WO2006087327A1

WO2006087327A1 - Method and apparatus for luminance non-homogeneity compensation in an am-oled

Info

Publication number: WO2006087327A1
Application number: PCT/EP2006/050930
Authority: WO
Inventors: Sébastien Weitbruch; Rainer Schweer; Andreas Hille
Original assignee: Thomson Licensing
Priority date: 2005-02-16
Filing date: 2006-02-14
Publication date: 2006-08-24

Abstract

The present invention relates to a method and an apparatus for compensating the luminance rendering difference between luminous elements of a display device such as an active matrix OLED (Organic Light Emitting Display). The luminance generated by each of the luminous elements is based on the intensity of a signal supplied to the luminous element for displaying the corresponding pixel of the picture and the intensity of the signal to be supplied to each luminous element is based on a set of reference signals. According to the invention, the intensity of the signal of the luminous elements of a second line is adjusted for compensating at least a part of luminance rendering difference with the luminous elements of a first line of the display device, said adjustment being made by adjusting the level of the set of the reference signals used by the luminous elements of the second line.

Description

METHOD AND APPARATUS FOR LUMINANCE NON-HOMOGENEITY COMPENSATION IN AN AM-OLED

The present invention relates to a method and an apparatus for compensating the luminance rendering difference between luminous elements of a display device such as an active matrix OLED (Organic Light

Emitting Display). This method has been more particularly but not exclusively developed for video application.

Background

The structure of an active matrix OLED (or AM-OLED) is well known. It comprises :

- an active matrix containing, for each cell, an association of several TFTs with a capacitor connected to an OLED material; the capacitor acts as a memory component that stores a value during a part of the video frame, this value being representative of a video information to be displayed by the cell during the next video frame or the next part of the video frame; the TFTs act as switches enabling the selection of the cell, the storage of a data in the capacitor and the displaying by the cell of a video information corresponding to the stored data;

- a row or gate driver that selects line by line the cells of the matrix in order to refresh their content;

- a column or source driver that delivers the data to be stored in each cell of the current selected line; this component receives the video information for each cell; and

- a digital processing unit that applies required video and signal processing steps and that delivers the required control signals to the row and column drivers.

Actually, there are two ways for driving the OLED cells. In a first way, each digital video information sent by the digital processing unit is converted by the column drivers into a current whose amplitude is proportional to the video information. This current is provided to the appropriate cell of the matrix. In a second way, the digital video information sent by the digital processing unit is converted by the column drivers into a voltage whose amplitude is proportional to the video information. This current or voltage is provided to the appropriate cell of the matrix.

From the above, it can be deduced that the row driver has a quite simple function since it only has to apply a selection line by line. It is more or less a shift register. The column driver represents the real active part and can be considered as a high level digital to analog converter. The displaying of a video information with such a structure of AM-OLED is the following. The input signal is forwarded to the digital processing unit that delivers, after internal processing, a timing signal for row selection to the row driver synchronized with the data sent to the column drivers. The data transmitted to the column driver are either parallel or serial. Additionally, the column driver disposes of a reference signaling delivered by a separate reference signaling device. This component delivers a set of reference voltages in case of voltage driven circuitry or a set of reference currents in case of current driven circuitry. The highest reference is used for the white and the lowest for the smallest gray level. Then, the column driver applies to the matrix cells the voltage or current amplitude corresponding to the data to be displayed by the cells.

In order to illustrate this concept, it will be taken the example of a voltage driven circuitry in the rest of the present document. The driver taken as example will use 8 reference voltages named VO to V7 and the video level are built as defined in the following table (table 1 ):

For computing the output voltages for the different input video levels, the following reference voltages can be used for example :

V0 = 3V V1 = 2,6V V2 = 2,2V V3 = 1,4V V4 = 0,6V V5 = 0,3V V6 = 0,16V V7 = 0V An output voltage can be thus computed for every input video level. These output voltages are given by the following table (table 2). The whole table is given by the annex 1.

