# Patent

## Title
OLED display system and method

## Abstract

A method and system control an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel. The method and system select a plurality of reference points in the pixel content domain with known color points and brightness levels. For each set of three sub-pixels of different colors, the method and system determine the share of each sub-pixel to produce the color point and brightness level of each selected reference point, and select the maximum share determined for each sub-pixel as peak brightness needed from that sub-pixel.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Patent Applications Nos. 61/976,909, filed Apr. 8, 2014, and 61/912,786, filed Dec. 6, 2013, each of which is hereby incorporated by reference in its entirety.


FIELD OF THE INVENTION
The present invention relates generally to OLED displays and, more particularly, to an OLED display system and method for improving color accuracy, power consumption or lifetime, and gamma and black level correction of OLED displays that have three or more sub-pixel of different colors and at least one white sub-pixel.
SUMMARY
In accordance with one embodiment, a method and system are provided for controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel. The method and system select a plurality of reference points in the pixel content domain with known color points and brightness levels. For each set of three sub-pixels of different colors, the method and system determine the share of each sub-pixel to produce the color point and brightness level of each selected reference point, and select the maximum share determined for each sub-pixel as the peak brightness needed from that sub-pixel.
In accordance with another embodiment, the method and system identify tri-color sets of three sub-pixels of different colors that encircle a desired color point, and, for each identified tri-color set of sub-pixels, determine the brightness shares of the sub-pixels in that tricolor set to produce the desired color point. The method and system select a set of share factors based on at least a pixel operation point and display performance, modify the brightness shares based on the share factors, and map the modified brightness shares to pixel input data. In one implementation, The method and system determine the efficiencies of the identified tri-color sets, increase the share factor of the tri-color set with the highest efficiency; decrease the share factor of the tri-color set with the lowest efficiency, as the gray scale of the desired color point increases, and decrease the share factor of the tri-color set with the highest efficiency, and increase the share factor of the tri-color set with the lowest efficiency, as the gray scale of the desired color point decreases.
A further embodiment provides an OLED display comprising san array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel for displaying desired color points and brightness levels. Each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, the sub-pixels having operating conditions that vary with the gray level displayed by the sub-pixel. The pixel has at least two sub-pixels for displaying the same color but having operating conditions that vary differently with the gray level being displayed. A controller selects one of the two sub-pixels displaying the same color, in response to a gray level input to that pixel.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.
FIG. 1 is a flow chart of a routine for calculating the peak brightness of each sub-pixel in a display.
FIG. 2 is a flow chart of a routine for calculating the brightness shares for a tri-color set of sub-pixels.
FIG. 3 is a flow chart of a routine for content mapping based on multiple sub-pixel colors in a display.
FIG. 4 is a diagram of a multiple sub-pixel display structure.
FIG. 5 is a graph of an example of share factors as a function of gray levels of a tricolor set with the lowest and highest efficiencies K1 and K2.
FIG. 6 is a block diagram of two locally optimized sub-pixels.
FIG. 7 is an electrical schematic diagram of a pixel circuit having two locally optimized sub-pixels.
FIG. 8A is a flow chart of a procedure for adjusting the black level of a display panel based on panel uniformity measurements.
FIG. 8B is a flow chart of a procedure for using a measured current response to determine a lookup table for initial compensation of a display panel.
FIG. 9 is a flow chart of a current response measurement procedure.
FIG. 10 is a flow chart of a map response to target curve procedure.



