Abstract:
The disclosed embodiments relate to a method and apparatus to align bit weights in a display device. There is provided a method for calibrating light output in a display device, the method comprising: displaying a first video pattern, the first video pattern comprising a first set of pixels divided into a first subset of pixels and a second subset of pixels, the first subset of pixels having a first intensity level, the second subset of pixels having an intensity level corresponding to a fully off state; measuring a first light output value associated with the first video pattern; displaying a second video pattern, the second video pattern comprising a second set of pixels, each of the second set of pixels having a second intensity level corresponding to a fraction of the first intensity level, the fractional value of second intensity level being determined so that a second light output value associated with the second video pattern is intended to equal the first light output; measuring the second light output value; and adjusting the fractional value of the LSB to converge the second light output value with the first light output value.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to digital imaging systems. More specifically, the present invention relates to a system and method for aligning bit weights in digital imaging systems that implement pixel shifting technology. 
     BACKGROUND OF THE INVENTION 
     This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     A common problem inherent with digital imaging systems is that they are limited in the number of bits that can be displayed. In other words, their bit depth is finite. This limitation in the number of bits is a resolution limitation causing contouring in the displayed images. Essentially, the number of colors that may be displayed, as well as the range of light intensity, is limited, precluding the displaying of smoother images. In order to increase the resolution of the image display systems the bit depth needs to be increased. 
     In digital micromirror devices (“DMD”) using pixel shift technology, one parameter limiting the bit depth is the value of the least significant bit (“LSB”). The LSB represents the minimum amount of time that a pixel can be switched on for a given frame of video. One technique to achieve better bit depth or to increase the number of bits that can be displayed is to create fractional bits. Parameters such as light intensity may be controlled over time intervals shorter than the time represented by the LSB by, for example, attenuating the light source during the interval an LSB is displayed. However, once these fractional bits are achieved they must be scaled to the LSB in order to obtain a proper video to light transfer curve. Without proper scaling or calibration of the fractional bits, contouring within the image displayed may persist even with the increase in bit depth. Therefore, a system and method for calibrating or properly scaling these fractional bits to the natural LSB is needed. 
     SUMMARY OF THE INVENTION 
     Certain aspects commensurate in scope with the disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below. 
     The disclosed embodiments relate to a system and method for bit weighting alignment in a display device. A method for calibrating light comprises: displaying a first video pattern, the first video pattern comprising a first set of pixels divided into a first subset of pixels and a second subset of pixels, the first subset of pixels having a first intensity level, the second subset of pixels having an intensity level corresponding to a fully off state; measuring a first light output value associated with the first video pattern; displaying a second video pattern, the second video pattern comprising a second set of pixels, each of the second set of pixels having a second intensity level corresponding to a fraction of the first intensity level, the fractional value of second intensity level being determined so that a second light output value associated with the second video pattern is intended to equal the first light output; measuring the second light output value; and adjusting the fractional value of the LSB to converge the second light output value with the first light output value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the invention may become apparent upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a block diagram of an exemplary digital imaging system in accordance with embodiments of the present invention; 
         FIG. 2  is an exemplary representation of light output in accordance with embodiments of the present invention; 
         FIG. 3  is an exemplary pixel shift video pattern in accordance with embodiments of the present invention; 
         FIG. 4  is an exemplary pixel shift video pattern in accordance with embodiments of the present invention; and 
         FIG. 5  is a flow chart illustrating an exemplary technique for bit weighting in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Referring to  FIG. 1 , a block diagram of an exemplary digital imaging system in accordance with one embodiment of the present invention is shown and generally designated by the reference numeral  10 . The digital imaging system  10  comprises a light engine  12  that shines or directs light to an imaging system  14 . In one embodiment the light engine  12  may include a metal halide lamp, such as an ultra high performance (“UHP”) lamp, configured to shine white light. 
     The imaging system  14  may be a digital micromirror device (“DMD”) or a liquid crystal device (“LCD”). In the case of a DMD device, the imaging system may comprise up to one-half million micromirrors or more. The micromirrors are mounted on microscopic hinges that are electrically actuated to tilt the micromirrors between an “on” position and an “off” position. Each micromirror represents a pixel displayed on a screen  20 . The minimum time a pixel can be switched on for a given frame of video is commonly referred to as an LSB. 
