Patent Grant
8619103
Reference is made to commonly-assigned, co-pending U.S. patent application Ser. No. 12/191,478, filed Aug. 14, 2008, entitled “OLED device with embedded chip driving” by Winters et al. and published as US 2010-0039030, to commonly-assigned, co-pending U.S. patent application Ser. No. 12/272,222, filed Nov. 17, 2008, entitled “Compensated drive signal for electroluminescent display” by Hamer et al. and published as US 2010-0123649, and to commonly-assigned, co-filed U.S. patent application Ser. No. 13/017,749, entitled “Electroluminescent device aging compensation with multilevel drive” by White, the disclosures of which are incorporated by reference herein.
The present invention relates to solid-state electroluminescent (EL) devices such as organic light-emitting diode (OLED) displays, and particularly to compensation for chromaticity shift of emitters in such devices.
Additive color digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal modulators, and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). Devices such as solid-state lamps are also being produced. In a common additive color display device, a pixel includes red, green, and blue colored subpixels. These subpixels correspond to color primaries that define a color gamut. By additively combining the illumination from each of these three subpixels, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. In one technology, OLEDs can be used to produce color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum, or alternatively, broadband emitting (apparently white) OLEDs can be attenuated with color filters to achieve red, green and blue.
It is possible to employ a white, or nearly white, subpixel along with the red, green, and blue subpixels to improve power efficiency or luminance stability over time. Other possibilities for improving power efficiency or luminance stability include the use of one or more additional non-white subpixels, such as yellow subpixels. However, images and other data destined for display on a color display device are typically stored or transmitted in three channels, that is, having three signals corresponding to a standard (e.g., sRGB) or specific (e.g., measured CRT phosphors) set of primaries. Therefore incoming image data will have to be converted for use on a display having four subpixels per pixel rather than the three subpixels used in a three channel display device.
In the field of CMYK printing, conversions known as undercolor removal or gray component replacement are made from RGB to CMYK, or more specifically from CMY to CMYK. At their most basic, these conversions subtract some fraction of the CMY values and add that amount to the K value. These methods are complicated by image structure limitations because they typically involve non-continuous tone systems, but because the white of a subtractive CMYK image is determined by the substrate on which it is printed, these methods remain relatively simple with respect to color processing. Attempting to apply analogous algorithms in continuous tone additive color systems would cause color errors if the additional primary is different in color from the display system white point.
In the field of sequential-field color projection systems, it is known to use a white primary in combination with red, green, and blue primaries. White is projected to augment the brightness provided by the red, green, and blue primaries, inherently reducing the color saturation of some or all of the colors being projected. A method proposed by Morgan et al. in U.S. Pat. No. 6,453,067 teaches an approach to calculating the intensity of the white primary dependent on the minimum of the red, green, and blue intensities, and subsequently calculating modified red, green, and blue intensities via scaling. However, the scaling cannot restore, for all colors, all of the color saturation lost in the addition of white. The lack of a subtraction step in this method ensures color errors in at least some colors. Additionally, Morgan's disclosure describes a problem that arises if the white primary is different in color from the desired white point of a display device, but does not adequately solve the problem. The method simply accepts an average effective white point, which effectively limits the choice of white primary color to a narrow range around the white point of the device.
A similar approach is described by Lee et al. (“TFT-LCD with RGBW Color System”, SID 03 Digest, pp. 1212-1215) to drive a color liquid crystal display having red, green, blue, and white pixels. Lee et al. calculate the white signal as the minimum of the red, green, and blue signals, then scale the red, green, and blue signals to correct some, but not all, color errors, with the goal of luminance enhancement paramount. The method of Lee et al. suffers from a similar color inaccuracy to that of Morgan.
In the field of ferroelectric liquid crystal displays, another method is presented by Tanioka in U.S. Pat. No. 5,929,843. Tanioka's method follows an algorithm analogous to the familiar CMYK approach, assigning the minimum of the R, G, and B signals to the W signal and subtracting the same from each of the R, G, and B signals. To avoid spatial artifacts, the method teaches a variable scale factor applied to the minimum signal which results in smoother colors at low luminance levels. Because of its similarity to the CMYK algorithm, it suffers from the same problem cited above, namely that a white pixel having a color different from that of the display white point will cause color errors.
