Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices; including televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently-colored emitters, typically red, green, and blue, to represent each image element represented within an input image signal. A variety of flat-panel display technologies are known, for example, plasma displays, field emissive displays (FEDs), liquid crystal displays (LCDs), and electro-luminescent (EL) displays, such as organic light-emitting diode (OLED) displays. To present images on these displays, the display typically receives an image input signal containing three-color-components for each image element which the display utilizes to drive each differently-colored emitter for each pixel.
In emissive displays, including plasma, field-emissive and electro-luminescent displays, the amount of visible radiant energy produced by the display is proportional to the amount of power that the display consumes. This same relationship does not exist in transmissive displays, such as certain LCDs, in which the energy provided to the light source is not modulated as these displays typically create enough light to provide the brightest possible image and then modulate the light rather than the input energy so that only the necessary portion of the light is transmitted to the user.
Displays are used in many professional and consumer electronic devices. In many professional and consumer electronic devices, the user is given the opportunity to adjust the color temperature of the display device. Typically, the color temperature of the display is adjusted to various white points representing different points on or near the Planckian Locus, including colors having color coordinates near the color of standard emitters such as D50, D65, D70 and D95. Further, some display devices provide the ability to change the color temperature of the display at a higher level. For example, the color temperature of the display can be adjusted to D65 when the display is placed in modes, such as those referred to by names such as “Cinema” and the display and to a color temperature such as D93 when placed in modes, such as those referred to by names such as “Standard”.
It is known to adjust the color temperature of a display device. For example, in the LCD art, it is known to use multiple illumination sources to enable the adjustment of color temperature in a display. For example, Cornelissen in U.S. Pat. No. 6,840,646, entitled “Illumination system and display device” discusses a liquid crystal display device with an adjustable backlight, comprising a low-pressure discharge lamp and additional blue LEDs. By changing the ratio of the power to these two light sources, the color of the LCD backlight can be modified to change the color temperature of the display as it passes through the liquid crystal and red, green, and blue color filters. Similarly, Evanicky in U.S. Pat. No. 6.535,190, entitled “Multiple light source color balancing system within a liquid crystal flat panel display” discusses an LCD having fluorescent lamps that serve as the backlight for the LCD, some lamps producing a different color of light than others. By adjusting the ratio of light produced by each of the lamps, the color temperature of the backlight and therefore the color temperature of the light that is passed through the LCD and its red, green, and blue color filters is modified. Although, this method adds cost to the overall LCD system, these displays make relatively efficient use of the light that is produced, as the light source is directly modified to create the color of light that is needed. Unfortunately, it is not possible to employ adjustable backlights in emissive displays and so this method cannot be applied directly in emissive displays, such as EL displays.
It is also known to adjust the color temperature of a display by adjusting the ratios of the red, green, and blue light-emitting elements to change the color temperature of the display. For example, Inohara et al. in U.S. Pat. No. 4,449,148, entitled “Image Display Apparatus” discusses a method for adjusting the color temperate of a CRT by adjusting the ratio of the electron power to the phosphors in the CRT, either by adjusting power or adjusting the on time to each of the red, green, and blue color channels of the CRT. Similar methods are used to adjust the luminance ratio of red, green, and blue emitters in emissive displays, this problem becomes somewhat more complex when additional emitters are added to the display.
However, not all displays have only red, green, and blue emitters. For example, Miller et al in U.S. Pat. No. 7,230,594, entitled “Color OLED display with improved power efficiency” discusses an OLED display having red, green, blue and an additional in-gamut emitter, wherein the efficiency of the in-gamut white emitter is significantly higher than the efficiency of the red, green, and blue emitters. As discussed in this patent, the presence of the in-gamut emitter significantly improves the power consumption of the display.
In the more than three-color emitter art, it is known to adjust the color temperature of a display having an in-gamut emitter. For example, Murdoch et al. in U.S. Pat. No. 6,897,876, entitled “Method for transforming three color input signals to four or more output signals for a color display” describes a method for driving an electro-luminescent display having red, green, blue, and white emitters to color temperatures other than the color of the white emitter. Also, in the LCD art, Higgins in U.S. Pat. No. 7,301,543 entitled “Systems and methods for selecting a white point for image displays” describes a method for adjusting the color temperature of an LCD having red, green, blue, and white emitters using a set of weighting coefficients. These methods allow the color temperature of the display to be adjusted, however, they have the disadvantage that they also have a significant effect on the power consumption of the display. For example, the method discussed by Higgins changes the ratio of light that is blocked by the LCD and in the emissive display case, the ratio of light produced by highly efficient white emitters to the ratio of light produced by lower efficiency red, green, and blue emitters of emissive displays with color filters is altered. As a result, the power consumption of the display can vary dramatically for displays having four or more emitters as the color temperature of the display is changed.
There is a need for an emissive display structure that permits the color temperature to be adjusted in emissive, specifically electro-luminescent displays having four or more color emitter systems, wherein the power efficiency of the resulting display does not vary significantly as a function of color temperature. It is further desirable to maintain high power efficiency, and therefore low display power consumption, regardless of display color temperature.