Patent Publication Number: US-7916104-B2

Title: Increased intensity resolution for pulse-width modulation-based displays with light emitting diode illumination

Description:
TECHNICAL FIELD 
     The present invention relates generally to a system and method for video display systems, and more particularly to a system and method for increasing intensity resolution (bit-depth) using LED illumination. 
     BACKGROUND 
     In a typical video display system, images can be created by either emitting or modulating light. The light forms picture elements (pixels), which, when viewed together with other pixels, form an image. The pixels in an image will typically have a variety of colors and/or intensities, with image quality being dependent upon a number of different intensity levels the pixels are capable of displaying. A binary spatial light modulator (binary SLM), such as a binary micromirror device (DMD), is digital in nature and is not capable of emitting light with different intensity levels. However, there may be other SLMs that can be digital in nature but are not binary. Instead, the binary SLMs will typically rely on a pulse width modulation (PWM) scheme to create light at various intensity levels by rapidly turning a light modulator on and off. The integration of the rapidly switching light by the eye provides an illusion of multiple intensity levels. 
     Being mechanical devices, there is a limit to how rapidly the light modulator can be turned on and off. For example, in a DMD, the time that is required to turn the light modulator (a mirror in the DMD) on and off corresponds to moving a mirror from the off state to an on state and then back to the off state. This time can be dominated by a time that the mirror (micromirror) takes to settle to a stable state after moving. This translates to a minimum amount of light that can be emitted within a single frame time. The minimum amount of light corresponds to a lowest intensity level that can be produced by the binary SLM and can be referred to as a bit-depth of the video display system. In general, the smaller the minimum amount of light, the higher the bit-depth and the finer the image quality produced by the video display system. 
     One technique that can be used to reduce the minimum amount of light produced by the binary SLM is to make use of a neutral density filter (NDF) to modulate the light for the short duration light pulses. The NDF can have different densities and therefore can attenuate the light to different levels. 
     Another technique that can be used to reduce the minimum amount of light produced by the binary SLM is to use dynamic aperture technology. Dynamic aperture technology makes use of adjustable apertures to reduce the intensity of the light. 
     One disadvantage of the prior art is that the use of the NDF causes loss of light during the entire time of reduced illumination. This time is far greater than the switching on/off time of the mirror. This loss of light results in a reduction of overall system brightness. 
     A second disadvantage of the prior art is that the use of the NDF or the dynamic aperture technology to modulate light amplitude can require modifications to existing binary SLM products and technologies, which can require significant redesign and redevelopment. This can lead to the expenditure of a large amount of time and money. 
     A third disadvantage of the prior art is that both the NDF and the dynamic aperture technology techniques are mechanical techniques, which also have physical limits on a minimum amount of light that can be emitted. Therefore, it may not be possible to reduce the minimum light intensity to a desired level if the physical limits are too high. Additionally, mechanical techniques may not be able to provide a desired level of flexibility when it comes to exactly producing a needed level of light. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provides a method for increasing intensity resolution using LED illumination. 
     In accordance with a preferred embodiment of the present invention, a method for displaying a bit in a spatial light modulator display system is provided. The method includes determining a display time for the bit based upon a weighting of the bit and using a light modulator to display the bit in response to a determination that the display time is substantially equal to or greater than a minimum display time. The method also includes using the light modulator and light source modulation to display the bit in response to a determination that the display time is less than the minimum display time. 
     In accordance with another preferred embodiment of the present invention, a system for displaying video images is provided. The system includes a spatial light modulator and a rapid switching light source that can optically illuminate the spatial light modulator. The spatial light modulator creates images made up of pixels by setting each light modulator in an array of light modulators into a state matching a corresponding pixel value, while the rapid switching light source is capable of switching at a faster rate than a rate of state switching by the spatial light modulator. 
     An advantage of a preferred embodiment of the present invention is that the use of LEDs for illumination can mean that LEDs can simply replace projector lamps in existing binary SLM designs. Therefore, implementation of the present invention can be achieved with very little modification to existing designs. 
     A further advantage of a preferred embodiment of the present invention is that when used in conjunction with a light modulator (for example, a mirror), the ability of the LEDs to rapidly turn on and off and produce light in a wide range of intensities can lead to a binary SLM with the capability to produce a wide range of light intensities. This can yield a binary SLM with an increased bit-depth. 
     Yet another advantage of a preferred embodiment of the present invention is that the ability to shorten the minimum amount of light producible by a binary SLM display system by using techniques other than shortening the light modulator switching time can result in a relaxation of design criteria and performance characteristics for the light modulators. The relaxation of the design criteria and performance characteristics can permit the use of lower cost and better tested manufacturing techniques and materials to create the light modulators, for example. Furthermore, the light modulators themselves may not have to be pushed as close to their performance limits to meet desired performance characteristics. Therefore, the light modulators may be cheaper and more reliable, as well as having higher manufacturing yields. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a pixel that is broken down into its RGB component values and a timing diagram for displaying the G component values of the pixel; 
         FIG. 2  is a timing diagram of the function of a binary SLM display system in response to control instructions; 
         FIG. 3  is a timing diagram of the function of a binary SLM display system, wherein a rapid switching light source is used to change the bit-depth of the binary SLM display system, according to a preferred embodiment of the present invention; 
         FIG. 4  is a timing diagram of the function of a binary SLM display system, wherein a rapid switching light source and a light modulator are used in conjunction to change the bit-depth of the binary SLM display system, according to a preferred embodiment of the present invention; 
         FIG. 5  is a timing diagram of the function of a binary SLM display system, wherein a rapid switching light source and a light modulator are used in conjunction to change the bit-depth of the binary SLM display system, according to a preferred embodiment of the present invention; 
         FIG. 6  is a timing diagram of the function of a binary SLM display system, wherein light output intensity is used to change the bit-depth of the binary SLM display system, according to a preferred embodiment of the present invention; 
         FIGS. 7   a  through  7   c  are timing diagrams illustrating the display of bits in a binary SLM display system, according to a preferred embodiment of the present invention; 
         FIGS. 8   a  through  8   e  are diagrams of algorithms for displaying an image in a binary SLM display system, according to a preferred embodiment of the present invention; and 
         FIG. 9  is a diagram of a portion of a binary SLM display system, according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely a binary SLM display system that makes use of mirrors as light modulators. The invention may also be applied, however, to other binary SLM display systems, wherein a light modulator is used to attenuate light intensity arising from a fixed light source. Examples of these binary SLM display systems can be display systems making use of liquid crystal display technology, liquid crystal on silicon technology, deformable mirror technology, actuated mirror technology, and so forth. 
