Patent Publication Number: US-9852701-B2

Title: Display device with improved luminance

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/446,198, entitled “Display Device with Improved Luminance” and filed on Apr. 13, 2012, which claims priority to Japanese Patent Application No. 2011-094626, filed in the Japanese Patent Office on Apr. 21, 2011, each of which are hereby incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present disclosure relates to a display device. 
     Reflective display devices that display an image by controlling reflectivity of external light, and transmissive display devices that display an image by controlling transmissivity of light from a backlight disposed on the back side thereof have been provided. Further, display devices having the advantages of both of the reflective display devices and the transmissive display devices, for example, transflective display devices having pixels including a reflective region and a transmissive region have been proposed. 
     In display devices such as color liquid crystal display devices, a color reproduction range has been expanded and luminance has been increased, and therefore devices having display pixels each including a group of subpixels for displaying three primary colors and a subpixel for displaying a different color (white, cyan, or the like) have been proposed. 
     For example, a color image display device disclosed in Japanese Patent No. 3167026 includes means for generating signals of three colors in an additive three primary colors process from an input signal, and means for generating an auxiliary signal by adding the color signals of the three hues at the same ratio, and supplying signals of the total four colors of the auxiliary signal and the three color signals obtained by subtracting the auxiliary signal from the signals of the three hues to a display device. The three color signals drive a red display subpixel, a green display subpixel, and a blue display subpixel, respectively. The auxiliary signal drives a white display subpixel. 
     SUMMARY 
     For example, in a case of a color-display reflective display device, when external light illuminance decreases, the luminance of a displayed image also decreases. In such a case, from a viewpoint of visibility of the image, it is preferable to display the image with saturation being suppressed to a low value, and luminance is increased to a high value. On the other hand, if the external light illuminance is sufficiently high, an adequate luminance of the displayed image can be obtained, and consequently, it is preferable to display the image of high luminance and high saturation. Accordingly, display devices that can adjust the relationship between the saturation and the luminance depending on the external light illuminance and can display an image having good visibility have been desired. 
     It is desirable to provide a display device that can adjust the relationship between the saturation and the luminance depending on the external light illuminance and can display an image having good visibility. 
     A display device according to an embodiment of the present disclosure includes a display unit having pixels arranged in a two-dimensional matrix, each pixel including additive mixture subpixels and a luminance adjustment subpixel, and a signal control unit controlling a luminance at a maximum gray scale in the luminance adjustment subpixel depending on an external light illuminance. 
     A display device according to an embodiment of the present disclosure includes a signal control unit that controls a luminance at a maximum gray scale in the luminance adjustment subpixel depending on an external illuminance. Accordingly, the display device can adjust the relationship between the saturation and the luminance depending on the external illuminance and can display an image having good visibility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view illustrating a display device according to a first embodiment. 
         FIG. 2  is a schematic circuit diagram illustrating a part of a display unit, the part including an (m, n)th pixel. 
         FIG. 3  is a schematic plan view illustrating a layout of various elements in the part including the (m, n)th pixel of the display unit. 
         FIG. 4  is a schematic cross-sectional view of the display unit taken along the line A-A in  FIG. 3 . 
         FIG. 5  is a schematic block diagram illustrating a signal control unit. 
         FIG. 6A  is a schematic graph illustrating a relationship between a voltage applied to a pixel electrode of a luminance adjustment subpixel at a maximum gray scale and an external light illuminance, and a relationship between an NTSC ratio and an external light illuminance in a color gamut of the display unit. 
         FIG. 6B  is a schematic graph illustrating a relationship between a voltage applied to a pixel electrode of a luminance adjustment subpixel and an external light reflectivity. 
         FIG. 7  is a schematic plan view illustrating a layout of elements in the part including the (m, n)th pixel of a display unit in a display device according to a second embodiment. 
         FIG. 8A  is a schematic graph illustrating a relationship between a voltage applied to a pixel electrode of a luminance adjustment subpixel at a maximum gray scale and an external light illuminance, and a relationship between an NTSC ratio and an external light illuminance in a color gamut of the display unit. 
         FIG. 8B  is a schematic graph illustrating a color variation when external light illuminance changed. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, with reference to the drawings, embodiments of the present disclosure are described. The scope of the present disclosure is not limited to the embodiments, and various numeric values and materials in the embodiments are only examples. In the description below, to the same elements and elements having similar functions, the same reference numerals are applied, and overlapping descriptions are omitted. The description will be made in the following order.
     1. Overall description of a display device according to the embodiments of the present disclosure   2. First embodiment   3. Second embodiment and others   