     A projection lens assembly  16  controlled by a modulator  18  receives light that passes through or is reflected from the imaging system  14  and directs it to the screen  20 . The modulator  18  and the projection lens assembly  16  enable the image display unit to implement pixel shift technology. In pixel shifting, a single pixel may be displayed at multiple positions on the screen  20  by adapting the modulator  18  to slightly tilt a projecting lens along an axis. The movement of the pixels is imperceptible to a human eye due to visual persistence. Thus, a single pixel appears to be multiple pixels and the pixel shifting effectively multiplies the number of pixels available to the digital imaging system  10 . 
     To perform pixel shifting, a video control system  24  may coordinate the movement of the lens through the modulator  18 . With a single modulator  18  at least two positions may be achieved for a single pixel. For example, the lens may direct pixels to a certain location on the screen  20 . The modulator  18  then manipulates the lens to direct the pixels to a secondary position on the screen  20  and subsequently back to the original position. The procedure is repeated so rapidly that a human eye is unable to detect the movement between the two positions and, as explained previously, the pixel shift technology effectively increases the resolution of image displayed on the screen. 
     The light passing through the projection lens assembly  16  is directed onto the screen  20  as an image seen by viewers. The image display unit  10  may be designed to provide an overscan, wherein light is projected to an area greater than that visible to users on the screen  20 . For example, light may be projected to areas behind a bezel (not shown) of the screen. A photodiode assembly  22  may be situated to receive this light without influencing the image displayed on the screen  20 . 
     The photodiode assembly  22  is configured to detect the amount of light output from the light source  12  in a given video frame. Upon receiving light from the projection lens assembly  16 , the photodiode assembly  22  produces a voltage corresponding to the light output. The voltage is converted to a corresponding digital signal and directed to the video control system  24 . 
     The video control system  24  controls images that are displayed on the screen  20 . Among other things, as described above and as shown, the video control system controls the light source  12 , the modulator and the imaging system  14  to produce the images on the screen  24 . In controlling the light source  12 , the video control system  24  controls the light output intensity. Specifically, it may decrease the intensity of the light output to achieve a fractional LSB value. Additionally, it may fine tune the light output of the light source  12  in order to properly scale a fractional LSB to a natural LSB value. 
     Returning to the photodiode assembly  22 , it communicates with the video control system  24 . Specifically, the photodiode assembly  22  provides feedback to the video control system  24  in the form of a digital signal corresponding to the amount of light output in a given frame of video. The video control system  24  compares the values of digital signals received and then determines whether to increase or decrease the amount of light output by light engine  12  when displaying a fractional LSB, as will be discussed in greater detail below. 
     Turning now to  FIG. 2 , an exemplary representation of light output in accordance with embodiments of the present invention is displayed and is generally designated by the reference numeral  26 . More specifically, the light output shown is relative to a full LSB, the full LSB represents 100% output by the light engine. The full LSB is attenuated to achieve fractional LSBs having ½ and ¼ the light output of an LSB. The light output of a full LSB is shown by LSB  28 , where there is no attenuation of the light. As illustrated, the light is attenuated by one half to produce a ½ LSB  30  and by three quarters to produce the ¼ LSB  32 . The attenuation of the light output to create fraction LSBs in this manner essentially increases the number of bits available to display an image. After creation, however, the fractional bits must be properly weighted with reference to a natural LSB to help ensure an accurate light transfer curve. 
     With reference to  FIG. 3 , an exemplary pixel shift video pattern is shown and is generally designated by the reference numeral  34 . Specifically, the pixel shift video pattern  34  comprises an array of diamond shaped pixels having a first pixel position  36   a  and a second pixel position  36   b  that is shifted from and partially superimposed upon the first pixel position  36   a.    
