Reference is made to commonly-assigned, copending U.S. Ser. No. (Kodak Docket 92893) filed concurrently herewith entitled “Method for Input-Signal Transformation for RGBW Displays With Variable W Color” by Hamer et al, the disclosure of which is incorporated herein by reference.
The present invention relates to calibrating flat-panel displays, and in particular to a method for calibrating color displays including at least one within-gamut emitter.
In today's digital infoimaging world, many images are previewed and manipulated on electronic flat panel displays. New display applications (i.e. cell phones, DVD, palm pilots, video games, GPS, TV, etc.) impose greater design requirements and improved imaging performance than other imaging display devices used previously. Displays are intended to provide a realistic representation of the images to the viewer, thus there is a need to correct display color and tonal responses to enhance the display image quality. The color and tonal enhancement must be implemented in the display's imaging chain.
Flat panel displays such as OLED displays have the potential for providing superior performance in brightness and color resolution, wide viewing angle, low power consumption, and compact and robust physical characteristics. However, unlike CRTs, these flat panel displays have a fixed white point and a chromatic neutral response that result from the manufacturing process, and are not adjustable. Variations in the manufacturing process result in variations in the white point and chromatic neutral, and therefore unwanted variations in display color reproduction. With manufacturing processing variability and the need to increase yield to reduce costs, it becomes imperative to develop robust and easily implemented color characterization and display driving techniques that accommodate manufacturing variations.
In a common OLED color display device, a pixel includes red, green, and blue colored OLEDs. These OLEDs correspond to color primaries that define a color gamut. By additively combining the illumination from each of these three OLEDs, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. OLEDs can be used to generate 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, OLED along with the red, green, and blue OLEDs to improve power efficiency and/or luminance stability over time.
Various methods of calibrating flat-panel displays have been proposed. For example, Cottone et al., in U.S. Pat. No. 6,677,958, disclose a method of calibrating a color flat panel display. Chiu et al., in US 2006/0038748, teach an image processing method for a plasma display panel. Evanicky et al., in U.S. Pat. No. 6,611,249, disclose a method of calibrating an LCD display with two different white light sources. Rykowski et al., in US 2004/0246274, provide a method for calibrating a display, including a light-emitting-diode display. Yasuda et al., in EP 1 681 668, describe a calibration method for a display, and in particular for an LCD display. Shimonishi, in US 2006/0044234, teaches a method of calibrating and adjusting a self-emissive display, e.g. an OLED or plasma display. Park, in US 2006/0012724, teaches a method of calibrating a flat panel display to produce color similar to a CRT display. Braudaway et al., in U.S. Pat. No. 6,690,383, teach a method of calibrating a display whose properties differ from a CRT display. However, all these methods only concern three gamut-defining emitters, e.g. red, green, and blue, and do not include a within-gamut emitter, such as white.
There is a need therefore for an improved method of calibrating and driving flat-panel displays that include within-gamut emitters.
In accordance with one embodiment, the invention is directed towards a method for calibrating a display device having four or more channels, including three main channels which include in their gamut a desired display white point, and one or more further channels, said display device also having one or more individual adjustment controls for each channel, said method comprising the steps of:
a) displaying a first target using a low level code value for each channel of the display;
(b) measuring and recording the luminance and chromaticity coordinates of the displayed first target;
c) displaying a second target using a minimum code value for each of the further channels, and a set of non-minimum code values, such set including one non-minimum code value for each of the three main channels;
d) measuring the luminance and chromaticity coordinates of the displayed second target;
e) adjusting the individual adjustment controls for each of the three main channels so that the chromaticity coordinates of the second target approximately match the chromaticity coordinates of the desired display white point;
f) recording the resulting values of the individual adjustment controls for each of the three main channels and the corresponding luminance and chromaticity coordinate measurements;
g) repeating steps c) through f) one or more times for each of a number of additional selected non-minimum code value sets;
h) displaying a third target using, for a first main channel, the value(s) of the individual adjustment control(s) for that channel recorded in step f) corresponding to a selected non-minimum code value set, and using, for each of the other channels, a minimum code value;
i) measuring and recording the luminance and chromaticity coordinates of the displayed third target;
j) repeating steps h) through i) for each remaining main channel;
k) displaying a fourth target using a selected code value for a first further channel, and a minimum code value for each of the other channels;
l) measuring and recording the luminance and chromaticity coordinates of the displayed fourth target;
m) repeating steps k) through l) for one or more of a number of additional selected code values of the first further channel; and
n) repeating steps k) through m) for each remaining further channel.