     Overall Description of a Display Device According to the Embodiments of the Present Disclosure 
     A display device according to the embodiments of the present disclosure may include a reflective display unit, a transmissive display unit, or a transflective display unit that has the features of the reflective display unit and the transmissive display. These display units include a display panel such as a liquid crystal display panel. Alternatively, the display units include a self-emitting display device. The self-emitting display device includes an electroluminescence display panel, a plasma display panel, and the like. 
     A signal control unit for controlling luminance at a maximum gray scale in a luminance adjustment subpixel depending on an external light illuminance includes, for example, a photo sensor for measuring an intensity of external light, and a signal control circuit that controls the value of a voltage for regulating the luminance at the maximum gray scale using an output from the photo sensor. The photo sensor includes existing sensors such as a photodiode and a phototransistor. The signal control circuit includes existing circuits such as an operational circuit, a digital-analog (D/A) converter, a voltage generation circuit, and the like. Such circuits include existing circuit elements. 
     As described above, a reflective, transmissive, or transflective display unit can be used. A display device employing the reflective type or the transflective type can display an image of good visibility depending on external light illuminance. 
     In the display device according to the embodiments of the present disclosure, a pixel includes subpixels for additive color mixture. Generally, color displaying is performed using an additive color mixture process of different three primary colors. For example, a pixel includes a first subpixel for displaying a first primary color (for example, red), a second subpixel for displaying a second primary color (for example, green), and a third subpixel for displaying a third primary color (for example, blue). However, the number of the subpixels for additive color mixture included in the pixel is not limited to three. For example, the pixel may include a fourth subpixel for displaying a fourth primary color for extending the color reproduction range. In addition to the subpixels, the pixel can further include a fifth subpixel for displaying a fifth primary color. In another example, in a configuration using an additive color mixture process of a color gamut of two colors to be displayed, a pixel may include two subpixels for additive color mixture. Generally, the term “primary color” means a color that is not obtained by mixing other colors. In the embodiments of the present disclosure, the definition of the term is not limited to the above-described definition. 
     In the display device according to the embodiments of the present disclosure including the above-described preferred configurations, the display device may be controlled such that the luminance at the maximum gray scale in the luminance adjustment subpixel is lowered as external light illuminance increases. For example, in a case where the external light illuminance is lower than a first reference value, the luminance at the maximum gray scale in the luminance adjustment subpixel may be set to a maximum value in the design. In another example, in a case where the external light illuminance is higher than a second reference value (the second reference value&gt;the first reference value), the luminance at the maximum gray scale in the luminance adjustment subpixel may be set to a minimum value in the design. 
     In the display apparatus according to the embodiments of the present disclosure including the above-described various preferred configurations, the gray scale of the luminance adjustment subpixel may be controlled using a signal indicating luminance information of the additive mixture subpixel. For example, in a case where the additive mixture subpixel includes a first subpixel, a second subpixel, and a third subpixel, the gray scale may be controlled using a signal indicating luminance information generated using each of three kinds of signals corresponding to the individual subpixels. In such a case, the signal indicating the luminance information may be a signal indicating a Y stimulus value. The Y stimulus value is a luminance value in the XYZ color system defined by the Commission internationale de l&#39;éclairage (CIE), or the like. For example, the Y stimulus value may be calculated by adding a predetermined coefficient to each of values of R, G, and B of a reference stimulus in a color equation and adding the values. 
     In the display apparatus according to the embodiments of the present disclosure including the above-described preferred various configurations, the luminance adjustment subpixel may display a color having a saturation lower than those of the colors displayed by the additive mixture subpixels. In such a case, the luminance adjustment subpixel may display white. 
     In another example, in the display apparatus according to the embodiments of the present disclosure including the above-described preferred configurations, the luminance adjustment subpixel may display a color different from those displayed by the additive mixture subpixels. In such a case, the luminance adjustment subpixel may display yellow or cyan. 
     In the individual embodiments described below, a color liquid crystal display panel of an active matrix type is used for the display unit. 
     The liquid crystal panel includes, for example, a front panel having a transparent common electrode, a rear panel having a pixel electrode, and a liquid crystal material disposed between the front panel and the rear panel. In the transmissive type, the pixel electrode is composed of a transparent conductive material. In the reflective type, the pixel electrode may be composed of a material that reflects light, or a reflector independent from the pixel electrode is provided, and the pixel electrode may be composed of a transparent conductive material. The transflective type liquid crystal panel may be similarly composed. 
     The operation mode of the liquid crystal display panel is not limited to a specific mode. For example, the liquid crystal display panel may be driven in a twisted nematic (TN) mode, a vertical alignment (VA) mode, or an in-plane switching (IPS) mode. Further, the liquid crystal display panel may be a normally white type or a normally black type. 
     More specifically, the front panel includes, for example, a substrate composed of glass, a transparent common electrode (for example, composed of indium tin oxide (ITO)) provided on the inner surface of the substrate, and a polarizing film provided on the outer surface of the substrate. On the inner surface of the substrate, a color filter covered with an overcoat layer composed of an acrylic resin or an epoxy resin is provided. On the front panel, further, on the overcoat layer, a transparent common electrode is formed. If necessary, an alignment layer may be formed on the transparent common electrode. 