     In  FIG. 3 , while the pixels are located in a first pixel position  36   a  they have a value of one, the one representing the value of one full LSB. Alternatively, while the pixels are located in a second pixel position  36   b  the pixels have a value of zero, the zero representing zero LSB or no light being directed to the screen. The result of the pixel shift video pattern  34  is that one half of the total pixels making up the video frame have an LSB value of one and the other half have a value of zero. Thus, the total light output of pixel shift video pattern  34  is one-half LSB. The total light output of the video pattern can be measured by the photodiode assembly  22 . This natural LSB value may be compared with the fractional bit weight values of fractional pixels, as discussed below. 
       FIG. 4  is an exemplary pixel shift video pattern in accordance with embodiments of the present invention and is generally designated by the reference numeral  38 . The pixel shift video pattern  38  is identical to the pixel shift video pattern  34  of  FIG. 3  in all respects except all of the pixels making up the video frame have a light output value of one-half LSB. Specifically, the pixels while in the first pixel position  36   a  have a value of one-half LSB and all of the pixels in the second pixel position  36   b  also have a value of one-half LSB. The total light output of the pixel shift video pattern  38  should equal the one-half LSB of video pattern  34 . The total light output from pixel shift video pattern  38  may be measured by the photodiode assembly  22 , which produces a voltage level corresponding to the amount of light it receives. The voltage level is converted to a digital signal and sent to the video control system  24 . 
     The video control system  24  compares the one-half LSB value produced from the video pattern  38  with the natural one-half LSB value obtained from the pixel shift video pattern  34  represented in  FIG. 3 . A comparison of these two pixel shift video patterns enables the video control system  24  to adjust the light output from the light source  12  to calibrate the fractional bits with the natural LSB. The calibration entails converging the value of video pattern  38  to the value of the pixel shift video pattern  34 . This weighting of the fractional bits to the natural LSB helps ensure a smoother image and reduces contouring. 
     Once the one-half LSB light output level is calibrated or properly scaled with the natural LSB, the procedure may be repeated to properly scale other fractions of the LSB such as one fourth LSB. For example, to scale the one-fourth LSB, the calibrated one-half LSB would be used in the place of one LSB in the video pattern  34  of  FIG. 3  and the one-fourth LSB would replace the one-half LSB of the video pattern  38  of  FIG. 3 . All of the other procedures remain as described above. 
     Turning to  FIG. 5 , a flow chart illustrating an exemplary technique for bit weighting in accordance with embodiments of the present invention is shown and generally designated by reference numeral  40 . Specifically, the flow chart shows the steps of ensuring proper bit weighting of fractional bits in an image display device using pixel shift technology. In one embodiment, the technique  40  may be performed upon start up of the image display unit  10 . Alternatively, the video control system  24  and the light engine  12  in conjunction with the photodiode assembly  22  may perform the technique  40  on the fly or while in operation to ensure continued proper fractional bit scaling. 
     As indicated by block  42 , the technique  40  may begin when the device is turned on. Initially, video pattern  34  is made wherein pixels are displayed in the first pixel position  36   a  with a light output of one LSB, then pixels are displayed in the second pixel position  36   b  with zero LSB or in a fully off state. As indicated by block  48 , a voltage output may be read from the photodiode at this time. This voltage output is representative of the total light output of the pixel shift video pattern  34 . 
     Next, video pattern  38  is made, wherein pixels may be displayed in their first pixel position  36   a  with one-half LSB and then shifted and displayed in their second pixel position  36   b , again with one-half LSB. A voltage may be read from the photodiode assembly  22  as indicated by block  54 . This voltage represents the total light output of the pixel shift video pattern  38 , where all of the pixels display a one-half LSB. This voltage level is compared with the voltage level of the pixel shift video pattern  34  which was obtained earlier. If the voltage levels do not match, the video control system  24  adjusts the light output of the light engine  12  to converge the voltage level represented in block  54  with the voltage level represented in block  48 . For example, if the video pattern  38  produced a higher voltage level than the video pattern  34 , the light output of the light source  12  should be further attenuated when displaying a fractional LSB. The technique is repeated until the voltage levels match and the bit weight of the fractional bits are scaled to the natural LSB. When the voltage levels are sufficiently close, the calibration ends. 
     As previously described, technique  40  may be performed upon an initial start up, or during use. Additionally, this calibration or scaling of the fractional bits may be performed in the factory before the image display unit is shipped to consumers. 
     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.