Patent Application
20080238336
This invention relates to back-light devices for an image display, and more particularly, to back-light devices for a liquid crystal display and a liquid crystal display comprising back-light devices. More specifically, although not solely limited thereto, this invention relates to a back-light device comprising a plurality of distributed light-emitting diodes (LEDs) and a liquid display comprising same.
Liquid crystal displays (LCD) have been increasingly used in display applications, for example, in televisions and computer monitors. The operation of an LCD video display panel typically requires a back-light device to illuminate an LCD panel from the backside of the LCD panel to facilitate image display, since liquid crystal itself does not generate light, but only passes or impedes the passage of light.
With the ever increasing demand on displays having a high image resolution, the number of pixels, or the pixel density which is the number of pixel per unit area, of a video display is also ever increasing. Typically, the pixels of an image display are arranged as a matrix of pixels which is organised into a plurality of rows and columns of pixels, such as LCD pixels. A video image is formed on a display by sequential line scanning of an image signal. Typically, a picture frame of a video image is formed by projecting an image signal sequentially from the left side to the right side of a display panel, and from the top to the bottom of a display panel, as is known by persons skilled in the art. The formation of an image on an LCD display by line scanning will strike a balance between providing a good quality display image and a reasonable power consumption. In general, video frames are currently formed at a rate of 60 frames-per-second or above, since it is known that a picture refreshing frequency at or above this rate is acceptable to the human eyes. However, a higher resolution means the number of rows of pixels will be ever increasing, and the fraction of time allocated for activation of each row of backlight to an LCD or like displays will be ever decreasing, since the sequential scanning of all the pixel rows will have to be completed within the time allocated for each video frame, that is, typically within 1/60 second or less for most applications. It will be appreciated that this imposes a severe limitation to further enhancing image resolution since the brightness of an backlight device will have to be extremely high in order to produce an appropriate luminance level acceptable for backlighting.
Furthermore, it is also known, for example in US 2005/0231978, that providing selective back-light to a display in accordance with the brightness of an image being displayed will enhance both image contrast and power consumption. For example, providing equal back lighting to a dark image section and a bright image section will make a dark image on a back-illuminated display portion less dark, a bright image less outstanding. Therefore, an equal level of back-light often could mean a waste of power coupled with performance degradation. With the demand for displays of ever increasing resolution, pixel density of an LCD display will keep on increasing and the issues of heat dissipation and image contrast will require particular attention.
Therefore, it is desirable if there can be provided an improved back-light device, and a display incorporating such a back-light device.
According to the present invention, there is provided a back-light arrangement for providing back-light to a display panel such as a liquid crystal video display panel, comprising a plurality of light emitting devices arranged and distributed for providing back-light to said display panel; electronic circuitry arranged for driving said plurality of light emitting devices to produce said back-light, wherein said electronic circuitry comprises a plurality of drivers each one of which is arranged to individually drive a corresponding one of said plurality of light emitting devices to emit light upon receipt of an actuating signal; and a controller arranged to multiplex an intensity signal to each one of said plurality of drivers for individually driving each one of said plurality of light emitting devices.
A backlight arrangement comprising a matrix of active drivers makes it possible to configure a driver so that its duration of operation is not entirely dependent on the actual actuation time received by the driver. In addition, the use of a multiplexing scheme, for example, a time division multiplexing scheme, to multiplex an intensity signal to an associated driver will help to mitigate adverse influence to the operation of backlight emitting devices of other, especially adjacent, pixel rows, while benefiting from power saving since the backlight emitting devices are operated as and when necessary.
In an exemplary embodiment, each said driver comprises a current source which is actuatable by a solid state switch, and said solid state switch is actuatable upon receipt of a said actuation signal from said controller. With a solid state switch arranged for the actuation of a current source, the current source can be isolated from the controller or other drivers upon deactivation of the solid state switch, so that inter-driver interference will be mitigated. Furthermore, isolation of the current source from other drivers means that the duration of operation of a driver could be extended independent of the operation of other drivers.
As an example, a capacitive member is coupled to the current source, and the capacitive member is arranged so that the duration of operation of the current source is extendable beyond the duration of the actuation signal. For example, a capacitor may be coupled to the input terminal of the driver or the current source so that the actuation of the driver or current source can be extended.
The current source may be isolatable from the actuation signal upon deactivation of the solid state switch. To facilitate an effective electrical isolation, for example between the controller and the driver when necessary, the solid state switch has a very high impedance upon de-activation.
For example, the current source may comprise a FET which is arranged for supply of a current to said light emit device, and said solid state switch comprises a FET actuatable by said actuation signal, and the capacitive member may be connected to the gate terminal of said FET of said current source.
According to another aspect of the present invention, there is provided a back-light arrangement for providing back-light to a display panel such as a liquid crystal video display panel, comprising a plurality of light emitting devices arranged into an array of a matrix of M columns and N rows, M and N being integers; and electronic circuitry comprising N row rails and M column rails, wherein said light emitting devices belonging to a row of said matrix is associated with a said row rail and said light emitting devices belonging to a column of said matrix is associated with a column rail, said row rails and said column rails being arranged for transmitting actuation signals to said light emitting devices; wherein said electronic circuitry further may comprise a plurality of solid state switches, each said light emitting device may be associated with a first solid state switch and a second solid state switch; wherein said first solid state switch may connect said light emitting device to a power source and is actuatable upon receipt of an actuation signal from said second solid state switch, said second solid states switches may be actuatable only when actuation signals are present on both a said row rail and a said column associated with said second solid state switch.
