FR2843225A1 - Active matrix image display device with compensation for trigger thresholds, uses measurement of current drawn by pixel driver to determine its threshold voltage and generates correction to command voltage to match threshold voltage - Google Patents

Abstract

The display has a network of light-emitting devices (1), each controlled by a current modulator, and a supply generator (3). An image data manager (44) corrects data as a function of pixel trigger threshold voltage for the modulator (Mn,p) to generate a corrected command voltage. The trigger threshold is determined (41) by measuring (42) the current delivered by the current generator.

Description

I

  DEVICE FOR VIEWING IMAGES WITH ACTIVE MATRIX AND A

  TRIGGERING THRESHOLD COMPENSATION.

  The invention relates to a device for displaying images partitioned in pixels to each of which image data is assigned, comprising: a display panel comprising a network of Enp light emitters each provided with control means comprising: O a current modulator Mnp controllable using a control voltage, O a storage capacity Cnp and a control switch Snp both connected to the control of said modulator, - a power supply generator of said network connected between a terminal common to all transmitters E and a terminal common to all nonp modulators Mnp. the other terminal of each transmitter and the other terminal of the corresponding modulator being interconnected, - control means of said Snp control switches adapted to select a plurality of Enp transmitters by simultaneously closing all the Snp control switches of these transmitters, - image data management means adapted to generate simultaneously at the level of the control of each modulator Mnp of a selected plurality of transmitters Enp, a data control voltage Vd-np which is a function of the data pixel or sub-pixel image

  corresponding to the Enp transmitter linked to said modulator.

  Such a device is for example described in documents WO 01/75852 PHILIPS - and US 5952789 - SARNOFF, where the emitters are organic electroluminescent cells; such a device is commonly called

  "OLED" ("Organic Light Emitting Diode" in English).

  The modulators as well as the control switches are active components for addressing the light emitting emitters of the panel; these are generally transistors produced using technology

  called "TFT" ("Thin Film Transistor" in English).

  Such transistors have a behavior which is similar to "MOSFET" type transistors; their characteristics represented by the current intensity curves between the source and the drain as a function of the control voltage applied between the gate and the source reveals a trigger threshold voltage Vth below which this modulator does not allow to pass

no current.

  As the transistors or modulators of the different electroluminescent cells or light emitters of an active matrix display panel have characteristics or trigger threshold voltages Vth substantially different from each other, it is found

  then defects in the visualization of the images.

  When a control voltage Vd is applied to the control of a modulator, the generator supplying the panel then delivers a current proportional to the control voltage Vd in this modulator and in the electroluminescent emitter, which in turn emits a quantity of light proportional to this current; when the control voltage of the modulator is lower than a trigger threshold Vth of this modulator, this modulator does not allow any significant current to pass and the transmitter

  correspondent stops emitting light.

  The same control voltage Vd will not cause the same amount of light emitted from one emitter to the other of the device, if the modulators of these emitters have different trigger threshold voltages Vth and V't, which is generally the case in the substrates of active matrices because they are generally based on polycrystalline semiconductor whose sizes and grain boundaries have very variable shapes, which are

  generated by laser recrystallization processes that are difficult to control.

  Thus, the same image data of a pixel or sub-pixel does not generate the same amount of light emitted from one emitter to another of the

  viewing, which degrades the viewing quality of the images.

  To avoid this drawback, the prior art, and in particular the document US Pat. No. 5,952,789 already cited, proposes adding at the level of each transmitter of such a device means for compensating for variations in triggering threshold voltages, which leads to costly complicate circuits

  control electronics at each transmitter of the device.

  The object of the invention is to more economically avoid this

  degraded viewing quality.

  To this end, the subject of the invention is a device for displaying images partitioned into pixels or sub-pixels to each of which image data is assigned, comprising: - a display panel comprising a network of light emitters each connected in series with a current modulator at the terminals of a power generator, each modulator being controllable using a control voltage and having a triggering voltage threshold Vthnpl - data management means d images suitable for simultaneously generating, at the level of the control of each modulator, a data control voltage Vd-np which is a function of the image data of the pixel or sub-pixel corresponding to the transmitter in series with said modulator , characterized in that it comprises: - means for generating at least one predetermined evaluation control voltage successively and separately at the level of the control of each fault modulator water, - means for measuring the current delivered by said power generator in the panel, - means for evaluating said triggering voltage threshold of each modulator from at least one measurement of said power current when an evaluation control voltage is applied only at the level of the control of said modulator, and in that said image data management means are also suitable for correcting the image data of each pixel or sub-pixel as a function of said rated trigger voltage threshold of the modulator

  in series with the transmitter corresponding to this pixel or sub-pixel.

