Patent Publication Number: US-2011063490-A1

Title: Display apparatus, portable information terminal, and display control method and display control program for portable information terminal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display apparatus including a so-called transparent display, a portable information terminal including the display apparatus, and a display control method and a display control program for the portable information terminal. 
     2. Description of the Related Art 
     In recent years, so-called transparent displays that may assume a state (display-on state) in which information is displayed on the display and a state (display-off state) in which a background is seen through the back surface side of the display have been developed. A display that uses a polymer-scattered liquid crystal (a polymer-dispersed liquid crystal or a polymer network liquid crystal), among various types of transparent displays being studied, has almost reached the level of practical use. The polymer-scattered liquid crystal scatters incident light when no voltage is applied, and allows incident light to pass through when a voltage is applied. 
     Japanese Unexamined Patent Application Publication No. 2003-150070 (FIG. 1) discloses a cellular phone including a display device in which a liquid crystal shutter member, in which liquid crystal molecules that change their orientation in accordance with an applied voltage are sealed, is disposed on the display surface side of a display that displays characters, images, and so forth. According to the cellular phone, characters, images, and so forth displayed on the display may be seen through the liquid crystal shutter member by applying no voltage to the liquid crystal molecules to make the liquid crystal shutter member transparent, and the liquid crystal shutter member may be used as a mirror by applying a voltage to the liquid crystal molecules to make the liquid crystal shutter member specular. 
     SUMMARY OF THE INVENTION 
     The transparent display described above involves the following issues. 
     A first issue is that information displayed on the display may be very difficult to see depending on the status (color, brightness, etc.) of a background seen through the transparent display in the case where a display apparatus including the transparent display is used under various circumstances such as in the field or in a dark place, for example. That is, in the case where white characters are displayed on the display, for example, the characters may be very difficult to see when the background seen through the display is white. Also, in the case where a polymer-scattered liquid crystal is used as the transparent display in a dark surrounding environment, for example, the polymer-scattered liquid crystal may be difficult to see with less incident light and hence less scattered light. 
     A second issue is a privacy issue that information displayed on the transparent display may be seen also from the back surface side of the display, for example, and may be seen by others. 
     According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-150070 (FIG. 1) described above, the display is provided under the liquid crystal shutter member, and information displayed on the display may be concealed by making the liquid crystal shutter member specular to protect the privacy. In this case, however, the displayed information is also not seen from a user himself/herself. The display device disclosed in Japanese Unexamined Patent Application Publication No. 2003-150070 (FIG. 1) does not allow the background to be seen through, and thus does not function as a transparent display. Other techniques according to the related art include providing a reflective sheet on the back side of a polymer-scattered liquid crystal. However, the technique also does not provide a transparent display. 
     It is therefore desirable to provide a display apparatus usable as a transparent display that addresses an issue of deteriorated viewability of information displayed on the display which may occur depending on the status of a background and a privacy issue that information displayed on the display is seen not only by a user himself/herself but also by others from the back surface side of the display, a portable information terminal, and a display control method and a display control program for a portable information terminal. 
     According to an embodiment of the present invention, there is provided a display apparatus including: a display section including a transparent scattering layer switchable in accordance with a first control signal between a state in which incident light is transmitted in units of dots and a state in which incident light is scattered in units of dots, a transparent reflective layer switchable in accordance with a second control signal between a state in which incident light is transmitted and a state in which incident light is reflected, and a bonding layer that bonds the transparent scattering layer and the transparent reflective layer to each other; and a control signal generation section configured to generate the first and second control signals. 
     According to an embodiment of the present invention, there is provided a portable information terminal including: a display section including a transparent scattering layer switchable in accordance with a first control signal between a state in which incident light is transmitted in units of dots and a state in which incident light is scattered in units of dots, a transparent reflective layer switchable in accordance with a second control signal between a state in which incident light is transmitted and a state in which incident light is reflected, and a bonding layer that bonds the transparent scattering layer and the transparent reflective layer to each other, the transparent scattering layer and the transparent reflective layer being provided on a front surface side and a back surface side, respectively, of the terminal; a circumstance determination section configured to determine a circumstance of use of the terminal; and a control signal generation section configured to generate the first and second control signals in accordance with results of determination of the circumstance of use of the terminal performed by the circumstance determination section. 
     The transparent scattering layer of the display section may include a polymer-scattered liquid crystal layer in which a plurality of liquid crystal molecules corresponding to the respective dots are arranged in a matrix, and two transparent plate material layers sandwiching the polymer-scattered liquid crystal layer and including an electrode that supplies the first control signal to the respective liquid crystal molecules, and the first control signal may cause an axis of a desired liquid crystal molecule to be oriented in a direction of scattering incident light or to be oriented in a direction of allowing incident light to pass through. Also, the transparent reflective layer may include a shutter liquid crystal layer in which a plurality of liquid crystal molecules are arranged in a matrix, two transparent plate material layers sandwiching the shutter liquid crystal layer and including an electrode that supplies the second control signal to the respective liquid crystal molecules, a polarization plate layer disposed on an outer side of one of the transparent plate material layers, and a reflective polarization film layer disposed on an outer side of the other of the transparent plate material layers, and the second control signal may cause a polarization direction of all the liquid crystal molecules for incident light to be aligned with a specific direction that allows incident light to pass through the polarization plate layer and the reflective polarization film layer or to be aligned with a direction different from the specific direction. 
