WO2007004155A2

WO2007004155A2 - Oled display with extended grey scale capability

Info

Publication number: WO2007004155A2
Application number: PCT/IB2006/052176
Authority: WO
Inventors: Jurgen J. L. Hoppenbrouwers
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-07-04
Filing date: 2006-06-29
Publication date: 2007-01-11
Also published as: TW200709159A; WO2007004155A3

Abstract

A method for extending grey scale capabilities of a display (104) is disclosed. The display (104) is divided into a plurality of blocks (126), each block (126) being driven by a corresponding column driver (124). In dependence on a brightness level of a part of a received image (120) to be displayed on a corresponding block (126), a supply voltage (Vdd) is generated for a column driver (124) for the corresponding block (126). The level of the supply voltage (Vdd) depends on the brightness level of each block (126). The present invention has the advantage over the prior art that it expands the grey scale capabilities of a display (104) without needing to extend the capabilities of the column drivers (124).

Description

OLED display with extended grey scale capability

FIELD OF THE INVENTION

This invention pertains in general to the field of organic light emitting devices (OLEDs). More particularly the invention relates to a method for extending grey scale capabilities of an OLED display, to a processing unit for carrying out such a method, and to a display device comprising the processing unit.

BACKGROUND OF THE INVENTION

Polymer and Small Molecule Organic Light Emitting Devices, also called OLEDs, have opened a new path to make high quality displays. The advantages of these displays are the self-emissive technology, the high brightness, the near-perfect viewing angle and the fast response time. These arguments show that OLEDs hold the promise of providing a better front screen performance than LCDs, also called Liquid Crystal Displays. Large size displays will use active matrix addressing, which means that each pixel has its own driving circuit that generates the current that is driven through the OLED. Because polymer and small-molecules OLED displays are emissive, a high brightness as well as a large contrast ratio can be obtained. Light is only generated where needed, ensuring a low black level. This also enables a high local peak brightness. Currently, display size and peak brightness are limited by the efficiencies of the materials used. As these efficiencies increase, larger displays with larger brightness become possible. Current TV sets (CRT as well as LCDs) are driven with 8-bit grey scale capability. This is sufficient for current brightness levels and broadcast material. However, the quality of the displayed video material is increasing with the introduction of high definition TV and this can only be shown on a display with a sufficiently large contrast and a sufficient grey scale capability. Furthermore, displays used in a professional environment like hospitals have heavier requirements regarding contrast/brightness/grey scale capability than TV. Therefore, high-dynamic range displays are attractive for professional as well as consumer markets.

Standard transmissive displays like LCDs modulate light emitted by a separate backlight. Such displays usually have a black level, which is not fully black due to light leakage (the liquid crystal cannot completely block the incoming light). Furthermore, the maximum brightness is equal over the entire screen and is determined by the backlight. This maximum brightness is limited because it must be generated over the entire backlight area and higher brightness values will result in an increase of power consumption. These factors limit the dynamic range of current LCD displays.

Several proposals are made to obtain a high dynamic range LCD display. For example, the University of Columbia proposed a LCD with a LED-backlight. In this example, separately controllable LEDs are used in the backlight. In this way, the light from the backlight can locally be controlled and a high-dynamic range backlight is obtained. The grey scale capability of the display is enhanced when the LEDs are driven in dependence on the grey scales to be reproduced. This option illustrates a display-behind-display principle. A drawback is that the backlight as well as the electronic driving is more complicated than in a conventional LCD, increasing the cost. However, a nice feature of this option is that the bit depth of the column drivers is not increased. Standard common-off-the-shelf column drivers with 8 bits can be used to drive the displays.

In active matrix OLED, also called AMOLED displays, light is generated when a current is driven through the light emitting diodes, also called LEDs, as OLEDs are current driven devices. This current is generated by a pixel driving circuit, which is located in the pixel (hence active matrix driving). The data is supplied to this pixel driving circuit through column or data lines either as a current or as a voltage. In the case of voltage programming, standard off-the-shelf column drivers that are comparable to the ones used for active matrix LCD, also called AMLCD displays, are used.

