In an active matrix liquid crystal display (LCD), an image is produced by controlling the light transmittance of a two-dimensional array of discrete image elements (subpixels). Control is performed by the conversion of digital image data, consisting of a data value for each subpixel of the image, into analogue voltages with values dependent on that data, and direction of those voltages to each pixel electrode in the array via an active matrix of source data lines, gate lines and thin film transistor (TFT) switching elements.
A block of three subpixels is termed a pixel. Each subpixel is associated with a color filter (typically Red, Green and Blue), and by controlling the amount of light being transmitted through these three color channels, any resultant linear combination of Red, Green and Blue light can be produced. In the case of one or more color channels being in the “off” state—that is, the liquid crystals are energized in such a way that no light is transmitted through them—the color is said to be “saturated” in that the color is as vivid as is possible with this display. Some LCDs are capable of producing very vivid saturated colors, while some are only able to produce pale colors, even with only one color channel in the on state. The vividness of the color is related to its light spectrum. A light with a very broad spectrum will appear pale, while a narrow spectral light will appear vivid. A monochromatic light (that is, light with only one wavelength, typically produced by a laser) is the most vivid light possible.
In conventional LCDs, a broad spectrum light is emitted by a backlight, and each color filter will only transmit a particular range of wavelengths. A narrow range will produce more vivid colors, but since more light is being absorbed by the filter, the brightness of the screen is reduced. In addition, LEDs with a broad spectral emission are in general more power-efficient than those with narrower emission windows.
There have been several methods tried to improve the power efficiency of high-color screens. In Akiyama, U.S. Pat. No. 7,106,276 (issued Sep. 12, 2006) the inventors supplement the white LED (W LED) with three separate LEDs each with a narrow spectral emission, in the three color primaries. However, the power requirement of this configuration is large, and outweighs the advantage of the high-color display. Bergquist, Publication US20080150864 (published Jun. 26, 2008) specifies using only Red Green and Blue LEDs (RGB LEDs), in such a way that for each color primary, if the image content allows it, the corresponding LED can be dimmed and the image data altered. In this way, the power requirement can be reduced while maintaining the color vividness. However, this still requires the RGB LEDs to be used even for pale colors, which is less efficient than using W LEDs.
Van Beek et al., US20090160756 (published Jun. 25, 2009) attempts to deal with this by combining RGB LEDs and W LEDs, and selectively choosing which LEDs to use at any one time. The inventors calculate the required drive current to each of the independently controllable R,G,B LEDs, and then make assumptions about the possibility of replacing R,G,B specific currents with a general W current. In this way they reduce the power consumption of the backlight device, but they also restrict the vividness of the image data to be displayed. Specifically, if their algorithm concludes that the Red, Green and Blue LEDs should all be driven at maximum current, then they will replace this with a White LED being driven at full current and the RGB LED not driven at all. This clearly reduces the vividness of the color that the panel can produce, even though the image content might require high saturation levels. More generally, for any color which fits inside the gamut of the White LED, this approach will dim the W LED to the smallest of the three colors, and supplement it with RGB LEDs, instead of the more efficient approach of using the W LED to supply all the required light, and not using the RGB LEDs.
Langendijk et al., U.S. Pat. No. 8,300,069 (issued on Oct. 30, 2012) discloses another means of balancing different backlights. The inventors do not use a W LED, but use a fourth (White) subpixel. They control the current going to the Red LED, and to the Green and Blue LED. To avoid “chromaticity dependence”—that is, Green light being transmitted through the Blue color filter and vice versa—the ratio of G:B current is held constant. For pale pixels, the White subpixel can be opened in such a way that the pixel will appear very bright; for vivid pixels, the White will be closed and only the desired primaries will be transmissive. In this way, the apparent brightness of the panel can be increased in pale areas so that the overall brightness appears greater; or the backlight powers can be scaled down to reduce the power increase necessitated by having the RGB LEDs. However, this gives a non-uniform distribution of brightnesses, contrary to standardized color spaces.
Many conventional devices (e.g., Nakano et al., U.S. Pat. No. 7,333,165 (issued Feb. 19, 2008), Keh et al., US20070103934 (published May 10, 2007), and Morishita, US20120242564 (published Sep. 27, 2012)) use a combination of different backlights, but the proportion of each backlight being used depends on a variety of different system conditions. For example, the ambient lighting, the desired brightness, or the display mode being used (e.g., Vivid, Low power), can all affect the balance.