Primerano et al., in U.S. Pat. No. 6,885,380, and Murdoch et al., in commonly-assigned U.S. Pat. No. 6,897,876, the disclosures of both of which are incorporated by reference herein, describe methods for transforming three color-input signals (R,G,B) into four color-output signals (R,G,B,W) which do not cause color errors when the white pixel has a color different from that of the display white point. Although useful, these methods assume that the color of the emitters and in particular the color of the W emitter (white, in these cases) is constant.
As described by Lee et al. in US 2006/0262053, the color of a white-emitting OLED can change with the controlling voltage. In other words, the color of a white-emitting OLED can vary with the intensity of emission. This problem can affect white subpixels in OLED or EL displays. It can also affect OLED or EL lamps, which can be considered to include a single, very large white subpixel. While a number of other methods have addressed the problem of transforming three color-input signals to four color-output signals, e.g., Morgan et al. in U.S. Pat. No. 6,453,067, Choi et al. in US 2004/0222999, Inoue et al. in US 2005/0285828, van Mourik et al. in WO 2006/077554, Chang et al. in US 2006/0187155, and Baek in US 2006/0256054, these methods cannot adjust for a white emitter with variable color. While Lee's method can adjust for a white emitter with variable color, it requires a set of six coefficients to apply a correction after the conversion from three color signals to four color signals. This method is computationally and memory intensive, and would be slow and difficult to implement in a large display. Gathering data for the method requires manual adjustments that can be time-consuming and labor-intensive. It requires gathering spectral data, which is more complex and time-consuming than colorimetric measurements. Further, it does not mathematically provide a colorimetric match between a desired RGB color and the RGBW equivalent.
Co-pending commonly-assigned U.S. Patent Application Publication No. 2008/0252797, filed Apr. 13, 2007, entitled “Method for input-signal transformation for RGBW displays” by Hamer et al., the disclosures of which are incorporated by reference herein, describes a method for transforming RGB to RGBW, where the W has color that varies with drive level.
US Patent Application Publication No. 2009/0189530 by Ashdown et al. describes feedback control of RGB LEDs by superimposing AM modulation on the PWM drive signal. However, the AM modulation does not provide control of chromaticity or luminance. It serves only to differentiate the R, G and B channels when sensed by a single photosensor.
US Patent Application Publication No. 2008/0185971 by Kinoshita describes adjusting current density and duty cycle of an EL emitter independently to vary chromaticity while keeping luminance constant. However, this scheme is limited to only chromaticities the EL emitter can produce natively. This is not sufficient for full-color displays, in which the desired chromaticity may not lie on the chromaticity locus of the EL emitter.
There is a need, therefore, for an improved method for compensating for chromaticity shift of an EL emitter in a single- or multi-color EL device or display.
According to one aspect of the present invention, there is provided a method for compensating for chromaticity shift of an electroluminescent (EL) emitter, comprising:
a) providing the EL emitter for receiving current and emitting light having a luminance and a chromaticity that both correspond to the density of the current;
b) providing a drive circuit electrically connected to the EL emitter for providing the current to the EL emitter;
c) receiving a designated luminance and selecting a chromaticity for the EL emitter;
d) selecting different black, first and second current densities based on the designated luminance and selected chromaticity, wherein
e) calculating respective black, first and second percentages of a selected emission time using the designated luminance, the selected chromaticity, and the black, first and second luminances and chromaticities, wherein the sum of the black, first and second percentages is less than or equal to 100%; and
f) providing the black, first and second percentages to the drive circuit to cause it to provide the black, first and second current densities to the EL emitter for the black, first and second percentages, respectively, of the selected emission time, so that the integrated light output of the EL emitter during the selected emission time has an output luminance and output chromaticity colorimetrically indistinct from the designated luminance and selected chromaticity, respectively, whereby the chromaticity shift of the EL emitter is compensated.
According to another aspect of the present invention, there is provided a method for compensating for chromaticity shift of an electroluminescent (EL) emitter, comprising:
a) providing the EL emitter for receiving current and emitting light having a luminance and a chromaticity that both correspond to the density of the current;
b) providing a drive circuit electrically connected to the EL emitter for providing the current to the EL emitter;
c) receiving a designated luminance and selecting a chromaticity for the EL emitter;
d) selecting different black, first, second and third current densities based on the designated luminance and selected chromaticity, wherein
e) calculating respective black, first, second and third percentages of a selected emission time using the designated luminance, the selected chromaticity, and the black, first, second and third luminances and chromaticities, wherein the sum of the black, first, second and third percentages is less than or equal to 100%; and
f) providing the black, first, second and third percentages to the drive circuit to cause it to provide the black, first, second and third current densities to the EL emitter for the black, first, second and third percentages, respectively, of the selected emission time, so that the integrated light output of the EL emitter during the selected emission time has an output luminance and output chromaticity colorimetrically indistinct from the designated luminance and selected chromaticity, respectively, whereby the chromaticity shift of the EL emitter is compensated.
