Patent Publication Number: US-2010118064-A1

Title: Driving Liquid Crystal Materials Using Low Voltages

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     This application is a divisional of U.S. patent application Ser. No. 10/675,648, filed Sep. 30, 2003. 
    
    
     BACKGROUND  
     The present invention relates to driving liquid crystal materials, and more particularly to driving such materials using low voltage techniques. 
     Various liquid crystal (LC) displays (LCDs), such as twisted-nematic (TN) mode LC (TNLC) displays, use a varying voltage ramp to drive the LC material to a selected gray scale level (i.e., percentage of optical rotation of polarized light). This varying voltage ramp is typically in excess of 5 volts. Since the desired result is a gray scale video rate image, a frame time of 16.7 milliseconds (ms) (i.e., 60 Hertz (Hz)) is required. Typical TNLC materials take several milliseconds to switch from one state to another, and barely respond at video rates, and therefore do not provide a response time needed to react to very short voltage pulses. 
     Thus, acceptable drive schemes for TNLC displays utilize full frame time variable voltage analog or analog-like systems. However, such analog or amplitude modulated (AM) systems suffer from performance issues. These performance issues include the need for high voltages (greater than 5 volts, and typically 7.5 volts) for driving the display and gray scale performance (i.e., shading steps from black to white) that is non-linear. Further, this non-linear performance curve has significant changes in voltage position and slope (i.e., change per voltage increment) over small temperature changes, and thus deleteriously affects display performance. 
     Also a variable voltage (VV) drive is extremely sensitive to capacitive effects such as those caused by pixels that have dielectric coatings over electrode surfaces. This typically causes an offset of performance as the LC cell is driven in one electrical polarity versus the reverse polarity. Thus a typical VV drive scheme provides an optical response that is very asymmetrical. This asymmetrical response can lead to charge buildup in the cell, causing image sticking and may further result in damage to the LC display over time. Thus a need exists to provide a display to overcome these drawbacks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross section of a liquid crystal cell in accordance with one embodiment of the present invention. 
         FIG. 2  is a graphical representation of relative output versus percentage of pulse width modulation of a display in accordance with one embodiment of the present invention. 
         FIG. 3  is a graphical representation of relative intensity versus pulse width modulation drive voltage of a display in accordance with one embodiment of the present invention. 
         FIG. 4  is a block diagram of a display system in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In various embodiments of the present invention, displays may be driven using variable width square wave pulses. In such manner, the pulses may be used to select a desired gray scale level at which an LC material of the display operates. In certain embodiments, low voltage pulse width modulated (LV-PWM) signals may be used to control a TNLC material, for example. While the voltage used to control a display may vary in different embodiments, a signal of less than five volts may be desired, and in certain embodiments a signal having a voltage of between approximately 2.0 volts and 4.3 volts may be used. As used herein, the term “low voltage” means a voltage of less than 5 volts. In particular embodiments, a signal of less than or equal to 3.3 volts may be desired, as current device design rules used in the integrated circuit (IC) industry are generally limited to a maximum of 3.3 volts. 
     The PWM signals may vary in different embodiments, but in certain embodiments a duty cycle of between approximately 40 and 90% may be used, and in particular embodiments, a duty cycle between approximately 70-85% may be used. In such manner, the LC material may respond with a fast rise time and a fast full time. 
     In certain embodiments, pixel array electronics may be utilized in accordance with current IC device rules. In such manner, a path to increased integration and fully digital drive electronics may be realized. Thus in certain embodiments, displays may be fabricated with pixel level digital drivers, and may be refreshed with frame updates, rather than progressive scanned updates to the pixels. Such frame updates may occur nearly instantaneously, giving a display a much higher duty cycle and making the resulting image much brighter. 
     In certain embodiments, a LV-PWM drive scheme in accordance with an embodiment of the present invention may exhibit much less sensitivity to capacitive effects, and provide an optical output from the LC cell that is substantially symmetrical. A PWM drive in accordance with an embodiment of the present invention may result in a nearly linear gray level response from high-speed TNLC materials driven in a bi-refringence mode. This linear response may eliminate the need for a lookup table for LC material response. 
