Abstract:
A method for adjusting a gamma setting of a display for a mobile device is disclosed. The display has two brightness settings, where the two brightness settings include a transmissive mode setting and a reflective mode setting. The method includes: a) changing the display to a selected brightness setting; and b) changing the gamma setting of the display to a predetermined gamma setting corresponding to the selected brightness setting. The predetermined gamma setting optimizes an image on the display at the selected brightness setting. The method also includes switching a back light connected to the display, where the back light is switched to the on position in the transmissive setting and to the off position in the reflective setting.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority from Canadian Patent Application No. 2,413,333, filed on Nov. 29, 2002, and also claims the benefit of U.S. Provisional Application No. 60/430,371, filed on Dec. 3, 2002.  
         FIELD OF THE INVENTION  
         [0002]    The invention relates to display devices, and in particular, to display devices having a transmissive mode and a reflective mode (also known as “transreflective” or “transflective” displays).  
         BACKGROUND OF THE INVENTION  
         [0003]    Transreflective displays are popular choices for consumer electronic devices, such as wireless phones, personal digital assistants (PDAs), and other mobile devices. Currently, available transreflective displays are liquid crystal displays (LCDs). Transreflective displays operate in two modes. One of the modes is a transmissive mode, where a source of illumination is placed behind the LCD layers to facilitate visualization of the resultant image. The other mode is a reflective mode where there is no illumination behind the LCD layers such that the viewer relies on the ambient light reflecting from the LCD to view the display. The reflective mode is especially helpful with LCDs for mobile devices, which have battery power limitations. By operating the display of the mobile device in the transmissive mode only when ambient light is insufficient for clear viewing, the battery life of a mobile device is extended.  
           [0004]    However, changing between the transmissive and reflective modes has an effect on the appearance of the color quality or saturation displayed on the LCD screen. This color saturation may be adjusted by changing the gamma setting of the display.  
           [0005]    Display devices, such as LCDs, display image colors in a linear fashion (with some amount of distortion), where one unit of input produces one unit of output. In contrast, human eyes see images logarithmically. The linear optical response of LCDs at the lower and higher ends of output (also referred to as “digital bit value”) changes too quickly for the human eye, causing some compression of the shadow detail where human eyes are very sensitive. Accordingly, instead of a linear response, the output of LCDs is adjusted from a linear setting to an exponential curve, referred to in the art as a “gamma curve”. The gamma curve, has a long and gradually curved beginning and end portions, and a substantially flat steeper middle portion. The gamma curve or setting is based on a polynomial equation describing any point on a brightness curve being output by a particular display. Its function is to correct for the non-linearity of the input signal and its corresponding luminance.  
           [0006]    The purpose of adjusting the output of displays in accordance with a gamma curve is to match the output of the display to the characteristics of the human eye (i.e. the increased sensitivity of the human eye at the lower and higher ends of output).  
           [0007]    Each of the display modes (i.e. the transmissive and reflective modes) has its own optimal gamma setting. If the gamma setting of the display is optimized for the reflective mode, colors may appear “washed out” in the transmissive mode. On the other hand, if the gamma setting is optimized for the transmissive mode, the colors may appear over-saturated or dark. This occurs because, in the reflective mode, light passes twice through the color filters in the LCD. Light passes through the color filters once from the ambient light source through the LCD to the reflector, and a second time from the reflector to the viewer&#39;s eye. In the transmissive mode, the light originates at a light source behind the LCD, and passes through the LCD once on its way to the viewer&#39;s eye.  
           [0008]    Accordingly, there is a need for a method and an apparatus for optimizing the image on a transreflective display by changing the gamma setting of the display.  
         SUMMARY OF THE INVENTION  
         [0009]    According to a first aspect of the invention, a method for adjusting a gamma setting of a display having a plurality of brightness settings is provided. The method comprises: (a) changing the display to a selected brightness setting, wherein the selected brightness setting is one of the plurality of brightness settings; and b) changing the gamma setting of the display to a predetermined gamma setting corresponding to the selected brightness setting, the predetermined gamma setting being adapted to optimize an image on the display.  
           [0010]    According to a second aspect of the invention, a computer readable medium, including a computer program that adjusts a gamma setting for a display having a plurality of brightness settings, is provided. The computer program causes the computer to perform the steps of:  
           [0011]    a) changing the display to a selected brightness setting, wherein the selected brightness setting is one of the plurality of brightness settings; and  
           [0012]    b) changing the gamma setting of the display to a predetermined gamma setting corresponding to the selected brightness setting, the predetermined gamma setting being adapted to optimize an image on the display.  