According to another aspect of the present invention, there is provided a method for compensating for chromaticity shift of an electroluminescent (EL) emitter, comprising:
a) providing a display substrate having a device side;
b) providing the EL emitter for receiving current and emitting light having a luminance and a chromaticity that both correspond to the density of the current, wherein the EL emitter is disposed over the device side of the display substrate;
c) providing an integrated circuit chiplet having a chiplet substrate different from and independent of the display substrate, wherein the chiplet includes a drive circuit electrically connected to the EL emitter for providing the current to the EL emitter, and the chiplet is located over, and affixed to, the device side of the display substrate;
d) receiving a designated luminance and selecting a chromaticity for the EL emitter;
e) selecting different black, first and second current densities based on the designated luminance and selected chromaticity, wherein
f) calculating respective black, first and second percentages of a selected emission time using the designated luminance, the selected chromaticity, and the black, first and second luminances and chromaticities, wherein the sum of the black, first and second percentages is less than or equal to 100%; and
g) providing the black, first and second percentages to the drive circuit to cause it to provide the black, first and second current densities to the EL emitter for the black, first and second percentages, respectively, of the selected emission time, so that the integrated light output of the EL emitter during the selected emission time has an output luminance and output chromaticity colorimetrically indistinct from the designated luminance and selected chromaticity, respectively, whereby the chromaticity shift of the EL emitter is compensated.
An advantage of this invention is an EL device that compensates for chromaticity shift of the organic materials in the device without requiring extensive lookup tables. A further advantage of this invention is that it can provide chromaticity-shift compensation for EL devices that have only a single color of EL emitter, such as EL lamps. It is an important feature of this invention that it makes productive use of changes in chromaticity with current density which have hitherto been considered undesirable. It permits the adjustment of luminance independently of chromaticity. In some embodiments, it can use lower bit depth than conventional digital drive schemes. It advantageously permits the reproduction of colors that lie off the chromaticity locus of a particular EL emitter.
EL display 10 includes a plurality of row select lines 20; each row of EL subpixels 60 has a corresponding select line 20. EL display 10 further includes a plurality of data lines 35 where each column of EL subpixels 60 has an associated data line 35 for readout. Each subpixel 60 includes an EL emitter 50 (
Three different current densities on each curve can be used to form a gamut analogous to a typical RGB color gamut. Gamut 101 uses three current densities from curve 100. Any chromaticity within gamut 101 can be reproduced by EL emitter 50.
In some embodiments, only the black, first and second current densities are used. For example, line 108 (
Hereinafter the term “primary” refers to the luminance (e.g., 132) and chromaticity (e.g., 102) produced at a particular current density (e.g., 136). For example, the “first primary” refers to the first luminance 133 and first chromaticity 103 produced by the EL emitter 50 when driven with current at first current density 137. The black point of the display at black current density 136 is referred to as the “black primary.” This corresponds to the conventional meaning of “primary” in the art, but expands the definition to permit using multiple current densities of the same EL emitter 50 as different primaries, rather than only using different EL emitters as different primaries. Expressions such as “the luminances of the primaries” refer to the respective luminances of the black, first, second and, in some embodiments, third primaries, i.e. the respective luminances produced by EL emitter 50 at the black, first, second and optionally third current densities.
Each primary is different from the other primaries in either its luminance or chromaticity. That is, no two primaries produce exactly the same luminance and chromaticity. This provides a color gamut. Some primaries can have the same chromaticities but different luminances, some can have the same luminances but different chromaticities, and some can have different luminances and chromaticities. Specifically, the respective luminance (132, 133, 134, 135) of each of the black 136, first 137, second 138 and third 139 current densities is colorimetrically distinct from the other luminances, or the respective chromaticity (102, 103, 104, 105) of each of the black 136, first 137, second 138 and third 139 current densities is colorimetrically distinct from the other chromaticities. In embodiments with only the black, first and second current densities, each of the three chromaticities is colorimetrically distinct from the other two or each of the three luminances is distinct from the other two. In embodiments with the black, first, second and third current densities, each of the four chromaticities is colorimetrically distinct from the other three, or each of the four luminances is colorimetrically distinct from the other three.