While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION
Sub-Pixel Mapping
To improve color accuracy, power consumption or lifetime, OLED displays may have more than three primary sub-pixel colors. Therefore, proper color mapping is needed to provide continuous color space despite transitions between different color elements. Each pixel in such OLED displays consists of n sub-pixels {SP1, SP2, SP3 . . . SPn}. The peak brightness that each sub-pixel should be able to create can be calculated, and used for the design of the display or for adjusting the gamma levels to required levels.
FIG. 1 is a flow chart of an exemplary routine for calculating the peak brightness for each sub-pixel. The first step 101 selects a plurality of reference points, with known color and brightness, such as peak white points, in the pixel content domain. Step 102 identifies all possible tri-color sets that include three of the sub-pixels. Then for each tri-color set, step 103 calculates the share of each sub-pixel to create the reference content point, i.e., the color and brightness. Step 104 selects the maximum value for each sub-pixel, from all the calculated shares, as the peak brightness that needs to be provided that sub-pixel.
The following is an example of calculating the brightness shares for a tri-color set of sub-pixels for a given white point and peak brightness:












function [Green Red Blue] = Color_Sharing_RGB (Rc, Gc, Bc, Wc)


%% Rc, Gc, Bc the color points of the tri-color sets


%% Wc is the white color point









L = 100; %% Peak Brightness



%% calculating the brightness share











WM= [Wc(1)−1
0
Wc(1);











0
1
0;



Wc(2)
0
Wc(2) ];









LM= [−Wc(1)*L;









L;



−[Wc(2)−1)*L];









x = inx (WM);



Wt = x* LM;












Mt = [Gc(1)/(Gc(2))

Rc(1)/(Rc(2))
Bc(1)/(Bc(2));














1


1
1 
;











(1−Gc(1)−Gc(2))/Gc(2)
(1−Rc(1)−Rc(2))/Rc(2)
(1−Bc(1)−









Bc(2))/Bc(2)];



x2 = inx (Mt) ;



CR = x2 * Wt; %% CR is the brightsess share of the trio-color set.



Green = CR(1);



Red = CR(2);



Blue = CR(3);







end









FIG. 2 is a flow chart of an exemplary routine for calculating the brightness shares for the sub-pixels in a tri-color set. The first step 201 finds a set of triangles, made with the tri-color sub-pixels Rc, Gc, Bc that encircle a wanted white point Wc. Step 202 then selects a sub-set of those triangles to be used in creating the wanted color point Wc. Then for each triangle in the subset of triangles, step 203 calculates the brightness share for each sub-pixel in each triangle to create the wanted color point Wc. Step 204 selects a set of sub-pixel brightness shares based on a pixel operation point, display performance and other parameters (K1, K2 . . . Kn). Step 205 then uses the outputs of steps 203 and 204 to modify the sub-pixel brightness shares, based on the calculated brightness shares and share factors. Finally, step 206 maps the modified brightness shares to the pixel input data.
Different standards exist for characterizing colors. One example is the 1931 CIE standard, which characterizes colors by a luminance (brightness) parameter and two color coordinates x and y. The coordinates x and y specify a point on a CIE chromatacity diagram, which represents the mapping of human color perception in terms of the two CIE parameters x and y. The colors that can be matched by combining a given set of three primary colors, such as red, green and blue, are represented by a triangle that joins the coordinates for the three colors, within the CIE chromaticity diagram.
The following is an example of the brightness shares:
The parameters x and y for the color points of the tri-color set and intended white point are as follows:


    Rc=[0.66 0.34]
    Bc=[0.14 0.15]
    Gc=[0.38 0.59]
    Wc=[0.31 0.33]
    [Green Red Blue]=Color_Sharing_RGB (Rc, Gc, Bc, Wc)

The color shares for the tri-color set are as follows:

    Green=59.8237%
    Red=17.7716%
    Blue=22.4047%


Each of the tri-color sets that encircles the pixel content will create a share of the pixel contents K1, K2 . . . Km, where the Ki's are the shares of the respective sub-pixels in each tri-color set in the pixel content. The value of each sub-pixel in each of the tri-color sets is calculated considering the share of each tri-color. One such method is based on the function illustrated in FIG. 3, where step 301 calculates the color point of the input signal for the pixels, and step 302 creates all possible tri-color sets that include three of the sub-pixels. Step 303 then selects the tri-color sets that encircle the pixel color point, and step 304 calculates the share of each color sub-pixel to create the ratio of the pixel content allocated to each selected tri-color set. Step 305 uses all the calculated values for each tri-color set to calculate the total value for each sub-pixel, e.g., the sum of all values calculated for each sub-pixel.
FIG. 4 shows an example of a display incorporating more than three sub-pixel colors (C1, C2, C3, C4, C5) and a wanted color point of Wc. As can be seen, the color point Wc can be created by any of {C1, C2, C4}, {C2, C4, C5}, {C2, C3, C5}, and {C1, C2, C3}. To create the wanted color Wc, one can use the algorithm described above. Also, one can use share factors to create the wanted color based on the sum of all the sets, such as:
Wc=K1*{C1, C2, C4}+K2*{C2, C4, C5}+K3*{C2, C3, C5}+K4*{C1, C2, C3},
where the Ki's are the share factors for the tri-color set.
Dynamic Share Factor Adjustment
The share of each tri-color set can be varied based on the pixel content. For example, some sets provide better characteristics (e.g., uniformity) at some grayscales, whereas other sets can be better for other characteristics (e.g., power consumption) at different grayscales.
In one example, a display consists of Red, Green, Blue and White sub-pixels. The white sub-pixel is very efficient and so it can provide lower power consumption at high brightness. However, due to higher efficiency, the non-uniformity compensation does not work well at lower gray scales. In this case, low gray scales can be created with less efficient sub-pixels (e.g., red, green, and blue). Thus, the share factor can be a function of gray scales to take advantage of different set strengths at each gray level. For example, the share factor of a tri-color set with the lowest efficiency (K1) can be reduced at higher gray levels and increased at lower gray scales. And the share factor of the tri-color set with the highest efficiency (K2=1−K1) can be increased as the gray scale increases. Thus, the display can have both lower-power consumption at higher brightness levels and higher-uniformity at lower gray scales. This function can be step, a linear function or any other complex function. However, a smoothing function can be used at large transitions to avoid contours. FIG. 5 shows an example of the share factors for a two tri-color set system.
Locally Optimized Sub-Pixels
Due to the wide range of specifications for display performance, the sub-pixels will have an optimum operation point, and diverging from that point can affect one or two specifications. For example, to achieve low power consumption, one can use drive TFTs that are as large as possible to reduce the operating voltage. On the other hand, at low current levels, the TFTs will operate in a non-optimized regime of operation (e.g., sub-threshold). On the other hand, using small TFTs to improve the low grayscale performance will affect the power consumption and lifetime due to using large operating currents.
To address the difficulty in having a single sub-pixel optimized across all gray levels and operation ranges (e.g. different environmental conditions, brightness levels, etc), one can add sub-pixels optimized for different operating ranges. To optimize the operation of each sub-pixel for a specific gray-level set, one can change the component size or use a different pixel circuit for each locally optimized sub-pixel. Here, one can share all or some components of the sub-pixel (e.g., OLEDs, bias transistors, bias lines, and others). FIG. 6 illustrates an example using two locally optimized sub-pixels with some shared components and some dedicated components to each sub-pixel. Also, one can have two different load elements (e.g., OLEDs). In this example, the current required for either shared load or combined separate load elements is generated by both sub-pixels 1 and 2 where I1=A1*I and I2=A2*I (I is the total current required for the load, I1 is the current generated by sub-pixel #1, 12 is the current generated by sub-pixel #2, and A2=(1−A1)). Here, A1 and A2 are adjusted for different gray-scales (or operating conditions) to adjust the ratio of each sub-pixel in generating the current.
One can add sub-pixels optimized for different operating ranges. Here, one can share all or some components of the pixel (e.g., OLED, bias transistors, bias lines, and others).
