Patent Publication Number: US-2011069051-A1

Title: Display

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese Priority Patent Application JP 2009-217182 filed in the Japan Patent Office on Sep. 18, 2009, the entire content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a display including a light-emitting element in a display panel. 
     Background of the Invention 
     In recent years, in the field of displays displaying an image, displays using current drive type optical elements of which light emission luminance changes depending on the value of a current flowing therethrough, for example, organic EL (Electro Luminescence) elements as light-emitting elements of pixels have been developed for commercialization. Unlike liquid crystal elements or the like, the organic EL elements are self-luminous elements. Therefore, in a display (an organic EL display) using the organic EL elements, a light source (a backlight) is not necessary, so compared to a liquid crystal display needing a light source, a reduction in the profile of the display and an increase in the luminance of the display are allowed. In particular, in the case where the display uses an active matrix system as a drive system, each pixel continuously emits light, resulting a reduction in power consumption. Therefore, the organic EL display is expected to become a mainstream of next-generation flat panel display. 
     An issues exists when using current EL Elements in that the luminance if reduces due to a degradation in the elements according to the value of a current passing therethrough. Therefore, in the case where the organic EL elements are used as pixels of a display, the pixels may have different degradation states. For example, in the case where information such as time or a display channel is displayed in a fixed area of a display with high luminance for a long time, degradation in pixels located in the area accelerates. As a result, in the case where a picture with high luminance is displayed in an area including prematurely degraded pixels of the display, a phenomenon called burn-in in which the picture is displayed dark in the area including the prematurely degraded pixels only occurs. Burn-in is irreversible, so once burn-in occurs, the burn-in is permanent. 
     A large number of techniques of preventing burn-in have been proposed. For example, as described in Japanese Unexamined Patent Application Publication No. 2002-351403, there is disclosed a method of estimating a degree of degradation in a dummy pixel which is arranged outside a display region by detecting a terminal voltage when the dummy pixel emits light and then correcting a picture signal with use of the estimated degree of degradation. Moreover, for example, as described in Japanese Unexamined Patent Application Publication No. 2008-58446 and International Publication WO2006/046196, there are disclosed methods of arranging a photosensor in each display pixel and correcting a picture signal with use of a photoreception signal outputted from the photosensor. 
     SUMMARY OF THE INVENTION 
     However, in the technique in Japanese Unexamined Patent Application Publication No. 2002-351403, the degree of degradation in a pixel in a display region is not estimated based on light emission information of the pixel in the display region, so a picture signal is not accurately corrected. Therefore, it is difficult to prevent burn-in. Moreover, in the techniques in Japanese Unexamined Patent Application Publication No. 2008-58446 and International Publication WO2006/046196, photoelectric conversion efficiency varies among photosensors in pixels. Therefore, for example, the magnitudes of photoreception signals from two pixels displaying with the same luminance may be different from each other. As a result, it is difficult to accurately prevent burn-in. 
     In accordance with principles of the invention, a display which allows accurate burn in prevention is provided. 
     According to one embodiment consistent with the present invention, there is provided a display including a display region including a plurality of luminescence elements, a non-display region including a plurality of luminescence elements and a photoreception element, a drive unit connected to each of the luminescence elements in the display region by a display region signal line, a photoreception drive circuit connected to the plurality of luminescence elements in the non-display region by a non-display signal line, and a photoreception processing unit which receives a signal output from each of the plurality of luminescence elements in the non-display region and outputs a degradation signal to the drive unit. Where the drive unit provides a signal to the plurality of luminescence elements in the display region based on the degradation signal. 
     In another embodiment consistent with the present invention, the drive unit adjusts the signal to the plurality of the luminescence elements in the display region based on the degradation signal. 
     In yet another embodiment consistent with the present invention, the photoreception unit determines the degradation signal based on the following equation: 
     
       
      
       D 
       i 
       =D 
       n(Yi, Ys)  
      
         
         
           
             where D i  is the degradation rate of one of the plurality of luminescence elements in the non-display region, D s  is the degradation rate of a reference luminescence elements, and n(Yi,Ys) is an exponentiation factor of luminance of one of the plurality of luminescence elements in the non-display region with respect to a reference luminescence element selected by the photoreception processing unit. 
           