“Different” and “colorimetrically-distinct” primaries are those separated visually, i.e. those that are at least 1 just-noticeable-difference (JND) apart. For example, the primaries can be plotted on the 1976 CIELAB L* scale, and any two primaries separated by at least 1 ΔE* are colorimetrically distinct. Distinct chromaticities can also be measured on the CIE 1976 u′v′ diagram as those points with Δ(u′, v′)≧0.004478 (the MacAdam JND, cited on pg. 1512 of Raymond L. Lee, “Mie Theory, Airy Theory, and the Natural Rainbow,” Appl. Opt. 37(9), 1506-1519 (1998), the disclosure of which is incorporated by reference herein), where Δ(u′, v′) is the Euclidian distance between two points on the CIE 1976 u′v′ diagram. Other methods of determining whether two colors or primaries are colorimetrically distinct are well-known in the color science art.
The black luminance 132 is less than a selected threshold of visibility 129, and the first 133, second 134 and third 135 luminances are greater than or equal to the selected threshold of visibility 129. The threshold of visibility 129 is selected based on the limits of the human visual system. For example, the threshold of visibility 129 can be 0.06 nits or 0.5 nits. The threshold of visibility 129 can be selected based on display peak luminance, display dynamic range, and display characteristics (e.g., ambient contrast ratio and surface treatment). The black luminance 132 is less than the threshold of visibility 129 so that the mathematical treatment of gamuts described herein corresponds to the mathematical treatment of conventional RGB gamuts. When using a standard primary matrix or phosphor matrix (“pmat”), intensities of 0 add no luminance or chromaticity to what the user perceives. In various embodiments, intensities of 0 in this treatment can correspond to black current density 136. Since black luminance 132 is less than threshold of visibility 129, black luminance 132 and black chromaticity 102 add no perceptible brightness or color to what the user perceives, so intensities of 0 behave as expected. To provide a black luminance 132 below threshold of visibility 129, black current density 136 can be less than a selected threshold current density (not shown), e.g., 0.02 mA/cm2.
To produce a color using gamut 101, a designated luminance is received and a chromaticity for the EL emitter 50 is selected. In one embodiment, the chromaticity is selected before mass-production of devices begins, and a device receives a sequence of designated luminances corresponding to the emission desired from different EL emitters 50 on the device. Designated luminances, hereinafter denoted “YW,” can be calculated from input RGB code values as known in the art, for example as shown in the above-referenced U.S. Pat. No. 6,885,380 and U.S. Pat. No. 6,897,876. For example, when an (R, G, B) code value triple is received, YW can be set equal to the minimum of the luminances corresponding to the R, G and B code values. An emission time 308 (
Respective black, first, second and, in some embodiments, third percentages of the selected emission time 308 are calculated using the designated luminance, the selected chromaticity, and the black, first, second and optionally third luminances and chromaticities. The sum of the black, first, second and optionally third percentages is less than or equal to 100%. The calculated percentages are the intensities [0,1] of the respective primaries. The intensities sum to ≦1 (the percentages to ≦100%) because only one EL emitter 50 is being used, and therefore time-division multiplexing is used. In some embodiments with only the black, first and second primaries, the black, first and second percentages can sum to 100%. In some embodiments also using the third primary, the black, first, second and third percentages can sum to 100%.
The black, first, second and optionally third percentages are provided to the drive circuit 700 (
Once the black 136, first 137, second 138 and optionally third 139 current densities of the primaries are selected based on the designated luminance and selected chromaticity (described below), the corresponding luminances and chromaticities of the primaries are used to calculate the percentages of the primaries to be used to produce the designated luminance and selected chromaticity. In embodiments which do not use the third current density 139, a virtual third primary is used to make a three-primary system. The virtual third primary can be selected having chromaticities which do not lay on the line between the first chromaticity 103 and second chromaticity 104, extended to infinity in both directions. The luminance of the virtual third primary can be selected arbitrarily. For example, the chromaticity of point 125 and the third luminance 135 can be selected as the virtual third primary.