     With reference now to  FIG. 1 , there is shown a diagram illustrating a pixel  105  and an exemplary technique that can be used to display the pixel  105  on a display screen of a binary SLM display system. In computer applications, a pixel, such as the pixel  105 , of a color image can be described by three values. The three values can represent amounts of three basic colors (red, green, and blue) present in the pixel. The three values are commonly denoted R (for red), G (for green), and B (for blue) and collectively are referred to as the RGB color space. Examples of other techniques to describe a pixel can include CYMK (cyan, yellow, magenta, and black), HSB (hue, saturation, and brightness), and LAB (luminance and chromatic components). 
     A numeric value can represent an amount of red, green, or blue present in the pixel  105 . For example, as displayed in  FIG. 1 , the pixel  105  can be represented in the RGB color space, where each value is represented as a four-bit binary number, with a red component equal to 7 (binary 0111 (box  110 )), a green component equal to 11 (binary 1011 (box  111 )), and a blue component equal to 9 (binary 1001 (box  112 )). The binary representation of the RGB values, for example, the green component, can be used by light modulators in the binary SLM display system to graphically display the pixel  105  on a display screen. 
     As shown in  FIG. 1 , the green component  111  of the pixel  105  can be represented in binary form as 1011. A light modulator, such as a micromirror, can be provided with the binary representation of the green component  111 , for example, in a bit-wise fashion and depending upon the value of the bit, the light modulator can be placed in a corresponding on or off state. For example, the binary form of the green component  111  can be represented as G 0 =1, G 1 =1, G 2 =0, and G 3 =1, wherein bit G 0  is the bit with the smallest weight (commonly referred to as being a least significant bit) and bit G 3  is the bit with the largest weight (commonly referred to as being a most significant bit). In addition to determining the state of the light modulator, the bit (more precisely, the weight of the bit) determines a period of time that the light modulator remains in the state. For instance, bit G 0  could have the light modulator in desired state for time T 0  and bit G 3  could have the light modulator in desired state for time T 3 , wherein time T 3  is 2 3  times longer in duration than time T 0 . In general, for each increase in bit weight, there is a two times increase in the time that the light modulator holds the desired state. Although the above example displays a binary (power of two) weighting system for the bits of the RGB components of the pixel  105 , weighting systems other than binary can be used. The example of the binary weighting system should not be construed as being limiting to the spirit of the invention. 
     The pixel information to be displayed for a single pixel may be in the binary form of the input data being provided to the binary SLM display system. However, the pixel information to be displayed for a single pixel may also be in a binary form that is a result of a transformation to a non-binary weighting of each bit in the representation. This transformation may occur within the binary SLM display system. For example, the binary form for the green component  111  in the system input data may be a standard four-bit binary weighting. In such a representation, the bit weights would be 1, 2, 4, and 8, respectively. Thus, a decimal 10 value would correspond to a binary representation of 1010 (2 3 +2 1 ). This input data, however, could be transformed into a six-bit weighting system with weights 1, 1, 2, 2, 4, and 5, for example. So, the same decimal 10 value could be represented as 011111 (4+2+2+1+1). 
     The time T 0 , which is a period of time that a light modulator maintains its state for the least significant bits of the RGB component values for each pixel being displayed, is also often a minimum state switch time for the light modulator being used in the binary SLM display system. For example, in a binary SLM display system using digital micromirror devices, a minimum amount of time that it takes to switch a micromirror from an off state to an on state and back to an off state is often assigned to time T 0 . The time T 0  then corresponds to a minimum amount of light intensity usable to illuminate a single pixel. 
     A trace  115  illustrates time allocated to each bit of an exemplary green (G) component of the RGB component values of the pixel  105  by a light modulator. Since bit G 0  represents the least significant bit of the green component of the pixel  105 , a minimum amount of time, T 0 , is used to display the bit G 0 . This is shown in the trace  115  as block  120 , labeled “G 0 .” Bit G 1  is referred to as being a second least significant bit, and since bit G 1  is twice as significant as bit G 0 , the light modulator should use twice the time to display bit G 1  as it did displaying bit G 0 . This is shown in the trace  115  as block  121 , labeled “G 1 .” Similarly, bits G 2  and G 3  are displayed with display times four times and eight times as long as the time to display bit G 0  (shown as block  122 , labeled “G 2 ,” and block  123 , labeled “G 3 ”), respectively. Again, the example discusses a binary weighting system only for discussion purposes and does not limit the present invention to the use of a binary weighting system. 
     While the above discussion presents specific examples of the RGB components of a single pixel in an image that is to be displayed by the binary SLM display system, the actual value of the bits being displayed does not actually factor into the behavior of the binary SLM display system. Regardless of the fact that a single bit of a component value of a pixel is a binary one or a binary zero, the binary SLM display system will dedicate a predetermined amount of time to the display of the bit (the amount of time being determined by a weight assigned to the bit) and provide the exact same sequence of instructions to each of the light modulators in the display system. 
     For a given image in an image stream, there is a time value, referred to as a frame time, within which the binary SLM display system must display all graphical information making up the image. For example, for an image containing the pixel  105 , the binary SLM display system must display all four bits representing the green component of the pixel  105 , as well as the bits representing the red and blue components of the pixel  105 , within the time span (shown as interval  125 ) that is less than the frame time. If the frame time is long compared to the number of bits that needs to be displayed, then it can be a relatively simple task to display all of the bits. In this case, it may be possible to scale up the display times for each bit to increase image intensity. However, if the number of bits is large, it may be possible that the time needed to display all of the bits will exceed the frame time. In this case, it may be necessary to reduce the time allocated to the display of certain bits. However, it may not be possible to reduce the time allocated to the display of the least significant bit if the minimum amount of time is already being used to display the least significant bit. 