     According to the display apparatus of the present invention, the display section includes a transparent scattering layer switchable between a state in which incident light is transmitted in units of dots and a state in which incident light is scattered in units of dots, and a transparent reflective layer switchable between a state in which incident light is transmitted and a state in which incident light is reflected. Thus, according to the present invention, it is possible to provide a display apparatus usable as a transparent display that addresses an issue of deteriorated viewability of information displayed on the display which may occur depending on the status of a background and a privacy issue that information displayed on the display is seen not only by a user himself/herself but also by others from the back surface side of the display. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an exemplary schematic external configuration of a cellular phone terminal according to an embodiment of the present invention, showing a schematic front view of the cellular phone terminal; 
         FIG. 1B  shows an exemplary schematic external configuration of the cellular phone terminal according to the embodiment of the present invention, showing a schematic back view of the cellular phone terminal; 
         FIG. 1C  shows an exemplary schematic external configuration of the cellular phone terminal according to the embodiment of the present invention, showing a schematic lateral view of the cellular phone terminal; 
         FIG. 1D  shows an exemplary schematic external configuration of the cellular phone terminal according to the embodiment of the present invention, showing a schematic lateral view of the cellular phone terminal; 
         FIG. 1E  shows an exemplary schematic external configuration of the cellular phone terminal according to the embodiment of the present invention, showing a schematic lateral view of the cellular phone terminal; 
         FIG. 1F  shows an exemplary schematic external configuration of the cellular phone terminal according to the embodiment of the present invention, showing a schematic lateral view of the cellular phone terminal; 
         FIG. 2  shows a schematic configuration of a polymer-scattered liquid crystal, showing a state in which incident light is transmitted; 
         FIG. 3  shows a schematic configuration of the polymer-scattered liquid crystal, showing a state in which incident light is scattered; 
         FIG. 4  shows a schematic configuration of a shutter liquid crystal, showing a state in which incident light is transmitted; 
         FIG. 5  shows a schematic configuration of the shutter liquid crystal, showing a state in which incident light is reflected; 
         FIG. 6  shows a schematic configuration of a display device according to the embodiment including the polymer-scattered liquid crystal and the shutter liquid crystal; 
         FIG. 7  is a table showing the correspondence between voltage application ON/OFF control signals for the polymer-scattered liquid crystal and the shutter liquid crystal and how the display device appears (display modes) during voltage application ON/OFF control; 
         FIG. 8  illustrates a state in which the display device according to the embodiment is in a display mode M 1 ; 
         FIG. 9  illustrates a state in which the display device according to the embodiment is in a display mode M 2 ; 
         FIG. 10  illustrates a state in which the display device according to the embodiment is in a display mode M 3 ; 
         FIG. 11  illustrates a state in which the display device according to the embodiment is in a display mode (privacy protection mode) M 4 ; 
         FIG. 12  shows a display image with the display device according to the embodiment in the display mode M 3 ; 
         FIG. 13  shows a display image with the display device according to the embodiment in the display mode (privacy protection mode) M 4 ; 
         FIG. 14  is a block diagram showing a schematic internal configuration of a cellular phone terminal according to the embodiment; and 
         FIG. 15  is a flowchart of a process for automatically setting the display mode to the privacy protection mode in the cellular phone terminal according to the embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
     In the embodiment, a cellular phone terminal equipped with a transparent display function is used as an example of a display apparatus, a portable information terminal, and a display control method and a display control program for a portable information terminal according to the present invention. It should be understood that the description made below in relation to the embodiment is merely illustrative and that the present invention is not limited thereto. 
     [Schematic External Configuration of Cellular Phone Terminal] 
       FIGS. 1A to 1F  show an exemplary schematic external configuration of a cellular phone terminal  20  according to the embodiment.  FIG. 1A  is a schematic front view,  FIG. 1B  is a schematic back view, and  FIGS. 1C to 1F  are each a schematic lateral view, respectively, of the cellular phone terminal  20  according to the embodiment. Respective devices provided with a reference numeral in  FIGS. 1A to 1F  are some of all the devices provided in the cellular phone terminal  20 . It is a matter of course that the cellular phone terminal  20  according to the embodiment is provided with various other devices provided in normal high-function cellular phone terminals, although not shown in  FIGS. 1A to 1F . 
     In the cellular phone terminal  20  according to the embodiment shown in  FIGS. 1A to 1F , a key device section  27  including key buttons such as numeric keys, call start/end keys, arrow keys, a clear key, and a mail key, a receiver speaker (receiver)  22 , a transmitter microphone  26 , a lens  24  of a sub camera device (in-camera), a display device  30  to be described later, a proximity sensor  23 , an electrostatic sensor and a temperature sensor (not shown), and so forth are disposed on a housing front surface side shown in  FIG. 1A . 
     In the cellular phone terminal  20  according to the embodiment, also, a lens  25  of a main camera device and an infrared ray sensor  29  to be described later are provided on a housing back surface side shown in  FIG. 1B . An interface connector  28  is provided on a housing lateral surface side shown in  FIG. 1F . A shutter button  21  is provided on a housing lateral surface side shown in  FIG. 10 . 
