Patent Publication Number: US-2020301139-A1

Title: Head-up display system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/822,464, filed Mar. 22, 2019, and U.S. Provisional Application No. 62/836,311, filed Apr. 19, 2019, each of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Various display systems may benefit from a suitable implementation of polarization. For example, a fast polarization switching apparatus can provide a polarized head-up display picture generation unit with vertical polarization and horizontal polarization output in a frame. 
     BACKGROUND 
     In a head-up display, a display mechanism can generate an image that is projected onto a screen, such as a windshield of a car. The display mechanism can have an adjustable brightness mechanism. The display mechanism can include a polarization system. For example, a fixed half-wave plate (HWP) or quarter-wave plate (QWP) can be used to provide the polarization. These approaches can introduce color shifts, produce visible interference flicker and are vulnerable to solar-induced internal heating. 
     SUMMARY 
     This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features, aspects or objectives. 
     A head-up display system is provided herein. The head-up display system includes a display device, a liquid crystal cell and a driver circuit. The display device is configured to generate a plurality of images at a frame rate. Each of the plurality of images includes a plurality of image segments with an initial polarization. The liquid crystal cell is disposed between the display device and a user of the head-up display system. The liquid crystal cell includes a plurality of addressable segments that are aligned with the plurality of image segments. The plurality of addressable segments are individually controllable to selectively adjust the initial polarization of the plurality of image segments between two different polarizations. 
     The driver circuit is in communication with the display device and the liquid crystal cell. The driver circuit is configured to control the plurality of addressable segments to pass each of the plurality of image segments that has visible content to the user with the initial polarization switched between the two different polarizations at a frequency greater than the frame rate of the plurality of images and attenuate sunlight that is incident on the display device at each of the plurality of image segments that has blank content. 
     A head-up display system is provided herein. The head-up display system includes a display device, a liquid crystal cell and a driver circuit. The display device is configured to generate a plurality of images at a frame rate and with an initial polarization. The liquid crystal cell is disposed between the display device and a user of the head-up display system. The liquid crystal cell is controllable to selectively adjust the initial polarization of the plurality of images between two different polarizations. 
     The driver circuit is in communication with the display device and the liquid crystal cell. The driver circuit is configured to control the liquid crystal cell to pass the plurality of images to the user with the initial polarization switched between the two different polarizations at a frequency greater than the frame rate of the plurality of images. 
     A head-up display system is provided herein. The head-up display system includes a display device, a liquid crystal cell and a driver circuit. The display device is configured to generate a plurality of images. Each of the plurality of images includes a plurality of image segments with an initial polarization. The liquid crystal cell is disposed between the display device and a user of the head-up display system. The liquid crystal cell includes a plurality of addressable segments that are aligned with the plurality of image segments. The plurality of addressable segments are individually controllable to selectively adjust the initial polarization of the plurality of image segments between two different polarizations. 
     The driver circuit is in communication with the display device and the liquid crystal cell. The driver circuit is configured to control the plurality of addressable segments to pass each of the plurality of image segments that has visible content to the user and attenuate sunlight that is incident on the display device at each of the plurality of image segments that has blank content. 
     In one or more embodiments of the head-up display system, the liquid crystal cell includes a reflective polarizer configured to reflect the sunlight that has one of the two different polarizations. 
     The above features and advantages and other features and advantages of the present teachings are readily apparent from the following detailed description of the best modes for carrying out the teachings when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of a vehicle having a head-up display system in accordance with one or more embodiments. 
         FIG. 2  illustrates a schematic diagram of a screen in accordance with one or more embodiments of the head-up display system. 
         FIG. 3  illustrates a schematic block diagram of a picture generation unit in accordance with one or more embodiments of the head-up display system. 
         FIG. 4  illustrates a flow diagram of a method of operation in accordance with one or more embodiments of the head-up display system. 
         FIG. 5  illustrates a graph of waveforms for a normal mode in accordance with one or more embodiments of the head-up display system. 
         FIG. 6  illustrates a graph of waveforms for a polarized sunglasses mode in accordance with one or more embodiments of the head-up display system. 
         FIG. 7  illustrates a schematic diagram of a segmented twisted nematic cell in accordance with one or more embodiments of the head-up display system. 
         FIG. 8  illustrates a schematic diagram of a picture generation unit in a non-rotation state in accordance with one or more embodiments of the head-up display system. 
         FIG. 9  illustrates a schematic diagram of the picture generation unit in a mixed-rotation state in accordance with one or more embodiments of the head-up display system. 
         FIG. 10  illustrates a schematic diagram of another picture generation unit in the rotation state in accordance with one or more embodiments of the head-up display system. 