     With reference now to  FIG. 2 , there is shown a diagram illustrating the function of a binary SLM display system in response to control instructions with a plurality of traces, wherein the binary SLM makes use of micromirrors to modulate a light. As shown in  FIG. 2 , a first trace  205  illustrates micromirror instructions issued by the binary SLM display system. At a first time, a first instruction  210  instructs the micromirror to move to an “ON” position and at a second time, a second instruction  212  instructs the micromirror to move to an “OFF” position. A second trace  215  illustrates micromirror state (position) as a function of time. Since the micromirror is a mechanical device, there is a certain amount of inertia that must be overcome prior to the micromirror being able to move. At the first time, when the first instruction  210  is issued, indicated by a first vertical line  216 , the micromirror can begin to move, but due to inertia, the micromirror does not begin to move until a later time, indicated by a second vertical line  217 . A time between when the first instruction  210  is issued and when the micromirror actually begins to move is shown as interval  218 . The second trace  215  is an idealized representation of the movement of the micromirror and does not display any ringing, vibration, or so forth that may actually be present in the movement of the micromirror. 
     Once the micromirror begins to move towards the “ON” position (at a time indicated by the second vertical line  217 ), the micromirror continues to move until it moves to a position consistent with being in an “ON” state. Once again, the second trace  215  does not display any over-travel, ringing, or so forth that may be present in the movement of the micromirror as it comes to a stop in the “ON” state. Once the micromirror passes a certain position along its movement path that is prior to it coming to a stop in the “ON” state, light reflecting off the micromirror can begin to show on a display screen. This micromirror position is shown in  FIG. 2  as horizontal line  219 . 
     A third trace  220  illustrates light present on the display screen, wherein the light originates from light that is reflected from the micromirror whose movement is displayed in the second trace  215 . The third trace  220  shows that until the position of the micromirror passes the position denoted by the horizontal line  219 , no light (or very little light due to ambient light) is present on the display screen. Once the micromirror passes the position shown as horizontal line  219  where some reflected light from the micromirror begins to show on the display screen at a time denoted by vertical line  225 , the light present on the display screen begins to increase. The light on the display screen will continue to increase until the micromirror stops in the “ON” state and a maximum amount of reflected light is displayed on the display screen. A time between when the micromirror “ON” instruction  210  is issued and when the light actually begins to appear on the display screen is shown as interval  222 . A complementary behavior is seen when the micromirror moves to an “OFF” state and the light on the display screen decreases until the micromirror passes the position where no light reflecting off the micromirror appears on the display screen. 
     Due to physical limitations such as inertia, vibrations, overshooting, ringing, and so forth, there is a minimum amount of time required for the micromirror to switch from the “OFF” state to the “ON” state and back to the “OFF” state. This minimum amount of time can correspond to a physical minimum amount of light displayed on a display screen. The physical minimum amount of time (physical minimum display time) for a binary SLM display system using a micromirror can be defined as the amount of time that elapses for a micromirror to switch from the “OFF” state to the “ON” state and back to the “OFF” state when micromirror instructions to move the micromirror to an “ON” state and then to an “OFF” state are issued as rapidly as possible, and is shown in  FIG. 2  as interval  230 . The interval  230  is commonly referred to as an effective light time (ELT). However, for a given binary SLM display system, the minimum amount of time (minimum display time) of the binary SLM display system may not be the same as the physical minimum amount of time. The minimum amount of time may be longer than the physical minimum amount of time due to design considerations, which must take into consideration factors such as system longevity, system reliability, frame time, number of bits to display, and so forth. It may be possible for the physical minimum amount of time to be equal to the minimum amount of time, but not necessarily. 
     With the binary SLM display system as described, the minimum amount of time (and its attendant minimum amount of light) corresponds to the least significant bit of a given RGB value of a given pixel. The log 2  of the ratio of this time value to the total time of all bits on for a given display color is referred to as being the bit-depth of the binary SLM display system for the given display color. For example, if this ratio is 1/256, the bit depth is eight (8). It is not possible to produce less light (increase the bit-depth) without the addition of additional hardware and software to the binary SLM display system, such as using the prior art techniques of neutral density filters and adaptive aperture technology. However, there is often a desire to reduce this minimum amount of time or the minimum amount of light or both to improve the quality of images displayed by the binary SLM display system, since there can be a corresponding increase in image quality when there is an increase in image bit-depth. 
     With reference now to  FIG. 3 , there is shown a diagram illustrating the function of a binary SLM display system, wherein a rapid switching light source is used to change the bit-depth of the binary SLM display system by shortening the ELT of the display system, according to a preferred embodiment of the present invention. In addition to using the light modulator (for example, a movable micromirror) to control when a pixel is displayed on a display screen, it may also be possible to actually turn a light source on and off to control the display of the pixel on the display screen. In a typical binary SLM display system, high-intensity light sources are used to provide lighting. However, these high-intensity lights typically have very long cycle times when they are turned on and off, on the order of seconds or minutes. This prohibits the use of turning the lights on and off as a way of controlling the display of the pixel on the display screen. However, it is possible to rapidly turn a light-emitting diode (LED) on and off. A typical LED can be turned on or off in less than a micro-second. This can result in a very short ELT. Although reference is made to a singular LED, it is possible for the binary SLM display system to make use of a plurality of LEDs. For example, a plurality of LEDs arranged in an array-like manner can be used in place of a single LED. Additionally, the plurality of LEDs may contain LEDs of different colors, such as an array of LEDs made up of white LEDs, red LEDs, green LEDs, blue LEDs, and so on. 