     Referring now to  FIG. 1 , shown is a cross-sectional view of a liquid crystal cell or display in accordance with one embodiment of the present invention. More specifically, cell  100  shown in  FIG. 1  is a liquid crystal on silicon (LCOS) display, although the scope of the present invention is not limited in this regard. 
     As shown in  FIG. 1 , cell  100  includes a liquid crystal material  110  that is enclosed between a first substrate  120  and a second substrate  130 . In one embodiment of a LCOS display, first substrate  120  may be a silicon backplane and second substrate  130  may be a cover glass. Liquid crystal material  110  may be enclosed by sealing members  115  which, in one embodiment, may be epoxy seals. 
     While not shown in  FIG. 1 , it is to be understood that pixel elements of cell  100  may be formed by providing a layer of reflective material, such as micromirrors patterned over first substrate  120  (and over control electrodes patterned thereon). Further, such pixel elements may also include a layer of antireflective material patterned over the reflective layer. Alternately in certain embodiments, second substrate  130  may include an antireflection coating. In one embodiment, a reflective layer may be formed from aluminum, for example, and an antireflective layer may be formed from silicon dioxide (SiO 2 ) and/or silicon nitride (Si 3 N 4 ), for example. 
     In a LCOS display, first substrate  120  may have electrodes patterned thereon to provide control signals that drive the display. Also, second substrate  130  may include an electrode (e.g., an indium tin oxide (ITO) ground electrode) such that an electric field may be established between electrodes on first substrate  120  and glass  130 . In certain embodiments, first substrate  120  and second substrate  130  may include alignment layers, such as polyimide layers. 
     In various embodiments, liquid crystal material  110  may be a twisted-nematic material, super twisted nematic material, ferroelectric liquid crystal material, surface-stabilized ferroelectric liquid crystal material, polymer dispersed liquid crystal material, electro-chromic liquid crystal material and the like. In certain embodiments, vertical aligned nematic (VAN), hybrid aligned nematic, electrically controlled birefringence, pi-cell or other alignment modes may be used. 
     Also shown in  FIG. 1  is a partial polarization rotation retardation material  140  which may be placed above second substrate  130  to provide a partial rotation of polarized light. While shown as being directly located on second substrate  130 , in certain embodiments an air gap may be present between second substrate  130  and material  140  via use of spacers. In certain embodiments, a user-controlled adjustment mechanism may be provided for a user to control the gap, thus controlling the rotation angle. In certain embodiments, the partial wave retardation material may be used to enhance the performance of the LC material by providing a complete rotation of polarized light from the incident polarization alignment to an output of 90 degrees to the incident alignment. In one such embodiment, a retardation film having less than a quarter wave retardation may be used. Alternately, the material may be deposited onto second substrate  130 , such as by a spin coating process. 
     The material used for partial wave retardation may vary in different embodiments. In one embodiment, the material may be an acetate material, such as a stretched acetate. For example, in a VAN mode display, a triacetyl-cellulose (TAC) film may be used. The thickness of such a film (or films) may vary, but in certain embodiments the thickness may be between approximately 60 nanometers (nm) and approximately 240 nm, and a film of 180 nm may be used in one desired embodiment. In another embodiment, the material may be a polycarbonate material. In different embodiments, a film may be between 0.2 mils and 1.0 mils thick, and in certain desired embodiments, may be between approximately 0.5 mils and 0.8 mils. Also, the amount of wave retardation may vary in certain embodiments. While in certain embodiments the amount of retardation may be minimal, in other embodiments polarization may be retarded by up to approximately a quarter wave. In certain embodiments however, the actual wave retardation may be much less than a quarter wave for example, ⅛ or 1/16. 
     Also, while shown in  FIG. 1  as being coupled to second substrate  130 , in other embodiments partial wave retardation material  140  may be coupled to adjacent optics, such as a prism of an optical engine (not shown in  FIG. 1 ). In other embodiments, such a retardation material need not be used, particularly if a drive with a voltage greater than approximately 4.0 volts, and desirably between approximately 4.0 and 4.5 volts is used. Further, such a retardation material need not be used with a VAN mode LC material. 