         FIG. 11  illustrates a schematic diagram of the other picture generation unit in the mixed-rotation state in accordance with one or more embodiments of the head-up display system. 
         FIG. 12  illustrates a schematic block diagram of a driver circuit and a liquid crystal cell in accordance with one or more embodiments of the head-up display system. 
     
    
    
     DETAILED DESCRIPTION 
     Those having ordinary skill in the art will recognize that terms such as “above,” “below,” “front,” “back,” “upward,” “downward,” “top,” “bottom,” etc., may be used descriptively herein without representing limitations on the scope of the disclosure. Furthermore, the present teachings may be described in terms of functional and/or logical block components and/or various processing steps. Such block components may be comprised of various hardware components, software components executing on hardware, and/or firmware components executing on hardware. 
     Embodiments of the disclosure may have various modifications and alternative forms, and some representative embodiments are shown by way of example in the drawings and will be described in detail herein. Novel aspects are not limited to the particular forms illustrated in the above-enumerated drawings. Rather, the disclosure is to cover modifications, equivalents, and combinations falling within the scope of the disclosure. 
     Certain embodiments generally relate to a head-up display system mountable in a motor vehicle. The head-up display system may be, for example, a fast polarization switching apparatus that provides polarized head-up display images (or pictures) with both vertical polarization and horizontal polarization. A frequency that the head-up display system switches between the vertical polarization and the horizontal polarization may be a higher frequency than the frame rate of the images being projected to a user (e.g., an occupant or a driver of the vehicle) to avoid an interference flicker perceptible to the user. 
     In various embodiments, the head-up display system may include an individually addressable segmented liquid crystal cell (or layer). The addressable segments may be independently controlled to rotate light or reflect light. In particular, some segments may be controlled to transmit portions of images generated by a display that contain visual content for the user. Other segments may be controlled to reflect sunlight entering the head-up display system from outside the vehicle. By reflecting some of the sunlight, a display device within the head-up display system may operate with a lower thermal load from the sun. 
     The human visual system generally experiences persistence of vision. More particularly, the human brain feels an illusion from a period of visual perception. In that period, the head-up display system may provide a vertical polarization state for a portion of the period and provide a horizontal polarization state in the other portion of the period. When the user is not wearing polarized sunglasses, the brain perceives intermediate brightness level from both the vertical polarization state brightness and the horizontal state brightness. When the user is wearing the polarized sunglasses, the brain perceives brightness mainly from the vertical polarization state brightness. The head-up display system may change the duty cycle of the vertical polarization state relative to the horizontal polarization state to better optimize the brightness for either a normal usage user or a polarized sunglasses wearing user. 
       FIG. 1  illustrates a schematic diagram of an example implementation of a vehicle  90  in accordance with one or more embodiments. The vehicle  90  generally comprises a head-up display system  92  positioned before the user  94 . The user  94  may optionally be wearing polarized sunglasses  96 . In various embodiments, the vehicle  90  may include a windshield  150 . The head-up display system  92  may comprise a picture generation unit  100 , a driver circuit  130 , an optical setup  140  and the windshield  150 . 
     A control signal (e.g., CNT) may be generated by the driver circuit  130  and transferred to the picture generation unit  100 . The control signal CNT may convey control information used to control the operations of the picture generate unit  100 . An optical signal (e.g., IMG) may be generated by the picture generation unit  100  and transferred to the user  94  through the optical setup  140 , reflected off the windshield  150 , pass through the polarized sunglasses  96  and seen by the user  94 . The optical signal IMG may carry a sequence of images at a frame rate. Another optical signal (e.g., SL) may be received by the head-up display system  92  from outside the vehicle  90 . The signal SL may be sunlight streaming into the optical setup  140  through the windshield  150 . The optical setup  140  may direct the sunlight SL to the picture generation unit  100 . The sunlight SL generally has multiple polarized states (e.g., a vertical polarized state and a horizontal polarized state). The terms vertical polarization and the horizontal polarization may be determined relative to the ground on which the vehicle  90  is situated. 
     The vehicle  90  may include mobile vehicles such as automobiles, trucks, motorcycles, boats, trains and/or aircraft. The vehicle  90  may receive the sunlight SL from the sun through at least the windshield  150 . 
     The head-up display system  92  may be implemented as a windshield head-up display or a screen head-up display. The head-up display system  92  is generally operational to generate and present visual information in the optical signal IMG to the user  94 . The visual information may include various data regarding the operation of the vehicle  90  in the forms of graphics, texts, and the like. The head-up display system  92  may receive a portion of the sunlight SL while the sunlight SL is entering the windshield  150 . 
