Abstract:
A display apparatus includes a spatial light modulator and an illumination unit for supplying light to the spatial light modulator. The power consumed by the illumination unit is reduced by adjusting both the intensity of light emitted by the illumination unit and video words that are supplied to the spatial light modulator in accordance with what is to be displayed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     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. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     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.  
         [0003]     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.  
         [0004]     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.  
         [0005]     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  
       SUMMARY OF THE INVENTION  
       [0006]     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.  
         [0007]     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.  
         [0008]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1A  schematically illustrates a back-lighted advertising billboard having an array of windows and adjustable shades in front of the windows, and is presented in conjunction with  FIG. 1B  to help explain the conceptual basis of the present invention;  
         [0010]      FIG. 2B  schematically illustrates the advertising billboard that is shown in  FIG. 1B , with the shades being adjusted to increase the transmission of light through the windows and with the intensity of the backlighting being reduced&#39; 
         [0011]      FIG. 2A  illustrates a first embodiment of the present invention, in which a display apparatus employs an illumination unit that emits white light and an LCD panel with a colored filter for each liquid crystal cell;  
         [0012]      FIG. 2B  illustrates a conversion unit used in the display apparatus of  FIG. 2A .  
         [0013]      FIG. 3  is a cross-sectional view of a portion of an illumination unit used in a second embodiment; and  
         [0014]      FIG. 4  is a schematic diagram of a conversion unit used in a third embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The conceptual basis of the present invention will first be explained, and then practical embodiments will be described.  
         [0016]     The Conceptual Basis  
         [0017]     The basic idea behind the present invention will now be explained with the aid of  FIGS. 1A and 1B  In  FIG. 1A , an advertising billboard  10  has an array of windows in it. A backlighting unit  14  bathes the back side of the billboard  10  with white light. An adjustable shade  16  is provided for each of the windows. The shades  16  are opaque, and attenuate the amount of light streaming through each window  16  by an amount corresponding to the portions of the windows  16  that they cover. A workman can change a black-gray-white image shown on the billboard  10  by manually adjusting the shades  16  to display a new black-gray-white image.  
         [0018]     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  12   11  in the drawings) is adjusted so as to permit the window  12   11  to pass only 80% of the light falling on the back side of the window  12   11 . 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  12   11  can be raised, as shown in  FIG. 1B , to permit the window  12   11  to pass 100% of the light. This is accompanied by a downward adjustment in the brightness of the light emitted by the backlighting unit  14 , so that the same amount of light streams through the window  12   11  in  FIG. 1B  as streams through in  FIG. 1A , and by an upward adjustment of the shades  16  for the other windows  12 .  
         [0019]     In particular, if a maximally bright display element in  FIG. 1A  (that is, window  12   11 ) transmits 80% of light, the same effect can be obtained (in  FIG. 1B ) by multiplying this 80% transmission factor by its receptacle (that is, 1/0.8, or 1.25), by multiplying the transmission factors for the other windows  12  by the same amount (that is, by 1.25), and by reducing the intensity of the light emitted by backlighting unit  14  so that its new intensity is 80% of its former value.  
       The First Embodiment  
       [0020]      FIG. 2A  illustrates a display unit  18  in accordance with a first embodiment of the present invention. A bus  20  supplies digital signals for the red, green, and blue components of a frame to an input unit  22 . The signals for the red, green, and blue components consist of multi-bit video data words (hereafter usually referred to as “video words”), each specifying one of a plurality of binary levels for the red, green, or blue brightness of spots that are to be displayed by an LCD array  24 . The LCD array  24  has display elements (individual liquid crystal cells) that received light colored by a red, green, or blue filter (only three of which are shown in  FIG. 2A ). The video words for the red, green, and blue components are coordinated with the display elements in such a manner that each of the video words for the red component designates a transmission factor for a corresponding one of the display elements with red filters, each of the video words for the green component designates a transmission factor for corresponding one of the display elements with green filters, and the same for the video words for the blue component. For the sake of convenience, the video words for the red, green, and blue components of a frame will hereafter be called red video words, green video words, and blue video words.  
         [0021]     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.  
         [0022]     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.  
         [0023]     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.  
         [0024]      FIG. 2B  shows the construction of the conversion unit  28 . A bus  36  carries the raw video words from the input unit  22  to a multiplier  38  and to a maximum value detector  40 . The maximum value detector  40 , as its name suggests, detects the maximum value of the video words for a frame. The maximum detected value is supplied to a conversion factor calculator  42 , which calculates a conversion factor for multiplying all of the raw video words and supplies this conversion factor to the multiplier  38 . After multiplication, the converted video words are stored in a memory  44 . Thereafter, the converted video words are read out by the control unit  26 , transformed to analog signals, and fed to the LCD driver unit  30 .  
         [0025]     The calculator  42  also calculates an intensity modification signal that is supplied by a line  46  to the driver unit  32 .  
       The Second Embodiment  
       [0026]     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  FIG. 2A . Furthermore, the illumination unit  34  in the second embodiment includes red, green, and blue light sources. The light sources may be a set of red LEDs, a set of green LEDs, and a set of blue LEDs. For example,  FIG. 3  is a side view of a portion of an illumination unit  34 ′ used in this embodiment. It includes LEDs  48 R,  48 G, and  48 B mounted on a support  50  and disposed behind an optical diffusion plate  52 .  
         [0027]     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 B R  (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.  
         [0028]     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  
       [0029]     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.  
         [0030]     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.  
         [0031]      FIG. 4  illustrates a conversion unit  28 ′ for use in the third embodiment. The bus  36  carries the raw video words for a given color component of a frame (red, for example) to a look-up table memory  54  and to a level detector  56 . The level detector  56  compares each video word for the color component to threshold values that divide the possible values for the video words into a hierarchy of ranges. This reduces the performance that is required of the conversion unit. It should be noted that this scheme would permit a table look-up memory with only eight address bits to be used to generate converted video words from five-bit video words that are compared to eight threshold values, instead of a hardware multiplier or multiplication routine. Zero may be used as one of the threshold values because if none of the video words of a given color component of a frame has a magnitude greater than zero, then all of these video words have a value of zero and it is not necessary to actuate the LEDs  48  for that color component at all.  
         [0032]     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 .  
         [0033]     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.