     The rear panel includes, for example, a substrate composed of glass, a switching element formed on the inner surface of the substrate, and a pixel electrode (for example, composed of ITO) whose conduction of electricity is controlled by the switching element. If necessary, on the whole area including the pixel electrode, an alignment layer may be formed, and a polarizing film or an optical compensation film may be provided on the outer surface of the substrate. 
     The members and materials constituting the liquid crystal display panel include existing members and materials. For the switching element, for example, a three-terminal element such as a thin-film transistor (TFT), or a two-terminal element such as a metal-insulator-metal (MIM) element, a varistor element, or a diode may be employed. To such a switching element, for example, a scanning line extending in the row direction or a signal line extending in the column direction is connected. 
     The shape of the display unit is not limited to a specific shape. For example, the display unit may be a landscape-oriented rectangular shape or a portrait-oriented rectangular shape. If the number of M×N pixels in the display unit is expressed as (M, N), for example, in a case where the display unit has a landscape-oriented rectangular shape, for example, the value (M, N) may be a resolution for image display such as (640, 480), (800, 600), (1024, 768) or the like. In a case where the display unit has a portrait-oriented rectangular shape, for example, the value (M, N) may be a resolution obtained by interchanging the values of the above-mentioned resolutions. The values are not limited to the examples. 
     When an illumination unit for illuminating the display unit with light is to be used, an existing illumination unit may be employed. The configuration of the illumination unit is not limited to a specific configuration. Generally, the illumination unit includes existing members such as a light source and a light guide plate. 
     The various conditions described in the embodiments of the present disclosure may be strictly satisfied or substantially satisfied. For example, a color “red” means a color that is recognized substantially as red, and a color “green” means a color that is recognized substantially as green. Similar descriptions can be applied to “blue”, “white”, “yellow” and “cyan”. Further, variations due to the design or the manufacturing process are allowed. 
     First Embodiment 
     A display device according to the first embodiment of the present disclosure is described. 
       FIG. 1  is a schematic perspective view illustrating the display device according to the first embodiment. 
     A display device  1  includes a display unit  10  having pixels  12  arranged in a two-dimensional matrix, each pixel  12  including additive mixture subpixels  12 A R ,  12 A G , and  12 A B  and a luminance adjustment subpixel  12 A AD . The display unit  10  is a reflective display unit. More specifically, the display unit  10  includes a reflective color liquid crystal display panel. 
     The display device  1  further includes a signal control unit  80  that controls a luminance at a maximum gray scale in the luminance adjustment subpixel  12 A AD  depending on an external light illuminance. The signal control unit  80  includes a photo sensor  82  and a signal control circuit  81 . The photo sensor  82  detects an intensity (illuminance) of external light (environmental light). The signal control circuit  81  performs control using an output from the photo sensor  82  or the like. The photo sensor  82  includes, for example, a photodiode. Due to photovoltaic effect, a photo sensor output (voltage) of the photo sensor  82  changes depending on the intensity of the external light. The photo sensor  82  is disposed at a place where the photo sensor  82  can receive the external light, and is not affected by light from an image displayed on the display unit  10 . In  FIG. 1 , a scanning circuit  101  illustrated in  FIG. 2  described below is omitted. 
     The additive mixture subpixels  12 A R ,  12 A G , and  12 A B  may be referred to as a first subpixel  12 A R , a second subpixel  12 A G , and a third subpixel  12 A B  respectively. The first subpixel  12 A R  displays red as a first primary color. The second subpixel  12 A G  displays green as a second primary color. The third subpixel  12 A B  displays blue as a third primary color. The luminance adjustment subpixel  12 A AD  displays a color having a saturation lower than those of the colors displayed by the additive mixture subpixels. Specifically, the luminance adjustment subpixel  12 A AD  displays white. 
     Based on the operation of the signal control unit  80 , the luminance at the maximum gray scale in the luminance adjustment subpixel  12 A AD  is controlled depending on the external light illuminance. More specifically, the luminance at the maximum gray scale in the luminance adjustment subpixel  12 A AD  is controlled such that the luminance decreases as the external light illuminance increases. The gray scale of the luminance adjustment subpixel  12 A AD  is controlled based on a signal indicating luminance information of the additive mixture subpixels  12 A R ,  12 A G , and  12 A B . More specifically, the signal indicating the luminance information is a signal indicating a Y stimulus value. The configuration and operation of the signal control unit  80  are described in detail below with reference to  FIGS. 5, 6A and 6B  described below. 
     In the description below, the additive mixture subpixels and the luminance adjustment subpixel may be simply referred to as “subpixels  12 A R ,  12 A G ,  12 A B , and  12 A AD ” without limiting the types of the subpixels. 
     In the description, it is assumed that a display region  11  of the display unit  10  is in parallel with the X-Z plane, and the direction in which images are to be observed is the +Y direction. As illustrated in the drawing, the display unit  10  includes a front panel in the +Y direction, a rear panel in the −Y direction, a liquid crystal material disposed between the front panel and the rear panel, and the like. For the purpose of illustration, in  FIG. 1 , the display unit  10  is illustrated as one panel. The display unit  10  has a rectangular shape, and the display region  11  where the pixels  12  are arranged also has a rectangular shape. Reference numerals  13 A,  13 B,  13 C, and  13 D indicate sides of the display unit  10 . In a display unit according to another embodiment illustrated in  FIG. 7  described below, the reference numerals similarly indicate sides of the display unit. 
     In the display region  11 , the total of M×N pixels  12 , i.e., M pixels in the row direction (X direction in the drawing), and N pixels in the column direction (Z direction in the drawing) are arranged. The pixel  12  of the m-th column (m=1, 2, . . . , M), and the n-th row (n=1, 2, . . . , N) is referred to as the (m, n)th pixel  12 , or the pixel  12   (m, n) . The number of pixels (M, N) in the display unit  10  is, for example, (768, 1024). To display units in the other embodiments, this description is similarly applied. 
     In the first embodiment, the pixel  12  includes a group of the reflective subpixels  12 A R ,  12 A G ,  12 A B , and  12 A AD . First, the display unit  10  is described in detail. Then, the configuration and operation of the signal control unit  80  are described in detail. 