Basically, the column drivers are D/A converters. Standard drivers have 6 or 8 bit grey scale capability. Although the row drivers can be integrated in the matrix plate when low temperature poly silicon is used, this is much more complicated for the column drivers. Usually, the column drivers are mounted on a foil that is connected to the display. Each driver IC drives a certain number of display lines. A typical driver can drive 384 columns.

The maximum voltage that is put on the columns and programmed in the pixel driving circuit is determined by the supply voltage of the driver IC. In AMOLED displays, it is possible to drive lines of a single color with one IC. This enables the color point of the display to be controlled by setting the supply voltages of the drivers individually for each IC.

The number of grey levels that can be displayed by the AMOLED display is determined by the number of bits of the column driver (6 or 8 bit). In high dynamic range display, the number of desired bits is larger because the number of grey levels that can be distinguished by a human viewer is larger. In other words, a high dynamic range display needs a larger number of grey scale values. A straightforward option is to design DA converters with a larger number of bits, but this will increase the complexity and cost of the driver as at the same time the column driver should be capable of switching within a fraction of the line-time of the display.

Thus, there is a need for a new advantageous method and system, which can expand the grey scale capabilities of a display while using standard column drivers.

SUMMARY OF THE INVENTION Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems at least partly by providing a method, a processing unit, and a display device that expands the grey scale capabilities of an OLED display, according to the appended independent claims. The dependent claims define advantageous embodiments.

The invention is based on the insight to divide the image to be displayed into a plurality of blocks. The brightness level required in each block is then determined. A higher or lower supply voltage is then applied to a column driver for each block, wherein the amount of voltage applied to each column driver depends on the determined brightness of each block.

The present invention has the advantage over the prior art that it expands the grey scale capabilities of a display without needing to extend the capabilities of the column drivers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which

Fig. 1 is a schematic diagram of an OLED display system according to an embodiment of the invention;

Fig. 2 is a flow chart illustrating a method for expanding the grey scale capabilities of an OLED display according to an embodiment of the invention; and

Fig. 3 is a block diagram for explaining a calculation of processed data, when gamma correction is required. DESCRIPTION OF EMBODIMENTS

The following description focuses on an embodiment of the present invention applicable to an OLED display. However, it will be appreciated that the invention is not limited to this application but may be applied to many other displays.

Briefly, the invention controls the supply voltage of the column drivers in such a way that the grey level range that can be displayed matches the range of the image information to be displayed. The image can be divided in segments or blocks. For each segment, the grey levels are divided from level 0 (the black level) to the maximum level that is present. If the part of the image corresponding to the segment is bright, the supply voltage is high and the full range of grey levels can be displayed. On the other hand, if the image part is dark, the supply voltage is lowered and dark grey levels can be displayed more accurately. This is because the number of grey levels is now divided over a smaller luminance range, resulting in a larger precision. In this way, the grey level capability is sufficient in dark image parts as well as bright image parts. If in a certain segment both a dark and bright region is present, the supply voltage should be chosen such that all levels can be displayed correctly. Otherwise, artifacts are introduced. If the segments are small enough, the bright part of the image will be more visible for viewers than the dark part and the limitation of grey scales in the dark part of the segment will not be directly visible. In the column direction, the segment size is determined by the number of columns that are connected to a single driver IC. In the row direction, the segment size can be chosen as small as a single line.

Fig. 1. illustrates a display device 100 according to an embodiment of the invention. The display device 100 comprises a display 104, e.g. an OLED display, and a data processing unit 102. In this illustrative example, the OLED display 104 is divided into a plurality of NxM blocks 126. A typical column driver IC drives 384 lines of the display, which means that N may equal 10. The display 104 has 768 lines, thus M can be as large as 768. A more practical value for M is 12, which means that one block consists of 64 lines and 384 columns and the display is divided into 120 blocks. It will be understood by those skilled in the art that N and M can be a variety of different values and the invention is not limited to any specific values. The display 104 is driven by row drivers 122 and column drivers 124. The column drivers 124 receive respective supply voltages Vdd and processed data PD from the processing unit 102.