  The display panel can be monochrome, in which case there is one emitter per pixel, or polychrome generally with three primary colors, in which case there are as many emitters as primary colors per pixel,

  each transmitter then corresponding to a sub-pixel.

  The evaluation control voltage or voltages are chosen to be greater than all of the trigger modulators of the panel modulators. Each panel transmitter is therefore connected in series with a modulator controlled by a control voltage; each series is connected to the terminals of the power generator via, generally, two common electrodes of the panel, one of them being generally connected to earth. The modulators are used to control the transmitters; the transmitter control means form what is generally called a

  active matrix based on TFT technology.

  Modulators are generally transistors whose operation is similar to that of "MOSFET" type transistors; they

  therefore have a trigger voltage threshold.

  In practice, the image data management means are adapted to memorize the threshold voltage values of each modulator, or to calculate an average threshold voltage value and to memorize the difference at the

  average value for each modulator.

  Thanks to the correction of the image data according to the invention, the same image data of a pixel or sub-pixel generates the same amount of light emitted from one emitter to the other of the display device, which provides very good viewing quality very economically

images.

  In addition, the device according to the invention advantageously makes it possible to compensate for the variations in service life of the different emitters of the panel, by maintaining the brightness of these different emitters at the same level during

the entire life of the panel.

  In addition, the device according to the invention advantageously makes it possible to compensate, after a long period of use, the variations in aging of the TFT components of the active matrix of the panel, in particular of the modulators. In summary, the subject of the invention is a device comprising a network of light emitters each controlled by a current modulator, a power generator, image data management means adapted to correct the image data. of each pixel as a function of the trigger voltage threshold of the modulator corresponding to this pixel and for generating at the level of the control of the modulators a data control voltage thus corrected, means for generating a control voltage for evaluation successively at the level control of each modulator, means for measuring the current supplied by the generator, and means for evaluating said trigger voltage threshold of each

  modulator from the measurement of this current.

  Preferably, the emitters of the panel are light-emitting diodes. Light-emitting diodes are elements which can be controlled by an electric current using a current modulator, unlike liquid crystals which can be controlled by a voltage. The electronic components at each transmitter are identical to those found in the case of active matrices of liquid crystal panels, with the difference that they here comprise a current modulator for converting the data voltage of a pixel or sub-pixel into a set electrical current; this modulator is generally a transistor conventionally comprising a gate or control electrode, a source and a drain; when a given emitter is selected, the data voltage applies to the gate of the transistor serving as a modulator; assuming that this transistor is biased in a saturation zone, the drain current then depends directly on the potential difference between the gate and the source of this transistor, therefore on the data voltage applied to the gate, since the applied voltage at the source of this transistor is otherwise constant; this data voltage is also stored in the storage capacity so that the current supplied to the emitter by the modulation transistor is proportional to the voltage between the gate and the source of said transistor and to the brightness of the transmitter is maintained for the duration of the rest of the image frame.

  Preferably, these light-emitting diodes are organic.

  Preferably, each transmitter is provided with control means which comprise, in addition to the modulator in series with this transmitter, a storage capacity and a control switch both connected to the control of the modulator, and the device also comprises control means of these control switches adapted to select a plurality of transmitters by simultaneously closing all the control switches

these transmitters.

  At each transmitter, the storage capacity and the control switch therefore have one terminal in common; the other terminal of the storage capacity is generally connected to one of the common terminals of the power supply of the transmitters; the other terminal of the control switch is connected to the control voltage generator generally via a

network of data electrodes.

  Therefore, the voltage across a storage capacity corresponds to the difference between the voltage Va at one of the terminals of the power generator and the control voltage Vd-np of the data electrode to which the corresponding control switch is connected, ie Vp-Vd-np The image data management means then make it possible to simultaneously generate a data control voltage Vd-t ,, p at the level of the

  control of each modulator of a selected plurality of transmitters.

  Preferably, the control means of each transmitter have no other active element than the modulator and the control switch. The term “active element” is understood to mean a component of the transistor type; compared to display devices provided with means for compensating for trigger thresholds of the prior art which generally comprise four transistors at each transmitter of the panel, such a device is therefore

particularly economical.

  Preferably, the light emitters of the panel being arranged in rows and columns, all of the control switches for the emitters of the same line being controlled by the same electrode called the line, for each line of emitters, the means control switches control elements of this line are formed by a

  driver of the corresponding line electrode.