       FIG. 2  is a schematic circuit diagram illustrating a part of the display unit  10 , the part including the (m, n)th pixel. 
     The display device  1  includes the reflective subpixels  12 A R ,  12 A G ,  12 A B , and  12 A AD  having N scanning lines  22  each extending in the row direction and one end is being connected to a scanning circuit  101 , 4×M signal lines  26  each extending in the column direction and one end is being connected to the signal control circuit  81 , and transistors (TFTs) being connected to the scanning lines  22  and the signal lines  26  and operating in response to a scanning signal from the scanning lines  22 . 
     To the pixel  12   (m, n) , the scanning line  22  (hereinafter, may be referred to as a scanning line  22   n ) of the n-th row is connected. To the subpixel  12 A R , the signal line  26  of the (4×m−3)th column is connected. To the subpixel  12 A G , the signal line  26  of the (4×m−2)th column is connected. To the subpixel  12 A B , the signal line  26  of the (4×m−1)th column is connected. To the subpixel  12 A AD , the signal line  26  of the (4×m)th column is connected. In the drawings and description below, the indication of “×” may be omitted. For example, the signal line  26  of the (4×m)th column may be expressed as  26   4m . 
     The liquid crystal capacitor LC 1  illustrated in  FIG. 2  includes a transparent common electrode provided on the front panel, a pixel electrode provided on the rear panel, and a liquid crystal material layer sandwiched between the front panel and the rear panel. The storage capacitor C 1  includes an auxiliary electrode conducted to the pixel electrode and the like. In  FIGS. 3 and 4  described below, the auxiliary electrode is omitted. 
     Input signals VD R , VD G , and VD B  corresponding to a color image to be displayed are externally supplied to the display device  1 . The input signals VD R , VD G , and VD B  are a signal for displaying red, a signal for displaying green, and a signal for displaying blue, respectively. According to the operation of the signal control circuit  81 , video signals VS R , VS G , VS B , and VS AD  for driving the subpixels  12 A R ,  12 A G ,  12 A B , and  12 A AD  are generated from the input signals VD R , VD G , and VD B . The relationship between the input signals VD R , VD G , and VD B  and the video signals VS R , VS G , VS B , and VS AD  is described in detail below with reference to  FIG. 5 . The video signal VS R  drives the subpixel  12 A R . The video signal VS G  drives the subpixel  12 A G . The video signal VS B  drives the subpixel  12 A B . The video signal VS AD  drives the subpixel  12 A AD . 
     In the description below, the input signals may be simply referred to as “input signals VD” without limiting the types of the input signals. Similarly, in the description below, the video signals may be simply referred to as “video signals VS” without limiting the types of the video signals. 
       FIG. 3  is a schematic plan view illustrating a layout of the various components in the part including the (m, n)th pixel of the display unit  10 .  FIG. 4  is a schematic cross-sectional view of the display unit taken along the line A-A in  FIG. 3 . 
     As illustrated in  FIG. 4 , the display unit  10  includes a rear panel  20 , a front panel  50 , and a liquid crystal material layer  40  sandwiched between the panels. 
     The front panel  50  includes, a substrate  51 , a transparent common electrode  54 , a quarter wavelength plate  61 , and a polarizing film  62 . The substrate  51  is, for example, composed of glass. The transparent common electrode  54  is, for example, composed of ITO, and provided on the inner surface of the substrate  51 . The quarter wavelength plate  61  is provided on the outer surface of the substrate  51 . The polarizing film  62  covers the quarter wavelength plate  61 . This structure is similar to those in the other embodiment described below. 
     On the liquid crystal material layer  40  side of the substrate  51 , black matrixes  52 , a color filter, the transparent common electrode  54 , and an upper alignment layer  55  are provided. The black matrixes  52  are disposed at corresponding positions between adjacent subpixels. The color filter is disposed within the region surrounded by the black matrixes  52 . The transparent common electrode  54  covers the whole surface including the black matrixes  52  and the color filter. The upper alignment layer  55  covers the whole surface including the transparent common electrode  54 . In  FIG. 4 , reference numeral  53   R  denotes a red color filter. 
     If  FIG. 4  is a schematic cross-sectional view illustrating the display unit taken along the line B-B in  FIG. 3 , reference numeral  12 A R  is replaced with reference numeral  12 A G , and the red color filter  53   R  is replaced with a green color filter  53   G . Similarly, if  FIG. 4  is a schematic cross-sectional view illustrating the display unit taken along the line C-C in  FIG. 3 , reference numeral  12 A R  is replaced with reference numeral  12 A B , and the red color filter  53   R  is replaced with a blue color filter  53   B . Similarly, if  FIG. 4  is a schematic cross-sectional view illustrating the display unit taken along the line D-D in  FIG. 3 , reference numeral  12 A R  is replaced with reference numeral  12 A AD , and the red color filter  53   R  is replaced with a white color filter (that is, simply, a transparent filter)  53   AD . 
     The rear panel  20  includes, a substrate  21 , a switching element, and a pixel electrode. The substrate  21  is, for example, composed of glass. The switching element is composed of a TFT, and the element is formed on the inner surface of the substrate  21 . The pixel electrode is, for example, composed of ITO, and the conduction of the electrode is controlled by the switching element. 
     More specifically, at the liquid crystal material layer  40  side of the substrate  21 , a first insulating layer  23  and a second insulating layer  25  are formed in a stacked structure. Between the substrate  21  and the first insulating layer  23 , the scanning line  22  is formed. Between the first insulating layer  23  and the second insulating layer  25 , a semiconductor thin layer  24  that forms the TFT is formed. On the second insulating layer  25 , the signal line  26  is formed. To one source-drain electrode of the TFT, a tongue region of the signal line  26  is connected. To the other source-drain electrode, through a conduction part  26 A, a pixel electrode  30  is connected. The conduction part  26 A is, for example, formed by patterning simultaneously with the formation of the signal line  26 . 
     The TFT functions as the switching element that operates according to a signal from the scanning line  22 . In response to the operation of the TFT according to the scanning signal from the scanning line  22 , from the signal control circuit  81  through the signal line  26 , the video signals VS R , VS G , VS B , and VS AD  are applied to the pixel electrode  30 . 
     On the second insulating layer  25 , a first insulating interlayer  27  is formed. On the front surface of the first insulating interlayer  27 , at parts corresponding to the subpixels, projections and depressions are formed. On the projections and depressions, for example, a reflector  28  is formed, for example, by evaporating aluminum. On the reflector  28 , a second insulating interlayer  29  is formed. On the second insulating interlayer  29 , the pixel electrode  30  is formed. Further, a lower alignment layer  31  that covers the whole surface including the pixel electrode  30  is provided. 