         
       
    
     In another embodiment consistent with the present invention, the photoreception unit determines the exponentiation factor based on the following equation 
     
       
         
           
             
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     where Ys(Tk) is a signal output from the reference luminescence element at a time Tk, Ys(Tk−1) is a signal output from the reference luminescence element at a time Tk−1, Yi(Tk) is a signal output from one of the plurality of luminescence elements in the non-display region at the time Tk, and Yi(Tk−1) is a signal output from one of the plurality of luminescence elements in the non-display region at the time Tk−1. 
     In another embodiment consistent with the present invention, the display unit includes a memory unit connected between the photoreception processing unit and the drive unit which stores the degradation signal before forwarding the signal to the drive unit. 
     In another embodiment consistent with the present invention, the photoreception drive circuit provides a constant signal to the plurality of luminescence elements in the non-display area. 
     In another embodiment consistent with the present invention, the reference luminescence element is one of the plurality of pixels in the non-display region. 
     In another embodiment consistent with the present invention, a constant sampling time period separates the time Tk from the time Tk−1 as defined by the following equation 
     
       
      
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     where ΔT is a constant time span. 
     In another embodiment consistent with the present invention, the time span ΔT is a variable time span. 
     Another embodiment consistent with the present invention provides method of adjusting the luminance of a display device which includes a display region having a plurality of luminescence elements and a non-display region having a plurality of luminescence elements with a photoreception element, the method comprising the steps of providing a control signal from a photoreception drive circuit to the plurality of luminescence elements in the non display region, receiving a signal output from each of the plurality of luminescence elements in the non-display region by a photoreception processing unit and determining a degradation rate of the luminescence elements in the non display region, outputting the degradation signal to the drive unit, and adjusting the signal sent from the drive unit to the luminescence elements in the display region by the degradation signal. 
     In another embodiment consistent with the present invention, the method includes the step of determining a degradation rate by the photoreception unit based on the following equation 
     
       
      
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     where D i  is the degradation rate of one of the plurality of luminescence elements in the non-display region, D s  is the degradation rate of a reference luminescence elements, and n(Yi,Ys) is an exponentiation factor of luminance of one of the plurality of luminescence elements in the non-display region with respect to a reference luminescence element selected by the photoreception processing unit. 
     In another embodiment consistent with the present invention, the exponentiation factor is determined by the photoreception unit based on the following equation 
     
       
         
           
             
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             where Ys(Tk) is a signal output from the reference luminescence element at a time Tk, Ys(Tk−1) is a signal output from the reference luminescence element at a time Tk−1, Yi(Tk) is a signal output from one of the plurality of luminescence elements in the non-display region at the time Tk, and Yi(Tk−1) is a signal output from one of the plurality of luminescence elements in the non-display region at the time Tk−1. 
           
         
       
    
     In another embodiment consistent with the present invention, the method includes the step of storing the degradation signal before forwarding the signal to the drive unit in a memory unit connected between the photoreception processing unit and the drive unit before the outputting step. 
     In another embodiment consistent with the present invention, the photoreception drive circuit provides a constant signal to the plurality of luminescence elements in the non-display area. 
     In another embodiment consistent with the present invention, the reference luminescence element is one of the plurality of pixels in the non-display region. 
     In another embodiment consistent with the present invention, a constant sampling time period separates the time Tk from the time Tk−1 as defined by the following equation 
     
       
      
