration image, a control voltage corresponding to a 128 level is applied. The correction value is subsequently defined from the current measurements performed for these two calibration images.
It goes without saying that, if there are differences in luminance between columns of cells owing to a laser scanning in a horizontal direction, a correction value will be determined for each column of cells instead of each row. Generally speaking, the device of the invention allows the luminance defects generated by the excimer laser to be corrected whatever its displacement direction relative to the substrate of the active matrix.
The processing by row and that by column may also be combined. Indeed, the device in Figure 2 only corrects the variations in electrical characteristics in the direction of displacement of the laser, i.e. the variations between rows in the case of a laser beam moving vertically. However, the laser beam is not always homogeneous and, over a given single row of cells, there may be variations in luminance between sections of the row, each row having identical row sections. Advantageously, a first calibration is performed over the rows of cells of the display screen, then a second calibration is performed over the columns of cells of the display screen, the video levels of the images used for said second calibration being corrected by the correction values obtained after the first calibration (the correction circuit 12 is no longer transparent during this second calibration). The correction values obtained at the end of the first calibration are replaced in the memory 11 by those obtained at the end of the second calibration.
The flow of data exchanged by the various elements of the display in Figure 2, during the normal operation phase of the device, is shown in Figure 4. During this phase, the multiplexer 8 transmits the signal coming from the video input circuit 5 to the correction circuit 12. The correction circuit 12 modifies the video signal received with the correction values stored in the memory 11. The video signal thus corrected is then transmitted to the OLED interface circuit 12 which generates the appropriate control voltages for obtaining an image without disparities in luminance.
The fabrication of this display is very simple since the calibration image generator 7, the multiplexer 8, the correction circuit 12 and the programmable memory 11 can very easily be implanted onto the control board of the active matrix. As regards the current measurement circuit 9, this can be implemented by a shunt resistor associated with an analogue/digital converter. This device can thus be made without having to modify the architecture of the active matrix or of the row or column drivers. Furthermore, the current measurements are more reliable since the currents measured are higher than in the prior art. Lastly, the number of correction values recorded in the memory is reduced since only one correction value is calculated per row of cells.
The present invention has been described with reference to an OLED active matrix display. It goes without saying that it can equally be applied to any type of active matrix display.

Claims

1. Device for displaying video images composed of pixels, to each of which is assigned an image data value, comprising:
- a plurality of light-emitting cells (1 ) organized in rows and in columns and each connected in series with a current modulator (Mn,p) across the terminals of a power supply generator (2), each row and each column comprising at least two cells, - image processing means (12, 6) for generating, at the control of each current modulator, a control voltage from said images to be displayed, which control voltage is a function of the data value of the image to be displayed by the cell in series with said modulator, characterized in that it also comprises: - means (7) for supplying at least one sequence of calibration images to the image processing means (12, 6), each calibration image comprising image data activating, via said image processing means, the cells disposed along a rectilinear axis belonging to said calibration image with a non-zero calibration control voltage, each cell of the device being activated only once and with the same image data value during said sequence,
- means (9) for measuring the current delivered by said power supply generator for each of said calibration images, and
- means (10) for determining, from each of said current measurements, a correction value for the cells disposed along the axis associated with the calibration image in question, said image processing means being designed to correct the image data values of each video image pixel to be displayed as a function of said correction value determined for the corresponding cells.
2. Device according to Claim 1 , in which each cell has control means that comprise, aside from the modulator (Mn,p) in series with said cell, a storage capacitor (Cn,p) and a control switch (Sn,p) both connected to the control of the modulator, said control means having been fabricated within a substrate made of amorphous semiconductor transformed into polycrystalline semiconductor by scanning the substrate with a laser beam, characterized in that the axis along which the cells are activated during a calibration image is an axis that is perpendicular to the direction of displacement of said laser beam with respect to the substrate.
3. Device according to either of Claims 1 and 2, characterized in that each calibration image comprises image data values for activating the cells belonging to one and the same row of cells.
4. Device according to either of Claims 1 and 2, characterized in that each calibration image comprises image data values for activating the cells belonging to one and the same column of cells.
5. Device according to either of Claims 1 and 2, characterized in that said means for supplying at least one sequence of calibration images supplies a first sequence of calibration images for successively and separately activating the rows of cells of the device and obtaining a correction value for each of the rows and a second sequence of calibration images for successively and separately activating the columns of cells of the device and obtaining a correction value for each of the columns, the calibration images of the second sequence being corrected by the correction values of the rows of cells.
6. Device according to one of the preceding claims, characterized in that the image data value used to activate said cells during said sequence of calibration images corresponds to a peak video level.
7. Device according to one of Claims 1 to 6, characterized in that said cells are organic light-emitting diodes.
8. Method for displaying images composed of pixels by means of the device according to any one of the preceding claims in which, for each image to be displayed, a data control voltage that is a function of the image data value is generated at the control of each modulator (Mn,p) for the cell in series with said modulator, characterized in that it comprises the initial steps consisting in:
- using the means (7) for supplying at least one sequence of calibration images and the image processing means (6,12) simultaneously applying a non-zero calibration control voltage to the control of the modulator (Mn,p) of the cells disposed along a rectilinear axis belonging to each calibration image, a calibration control voltage being applied only once to all of the cells during said sequence, - during the application of a calibration control voltage to the modulators (Mn,p) of the cells disposed along the same rectilinear axis, measuring the current output from said power supply generator (2), using said current measurement means (9),
- using the means (10) for determining the correction values and from the measurement of said current for each calibration image, deducing a correction value for the cells disposed along the rectilinear axis belonging to the calibration image in question, the same correction value being assigned to the whole of the cells disposed along said axis, and in that, for each image to be displayed, each image data control voltage associated with a pixel of this image is corrected using the correction value determined for the cell corresponding to said pixel.
PCT/EP2006/062041 2005-05-24 2006-05-04 Active matrix video image display device with correction for luminance non-uniformities WO2006125718A1 (en)

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