           [0013]    According to a third aspect of the invention, an apparatus for adjusting a gamma setting of a display having a plurality of brightness settings is provided. The apparatus comprises:  
           [0014]    a) a means for changing the display to a selected brightness setting, wherein the selected brightness setting is one of the plurality of brightness settings; and  
           [0015]    b) a means for changing the gamma setting of the display to a predetermined gamma setting corresponding to the selected brightness setting, the predetermined gamma setting being adapted to optimize an image on the display. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will now be described by way of example only with reference to the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a block diagram of an apparatus according to the preferred embodiment of the invention; and  
         [0018]    [0018]FIG. 2 is a chart showing two gamma settings for the apparatus shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 shows an apparatus according to the preferred embodiment of the invention. Preferably, the apparatus is a mobile computing device  10 , but may be any other suitable device having a transreflective display  12 . The display  12  includes a back light  14  to control the illumination of the display  12 . When the back light  14  is on, the display  12  is in a transmissive mode, and when it is off, the display  12  is in a reflective mode. Preferably, the display  12  is a color LCD screen.  
         [0020]    In an alternative embodiment the back light  14  may have several illumination levels to provide several transmissive modes.  
         [0021]    Referring to FIG. 1, the display  12  is in communication with a driver  16 . The driver is in communication with a central processing unit (CPU)  18  of the mobile computing device  10 . The CPU  18  sends commands to the driver  16  which controls the images appearing on the display  12 , as well as other characteristics of the display  12 , as is well known in the art. In particular, the CPU  18  sends commands to the driver  16  to turn the back light  14  on and off, and to change the gamma setting on the display  12 .  
         [0022]    In an alternative embodiment where the back light  14  has several illumination levels, the driver  16  switches the illumination levels of the back light  14  in response to commands received from the CPU  18 .  
         [0023]    Continuing to refer to FIG. 1, the CPU  18  is in communication with software  20  which communicates with the hardware components of the mobile computing device  10 . Depending on the type of mobile device, the software may be an operating system, a Java Virtual Machine, Java applications, or any other suitable software, as is well known in the art. The software  20  receives a signal indicating a selected brightness setting for the display  12  from an input device  22  and translates the signal to a command to the CPU  18 . Any suitable number of brightness settings may be provided in the mobile computing device  10  to correspond to varying levels of ambient light.  
         [0024]    The user of the mobile computing device  10  may change the brightness setting of the display  12  preferably in response to variation in the ambient light. The user chooses the selected brightness setting by actuating the input device  22 . The input device  22  may be a button or buttons on the device, a touch screen, or any other suitable device.  
         [0025]    Alternatively, the input device  22  may be a light sensor which detects the level of ambient light and sends this information to the software  20 . The software  20  may then select the optimal brightness setting for the ambient light and communicate the selected brightness setting to the CPU  18 , as is well known in the art. The selection algorithm may be programmed into the software in any number of known ways.  
         [0026]    The operation of the method according to a preferred embodiment of the present invention will now be described with reference to FIG. 1.  
         [0027]    The user of the mobile computing device  10  may choose the brightness setting for the display  12  using the input device  22 . Alternatively, a light sensor (not shown) may sense the level of ambient light and communicate the information to the software  20 , as discussed above. The selected brightness setting may be chosen automatically by the software  20  without participation of the user, as described above.  
         [0028]    Each brightness setting has a corresponding predetermined gamma setting. In low ambient light, the predetermined gamma setting for the selected brightness setting is preferably optimized for a transmissive mode setting where the back light  14  is on. In brighter ambient light, the predetermined gamma setting for selected brightness setting may be optimized for a reflective mode setting where the back light  14  is off.  
         [0029]    An example of a gamma setting for each of the modes is illustrated in FIG. 2. As shown in FIG. 2, the predetermined gamma setting (gama=3) for the transmissive mode is higher than the predetermined gamma setting (gamma=1.5) for the reflective mode.  
         [0030]    In an alternative embodiment of the invention, several transmissive mode settings may be provided to optimize the display  12  for different levels of low ambient light. In such an embodiment, the back light  14  has several illumination levels, where each of the levels is a different transmissive mode setting. The back light  14  may be set for maximum brightness in the transmissive mode setting optimized for little or no ambient light, with each of the other transmissive mode settings being set to consecutively lower illumination levels from the back light  14 , to match viewing conditions with greater amounts of ambient light.  