     As shown in  FIG. 1 , in operation incoming light L V  having a vertical polarization may be directed onto display  100 , and the resulting light L h  exiting display  100  may be reflected with a horizontal polarization, if no voltage is applied to the pixels of the display. In one embodiment, a display in accordance with an embodiment of the present invention may be a digitally driven LCOS microdisplay having an active matrix backplane. 
     While not shown in  FIG. 1 , it is to be understood that display  100  may include additional standard features of an LC display. For example, in certain embodiments (e.g., an LCD embodiment) one or both of first substrate  120  and second substrate  130  may include polarizing films having polarities separated by 90°. 
     In certain embodiments, a relatively thin cell gap may be provided in an LC material. The cell gap may be the thickness or distance between first substrate  120  and second substrate  130 , as shown in  FIG. 1 . Thus, the cell gap may refer to the thickness of the liquid crystal material sandwiched between respective enclosures. While cell gaps may vary in different embodiments, a cell gap of approximately 1 micron may be used in one embodiment, and the thickness of liquid crystal material  110  may vary between approximately 0.5 microns and 1.5 microns in other embodiments. As shown in  FIG. 1 , epoxy seals  115  (and spacers in the cell gap, not shown in  FIG. 1 ) may be used to form a cell gap such that a substantially uniform thickness exists in liquid crystal material  110 . 
     In such manner, a lower voltage may be used to drive the material, due to the relativity thin cell gap. The material may exhibit a faster response time for the same reason. 
     Referring now to  FIG. 2 , shown is a transition curve of a display in accordance with one embodiment of the present invention. As shown in  FIG. 2 , when driven using a LV-PWM signal, a linear response results and little change with temperature occurs. Specifically, at temperatures of 45° Celsius (C), 50° C., and 55° C., the relative output (i.e., relative intensity) is substantially linear as a function of a LV-PWM signal. Further, the relative output is approximately 0 at 100% duty cycle, and linearly progresses to approximately 8 (or 80%) at a 0% duty cycle. In such an embodiment, a single variable linear function may be used to fully compensate for the performance shift, instead of a multi-variable curve function lookup table, thus minimizing any necessary memory in the control system for thermal stabilization. 
     Such thermal stabilization may aid in performing rapid color gamut control. Because no tables are needed for temperature compensation, digital control information may be provided to a gamut control table to select desired color values. The output of such a gamut control table may be used in providing control signals to the electrodes of the display. 
     Referring now to  FIG. 3 , shown is a graphical representation of relative intensity versus PWM drive voltage in accordance with one embodiment of the present invention. As shown, a relative intensity of less than approximately 1 microwatt (μW) occurs with a drive voltage between approximately 3.0 volts and 3.3 volts, and a minimum relative intensity occurs at a drive voltage of approximately 3.15 volts. Also shown in  FIG. 3  is a point marked “AM”, which shows the minimum intensity of an analog voltage drive at approximately 7.5 volts on the same material in the same optical conditions as compared to a LV-PWM drive in accordance with one embodiment of the present invention. 
     In certain embodiments, the combination of a thin cell gap and a LV-PWM drive signal may provide a nearly linear gray scale response at video frame rates of 60 Hz. Further, the combination of a thin cell gap with a partial optical wave plate retarder and a LV-PWM drive signal may allow the LC material to respond with an optical response that is similar to the drive signal. Such an optical PWM output may result in a linear gray scale response that is nearly an image of the electrical drive pulse that generates it. In such manner, an optically digital response to an electrically digital drive may be effected. 
     Because of the high speed switching afforded by embodiments of the present invention, displays in accordance with various embodiments may be switched at speeds much higher than video rates (i.e., 60 Hz). For example, in certain embodiments, switching may be effected between approximately 120 Hz and 360 Hz, although the scope of the present invention is not limited in this regard. In such manner, a color sequential system may be implemented, and images of two or three colors may be provided out of a display during each video cycle. 
     LC cell  100  may be part of an optical projection device in one embodiment. In such an embodiment, LC cell  100  may be a LCOS light modulator, such as a LCOS cell to reflect a single color, such as red, green, or blue (or other color schemes). Alternately, a LCOS cell may be adapted to modulate light of two colors or three colors. It is to be appreciated that in other embodiments, LC cell  100  may be used in connection with other types of optical devices. These optical devices may include, but not be limited to, rear and front-end projectors, virtual near-to-eye devices, and the like. 