One problem of the current AMOLED screens is that the luminance rendering of the different cells or luminous elements is not homogeneous on the whole screen. In other words, a pattern can be visible on the screen while displaying homogeneous areas (e.g. a white page). An example of such a non-homogeneity issue is illustrated on figure 1.

Figure 1 shows the display of two various sources on a display suffering from line non-homogeneity. In other words, there are luminance rendering (or efficiency) differences among the vertical direction of the screen. These homogeneity problems are mainly related to the active matrix backplane containing the thin-film-transistors arrays or can be generated by printing technologies such as those used with polymer OLED materials.

In the field of TFT non-homogeneity, one problem can be linked to the technology used for manufacturing the active matrix. Indeed, today there are two main materials used for the thin-film-transistors:

- Amorphous Silicon (a-Si), as used for the active matrix in liquid crystal displays, that is quite homogeneous but less stable when current is flowing through the OLED; furthermore, it is difficult to integrate driver with such thin-film-transistors; so, it is not so suitable for very small applications with high integration level; however, the technology is mature since it is used for many years for Liquid Crystal Displays;

- Low-temperature-Poly-Silicon (LTPS) that is produced from a-Si by re-crystallization with an excimer laser; because of its high carrier mobility, LTPS provides thin-film-transistors with high-current carrying capability and high switching speed; the utilization of polysilicon enables to achieve the coverage of large surfaces but homogeneity is more critical; the process for producing Low Temperature Poly Silicon required for AMOLED takes place at a temperature around 45O⁰C; it requires the utilization of a excimer laser for transforming the amorphous silicium into polysilicon; the excimer laser wavelength (308nm) is absorbed efficiently by the amorphous silicon without strong increasing of the substrate temperature; moreover, because of the nature of the fabrication process of Poly-Silicon thin-film-transistors, a dispersion in the electrical characteristics of the thin-film-transistors over the substrate area is always observed; indeed, the laser beam must scan the screen to crystallize the a-Si into PoIy-Si and the overlapping areas between the scanned areas shows behavioral variations; on the other hand, such a technology makes driver integration possible and is therefore suitable for very small displays with a high integration level; this technology is not so mature compared to the Amorphous Silicon one. It is an object of the present invention to propose a compensation method against the lack of homogeneity seen line by line independently of its source (LTPS manufacturing, screen printing...). For horizontal homogeneity problems, further methods can be applied on the top of the inventive method.

The homogeneity problem is following a statistical distribution and is therefore different from one screen to another one. This random behavior requires the use of calibration methods for luminance non-homogeneity compensation. The calibration method can be done via a CCD camera or other systems. The main idea behind the calibration is to use reference pictures (e.g. full white page, full red page, full green page and full blue page) and to measure the luminance perceived at any point of the screen in order to determine a set of compensation factors for modifying pictures to be displayed so that the displayed pictures look homogeneous. For example, considering a screen presenting a luminance non- homogeneity between its lines, all the pixels of a line having the same behavior. A known method consists of determining compensation factors for each line and applying these factors to the video signal of the pictures to be displayed for adjusting the line luminance to a predefined screen behavior. In this case, the darker line (the line with the lowest luminance rendering) is used as a reference line and the associated compensation factor is 1. All the other lines will be compensated with a factor lower than 1.

For instance, if a line is 20% more luminous than the reference line, the signal intensity of all pixels from this line will be multiplied by a factor 0.8. This example is illustrated by figure 2.

Figure 2 shows the behavior of a screen suffering from vertical homogeneity problems and having a dynamic difference of 20% between the most luminous line (line 60) and less luminous one (line 105). With the current approach, the most luminous line will be reduced to 80% of video amplitude whereas the less luminous one is using the full video dynamic range. In order to implement such a solution, compensation factors are for example stored in a memory and then applied line by line to the video signal of the picture to be displayed as illustrated by figure 3. In this figure, the source picture is a full white picture and is modified by compensation factors in order to generate a modified picture comprising luminance distortions inverse to those of the screen.