     As shown in  FIG. 3 , the first trace  205  illustrates micromirror instructions as issued by the binary SLM display system. The binary SLM display system issues the first instruction  210  to move the micromirror to an “ON” position and the second instruction  212  to move the micromirror to an “OFF” instruction. The second trace  215  illustrates graphically the state (position) of the micromirror as a function of time and the third trace  220  illustrates light present on the display screen. 
     A fourth trace  305  illustrates LED (light) instructions as issued by the binary SLM display system. The binary SLM display system can issue at least two types of LED instructions, turn LED “ON” and turn LED “OFF.” To ensure that no unintended light is reflected onto the display screen, a first LED instruction  307  is issued to turn LED “OFF.” Although as shown in  FIG. 3  the first LED instruction  307  is issued at approximately the same time as the first instruction  210  to move the micromirror to an “ON” position, the first LED instruction  307  may be issued at any time as long as the LED is off prior to the micromirror actually assuming the “ON” position. A second LED instruction  308  and a third LED instruction  309  can be used to turn the LED “ON” and then “OFF” for a desired amount of time. Since the micromirror was already in the “ON” position (from instruction  210 ), the light from the LED is displayed on the display screen for a time span (shown as highlight  315 ) that is substantially equal to the time between the second LED instruction  308  and the third LED instruction  309 . Since the time to turn the LED on and off is short (on the order of less than a micro-second), light immediately appears (and disappears) from the display screen as the second LED instruction  308  (and the third LED instruction  309 ) is issued. The use of the LED light source can permit the binary SLM display system to generate light with shorter durations than with a light source that cannot rapidly switch on and off and therefore increase its bit-depth. 
     With reference now to  FIG. 4 , there is shown a diagram illustrating the function of a binary SLM display system, wherein a rapid switching light source and a light modulator are used in combination to change the bit-depth of the binary SLM display system by shortening the ELT of the display system, according to a preferred embodiment of the present invention. It is possible that current drivers for the LED may be a limiting factor in the ELT, for example, there may be a limit to the rate of change in the current that the current drivers can provide to the LED during the LED turn on transition. If this is the case, then the light produced by the LED may increase at a relatively slow rate (compared to a light turn off time, for example). Furthermore, the behavior of the LED while being turned on may be unstable or unpredictable. In these situations (and perhaps others), it can be possible to use the light modulator, such as a micromirror, in conjunction with the LED to shorten the ELT. The turn on behavior of the LED can be referred to as being undesirable. 
     A first trace  205  illustrates micromirror instructions as issued by the binary SLM display system and a second trace  215  illustrates the micromirror state (position) as a function of time. A third trace  305  illustrates LED instructions as issued by the binary SLM display system. Due to a relatively slow LED turn on rate, the second LED instruction  308  is issued prior to the micromirror being moved into the “ON” state. The LED may have already been on from displaying an earlier bit. A scheduler (referred to as a sequence controller) can insert proper instructions to control LED state, mirror state, and so forth, and if the LED was already on from displaying an earlier bit, the sequence controller may not have inserted the second LED instruction  308 . However, depending upon binary SLM display system implementation, an instruction to turn the LED on may have no effect on the LED if it is already on. In this situation, the presence of the second LED instruction  308  is substantially harmless. Since an extended amount of time may elapse between the instruction  210  to turn the micromirror “ON” to when the micromirror actually moves into the “ON” state, the second LED instruction  308  can be issued at anytime such that the LED will be on prior to the micromirror moves into the “ON” state. 
     A fourth trace  405  illustrates LED illumination. Since the LED turn on rate is slow, a curve with a positive slope  407  is used to indicate the increasing illumination of the LED. A fifth trace  410  illustrates light on the display screen. Although the LED begins to produce light at a time prior to a time when light begins to appear on the screen, since the micromirror has not moved into the “ON” state, light from the LED is not reflected onto the display screen. As the micromirror moves into the “ON” state, it passes the position wherein some of the light reflected from the micromirror begins to show on the display screen. This position is indicated on the second trace  215  as the horizontal line  219  (a vertical line  411  indicates the timing relationship). The ELT of the binary SLM display system is shown as interval  412 . 
     While the current driver may be a limiting factor on the turn on of the LED, it may not be a limiting factor on the turn off of the LED. As shown in  FIG. 4 , when the third LED instruction  309  is issued to turn off the LED, the light produced by the LED almost immediately turns off and this is reflected in the light shown on the display screen. 
     With reference now to  FIG. 5 , there is shown a diagram illustrating the function of a binary SLM display system, wherein a rapid switching light source and a light modulator is used in combination to change the bit-depth of the binary SLM display system by shortening the ELT of the display system, according to a preferred embodiment of the present invention. It is possible that current drivers for the LED may be a limiting factor in the ELT, for example, there may be a limit to the rate of change in the current that the current drivers can provide to the LED during the LED turn off transition. If this is the case, then the light produced by the LED may decrease at a relatively slow rate. Furthermore, the behavior of the LED while being turned off may be unstable or unpredictable. In these situations (and perhaps others), it can be possible to use the light modulator, such as a micromirror, in conjunction with the LED to shorten the ELT. The turn off behavior of the LED can be referred to as being undesirable. 
     A first trace  205  illustrates micromirror instructions as issued by the binary SLM display system and a second trace  215  illustrates the micromirror state (position) as a function of time. A third trace  305  illustrates LED instructions as issued by the binary SLM display system. The first LED instruction  307  turns the LED off prior to the micromirror assuming the on position. 
     A fourth trace  505  illustrates LED illumination. A fifth trace  510  illustrates light on the display screen. Although the LED is still on, the light on the display screen begins to drop as the micromirror begins to move away from the “ON” state and then turns off completely when the micromirror moves out of the “ON” state and crosses the threshold where light reflected from the micromirror begins to show on the display screen (shown in the second trace  215  as the horizontal line  219 ). Since light reflected by the micromirror no longer shows on the display screen, any remaining light from the LED is not reflected on the display screen even if the LED is still outputting light. The ELT of the binary SLM display system is shown as interval  512 . Depending upon implementation, the sequence controller may elect to turn off the LED at this point, however, it is not necessary for it to do so. 