  By driver is meant a control element generally comprising at least one switch generally formed by a transistor; each line driver makes it possible to select all of the transmitters of this line, which then forms a plurality of transmitters; each line driver is connected to the "gates" of the control transistors of the same line by means of the corresponding line electrode; to view the images, we select

  successively each line of the panel.

  Preferably, all of the control switches for the transmitters of the same column have a common terminal connected to the same column electrode, and each column electrode is provided with a column driver. Each column driver is connected to the "drains" of the control transistors of the same column by means of the corresponding column electrode; to view the images, a line of transmitters in the panel being selected, the voltage of each column is applied simultaneously

  data command which is a function of the image data of the pixel or subpixel at the intersection of this line and this column.

  The means for generating the at least one predetermined evaluation load voltage successively at each modulator of the panel then comprise said row drivers and said column drivers; thus, by selecting a single line n of the panel by activating only the driver corresponding to this line, and by applying the evaluation control voltage only to a column p of this panel, the generator of the panel does not supply current that in the single transmitter corresponding to the crossing of line n and column p, and the measured current essentially depends, apart from electrical leaks, on the operating characteristics of the modulator

corresponding to this transmitter.

  By applying the evaluation control voltage successively to each column of the panel, by successively selecting each row of the panel, it is thus possible to generate the evaluation control voltage successively at the level of the control of each modulator of the panel. According to the invention, column drivers are therefore used to send the predetermined evaluation control voltage signal; this advantageously avoids a set of drivers dedicated specifically to measurement

  of the currents debited in the transmitters.

  The subject of the invention is also a method of displaying images partitioned into pixels or sub-pixels using the device according to the invention in which, for each image to be displayed, the command of each modulator is generated of the panel, a data control voltage Vd np which is a function of the image data of the pixel or sub-pixel of this image corresponding to the transmitter in series with said modulator, characterized that it comprises the prior steps consisting in : - using the means for generating the evaluation control voltage, successively applying at least one evaluation control voltage Vi successively and separately at the level of the control of each modulator of the panel, - during the application of d '' a control voltage Vin evaluation to a modulator, measure the supply current linl np delivered by the power supply generator of the panel, using the current measurement means, - using evaluation means and from the measurement of this current in -np for each predetermined evaluation control voltage Vm, deduce the value V of the triggering voltage threshold of this Vth-n, p modulator, and in this that, for each image to be displayed, each data control voltage V n associated with a pixel or sub-pixel of this image is corrected as a function of the value Vth-np of the voltage threshold of

  triggering of the transmitter modulator corresponding to said pixel or subpixel.

  The at least one evaluation control voltage is greater than all of the trigger voltages of the modulators of the panel; the predetermined evaluation control voltage is chosen to be greater than all of the trigger voltages of the modulators of the panel so that each modulator lets an electric current flow through the transmitter which corresponds to it each time a signal from

  Evaluation control voltage is applied to his order.

  Preferably, the value Vth-np is deduced from the triggering voltage threshold of each modulator from the at least one measurement of the current I np using the relation 1m-np = K. (Vm - Vth-nP) 2 'or K is a constant. Preferably, the method according to the invention further comprises a prior step in which, while no evaluation control voltage VM is applied to any modulator, using current measurement means, the supply current I supplied by the supply generator; the value thus measured 10 of the leakage current is then subtracted from the measured values of current Imn-p to deduce the values Vth-np of the

  tripping voltage thresholds for the panel modulators.

  Preferably, for each image to be displayed: - successively different pluralities of transmitters of the panel are selected by closing the corresponding control switches using their control means, - each time that a plurality of transmitters is thus selected, using the image data management means, said voltage is simultaneously generated at the control of each modulator of these transmitters

corrected data order.

  This plurality of transmitters generally corresponds to lines of transmitters of the panel; to fully view an image, we therefore successively select each line of the panel using line drivers; during the selection of a line, the

  different control voltages via, generally, column drivers.

  The invention will be better understood on reading the description which will

  follow, given by way of nonlimiting example, and with reference to the appended figures in which: - Figure 1 describes the control means of a transmitter of a display panel according to an embodiment of the invention, - FIG. 2 illustrates the current-voltage characteristics of different modulators of a display panel, FIG. 3 describes a display device according to an embodiment of the invention, where each transmitter is provided with control means according to FIG. 1 , - Figure 4 illustrates a timing diagram of application of an evaluation control voltage to the various modulators of the panel of Figures 1 and 3, according to a mode of application of the method according to the invention; this figure does not take into account a scale of values in order to better reveal certain details which would not appear

  clearly if the proportions had been respected.