     The key device section  27 , the receiver speaker (receiver)  22 , the transmitter microphone  26 , the lenses  25  and  24  of the camera devices, and the interface connector  28  are the same as those provided in normal cellular phone terminals, and therefore are not described herein. The proximity sensor  23 , the infrared ray sensor  29 , the electrostatic sensor, and the temperature sensor will be described in detail later. 
     In the cellular phone terminal  20  according to the embodiment, the display device  30  has a function of displaying images, characters, symbols, and so forth on its display surface. In the case of the embodiment, the display device  30  is formed, for example, from a polymer-scattered liquid crystal and a shutter liquid crystal as described later, and also has a function of becoming transparent so that the housing front surface side is viewable from the housing back surface side and vice versa, a function of making the display content viewable from both the housing front surface side and the housing back surface side, a function of becoming substantially specular when seen from the housing front surface side, and a function of making the display content viewable from only the housing front surface side, the functions being appropriately switchable between each other. 
     Before describing each function of the display device  30 , the schematic configuration of the polymer-scattered liquid crystal and the shutter liquid crystal will be described. 
     [Schematic Configuration of Polymer-Scattered Liquid Crystal] 
       FIGS. 2 and 3  show a schematic configuration of the polymer-scattered liquid crystal. 
     As shown in  FIGS. 2 and 3 , the polymer-scattered liquid crystal has a structure in which a polymer-scattered liquid crystal layer  2  in which a large number of liquid crystal molecules corresponding to display dots are arranged in a matrix is sandwiched between two transparent plate material layers  1  and  3  made of glass or the like. One of the two transparent plate material layers  1  and  3  is disposed on the housing front surface side shown in  FIG. 1A  and the other is disposed on the housing back surface side shown in  FIG. 1B  so as to be seen from the outer side. The transparent plate material layers  1  and  3  are provided with an electrode corresponding to each liquid crystal molecule. A control signal for driving the axis of a liquid crystal molecule is sent from a control signal generator  10  via the electrode to orient the axis of a desired liquid crystal molecule in the direction of scattering incident light or in the direction of allowing incident light to pass through. 
     In the case where the axis of a liquid crystal molecule is oriented in the direction of allowing incident light to pass through in the polymer-scattered liquid crystal, both incident light LF from the housing front surface side and incident light LB from the housing back surface side are allowed to pass through to the other side as shown in  FIG. 2 . The polymer-scattered liquid crystal in this state becomes transparent so that the housing front surface side is viewable from the housing back surface side and vice versa. On the other hand, in the case where the axis of a desired liquid crystal molecule is oriented in the direction of scattering light (LS), a plurality of liquid crystal molecules related to the scattering make it possible to display images, characters, symbols, and so forth such that the display content is seeable from both the housing front surface side and the housing back surface side as shown in  FIG. 3 . In the example shown in  FIGS. 2 and 3 , the axis of a liquid crystal molecule is oriented in the direction of allowing incident light to pass through when the control signal generator  10  applies a voltage (ON), and the axis of the liquid crystal molecule is oriented in the direction of scattering incident light when the control signal generator  10  applies no voltage (OFF). 
     [Schematic Configuration of Shutter Liquid Crystal] 
       FIGS. 4 and 5  show a schematic configuration of the shutter liquid crystal. 
     As shown in  FIGS. 4 and 5 , the shutter liquid crystal has a structure in which a shutter liquid crystal layer  7  in which a large number of liquid crystal molecules are arranged in a matrix is sandwiched between two transparent plate material layers  6  and  8  made of glass or the like, a polarization plate layer  5  is provided on a side of the transparent plate material layer  6  opposite the shutter liquid crystal layer  7 , and a reflective polarization film layer  9  is provided on a side of the transparent plate material layer  8  opposite the shutter liquid crystal layer  7 . The polarization plate layer  5  is disposed on the housing front surface side shown in  FIG. 1A  and the reflective polarization film layer  9  is disposed on the housing back surface side shown in  FIG. 1B  so as to be seen from the outer side. The transparent plate material layers  6  and  8  are provided with an electrode corresponding to each liquid crystal molecule. A control signal for driving the liquid crystal polarization surfaces is sent from the control signal generator  10  via the electrodes to control the polarization direction of the liquid crystal molecules. In the case of the shutter liquid crystal, the polarization plate layer  5  allows only light components of incident light in a specific polarization direction to pass through, and the reflective polarization film layer  9  is transparent to only light components in a specific polarization direction and reflects light components in other polarization directions. The specific polarization direction of the polarization plate layer  5  and the specific polarization direction of the reflective polarization film layer  9  are substantially the same as each other. 