A primary matrix (“pmat”) is formed using the first, second and third luminances and chromaticities. The primaries' luminances and chromaticities are transformed into the primaries' XYZ tristimulus values (e.g., using the inverse of CIE 15:2004, 3rd. ed., ISBN 3-901-906-33-9, pg. 15, Eq. 7.3) as in Eq. 1:
Xp=xpYp/yp; Zp=(1−xp−yp)Yp/yp (Eq. 1)
where p=1, 2 or 3 for the first, second or third primary respectively. If the third current density 139 is not being used, the virtual third primary is employed for x3, y3, Y3. The XYZ tristimulus values of the three primaries are then formed into a pmat according to Eq. 2:
Unlike conventional RGB-gamut systems, this pmat has no white point and no normalization. The tristimulus values produced by intensities of (1,0,0), (0,1,0), or (0,0,1) are simply those corresponding to the primaries' luminances and chromaticities, not to scaled versions of the luminances. Conventional pmats are described by W. T. Hartmann and T. E. Madden in “Prediction of display colorimetry from digital video signals”, J. Imaging Tech, 13, 103-108, 1987, the disclosures of which are incorporated by reference herein.
Designated tristimulus values are then calculated from the designated luminance and chromaticity using Eq. 1, above, to produce Xd, Yd, Zd. Intensities for the three primaries are then calculated using Eq. 3:
As in conventional systems, any intensity Ip outside of the range [0, 1] is not reproducible. In embodiments without the third current density 139, any substantially non-zero value of I3 (e.g., outside of [−0.01, 0.01]) indicates a non-reproducible color, since the virtual third primary is being used. Note that the intensities Ip of the three primaries are of the three primaries of EL emitter 50, as discussed above, not intensities of R, G and B emitters on the EL device.
I1, I2 and I3 are, respectively, the first, second and third percentages which are provided to the drive circuit 700. The EL emitter 50 is driven to emit light at the first, second and optionally third current density for the percentage of the emission time tf 308 specified by the respective Ip. ΣIp does not have to be 1 (100%); if it is less than 1, the black current density can be provided for the remainder tr of the emission time 308, or a time less than tr, with tr being calculated according to Eq. 4:
tr=tf−ΣIp. (Eq. 4)
In this way, a designated color is produced using the black 136, first 137, second 138 and optionally third 139 current densities selected based on the measured age of EL emitter 50. Consequently, various designated luminances can be produced at the selected chromaticity using different selected primaries. This permits compensation for the chromaticity shift of the EL emitter 50 with current density. The primaries can be selected using a lookup table which maps the designated luminance of EL emitter 50, and optionally the selected chromaticity, to the selected black 136, first 137, second 138 and optionally third 139 current densities. The EL device can include different lookup tables for different selected chromaticities, in which case each table maps designated luminance to the selected current densities. In various embodiments, more than three primaries are used. The pmat is extended to 3×4 or wider, and other transformations, such as white replacement, are used to calculate Ip. An example of such a technique useful with various embodiments is given in U.S. Pat. No. 6,885,380, referenced above.
Referring to
Solid-line waveform 310 is a drive waveform using three primaries plus black. At the beginning of the emission time 308, the first current density 137 is provided. At time 301, the second current density 138 is provided. At time 302, the third current density 139 is provided. At time 303, the black current density 136 is provided. Here ΣIp<1, and specifically ΣIp equals time 303. In some embodiments, waveforms such as waveform 310 provide a desired color with a lower bit depth than would be required for conventional digital drive, as different non-zero luminances can be combined to produce the desired color, rather than producing the color using entirely a single luminance. For example, low-luminance colors require very high bit depths in digital drive systems, because a very high luminance is emitted for a very short time. The short times are small fractions of the emission time, but require large numbers of bits to represent them. In various embodiments, a lower luminance is emitted for a longer time that is a larger fraction of the emission time and so requires fewer bits (one-half requires one bit, one-fourth two bits, one-eighth three bits and so on, so increasing the minimum time slice from one-eighth to one-fourth saves one bit).
Dashed-line waveform 320 shows a drive waveform like waveform 310, except with ramps between current densities. The Ip values for waveform 320 are the times that the current density being provided to the EL emitter 50 is substantially steady (e.g., within ±5%) of the corresponding selected current density. For example, I2 on waveform 320 is equal to time 305 minus time 304. I2 for waveform 310, however, is equal to time 302 minus time 301. Here the black current density 136 is provided for a time less than tr of Eq. 4, because some of the emission time is occupied by ramps, e.g., from time 305 to time 306. Specifically, the sum of the black, first and second percentages is less than 100%, and the drive circuit 700 provides current ramps between consecutive current densities to the EL emitter 50. The ramps can be linear, quadratic, logarithmic, exponential, sinusoidal, or other shapes. The actual currents of the ramps can vary ±10% from ideal values. Sinusoidal ramps are sections of a sinusoid, e.g., sin(θ) for θ on [−π/2, π/2] scaled to fit between the current density levels. For example, the current density J(t) of a sinusoidal ramp from second current density 138 (J2) to third current density 139 (J3) from time 305 (t305) to time 306 (t306) centered on time 302 (t302) can be calculated using Eq. 5:
Ramps, especially sinusoidal ramps, provide smoother transitions between current densities, reducing inductive kick as the current density changes. In an embodiment, no direct control of the ramp is provided. In between one current density and another, there is a transition period including an exponential ramp as capacitive loads charge under a constant applied voltage. In another embodiment, the transition period includes a linear ramp as capacitive loads charge under a constant applied current.
