Abstract:
A device and a method are provided for establishing a monochromatic background light source in an electronic device with a field sequential liquid crystal display. The device and method provide for the continuous illumination of one or more of a plurality of color backlights of a field sequential liquid crystal display to provide a monochromatic source of light behind the liquid crystal layer of the display. The intensities of the one or more of the plurality of color backlights may be selected to achieve a user selected color, or the intensities may be chosen to reduce power consumption. The monochromatic mode may be selected while in another mode of operation.

Description:
This application claims the benefit of U.S. Ser. No. 60/494,398 filed Aug. 12, 2003, which disclosure is incorporated herein by reference. 
    
    
     BACKGROUND 
     Several types of color displays are known for use in mobile devices. These known devices have limitations however, including high power consumption requirements and limited color saturation capabilities. Limited color saturation refers to situations in which the display cannot distinctly display subtle color changes. An example of such a known display is an Organic Light-Emitting Diode (OLED) display. A single pixel  10  of an OLED is shown in  FIG. 1 . Each pixel of an OLED has a set of three color emitters  12 : red  12   a , green  12   b , and blue  12   c . Colors other than red, blue and green are generated by illuminating more than one emitter at different intensities. OLED is an emissive display technology, so no backlight is required, but when the OLED is turned off the display is no longer readable. OLED displays generally demonstrate good color saturation, but they consume significant power. 
     Another type of known color display is a field sequential liquid crystal display (FS LCD). An illustration of an FS LCD  20  is shown in  FIG. 2 . FS LCD technology does not utilize OLED type color emitters or other known types of filters. An FS LCD panel utilizes a tri-color backlight  22 , typically with red  24 , green  26 , and blue  28  colors and a light guide  30 . Behind the light guide  30  is a reflector  32  and in front of the light guide  30  is a liquid crystal layer  34  between top  36  and rear  38  pieces of glass. Liquid crystal layer  34  can be, for example, a monochrome thin film transistor (TFT) display. As illustrated in  FIG. 3 , in an FS LCD, the tri-color backlight  22  turns on and off individual colors one by one at a rate higher than the human eye can differentiate so that the viewer perceives a composite color made of the individual colors lit during a cycle. As shown in  FIG. 3 , different fields of the liquid crystal layer  34  can be set to pass light as the individual backlights are illuminated.  FIG. 3  shows red  40 , blue  42 , and green  44  fields being sequentially formed as the respective backlight is illuminated to form a composite image  46 . A wide array of colors can be created with this technique. 
     The rate of the sequence and the time that each backlight is illuminated is a function of, and limited by, the response time of the liquid crystal layer  34 . A sixty (60) Hertz frame rate is achieved in the example shown in  FIG. 3  by tripling the frame rate of the liquid crystal to 180 Hertz and displaying each color for one-third of the time or 60 or 180 cycles in a second. By this method the human eye perceives a composite image  46  as shown in the center of  FIG. 3 . If the response time of a liquid crystal is slowed, then eventually the user will be able to see the sequence of the backlight colors. When the rate is slow enough for the user to perceive the sequence of backlights, the user will have difficulty perceiving composite colors and will most likely see fragments of color. Color fragmentation also occurs or becomes more severe when the user either moves with respect to the display or experiences certain vibrations, such as on a bumpy car or train ride. Any degree of color fragmentation makes it difficult for the user to perceive the data being displayed, as individual images or characters may appear blurred. An ideal liquid crystal layer  34  for an FS LCD  100  would have a response time fast enough that users would not see the individual sequencing of the primary colors. 
     When color fragmentation becomes a problem for the user, one solution is to turn off the multi-color backlight  22 , and use the FS LCD  20  as a black on “white” display. The “white” background in this mode is created by ambient light being reflected off the reflector  32  located at the back of the display. In this mode of operation, however, the black characters created by the liquid crystal have shadows caused by reflections of the characters off the reflector  32 . Due to shadows and the passive nature of reflected ambient light this mode also has a low contrast ratio. 
     SUMMARY 
     A device and a method for establishing a monochromatic background light source in an electronic device with a field sequential liquid crystal display are provided. The device comprises a field sequential liquid crystal display with a liquid crystal layer and a plurality of color backlights, and a control module. To achieve a monochromatic background light source behind the liquid crystal display, the control module controls the continuous illumination of one or more of the plurality of color backlights. The method comprises continuously illuminating one or more of the plurality of color backlights to provide a monochromatic background light behind the liquid crystal display. The intensities of the one or more of the plurality of color backlights may be selected to achieve a user selected color, or the intensities may be chosen to reduce power consumption. The monochromatic mode may be selected while in another mode of operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an organic light emitting diode (OLED). 
         FIG. 2  is a diagram showing a field sequential liquid crystal display (FS LCD). 
         FIG. 3  is a diagram showing a FS LCD scanning sequence. 
         FIG. 4  is a block diagram of a FS LCD device using simultaneous rather than sequential backlighting. 
         FIG. 5  is a block diagram of the FS LCD device shown in  FIG. 4  with only the red backlight active. 