     In the case where the polarization direction of the shutter liquid crystal layer  7  is aligned with a specific direction matching the specific polarization direction of the polarization plate layer  5  and the reflective polarization film layer  9  in the shutter liquid crystal, both incident light L 1  from the housing front surface side and incident light L 2  from the housing back surface side are allowed to pass through to the other side as shown in  FIG. 4 . This allows the shutter liquid crystal to become transparent so that the housing front surface side is viewable from the housing back surface side and vice versa. On the other hand, in the case where the polarization direction of the shutter liquid crystal layer  7  is aligned with a direction different from the specific polarization direction of the polarization plate layer  5  and the reflective polarization film layer  9  in the shutter liquid crystal, incident light L 1  from the housing front surface side is reflected (LR) by the reflective polarization film layer  9  and incident light L 2  from the housing back surface side is blocked by the polarization plate layer  5  so as not to be output to the housing front surface side as shown in  FIG. 5 . In the example shown in  FIGS. 4 and 5 , the polarization direction of the liquid crystal molecules is aligned with the specific direction when the control signal generator  10  applies a voltage (ON), and the polarization direction of the liquid crystal molecules is aligned with a direction different from the specific direction when the control signal generator  10  applies no voltage (OFF). 
     [Schematic Configuration of Display Device According to Embodiment] 
       FIG. 6  shows a schematic configuration of a main portion of the display device according to the embodiment formed using the polymer-scattered liquid crystal illustrated in  FIGS. 2 and 3  and the shutter liquid crystal illustrated in  FIGS. 4 and 5  described above. 
     As shown in  FIG. 6 , the display device according to the embodiment has a structure in which the polymer-scattered liquid crystal and the shutter liquid crystal are bonded to each other via a bonding sheet layer  4 . That is, the display device according to the embodiment includes the transparent plate material layer  1 , the polymer-scattered liquid crystal layer  2 , the transparent plate material layer  3 , the bonding sheet layer  4 , the polarization plate layer  5 , the transparent plate material layer  6 , the shutter liquid crystal layer  7 , the transparent plate material layer  8 , and the reflective polarization film layer  9 , which are disposed in this order with the transparent plate material layer  1  disposed on the housing front surface side shown in  FIG. 1A  and with the reflective polarization film layer  9  disposed on the housing back surface side shown in  FIG. 1B  so as to be seen from the outer side. 
     The transparent plate material layers  1  and  3  are provided with an electrode corresponding to each liquid crystal molecule of the polymer-scattered liquid crystal layer  2 . A control signal for driving the axis of a liquid crystal molecule is sent from a control signal generator  10 A via the electrode to orient the axis of a desired liquid crystal molecule in the direction of scattering incident light or in the direction of allowing incident light to pass through. 
     The transparent plate material layers  6  and  8  are provided with an electrode corresponding to each liquid crystal molecule of the shutter liquid crystal layer  7 . A control signal for driving the liquid crystal polarization surfaces is sent from a control signal generator  10 B via the electrodes to control the polarization direction of the liquid crystal molecules. 
     In the embodiment, the polymer-scattered liquid crystal and the shutter liquid crystal are bonded to each other via the bonding sheet layer  4  because the presence of an air layer between the transparent plate material layer  3  of the polymer-scattered liquid crystal and the polarization plate layer  5  of the shutter liquid crystal would refract light at the interface between the transparent plate material layer  3  and the air layer and at the interface between the polarization plate layer  5  and the air layer to cause a loss of light and deteriorate the transmittance and the viewability. That is, the bonding sheet layer  4  is formed from a material having a refractive index that is the same as or very close to the refractive index of the material forming the transparent plate material layer  3  and the polarization plate layer  5 . Providing the bonding sheet layer  4  substantially eliminates refraction of light and prevents a loss of light and deterioration of the transmittance and the viewability. 
     [Control Signals from Control Signal Generator and How Display Device Appears] 
       FIG. 7  is a table showing the correspondence between voltage application ON/OFF control signals for the polymer-scattered liquid crystal and the shutter liquid crystal and how the display device appears (display modes) during voltage application ON/OFF control.  FIGS. 8 to 11  show how the display device transmits, scatters, or reflects incident light in each display mode shown in the correspondence table of  FIG. 7 . 
     In the case of a display mode M 1  of  FIG. 7 , in which the polymer-scattered liquid crystal is applied with a voltage (ON) and the shutter liquid crystal is applied with a voltage (ON), the axis of the liquid crystal molecule is oriented in the direction of allowing incident light to pass through in the polymer-scattered liquid crystal portion, and the polarization direction of the shutter liquid crystal layer  7  is aligned with the specific direction in the shutter liquid crystal portion. In this case, in the display device according to the embodiment, both incident light LF from the housing front surface side and incident light LB from the housing back surface side are allowed to pass through to the other side as shown in  FIG. 8 . This allows the display device to become transparent so that the housing front surface side is viewable from the housing back surface side and vice versa. 
     In the case of a display mode M 2  of  FIG. 7 , in which the polymer-scattered liquid crystal is applied with no voltage (OFF) but the shutter liquid crystal is applied with a voltage (ON), the axis of the liquid crystal molecule is oriented in the direction of scattering incident light (LS) in the polymer-scattered liquid crystal portion, and the polarization direction of the shutter liquid crystal layer  7  is aligned with the specific direction in the shutter liquid crystal portion. In this case, in the display device according to the embodiment, it is possible to display images, characters, symbols, and so forth on the polymer-scattered liquid crystal portion such that the display content is seeable from both the housing front surface side and the housing back surface side as shown in  FIG. 9 . 