     In one embodiment, a LC cell may be used as a spatial light modulator (SLM), which is a multipixel opto-electronic device that modulates light intensity that is imaged by its pixels by reflecting (or in some embodiments by transmitting) controllable amounts of light independently at each pixel. In one embodiment, such an SLM may be a LCOS microdisplay. Alternately such an SLM may be a LCD, digital mirror device (DMD), grating light valve (GLV) or the like. 
     Referring now to  FIG. 4 , shown is a block diagram of a display system in accordance with one embodiment of the present invention. The display system may be any desired display such as a rear or front projection screen, a heads-up display, virtual near-to-eye device and the like. 
     As shown in  FIG. 4 , display system  200  may include one or more light sources  210 . The light sources may be any desirable form of light, including, for example arc or plasma lamps, lasers, light emitting diodes or the like. 
     As shown in  FIG. 4 , light source  210  is provided to optics  220 . Such optics may include, for example, a condensing lens, a shaping lens, and other optical devices. Also, in certain embodiments, optics  220  may include a color filter or a color switching mechanism, such as a color wheel, to provide one or more desired colors to LC cell  100 . Also in an embodiment in which LC cell  100  is LCOS cell, optics  220  may also include a polarizer to polarize incident light on LC cell  100 . 
     A driver board  230  may coupled to provide drive signals to LC cell  100  to modulate the incident light into a desired image. In one embodiment, driver board  230  may include a processor and one or more memories. Driver board  230  may be coupled to LC cell  100  via, for example, a flexible cable or the like. In various embodiments, the processor may be a general-purpose microprocessor, or a special-purpose processor such as a microcontroller, application specific integrated circuit (ASIC), a programmable gate array (PGA) and the like. Further, the memory or memories may be static random access memories (SRAMs), in one embodiment. Driver board  230  or another location in display system  200  may include one or more computer programs stored on a storage medium having instructions to operate the system in accordance with an embodiment of the present invention. The storage medium may include, but is not limited to, any type of disk including floppy disks, optical disks, compact disk read-only memories (CD-ROMs), compact disk rewritables (CD-RWs), and magneto-optical disks, semiconductor devices such as read-only memories (ROMs), random access memories (RAMs) such as dynamic and static RAMs, erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), flash memories, magnetic or optical cards, or any type of media suitable for storing electronic instructions. 
     In operation, the processor of driver board  230  may provide signals to the one or more memories to form a representation of an image. The memories of driver board  230  may act as buffers to store alternating frames of the image. In turn, each memory may be read out to LC cell  100  to enable electrodes controlling the cell to activate the desired pixel elements of the cell. In such manner, frame updates may be provided to LC cell  100 , thereby allowing high speed switching of images on LC cell  100 . 
     Light exiting LC cell lop having the formed image therein is provided to a projection lens  240 , which projects the image on a display screen  250 . In an embodiment using a LCOS cell, projection lens  240  may also include a polarizer to polarize the light. In one embodiment, projection lens  240  may also include a turning mirror to reflect projected light onto display screen  250 . 
     While shown in  FIG. 4  as including a single LC cell  100 , it is to be understood that in other embodiments a plurality of such cells may be present. For example, in one embodiment three LC cells may be present in a display system such that each cell is adapted to modulate light of a given color. In such manner, in a red, green, blue (R, G, B) color space each cell may receive incident light of a given color and provide a modulated image of the same color. In yet another embodiment, two LC cells may be present. In such an embodiment, for example, one LC cell may be dedicated to modulating green light, and the other cell may modulate both red and blue light. In such an embodiment, a color wheel having red and blue components may be located in the incident light path of the red and blue LC cell. In other embodiments, a single LC cell may be dedicated to the red color and a separate LC cell may modulate both blue and green colors. 
     In one embodiment, optics  220  may include a polarizing beam splitter (PBS), and light exiting LC cell  100  may be reflected back through the PBS of optics  220  and provided therefrom to projection lens  240 . 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.