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     where ΔT is a constant time span. 
     In another embodiment consistent with the present invention, the time span ΔT is a variable time span. 
     Other systems, methods, features, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating an example of a configuration of a display according to an embodiment of the invention. 
         FIG. 2  is a schematic view illustrating an example of a configuration of a pixel circuit. 
         FIG. 3  is a top view illustrating an example of a configuration of a display panel in  FIG. 1 . 
         FIG. 4  is a plot illustrating an example of a temporal change in luminance degradation rate of each initial luminance. 
         FIG. 5  is a plot illustrating an example of a relationship between a luminance degradation rate and a luminance degradation rate of a dummy pixel with initial luminance Y S . 
         FIG. 6  is a plot illustrating an example of a relationship between an exponentiation factor n (Y i , Y s ) and an initial luminance ratio Y i /Y s . 
         FIG. 7  is a plot illustrating an example of a relationship between an estimated value Y S2  of a luminance degradation rate at a time T k  and a measured value Y S1  of the luminance degradation rate at the time T k . 
         FIG. 8  is a plot illustrating an example of a relationship between a luminance degradation function F s (t) at a time T k−1  and a luminance degradation function F s (t) at the time T k . 
         FIG. 9  is a conceptual diagram for describing an example of a method of calculating an exponentiation factor. 
         FIG. 10  is a plot illustrating an example of a relationship between an exponentiation factor n(Y i , Y s ) at the time T k−1  and an exponentiation factor n(Y i , Y s ) at the time T k . 
         FIG. 11  is a conceptual diagram for describing an example of a method of calculating a luminance degradation function F i (t). 
         FIG. 12  is a conceptual diagram for describing an example of a method of deriving an accumulated light emission time T xy  with reference luminance 
         FIG. 13  is a conceptual diagram for describing an example of a method of deriving a correction amount ΔS xy . 
         FIG. 14  is a conceptual diagram for describing a correction method in related art. 
         FIG. 15  is a plot illustrating an example of a relationship between an acceleration factor α and a luminance degradation rate. 
         FIG. 16  is a plot illustrating another example of a relationship between an acceleration factor α and a luminance degradation rate. 
         FIG. 17  is an external perspective view of Application Example 1 of the display according to the above-described embodiment. 
         FIGS. 18A and 18B  are an external perspective view from the front side of Application Example 2 and an external perspective view from the back side of Application Example 2, respectively. 
         FIG. 19  is an external perspective view of Application Example 3. 
         FIG. 20  is an external perspective view of Application Example 4. 
         FIGS. 21A to 21G  illustrate Application Example 5,  FIGS. 21A and 21B  are a front view and a side view in a state in which Application Example 5 is opened, respectively, and  FIGS. 21C ,  21 D,  21 E,  21 F and  21 G are a front view, a left side view, a right side view, a top view and a bottom view in a state in which Application Example 5 is closed, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While various embodiments of the present invention have been described, it will be apparent to those of skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents. 
       FIG. 1  illustrates a schematic configuration of a display  1  according to one embodiment consistent with the present invention. The display  1  includes a display panel  10  and a drive circuit  20  driving the display panel  10 . 
     The display panel  10  includes a display region  12  in which a plurality of organic EL elements  11 R,  11 G and  11 B are two-dimensionally arranged. In the embodiment, three adjacent organic EL elements  11 R,  11 G and  11 B configures one pixel (one display pixel  13 ). In addition, the organic EL elements  11 R,  11 G and  11 B are collectively called organic EL elements  11  as necessary. The display panel  10  also includes a non-display region  15  in which a plurality of organic EL elements  14 R,  14 G and  14 B are two-dimensionally arranged. In this embodiment, three adjacent organic EL elements  14 R,  14 G and  14 B configures one pixel (one dummy pixel  16 ). In addition, the organic EL elements  14 R,  14 G and  14 B are collectively called organic EL elements  14  as necessary. In