     The polarized sunglasses  96  may be employed by the user  94  in some situations. For example, the user  94  may wear the polarized sunglasses  96  on bright sunny days. The polarized sunglasses are generally operational to pass light (e.g., the optical signal IMG and the sunlight SL) having the vertical polarization. Light having non-vertical polarization may be attenuated by the polarized sunglasses  96  while the eyes of the user  94  are oriented horizontally relative to each other. 
     The picture generation unit  100  may implement a projection device. The picture generation unit  100  is generally operational to generate and present the optical signal IMG in response to the control information received in the control signal CNT. The picture generation unit  100  may be aligned with the optical setup  140  to transfer the optical signal IMG. The sunlight SL received by the picture generation unit  100  generally causes localized heating within the picture generation unit  100 . To help reduce the heating, the picture generation unit  100  may be operable in one or more modes to partially reflect the sunlight SL where and whenever possible. 
     The driver circuit  130  may implement one or more microcontrollers. The driver circuit  130  is generally operational to control the picture generation unit  100  to create the optical signal IMG. The driver circuit  130  may be configured to alter a duty cycle of the picture generation unit  100  based on an active mode (e.g., a polarized sunglasses mode or a normal mode) in the head-up display system  92 . 
     Each microcontroller may include one or more processors, each of which may be embodied as a separate processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a dedicated electronic control unit. 
     The microcontrollers may be any sort of electronic processors (implemented in hardware, software executing on hardware, or a combination of both). The microcontrollers may also include tangible, non-transitory memory, (e.g., read only memory in the form of optical, magnetic, and/or flash memory). For example, a microcontroller may include application-suitable amounts of random-access memory, read-only memory, flash memory and other types of electrically-erasable programmable read-only memory, as well as accompanying hardware in the form of a high-speed clock or timer, analog-to-digital and digital-to-analog circuitry, and input/output circuitry and devices, as well as appropriate signal conditioning and buffer circuitry. 
     Computer-readable and executable instructions embodying the present disclosure may be stored in the memory and executed as set forth herein. The executable instructions may be a series of instructions employed to run applications on the microcontrollers (either in the foreground or background). The microcontrollers may receive commands and information, in the form of one or more input signals from various controls or components in the vehicle  90  and communicate the commands to the picture generation unit  100  through the control signal CNT. 
     The optical setup  140  may implement one or more mirrors and/or lenses that direct the optical signal IMG onto the windshield  150 . The mirrors may be curved to adjust a focal point of the images. 
     The windshield  150  implement an automotive windshield. The windshield  150  may be operational to reflect the images received from the optical setup  140  toward the user  94 . The windshield  150  is transparent and so allows the sunlight SL to enter an interior of the vehicle  90 . Because of an inclination of the windshield  150 , the vertical component (or P-component) of the sunlight energy may be larger than the horizontal component (or S-component), in a ratio of approximately 1.75:1. 
       FIG. 2  illustrates a schematic diagram of an example implementation of a screen  152  in accordance with one or more embodiments of the head-up display system  92 . In some embodiments of the head-up system  92 , the screen  152  may be disposed between the optical setup  140  and the windshield  150 . The screen  152  may provide an optical surface from which the optical signal IMG is reflected toward the user  94 . Since the screen  152  is generally transparent for the sake of the user&#39;s forward field of view, the screen  152  also allows the sunlight SL to enter the optical setup  140 . 
       FIG. 3  illustrates a schematic block diagram of an example implementation of the picture generation unit  100  in accordance with one or more embodiments of the head-up display system  92 . The picture generation unit  100  generally comprises a display device  110  and a liquid crystal cell  120 . The display device  110  and the liquid crystal cell  120  may be in electrical communication with the driver circuit  130 . The optical signal IMG may be projected by the display device  110 , through the liquid crystal cell  120 , through the optical setup  140  to the windshield  150  or the screen  152 . 
     The display device  110  may implement an active display. The display device  110  is generally operational to generate and present a sequence of images in the optical signal IMG. The images may be generated at a frame rate controllable by the driver circuit  130 . The images may also have an initial polarization (e.g., an S-polarization or a P-polarization). In various embodiments, the display device  110  may be implemented as a thin-film transistor display device with a built-in backlight. In some embodiments, the display device  110  may be implemented as a liquid crystal display device with a built-in backlight. Other display types, including, but not limited to, organic light emitting diode displays may also be implemented to meet the design criteria of a particular application. 
