# Patent

## Title
Compensation technique for color shift in displays

## Abstract

A system for maintaining a substantially constant display white point over an extended period of operation of a color display formed by an array of multiple pixels in which each of the pixels includes multiple subpixels having different colors, and each of the subpixels includes a light emissive device. The display is generated by energizing the subpixels of successively selected pixels, and the color of each selected pixel is controlled by the relatives levels of energization of the subpixels in the selected pixel. The degradation behavior of the subpixels in each pixel is determined, and the relative levels of energization of the subpixels in each pixel are adjusted to adjust the brightness shares of the subpixels to compensate for the degradation behavior of the subpixels. The brightness shares are preferably adjusted to maintain a substantially constant display white point.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to and is a continuation of U.S. patent application Ser. No. 12/816,856, filed Jun. 16, 2010, which claims priority to Canadian Application No. 2,669,367 which was filed Jun. 16, 2009, each of which is incorporated herein by reference in their entireties.


FIELD OF THE INVENTION
This invention is directed generally to color displays that use light emissive devices such as OLEDs and, more particularly, to compensating for color shifts in such displays as the light emissive devices age.
BACKGROUND OF THE INVENTION
Previous compensation technique for OLED displays considered backplane aging and OLED efficiency lost. The aging (and/or uniformity) of the panel was extracted and stored in lookup tables as raw or processed data. Then a compensation block used the stored data to compensate for any shift in the electrical parameters of the backplane (e.g., threshold voltage shift) or the OLED (e.g., shift in the OLED operating voltage). Such techniques can be used to compensate for OLED efficiency losses as well. These techniques are based on the assumption that the OLED color coordinates are stable despite reductions in the OLED efficiency. Depending on the OLED material and the required device lifetime, this can be a valid assumption. However, for OLED materials with low stability in color coordinates, this can result in excessive display color shifts and image sticking issues.
The color coordinates (i.e., chromaticity) of an OLED shift over time. These shifts are more pronounced in white OLEDs since the different color components that are combined in an OLED structure used to create white light can shift differently (e.g., the blue portion may age faster than the red or green portion of the combined OLED stack), leading to undesirable shifts in the display white point, which in turn lead to artifacts such as image sticking. Moreover, this phenomenon is applicable to other OLEDs as well, such as OLEds that consist of only single color components in a stack (i.e., single Red OLED stack, single GREEN OLED stack, etc.). As a result, color shifts that occur in the display can cause severe image sticking issues.
SUMMARY
Additional aspects of the invention will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments, which is made with reference to the drawings, a brief description of which is provided below.
In accordance with one embodiment, a system is provided for maintaining a substantially constant display white point over an extended period of operation of a color display formed by an array of multiple pixels in which each of the pixels includes multiple subpixels having different colors, and each of the subpixels includes a light emissive device. The display is generated by energizing the subpixels of successively selected pixels, and the color of each selected pixel is controlled by the relatives levels of energization of the subpixels in the selected pixel. The degradation behavior of the subpixels in each pixel is determined, and the relative levels of energization of the subpixels in each pixel are adjusted to adjust the brightness shares of the subpixels to compensate for the degradation behavior of the subpixels. The brightness shares are preferably adjusted to maintain a substantially constant display white point.
In one implementation, the light emissive devices are OLEDs, and the degradation behavior used is a shift in the chromaticity coordinates of the subpixels of a selected pixel, such as a white pixel in an RGBW display. The voltage at a current input to each OLED is measured and used in the determining the shift in the chromaticity coordinates.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings.
FIG. 1 is a functional block diagram of system for compensating for color shifts in the pixels of a color display using OLEDs.
FIG. 2 is a CIE chromaticity diagram.
FIG. 3 is a flow chart of a procedure for compensating for color shifts in the system of FIG. 1.
FIG. 4A is a pair of graph representing variations in the chromaticity coordinate Cx of the measured brightness values of two white OLEDs subjected to two different stress conditions, as a function of the difference between the measured OLED voltages and a non-aged reference OLED.
FIG. 4B is a pair of graph representing variations in the chromaticity coordinate Cy of the measured brightness values of two white OLEDs subjected to two different stress conditions, as a function of the difference between the measured OLED voltages and a non-aged reference OLED.
FIG. 5 is a graph representing variations in a brightness correction factor as a function of the OLED voltage a white OLED subjected to one of stress conditions depicted in FIG. 4.
FIG. 6 is a functional block diagram of a modified system for compensating for color shifts in the pixels of a color display using OLEDs.



DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
Although the invention will be described in connection with certain preferred embodiments, it will be understood that the invention is not limited to those particular embodiments. On the contrary, the invention is intended to include all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of the invention as defined by the appended claims.
FIG. 1 illustrates a system in which the brightness of each subpixel is adjusted, based on the aging of at latest one of the subpixels in each pixel, to maintain a substantially constant display white point over time, such as the operating life of a display, e.g., 75,000 hours. For example, in an RGBW display, if the white OLED in a pixel loses part of its blue color component, thus producing a warmer white than desired, the blue OLED in that same pixel may be turned on along with the white OLED in that same pixel, during a white display. Similarly, in an RGB display, the brightness shares of the red, green and blue OLEDs may be dynamically adjusted over time in response to each OLED's degradation behavior, to keep the white point of the display substantially constant. In either case, the amount of change required in the brightness of each subpixel can be extracted from the shift in the color coordinates of one or more of the subpixels. This can be implemented by a series of calculations or by use of a look-up table containing pre-calculated values, to determine the correlation between shifts in the voltage or current supplied to a subpixel and/or the brightness of the light-emitting material in that subpixel.
Fixed initial color points of the subpixels may be used to calculate the brightness shares of the subpixels in each subpixel. Then during operation of the display, a correction unit determines a correction factor for each subpixel, e.g., by use of a lookup table. In FIG. 1, the initial subpixel color points and the video input signal for the display are supplied to an initial brightness share calculation unit 10, which determines the brightness shares for the red, green blue and white subpixels. These brightness shares are then adjusted by respective values ΔR, ΔG, ΔB and ΔW derived from a signal ΔWOLED that represents the aging of the white subpixel. The adjusted brightness shares are sent to a compensation unit 11, which adjusts the video signal according to the adjusted brightness shares and sends the adjusted video signals to a driver 12 coupled to an OLED display 13. The driver 12 generates the signals that energize the various subpixels in the display 13 to produce the desired luminance from each subpixel.
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, as illustrated in FIG. 2, 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 in FIG. 2 by the triangle T that joins the coordinates for the three colors, within the CIE chromaticity diagram of FIG. 2.
FIG. 3 is a flow chart of a procedure for determining the brightness shares for the subpixels in an RGBW display from initial subpixel color points and the video input signal for the image to be displayed, which are the two inputs to the initial brightness share calculation unit 10 in FIG. 1. The procedure of FIG. 3 begins at step 101 by choosing two subpixels from the red, green and blue subpixels, such that the desired display white point is inside a triangle that can be formed with the color points of the two selected subpixels and the white subpixel. For example, the triangle T in FIG. 2 is defined by the red, green and white subpixel values from the following set of chromaticity coordinates of four RGBW subpixels and a display white point:
Blue subpixel=[0.154, 0.149]
Red subpixel=[0.67, 0.34]
Green subpixel=[0.29, 0.605]
White subpixel=[0.29, 0.31]
Display white point=[0.3138, 0.331]
It can be seen that the display white point falls inside the triangle T formed by connecting the chromaticity coordinates of the red, green and white subpixels.
After choosing two subpixels at step 101, it is assumed that the white subpixel is the third primary color, and then at step 102 the chromaticity coordinates of the red, green and blue subpixels (considering the blue and white subpixels to be the same at this stage) are converted to tristimulus parameters to facilitate calculation of the brightness shares of the red, green and blue subpixels to achieve the desired display white point. Any color on a CIE chromaticity diagram can be considered to be a mixture of three CIE primaries, which can be specified by three numbers X, Y and Z called tristimulus values. The tristimulus values X, Y and Z uniquely represent a perceivable hue, and different combinations of light wavelengths that give the same set of tristimulus values are indistinguishable to the human eye. Converting the chromaticity coordinates to tristimulus values permits the use of linear algebra to calculate a set of brightness shares for the red, green and blue subpixels to achieve the desired display white point.
Step 103 uses the tristimulus values to calculate the brightness shares for the red, green and blue subpixels to achieve the desired display white point. For the exemplary set of chromaticity coordinates and desired display white point set forth above, the brightness shares of the red, green and blue subpixels are BRW=6.43%, BGW=11.85% and BWW=81.72%, respectively. The same calculation can be used to calculate the brightness shares BR, BG and BB for the red, green and blue subpixels in an RGB display.
Step 104 assigns to the white subpixel the brightness share calculated for the blue subpixel, and these brightness shares will produce the desired display white point in an RGBW system. Video signals, however, are typically based on an RGB system, so step 105 converts the video signals Rrgb, Grgb and Brgb to modified RGBW values Wm, Rm, Gm and Bm by setting Wm equal to the minimum of Rrgb, Grgb and Brgb and subtracting the white portion of the red, green and blue pixels from the values of the signals Rrgb, Grgb and Brgb, as follows:

Wm=minimum of Rrgb, Grgb and Brgb 

Rm=Rrgb−W 

Gm=Grgb−W 

Bm=Brgb−W 

Step 106 then uses the calculated brightness shares for BRW, BGW and BWW to translate the modified values Wm, R, Gm, and Bm to actual values W, R, G and B for the four RGBW subpixels, as follows:

W=Wm*BWW 

R=Rm+Wm*BRW/BR 

G=Gm+Wm*BGW/BG 

B=Bm+Wm*BBW/BB 

The values W, G, R and B are the gray scales for the white, green, red and blue subpixels w, r, g, and b.
FIGS. 4A and 4B are graphs plotted from actual measurements of the brightness of two white OLEDs while being aged by passing constant currents through the OLEDs. The currents supplied to the two OLEDs were different, to simulate two different stress conditions #1 and #2, as indicated in FIGS. 4A and 4B, As the OLED material ages, the resistance of the OLED increases, and thus the voltage required to maintain a constant current through the OLED increases. For the curves of FIGS. 4A and 4B, the voltage applied to each aging OLED to maintain a constant current was measured at successive intervals and compared with the voltage measured across a non-aged reference OLED supplied with the same magnitude of current and subjected to the same ambient conditions as the aging OLED.
The numbers on the horizontal axes of FIGS. 4A and 4B represent ΔVOLED, which is the difference between the voltages measured for the aging OLED and the corresponding reference LED. The numbers on the vertical axes of FIGS. 4A and 4B represent the respective chromaticity coordinates Cx and Cy of the measured brightness values of the aging white OLEDs.
In order to compensate for the brightness degradation of a white subpixel as the white subpixel ages, the brightness shares of the red, green and blue subpixels can be to be adjusted to BRW=7.62%, BGW=8.92% and BWW=83.46%, respectively, at ΔVOLED=0.2; to BRW=8.82%, BGW=5.95% and BWW=85.23%, respectively, at ΔVOLED=0.4; and to BRW=10.03%, BGW=2.96% and BWW=87.01%, respectively, at ΔVOLED=0.6. These adjustments in the brightness shares of the subpixels are used in the compensation unit 11 to provide compensated video signals to the driver 12 that drives successive sets of subpixels in the display 13.
FIG. 6 illustrates a compensation system using OLED data extracted from a display 200 (in the form of either OLED voltage, OLED current, or OLED luminance) and corrects for color shifts. This system can be used for dynamic brightness share calculations in which the chromaticity coordinates of the subpixels do not remain fixed, but rather are adjusted from time to time to compensate for changes in the color point of each subpixel over time. These calculations can be done in advance and put into a lookup table.
FIG. 6 illustrates a system in which OLED data, such as OLED voltage, OLED current or OLED luminance, is extracted from an OLED display 200 and used to compensate for color shifts as the OLEDs age, to maintain a substantially constant display white point over time. A display measurement unit 201 measures both OLED data 202 and backplane data 203, and the backplane data 203 is sent to a compensation unit 206 for use in compensating for aging of backplane components such as drive transistors. The OLED data 202 is sent to a subpixel color point unit 204, a subpixel efficiency unit 205 and a compensation unit 206. The subpixel color point unit determines new color points for the individual subpixels based on the OLED data (e.g., by using a lookup table), and the new color points are sent to a subpixel brightness share calculation unit 207, which also receives the video input signal for the display. The brightness shares may be calculated in the same manner, described above, and are then used in the compensation unit 206 to make compensating adjustments in the signals supplied to the four subpixels in each pixel. Lookup tables can be used for a simpler implementation, and lookup tables for the color points and the color shares can even be merged into a single lookup table.
To compensate for the optical aging of the individual subpixels, the gray scales may be adjusted using the following value ΔVCL—W as the compensating adjustment for the white pixels:

ΔVCL—W=GmW(W)·KCL—W 

where




  
    
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KCL—W is a brightness correction factor for the white subpixels and may be determined from the empirically derived interdependency curves shown in FIG. 4 that relate OLED color shift to ΔVOLED. That measured data can be used to generate the graph of FIG. 5, which plots the brightness correction factor KCL—W as a function of ΔVOLED for a white pixel. Then assuming that any color shifts in the red, green and blue OLEDs are negligible, brightness correction factors Kb, Kr and Kg are computed from the KCL—W curve, using the same brightness shares for red, green and blue described above. The compensating adjustments for the red, green and blue OLEDs can then be calculated as follows:

ΔR=Kr(R)*ΔVCL—W 

ΔG=Kg(G)*ΔVCL—W 

ΔB=Kb(B)*ΔVCL—W 

The final adjusted values of the gray scales for the red, green and blue OLEDs are calculated by adding the above values ΔR, ΔG and ΔB to the values derived from the original gray-scale values.
While particular embodiments, aspects, 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 may 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 maintaining a substantially constant display white point over an extended period of operation of a color display formed by an array of multiple pixels, each of said pixels including multiple subpixels having different colors and white, and each of said subpixels including a light emissive device, said method comprising
receiving a video signal,
controlling the color of a selected pixel by adjusting relative levels of energization of the subpixels in the selected pixel based on the video signal,
determining a color shift degradation behavior based on aging of a white subpixel in the selected pixel, and
re-adjusting the relative levels of energization of at least one of the color subpixels in the selected pixel to adjust brightness shares of said subpixels in said selected pixel to compensate for said color shift degradation, due to aging, of said white subpixel independently of other pixels in said array and based on said determined color shift degradation behavior of said white subpixel based on aging, said brightness shares being adjusted to maintain a substantially constant display white point, and
in which said controlling is carried out according to a lookup table mapping the video signal to the relative levels of energization of the subpixels and in which said adjusting is carried out by dynamically adjusting the lookup table based on the determined color shift degradation behavior based on aging.

2. The method of claim 1 in which said color shift degradation behavior is a shift in the chromaticity coordinates of the said white subpixel.

3. The method of claim 2 in which said brightness shares of said subpixels are adjusted by respective values ΔR, ΔG, ΔB, and ΔW derived from a signal ΔWOLED that represents the aging of the white subpixel, wherein ΔR, ΔG, ΔB, and ΔW are the adjustment values for red, green, blue and white subpixels in said selected pixel.

4. The method of claim 1 in which said light emissive device in each subpixel is an OLED.

5. The method of claim 1 in which said display is an RGBW display.

6. The method of claim 1 in which said color shift degradation behavior based on aging is detected by measuring a voltage across said light emissive device in said white subpixel, and in which said color shift degradation behavior is related to the voltage across said light emissive device by an empirically determined dependency curve.

7. The method of claim 1 which includes determining a shift in the chromaticity coordinates of the said white subpixel in the selected pixel as said white subpixel ages.

8. A system for maintaining a substantially constant display white point over an extended period of operation of a color display, said system comprising
a color display formed by an array of multiple pixels, each of said pixels including multiple subpixels having different colors and white and each of said subpixels including a light emissive device,
drive circuitry for energizing the subpixels of a selected pixel and controlling the color of each selected pixel by adjusting relative levels of energization of the subpixels in the selected pixel based on a video signal, and
a controller monitoring a color shift degradation behavior based on aging of a white subpixel in the selected pixel and re-adjusting the relative levels of energization of at least one of the color subpixels in the selected pixel to adjust brightness shares of said subpixels in said selected pixel to compensate for said color shift degradation, due to aging, of said white subpixel independently of other pixels in said array and based on said determined color shift degradation behavior of said white subpixel based on aging, said brightness shares being adjusted to maintain a substantially constant display white point, and
in which said controlling is carried out according to a lookup table mapping the video signal to the relative levels of energization of the subpixels and in which said adjusting is carried out by dynamically adjusting the lookup table based on the determined color shift degradation behavior based on aging.

9. The system of claim 8 in which said color shift degradation behavior is a shift in the chromaticity coordinates of the said white subpixel.

10. The system of claim 9 in which the controller adjusts the brightness shares of said subpixels by respective values of ΔR, ΔG, ΔB, and ΔW derived from a signal ΔWOLED that represents the aging of the white subpixel, wherein ΔR, ΔG, ΔB, and ΔW are the adjustment values for red, green, blue and white subpixels in said selected pixel.

11. The system of claim 8 in which said light emissive device in each subpixel is an OLED.

12. The system of claim 8 in which said display is an RGBW display.

13. The system of claim 8 in which said color shift degradation behavior is detected by measuring a voltage across said light emissive device in said white subpixel, and in which said color shift degradation behavior based on aging is related to the voltage across said light emissive device by an empirically determined dependency curve.