the non-display region  15 , a photoreception element group  17  (a photoreception section) receives light emitted from the organic EL elements  14 R,  14 G and  14 B. The photoreception element group  17  is configured of, for example, a plurality of photoreception elements (not illustrated). For example, the plurality of photoreception elements are two-dimensionally arranged so as to be paired with the organic EL elements  14 , respectively, and each of the photoreception elements detects light (emission light) emitted from each dummy pixel  16  (each organic EL element  14 ) to output a photoreception signal  17 A (luminance information) of each dummy pixel  16 . Each photoreception element may include, but is not limited to, a photodiode or any other device capable of detecting light and outputting a photoreception signal. 
     The drive circuit  20  includes a timing generation circuit  21 , a picture signal processing circuit  22 , a signal line drive circuit  23 , a scanning line drive circuit  24 , a dummy pixel-photoreception element group drive circuit  25 , a photoreception signal processing circuit  26  and a memory circuit  27 . 
       FIG. 2  illustrates one configuration of a circuit configuration in the display region  12 . In the display region  12 , a plurality of pixel circuits  18  are two-dimensionally arranged so as to be paired with the organic EL elements  11 , respectively. Each of the pixel circuits  18  is configured of, for example, a drive transistor Tr 1 , a writing transistor Tr 2  and a retention capacitor C s , that is, each of the pixel circuits  18  has a 2Tr1C circuit configuration. The driving transistor Tr 1  and the writing transistor Tr 2  each are configured of, for example, an n-channel MOS type thin film transistor (TFT). The drive transistor Tr 1  or the writing transistor Tr 2  may be configured of, for example, a p-channel MOS type TFT. 
     In the display region  12 , a plurality of signal lines DTL are arranged in a column direction, and a plurality of scanning lines WSL and a plurality of power supply lines Vcc are arranged in a row direction. One (one sub-pixel) of the organic EL elements  11 R,  11 G and  11 B is arranged around each of intersections of the signal lines DTL and the scanning lines WSL. Each of the signal lines DTL is connected to an output end (not illustrated) of the signal line drive circuit  23  and a drain electrode of the writing transistor Tr 2 . Each of the scanning lines WSL is connected to an output end (not illustrated) of the scanning line drive circuit  24  and a gate electrode of the writing transistor Tr 2 . Each of the power supply lines Vcc is connected to an output end (not illustrated) of a power supply and a drain electrode of the drive transistor Tr 1 . A source electrode of the writing transistor Tr 2  is connected to a gate electrode of the drive transistor Tr 1  and an end of the retention capacitor C s . A source electrode of the drive transistor Tr 1  and the other end of retention capacitor C s  are connected to an anode electrode of the organic EL element  11 . A cathode electrode of the organic EL element  11  is connected to, for example, a ground line GND. 
       FIG. 3  illustrates one embodiment of a top configuration of the display panel  10  consistent with the present invention. The display panel  10  has, for example, a configuration in which a drive panel  30  and a sealing panel  40  are bonded together with a sealing layer (not illustrated) in between. 
     The drive panel  30  includes a plurality of organic EL elements  11  (not illustrated in  FIG. 3 ) which are two-dimensionally arranged and a plurality of pixel circuits  18  (not illustrated in  FIG. 3 ) which are arranged adjacent to the organic EL elements  11 , respectively, in the display region  12 . The drive panel  30  further includes a plurality of organic EL elements  14  (not illustrated in  FIG. 3 ) which are two-dimensionally arranged and a plurality of photoreception elements (not illustrated in  FIG. 3 ) which are arranged adjacent to the organic EL elements  14 , respectively, in the non-display region  15 . 