     The liquid crystal cell  120  may implement an active polarization modulator. The liquid crystal cell  120  is generally operational to rotate the initial polarization of the images in the optical signal IMG in response to a control voltage in the control signal CNT. In various embodiments, the liquid crystal cell  120  may be a twisted nematic liquid crystal cell. In other embodiments, the liquid crystal cell  120  may be a Pi-cell, a vertical alignment liquid crystal cell, or a ferroelectric liquid crystal cell. Other types of active polarization modulators may be implemented to meet the design criteria of a particular application. 
     The control voltage carried by the control signal CNT that corresponds to the liquid crystal cell  120  may be generated over a range of voltages (e.g., zero volts to 5 volts). The liquid crystal cell  120  may respond to the zero volts by passing the optical signal IMG without a rotation of the initial polarization. The liquid crystal cell  120  may respond to the 5 volts by rotating the initial polarization of the optical signal IMG by approximately 90 degrees. To achieve a rotation between zero degrees and 90 degrees, either an intermediate voltage may be applied to the liquid crystal cell  120  and/or the applied control voltage may be modulated between zero volts and 5 volts. For example, applying a 2.5 volt control signal may result in an approximately 45 degree rotation of the optical signal IMG. In another example, modulating the applied control voltage with a duty cycle of 50% at zero volts and 50% at 5 volts may be perceived by the user  94  as a 45 degree rotation of the polarization. 
       FIG. 4  illustrates a flow diagram of an example method  200  of operation in accordance with one or more embodiments of the head-up display system  92 . The operational method (or process)  200  may be implemented by the components of the head-up display system  92 . The operational method  200  generally comprises a step  202 , a step  204 , a step  206  and a step  208 . 
     In the step  202 , the display device  110  may be driven by the driver circuit  130  to create the optical signal IMG. The optical signal IMG may be generated at a frame rate established by the driver circuit  130 . The driver circuit  130  may control the liquid crystal cell  120  in the step  204  to switch between the two different polarizations at a frequency greater than the frame rate of the images in the optical signal IMG. For example, one of the polarizations may be a non-rotated polarization where the liquid crystal cell  120  passes the images in the optical signal IMG without altering the initial polarization of the images. The other polarization may be a rotated polarization where the liquid crystal cell  120  rotates the initial polarization of the images by approximately 90 degrees as the images transit the cell. 
     The switching frequency between the two different polarizations may be controlled to avoid a generation of visible interference flickering (or beat frequency) in the images as seen by the user  94 . In various embodiments, the switching frequency may be certain harmonics of the frame rate of the images. For example, the switching frequency may be twice that of the frame rate such that each image frame is transferred with the initial polarization for part of the frame period and with the rotated polarization for the remainder of the frame period. Various harmonic frequencies of the frame rate that would result in the flickering may be avoided. Other switching frequencies may be implemented to meet a design criteria of a particular application. 
     In the step  206 , the optical signal IMG may be transferred through the optical setup  140  and directed toward the windshield  150  and/or the screen  152 . The windshield  150  or the screen  152  may direct the optical signal IMG toward the user  94  in the step  208 . 
     The switching of the liquid crystal cell  120  in each image frame generally results in the optical signal IMG reaching the user  94  in both the horizontal polarization state and the vertical polarization state at slightly different times. The temporal shift between the two polarization states may be imperceptibly short to the user  94 . In a normal mode of operation of the head-up display system  92 , a duration of the horizontal polarization state and a duration of the vertical polarization state of the optical signal IMG may be approximately the same (e.g., a first duty cycle). Activation of the normal mode may be suitable while the user  94  is not wearing the polarized sunglasses  96 . 
     While the user  94  is wearing the polarized sunglasses  96 , the horizontally polarized components of the images are attenuated by the polarized sunglasses  96 . Therefore, the information being presented to the user  94  by the head-up display system  92  may appear dimmer than normal. To compensate for the presence of the polarized sunglasses  96 , the head-up display system  92  may include a polarized sunglasses mode. While the polarized sunglasses mode is active, the driver circuit  130  may control the switching frequency of the liquid crystal cell  120  to emphasize the vertical polarization state more often than the horizontal polarization state (e.g., a second duty cycle). For example, the images adjusted by the liquid crystal cell  120  may be presented to the user  94  with the vertical polarization a majority of the period (e.g., 70% to 100%) and with the horizontal polarization during the rest of the period (e.g., 0% to 30%). 
       FIG. 5  illustrates a graph  220  of example waveforms for the normal mode in accordance with one or more embodiments of the head-up display system  92 . The image frames may be illustrated by a waveform  222 . Each image frame may persist for a single period. Successive image frames (e.g., N, N+1, etc.) may be temporally adjacent (or adjoining) each other. The liquid crystal cell  120  may be controlled per a waveform  224 . During approximately a first half of each period, the liquid crystal cell  120  may be controlled to present the corresponding image such that the user  94  sees the vertical polarization. During approximately a second half of each period, the liquid crystal cell  120  may be controlled to present the corresponding image such that the user  94  sees the horizontal polarization. 