Such a solution introduces a global loss of luminance and video dynamic. Indeed, in our example video dynamic as well as luminance is decreased by 20% for some parts of the screen and this can be even more depending on technology issues. Today, screens require high video dynamic range and even more than

8-bit to satisfy new demanding applications. Therefore, solutions as disclosed in previous paragraph should be avoided in order not to reduce the video range and luminance.

Invention

In order to solve these problems, it is proposed a method for compensating the luminance rendering difference between the luminous elements of a display device comprising a plurality of luminous elements organised in lines, each luminous element being used for displaying a pixel of a picture, wherein the luminance generated by each of said luminous elements is based on the intensity of a signal supplied to the luminous element for displaying the corresponding pixel of the picture and wherein the intensity of the signal to be supplied to each luminous element is based on a set of reference signals, characterized in that the intensity of the signal of the luminous elements of a second line is adjusted for compensating at least a part of luminance rendering difference with the luminous elements of a first line of the display device, said adjustment being made by adjusting the level of the set of the reference signals used by the luminous elements of the second line. Thus, the compensation is made by modifying the reference signals and not by modifying the video signal of the picture to be displayed.

In particular, a basic set of reference signals is defined for said first line of luminous elements and the set of reference signals to be used for the second line of luminous elements is determined by multiplying the reference signals of the basic set by a compensation factor allocated to the second line of luminous elements.

In a preferred embodiment, the basic set of reference signals is defined for the line having the highest luminance rendering and in that the compensation factor allocated to the other lines are higher than 1 , said compensation factors decreasing with the luminance rendering of the associated line.

The invention concerns also an apparatus for compensating the luminance rendering difference between the luminous elements of a display device comprising a plurality of luminous elements organised in lines, each luminous element being used for displaying a pixel of a picture. The luminance generated by each of said luminous elements is based on the intensity of a signal supplied to the luminous element for displaying the corresponding pixel of the picture and the intensity of the signal to be supplied to each luminous element is based on a set of reference signals,

According to the invention, the apparatus comprises means for adjusting the intensity of the signal of the luminous elements of a second line in order to compensate at least a part of luminance rendering difference with the luminous elements of a first line of the display device, the adjustment being made by adjusting the level of the set of the reference signals used by the luminous elements of the second line.

Brief description of the drawings Exemplary embodiments of the invention are illustrated in the drawings and in more detail in the following description.

In the figures :

Fig.1 illustrates the problem of luminance rendering non- homogeneity of a display device for two pictures;

Fig.2 shows the luminance rendering difference between different lines of luminous elements in a display device;

Fig.3 shows a known solution for compensating said luminance homogeneity problem; and

Fig.4 shows a device implementing the method of the invention;

Description of preferred embodiments

The invention proposes a new solution is based on a vertical voltage reference adjustment in order to compensate vertical screen luminance variation while keeping full 8-bit video range.

The video level encoding is based on voltage reference signaling that defines for each 8-bit video level the voltage to be applied to the screen columns as defined in table 3.

According to the invention, for compensating the luminance difference between line 60 and line 105, line 105 should be 20% more luminous and line 60 should not be modified.

To this end, the voltage references for each line are modified as defined in the following table (table 4).

This table shows two sets of reference voltages that are used for compensating luminance non-homogeneity between line 60 and line 105. One set, called basic set, is defined for line 60 and a second set is defined for line 105.

The following table (table 5) shows the theoretical luminance values obtained with these voltage references. The whole table is given by the annex 2.

There is a theoretical increase of 20% between line 105 and line 60 but since line 105 is 20% less efficient, a same luminance value is obtained. The voltage references have been increased by a factor VL2 = 1.095 since the relation between luminance and voltage is the same as between current and voltage : a square function under the form Luminance= k x (V-V_th)² where k is a multiplication coefficient and V_th is a threshold voltage (0 in this example).

In order to implement such a concept, a new set of voltage references has to be defined for each line and must be updated at each line during the vertical scanning of the screen. The basic voltage references defining the full-white screen luminance are adapted to the more efficient line, all other voltage references must be computed according this reference to target equal screen luminance.