     As illustrated in  FIG. 3 , the pixel electrode  30  is formed in a rectangular shape. As illustrated in  FIGS. 3 and 4 , the pixel electrode  30  is connected to the conduction part  26 A through the contact penetrating the insulating interlayers  29  and  27 . 
     The liquid crystal material layer  40  is in contact with the lower alignment layer  31  and the upper alignment layer  55 . The alignment layers  31  and  55  define the direction of the molecular axis of liquid crystal molecules in a state in which an electric field is not applied. 
     A voltage V com  (for example, 0 V) illustrated in  FIG. 2  is applied to the transparent common electrode  54  illustrated in  FIG. 4 . Accordingly, the intensity of the magnetic field generated between the pixel electrode  30  and the transparent common electrode  54  can be controlled by a voltage (that is, the video signals VS) applied to the pixel electrode  30 . Further, the electric field generated between the pixel electrode  30  and the transparent common electrode  54  controls the alignment state of the liquid crystal molecules composing the liquid crystal material layer  40 . 
     In  FIG. 4 , the thickness of the liquid crystal material layer  40  is denoted by reference numeral d 1  and held at a predetermined value by a spacer, or the like (not illustrated). The liquid crystal material layer  40  functions as a quarter wavelength plate when no voltage is applied. As the absolute value of the applied voltage increases, the function as the quarter wavelength plate decreases. When the absolute value of the applied voltage is a certain large value, the liquid crystal material layer  40  simply functions as a transparent layer. 
     External light passes through the polarizing film  62 , turns into linearly polarized light, and enters the quarter wavelength plate  61 . Then, in a state the phase is shifted by a quarter wavelength, the light enters the liquid crystal material layer  40 . 
     When no voltage is applied to the liquid crystal material layer  40 , entered light is transmitted through the liquid crystal material layer  40  and the phase of the light further shifts by a quarter wavelength. In this state, the light reaches the reflector  28  and is reflected. The phase of the reflected light further shifts by a quarter wavelength when the light is transmitted through the liquid crystal material layer  40 . In this state, the light enters the quarter wavelength plate  61 . The total of the phase differences of the light that is transmitted through the quarter wavelength plate  61  and enters the polarizing film  62  is one wavelength. This means no phase difference exists. Consequently, the light is directly transmitted through the polarizing film  62 , and exits toward the observer side in a state in which the luminance of the subpixel is high. 
     On the other hand, when a voltage of an adequate value is applied and the liquid crystal material layer  40  simply functions as a transparent layer, the phase of the light being transmitted through the liquid crystal material layer  40  does not change. As described above, the external light passes through the polarizing film  62 , turns into the linearly polarized light, and enters the quarter wavelength plate  61 . Then, in the state in which the phase is shifted by the quarter wavelength, the light enters the liquid crystal material layer  40 . When the light reflected by the reflector  28  enters the quarter wavelength plate  61  again, the phase shift remains by the quarter wavelength. Consequently, the total of the phase differences of the light that is transmitted through the quarter wavelength plate  61  and enters the polarizing film  62  is half the wavelength. This means that the light is linearly polarized light rotated by 90 degrees, and consequently, the polarization direction of the light is perpendicular to the polarizing axis of the polarizing film  62 . As a result, the light is not emitted toward the observer side, and the luminance of the subpixel is low. 
     As described above, the luminance (in other words, the reflectivity of the external light) of the subpixel increases as the absolute value of the voltage applied to the liquid crystal material layer  40  decreases. That is, the display unit  10  operates as a normally white display unit. Meanwhile, a display unit that operates as a normally black display unit can be employed. In such a case, the display unit is to be controlled such that the relationship between the applied voltage and the luminance becomes opposite. 
     The configuration and operation of the signal control unit  80  are described in detail. 
       FIG. 5  is a schematic block diagram illustrating the signal control unit  80 . 
     As described above, the signal control unit  80  includes the photo sensor  82  and the signal control circuit  81 . The photo sensor  82  detects an intensity of external light. The signal control circuit  81  performs control using an output S 1  or the like from the photo sensor  82 . 
     The signal control circuit  81  includes a luminance adjustment subpixel input signal generator  83 , D/A converters  84 A and  84 B, and a reference voltage generator  85 . These elements include a logic circuit, an operational circuit, and the like, and can include an existing circuit element. Each part constituting the signal control circuit  81  and the operational timing of the scanning circuit  101  illustrated in  FIG. 2  are controlled by a timing controller (not illustrated). 
     The luminance adjustment subpixel input signal generator  83  generates the input signal VD AD  corresponding to the luminance adjustment subpixel  12 A AD  using the input signals VD R , VD G , and VD B  that are externally inputted corresponding to the color image to be displayed. The gray scale of the luminance adjustment subpixel  12 A AD  is controlled by the signal VD AD  generated using the three signals VD R , VD G , and VD B  that correspond to the additive mixture subpixels  12 A R ,  12 A G , and  12 A B  respectively. More specifically, the signal VD AD  generated using the three signals VD R , VD G , and VD B  indicates a Y stimulus value. 
     In the description, it is assumed that the input signals VD R , VD G , and VD B  are discrete gray scale values of 0 to 255 in 8 bits, respectively. The values are not limited to the discrete values in 8 bits, but can be appropriately selected depending on the design or the like of the display device. 
     The input signals VD R , VD G , and VD B  are inputted to the luminance adjustment subpixel input signal generator  83 . The luminance adjustment subpixel input signal generator  83  calculates a Y stimulus value shown in the following equation (1) using the input signal VD R  for a stimulus value R, the input signal VD G  for a stimulus value G, and the input signal VD B  for a stimulus value B. The values of coefficients shown in the equation (1) are an example in a case of a standard RGB (sRGB) color space, and the values are not limited to the example. 
     