       FIG. 6  illustrates a graph  230  of example waveforms for the polarized sunglasses mode in accordance with one or more embodiments of the head-up display system  92 . The image frames may be illustrated by the waveform  222 . The liquid crystal cell  120  may be controlled per a waveform  226 . During a majority of the first half of each period, the liquid crystal cell  120  may be controlled to present the corresponding image such that the user  94  sees the vertical polarization. During the minority remainder of each period, the liquid crystal cell  120  may be controlled to present the corresponding image such that the user  94  sees the horizontal polarization. 
       FIG. 7  illustrates a schematic diagram of an example implementation of a twisted nematic cell  240  in accordance with one or more embodiments of the head-up display system  92 . The twisted nematic cell  240  may be used in the liquid crystal cell  120 . In some embodiments, the twisted nematic cell  240  may comprise several addressable segments. The addressable segments may include a first addressable segment  242 , a second addressable segment  244 , a third addressable segment  246 , a fourth addressable segment  248 , a fifth addressable segment  250  and a sixth addressable segment  252 . Other numbers and spatial arrangements of the addressable segments  242 - 252  may be implemented to meet a design criteria of a particular application. 
     Each addressable segment  242 - 252  may have a size and position within the twisted nematic cell  240  to align with information presented in corresponding image segments of the images. For example, the first addressable segment  242  may be aligned with a speedometer graphic image while the second addressable segment  244  may be aligned with a tachometer graphic image. 
     Each addressable segment  242 - 252  may be independently controlled by the control signal CNT to either the rotation state or the non-rotation state. The addressable segments  242 - 252  may respond to the zero volts by passing the corresponding image segment in the optical signal IMG without a rotation of the initial polarization. The addressable segments  242 - 252  may respond to the 5 volts by rotating the initial polarization of the corresponding image segment of the optical signal IMG by approximately 90 degrees. To achieve a rotation between zero degrees and 90 degrees, either an intermediate voltage may be applied to the addressable segments  242 - 252  and/or the applied voltage may be modulated between zero volts and 5 volts. 
     In some situations (e.g., the normal mode), the addressable segments  242 - 252  may be controlled together such that the user  94  sees entire images in the optical signal IMG with the horizontal polarization and the vertical polarization. In other situations (e.g., a cool mode), some of the addressable segments  242 - 252  may be held in just one of the rotation state or the non-rotation state. For example, while the image segments that corresponds to the addressable segment  246  and  248  do not contain any visible information, the addressable segments  246  and  248  may be controlled to cause the sunlight SL from reaching the display device  110 . Keeping parts of the display device  110  shaded from the sunlight SL generally helps the display device  110  to remain cooler than if the sunlight SL were allowed through the addressable segments  246  and  248 . 
     In the normal mode, the addressable segments  242 - 252  that have corresponding image segments with visible content may be switched between the rotation state and the non-rotation state at the first duty cycle (e.g., 50% V-50% H). Therefore, the user  94  not wearing the polarized sunglasses  96  may perceive the visible content in the vertical polarized state and the horizontal polarized state. 
     In the (first) polarized sunglasses mode, the addressable segments  242 - 252  that have corresponding image segments with visible content may be switched between the rotation state and the non-rotation state with the second duty cycle (e.g., 80% V-20% H). Therefore, the user  94  may perceive the visible content in predominantly the vertical polarized state (which easily passes through the polarized sunglasses  96 ). 
     In a second polarized sunglasses mode, the addressable segments  242 - 252  that have corresponding image segments with visible content may be controlled to one of the rotation state or the non-rotations state with a third duty cycle (e.g., 100% V-0% H). Therefore, the user  94  while wearing the polarized sunglasses  96  may perceive the visible content in solely the vertical polarized state. 
       FIG. 8  illustrates a schematic diagram of an example implementation of a picture generation unit  100   a  in the non-rotation state in accordance with one or more embodiments of the head-up display system  92 . The picture generation unit  100   a  generally comprises a display device  110   a  and a liquid crystal cell  120   a . The liquid crystal cell  120   a  may comprise a reflective polarizer  122   a  and a twisted nematic cell  124 . The picture generation unit  100   a  may be a variation of the picture generation unit  100 . The display device  110   a  may be a variation of the display device  110 . The twisted nematic cell  124  may be a variation of the twisted nematic cell  240 . 