FIG. 7 is a circuit diagram of an exemplary embodiment in which the drive TFT (T1), the programming switch TFT (T2), and the storage element (CS) are optimized for each sub-pixel. Also, the TFT T3, the bias line, the select line (SEL) and the power line (VDD) are shared. In one case, different sizes of drive TFTs can be used to optimize the sub-pixels for different ranges of operation. For example, one can use a smaller drive TFT for one sub-pixel to be used for lower gray scales, and a larger drive TFT for the other sub-pixel to be used for higher gray scales.
Selecting each sub-pixel can be done either through a switch that activates or deactivates the sub-pixel, or through programming a sub-pixel with an off voltage to deactivate it.
The locally optimized sub-pixel method can be used for all sub-pixels or for only selected sub-pixels. For example, in the case of a RGBW sub-pixel structure, optimizing white sub-pixels across all gray levels is very difficult due to high OLED efficiency, while other sub-pixels can be optimized more easily. Thus, one can use a locally optimized sub-pixel method only for the white sub-pixel.
Gamma and Black Level Correction
A gamma calibration procedure ensures that colors displayed by a panel are accurate to the desired gamma curve, usually 2.2. The procedure has now been largely automated. The target white-point and curve are parameterized. The high level process is shown in FIGS. 8A and 8B. This procedure assumes that initial uniformity compensation for the panel has already been applied.
In the procedure of FIG. 8A, step 801 measures the display panel for uniformity compensation, and then curve fits the measured data. A black level is applied to the panel, and the threshold parameter for each sub-pixel is adjusted until the panel is black. In the procedure of FIG. 8B, the current response is measured at step 804, and then mapped to a target curve in step 805. Step 806 applies the resulting lookup table to initial compensation.
One advantage of emissive displays is deep black level. However, due to the non-linear behavior of the pixels and non-uniformity in the pixels, it is difficult to achieve black levels based on a continuous gamma curve. In one method, the worst case is chosen, and the off voltage is calculated based on that. Then that voltage, with some margin, is assigned to the black gray level, which generally puts the panel in a deep negative biasing condition. Since some backplanes are sensitive to negative bias conditions, the panel will develop image burn-in and non-uniformity over time.
To avoid that, the black level can be adjusted based on panel uniformity information. In this case, the uniformity of the pixel is measured at step 801 in FIG. 8A, and the threshold voltage (at which the pixel current is assumed to be off) is calculated at step 802. However, since simplified models are used to reduce the calculation and compensation complexity, the calculated threshold voltage will have some error. To assign a black voltage, the threshold voltage of the pixel is reduced at step 803 until the panel turns black. This can be done for each color individually, and the new modified threshold voltage is used for black voltage level.
In another aspect of this invention, a plurality of sensors are added to the panel, and the voltage of the black level is adjusted until all sensors provide zero readings. In this case, the initial start of the black level can be the calculated threshold voltage.
In another aspect of this invention, the black level for each sensor is adjusted individually, and a map of black level voltage is created based on each sensor data. This map can be created based on different methods of interpolation.
In another aspect of the invention, the black level has at least two values. One value is used for dark environments and another value is used for bright environments. Since the lower black level is not useful in bright environments, the pixel can be slightly on (at a level that is less than or similar to the reflection of the panel). Therefore, the pixel can avoid negative stress which is accelerated under higher brightness levels.
In another aspect of the invention, the black level has at least two values. One value is used when all the sup-pixels are off, and another value is used when at least one sub-pixel is ON. In this case, there can be a threshold for the brightness level of the ON sub-pixels required to switch to the second black level value for the OFF sub-pixels. For example, if the blue sub-pixel is ON and its brightness is higher than 1 nit, the other sub-pixels can be slightly ON (for example, less than 0.01 nit). In this case, the OFF sub-pixels can eliminate the negative bias stress under illumination.