  Referring to FIG. 3, the image display device according to the invention comprises: - an active matrix OLED panel 1 comprising a network of organic emitting emitters Enp arranged in rows and columns; with reference to FIG. 1, each transmitter comprises: O a current modulator M controllable using an np control voltage formed by a first transistor, O a storage capacity Cnp and a control switch S formed by a second transistor; these two elements are not both connected to the control of the modulator; a power generator 3 of this network (not shown) delivering a voltage V between a terminal common to all the modulators M and a terminal common to all the transmitters Enp p the latter being connected to earth; the other terminal of each transmitter and the other terminal of the corresponding modulator

are interconnected.

  As illustrated in FIG. 2, the different modulators Mnp have behaviors comparable to MOS transistors and have different thresholds for triggering voltage Vth-n.p. varying here between 0.9 V and 1.1 V. This heterogeneity of the triggering voltage threshold is due to the fact that, in the crystalline poly-silicon transistors which compose the active matrix intended for the addressing of the electroluminescent emitters of the panel , the size of the silicon grains and the grain boundaries are particularly difficult to control during the manufacture of the panel, in particular during the phase of

  recrystallization of silicon generally carried out by laser.

  When a MOS type transistor is used in its so-called saturation zone where the intensity Id which passes through it is approximately constant whatever the voltage applied between the drain and the source, it is known that the intensity d which crosses it then varies according to the potential difference V0s applied between the grid and the source according to the relation R1: Id = K. (Vgs - Vth) 2 o K is a constant which is worth kW / L, and o: - W and L are respectively the width and the length of the transistor passage channel, - k is a constant which depends on the manufacturing technology, - Vth is the triggering threshold voltage, below which no current flows through the transistor ; it is this value which varies from one modulator of the

panel to another.

  The OLED panel also includes an array X of column X electrodes and an array Y of row electrodes Yn; each line electrode connects the control of the various control switches linked to the transmitters of this line; each column electrode connects one of the terminals of the

  different control switches linked to the transmitters in this column.

  The display device comprises a series of line drivers (not shown) positioned on a vertical side of the panel; each line driver makes it possible to control a line electrode of the panel; these drivers form means for controlling the panel control switches which are suitable for selecting the panel transmitters by line, by simultaneously closing all the control switches of a

same line.

  The display device comprises a series 21 of column drivers positioned on a horizontal side of the panel; each driver of

  column is used to control a panel column X electrode.

  The display device comprises image data management means 44 adapted to generate, for each transmitter Enp linked to a pixel or sub-pixel of an image to be displayed, a data control voltage Vd-np which is a function the image data of this pixel or sub-pixel; he

  this is a classic element of an electroluminescent device.

  With the help of the row and column drivers, it is then possible to address to each transmitter of the panel the data control voltage which

  matches; these addressing links are not shown in FIG. 3.

  According to the invention, the display device further comprises: - means 41 for generating at least one evaluation control voltage VmiP Vm2, ... predetermined, - means 42 for measuring the current supplied by the generator panel feed; these means include for example a calibrated resistor connected in series with the panel across the power generator, a high impedance amplifier to measure the voltage across the calibrated resistor, and an analog / digital converter to convert the voltage measurement into a numeric value; means of evaluation 43 of the triggering voltage threshold Vth-np of each modulator Mnp of the panel from at least one supply current measurement Im np carried out when an evaluation control voltage Vm is applied only at the command of this Mn'p modulator; these means are adapted in a manner known per se to deduce a threshold value Vth-np from a value of intensity Im n p using the relation R1 already mentioned; these means integrate a memory

  to store the Vth-np results of all the modulators of the panel.

  Finally, according to the invention, the image data management means 44 are also suitable for correcting the image data of each pixel or sub-pixel to be addressed to each modulator as a function of the voltage threshold of

  triggering of this modulator evaluated by the evaluation means 43.

  We will now describe the implementation of the method according to the invention with reference to the timing diagram of FIG. 4: Before a series of visualization of images using the panel, we proceed to the prior evaluation of the voltage threshold of tripping of each modulator of the panel: - as a preamble to this evaluation phase, a leakage current is measured in the panel as follows: no line of the panel being selected, using the measurement means of current 42, the current 10 supplied by the power generator 3 of the panel is measured; the current 10 integrates all the ohmic electrical losses of the panel, in particular through the resistive elements of the active matrix; - using the line drivers, a line of modulators Mn .., Mn'p is then selected. MnP + 1, ... of the panel by applying a voltage Vs to the line electrode Yn) so that all of the control switches Snjl .., Sn, p X ... of this line are closed during that this line is selected, using the means 41 and the column drivers 21, a voltage Vm is applied for a time ta

  successively and separately to each of the column electrodes Xi, ...