In some embodiments, luminance range 112 (
The different black, first, second and optionally third current densities are selected based on the designated luminance and selected chromaticity (hereinafter “xyYd”). One way to do this is to characterize an EL emitter 50 before mass-production. Based on measurements of the luminance and chromaticity of the W emitter at various current densities, appropriate primaries can be selected for each xyYd. However, given limitations typically placed on the resolution (i.e. driver bit depths) of current densities and intensities, it is not always possible to reproduce exactly the selected chromaticity at a particular designated luminance (e.g., point 125 of
The different black 136, first 137, second 138 and optionally third 139 current densities based on the measured age of EL emitter 50 can be selected as follows. The luminances and chromaticities of any number of points are received, those points being measured along a current density sweep of EL emitter 50 at any number of ages. The number of combinations of these points is determined by the resolution with which current densities can be supplied to EL emitter 50. For example, there are sixteen possible combinations of current densities available for two, two-bit current supplies. A set of test intensities to try is also selected. The number of test intensities is determined by the resolution of intensities, i.e. how finely the emission time 308 can be subdivided. Respective test tristimulus values are calculated for the test intensities for each possible pmat. Test CIELAB values are then calculated from the test tristimulus values.
A set of aim designated luminances is then selected. For each aim designated luminance, the CIELAB ΔE* is computed between the entire test CIELAB values and the aim designated luminance at the selected chromaticity. The intensity combination having the lowest ΔE* is selected as the intensity for that aim designated luminance, and the ΔE* is recorded. The ΔE* in the selection can be weighted, e.g., to weight luminance error more heavily than chromaticity error, or vice versa. Additionally, any test CIELAB value (and corresponding test intensities) having ΔE*>1 JND (e.g., >1.0 or >2.0) can be omitted from consideration, as the result would not be colorimetrically indistinct from the desired luminance at the selected chromaticity. Alternatively or additionally, the test intensities corresponding to any test tristimulus value that are not within 1 JND u′v′ of the selected chromaticity can be omitted. The recorded ΔE* values for the (non-omitted) test intensities of a particular combination of current densities are combined, e.g., by taking the mean and maximum ΔE*. The combination with desired ΔE* characteristics for the test intensities is then selected as the set of primaries to use. For example, the combination with the lowest max(ΔE*) or rms(ΔE*) can be selected.
This method will select a single black, first, second and optionally third primary current density to be used for designated luminances. Alternatively, different primaries can be selected for different designated luminances or ranges of designated luminances. The selection can be performed at manufacturing time and stored in the EL device (e.g., EL display 10), or performed during operation of the EL device.
Selected primaries were calculated from measured data of a representative OLED emitter. This example was calculated with three-bit intensities and approximately four-bit current densities. The producible luminance range for this example is approximately 0 nits to 10,840 nits. The chromaticity locus passes through the measured points given in Table 1.
The pmat for gamut 101 is (no scaling; luminances in nits):
This pmat can be used to calculate Ip values as described above.
For example, to four significant figures, in gamut 101, intensities (0.2857, 0.1429, 0) produce approximately 1958 nits at (x,y)=(0.2936, 0.3040) (a neutral with CCT=8154K), or (u′,v′)=(0.1938, 0.4514). This point is Δxy=0.0002171 away from the closest point on a linear interpolation of the locus between each pair of adjacent points in Table 1, above. The two closest points are (0.2937, 0.3047) and (0.2919, 0.3003), and the closest point on the line between them to (0.2936, 0.3040) is (0.2934, 0.3040). Although the Δxy is small for this example, it is nonzero, demonstrating that colors that lie off the chromaticity locus of a particular EL emitter can be reproduced using that emitter, as described herein. The value of Δxy for any particular emitter and reproduced color depends on the shape of the locus and the selected color. For example, a semi-circular locus has a Δxy to a point at the center of the locus equal to the radius of the locus.