In another aspect of the invention, the brightness of neighboring sub-pixel can be used to switch between different black level values. In this case, a weight can be assigned to the sub-pixels based on their distance from the OFF sub-pixels. In one example, this weight can be a fixed value, dropping to zero after a distance of a selected number of pixels. In another example, the weight can be a linear drop from one to zero. Also, different complex functions can be used for the weight function.
Measure Current Response
The steps for a measure-current-response process are summarized in FIG. 9. The initial step 901 sets a timing controller, which ensures that measurements are taken with the display in the correct mode. Specifically, it ensures that the most recent compensation is being displayed on the panel. It also ensures that TFT and OLED corrections required before a gamma function is applied, are enabled while gamma correction and luminance correction are disabled. To avoid having to write the entire frame buffer to a single value, special flat-field registers can be implemented in the timing controller. When the timing controller is placed in this mode, step 902 writes the desired grey scale to the corresponding colors register, which is sufficient to display the desired color. Since characterizing the panel, especially at higher levels, with the entire panel on can lead to lower brightness and/or current limiting, step 903 sets only part of the panel to show the desired color level.
As pre-set list of grey scales is used to determine the measurement points that will be used. In one implementation, a list of 61 levels is used for characterization. These points are not linearly spaced; they are positioned more densely toward the low end of the curve, becoming sparser as the grey level increases. This is done to generally fit a 2.2 curve, not a linear one, and can be adjusted for other gamma curves. The list is ordered from the lowest target level (e.g., 0) to the highest target (e.g., 1023). Also, it can be in any other order. After applying each color level, the resulting luminance and/or color point (CIE-XY) are then recorded at step 904. Multiple measurements are taken, and error checking is employed to ensure the validity of the readings. For example, if the variation in the reading is too great, the setup is not working properly. Or if the reading shows an increasing or decreasing trend, it means the values have not settled yet. If luminance only is measured by a calibrated sensor, these readings are converted to luminance and color point data during processing based on a calibration curve of the sensor. The order of steps can be changed and still obtain valid results. Steps 903 and 904 are repeated until the last color is detected at step 905, after which steps 902-905 are repeated until the last gray color is detected at step 906.
Map Response to Target Curve
The target curve (e.g., the required gamma response) and white-point are specified as input parameters to the mapping function. The steps of this process are summarized in FIG. 10.
The first step is to load the measured data from the generated by the characterization procedure. If the data to be processed is from a calibrated sensor, one additional step is required. The calibration files for the sensor are used to convert the raw sensor readings to luminance and color point values.
Once the data is loaded, the target color point and peak luminance are used to calculate the peak target luminance for each color. Step 1001 finds the grey scale which results in this luminance, which allows the new maximum grey scale for each color to be determined. If any of the colors are not able to achieve the target, the target is adjusted such that the highest achievable brightness is targeted instead. Then the luminance readings are normalized to one, with respect to this new maximum grey scale, at step 1002.
This normalized data can now be used to map the measurements to the target curve, generating a look up table at step 1003. Linear interpolation is used to estimate the luminance between the measurement points. However, different known curve fitting processes can be used as well. The target curve is created by normalizing the target curve and finding the values for each of the points from lowest gray level (e.g., 0) to the highest gray level (e.g., 1023).
Some cases, like the standard sRGB curve, are actually piece wise. In these cases, a different component is used for each part of the curve. For example, for the standard sRGB, there is a linear component at the low end while the remainder of the curve is exponential. As a result, linearization is applied to the low end of the lookup table at step 1004. The point where linearization needs to be applied can be extracted from mapping the measured data to the standard. For example, the linearization can be applied to the first 100 grey scales where gray 100 represents the brightness points that the standard identifies and the change in the curve.
After the linearization is applied, all that remains is to write the resulting lookup table (LUT) to the appropriate output formats, at step 1005.
While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.