     In the case of a display mode M 3  of  FIG. 7 , in which the polymer-scattered liquid crystal is applied with a voltage (ON) but the shutter liquid crystal is applied with no voltage (OFF), the axis of a liquid crystal molecule is oriented in the direction of allowing incident light to pass through in the polymer-scattered liquid crystal portion, and the polarization direction of the shutter liquid crystal layer  7  is aligned with a direction different from the specific direction in the shutter liquid crystal portion. In this case, in the display device according to the embodiment, incident light LF from the housing front surface side is reflected by the reflective polarization film layer  9  and incident light LB from the housing back surface side is blocked by the polarization plate layer  5  so as not to be output to the housing front surface side as shown in  FIG. 10 . That is, in the case of the display mode M 3 , light reflected by the reflective polarization film layer  9  is seeable from the housing front surface side, but not seeable from the housing back surface side. 
     In the case of a display mode M 4  of  FIG. 7 , in which both the polymer-scattered liquid crystal and the shutter liquid crystal are applied with no voltage (OFF), the axis of the liquid crystal molecule is oriented in the direction of scattering incident light (LS) in the polymer-scattered liquid crystal portion, and the polarization direction of the shutter liquid crystal layer  7  is aligned with a direction different from the specific direction in the shutter liquid crystal portion. In this case, in the display device according to the embodiment, it is possible to display images, characters, symbols, and so forth on the polymer-scattered liquid crystal portion such that the display content is seeable from only the housing front surface side and incident light LB from the housing back surface side is blocked by the polarization plate layer  5  so as not to be output to the housing front surface side as shown in  FIG. 11 . That is, in the case of the display mode M 4 , the display content is seeable from the housing front surface side, but not seeable from the housing back surface side. In the embodiment, the display mode M 4  is appropriately referred to as a “privacy protection mode”. 
     [Display Image] 
       FIGS. 12 and 13  show exemplary display images on the display device according to the embodiment of the present invention. 
     In the embodiment, voltage application to each of the liquid crystal molecules of the polymer-scattered liquid crystal is individually turned on and off in accordance with a control signal to enable the liquid crystal molecules to make a dot representation. For the shutter liquid crystal, voltage application to all the liquid crystal molecules is collectively turned on and off. 
       FIG. 12  shows a display image in the case of the display mode M 3  of  FIG. 7 . In the example of  FIG. 12 , voltage application to the shutter liquid crystal is turned off, and liquid crystal molecules of the polymer-scattered liquid crystal corresponding to respective dots forming an image DG are applied with a voltage (ON) and the other liquid crystal molecules of the polymer-scattered liquid crystal are applied with no voltage (OFF). Thus, in the example of  FIG. 12 , a background BG, together with the image DG, is seen from both the housing front surface side and the housing back surface side. 
       FIG. 13  shows a display image in the case of the display mode (privacy protection mode) M 4  of  FIG. 7 . In the example of  FIG. 13 , liquid crystal molecules of the polymer-scattered liquid crystal corresponding to respective dots forming an image DG are applied with a voltage (ON) and the other liquid crystal molecules of the polymer-scattered liquid crystal are applied with no voltage (OFF), and voltage application to the shutter liquid crystal is turned on. Thus, in the example of  FIG. 13 , the image DG is seeable from the housing front surface side, but not seeable from the housing back surface side. That is, in this case, the display content is not seen at all from the housing back surface side to protect the secrecy of the display content. Also in this case, the display content is very easy to see from the housing front surface side with a background on the housing back surface side not seen through the display content and with a reflective surface formed on the housing back surface side of the polymer-scattered liquid crystal, although the background is not seeable from the housing front surface side. 
     [Schematic Internal Configuration of Cellular Phone Terminal] 
       FIG. 14  shows a schematic internal configuration of the cellular phone terminal  20  according to the embodiment. 
     In  FIG. 14 , a display device  42  corresponds to the display device  30  described above in which the polymer-scattered liquid crystal and the shutter liquid crystal according to the embodiment described above are bonded to each other. A display device controller  43  is a device controller that controls operation, display, and so forth of the display device  42 . 
     LEDs (light emitting diodes)  44  are various illumination light sources provided in the cellular phone terminal according to the embodiment such as an incoming lamp and an illumination lamp in the key device section  27  shown in  FIG. 1A , for example. An LED driver  45  controls driving of the plurality of LEDs  44 . 
     An electrostatic sensor  46  detects variations in capacitance. In the case of the cellular phone terminal  20  according to the embodiment, the electrostatic sensor  46  is disposed on the housing front surface side on which the key device section  27  shown in  FIG. 1A  is provided, for example in the vicinity of the receiver  22 . The electrostatic sensor  46  is adapted to detect that an ear or a part of the face of the user is in contact with the electrostatic sensor  46  when the user is engaged in a call with his/her ear etc. put on the receiver  22 , for example. In the case of the embodiment, in particular, the electrostatic sensor  46  is adapted to detect that the ear or a part of the face is in contact for a specific amount of area or more. 
     A proximity sensor  47  is formed by an ultrasonic sensor or an optical position sensor, and corresponds to the proximity sensor  23  of  FIG. 1A . An ultrasonic sensor detects the distance to an object by emitting ultrasonic waves and measuring the arrival time of the ultrasonic waves reflected by the object. An optical position sensor detects the distance to an object by emitting light and measuring the incident angle of the light reflected by the object. In the case of the cellular phone terminal  20  according to the embodiment, the proximity sensor  47  is disposed on the housing front surface side on which the key device section  27  shown in  FIG. 1A  is provided, for example in the vicinity of the receiver  22 . The proximity sensor  44  is adapted to detect that an ear or a part of the face of the user is proximate when the user is engaged in a call with his/her ear etc. put on the receiver  22 , for example. In the case of the embodiment, in particular, the proximity sensor  47  is adapted to detect that the ear or a part of the face is close to the cellular phone terminal  20  within a specific distance from the cellular phone terminal  20 . 