         FIG. 6  is a block diagram of a mobile device with an FS LCD display using simultaneous rather than sequential backlighting. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  is a block diagram of a FS LCD device  100  using a continuous monochromatic display mode rather than the standard sequential color FS LCD mode. For simplicity,  FIG. 4  shows a liquid crystal layer  102  on top of red  104 , green  106 , and blue  108  backlights. It should be understood, however, that the red  104 , green  106 , and blue  108  backlights may be located remote from each picture element and a light guide may transmit the light components to the picture elements (as shown in  FIG. 2 ). Liquid crystal layer  102  can be, for example, a thin film transistor (TFT) display. A control module  110  controls the power levels of each backlight, and also controls the liquid crystal layer  102  using control lines  112 . The control module may be a dedicated unit or may be integrated with other functional components of an electronic device. 
     In  FIG. 4 , each of the three backlights is outputting a different power level simultaneously, as indicated by the wavelength intensity bars for blue  114 , red  116 , and green  118 . In this embodiment, the blue wavelength intensity bar  114  is the brightest, the green wavelength intensity bar  116  the next brightest, and the red wavelength intensity bar  118  the least brightest. When the intensity of each color is fixed and the backlights are illuminated continuously, the user perceives a single composite color. Under these conditions, characters formed by the liquid crystal layer  102  are contrasted by a monochromatic display color. This continuous mode of operation of the backlights provides a constant background color that does not flicker. 
     By adjusting the intensity of the red  104 , green  106 , and blue  108  backlights, the control module  110  can select a wide range of colors to be displayed as a background, and allows the FS LCD  100  to operate in a transmissive monochromatic display mode. The contrast of a transmissive display is significantly higher than the contrast of a reflective display. Additionally, because the backlight is providing the light source, the shadow effect caused by characters formed on the liquid crystal reflecting off a reflector may be eliminated. 
       FIG. 5  shows an alternative continuous monochromatic display mode. In  FIG. 5 , only the red  116  backlight is active and the user of the display will see a monochromatic red background on the FS LCD screen. In this mode, the control module  110  has only activated the red  116  backlight. By selectively activating a single backlight, power may be conserved. Other power conservation modes are possible by, for example, selectively activating the most power efficient color backlight, lowering the intensity of a single backlight, or by forming a composite color of multiple backlights illuminated at a low intensity. The intensity level of the backlights can be specified by the user. The contrast afforded characters formed on the liquid crystal of the display may depend on the intensity level of the backlights, which may be specified by the user to provide an acceptable contrast level. 
     The continuous monochromatic display modes described above can be selected while in another mode of operation. For example, if the user wanted to conserve power in order to extend battery life, he could switch to the continuous monochromatic display mode. Further, if the user was experiencing color separation in a standard FS LCD mode due to movement or vibration, he could switch to the continuous monochromatic display mode. 
     The frame rate frequency in the continuous monochromatic display modes described above can be any rate achievable by the liquid crystal. For example, the frame rate frequency in regular sequential color operation of an FS LCD may be 180 Hertz and the monochromatic display mode may continue this frame rate frequency. As a further example, because the backlights are operating continuously rather than sequentially, the frame rate frequency could be reduced. The frame rate frequency of the liquid crystal can be reduced to any level, however, below approximately 24 Hertz the human eye can detect individual frames. Preferably, the frame rate frequency is decreased to between about 24 and about 70 Hertz, more preferably between about 24 and about 40 Hertz, and even more preferably to about 24 Hertz. Reducing the frame rate of the liquid crystal also provides power savings. 
       FIG. 6  is a schematic diagram of a mobile device  200  that could be used with an FS LCD  100  as described above. The mobile device  200  may, for example, be a two-way communication device having voice and data communication capabilities. The mobile device may also be operable to communicate with other computer systems on the Internet. Depending on the functionality provided by the device, the device may be referred to as a data messaging device, a two-way pager, a cellular telephone with data messaging capabilities, a wireless Internet appliance, a data communication device, or by other names 
     Where the mobile device  200  is enabled for two-way communications, it incorporates a communication subsystem  202 , including a receiver  204  and a transmitter  206 , as well as associated components such as one or more, preferably embedded or internal, antenna elements  208  and  210 , local oscillators (LOs)  212 , and a processing module such as a digital signal processor (DSP)  214 . The particular design of the communication subsystem  202  may be dependent upon the communication network in which the device is intended to operate. For example, a mobile device  200  may include a communication subsystem  202  designed to operate within the Mobitex™ mobile communication system, the DataTAC™ mobile communication system, a CDMA network, an iDen network, or a GPRS network. 
     Network access requirements may also vary depending upon the type of network  216 . For example, in the Mobitex and DataTAC networks, mobile devices  200  are registered on the network using a unique identification number associated with each mobile device. In GPRS networks however, network access is associated with a subscriber or user of a mobile device  200 . A GPRS mobile device therefore requires a subscriber identity module, commonly referred to as a SIM card, in order to operate on a GPRS network. Without a valid SIM card, a GPRS mobile device may not be fully functional. Local or non-network communication functions, as well as legally required functions (if any) such as “911” emergency calling, may be operable, but the mobile device  200  may be unable to carry out any other functions involving communications over the network  216 . 