The processing unit 102 controls the operation of the display 104. The processing unit 102 comprises, a brightness determination unit 106, a frame memory 108, a video adaptation unit 110, a data formatting unit 112 and a synchronization unit 114. It will be understood that the processing unit 102 may be comprised of one or more processors and the invention is not limited thereto. Moreover the processing unit 102 may comprise elements other than the ones mentioned in this description. The operation of the display device 100 will now be described with reference to Fig. 1 and Fig. 2. As mentioned above, the display 104 is divided into a plurality of blocks 126 in step 201. Each block 126 is driven by a corresponding column driver 124. When input data representing an image 120 is received by the processing unit 102 in step 203, the processing unit 102 first determines which part of the image 120 will be displayed in which block or blocks 126 on the display 104 in step 205. The input data are stored in the frame memory 108.

The brightness levels of parts of the image 120 corresponding to respective blocks 126 are checked by the brightness determination unit 106 and a maximum drive level is determined for each block in step 207. The maximum drive level for each block can be determined in a variety of ways. For example, the brightness determination unit 106 can determine the maximum required drive level to correspond to the brightness of the pixel with the highest brightness value present in such a part of the image 120, but the invention is not limited thereto. For example, it is also possible to clip the brightness level of a certain number of pixels with the highest brightness values to a lower grey level. In this illustrative embodiment, the maximum drive level is determined by classifying it as either high (bright) or low (dark). However, the invention can have any number of classification levels and the invention is not limited to high and low. If the part of the image 120 to be displayed on a particular block 126 is bright, the supply voltage is Vdd of the corresponding column driver 124 is high and the full range of grey levels is used. On the other hand, if the part of the image 120 is dark, the corresponding supply voltage Vdd is lowered and dark grey levels can be displayed more accurately, because the available number of grey levels is now divided over a smaller luminance range. In this way, the grey level capability is sufficient in dark image parts as well as bright image parts. If in a part of an image 120 both a dark and a bright region is present, the corresponding supply voltage Vdd should be chosen such that all levels can be displayed correctly. Otherwise, artefacts are introduced.

In step 208, the supply voltage Vdd for each of the column drivers 124 is determined based on the determined maximum drive level. Most voltage-programmed AMOLED displays have a transfer characteristic, which is approximately the inverse of the gamma that is applied to the image 120 before it is recorded or transmitted to the display device 100. In this case, the transfer function from the input data to the output voltage of the column drivers 124 is preferably linear, i.e. the output voltage is proportional to the input data. As a result, the applied supply voltage Vdd for a column driver 124 is simply proportional to the determined maximum drive level. Expressed as formula: Vdd = Vdd_maχ*MDL/ID_max, with Vddm_ax the supply voltage when the full range of the column driver is to be used, ID max the maximum data level (255 for 8 bits data), and MDL the determined maximum drive level (a level of 255 or lower for 8 bits data). Displays that have a different gamma characteristic require a different calculation. In such displays, a part of the gamma is compensated in the display, and a part in the transfer function (which is not linear in this case). The non- linear transfer function is stored in a look-up-table. The determined maximum drive level for a block 126 is used as input to this table and the output is then proportional to the desired supply voltage Vdd of the column driver 124.

The desired supply voltages Vdd to drive the respective column drivers 124 are stored in a table for each block. In the processing unit 102, a counter determines which line of the display is being addressed. The corresponding desired supply voltages Vdd for the column drivers 124 are read from the table. The processing unit 102 alters the supply voltages Vdd for each column driver 124 accordingly by sending a control signal to a voltage generator (not shown) that provides the column drivers 124 with their supply voltages Vdd. The input data, stored in the frame memory 108, are processed by the video adaptation unit 110 and the data formatting unit 112, basically in a known manner, except for some additional processing as described below, to prepare the data for displaying on the display 104. The synchronization unit 114 receives horizontal sync signals 116 and vertical sync signals 118. The synchronization unit 114 sends the appropriate sync signals to the column drivers 124 and the row drivers 122. The video adaptation unit 110 processes in step 210 the data in such a way that the image 120 is displayed correctly. If the transfer function should be linear, the data is simply scaled in dependence on the determined maximum drive level: PDL = ID * ID max/MDL, with PDL being the level of the processed data PD, ID being the level of the input data. In this way, any change of brightness of the displayed image 120 due to a change of the supply voltage Vdd is compensated by a correction of the processed data PD.