     A temperature sensor  48  detects the temperature of a surface or a vicinity of the cellular phone terminal. In the case of the cellular phone terminal  20  according to the embodiment, the temperature sensor  48  is disposed on the housing front surface side on which the key device section  27  shown in  FIG. 1A  is provided. The temperature sensor  48  is adapted to detect a temperature rise caused when an ear or a part of the face of the user contacts the cellular phone terminal  20  when the user is engaged in a call with his/her ear etc. put on the receiver  22 , for example. In the case of the embodiment, in particular, the temperature sensor  48  is adapted to detect a temperature rise up to a specific temperature, such as a body temperature, or more. 
     An infrared ray sensor  49  corresponds to the infrared ray sensor  29  of  FIG. 1B , and detects an infrared ray. In the case of the cellular phone terminal  20  according to the embodiment, the infrared ray sensor  49  is disposed on the housing back surface side on which the lens of the main camera is provided, for example. The infrared ray sensor  49  is adapted to detect an infrared ray emitted from a human body surface or the like to the housing back surface side of the cellular phone terminal  20  according to the embodiment. 
     A receiver  50  is equivalent to the receiver  22  of  FIG. 1A . The receiver  50  is a speaker that outputs a received sound when a voice call is being made on the cellular phone terminal according to the embodiment. 
     A microphone  51  is equivalent to the microphone  26  of  FIG. 1A . The microphone  51  is a sound input device that is used to input a sound to be transmitted when a voice call is being made on the cellular phone terminal according to the embodiment. 
     An external interface (I/F)  52  includes an external connector of various types and an external connection section that performs signal transfer with the external connector. The external connector includes a connector according to a so-called USB 2.0 (Universal Serial Bus 2.0) standard. Therefore, the cellular phone terminal according to the embodiment also includes a USB 2.0 controller  53 . 
     A USIM card slot  54  is an IC card slot that receives a so-called USIM (Universal Subscriber Identity Module) card storing subscriber information (contractor information) and so forth. 
     An infrared communication module  55  is a communication device that communicates information using an infrared ray. 
     A vibration motor  56  is a so-called vibrator that generates vibration in the cellular phone terminal according to the embodiment. 
     A battery  57  is a power source that supplies electric power to be used by the respective sections of the cellular phone terminal according to the embodiment. 
     A peripheral IC+power IC  58  is connected to the USIM card slot  54 , the infrared communication module  55 , the vibration motor  56 , the battery  57 , and the external interface  52  to perform control and signal processing for the respective sections and to control power supply from the battery  57 . 
     A main camera  60  corresponds to the main camera device of  FIG. 1B , and includes an imaging optical system such as a camera lens, an imaging element (imaging device), and so forth. A flash (flash light)  62  is a light source lamp that emits fill light when the main camera  60  performs capturing. A flash light controller  61  controls light emission performed by the flash  62  in conjunction with the capturing performed by the main camera  60 . 
     A sub camera  63  corresponds to the sub camera device described above, and is provided separately from the main camera  60 . The sub camera  63  is used to capture an image of the user for himself/herself or capture an image of the user during a video call, for example. 
     An RF circuit  65  is a wireless communication circuit used when the cellular phone terminal according to the embodiment communicates with a base station of a cellular phone network. An antenna  66  is a wireless communication antenna used when the cellular phone terminal according to the embodiment communicates wirelessly with a base station. 
     A contactless communication card controller  67  performs various control and signal processing during signal communication with a contactless communication IC card through a so-called electromagnetic induction scheme. A contactless communication card antenna  68  is an antenna for contactless communication through the electromagnetic induction scheme. 
     A close-range wireless controller  69  performs various control and signal processing for close-range wireless communication through a so-called Bluetooth (registered trademark) scheme, for example. A close-range wireless antenna  70  is an antenna for close-range wireless communication. 
     A digital TV tuner  71  is a tuner for so-called digital television broadcasting. A digital TV antenna  72  is an antenna that receives radio waves of the digital television broadcasting. 
     A memory card slot  74  is a slot that removably receives an external memory card such as a so-called SD (Secure Digital) card, for example. A memory card controller  73  performs control and signal processing for writing/reading of data into/from a memory card loaded in the memory card slot  74 . 
     A keypad  75  corresponds to the key device section  27  described above. The keypad  75  includes various keys and various buttons such as numeric keys, arrow keys, and a shutter button provided in the cellular phone terminal according to the embodiment, and generates a key/button output signal when such keys and buttons are operated by the user. The shutter button  21  of  FIG. 1C  is one of the buttons belonging to the keypad  75 . 
