Patent Application
20060049781
Liquid crystal display (LCD) backlighting using light emitting diodes (LEDs) poses a few advantages over LCD backlighting using a cold cathode fluorescent lamp (CCFL). Namely, an LED-based backlight can provide a wider color gamut, a selectable white point, a longer operating life, and a mercury-free lighting means. However, LED backlights also present a few difficulties. For example, the optical characteristics of LEDs vary with temperature, drive current and aging. LED optical characteristics can also vary from batch to batch within the same fabrication process.
Typically, LED backlighting is accomplished via a side-firing group or array of red, green and blue (RGB) LEDs that, together, produce a substantially white light. To maintain the uniformity of the white light, the RGB LEDs are usually mounted on a common substrate along with a light sensor. Brightness information (and sometimes chrominance information) obtained from the light sensor is then used to adjust the drive signals of the RGB LEDs, thereby controlling the intensity and chrominance of the backlight and maintaining the uniformity of the white light.
In one embodiment, apparatus comprises a direct-firing backlight for a display, a plurality of light sensors, and a control system. The direct-firing backlight has a plurality of light emitting regions, and each of the light sensors is positioned to sense light produced by a corresponding one of the light emitting regions. The control system is operatively associated with the light sensors and light emitting regions, to receive information from the light sensors and, in response thereto, regulate light emitted from regions of a display. The regions of the display correspond to the light emitting regions of the direct-firing backlight.
In another embodiment, a direct-firing backlight for a display is divided into a plurality of independently controllable light emitting regions. The light emitted from each of the light emitting regions is measured and compared to at least one reference value. In response to the comparisons, the light emitted from each light emitting region is independently regulated.
Other embodiments are also disclosed.
Illustrative and presently preferred embodiments of the invention are illustrated in the drawings, in which:
For LCD displays of larger size, conventional side-firing LEDs have difficulty producing a well-dispersed backlight of uniform intensity and chrominance. One solution is to simply move the substrate holding the side-firing LEDs to a position that is behind the LCD display, thereby turning a side-firing LED backlight into a direct-firing LED backlight. However, the design of a light guide that sufficiently disperses the light produced by a direct-firing LED backlight is difficult; and, if the light guide is not adequate, the uniformity (i.e., intensity and chrominance) of the backlight suffers. Furthermore, the light sensed by a single light sensor mounted in close proximity to a group of LEDs forming a direct-firing backlight may not be a good indicator of the uniformity of the backlight as a whole. Although a light guide could be designed to not only disperse the light produced by the direct-firing LED backlight, but to also channel the dispersed light back to a light sensor, such a light guide is difficult to design and fabricate.
As shown in
In one embodiment, all of the light emitting regions 102-118 are formed on a common substrate. In another embodiment, each light emitting region 102-118 is formed on its own substrate. It should also be noted that, together, the light sources (e.g., LEDs 120-136) of the different light emitting regions 102-118 may form a substantially uniform matrix of light sources; or, the light sources 120-136 of each light emitting region 102-118 may be grouped in closer proximity to each other.
Interspersed with the light emitting regions 102-118 of the backlight 100 is a plurality of light sensors 138-154, each sensor of which is positioned to sense light produced by at least one of the light emitting regions 102-118. In one embodiment, the number of light sensors 138-154 equals the number of light emitting regions 102-118, and the light sensors 138-154 are spaced substantially equidistant from one another. Each light sensor 138-154 may be placed within its corresponding light emitting region 102-118, as shown in
The light sensors 138-154 may take various forms. In one embodiment, the light sensors 138-154 measure light brightness. In another embodiment, the light sensors 138-154 measure both light brightness and light chrominance. In the former case, each light sensor 138-154 need only comprise a single photodiode. In the latter case, each light sensor 138-154 could take the form of a plurality of photodiodes, each of which is associated with a filter that enables the brightness of only a predetermined wavelength (or wavelengths) of light to be measured. For example, three different photodiodes could be used to take readings related to Commission Internationale de l'Éclairage (CIE) tristimulus values. Alternately, the same readings could be taken serially, using a single photodiode associated with an adjustable light filter.
A control system 156 is operatively associated with both the light sensors 138-154 and light emitting regions 102-118. The control system 156 receives information from the light sensors 138-154 (e.g., brightness and/or chrominance information) and, in response thereto, regulates light emitted from regions of a display 200 (see
The control system 156 may regulate the light emitted from regions of a display 200 in a number of ways. In one embodiment, the control system 156 regulates the light by comparing information received from the light sensors 138-154 to at least one reference value and, in response thereto, regulating the light intensity of each light emitting region 102-118. Alternately, the control system 156 may regulate both the intensity and chromaticity of each light emitting region 102-118. In this second embodiment, the light sources of each light emitting region 102-118 may comprise different colored LEDs (e.g., RGB LEDs 120-136); the at least one reference value may comprise Commission Internationale de l'Éclairage (CIE) tristimulus values; and the control system 156 may regulate the LEDs 120-136 by regulating their drive signals.
In a third embodiment, the control system 156 regulates light emitted from regions of a display by comparing information received from the light sensors 138-154 to at least one reference value and, in response thereto, outputting required video signal adjustments for the display 200. In this embodiment, the light emitted by the backlight 100 remains uniform, and adjustments in the colors defined by a video signal are used to offset chrominance disparities between the regions 102-118 of the backlight 100. Video signal adjustments may also be used to compensate for slight disparities in the brightness of light emitted by different backlight regions 102-118. However, if the display 200 is an LCD, compensation for brightness variations will likely be limited.
The control system 156 preferably initiates its light comparisons and regulation on a periodic basis, during normal operation of the backlight 100. However, the control system 156 could also regulate the backlight 100 during a configuration mode, on power-up, or at other times.
The control system 156 may be a central control system (as shown), or may alternately comprise a plurality of distributed controllers (e.g., one for each light emitting region 102-118 of the backlight 100). The control System 156, or each controller thereof, may comprise a processing unit 158 and a memory 160. The memory 160 may store the one or more reference values, which may take the form of fixed values (e.g., values burned in a read-only memory (ROM)) or programmable values (e.g., user-configured values loaded in a random-access memory).
By way of example,