     In addition to having the ability to turn on and off rapidly, the light output of LEDs is often dependent upon a current provided to the LEDs. Therefore, it is possible to increase the intensity of the light produced by the LED (up to a certain limit) or decrease the intensity of light produced by the LED (again, to a certain limit) by changing the magnitude of a current provided to the LED. Furthermore, if a plurality of LEDs are used to provide light for the binary SLM display system, light intensity can be varied by turning on (or off) a certain number of LEDs. For example, if maximum light intensity is desired, then all LEDs in the plurality can be turned on, while for less light intensity, some subset of LEDs in the plurality of LEDs can be turned on. 
     With reference now to  FIG. 6 , there is shown a diagram illustrating the function of a binary SLM display system, wherein light output intensity of LEDs can be used to change the bit-depth of the binary SLM display system, according to a preferred embodiment of the present invention. As shown in  FIG. 6 , the diagram illustrates the use of varying LED light output intensity to change the bit-depth of the binary SLM display system. In a situation where a plurality of LEDs are used as a light source, rather than changing LED light output intensity (or in conjunction with) it may be possible to change a number of LEDs turned on (or off) in an array of LEDs. For example, to increase light output intensity, additional LEDs can be turned on, while LEDs can be turned off to decrease light output intensity. 
     As shown in  FIG. 6 , a first trace  215  illustrates micromirror state (position) as a function of time and a second trace  605  illustrates light showing on a display screen in a situation wherein the light is from a fully illuminated LED that was turned on while the micromirror was moving from an “OFF” state to an “ON” state and back to an “OFF” state. A third trace  610  illustrates a series of LED instructions to control light output of the LED. A first LED instruction  611  can be used to turn the LED on and a second LED instruction  612  reduces the light output of the LED. It may be possible to turn on an LED at a desired light output, thereby potentially reducing one LED instruction. For example, the first LED instruction  611  may have an argument that specifies a light output level, thereby eliminating the second LED instruction  612 . A third LED instruction  613  restores the light output of the LED, while a fourth LED instruction  614  turns off the LED. Again, variations in LED instructions are possible. The actual implementation of the LED instructions are not important in describing the spirit of the present invention. 
     A fourth trace  615  illustrates LED light output resulting from the execution of the four LED instructions  611 ,  612 ,  613 , and  614 . Finally, a fifth trace  620  illustrates light showing on the display screen with LED light output as shown in the fourth trace  615 . As shown in  FIG. 6 , the light output of the LED was decreased prior to the micromirror moving to a position where light reflected from the micromirror begins to show on the display screen (the position is shown in the first trace  215  as the horizontal line  219 ). Therefore, the light showing on the display screen is the reduced intensity light output of the LED. Comparing the second trace  605  to the fifth trace  620 , the magnitude of the fifth trace  620  is smaller than the magnitude of the second trace  605 . The smaller magnitude indicates a lower light intensity. 
     It can be possible to combine the above discussed techniques to provide a greater degree of flexibility when it comes to reducing the amount of light produced by a binary SLM display system. To further reduce the amount of light, a shortened ELT can be combined with lower LED intensity. For example, if the ELT is shortened by 50% and the LED intensity is also lessened by 50%, then the net amount of light produced is approximately 25% of the amount of light producible within the same display period with full LED intensity. The additional flexibility can enable more possibilities when it comes to scheduling the display of bits in the binary SLM display system. This may lead to positive effects such as longer component life due to less frequent switching of LED state, for example. This is in addition to the positive benefit of greater image quality due to increased bit-depth. 
     Furthermore, with the technique of varying LED output light intensity, it can be possible to reduce changes to LED state and output light intensity levels by varying display time. Any reduction in the number of times the LED state and/or output light intensity is changed can lead to an increased useful life of the LED. For example, if a minimum display time is 20 micro-seconds and with the LED on at full intensity, it is possible to display 20 units of light, then to display 15 units of light, it is possible to have the LED on at one-half intensity for 30 micro-seconds. If the LED was already set at one-half intensity to display a previous bit, a change in LED output light intensity is saved by using this technique. 
     With reference now to  FIGS. 7   a  through  7   c , there are shown diagrams illustrating time used by a light modulator to display bits of a component value of a pixel in an image, according to a preferred embodiment of the present invention. The diagram shown in  FIG. 7   a  illustrates an allocation of time to display of the bits of a green component value of a pixel in an image by a binary SLM display system, wherein a variety of bit-depths are used to represent the green component value. Similar allocations of time may be used to display the bits of blue and red components of a pixel in the binary SLM display system. A first trace  705  illustrates the display of the green component value, wherein three bits are used to represent the green component value. Three time intervals are used to display the three bits of the green component value, a first time interval “G 0 ”  706 , a second time interval “G 1 ”  707 , and a third time interval “G 2 ”  708 , with the first time interval “G 0 ”  706  being substantially equal to the minimum display time of the binary SLM display system. A total time required to display the three bits of the green component value is shown as interval  709 . 
     A second trace  715  illustrates the display of the green component value, wherein four bits are used to represent the green component value with a fourth bit (GX 0 ) being used to represent a new least significant bit. The addition of an additional bit can improve image quality, however, since the minimum display time has already been assigned to display the least significant bit of the case where three bits are used to represent the green component value, G 0 , the display of the fourth bit GX 0  cannot be accomplished by simply halving the time used to display the least significant bit G 0 . Therefore, the time used to display the fourth bit GX 0  has to be substantially the same as the time used to display the second least significant bit (G 0 ). A fourth time interval “GX 0 ”  716  displays the time allocated to display the least significant bit “GX 0 .” A total time required to display the four bits of the green component value is shown as interval  720 . If the time required to display the four bits of the green component value is greater than a frame time, it may be necessary to scale the time spent on each of the bits (except bits G 0  and GX 0  since they are already assigned minimum times) so that the overall time is less than or equal to the frame time. 
