Patent Application
20060250323
The present invention is directed to techniques for providing a display apparatus having reduced energy consumption. Such reduced-energy display apparatuses would be particularly useful in battery-powered applications, such as display apparatuses in laptop computers, digital cameras, PDAs, or cell phones.
Some display apparatuses generate their own light. Examples include cathode ray tube monitors, plasma display panels, and field emission displays. Other display apparatuses employ spatial light modulators (SLMs) to modulate light emitted by an illumination unit. Known types of SLMs include LCD panels, liquid-crystal-on-silicon chips, and digital micromirror devices. The illumination units that are currently (2005) used with LCD panels typically emit white light at a constant intensity, and each display element (that is, liquid crystal cell) of the panel is provided with its own color filter. However, it is also known to use an illumination unit that bathes the back of an LCD-panel with flashes of red, green, and blue light. This triples the resolution of the display apparatus, because a single display element can be used to modulate all three primary colors (instead of three filtered display elements being needed in order to produce the entire spectrum), but the LCD panel must be capable of operation at a higher-speed. Display apparatuses of this type are frequently called field sequential displays.
Display apparatuses that employ digital micromirror devices may use an illumination unit that includes a color wheel in order to expose a single DMD to light of three different colors, or the illumination unit may include different light sources for each color. Display apparatuses are also known that use three DMDs or three LCOS chips, with each DMD or LCOS chip being exposed to one of the three primary colors and with the images produced by the three DMDs or LCOS chips being combined so as to yield a single colored image.
In all of these display apparatuses, the illumination unit emits light in some predetermined manner, regardless of how this light is modulated by the SLM. For example, many LCD display apparatuses that are currently used in digital cameras or laptop computers employed backlighting units that are driven at a constant intensity from frame to frame, even if the scene that is shown on the display apparatus is a dark one. In effect, more light is generated that is needed, and the LCD panel then attenuates the unnecessary light
The object of the present invention is to reduce the amount of power required by a display apparatus of the type that employs an illumination unit and a spatial light modulator. This object can be attained by adjusting video words that specify what is shown on the spatial light modulator so as to permit a temporary reduction in the amount of light produced by the illumination unit.
In accordance with one aspect of the invention, a set of video words is examined to determine whether substantially all of them have values less than a predetermined value. If so, the values of at least some of the video words in the set (those with values greater than zero, for example) are increased to form a set of converted video words. The converted video words are supplied to the spatial light modular for display. The light emitted by the illumination unit is adjusted to as to compensate for the increase in the values of the video words during the formation of the set of converted video words.
The examination of the video words may involve finding the maximum value of the video words in the set. Alternatively, the examination may involve comparing the video words in the set to a set of threshold values.
The conceptual basis of the present invention will first be explained, and then practical embodiments will be described.
The Conceptual Basis
The basic idea behind the present invention will now be explained with the aid of
Suppose that the image that is shown on billboard 10 is a dark one, and that the shade 16 of the window at the first row and first column of the array (marked 1211 in the drawings) is adjusted so as to permit the window 1211 to pass only 80% of the light falling on the back side of the window 1211. Also suppose that no more light is transmitted through any of the other windows 12 of the billboard 10 (that is, all other windows 12 transmit 80% of the light or less). Then the shade 16 of the window 1211 can be raised, as shown in
In particular, if a maximally bright display element in
The input unit 22 stores the video words received via bus 20 in accordance with control signals received from a control unit 26. A conversion unit 28 receives the stored video words, locates one or more video words in the frame having the largest value, and calculates a conversion factor for multiplying all of the video words. These converted video words are then supplied to an LCD driver unit 30, which addresses the display elements in the array 24 in a row by row manner and supplies the converted video words to the display elements to which they correspond.
A simple example will help illustrate this. Suppose that each of the video words has five bits, so the video words can have values ranging from 00000 (zero in decimal) to 11111 (31 in decimal). This provides 32 intensity levels. Suppose also that one or more of the video words for a given frame has a value of 11000 (24 in decimal), and that none of the video words for the frame has a higher value. Then the video words with the largest values specify a transmissivity for the corresponding display elements that is 24/32 (or ¾) of 100% transmission. The video words with the largest values can be multiplied by the reciprocal of ¾ (that is, 4/3) to bring the transmissivity of the display elements corresponding to the video words with the largest values up to 100%, and the remaining video words can be multiplied by the same conversion factor (4/3) to raise them proportionately.