     The display device  110   a  may implement a display that generates the optical signal IMG with the S-polarization (or horizontal polarization). The display device  110   a  is generally controlled by the driver circuit  130  to present visible information arranged in predetermined positions and/or areas that define the multiple image segments. In the example, the display device  110   a  is illustrated with an “on” image segment  112  having visible content and an “off” image segment  114  having no visible content (e.g., no light emitted or blank). The image segment  112  may contribute to the optical signal IMG that is projected toward the reflective polarizer  122   a.    
     The reflective polarizer  122   a  may be operational to pass S-polarized light (or horizontally polarized) and reflect P-polarized (or vertically polarized) light. In the example, the optical signal IMG generated by the “on” image segment  112  of the display device  100   a  may pass through the reflective polarizer  122   a . The sunlight SL entering the picture generation unit  100   a  may be essentially P-polarized and so is reflected by the reflective polarizer  122   a.    
     The twisted nematic cell  124  may have multiple addressable segments. The example illustrates addressable segments  126  and  128 . The addressable segment  126  is aligned with the image segment  112 . The addressable segment  128  is aligned with the image segment  114 . While the addressable segments  126  and  128  are in an “on” state, the addressable segments  126  and  128  do not rotate the polarization of the optical signal IMG and the polarization of the sunlight SL. Therefore, the addressable segment  126  passes the visible content in the image segment  112  of the optical signal IMG unrotated to the optical setup  140 . The addressable segments  126  and  128  pass the sunlight SL unrotated to the reflective polarizer  122   a . The reflective polarizer  122   a  subsequently reflects the P-polarized sunlight SL back to the addressable segments  126  and  128 . The addressable segments  126  and  128  again pass the P-polarized sunlight SL out of the picture generation unit  100   a . Therefore, the P-polarized sunlight SL does not contribute to a heating of the display device  110   a.    
       FIG. 9  illustrates a schematic diagram of an example implementation of the picture generation unit  100   a  in a mixed-rotation state in accordance with one or more embodiments of the head-up display system  92 . The addressable segment  128  of the twisted nematic cell  124  may remain in the on (or non-rotation) state and the addressable segment  126  may be controlled to an off (or rotation) state. 
     The optical signal IMG may be generated by the image segment  112  of the display device  110   a  with the S-polarization. The S-polarized optical signal IMG may pass through the reflective polarizer  122   a . While transiting through the “off” addressable segment  126 , the S-polarization of the optical signal IMG may be rotated to P-polarization. Therefore, the user  94  receives the optical signal IMG with the P-polarization. 
     The P-polarized sunlight SL entering the “on” addressable segment  128  may continue through the twisted nematic cell  124  to the reflective polarizer  122   a  . The P-polarized sunlight SL may be reflected by the reflective polarizer  122   a  and back out of the head-up display system  92 . The P-polarized sunlight SL entering the “off” addressable segment  126  may be rotated by the twisted nematic cell  124  to the S-polarization. Therefore, the S-polarized sunlight SL passes through the reflective polarizer  122   a  and may radiantly heat the display device  110   a  in the content area. 
       FIG. 10  illustrates a schematic diagram of an example implementation of a picture generation unit  100   b  in the rotation state in accordance with one or more embodiments of the head-up display system  92 . The picture generation unit  100   b  generally comprises a display device  110   b  and a liquid crystal cell  120   b . The liquid crystal cell  120   b  may comprise a reflective polarizer  122   b  and the twisted nematic cell  124 . The picture generation unit  100   b  may be a variation of the picture generation unit  100 . The display device  110   b  may be a variation of the display device  110 . The twisted nematic cell  124  may be a variation of the twisted nematic cell  240 . 
     The display device  110   b  may implement a display that generates the optical signal IMG with the P-polarization (or vertical polarization). The display device  110   b  is generally controlled by the driver circuit  130  to present visible information arranged in predetermined positions and/or areas that define the multiple image segments. In the example, the display device  110   b  is illustrated with the “on” image segment  112  having visible content and the “off” image segment  114  having no visible content (e.g., no light emitted or blank). The image segment  112  may contribute to the optical signal IMG that is projected toward the reflective polarizer  122   b.    
     The reflective polarizer  122   b  may be operational to pass P-polarized (or vertically polarized) light and reflect S-polarized (or horizontally polarized) light. In the example, the optical signal IMG generated by the “on” image segment  112  of the display device  110   b  may pass through the reflective polarizer  122   b . The sunlight SL entering the picture generation unit  100   b  may be essentially P-polarized, is rotated to the S-polarization by the twisted nematic cell  124  and so is reflected by the reflective polarizer  122   a.    