     A third trace  725  illustrates the display of the green component value, wherein five bits are used to represent the green component value. The first three bits of the green component value (G 0 , G 1 , and G 2 ) are as in the three-bit case with two additional bits GX 1  and GX 0 , where GX 0  is the least significant bit. Since the time allocated to display bit G 0  is already the minimum display time, the time used to display the two additional bits GX 1  and GX 0  must also be substantially equal to the minimum display time (shown as intervals  726  and  727 ). A total time required to display the five bits of the green component value is shown as interval  731 . A fourth trace  735  illustrates the display of the green component value with six bits being used to represent the green component value. The first three bits of the green component value (G 0 , G 1 , and G 2 ) are as in the three-bit case with three additional bits GX 2 , GX 1 , and GX 0 , again with GX 0  being the least significant bit. Again, since the time allocated to display bit G 0  is already the minimum display time, the time used to display the three additional bits GX 2 , GX 1 , and GX 0  must also be substantially equal to the minimum display time (shown as intervals  736 ,  737 , and  738 ). A total time required to display the six bits of the green component value is shown as interval  742 . If the time required to display the five and six bits of the green component value is greater than the frame time, it may be necessary to scale the time spent on each of the bits (except bits G 0 , GX 0 , GX 1  of the five bit example and bits G 0 , GX 0 , GX 1 , GX 2  of the six bit example since they are already assigned minimum times) so that the overall time is less than or equal to the frame time. 
     A total amount time allowed to display the pixel of an image may be limited by the frame time, which is the amount of time that an image is displayed on the display screen before it is replaced with another image (the replacement image may be the same image). The addition of extra bits may push the total time required to display all of the bits beyond the frame time. Since the frame time is fixed, the total time may need to be shortened to fit within the frame time. However, the minimum display time is fixed, therefore, the time to display the bit G 0  and any additional bits, such as GX 2 , GX 1 , and GX 0  cannot be shortened. Therefore, the time to display the more significant bits, such as bits G 1  and G 2  may need to be shortened to have the total time fit within the frame time. 
     As described above, the present invention can permit the light output intensity of the light source of the binary SLM display system (an LED) to be varied or to turn the light source on and off while the light modulator is in an “ON” state, thus the minimum amount of light producible by the binary SLM display system can be reduced. For example, to halve the amount of light produced by the binary SLM display system, it is possible keep the light source on for an equivalent amount of time, but reduce the light output of the light source by two (2), or maintain a constant light output, but keep the light source on for one-half the amount of time. 
     The diagrams shown in  FIGS. 7   b  and  7   c  illustrate two techniques for reducing the minimum amount of light producible by the binary SLM display system. The diagrams display light source behavior for time intervals  736 ,  737 ,  738 , and  706  from the fourth trace  735  (FIG.  7   a ). The diagram shown in  FIG. 7   b  illustrates the technique of maintaining constant light output while shortening the light source on time. Interval  706  represents the display of bit G 0 , the least significant bit that may be displayed solely via light modulator control. The light source is left on the entire time represented by interval  706  (shown as block  750 ). Interval  738  represents the display of bit GX 2 . The bit GX 2  is half as significant as bit G 0 , so the light representing bit GX 2  should be one-half the intensity of the light representing bit G 0 . To accomplish this, the light source is on for one-half of the time represented by interval  738  (shown as block  755 ). Similarly, bit GX 1  is one-quarter as significant as bit G 0 , so the light representing bit GX 1  should be one-quarter the intensity of the light representing bit G 0 . This is accomplished by having the light source on for one-quarter of the time represented by interval  737  (shown as block  760 ). The bit GX 0  is one-eighth the significance of the bit G 0 , so the light source is on for one-eighth of the time represented by interval  736  (shown as block  765 ). 
     The diagram shown in  FIG. 7   c  illustrates the technique of maintaining light source on time while varying light output intensity. Interval  706  represents the display of bit G 0 , the least significant bit that may be displayed solely via light modulator control. The light source is producing light at a maximum intensity for the entire time represented by interval  706  (shown as block  775 ). Interval  738  represents the display of bit GX 2 . The bit GX 2  is half as significant as bit G 0 , so the light representing bit GX 2  should be one-half the intensity of the light representing bit G 0 . To accomplish this, the light source is set to produce light at one-half of maximum intensity for the entire time represented by interval  738  (shown as block  780 ). Similarly, bit GX 1  is one-quarter as significant as bit G 0 , so the light representing bit GX 1  should be one-quarter the intensity of the light representing bit G 0 . This is accomplished by having the light source set to produce light at one-quarter of maximum intensity for the entire time represented by interval  737  (shown as block  785 ). The bit GX 0  is one-eighth the significance of the bit G 0 , so the light source is set to produce light at one-eighth of maximum intensity for the entire time represented by interval  736  (shown as block  790 ). 
     As discussed above, the two techniques for increasing bit-depth by reducing light output illustrated in  FIGS. 7   b  and  7   c  can be combined to provide greater flexibility in light output production. The combination technique can make use of both LED light output intensity modulation and light source on time. For example, to halve the amount of light produced in interval  738  ( FIG. 7   c ), the LED light can be turned off for one-half the duration of the interval  738 . This will result in halving the light output of the original interval  738 . 
     The discussion of the two techniques for reducing the minimum amount of light produced by the binary SLM display system uses, for discussion purposes, a binary weighting system to assign significance to bits. This should not be construed as limiting the present invention to the use of the binary weighting system. Other weighting systems, including arbitrary and variable weighting systems, can be used in the present invention. 
