Patent Application
20050263674
Two common backlighting solutions are white light emitting diodes (LEDs) and the cold cathode fluorescent lamp (CCFL). In theory, the wider color gamut of red-green-blue (RGB) LEDs would provide a better backlighting solution. However, factors such as cost, light efficiency and power dissipation are currently prohibiting the effective commercialization of RGB LEDs as a backlighting solution.
In one embodiment, a mixed light is produced using first and second light sources of first and second colors. A determination is then made as to whether feedback tristimulus values, representative of the mixed light, are within a range of reference tristimulus values. If one or more of the feedback tristimulus values is out of range, a luminance ratio between the first and second light source is adjusted.
In another embodiment, apparatus comprises a liquid crystal display (LCD), a backlight for the LCD, a sensing system, and a control system. The backlight comprises first and second light sources of first and second colors. The sensing system acquires feedback tristimulus values that are representative of a mixed light produced by the backlight. The control system is provided to 1) determine whether the feedback tristimulus values are within a range of reference tristimulus values, and 2) if one or more of the feedback tristimulus values is out of range, adjust a luminance ratio between the first and second light source.
Other embodiments of the invention are also disclosed.
Illustrative and presently preferred embodiments of the invention are illustrated in the drawings, in which:
A backlighting solution that provides wider color gamut than a white light source, but which is more practical to implement than RGB LEDs, is a bicolor light source comprised of first and second light sources of first and second colors. By way of example, the first and second light sources may be a white light source and a colored light source. The white light source can take the form of a CCFL or white LEDs, and the colored light source can take the form of red LEDs (i.e., a red light source).
A problem with backlighting via a bicolor light source is that the optical characteristics of its two light sources may vary with temperature, drive current, aging and other factors. When this occurs, the color of the mixed light produced by the bicolor light source can drift. If one or both of the light sources is implemented using LEDs, differences in LED characteristics can further aggravate the problem of color drift (since LED characteristics can vary from batch to batch within the same fabrication process).
In applications such as liquid crystal display (LCD) backlighting, color consistency and uniformity are very important. A means for adjusting the color of a bicolor light source is therefore needed.
To this end,
The chromaticity diagrams shown in
In
Now consider a change in the temperature of the red light source, which causes a shift in its tristimulus values. Point B2 is indicative of the tristimulus values of light produced by the red light source at Temperature_2. Due to this change in the tristimulus values of the red light source, color set point D1 is no longer achievable. The closest achievable color is now defined by point D2.
Backlights consisting of only a CCFL or white LEDs assume that the color set point of the backlight is not adjustable, and only the intensity of the backlight is adjustable. On the other hand, backlights comprised of RGB LEDs assume that the backlight's color set point can be precisely defined (i.e., because the intensities of three adjustable light sources (i.e., red, green and blue light sources) can be adjusted to achieve any color set point falling within a triangulated area between the colors of the three light sources). However, neither of these assumptions is applicable to a bicolor light source. This is because, under any given set of conditions under which the first and second light sources operate, each of the light sources produces only a single set of tristimulus values. Thus, the mixed light produced by the two light sources can only be adjusted along a line of colors connecting the two sets of tristimulus values.
If past color setting methods are applied to a bicolor light source, they can result in a control system “hunting” for a color set point that cannot be obtained. To someone viewing a display that is backlit via a bicolor light source, this “hunting” can appear as visible oscillations in the display's color. The method 100 prevents (or at least mitigates) this hunting by adjusting a luminance ratio between first and second light sources only when feedback tristimulus values of a mixed light are “out of range” with respect to reference tristimulus values.
In one embodiment of the method 100, the range D is established as those values falling within an ellipse about a defined set of reference tristimulus values. The set of reference tristimulus values may be predefined, or may be obtained from user input.
The luminance ratio (i.e., the ratio of the intensity of the first light source in comparison to the intensity of the second light source) may be adjusted in a variety of ways. In one embodiment of the method 100, the luminance ratio is adjusted by adjusting drive signals supplied to the first and second light sources. That is, the intensity of either or both of the light sources may be adjusted. In another embodiment of the method 100, the luminance ratio is adjusted by adjusting a drive signal of only one of the light sources.
The method 100's actions of determining and, if necessary, adjusting may be undertaken continuously or, preferably, at predetermined time intervals.
In one embodiment, the control system 410 uses fuzzy feedback to determine whether the feedback tristimulus values are within the range of reference tristimulus values.
The apparatus 400 may further comprise a computing system 412 to display a graphical user interface (GUI) on the LCD 402. The GUI may prompt a user to define parameters of images that are generated by the LCD (e.g., color temperature, color intensity, etc.). The computing system 412 then 1) derives a set of reference tristimulus values from the user-defined parameters, and 2) provides the set of reference tristimulus values to the control system 410.
In another embodiment, the apparatus 400 may further comprise a manually-adjustable user control 414 that is coupled to supply the control system 410 with a state of the control. In response to the control's state, the control system 410 may then update its range of reference tristimulus values.
While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed, and that the appended claims are intended to be construed to include such variations, except as limited by the prior art.