The use of a pair of solid state switches to cooperatively and selectively operate a light emitting source, such as an LED or an ensemble of LEDs, makes line scanning possible even with an increasing number of pixel rows.
According to another aspect of the present invention, there is provided a back-light arrangement for providing back-light to a display panel such as a liquid crystal video display panel, comprising a plurality of light emitting devices arranged and distributed for providing back-light to said display panel; electronic circuitry arranged for driving said plurality of light emitting devices to produce said back-light, wherein said electronic circuitry comprises a plurality of drivers each one of which is arranged to individually drive a corresponding one of said plurality of light emitting devices to emit light upon receipt of an actuating signal, and wherein the brightness of a said light emitting device is controllable by varying the amplitude of an intensity signal; and a controller arranged to transmit said intensity signal to each one of said plurality of drivers for individually driving each one of said plurality of light emitting devices, wherein the amplitude of each said intensity signal is individually adjustable in response to changes in image intensity distribution on said display panel.
By providing a back-light arrangement in which the brightness of each of the light emitting source is individually controllable, an appropriate level of back-light can be provided to a particular pixel or a pixel region, thereby enhancing image contrast and at the same time mitigating power wastage. This arrangement is also beneficial for a back-light arrangement in which a uniform brightness is required, since the amount of current required to produce a certain brightness is variable among light emitting sources, such as LEDs, due to manufacturing tolerance. By permitting individual brightness control of the light emitting sources, a uniform brightness can be produced by variation of LED drive current.
To provide the intensity signals to the controller, the arrangement may comprise an image intensity analysing device for analysing image intensity distribution information of an image to be displayed on said display panel, wherein said controller adjusts the amplitude of said intensity signals in response to intensity distribution information provided by said image intensity analysing device.
As an example, said light emitting device may be allocated for back-illumination of a pre-determined portion of said display, and the amplitude of a said intensity signal associated with a said light emitting device may increase with the brightness of an image to be displayed on that pre-determined portion of said display panel.
In an embodiment, the intensity signals are arranged into a plurality of intensity signal groups and each signal group comprises a plurality of intensity signals of non-identical amplitudes arranged in time sequence, and wherein the plurality of intensity signals in an intensity signal group are sequentially multiplexed to said plurality of drivers.
Time division multiplexing of the intensity signals (or intensity data) means that the intensity signals are delivered to the drivers only when necessary and this means power saving as well as enhanced contrast due to mitigation of light contamination by unnecessary back-light.
The brightness of each one of said plurality of light emitting devices may be gradually variable by adjusting the amplitude of a said intensity signal associated with said light emitting device. A gradually variable intensity signal means the brightness of a back-light source can closely follow the change of a corresponding image pixel or image pixel portion.
The drivers are actuated in synchronisation with the line scanning signal of a video image to be projected on said display panel.
Each said intensity signal may comprise a pulse of an amplitude determined by said controller.
The intensity signals may be time-multiplexed to each one of said plurality of drivers.
The light emitting devices may be arranged into a matrix of M rows and N columns, M and N being integers; wherein said intensity signals are arranged into M groups each comprising N intensity signals in sequence; and wherein each said intensity signal is a pulse having a duration T of 1/Nf, where f is the number of video frames per second.
Each light emitting device may be configured for back-illumination of a pre-determined portion of said display panel, the brightness level of each said light emitting device being individually controllable by said electronic circuitry responsive to the brightness level of the portion of said display panel being under back-illumination by said light emitting device.
The plurality of light illuminating devices may be configured for back-illumination a liquid crystal display.
Each said light emitting device may comprise at least one light emitting diode.
Each said light emitting device may be within a discrete chip package.
Each light emitting device may comprise a plurality of light emitting diodes within a discrete chip package.
The plurality of light emitting devices may be arranged in a matrix of light emitting diodes.
In an embodiment, the separation between adjacent light emitting devices is uniform.
The driver may be a current source.
A capacitive component may be coupled to said current source for extending duration of illumination of an associated one of said light emitting devices.
The output current of said current source may be controllable by varying the amplitude of said intensity signal.
The current source may comprise a FET in series connection with an associated one of said light emitting source, the amount of current to flow through said FET may be controllable by varying the amplitude of said intensity signal.
In another aspect of the present invention, there is provided a liquid crystal display comprising a back-light arrangement of any of the aforesaid features.