     As illustrated in  FIG. 3 , a plurality of picture signal supply TABs  51 , a control signal supply TCP  54  and a photoreception signal output TCP 55  are mounted on one side (a long side) of the drive panel  30 . For example, scanning signal supply TABs  52  are mounted on another side (a short side) of the drive panel  30 . Moreover, for example, a power supply TCP  53  is mounted on a side (a long side) different from the long side where the picture signal supply TABs  51  are mounted of the drive panel  30 . The picture signal supply TABs  51  each are formed by interconnecting an integrated IC of the signal line drive circuit  23  to an opening of a film-shaped wiring board. The scanning signal supply TAB  52  is formed by interconnecting an integrated IC of the scanning line drive circuit  24  to an opening of a film-shaped wiring board. The power supply TCP  53  is formed by forming a plurality of wires which are electrically connected between an external power supply and the power supply lines Vcc on a film. The control signal supply TCP  54  is formed by forming a plurality of wires which are electrically connected between the external dummy pixel-photoreception element group drive circuit  25  and the dummy pixels  16  and between the dummy pixel-photoreception element group drive circuit  25  and the photoreception element group  17  on a film. The photoreception signal output TCP  55  is formed by forming a plurality of wires which are electrically connected between the external photoreception signal processing circuit  26  and the photoreception element group  17  on a film. In addition, the signal line drive circuit  23  and the scanning line drive circuit  24  are not necessarily formed with a TAB structure, and may be formed on, for example, the drive panel  30 . 
     The sealing panel  40  includes, for example, a sealing substrate (not illustrated) sealing the organic EL elements  11  and  14  and a color filter (not illustrated). The color filter is provided in a region allowing light from the organic EL elements  11  to pass therethrough of a surface of the sealing substrate. The color filter includes, for example, a red filter, a green filter and a blue filter (all not illustrated) corresponding to the organic EL elements  11 R,  11 G and  11 B, respectively. The sealing panel  40  further includes, for example, a light reflection section (not illustrated). The light reflection section reflects light emitted from the organic EL elements  14  so that the light enters into the photoreception element group  17 , and the light reflection section is provided, for example, in a region allowing light from the organic EL elements  14  to pass therethrough of the surface of the sealing substrate. 
     Next, each circuit in the drive circuit  20  will be described below referring to  FIG. 1 . The timing generation circuit  21  controls the picture signal processing circuit  22 , the signal line drive circuit  23 , the scanning line drive circuit  24 , the dummy pixel-photoreception element group drive circuit  25  and the photoreception signal processing circuit  26  to operate in synchronization with one another. 
     For example, the timing generation circuit  21  outputs a control signal  21 A to each of the above-described circuits in response to (in synchronization with) a synchronization signal  20 B inputted from outside. The timing generation circuit  21  is formed on a control circuit board (not illustrated) which is different from the display panel  10  together with the picture signal processing circuit  22 , the dummy pixel-photoreception element group drive circuit  25 , the photoreception signal processing circuit  26 , the memory circuit  27  and the like. 
     As an illustrative example, the picture signal processing circuit  22  corrects a digital picture signal  20 A inputted from outside in response to (in synchronization with) input of the control signal  21 A, and converts the corrected picture signal  20 A into an analog signal to output the analog signal to the signal line drive circuit  23 . In the embodiment, the picture signal processing circuit  22  corrects the picture signal  20 A with use of correction information  26 A (which will be described later) read out from the memory circuit  27 . The picture signal processing circuit  22  reads out, as the correction information  26 A, a correction amount ΔS xy  (which will be described later) of each of display pixels  13  for one line from the memory circuit  27  in each horizontal period, and then corrects the picture signal  20 A with use of the read correction amount ΔS xy  to output a picture signal  22 A which is obtained by correction to the signal line drive circuit  23 . 
     The signal line drive circuit  23  outputs the analog signal  22 A inputted from the picture signal processing circuit  22  to each signal line DTL in response to (in synchronization with) input of the control signal  21 A. For example, as illustrated in  FIG. 3 , the signal line drive circuit  23  is provided in each of the picture signal supply TABs  51  mounted on a side (a long side) of the drive panel  30 . The scanning line drive circuit  24  sequentially selects one scanning line WSL from a plurality of scanning lines WSL in response to (in synchronization with) input of the control signal  21 A. For example, as illustrated in  FIG. 3 , the scanning line drive circuit  24  is provided in each of the scanning signal supply TABs  52  mounted on another side (a short side) of the drive panel  30 . 