Xp, Xp + i1 ....

  Using row and column drivers, it is thus possible to apply the voltage V. successively and separately at the level of the control of each Mnp modulator of the panel; in other words, we manage to address the data VM separately to each pixel or sub-pixel of the panel, which means that, while a pixel or sub-pixel is thus addressed, all the other pixels or sub-pixels of the panel are assigned the value zero or almost zero, so that no other current than the leakage currents previously

  measured does not pass through unaddressed pixels.

  The voltage V is chosen in a manner known per se greater than all the probable triggering voltage thresholds of the different

panel modulators.

  This voltage Vm is called data because it is addressed to each pixel in the same way as image data; it is called evaluation because it will be used to evaluate the triggering voltage threshold of each modulator of the panel, as follows: - while applying the voltage Vm to the control of a modulator Mnp of the panel, at using the current measurement means 42, the current I-np delivered by the power supply generator 3 of the panel is measured mainly through this modulator and the corresponding transmitter P; to obtain the actual current passing through this modulator, the value 10 of the previously measured leakage currents is subtracted from the measured value I; mn, p - using the evaluation means 43, more precisely using the relation Rl already cited o (î n p-lo) = K (Vm - Vth-n p) 2, we deduce the value Vthfl p of trigger threshold of the modulator M and this value is memorized. n, p This is done for each of the modulators of the panel so as to obtain a complete memorized table of triggering voltage threshold values. As illustrated in FIG. 4, the intensity measurement starts a little after the start of the application of the voltage VM and ends a little before the end of the application of this evaluation voltage; the resulting time and 8te time offsets make it possible to avoid taking into account transient currents, in particular capacitive or inductive; the duration tm of each measurement is therefore less than the duration ta of applying the evaluation voltage Vm to the

control of a modulator.

  After the evaluation phase which has just been described, the average value Vthm of these trigger voltage threshold values is calculated, and each value Vthfn.p of the table is replaced by the arithmetic difference of this value with respect to at the average value, either by AVth-np = Vth-npVthm The second phase of the process is that of viewing a series of images; for each image, the procedure is as follows: - for each transmitter Enp linked to a pixel or sub-pixel of this image, using the image data management means 44, one generates in a manner known in it even a data control voltage Vd-np which is a function of the image data of this pixel or sub-pixel; according to the invention, this value Vd-np is corrected by replacing it with Vd-np + AVth-np - for the other steps of viewing this image, we then proceed in a conventional manner as follows: ò we successively select each line n of transmitters of the panel by applying to the corresponding line electrode Yn a voltage Vs adapted to close the control switches S, p of this line, ò each time a line of transmitters is thus selected, at with the aid of the image data management means 44 and of the column drivers, one generates, simultaneously at the level of the control of each modulator Mn, ..., Mnp ... of this line, the voltage of

  data command Vd-np + to Vth n p corrected according to the invention.

  Thanks to the correction of the image data according to the invention, the same image data of a pixel or sub-pixel generates the same amount of light emitted from one emitter to the other of the display device, which provides very good quality image viewing. The method of compensating for variation of the triggering voltage threshold according to the invention advantageously compensates for the drifts as a function of the time of these thresholds, and the differences between the drifts of the different modulators of the panel; this process therefore makes it possible to ensure a more constant image viewing quality over the life of the panel. According to a variant of the method of the invention, two values VMI, V1-2 of evaluation data voltage are used, which are successively applied to the control of each modulator of the panel, as previously described; each modulator Mnp is then associated with two current measurements lr np and ln2np, which makes it possible, for each modulator, to calculate a more precise value of the triggering voltage threshold Vth-np According to another variant of the method of the invention, the image data Vd.np of each pixel or sub-pixel is directly corrected by the triggering voltage threshold Vth-np by replacing this data with the value Vd + Vth n, p The present invention has been described with reference to an OLED type panel; it is obvious to those skilled in the art that it can be applied to other types of display panels, electroluminescent or not, without leaving the

The scope of the claims below.