In this case, the compensation is done by mathematical function like for instance M(y) x Ref_n + A(y) that is changing from line to line (y). The coefficient M(y) and A(y) are increasing with the luminance rendering of the line y is increasing. A more complicated function can be used. More generally, the various voltage references can be adjusted or computed line by line to the display during calibration phase and then stored in a LUT inside a digital processing unit. If there are also a horizontal homogeneity variation, the darker pixel from a line (except black that means defect pixel) is used to calibrate the whole line. On the top of that, it is then possible to use simple horizontal coefficient to adjust the video level on a line. This solution assumes that the vertical variation is more important than the horizontal one so that less video dynamic range is lost by this method.

Furthermore, it is also possible to adjust a global lifetime compensation on the top of this system in order to adjust the voltage references to a luminous efficacy degradation.

Figure 4 illustrates a possible implementation of the inventive compensation method. The input picture is forwarded to a standard video correction performing contrast, luminance and color space conversion block 10. At this location, an optional block 11 can be added to adjust the video coefficient in order to compensate a possible horizontal luminance non- homogeneity. The output signals of the block 10 are forwarded to a standard OLED driving unit 12. The standard OLED driving unit 12 controls the operation of the column driver 13 and the row driver 14 of the active matrix 15 of the display device. In parallel to that, a specific vertical non-homogeneity compensation block 16 extracts compensation factors from a LUT (not shown) for each line and multiplies them to basic reference voltages in order to obtain the voltage references to be used by the column driver for displaying the corresponding line. Thus, the compensation factors are updated line by line during the scanning of the screen. To this end, this block 16 is synchronized with the row driver 14. Optionally, an ageing compensation block 17 taking into account the total display running time can further adjust the voltage references in order to increase the voltage references while the global display efficacy is reducing. This compensation is limited and can only extend a bit the global display lifetime.

Claims

WHAT IS CLAIMED IS :

1 ) Method for compensating the luminance rendering difference between the luminous elements of a display device comprising a plurality of luminous elements organised in lines, each luminous element being used for displaying a pixel of a picture, wherein the luminance generated by each of said luminous elements is based on the intensity of a signal supplied to the luminous element for displaying the corresponding pixel of the picture and wherein the intensity of the signal to be supplied to each luminous element is based on a set of reference signals, characterized in that the intensity of the signal of the luminous elements of a second line is adjusted for compensating at least a part of luminance rendering difference with the luminous elements of a first line of the display device, said adjustment being made by adjusting the level of the set of the reference signals used by the luminous elements of the second line.

2) Method according to claim 1 , characterized in that a basic set of reference signals is defined for said first line of luminous elements and in that the set of reference signals to be used for the second line of luminous elements is determined by multiplying the reference signals of the basic set by a compensation factor allocated to the second line of luminous elements.

3) Method according to claim 2, characterized in that the basic set of reference signals is defined for the line having the highest luminance rendering and in that the compensation factor allocated to the other lines are higher than 1 , said compensation factors decreasing with the luminance rendering of the associated line. 4) Method according to claim 2 or 3, characterized in that the luminance renderings of lines of luminous elements of the display device are determined by:

- displaying a reference picture by the display device, each pixel of the reference picture having the same video level, and

- measuring the luminance rendering value of each luminous element by a CCD system, and

- computing the luminance rendering value for each line of luminous element.

5) Apparatus for compensating the luminance rendering difference between the luminous elements of a display device comprising a plurality of luminous elements organised in lines, each luminous element being used for displaying a pixel of a picture, wherein the luminance generated by each of said luminous elements is based on the intensity of a signal supplied to the luminous element for displaying the corresponding pixel of the picture and wherein the intensity of the signal to be supplied to each luminous element is based on a set of reference signals, characterized in that it comprises means (16) for adjusting the intensity of the signal of the luminous elements of a second line in order to compensate at least a part of luminance rendering difference with the luminous elements of a first line of the display device, said adjustment being made by adjusting the level of the set of the reference signals used by the luminous elements of the second line.