     The twisted nematic cell  124  may have multiple addressable segments. The example illustrates addressable segments  126  and  128 . The addressable segment  126  is aligned with the image segment  112 . The addressable segment  128  is aligned with the image segment  114 . While the addressable segments  126  and  128  are in the “off” state, the addressable segments  126  and  128  rotate the polarization of the optical signal IMG and the polarization of the sunlight SL. Therefore, the addressable segment  126  rotates the visible content in the image segment  112  of the optical signal IMG from the P-polarization to the S-polarization. The addressable segments  126  and  128  rotate the sunlight SL from the P-polarization to the S-polarization. The reflective polarizer  122   b  subsequently reflects the S-polarized sunlight SL back to the addressable segments  126  and  128 . The addressable segments  126  and  128  rotate and pass the sunlight SL out of the picture generation unit  100   a . Therefore, the P-polarized sunlight SL does not contribute to heating the display device  110   b.    
       FIG. 11  illustrates a schematic diagram of an example implementation of the picture generation unit  100   b  in the mixed-rotation state in accordance with one or more embodiments of the head-up display system  92 . The addressable segment  128  of the twisted nematic cell  124  may remain in the off (or rotation) state and the addressable segment  126  may be controlled to an on (or non-rotation) state. 
     The optical signal IMG may be generated by the image segment  112  of the display device  110   b  with the P-polarization. The P-polarized optical signal IMG may pass through the reflective polarizer  122   b . While transiting through the “on” addressable segment  126 , the P-polarization of the optical signal IMG may remain unrotated. Therefore, the user  94  receives the optical signal IMG with the P-polarization. 
     The P-polarized sunlight SL entering the “off” addressable segment  128  may be rotated by the twisted nematic cell  124  from the P-polarization to the S-polarization. The S-polarized sunlight SL may be reflected by the reflective polarizer  122   b  and back out of the head-up display system  92 . The P-polarized sunlight SL entering the “on” addressable segment  126  may remain unrotated by the twisted nematic cell  124 . Therefore, the P-polarized sunlight SL passes through the reflective polarizer  122   b  and may radiantly heat the display device  110   b  in the content area. 
       FIG. 12  illustrates a schematic block diagram of an example implementation of the driver circuit  130  and the liquid crystal cell  120  in accordance with one or more embodiments of the head-up display system  92 . The driver circuit  130  generally comprises a liquid crystal cell controller  132  and a display controller  134 . The liquid crystal cell  120  generally comprises the twisted nematic cell  124  and a reflective polarizer  122 . The reflective polarizer  122  may be representative of the reflective polarizer  122   a  and/or the reflective polarizer  122   b.    
     The liquid crystal cell controller  132  may be in electrical communication with the twisted nematic cell  124  and the display controller  134 . The liquid crystal cell controller  132  is generally operational to control the individual addressable segments of the twisted nematic cell  124 . The liquid crystal cell controller  132  may coordinate with the display controller  134  to control the addressable segments of the twisted nematic cell  124  to switch the polarization of the active image segments having visible content. The liquid crystal cell controller  132  may control the addressable segments corresponding to the blank image segments based upon the mode of the head-up display system  92 . For example, in the normal mode the liquid crystal cell controller  132  may control the addressable segments of the twisted nematic cell  124  the same with the first duty cycle so that all of the image segments are treated the same. In a secondary normal mode, the liquid crystal cell controller  132  may control the addressable segments of the twisted nematic cell  124  corresponding to the blank image segments to set the polarization of the sunlight SL to be reflected by the reflective polarizer  122 . In the sunglasses mode, the liquid crystal cell controller  132  may control the addressable segments of the twisted nematic cell  124  the same with the second duty cycle so that all of the image segments are treated the same. In the secondary polarized sunglasses mode, the liquid crystal cell controller  132  may control the addressable segments of the twisted nematic cell  124  corresponding to the blank image segments to set the polarization of the sunlight SL to be reflected by the reflective polarizer  122 . 
     The display controller  134  may be implemented as a video display driver. The display controller  134  may be in electrical communication with the display device  110  and the liquid crystal cell controller  132 . The display controller  134  is generally operational to present the images to the display device  110  at the frame rate in all modes. The display controller  134  may also inform the liquid crystal cell controller of which image segments of the images contain visible content. The information may include an indication of a current display mode comprising one or more of an all on mode, the normal mode, the secondary normal mode, an alternative mode, a lane keep assist (LKA) mode, a second LKA mode, a navigation mode, the sunglasses mode and/or the secondary sunglasses mode. 