EL devices can be implemented on a variety of device substrates with a variety of technologies. For example, EL displays can be implemented using amorphous silicon (a-Si) or low-temperature polysilicon (LTPS) on glass, plastic or steel-foil display substrates. In one embodiment, an EL device is implemented using chiplets, which are control elements distributed over a device substrate. A chiplet is a relatively small integrated circuit compared to the device substrate and includes a circuit including wires, connection pads, passive components such as resistors or capacitors, or active components such as transistors or diodes, formed on an independent chiplet substrate. Details concerning chiplets and the processes for preparing them can be found, for example, in U.S. Pat. No. 6,879,098; U.S. Pat. No. 7,557,367; U.S. Pat. No. 7,622,367; US20070032089; US20090199960 and US20100123268, the disclosures of all of which are incorporated by reference herein.
For example, an eight-bit counter can count 256ths of the emission period [0, tf), starting at 0, crossing over to 255 at tf−tf/256, and rolling over back to 0 at tf. When the counter value is 0 to the value stored in register 735a minus one, comparator 730a can output TRUE, and the other comparators output FALSE, to cause the mux 710 to pass the value from capacitor 716a to the gate of drive transistor 70. From the register 735a value to the register 735b value minus one, comparator 730b can output TRUE and the others FALSE, and from the register 735b value to the register 735c value, comparator 730c can output TRUE and the others FALSE. As indicated by the dashed arrows, comparators 730a, 730b and 730c can communicate with each other to indicate when the next comparator should output TRUE. This is one of many possible drive circuits which can be employed with various embodiments;
Referring back to
Since the chiplets 410 are formed in a semiconductor substrate, the circuitry of the chiplet 410 can be formed using modern lithography tools. With such tools, feature sizes of 0.5 microns or less are readily available. For example, modern semiconductor fabrication lines can achieve line widths of 90 nm or 45 nm and can be employed in making the chiplets 410. The chiplet 410, however, also requires connection pads 412 for making electrical connection to the metal layer 403 provided over the chiplets 410 once assembled onto the device substrate 400. The connection pads 412 are sized based on the feature size of the lithography tools used on the device substrate 400 (for example 5 μm) and the alignment of the chiplets 410 to any patterned features on the metal layer 403 (for example ±5 μm). Therefore, the connection pads 412 can be, for example, 15 μm wide with 5 μm spaces between the pads 412. The pads 412 will thus generally be significantly larger than the transistor circuitry formed in the chiplet 410.
The pads 412 can generally be formed in a metallization layer on the chiplet 410 over the transistors. It is desirable to make the chiplet 410 with as small a surface area as possible to enable a low manufacturing cost.
By employing chiplets 410 with independent chiplet substrates 411 (e.g., comprising crystalline silicon) having circuitry with higher performance than circuits formed directly on the device substrate 400 (e.g., amorphous or polycrystalline silicon), an EL device with higher performance is provided. Since crystalline silicon has not only higher performance but also much smaller active elements (e.g., transistors), the circuitry size is much reduced. A useful chiplet 410 can also be formed using micro-electro-mechanical (MEMS) structures, for example as described in “A novel use of MEMs switches in driving AMOLED”, by Yoon, Lee, Yang, and Jang, Digest of Technical Papers of the Society for Information Display, 2008, 3.4, p. 13.
The device substrate 400 can include glass and the metal layer or layers 403 can be made of evaporated or sputtered metal or metal alloys, e.g., aluminum or silver, formed over a planarization layer 402 (e.g., resin) patterned with photolithographic techniques known in the art. The chiplets 410 can be formed using conventional techniques well established in the integrated circuit industry.
Electroluminescent (EL) devices include EL displays and EL lamps. The present invention is applicable to both, and will be discussed first with reference to an EL display.