Claims (10)

  i.- Device for displaying images partitioned in pixels or subpixels to each of which image data is assigned, comprising: - a display panel (1) comprising a network of light emitters (In p) each connected in series with a current modulator (Mn P) at the terminals of a power generator (3), each modulator being controllable using a control voltage and having a triggering voltage threshold V thnp '- des image data management means (44) adapted to generate simultaneously, at the level of the control of each modulator (Mn p), a data control voltage Vd-np which is a function of the image data of the pixel or sub-pixel corresponding to the transmitter (E np) in series with said modulator, characterized in that it comprises: - means (41) for generating at least one predetermined evaluation control voltage successively and separately at the level of the control of each modulator (Mn p) of the panel, - means (42) for measuring the current delivered by said power generator (3) in the panel, - evaluation means (43) of said triggering voltage threshold of each modulator ( Mnp) from at least one measurement of said supply current when an evaluation control voltage is applied only at the level of the control of said modulator (Mn p), and in that said data management means d images (44) are also adapted to correct the image data of each pixel or subpixel as a function of said evaluated trigger voltage threshold of the modulator   (Mn p) in series with the transmitter (Enp) corresponding to this pixel or subpixel.   2.- Device according to claim 1, characterized in that said   emitters are light emitting diodes.   3.- Image display device according to any one of   claims 1 to 2 characterized in that each transmitter is provided with   control means which comprise, in addition to the modulator in series with said transmitter, a storage capacity (Cnp) and a control switch (Sn p) both connected to the control of said modulator, and in that it also comprises means for controlling said control switches (SP) adapted to select a plurality of transmitters (Enp) by closing   simultaneously all the control switches (Sn p) of these transmitters.   4.- Device according to claim 3 characterized in that said control means of each transmitter (Enp) have no other element   active than said modulator (M p) and said control switch (Sn p).   5.- Device according to any one of claims 3 to 4 characterized   in that, said light emitters being arranged in rows and columns, all the switches for controlling the emitters of the same line being controlled by the same electrode called line, for each line of emitters, said means of control of the control switches of the elements of this line are formed by a driver of said corresponding line electrode.   6.- Device according to claim 5 characterized in that all of the control switches of the transmitters of the same column has a common terminal connected to the same column electrode, and in that each   column electrode has a column driver.   7.- Method for viewing images partitioned into pixels or subpixels using the device according to any one of the claims   previous in which, for each image to be displayed, a command control voltage Vd-np is generated at the command of each modulator (Mnp) of the panel which is a function of the image data of the pixel or sub-pixel of this image corresponding to the transmitter (In p) in series with said modulator, characterized that it comprises the prior steps consisting in: - using the means (41) for generating the evaluation control voltage, applying successively at least one evaluation control voltage Vi successively and separately at the level of the control of each modulator (Mn) of the panel, - during the application of a control voltage VM evaluation to a modulator (Mnp), measuring the supply current iml np supplied by said supply generator (3), using said current measuring means (42), - using said evaluation means (43) and from the measurement of said current Inl np for each voltage V. of comm ande of predetermined evaluation, deduce the value Vth-np of the triggering voltage threshold of said modulator (Mn p), and in that, for each image to be displayed, each data control voltage Vd-np associated with a pixel or sub-pixel of this image is corrected as a function of the value Vth-np of the triggering voltage threshold of the modulator (Mnp) of the transmitter (Enp) corresponding to said pixel or sub-pixel.   8.- Method according to claim 7 characterized in that the at least one evaluation control voltage is greater than all the voltage thresholds   triggering said modulators.   9.- Method according to any one of claims 7 to 8 characterized   in that the value Vth-np is deduced from the triggering voltage threshold of each modulator (Mn p) from the at least one measurement of the current limnp in   using the relation 1m-np = K. (Vm - Vth-np) 2 'or K is a constant.   10.- Method according to any one of claims 7 to 9 characterized   in that it further comprises a preliminary step in which, while no evaluation control voltage V. is applied to any modulator (Mnp), using said current measurement means (42) , the supply current 10 supplied by said supply generator (3) is measured, and in that the value thus measured 10 of the leakage current is subtracted from the measured values of current Imn-p to deduce the values Vth- np voltage thresholds   triggering modulators (Mn p) 5 11.- Method according to any one of claims 7 to 10 when   depends on any one of claims 3 to 6 characterized in that,   for each image to be displayed: - successively different pluralities of transmitters of the panel are selected by closing the corresponding control switches using their control means, - each time a plurality of transmitters is thus selected, using the image data management means, said voltage is simultaneously generated at the control of each modulator of these transmitters corrected data order.