If the gamma relation is partly in the transfer function and partly in the display gamma characteristic, the calculation is more complex. This calculation is explained using Fig. 3. Again, the relation between input data ID and output voltage of the column driver 124 is stored in a look-up-table. First, the desired output voltage of the column driver 124 is determined by a first look-up operation TF-LUT, the input data value being used as input for the look-up table. The result is then scaled in a multiplyer MU by a factor MDL/ID max to compensate for the adapted range of the column driver 124 as a result of its determined supply voltage Vdd. In a second look-up-table ITF-LUT, the inverse transfer function is stored, and this table is used to determine, based on the scaled results the levels of the processed data PD.

Once the input data have been processed by the processing unit 102, the processed data PD as well as the corresponding determined supply voltages Vdd are sent to the appropriate column drivers 124 in step 209. The determined supply voltages Vdd used by the column drivers 124 in combination with the processed data PD provide the appropriate drive to display each part of the image 120 correctly on corresponding blocks 126 in step 211.

In another embodiment of the invention a computer-readable medium has embodied thereon a computer program for processing by a computer, the computer program comprising code segments for extending the grey scale capabilities of a display. The computer program comprises a code segment for dividing the display into a plurality of blocks, a code segment for determining a brightness level for each block on which a corresponding part of an image is to be displayed, and a code segment for generating a supply voltage which is supplied to a corresponding column driver for each block, wherein the amount of voltage supplied depends on the determined maximum drive level of each block.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A method for extending grey scale capabilities of a display (100), the display (100) being formed by a plurality of blocks (126), the display (100) further comprising column drivers (124), each column driver (124) driving a corresponding block (126), said method comprising: - determining a parameter value related to a brightness level of a part of an image (120) to be displayed on the corresponding block (126); generating a supply voltage (Vdd) for the corresponding column driver (124) in dependence on the determined parameter value of the part of the image (120); and processing data (ID) corresponding to brightness levels of the part of the image in dependence on the supply voltage (Vdd) of the corresponding column driver (124) to which the processed data are supplied.

2. The method according to claim 1, wherein the data (ID) are processed for substantially compensating a change of brightness levels of a displayed part of the image (120) caused by the generated supply voltage (Vdd).

3. The method according to claim 1, comprising providing a first supply voltage to column drivers (124) for blocks (126) on which parts of an image (120) have to be displayed that have a parameter value above a predetermined level.

4. The method according to claim 3, comprising providing a second supply voltage to column drivers (124) for blocks (126) on which parts of an image (120) have to be displayed that have a parameter value below the predetermined level.

5. The method according to claim 1, the parameter value corresponding to the highest value of the brightness level present in the part of the image or to a lower value.

6. The method according to claim 1, wherein the display (100) is an organic light emitting device display.

7. Processing unit (102) for driving a display device with enhanced grey scale capabilities, the display device comprising a display (104) being formed by a plurality of blocks (126), and column drivers (124) each driving a corresponding block, said processing unit (102) having an input for receiving an image (120) to be displayed, the processing unit (102) comprising: means (106) for determining a parameter value related to a brightness level of a part of the image (120) to be displayed on the corresponding block; means (106) for generating a supply voltage (Vdd) for the corresponding column driver (124) in dependence on the determined parameter value of the part of the image (120); and means (110) for processing data (ID) corresponding to brightness levels of the part of the image in dependence on the supply voltage (Vdd) of the corresponding column driver (124) to which the processed data (PD) are supplied.

8. A display device (100) comprising: the processing unit (102) of claim 7 the display (104); and the column drivers (124).