A compensator 191 receives the designated luminance and selected chromaticity on input line 85. Compensator 191 selects the current densities of the primaries using the designated luminance and selected chromaticity and calculates the percentages Ip using the designated luminance and chromaticity and the selected current densities. It then provides information corresponding to the selected current densities and the calculated percentages on control line 95. Source driver 155 receives the information and produces a drive transistor control waveform on data line 35. The drive transistor control waveform includes the gate voltages necessary to cause the drive transistor to produce a current-density waveform such as those illustrated in
In one embodiment, the drive transistor control waveform includes a first gate voltage, a second gate voltage, and a black gate voltage in sequence for the percentages of the emission time corresponding to the black, first and second primaries. Thus, compensator 191 can provide compensated data during the display process. As known in the art, the designated luminance and chromaticity can be provided by a timing controller (not shown). The designated luminance and chromaticity can correspond to an input code value. The input code value can be digital or analog, and can be linear or nonlinear with respect to commanded luminance. If analog, the input code value can be a voltage, a current, or a pulse-width modulated waveform. Compensator 191 can optionally be connected to memory 195 for storing information used in selecting the primaries, such as the primaries themselves, if pre-selected primaries are used for designated luminances at the selected chromaticity, or tables mapping selected chromaticities and designated luminances or luminance ranges to primaries. Memory 195 can be non-volatile storage such as Flash or EEPROM, or volatile storage such as SRAM.
Source driver 155 can include a digital-to-analog converter or programmable voltage source, a programmable current source, or a pulse-width modulated voltage (“digital drive”) or current driver, or another type of source driver known in the art, provided that it can cause the a current-density waveform, e.g.,
In one embodiment, before mass-production of the EL device, one or more representative devices can be characterized to produce an product model mapping the designated luminance and the selected chromaticity to the corresponding selected black 136, first 137, second 138, and optionally third 139 current densities. More than one product model can be created. For example, different regions of the device can have different product models. The product model can be stored in a lookup table or used as an algorithm. These models can be combined, or the boundaries between them smoothed, by regression techniques known in the statistical art such as spline fitting. Compensator 191 can store the product model(s), e.g., in memory 195.
In a preferred embodiment, the EL device includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292 and U.S. Pat. No. 5,061,569. Many combinations and variations of organic light emitting materials can be used to fabricate such a device. Referring to
Transistors 70, 80 and 90 can be amorphous silicon (a-Si) transistors, low-temperature polysilicon (LTPS) transistors, zinc oxide transistors, or other transistor types known in the art. They can be N-channel, P-channel, or any combination. The OLED can be a non-inverted structure (as shown) or an inverted structure in which EL emitter 50 is connected between first voltage source 140 and drive transistor 70.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that combinations of embodiments, variations, and modifications can be effected within the spirit and scope of the invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4769292 | Tang et al. | Sep 1988 | A |
| 5061569 | VanSlyke et al. | Oct 1991 | A |
| 5929843 | Tanioka | Jul 1999 | A |
| 6329764 | Van De Ven | Dec 2001 | B1 |
| 6453067 | Morgan et al. | Sep 2002 | B1 |
| 6879098 | Buchwalter et al. | Apr 2005 | B2 |
| 6885380 | Primerano et al. | Apr 2005 | B1 |
| 6897876 | Murdoch et al. | May 2005 | B2 |
| 7557367 | Rogers et al. | Jul 2009 | B2 |
| 7622367 | Nuzzo et al. | Nov 2009 | B1 |
| 20050168564 | Kawaguchi et al. | Aug 2005 | A1 |
| 20060114200 | Yamashita et al. | Jun 2006 | A1 |
| 20060262053 | Lee et al. | Nov 2006 | A1 |
| 20070032089 | Nuzzo et al. | Feb 2007 | A1 |
| 20070057263 | Kahen | Mar 2007 | A1 |
| 20080185971 | Kinoshita | Aug 2008 | A1 |
| 20080252797 | Hamer et al. | Oct 2008 | A1 |
| 20090189530 | Ashdown et al. | Jul 2009 | A1 |
| 20090199960 | Nuzzo et al. | Aug 2009 | A1 |
| 20100123268 | Menard | May 2010 | A1 |
| 20100123694 | Cok et al. | May 2010 | A1 |
| 20100148683 | Zimmermann et al. | Jun 2010 | A1 |
| 20110273102 | van de Ven et al. | Nov 2011 | A1 |
| Entry |
|---|
| Lee, “TFT-LCD with RGBW Color System”, SID 03 Digest, 40.5LB/-w. Lee, 40.5L: Late-News Paper, pp. 1212-1215, Copyright 2003. |
| Broadcast Monitor Buyer's Guide, Library maintained by Chan, Glenn “Understanding Black Level” http://www.glennchan.info/broadcast-monitors/understanding/black-level.htm Jun. 23, 2009 archive. |
| Number | Date | Country | |
|---|---|---|---|
| 20120194565 A1 | Aug 2012 | US |