     With reference now to  FIGS. 8   a  through  8   e , there are shown diagrams illustrating algorithms for displaying an image in a binary SLM display system, according to a preferred embodiment of the present invention. The diagram shown in  FIG. 8   a  illustrates an algorithm  800  that can be used to display bits representing RGB component values of pixels of an image. The algorithm  800  illustrates the operations necessary to display bits for a single RGB component value, such as red, green, or blue. However, the operations for displaying the other component values are similar. According to a preferred embodiment of the present invention, the algorithm  800  may execute in a sequence controller (not shown) located in the binary SLM display system. Alternatively, the algorithm  800  may be implemented in an instruction compiler that generates instruction sequences to handle specific situations that may arise in the operation of the binary SLM display system. The instruction compiler may generate different instruction sequences that are to be used when specific situations arise. For example, if the time to display a bit is less than the minimum display time of the binary SLM display system, then use a first sequence of instructions, else use a second sequence of instructions. In such a situation, the sequence controller may have limited processing capabilities and may simply have the capability to execute instructions provided to it. The discussion below describes the operation of a sequence controller that is capable of making decisions regarding the instructions that it issues. However, with minor changes, the algorithms presented can be applied to an instruction compiler used to pre-generate instruction sequences. 
     The sequence controller may be responsible for scheduling binary SLM display system control instructions, executing instructions for providing the component bit values to the light modulators (such as the micromirrors), controlling the state of the light source, controlling the light output intensity of the light source, resetting the light modulators, and so forth. The sequence controller can further enhance the operation of the binary SLM display system, by optimizing the scheduling of control instructions to minimize LED light state changes, for example. The sequence controller may be a custom designed integrated circuit, a micro-controller, a general purpose processor, or so on. The sequence controller can begin with a bit of a component value of a pixel in an image that it is to display. The order in which the bits of the component value are displayed in the binary SLM display device can have an impact upon the image quality, therefore, the sequence controller may be programmable to allow custom ordering of bits. Upon selecting the bit to be displayed, the sequence controller can determine a display time for the bit (block  802 ). The computation of the display time can be dependent upon a weight of the bit, a desired LED light output intensity, a minimum display time, and so forth. With a given weight of the bit, it can be possible to fix the desired LED light output intensity and compute a display time or fix a display time and then compute an LED light output intensity. To normalize display time computations and to simplify comparisons, the display time computations may be for a single LED light output intensity, such as with maximum intensity. 
     The sequence controller can then determine if the time required to display the bit (the display time) is shorter than the binary SLM display system&#39;s minimum display time (block  805 ). As discussed earlier, the minimum display time can be a physically limited time duration that corresponds to a minimum amount of time that it takes for the binary SLM display system&#39;s light modulator to switch from an “OFF” state to an “ON” state and back to an “OFF” state or it may be a value specified by a designer of the binary SLM display system. The minimum display time can translate to a minimum amount of light that can be displayed on a display screen for a single LED light output intensity. Without resorting to the use of additional hardware, such as neutral density filters or dynamic apertures, it may not be possible to put less light on the display screen. 
     One way to determine a bit&#39;s display time is to use a binary weighting of the bits. Again, the discussion of the use of the binary weighting system is for discussion purposes only and is not intended to limit the present invention. For example, if the least significant bit is assigned a weight of one (1), then the next bit can be assigned a weight of two (2), and so on. One of the bits in the component value will be assigned a display time that is substantially equal to the minimum display time. Then, using a ratio of the weight of a bit to be displayed with the weight of the bit with the display time that is substantially equal to the minimum display time, the display time of the bit to be displayed can be computed. For example, if the bit to be displayed has a weight of two (2) and the bit with the display time substantially equal to the minimum display time has a weight of eight (8), then the display time of the bit to be displayed is 2/8*minimum display time=¼ of the minimum display time. The display times may be pre-computed and stored for rapid access. 
     If the time required to display the bit is less than the minimum display time, then the sequence controller can make use of light modulating techniques enabled by the use of a rapid switching light source, such as an LED, to display the bit (block  810 ). Detailed discussions of several such techniques are presented below. If the time required to display the bit is substantially equal to or greater than the minimum display time, then the sequence controller can make use of light modulator state to display the bit (block  815 ). In order to display a bit whose required display time is equal to or greater than the minimum display time, the sequence controller may simply provide the value of the bit to light modulator, issue an instruction to have the light modulator assume the value of the bit, and then after the required display time has expired, issue another instruction to reset the light modulator. Since there may be a time delay in between the issuance of an instruction to the light modulator, the actual time of the issuance of an instruction and the required display time may not be equal. Once the bit has been displayed, the sequence controller can prepare to display another bit of the component value of the pixel (block  820 ). 
     The diagram shown in  FIG. 8   b  illustrates a technique that can use both modulation of light from the light source and a light modulator&#39;s state to display a bit, wherein a light on time is varied to change the amount of light displayed on the display screen. The technique may be an implementation of block  810  of the algorithm  800  ( FIG. 8   a ). Since the light modulator cannot be used to further reduce the light displayed on the display screen (due to the minimum display time), further reduction of the light displayed on the display screen may arise from modulating the light from the light source itself. The sequence controller can begin by providing the value of the bit to the light modulator (block  840 ). This may involve the writing of the bit value to a memory location associated with the light modulator. 
     The providing of the bit value to the light modulator may initiate a change of state of the light modulator or an additional instruction may be needed to initiate the change of state of the light modulator. Then, the sequence controller can issue an instruction to configure the light source to produce light at a maximum intensity (block  845 ). The maximum intensity light level may not be the light source&#39;s maximum light output, but it may be a maximum calibrated light level set during configuration. Once the light modulator has changed to a state corresponding to the bit value, the sequence controller may then issue a command to turn on the light source (block  850 ) and after the desired amount of time, the sequence controller may issue another command to turn off the light source. Alternatively, the command to turn on the light source may have an argument specifying a period of time that the light source is to remain on. Once the light source is off, the sequence controller can issue a command to reset the light modulator (block  855 ). 
     The diagram shown in  FIG. 8   c  illustrates a technique that can use both modulation of light from the light source as well as a light modulator&#39;s state to display a bit, wherein light output intensity is varied to change the amount of light displayed on the display screen. The technique may be an implementation of block  810  of the algorithm  800  ( FIG. 8   a ). Rather than switching the light source on and off for a duration that is less than the minimum display time as described in  FIG. 8   b , it can be possible to reduce the light output of the light source. The sequence controller can begin by providing the value of the bit to the light modulator (block  860 ). This may involve the writing of the bit value to a memory location associated with the light modulator. 