     An internal memory  76  includes a DDR SDRAM (Double Data Rate SDRAM)  77  and a NAND-type flash memory  78 , for example. The NAND-type flash memory  78  stores a program for an OS (Operating System), a control program for a CPU  40  to control the respective sections, a control program for the display device according to the present invention, various application programs, compression-encoded data contents such as musical pieces, moving images, and still images, various setting values, font data, various dictionary data, model information, terminal identification information, and so forth. The DDR SDRAM  77  serves as a work area for the CPU  40  to perform various data processing and computation, and stores data any time. 
     A sound chip  79  is an IC for signal processing such as music playback, for example. A right speaker  80  and a left speaker  81  for stereo sound output are connected to the sound chip  79  via an amplifier (not shown). 
     The CPU (Central Processing Unit)  40  performs communication control, sound processing and control, image processing and control, camera capture control, and various other signal processing and control for the respective sections. The CPU  40  also executes various control programs and application programs stored in the internal memory  76 , and processes various data accompanying the programs. In the case of the embodiment, in particular, the CPU  40  executes the display control program according to the present invention to perform display control for each display mode of the display device described above, automatic control of the display mode based on detection outputs from the various sensors to be described later, and so forth. 
     It is a matter of course that the cellular phone terminal according to the embodiment additionally includes various constituent elements provided in general cellular phone terminals, although not shown in  FIG. 14 . 
     [Display Control according to Display Mode (Automatic Setting to Privacy Protection Mode)] 
     The cellular phone terminal according to the embodiment not only allows the user to manually set the display mode described above but also enables automatic setting of the display mode according to the result of detecting the circumstance of use of the cellular phone terminal. Automatic setting of the display mode may be performed by, for example, setting the display mode to the privacy protection mode (M 4 ) in accordance with the circumstance of use of the cellular phone terminal, which prevents a privacy issue that information displayed on the display is seen by others from the back surface side of the display. 
     Information displayed on the display should not be seen from the back surface side of the display in circumstances of use where a person stays on the housing back surface side of the cellular phone terminal (first use case), where the user is engaged in a call with the cellular phone terminal put on his/her ear (second use case), and where the user is seeing a display content that he/she does not want others to see (third use case), for example. It should be understood that these use cases are merely exemplary and that the present invention is not limited thereto. 
     In the embodiment, when the main camera device ( 25 ,  60 ) provided on the housing back surface side captures an image and a face image of a person is recognized in the image using a so-called face image detection technology, for example, it is determined that a person stays on the housing back surface side of the cellular phone terminal (first use case) so that the display mode is automatically switched to the privacy protection mode M 4 . 
     Also, when the infrared ray sensor ( 29 ,  49 ) provided on the housing back surface side detects an infrared ray and it is determined that the detected infrared ray is within a threshold range determined in correspondence with infrared rays emitted from human bodies, for example, it is determined that a person stays on the housing back surface side of the cellular phone terminal (first use case) so that the display mode is automatically switched to the privacy protection mode M 4 . 
     Also in the embodiment, when the electrostatic sensor  46  provided in the vicinity of the receiver  22  detects a contact with an ear or a face and it is determined that the detected contact area is within a certain threshold area range determined in advance, for example, it is determined that the user is engaged in a call with the cellular phone terminal put on his/her ear (second use case) so that the display mode is automatically switched to the privacy protection mode M 4 . 
     Also, when the proximity sensor ( 23 ,  47 ) provided in the vicinity of the receiver  22  detects an approach to an ear or a face and it is determined that the detected distance is within a certain threshold distance determined in advance, for example, it is determined that the user is engaged in a call with the cellular phone terminal put on his/her ear (second use case) so that the display mode is automatically switched to the privacy protection mode M 4 . 
     Also, when the temperature sensor ( 48 ) provided in the vicinity of the receiver  22  detects a temperature and it is determined that the detected temperature is within a certain threshold temperature range around body temperatures (for example, 30 to 36 degrees), for example, it is determined that the user is engaged in a call with the cellular phone terminal put on his/her ear (second use case) so that the display mode is automatically switched to the privacy protection mode M 4 . 
     Also, when a browser application program, a mail application program, or a viewer application program for moving images or still images is being run and the content being displayed on the display by such an application program has been set in advance as a content to be protected for the security of privacy, or simply when such an application program is being run, for example, it is determined that the user is seeing a display content that he/she does not want others to see (third use case) so that the display mode is automatically switched to the privacy protection mode M 4 . The display mode may be switched to the privacy protection mode M 4  in a circumstance of use where a call line is actually connected with a voice calling application program run. Whether or not a display content should be protected for the security of privacy may be determined in accordance with whether or not the content is filtered by a so-called filtering function provided by a website, for example, in addition to whether or not the content has been registered in advance by the user. 
     [Flow of Display Control According to Display Mode (Automatic Setting to Privacy Protection Mode)] 
       FIG. 15  is an exemplary flowchart of a process for automatically setting the display mode to the privacy protection mode in accordance with the results of the determination as to the various use cases described above. The process of  FIG. 15  is performed when a control section  40  executes a display control program according to the present invention. 
     In  FIG. 15 , the control section  40  starts the display control program according to the present invention when any information is to be displayed on a display screen, for example. 