       
         
           
             
               
                 
                   
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                             0.412424 
                           
                           
                             0.357579 
                           
                           
                             0.180464 
                           
                         
                         
                           
                             0.212656 
                           
                           
                             0.715158 
                           
                           
                             0.072186 
                           
                         
                         
                           
                             0.019332 
                           
                           
                             0.119193 
                           
                           
                             0.950444 
                           
                         
                       
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                             B 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   1 
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     As described above, the Y stimulus value means a luminance value in the XYZ color system defined by the CIE, or the like. The Y stimulus value is zero when all of the input signals VD R , VD G , and VD B  are at zero gray scale, and the Y stimulus value is 255 when all of the input signals VD R , VD G , and VD B  are at 255 gray scale. The luminance adjustment subpixel input signal generator  83  outputs the Y stimulus value as the input signal VD AD  for the luminance adjustment subpixel. Similarly to the input signals VD R , VD G , and VD B , the input signal VD AD  is a value at a gray scale from 0 to 255. 
     Now, the video signals VS R , VS G , VS B , and VS AD  are described. 
     The input signals VD R , VD G , and VD B  are inputted to the D/A converter  84 A. The D/A converter  84 A outputs the video signals VS R , VS G , and VS B  that are voltage signals corresponding to the gray scale values of the input signals VD R , VD G , and VD B . 
     To the D/A converter  84 A, voltages V REF   _   H  and V REF   _   L  are applied as reference voltages for performing the D/A conversion. The voltage V REF   _   H  defines the voltage at the maximum gray scale (255 level), and the value is, for example, about 0 V. The voltage V REF   _   L  defines the voltage at the minimum gray scale (0 level), and the value is, for example, about 4 V. 
     Practically, in order to operate the liquid crystal material layer  40  in alternating current driving, the polarity of, for example, the voltage V REF   _   L  is switched, for example, for each display frame. In the description, the voltage polarity reversal is not taken into consideration. 
     The video signals VS outputted by the D/A converter  84 A take values closer to the voltage V REF   _   H  as the gray scale values of the input signals VD become closer to 255. On the other hand, the video signals VS take values closer to the voltage V REF   _   L  as the gray scale values of the input signals VD become closer to zero. 
     To the D/A converter  84 B, the above-mentioned input signal VD AD  is inputted. The D/A converter  84 B outputs the video signal VS AD  that is the voltage signal corresponding to the gray scale value of the input signal VD AD . The D/A converter  84 B controls the luminance at the maximum gray scale of the luminance adjustment subpixel  12 A AD  depending on the external light illuminance. Consequently, in the D/A converter  84 B, the control corresponding to the external light illuminance is performed. 
     To the D/A converter  84 B, the above-described voltage V REF   _   L  and a voltage V REF   _   Hval  from the reference voltage generator  85  are applied. 
     To the reference voltage generator  85 , from the photo sensor  82 , the photo sensor output S 1  corresponding to the external light illuminance is inputted. In the description, it is assumed that the value of the photo sensor output S 1  increases depending on the external light illuminance, for example, when the external light illuminance is 1×10 2  lux, the value reaches a first reference value L 1 , and when the external light illuminance is 1×10 4  lux, the value reaches a second reference value L 2 . 
     If the photo sensor output S 1  is lower than or equal to the first reference value L 1 , the reference voltage generator  85  sets the value of the voltage V REF   _   Hval  to about 0 V similarly to the voltage V REF   _   H , and if the photo sensor output S 1  is higher than the second reference value L 2 , the reference voltage generator  85  sets the value of the voltage V REF   _   Hval  to about 4 V similarly to the voltage V REF   _   L . 
     If the photo sensor output S 1  is higher than the first reference value L 1  and lower than or equal to the second reference voltage L 2 , the reference voltage generator  85  increases the value of the voltage V REF   _   Hval  depending on the value of the photo sensor output S 1 . In such a case, the value of the voltage V REF   _   Hval  takes a value between the voltage V REF   _   H  and the voltage V REF   _   L  depending on the external light illuminance. 
     The operation of the D/A converter  84 B is similar to that in the D/A converter  84 A, except that the value of the voltage V REF   _   Hval  is controlled depending on the external light illuminance. The voltage value of the video signal VS AD  outputted by the D/A converter  84 B takes a value closer to the voltage V REF     —Hval    as the gray scale value of the input signal VD AD  becomes closer to 255. On the other hand, the voltage value of the video signal VS AD  takes a value closer to the voltage V REF   _   L  as the gray scale value of the input signal VD AD  becomes closer to zero. 
     In the D/A converter  84 B, as described above, the value of the voltage V REF   _   Hval  defining the voltage at the maximum gray scale (255 level) is controlled depending on the external light illuminance. By the control, the luminance at the maximum gray scale of the luminance adjustment subpixel  12 A AD  is controlled depending on the external light illuminance. 