Preferred embodiments of the present invention will be explained in further detail below by way of examples and with reference to the accompanying drawings, in which:—
Referring to
Since liquid crystal does not in itself emit light, back-light is provided to make an image on the LCD display visible to a viewer. A back-light for an LCD panel is usually a white light source. In the case of a colour LCD display, a white back-light will be filtered by a colour filter of a sub-pixel to form a sub-pixel of that colour. The output of the sub-pixels will then produce a coloured pixel. The back-light device is located behind the LCD panel and comprises a plurality of light emitting sources (which are light emitting diodes (LED) in this embodiment) arranged into an array of matrix of light sources. To provide a compact and thin display panel, the LEDs are in a discrete chip package. As can be seen in
The matrix of light emitting sources 120 is organised into an array of matrix with M columns and N rows so that each light emitting source is responsible for illuminating one of M×N LCD pixels or one of M×N pixel sections on the LCD display. To provide necessary operating conditions or currents for the LEDs, electronic circuitry comprising a plurality of drivers 140 for individually driving each one of the LEDs is provided. The electronic circuitry further comprises a row rail 142 and a column rail 144 which collectively forms a back bone signal switching rail of the driver matrix.
As shown in
Each driver of
To mitigate the amount of wasted power and to enhance image contrast, each one of the LEDs of this invention is individually driven with an individual intensity data or signal. In addition, the drivers and current sources of this invention are arranged so that the brightness of each individual LED is determined by the amplitude of the intensity signal received by that LED. This is exemplified by the driver and current source arrangement shown
In order to provide an individual intensity signal for each individual LED, it will be appreciated that a total of M×N intensity signals or data will be required for each video frame. Although it is possible to transmit all the M×N intensity data to the drivers at the same time, it will be more efficient to time multiplex the intensity data to the drivers since it is known that a video frame is usually formed by sequential line scanning. More particularly, it is known that a video image frame is formed by sequential line scanning, and a complete video frame is formed in a period of time, namely, T, where T=1/f, and f is the frame refreshing frequency, that is, number of video frame per second, as is known to persons skilled in the art. By actuating a driver only when a corresponding image has just been formed or is just to be formed on the LCD display will be more power efficient, as well as minimising possible contamination due to back lighting to an adjacent, non-image forming pixel.
To facilitate multiplexing of the M×N intensity signals onto the individual drivers for back illumination of the M×N LCD pixels or LCD pixel regions, and assuming that the display can be considered as dividing into N rows or N horizontal or axial regions of images, the intensity signals are divided into M groups each comprising N intensity data pulses arranged in a sequence. Since a complete video image frame is formed within the period (T=1/f), all the M×N intensity data must be transmitted to the drivers within the period (T). Furthermore, since a video image frame can be considered as formed by sequential line scanning from row 1 to row N, it will be appreciated that the N rows of LEDs could be sequentially driven without loss of generality.
Referring to
When a new video frame starts, an image will be formed by line scanning starting from the top LCD pixel row 1 and finishing at the bottom LCD pixel row N. Since there are a total of N rows of pixels or N rows of pixel sections in a video image frame, the scanning of each row to form a complete image line or row will have to complete in a time of 1/Nf, and the scanning of a second image row will begin at a time of 2/Nf, and etc. By arranging the N data pulses so that the pulse width of each of the data pulses is 1/Nf, or so that the pulse centre spacing between adjacent data pulses is at t=1/Nf, the row intensity data can be multiplexed onto the individual drivers sequentially and in synchronisation with the line scanning of an image frame. To facilitate multiplexing of the row intensity data onto the correct or the corresponding driver row, actuation or enabling data as shown adjacent the “Row Driver” of
For example, and as shown in
To extend the lighting persistence of an LED, a capacitive device 170 is connected to the actuation gate of the current source. The capacitive device is arranged so that the voltage at the actuation gate of the FET of the current source is maintained at a desired intensity level for an extended period of time T sufficient to alleviate premature vanishing of an image or excessive flashing for enhanced viewing, as better understood with reference more particularly to
The back-light arrangement of this invention also facilitates the device of a more compact LCD display since only a small number of components is required. For example, in the first exemplary arrangement of
To facilitate individual controlling of the LEDs, and to generate individual intensity data, a video signal analysing device in the form of a data processing unit 220 is provided and as shown in
In an alternative configuration, the back-light arrangement is identical to that of the above mentioned arrangement, except that the FET 152 of the current source is configured as a solid state switch. In such a configuration and as shown in
While the present invention has been explained by reference to the examples or preferred embodiments described above, it will be appreciated that those are examples to assist understanding of the present invention and are not meant to be restrictive. Variations or modifications which are obvious or trivial to persons skilled in the art, as well as improvements made thereon, should be considered as equivalents of this invention.
For example, although the present invention has been explained with reference to an LCD display, the back-light device of this invention would be equally useful for other non-light emitting display members available from time to time.
Furthermore, while the present invention has been explained by reference to using discrete LED chip components as an example of light emitting sources for back lighting, it should be appreciated that the invention can apply, whether with or without modification, to other discrete light emitting devices without loss of generality. Also, although this invention has been illustrated with reference to a LCD display, it should be appreciated that the invention is not limited to LCD displays without loss of generality.
As a further note, although the above embodiments have been explained with reference to a light emitting source each comprising a single light emitting source, such as a white LED, it will be appreciated that each driver may be configured to drive a plurality of light emitting sources having the same colour without loss of generality.