It is an advantage of this invention that it performs a calibration for a display device having four or more channels resulting in an achromatic neutral scale that is more representative of real operating conditions than prior art methods. It is a further advantage of this invention that it leads to a simpler calibration method, as it does not require measuring individual red, green, and blue main channel scales, and thus requires fewer steps than prior art methods. It is a further advantage that no additional computation is required to get an achromatic neutral scale. It is a further advantage that the calibration method of the invention can be easily automated. It is a further advantage of this invention that it reduces error due to additivity failure to a greater extent than other methods.
Turning now to
A display calibration procedure typically starts with establishment of the desired display white and black points. The desired display white point is established in terms of x, y, and Y, where x and y are 1931 CIE chromaticity coordinates and Y is the 1931 CIE luminance in units of cd/m2. The chromaticity coordinates of the desired display white point will also be referred to herein as a neutral, which can include lower luminance points, e.g. gray and black. The desired display black point is established in terms of the 1931 CIE luminance in units of cd/m2. Ideally, the desired display black point has the same chromaticity coordinates as the desired display white point, but the black luminance level is often so low that it can be difficult to achieve the same chromaticity coordinates as the desired display white point. There is also a peak display white point, which is herein defined as the maximum possible luminance at the desired chromaticity coordinates. Depending on the application, the desired display white point and the peak display white point can be the same or different. For example, one may choose to set the desired display white point at a lower luminance than the peak display white point to leave some headroom for display luminance or chromaticity coordinate changes over time. It is also important to define the peak display luminance point as the point at which all main channels are driven to their maximum level. This peak display luminance point may not be at the same chromaticity coordinates as the desired display white point or the peak display white point.
The typical color imaging system including the hardware necessary to calibrate the display, as illustrated in
Turning now to
Turning now to
Turning now to
It will be understood that other adjustment controls can also be used for the adjustment in Step 230, for example gamma voltages as taught by Park et al. in U.S. Pat. No. 6,806,853, analog gains and/or offsets as taught by Cottone et al. in U.S. Pat. No. 6,677,958, and linear processing methods, such as digital gain and offset, as described in “A Technical Introduction to Digital Video” by C. Poynton, John Wiley & Sons, New York, 1996, chapters 5 & 6. However, code values are a convenient adjustment because they can also be used to set the display's channel luminances and chromaticity coordinates, so that the adjustment can be done by the same apparatus (e.g. computer 40 in
Turning now to
An advantage of selecting the set of non-minimum code values corresponding to the peak display white point from Step 240 of
Turning now to
In measurements of this type, it can be a problem to make the display luminance directly proportional to the current at all levels, for several reasons. One reason is that the display's peripheral circuitry has a resistance. At a high display luminance, which requires a high display load for the current, the voltage loss through the peripheral circuitry will be greater than at low luminance/low load, changing the voltage across the displayed pixels and introducing non-linearity into the luminance-current response of the display. It is desirable to maintain a constant display load to minimize this effect. It can also be desirable that the constant display load approximately matches a display reference load condition, e.g. the average display load over the lifetime of the display. Since the world integrates to an 18% gray (van der Weijer, J. and Gevers, T., “Color Constancy Based on Grey-Edge Hypothesis”, IEEE International Conference on Image Processing, ICIP, 2005), this can be used to represent the average display luminance over the lifetime of the display. Turning now to
An alternative method of maintaining a display load approximately matching a display reference load condition at relatively high luminance is by displaying one or more buck patterns on the display, as shown in
These patterns can be used together. For example, for a target at a relatively low luminance, a boost pattern can be displayed on the display to increase the display load. For a different target at a relatively high luminance, a reduction pattern or a buck pattern can be displayed on the display to decrease the display load. Thus, for different targets at different relative luminances, the display load can be made to approximately match the display reference load condition.