     During execution of the display control program, the control section  40  determines in step S 1  whether or not the user has enabled automatic setting of the display mode to the privacy protection mode in the cellular phone terminal according to the embodiment. If the control section  40  determines in step S 1  that automatic setting to the privacy protection mode is enabled, the process proceeds to step S 5 . On the other hand, if it is determined that automatic setting to the privacy protection mode is not enabled, the process proceeds to step S 2 . Even if it is determined that automatic setting to the privacy protection mode is enabled, the process may proceed to step S 2  in the case where the user explicitly issues a command for shifting to a manual setting mode, for example. Conversely, even if it is determined that automatic setting to the privacy protection mode is not enabled, the process may proceed to step S 5  in the case where the user explicitly issues a command for shifting to an automatic setting mode, for example. 
     In the case where it is determined in step S 1  that automatic setting to the privacy protection mode is not enabled and the process proceeds to step S 2 , the control section  40  determines whether or not the user has manually set the display mode through the keypad  75  or the like. 
     If it is determined in step S 2  that the display mode is manually set, the control section  40  causes the control signal generators  10 A and  10 B to generate a control signal for setting the display mode set by the user in step S 3 . After step S 3 , the control section  40  returns to the beginning of the process. 
     On the other hand, if it is determined in step S 2  that the display mode is not manually set, the control section  40  maintains the previously set display mode in step S 4 . After step S 4 , the control section  40  returns to the beginning of the process. 
     In the case where it is determined in step S 1  that automatic setting to the privacy protection mode is enabled and the process proceeds to step S 5 , the control section  40  determines whether or not the display content being currently displayed on the display has been registered as a content to be protected for the security of privacy, whether or not the application program being run has been registered as an application program that may display a content to be protected for the security of privacy, or the like. If it is determined that privacy protection should be performed, the control section  40  proceeds to step S 8 . On the other hand, if it is determined that the necessity of privacy protection is not registered, the control section  40  proceeds to step S 6 . 
     In step S 6 , the control section  40  causes the main camera device ( 25 ,  60 ) to capture an image and determines whether or not a face image of a person is recognized in the image. If a face image is recognized, the process proceeds to step S 8 . If a face image is not recognized, the process proceeds to step S 7 . 
     In step S 7 , the control section  40  compares a detection signal generated by the electrostatic sensor ( 46 ), the proximity sensor ( 23 ,  47 ), the temperature sensor ( 48 ), or the infrared ray sensor ( 29 ,  49 ) described above with a threshold determined in advance as described above. If it is determined from the results of the comparison between the detection signal and the threshold that the user is engaged in a call or that a person stays on the housing back surface side of the cellular phone terminal, the process proceeds to step S 8 . In other cases, the control section  40  returns to the beginning of the process. 
     In step S 8 , the control section  40  causes the control signal generators  10 A and  10 B to generate a control signal for setting the display mode to the display mode (privacy protection mode) M 4  described above. After step S 8 , the control section  40  returns to the beginning of the process. 
     CONCLUSION 
     According to the cellular phone terminal of the embodiment described above, a polymer-scattered liquid crystal usable as a transparent display and a shutter liquid crystal equipped with a mirror function which is the most effective in improving the viewability are combined with each other to form a display device shown in  FIG. 6 , and the display device is controlled so as to be driven in accordance with the display modes of  FIG. 7  described above. 
     Thus, specifically in the display mode M 4 , it is possible to prevent what is displayed on the polymer-scattered liquid crystal from being seen by others from the back surface side while improving the viewability of the polymer-scattered liquid crystal for easy recognition by the user. 
     Also, the cellular phone terminal according to the embodiment allows automatic setting of the display mode. Therefore, what is displayed on the display is not seen by others, which protects the secrecy of the display content. 
     According to the embodiment, in addition, the bonding sheet layer described above is provided between the polymer-scattered liquid crystal and the shutter liquid crystal, which eliminates a loss of light and prevents deterioration of the transmittance and the viewability. 
     The embodiment described above is merely illustrative of the present invention. It is therefore a matter of course that the present invention is not limited to the embodiment described above and may be modified in various ways in design or the like without departing from the technical spirit of the present invention. 
     For example, the polymer-scattered liquid crystal described above may be replaced with a so-called STN (Super Twisted Nematic) liquid crystal or an organic EL (Electro Luminescent), for example. While the liquid crystals become transparent when voltage application is turned on in the embodiment described above, a liquid crystal that becomes transparent when voltage application is turned off may also be used. In this case, “ON” and “OFF” in  FIG. 7  are reversed. 
     The bonding sheet layer is not limited to that used in the embodiment, and may be formed from a liquid ultraviolet-curable resin by irradiating the ultraviolet-curable resin with ultraviolet rays to cure the ultraviolet-curable resin, for example. That is, the bonding sheet layer may be formed from any material that eliminates an air layer between the polymer-scattered liquid crystal and the shutter liquid crystal. It is desirable, however, that the refractive index of the material forming the bonding sheet layer should be the same as or very close to the refractive index of the material forming the transparent plate material layer  3  and the polarization plate layer  5 . 
     In the embodiment described above, the sensors perform detecting operations at all times. However, the sensors may start detecting operations only when an application program determined in advance such as a voice calling application program or a browser application program is executed, for example. In this case, the sensors are activated only when the application program is executed and do not function when the application program is not executed, which reduces the power consumption. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-211976 filed in the Japan Patent Office on Sep. 14, 2009, the entire content of which is hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.