     The locally-switched twisted nematic cell  124  in the picture generation unit  100  may operate in one or more of the different modes. In some embodiments of the head-up display system  92 , the modes may be selected by the user  94 . In other embodiments, a detection method may be used to detect when the user  94  is wearing sunglasses and conclude that the sunglasses are the polarized sunglasses  96 . 
     When used in a windshield-reflecting head-up display system  92 , the sunlight SL received by the optical setup  140  may result in heating of the display device  110 . As a result of the windshield inclination, the P-component (vertically polarized) of the sunlight energy is generally larger than the S-component (horizontally polarized), in a ratio of approximately 1.75:1. Due to the ratio, blocking the P-component of the sunlight energy may result in a more effective cooling of the display device  110  than blocking the S-component. By dividing the twisted nematic cell  124  into several addressable segments (and thereby allowing local switching), the addressable segments overlapping with or corresponding to areas of no content on the display device  110  may remain in the S-polarization state thus attenuating the sunlight SL intrusion and reducing solar heating of the display device  110 . 
     The above-described approaches may be combined with further sun radiation mitigation strategies. For example, the display device  110  may be placed in an area that is expected to seldom receive direct sunlight, such as embedded deeply within a dashboard and pointing upward toward the dashboard. Furthermore, an intermediate mirror may be used, which may reduce the thermal load to the display device  110 . Additionally, infrared absorbent material may be placed at a location toward which the display device  110  faces (e.g., in the optical setup  140 ). Other modifications, adaptations, and variants may be implemented to meet the design criteria of a particular application. 
     According to certain embodiments, the head-up display system  92  may include a picture generation unit  100 / 100   a / 100   b . The head-up display system  92  may also include a liquid crystal cell  124 / 124   a / 124   b  disposed in front of the display device  110  in a picture display direction. The head-up display system  92  may further include a driver circuit  130  configured to drive the liquid crystal cell  120  at a switching frequency greater than a switching frequency of the frame rate of the display device  110 . 
     In various embodiments, the driver circuit  130  may be configured to provide both a vertical polarization state and a horizontal polarization state of the optical signal IMG exiting the liquid crystal cell  120  in a single picture frame period of the picture generation unit  100 . The driver circuit  130  may be configured to alter a duty cycle of the addressable segments in the liquid crystal cell  120  based on which of the multiple modes is currently active. 
     Certain embodiments may relate to a method. The method may be configured to operate in accordance with the various system embodiments described above. The method generally includes, displaying a sequence of images using the picture generation unit  100 . The images may include the entire visible content of the head-up display system  92 , including graphics, texts, and the like. 
     The method may further include driving a liquid crystal cell  120  disposed in front of the display device  110  in the picture display direction. The driving may be performed at a switching frequency greater than frame rate of the optical signal IMG. The switching frequency of the liquid crystal cell  120  generally prevents visible interference flicker in the optical signal IMG. 
     The driving may include providing both a vertical polarization state of and a horizontal polarization state of the optical image IMG in a single picture frame period of the picture generation unit  100 . The method may further include altering a duty cycle of the liquid crystal cell  120  based on an active mode of the head-up display system  92 . The method generally includes determining whether to use the polarized sunglasses mode or the secondary sunglasses mode. The alteration of the duty cycle may be dependent on whether polarized sunglasses mode or the secondary sunglasses mode is active. 
     The method may be implemented by a computer processor. For example, a non-transitory computer-readable medium may be encoded with instructions that, when executed in hardware, perform a process. The process generally includes displaying an image using a display device  110 . The process may drive the liquid crystal cell  120  disposed in front of the display device  110  in the picture display direction. The driving may be performed at a switching frequency greater than a frame rate of the display device  110 . 
     Certain embodiments are directed toward a new twisted nematic cell configuration that may reduce the heating of the head-up display system  92 . More generally, certain embodiments generally relate to any liquid crystal cell configuration, of which twisted nematic (TN), vertically aligned (VA), in-plane switching (IPS) are examples. Any other liquid crystal (LC) modes that change the polarization angle, for example changes the polarization by 90 degrees may also be implemented. Heating of the head-up display system  92  may occur due to the sunlight SL intrusion through the twisted nematic cell  124  to the display device  110 . By dividing the nematic cell  124  into several segments and overlapping the segments with areas of no content on the display device  110 , the heating of the display device  110  may be reduced, as the areas of no content on the display device  110  may still be protected from the sunlight SL intrusion. 
     The foregoing detailed description and the drawings are supportive and descriptive of the disclosure, but the scope of the disclosure is defined solely by the claims. As will be appreciated by those of ordinary skill in the art, various alternative designs and embodiments may exist for practicing the disclosure.