A driver unit 32 for illumination unit 34 receives a brightness signal B, which establishes a desired or nominal level for the brightness of the images that are to be displayed. The signal B may be a preset value, or may be a value that is adjustable by the user of the display apparatus 18, or it may be a value that is both user-adjustable and dependent on the intensity of the ambient light. The conversion unit 28 supplies a signal to the driver unit 32 for modifying the magnitude of the signal B in accordance with the maximum value of the video words for the frame. In the example above, where the maximum value was 11000 (or ¾ of the potential maximum, 11111 plus the zero level), the signal B would be multiplied by ¾ to provide a converted signal for driving the illumination unit 34 at a reduced level.
The calculator 42 also calculates an intensity modification signal that is supplied by a line 46 to the driver unit 32.
The second embodiment differs from the first embodiment in that the second embodiment is directed to a field sequential liquid crystal display apparatus. The LCD array 24 in the second embodiment lacks the red, green, and blue filters that are shown in
During operation, the input unit 22 in the second embodiment transmits the red video words for an entire frame to the conversion unit 28, which then detects one or more red video words having the largest value. Based on this largest detected value, the conversion unit 28 calculates a conversion factor for multiplying all of the red video words before they are converted to analog and supplied to the LCD driver unit 30. The conversion unit 28 also calculates a conversion factor for multiplying a brightness signal BR (a brightness signal for the red component). The converted brightness signal is supplied to the illumination unit 34, and the illumination unit 34 emits an amount of red light designated by the converted brightness signal to the back of the LCD array 24. The green and blue components of the frame are displayed in the same way.
A significant advantage of the second embodiment, over the first embodiment, is that the maximum-value video words for each color component are detected individually. It may happen that an image to be displayed is primarily red and green and has very little blue in it. If the maximal blue video word (or words) for the frame has a relatively small value, the amount of blue light emitted by the illumination unit 34 can be reduced considerably. In contrast, in the first embodiment, the red, green, and blue video words were all considered together when the maximum value was detected.
The third embodiment is similar to the second embodiment in that it is directed to a field-sequential display. The difference is that the video words are not examined to determine their maximum value in the third embodiment; instead, they are compared to a set of predetermined threshold values. Using again video words with five bits as an example, the 32 possible intensity values afforded by five bits might be divided into eight ranges by comparing the video words to 7/8×32=28 (11100 in binary), 6/8×32=24 (11000 in binary), 5/8×32=20 (10100 in binary), 4/8×32=16 (10000 in binary), and so on. Using the binary numbers 11100, 11000, 10100, 11100, and so on as threshold values, it is first determined whether any of the video words for the relevant color component of a frame (red, for example) lies in the highest intensity range (that is, whether any of the video words for the color component has a value higher than 7/8, or 11100 in binary). If so, the raw video words are multiplied by one and the “converted” video words that are fed to the LCD driver unit 30 are the same as the raw video words. If none of the video words for the color component of the frame lies in the highest intensity range, it is then determined if any lie in the second highest (that is, if 7/8 or 11100 in binary is the minimum threshold level that is not exceed by any of the video words for the relevant color component). If so, the raw video words are multiplied by 8/7 to form the converted video words and the brightness signal supplied to the driver unit 32 is multiplied by 7/8. If none of the video words for the color component of the frame lies in the top two intensity ranges, it is then determined if any lie in the third-highest intensity range (that is, if 6/8 or 11000 in binary is not exceeded by any of the video words for the relevant color component). If so, the raw video words are multiplied by 4/3 to form the converted video words, and the brightness signal is multiplied by ¾. The lower intensity ranges are examined in the same way if none of the video words for the relevant color component of the frame lies in the three highest intensity ranges.
In short, instead of examining all of the video words for each color component of a frame in order to detect the maximum value of the video words for each color component (as in the second embodiment), in the third embodiment it is only necessary to detect the minimum threshold level that is not exceeded by at least one video word for each color component of a frame.
A digital signal identifying the largest threshold value that has not been exceeded by any of the video words for the relative color component is conveyed by a bus 58 to the look up table 54 and by a bus 46′ to the driver unit 32. The signals on busses 36 and 58 serve as address signals for the look up table 54, which stores the products of the possible values of the video words times conversion factors that are determined by the highest threshold value not exceeded. Converted video words from the look up table 54 are stored in a memory 60. When they are read out of the memory 60 by the control unit 26, they are converted to analog signals and supplied to the LCD driver unit 30.
It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
This application claims the benefit of priority under 35 USC 119 of U.S. provisional application No. 60/678,788, filed on May 9, 2005.
| Number | Date | Country | |
|---|---|---|---|
| 60678788 | May 2005 | US |