     That is, in a case where the external light illuminance is lower than or equal to 1×10 2  lux, the voltage V REF   _   Hval  takes a value similar to the voltage V REF   _   H . Consequently, the subpixels  12 A R ,  12 A G ,  12 A B , and  12 A AD  are driven in the same condition, and as a result, no difference is generated in the reflectivities of the external light at the maximum gray scale value. Accordingly, basically, the luminances of the individual subpixels at the maximum gray scale take similar values. 
     In a case where the external light illuminance is higher than 1×10 2  lux and lower than or equal to 1×10 4  lux, the voltage V REF   _   Hval  takes a value between the voltage V REF   _   H  and the voltage V REF   _   L . Consequently, as the external light illuminance increases, the luminance of the luminance adjustment subpixel  12 A AD  at the maximum gray scale decreases. 
     In a case where the external light illuminance is higher than 1×10 4  lux, the voltage V REF   _   Hval  takes a value similar to the voltage V REF   _   L  that defines the minimum gray scale (0 level). Consequently, the luminance adjustment subpixel  12 A AD  is driven in a condition different from those for the subpixels  12 A R ,  12 A G , and  12 A B . The reflectivity of the external light in the luminance adjustment subpixel  12 A AD  at the maximum gray scale is substantially zero, and accordingly, the luminance adjustment subpixel  12 A AD  is in a substantially black display state irrespective of the gray scale value. 
     As described above, based on the operation of the signal control unit  80 , the luminance at the maximum gray scale in the luminance adjustment subpixel  12 A AD  is controlled depending on the external light illuminance. More specifically, the luminance at the maximum gray scale in the luminance adjustment subpixel  12 A AD  is controlled such that the luminance decreases as the external light illuminance increases. The control of the luminance is described with reference to  FIGS. 6A, 6B, and 7 . 
       FIG. 6A  is a schematic graph illustrating the relationship between the voltage applied to the pixel electrode of the luminance adjustment subpixel at the maximum gray scale and the value of the external illuminance, and the relationship between an NTSC ratio and the value of the external illuminance in the color gamut of the display unit.  FIG. 6B  is a schematic graph illustrating the relationship between the voltage applied to the pixel electrode of the luminance adjustment subpixel and the external light reflectivity. 
     As illustrated in  FIG. 6A , as the external light illuminance Ei increases, the voltage applied to the pixel electrode  30  in the luminance adjustment subpixel  12 A AD  at the maximum gray scale increases. As illustrated in  FIG. 6B , as the voltage applied to the pixel electrode  30  in the luminance adjustment subpixel  12 A AD  increases, the external light reflectivity decreases. In  FIG. 6B , the unit of the vertical axis is an arbitrary unit normalized by the maximum reflectivity equal to one. 
     Qualitatively, if display using a luminance adjustment subpixel having a high lightness and a low saturation such as white is performed, the luminance of the displayed image increases and the saturation of the image decreases. Consequently, an NTSC ratio (a ratio to a region in a triangle color gamut in the NTSC system in the 1976 UCS chromaticity) varies depending on the voltage applied to the pixel electrode  30  in the luminance adjustment subpixel  12 A AD  at the maximum gray scale. In the first embodiment, the NTSC ratio is about 40% when the external light illuminance exceeds 1×10 4  lux, and as the external light illuminance decreases, the NTSC ratio decreases. When the external light illuminance is lower than or equal to 1×10 2  lux, the NTSC ratio decreases to about 5%. 
     As a result, in a bright place, the image having the high luminance and the high saturation can be displayed. On the other hand, in a dark place, the image having the low saturation but having the higher luminance can be displayed. As described above, depending on the external light illuminance, the relationship between the saturation and the luminance can be adjusted, and the image having excellent visibility can be displayed. 
     Second Embodiment 
     The second embodiment is a modification of the first embodiment. In the second embodiment, as compared to the first embodiment, the color displayed by the luminance adjustment subpixel differs, and setting of the areas of the subpixels differs. 
     In a schematic perspective view illustrating a display device according to the second embodiment, the display unit  10  illustrated in  FIG. 1  is replaced with a display unit  210 , and the display device  1  is replaced with a display device  2 . In a schematic circuit diagram illustrating a part of the display unit  210 , the part including the (m, n)th pixel, is similar to the circuit diagram illustrated in  FIG. 2 . 
     As described above, a pixel includes, as the additive mixture subpixels, the first subpixel  12 A R  that displays red as the first primary color, the second subpixel  12 A G  that displays green as the second primary color, and the third subpixel  12 A B  that displays blue as the third primary color. The luminance adjustment subpixel  12 A AD  displays a color different from the color displayed by the additive mixture subpixels. More specifically, the luminance adjustment subpixel  12 A AD  displays yellow. Alternatively, the luminance adjustment subpixel  12 A AD  can display cyan. 
       FIG. 7  is a schematic plan view illustrating a layout of the various components of a part in the display unit in the display device according to the second embodiment, the part including the (m, n)th pixel. 
     In the second embodiment, the luminance adjustment subpixel  12 A AD  displays yellow. Consequently, qualitatively, when the luminance adjustment subpixel  12 A AD  operates, the color of the image shifts to the yellow side. Accordingly, the display by the additive mixture subpixels is set to shift to the blue side where the relationship of complementary colors is established. More specifically, as illustrated in  FIG. 7 , the size of the third subpixel  12 A B  that displays blue is set to a size larger than those of the first subpixel  12 A R  and the second subpixel  12 A G . The ratio of the size of each subpixel to the entire pixel size can be appropriately set depending on the design of the display device. 