     When required network registration or activation procedures have been completed, a mobile device  200  may send and receive communication signals over the network  216 . Signals received by the antenna  208  through a communication network  216  are input to the receiver  204 , which may perform such common receiver functions as signal amplification, frequency down conversion, filtering, channel selection and the like, and in the example system shown in  FIG. 6 , analog to digital conversion. Analog to digital conversion of a received signal allows more complex communication functions, such as demodulation and decoding, to be performed in the DSP  214 . In a similar manner, signals to be transmitted are processed by the DSP  214  and input to the transmitter  206  for digital to analog conversion, frequency up conversion, filtering, amplification and transmission over the communication network  216  via the antenna  210 . 
     The DSP  214  may also provide receiver and transmitter control. For example, the gains applied to communication signals in the receiver  204  and transmitter  206  may be adaptively controlled through automatic gain control algorithms implemented in the DSP  214 . 
     The mobile device  200  may include a microprocessor  222 , which controls the overall operation of the device. Communication functions, such as data and voice communications, are performed through the communication subsystem  202 . The microprocessor  222  also interacts with further device subsystems such as the FS LCD  100 , flash memory  224 , random access memory (RAM)  226 , auxiliary input/output (I/O) subsystems  228 , serial port  230 , keyboard  232 , speaker  234 , microphone  236 , a short-range communications subsystem  238  and any other device subsystems generally designated as  240 . 
     Some of the subsystems shown in  FIG. 6  perform communication-related functions, whereas other subsystems may provide “resident” or on-device functions. Some subsystems, such as keyboard  232  and FS LCD  100 , may be used for both communication-related functions, such as entering a text message for transmission over a communication network, and device-resident functions such as a calculator or task list. 
     Operating system software used by the microprocessor  222  may be stored in a persistent store, such as flash memory  224 , a read only memory (ROM), or similar storage element. The operating system, specific device applications, or parts thereof, may be temporarily loaded into a volatile store such as RAM  226 . Received communication signals may also be stored to RAM  226 . 
     As shown, the flash memory  224  can be segregated into different areas for computer programs and program data storage  242 . These different PIM storage types indicate that each program can allocate a portion of flash memory  224  for its database requirements. The microprocessor  222 , in addition to its operating system functions, may enable execution of software applications on the mobile device. A predetermined set of applications that control basic operations, such as data and voice communication applications may normally be installed on the mobile device  200  during manufacturing. For example, one software application may be a personal information manager (PIM) application operable to organize and manage data items relating to the user of the mobile device such as, but not limited to, e-mail, calendar events, voice mails, appointments, task items, or others. One or more memory stores may be available on the mobile device to facilitate storage of PIM data items. Such PIM application may have the ability to send and receive data items via the wireless network  216 . In a preferred embodiment, the PIM data items are seamlessly integrated, synchronized and updated, via the wireless network  216 , with the mobile device user&#39;s corresponding data items stored or associated with a host computer system. Further applications may also be loaded onto the mobile device  200  through the network  216 , an auxiliary I/O subsystem  228 , serial port  230 , short-range communications subsystem  238  or any other suitable subsystem  240 , and installed by a user in the RAM  226  or preferably a non-volatile store for execution by the microprocessor  222 . 
     In a data communication mode, a received signal such as a text message or web page download is processed by the communication subsystem  202  and input to the microprocessor  222 , which may further processes the received signal for output to the display  100  or to an auxiliary I/O device  228 . A user of mobile device  202  may also compose data items, such as email messages, using the keyboard  232 , which is preferably a complete alphanumeric keyboard or telephone-type keypad, in conjunction with the display  422  and possibly an auxiliary I/O device  228 . Such composed items may be transmitted over a communication network through the communication subsystem  202 . 
     For voice communications, overall operation of the mobile device  200  is similar, except that received signals may be output to a speaker  234  and signals for transmission may be generated by a microphone  236 . Alternative voice or audio I/O subsystems, such as a voice message recording subsystem, may also be implemented on the mobile device  200 . Although voice or audio signal output is preferably accomplished primarily through the speaker  234 , the FS LCD  100  may also be used to provide an indication of the identity of a calling party, the duration of a voice call, or other voice call related information for example. 
     The serial port  230  may be implemented in a personal digital assistant (PDA)-type mobile device to synchronize with a user&#39;s desktop computer. A serial port  230  may enable a user to set preferences through an external device or software application and may provide a path for information or software downloads to the mobile device  200  other than through a wireless communication network. The serial port  230  may, for example, be used to load an encryption key onto the device through a direct and thus reliable and trusted connection to thereby enable secure device communication. 
     A short-range communications subsystem  238  may be included to provide communication between the mobile device  200  and different systems or devices. For example, the subsystem  238  may include an infrared device and associated circuits and components or a Bluetooth™ communication module to provide for communication with similarly-enabled systems and devices.