While a display load approximately matching the display reference load condition can also be achieved with a flat field reduction pattern as described above, a buck pattern has the additional advantage of being able to maintain the display reference load condition across the entire display. A reduction pattern of
The measured data and individual adjustment control values for each channel obtained from the method described herein can be used to compute values used by an image processing path to drive the display device. Such a method of computing values used by an image processing path has been described, e.g., by Giorgianni and Madden in Digital Color Management: encoding solutions, Reading: Addison-Wesley, 1998.
Turning now to
The relationship given in Eq. 1 was derived by W. T. Hartmann and T. E. Madden, “Prediction of display colorimetry from digital video signals”, J. Imaging Tech, 13, 103-108, 1987. The 3×3 matrix is known as the inverse primary matrix, where the columns of the matrix XR, YR, and ZR are the tristimulus values for the red gamut-defining primary, XG, YG, and ZG are the tristimulus values for the green gamut-defining primary, and XB, YB, and ZB are the tristimulus values for the blue gamut-defining primary. Colorimetric measurements resulting in XYZ tristimulus values of each gamut-defining channel were the data collected in
Once determined, the relationship between code value of the further channel and intensities of the three main channels can be employed to transform the common three color-input signals (e.g. R, G, and B) corresponding to the three main channels of the display to four color-output signals, corresponding to the main channels and the further channel of the display, which can be labeled R′, G′, B′, and W. Typically, one starts with a desired color specified as three color-input signals wherein each of the three components is linear with respect to intensity for red, green, and blue, and corresponds to the main channels of the display. If the color-input signals are non-linear with respect to intensity, they can first be converted to a linear signal, for example by a conversion such as sRGB (IEC 61966-2-1:1999, Sec. 5.2). The relationship can be employed with the three color-input signals (R, G, B) to determine a drive value W (which can be a code value) of the four color-output signals and modification values to be applied to one or more of the R, G, B components of the three color-input signals to form the R′, G′, B′ color-output signals, as further described in Hamer et al. U.S. Ser. No. (Kodak Docket 92893). The display can then be driven with the four color-output signals, or transformed values thereof (e.g. the R′, G′, and B′ components of the four color-output signals, which are linear in intensity, can be transformed into display code values).
Each code value is typically associated with a voltage used to drive the display to a particular luminance. It can be necessary to adjust the voltages associated with one or more of the code values. This can be accomplished in the case where a display has one or more global adjustment controls, which affect all channels. One would use the global adjustment controls to make preliminary adjustments to the display before using the method of this invention. That is, such a preliminary adjustment would be done before Step 100 of
The desired display black point can then be adjusted. In this example, desired display black point 520 is at an aim code value of 0, which has a data voltage associated with it. The difference between the data voltage and the supply voltage, which is called black point voltage 550, determines the luminance of the display at that code value. The lowest of the global gamma voltages can be set so that the display produces the desired display black point when driven at the selected low level code values (Step 580 of
There can be more global gamma voltages between the desired display white and black points, e.g. at display points 530a, 530b, and 530c. The global gamma voltages can be adjusted for each of these points (Step 590 of
Depending on the innate characteristics and drive electronics of a particular display device, gamma voltage curves may need different shapes to accomplish the desired effect of luminance resolution corresponding to eye sensitivity. For example, display devices such as OLEDs may be driven by current provided by drive transistors, and there is a nonlinear relationship between voltage on a drive transistor and current through the device. This nonlinearity can innately provide luminance resolution corresponding to eye sensitivity, so the gamma voltage curve can be linear. In other cases, achieving the desired display black point may require lower currents than the rest of the range would suggest, for example to place the drive transistor in its subthreshold operating region, so the gamma voltage curve can be concave down. In another example, conventional twisted-nematic LCDs as known in the art can have a variety of shapes of transmittance curve as a function of voltage; see for example Leenhouts in U.S. Pat. No. 4,896,947,
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.