         [0031]    In an embodiment where the input device  22  is a light sensor, several reflective mode settings may also be provided to optimize the gamma setting for different levels of ambient light where the back light  14  is not necessary for viewing. For example, one reflective mode setting may be provided for bright sunlight and another for normal light conditions.  
         [0032]    The input device  22  sends a signal to the software  20  indicating the selected brightness setting. The software translates the signal, as is well known in the art, and forwards the command to the CPU  18 . Preferably, the CPU  18  sends two simultaneous commands on a data bus (not shown) connecting the CPU  18  and driver  16  for the display  12 . As used herein, “simultaneously” means any period of time which is substantially imperceptible to the viewer. However, it will be understood by those skilled in the art, that the commands do not have to be simultaneous, and that there may be a measurable delay between the change in the selected brightness setting and the change in gamma setting.  
         [0033]    The data bus may be a 16 bit data bus. Alternatively, the connection between the CPU  18  and the driver  16  may be made serially via a series of discrete outputs (i.e. a dedicated pin that enables/disables the backlight, or sets the brightness). In another alternative embodiment, a graphics co-processor (not shown) may be provided. The graphics co-processor may take commands from the CPU  18 , and interpret them for the display  12 .  
         [0034]    The first command from the CPU preferably changes the state of a register (not shown) on the driver  16  responsible for switching between the brightness levels of the back light  14 . In the embodiment where there is only one transmissive and one reflective mode, the driver  16  toggles the back light  14  on and off. The second command from the CPU  18  writes to another register (not shown) on the driver  16  responsible for changing the gamma setting of the display  12  to a predetermined gamma setting corresponding to the selected brightness setting. It will be understood by those skilled in the art, that the order of the commands is not essential to the invention, particularly if the commands are executed simultaneously, as discussed above. The commands may have the order discussed above or they may be reversed. Specifically, the gamma setting of the display  12  may be adjusted prior to switching of the back light  14 .  
         [0035]    In the embodiment where more than one reflective mode setting is provided and the switch is from one reflective mode setting to another reflective mode setting, no command to the back light  14  may be necessary.  
         [0036]    In response to the commands from the CPU  18 , the driver  16  sends an electrical signal to enable the back light  14 . If the command is to select one of the transmissive mode settings, the back light  14  is switched to the illumination level corresponding to the selected transmissive mode setting. If the command is to select the reflective mode setting, the back light  14  is switched to the off position, unless it was already off (i.e. in the embodiment where more than one reflective mode settings are provided). The driver  16  also changes the voltage level and characteristics to the display  12  in order to achieve a predetermined gamma setting optimal for the selected mode. Preferably, the illumination level change (if necessary) and the gamma setting change on the display  12  are carried out simultaneously. To achieve optimal display appearance for the user, the gamma setting increases with the illumination level of the back light, such that the gamma is higher in the transmissive modes and lower in the reflective mode (as shown in FIG. 2). The exact gamma setting for providing optimal color saturation in each mode varies widely with each device and the effect desired by the manufacturer.  
         [0037]    In order to create a more smooth transition in the color saturation during a display mode change, the gamma setting may be adjusted gradually. As used herein, “gradually” is any period of time in which the user will not notice the change in color saturation (i.e. gamma setting) of the display  12 . Preferably, the gradual adjustment is made by dividing the required adjustments into a series of partial adjustments carried out over a suitable period of time. The gamma setting change in each partial adjustment is imperceptible to the user. Depending, on the severity of the required adjustment, the time to make the required adjustment is extended so that it is imperceptible to the user. For example, the period of time for an adjustment may be one second, with each step being implemented in milliseconds.  
         [0038]    The gradually shifting gamma setting may be used in conjunction with a backlight that tends to “warm up” or change illumination level gradually (e.g. a backlight which utilizes EL (electro luminescent) panels).  
         [0039]    In an alternative device configuration (not shown), the back light  14  may be a separate hardware component from the display  12 . In this case, the back light  14  is not controlled by the driver  16 , but may be controlled directly by the CPU  18 .  
         [0040]    The method according to the present invention improves the picture on the display  12  by optimizing the gamma setting for different ambient light conditions.  
         [0041]    While the present invention as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and thus, is representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described preferred embodiment that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it is to be encompassed by the present claims.