## Claims

1. A method of controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, said method comprising
selecting a plurality of reference points in a pixel content domain with known color points and brightness levels,
identifying all possible tri-color sets of three sub-pixels from the at least three sub-pixels having different colors and the at least one white sub-pixel;
for each tri-color set of three sub-pixels, determining a share of each sub-pixel to produce the color point and brightness level of each selected reference point, and
selecting the maximum share determined for each sub-pixel as the peak brightness needed for that sub-pixel.

2. The method of claim 1 wherein the at least three sub-pixels having different colors of each pixel comprises red, green and blue sub-pixels.

3. A system for controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, said system comprising
a processor configured to
select a plurality of reference points in a pixel content domain with known color points and brightness levels,
identify all possible tri-color sets of three sub-pixels from the at least three sub-pixels having different colors and the at least one white sub-pixel,
determine, for each tri-color set of three sub-pixels, a share of each sub-pixel to produce the color point and brightness level of each selected reference point, and
select the maximum share determined for each sub-pixel as the peak brightness needed for that sub-pixel.

4. The system of claim 3 wherein the at least three sub-pixels having different colors of each pixel comprises red, green and blue sub-pixels.

5. A method of controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, said method comprising
identifying two or more tri-color sets of three sub-pixels of different colors that encircle a desired color point,
for each identified tri-color set of sub-pixels, determining the brightness shares of the sub-pixels in that tricolor set to produce the desired color point,
for each identified tri-color set of sub-pixels, selecting a set of share factors based on at least one of a pixel operation point and display performance,
modifying said brightness shares based on said share factors, and
mapping the modified brightness shares to pixel input data.

6. The method of claim 5 which includes
determining the efficiencies of the identified tri-color sets,
increasing the set share factor of one of the tri-color sets with the highest efficiency,
decreasing the set share factor of one of the tri-color sets with the lowest efficiency, as the gray scale of the desired color point increases, and
decreasing the share factor of the tri-color set with the highest efficiency, and increasing the share factor of the tri-color set with the lowest efficiency, as the gray scale of the desired color point decreases.

7. The method of claim 5 in which each tricolor set of sub-pixels of different colors is a set of red, green and blue sub-pixels.

8. A system for controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, said system comprising
a processor configured to
identify two or more tri-color sets of three sub-pixels of different colors that encircle a desired color point,
determine, for each identified tri-color set of sub-pixels, the brightness shares of the sub-pixels in that tricolor set to produce the desired color point,
select, for each identified tri-color set of sub-pixels, a set share factor based on at least one of: a pixel operation point and display performance,
modify said brightness shares based on said set share factors, and
map the modified brightness shares to pixel input data.

9. The system of claim 8 which said processor is configured to:
determine the efficiencies of the identified tri-color sets,
increase the set share factor of one of the tri-color sets with the highest efficiency, and decrease the set share factor of one of the tri-color sets with the lowest efficiency, as the gray scale of the desired color point increases, and
decrease the set share factor of the tri-color set with the highest efficiency, and increase the set share factor of the tri-color set with the lowest efficiency, as the gray scale of the desired color point decreases.

10. The system of claim 8 in which each tri-color set of sub-pixels of different colors is a set of red, green and blue sub-pixels.

11. A method of controlling an OLED display to achieve desired color points and brightness levels in an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel, said method comprising
determining the color point of an input signal for a selected pixel,
identifying all tri-color sets of three sub-pixels of different colors,
selecting two or more tri-color sets that encircle said color point of said input signal,
for each selected tri-color set of sub-pixels, determining brightness shares of the three sub-pixels of that tri-color set to produce said color point of said input signal,
selecting, for each identified tri-color set of sub-pixels, a set share factor based on at least one of: a pixel operation point and display performance,
modifying said brightness shares based on said set share factors, and
mapping the modified brightness shares to pixel input data.

12. The method of claim 11 in which each tri-color set of sub-pixels of different colors is a set of red, green and blue sub-pixels.

13. An OLED display comprising
an array of pixels in which each pixel includes at least three sub-pixels having different colors and at least one white sub-pixel for displaying desired color points and brightness levels,
said sub-pixels having operating conditions that vary with the gray level displayed by the sub-pixel,
said pixel having at least two sub-pixels for displaying the same color but having operating conditions that vary differently with the gray level being displayed, and

a controller for selecting one of the two sub-pixels displaying the same color, in response to a gray level input to that pixel.

14. The OLED display of claim 13 wherein the at least three sub-pixels having different colors of each pixel comprises red, green and blue sub-pixels.