1. Field of the Invention
The current invention relates to backlighting of liquid crystal displays (LCDs). More particularly, the invention relates to a backlight device for providing backlighting to a liquid crystal display panel and to a method of controlling brightness of a liquid crystal display panel.
2. Background Information
A liquid crystal display (LCD) panel is not a spontaneous light emitting device. A voltage applied to the LCD panel changes the light transmittance of liquid crystal elements (pixels) in the panel. The LCD panel can be light reflective so that an image produced on the panel is seen by ambient light reflection. However, this does not work for large size or high contrast LCD panels.
For use in applications such as televisions, computer monitors and head-held electronic devices LCD panels are illuminated from behind by a backlight. In most applications the backlight has an even and constant light output with changes in the brightness of the displayed image being controlled by changing the light transmittance of the liquid crystal elements within the display panel. In order to produce good view ability in high ambient light conditions the backlight must have a high brightness level. There are a number of disadvantages in this including high power consumption, excess heat generation. Another disadvantage of a constant backlight is that it leads to limited dynamic contrast of an LCD display because of light leakage through the LCD panel from the backlight when the pixels are in a dark or off state. This light leakage causes the dark areas to have a gray appearance instead of a solid black appearance.
One technique intended to improve the dynamic range of an LCD display is to dynamically adjust the overall backlight brightness in accordance with brightness of the video image. If the image is relatively bright, the backlight control operates the light source at high intensity. If the image is darker, the backlight output is dimmed to reduce leakage and help darken the image. One benefit to this backlight technique is to reduce the backlight power consumption. Although this technique can improve the LCD contrast range and slightly save the backlight power, it can create image S distortion and induce image brightness fluctuations.
More recently, attempts have been made to dynamically vary different areas of the backlight at different light intensities depending on the brightness of different parts of the displayed image. Such a method is described in US patent application publications 2005/0231978 and 2006/0007103.
Both of the above US patent publications described a backlighting system in which the backlight comprises an array of light emitting diodes (LEDs) arranged behind a LCD panel divided into two or more division areas. A controller of the backlight system determines the peak brightness of the displayed image within each division area and individually controls the light intensity of the LEDs behind that division area in accordance with the required brightness. Thus, when one part of a displayed image has dark colors or low brightness levels the backlight LEDs behind that portion of the image have a low light output whereas in another part of the displayed image with light colors or high brightness the corresponding LEDs of the backlight have a highlight light output.
The above described methods, and other methods practiced hitherto, still suffer from several drawbacks including undesirable image color distortion and brightness distortion.