     The providing of the bit value to the light modulator may initiate a change of state of the light modulator or an additional instruction may be needed to initiate the change of state of the light modulator. Then, the sequence controller can issue an instruction to turn on the light source with a specified light output intensity (block  865 ). The specified light output intensity can be computed in a manner similar to the technique discussed for computing display time. For example, if the light source is an LED, it can be possible to change the light output intensity of the LED by changing a current provided to the LED. Alternatively, if the light source is a plurality of LEDs, it can be possible to change the light output intensity by turning on a specified number of LEDs. After the expiration of the minimum display time, the sequence controller may issue a command to reset the light modulator (block  870 ). 
     The diagram shown in  FIG. 8   d  illustrates a variation of the technique described in  FIG. 8   c , wherein it is possible that a desired LED light output intensity is specified prior to a computation of a desired display time. Depending upon the desired LED light output intensity, the desired display time may actually be longer than the minimum display time of the binary SLM display system. For example, if the minimum display time for a binary SLM display system is 20 micro-seconds, then for a maximum LED light output intensity, the binary SLM display system can display 20 units of light. If it is desired to display 15 units of light, then if the desired LED light output intensity is 75%, then the desired display time is 20 micro-seconds. However, if the desired LED light output intensity is 50%, then the desired display time is 30 micro-seconds. A reason for specifying the desired LED light output intensity is that if the LED was already on with the desired LED light output intensity, then the bit can be displayed without changing the LED light output intensity, which may result in a longer useful lifespan for the LED. However, if the desired display time is less than the minimum display time, then the technique described in  FIG. 8   c  will need to be used. The sequence controller will know which technique to use since it knows the LED light output intensity. If the LED light output intensity is at a maximum, then the technique described in  FIG. 8   c  will be used. Only if the LED light output intensity is below some known value (such as 75% in the above example) can the technique described in  FIG. 8   d  be used. 
     The sequence controller can begin by specifying a desired LED light output intensity (block  872 ). The desired LED light output intensity may be equal to a current LED light output intensity. The sequence controller can the compute a desired display time (block  874 ) based upon the desired LED light output intensity. With the desired display time computed, the sequence controller can begin by providing the value of the bit to the light modulator (block  875 ). This may involve the writing of the bit value to a memory location associated with the light modulator. The providing of the bit value to the light modulator may initiate a change of state of the light modulator or an additional instruction may be needed to initiate the change of state of the light modulator. Then, the sequence controller can issue an instruction to turn on the light source with a specified light output intensity (block  876 ). The specified light output intensity can be computed in a manner similar to the technique discussed for computing display time. For example, if the light source is an LED, it can be possible to change the light output intensity of the LED by changing a current provided to the LED. Alternatively, if the light source is a plurality of LEDs, it can be possible to change the light output intensity by turning on a specified number of LEDs. After the expiration of the desired display time, the sequence controller may issue a command to reset the light modulator (block  878 ). 
     The diagram shown in  FIG. 8   e  illustrates a combination technique that modulates light from the light source to display a bit by varying the light on time and the light output intensity as well as a light modulator&#39;s state to change the amount of light displayed on the display screen. The technique may be an implementation of block  810  of the algorithm  800  ( FIG. 8   a ). Rather than just switching the light source on and off for a duration that is less than the minimum display time and reducing the light output of the light source, the technique does both. The sequence controller can begin by providing the value of the bit to the light modulator (block  880 ). This may involve the writing of the bit value to a memory location associated with the light modulator. 
     The providing of the bit value to the light modulator may initiate a change of state of the light modulator or an additional instruction may be needed to initiate the change of state of the light modulator. Then, the sequence controller can issue an instruction to configure the light source to produce light at a desired intensity (block  885 ). The desired intensity light level of the light source may be multiple output light levels. Once the light modulator has changed to a state corresponding to the bit value, the sequence controller may then issue a command to turn on the light source (block  890 ) and after the desired amount of time, the sequence controller may issue another command to turn off the light source. Alternatively, the command to turn on the light source may have an argument specifying a period of time that the light source is to remain on. Once the light source is off, the sequence controller can issue a command to reset the light modulator (block  895 ). 
     The technique described in  FIG. 8   e  makes use of the light intensity modulation technique described in  FIG. 8   c , wherein the LED light output intensity is computed based upon the minimum display time of the binary SLM display system. However, the technique can be readily modified to use the technique described in  FIG. 8   d , wherein a desired display time is computed based upon a desired LED light output intensity, by persons of ordinary skill in the art of the present invention. 
     With reference now to  FIG. 9 , there is shown a diagram illustrating a portion of a binary SLM display system  900 , according to a preferred embodiment of the present invention. As shown in  FIG. 9 , the portion of the binary SLM display system  900  includes a spatial light modulator  905 , such as a digital micromirror device (DMD) array, and a rapid switching light source  910 . In addition to micromirrors, other light modulator technology, such as liquid crystal, liquid crystal on silicon, deformable mirrors, actuated mirrors, and so forth, can be used in the spatial light modulator  905 . The rapid switching light source  910  should be able to switch from on to off and off to on at a faster rate than the light modulators in the spatial light modulator  905  are capable of changing state. The rapid switching light source  910  may be a single LED or an array of LEDs. The array of LEDs may be made up of LEDs of a single color or differently colored LEDs may be used. Furthermore, the rapid switching light source  910  is capable of producing light at various intensities. Light from the rapid switching light source  910  reflects from the spatial light modulator  910  and onto a display screen  915 . 
     A sequence controller  920  can provide instructions to the rapid switching light source  910  to control LED state. The sequence controller  920  can also access a memory  925 , which can contain the data (pixel information) of images to be displayed via the spatial light modulator  905 . A reset controller  930 , also controlled by instructions provided by the sequence controller  920 , places the spatial light modulator  905  into a mode that allows it to accept new state change instructions from the sequence controller  920 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.