     A schematic cross-sectional view of the display unit taken along the line A-A in  FIG. 7  is similar to the cross-sectional view illustrated in  FIG. 4 . Similarly to the description in the first embodiment, the line B-B and the line C-C in  FIG. 7  are to be appropriately replaced with the cross-sectional view illustrated in  FIG. 4 . In a schematic cross-sectional view illustrating the display unit taken along the line D-D in  FIG. 7 , reference numeral  12 A R  in  FIG. 4  is replaced with reference numeral  12 A AD , and the red color filter  53   R  in  FIG. 4  is replaced with a yellow color filter  53   AD . 
     The operation of the signal control unit  80  is similar to that described in the first embodiment. The yellow luminance adjustment subpixel  12 A AD  is, similarly to that in the first embodiment, driven by the input signal VD AD  for the luminance adjustment subpixel. 
       FIG. 8A  is a schematic graph illustrating the relationship between the voltage applied to the pixel electrode of the luminance adjustment subpixel at the maximum gray scale and the value of the external illuminance, and the relationship between an NTSC ratio and the value of the external illuminance in the color gamut of the display unit.  FIG. 8B  is a schematic graph illustrating a color variation when the external light illuminance changed. 
     In the second embodiment, the NTSC ratio is about 15% when the external light illuminance exceeds 1×10 4  lux, and as the external light illuminance decreases, the NTSC ratio decreases. When the external light illuminance is lower than or equal to 1×10 2  lux, the NTSC ratio decreases to about 5%. 
     As described above, similarly to the description in the first embodiment, in a bright place, the image having the high luminance and the high saturation can be displayed. On the other hand, in a dark place, the image having the low saturation but having the higher luminance can be displayed. As described above, depending on the external light illuminance, the relationship between the saturation and the luminance can be adjusted, and the image having excellent visibility can be displayed. 
     In the second embodiment, as the external light illuminance increases, the hue in the white display varies in the blue direction.  FIG. 8B  illustrates the relationship between the external light illuminance and the variation in the chromaticity coordinates in a L*a*b* color system. As illustrated in the graph in  FIG. 8B , as the external light illuminance Ei increases, the color coordinates vary in the +a* direction and in the −b* direction. 
     Generally, reflective liquid crystal display panels tend to have a yellowish tint in the white display due to the constituent materials. Such a tendency can be corrected by adjusting a spectral transmittance in a color filter. However, the correction may cause decrease in the efficiency in the use of the light. According to the second embodiment, when the external light illuminance is high, the hue in the white display shifts in the blue direction. Consequently, there is an advantage that the yellowish tint in the white display becomes less noticeable. 
     While the present disclosure has been specifically described with reference to the embodiments, it is to be understood that the present disclosure is not limited to the disclosed embodiments, and various modifications and changes can be made within the technical scope of the disclosure. 
     For example, in the above-described embodiments, a transflective display unit may be employed as the display unit. When the transflective display unit is employed, for example, each subpixel may include a reflective region and a transmissive region. For example, the transmissive region can be formed by removing a part of the second insulating interlayer  29  and the reflector  28  illustrated in  FIG. 4 , and making the thickness of the liquid crystal material layer  40  in the part function as a half-wavelength plate. On the outside (backlight side) of the rear panel, in addition to the polarizing film, a necessary optical compensation film may be provided. 
     Further, the present technology may be provided as follows: 
     (1) A display device including: 
     a display unit having pixels arranged in a two-dimensional matrix, each pixel including additive mixture subpixels and a luminance adjustment subpixel; and 
     a signal control unit controlling a luminance at a maximum gray scale in the luminance adjustment subpixel depending on an external light illuminance. 
     (2) The display device described in (1), wherein the display unit is a reflective or transflective display unit. 
     (3) The display device described in (1) or (2), wherein the luminance at the maximum gray scale in the luminance adjustment subpixel is controlled to decrease as the external light illuminance increases. 
     (4) The display device described in any one of (1) to (3), wherein the gray scale of the luminance adjustment subpixel is controlled using a signal indicating luminance information of the additive mixture subpixels. 
     (5) The display device described in (4), wherein the signal indicating the luminance information indicates a Y stimulus value. 
     (6) The display device described in any one of (1) to (5), wherein the luminance adjustment subpixel displays a color having a saturation lower than those of colors displayed by the additive mixture subpixels. 
     (7) The display device described in (6), wherein the luminance adjustment subpixel displays white. 
     (8) The display device described in (1), wherein the luminance adjustment subpixel displays a color different from those displayed by the additive mixture subpixels. 
     (9) The display device described in (8), wherein the luminance adjustment subpixel displays yellow or cyan. 
     The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2011-094626 filed in the Japan Patent Office on Apr. 21, 2011, the entire contents of which are hereby incorporated by reference.