     Referring again to  FIG. 1 , the photoreception signal processing circuit  26  derives the correction information  26 A based on the photoreception signal  17 A inputted from the photoreception element group  17 , and then outputs the derived correction information  26 A to the memory circuit  27  in response to (in synchronization with) input of the control signal  21 A. In addition, a method of deriving the correction information  26 A will be described later. The memory circuit  27  stores the correction information  26 A inputted from the photoreception signal processing circuit  26 . The memory circuit  27  is allowed to read out the stored correction information  26 A by the picture signal processing circuit  22 . 
     The dummy pixel-photoreception element group drive circuit  25  allows constant currents with different magnitudes to flow through the dummy pixels  16 , respectively, so that the dummy pixels  16  emit light in response to (in synchronization with) input of the control signal  21 A. In the case where the number of dummy pixels  16  is n, the dummy pixel-photoreception element group drive circuit  25  allows a constant current with a magnitude allowing a pixel to have initial luminance Y 1  to flow through a first dummy pixel  16 , and allows a constant current with a magnitude allowing a pixel to have initial luminance Y 2 (&gt;Y 1 ) to flow through a second dummy pixel  16 . Moreover, the dummy pixel-photoreception element group drive circuit  25  allows a constant current with a magnitude allowing a pixel to have initial luminance Y i (&gt;Y i−1 ) to flow an ith dummy pixel  16 , and allows a constant current with a magnitude allowing a pixel to have initial luminance Y n (&gt;Y n−1 ) to flow through an nth dummy pixel  16 . For example, the dummy pixel-photoreception element group drive circuit  25  measures a time when a current flows through each dummy pixel  16 . 
     In addition, even if a constant current continuously flows through each dummy pixel  16 , for example, as illustrated in  FIG. 4 , the luminance of each dummy pixel  16  is gradually reduced over time, because the organic EL element  14  included in each dummy pixel  16  degrades with an increase in a current-carrying time (an accumulated light emission time). As a result, the light emission luminance is reduced according to a progress degree of degradation in the organic EL element  14 . In addition, Y s  in  FIG. 4  is initial luminance of a pixel selected as a reference pixel (which will be described later) from the dummy pixels  16 . 
     Moreover, the transition of the luminance degradation rate of each dummy pixel  16  is not uniform. For example, as illustrated in  FIG. 5 , in the case where a horizontal axis in  FIG. 5  indicates the luminance degradation rate of the pixel (the dummy pixel  16 ) set as the reference pixel, it is obvious that at first, the transition of the luminance degradation rate of a dummy pixel  16  with smaller initial luminance than the initial luminance Y s  of the reference pixel is more moderate than the transition of luminance degradation in the reference pixel. On the other hand, it is obvious that at first, the transition of the luminance degradation rate of a dummy pixel  16  with larger initial luminance than the initial luminance Y s  of the reference pixel is steeper than the transition of luminance degradation in the reference pixel. The transition of the luminance degradation rate of each dummy pixel  16  exemplified in  FIG. 5  is represented by the following expression. 
         D   i   =D   s   n(Yi, Ys)    Mathematical Expression 1
 
     In Mathematical Expression 1, D i  represents a luminance degradation rate of the ith dummy pixel  16 . D s  represents a luminance degradation rate of the reference pixel. Moreover, n(Y i , Y s ) represents an exponentiation factor of luminance of the ith dummy pixel  16  with respect to luminance of the reference pixel. For example, as illustrated in the following expression, the exponentiation factor n(Y i , Y s ) is derived by dividing (Log(Y i (T k ))−Log(Y i (T k−1 ))) by (Log(Y s (T k )−Log(Y s (T k−1 ))). 
     
       
         
           
             
               
                 
                   
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     In Mathematical Expression 2, Log(Y s (T k )), Log(Y s (T k−1 )), Log(Y i (T k )) and Log(Y i (T k−1 )) represent a logarithm of Y s (T k ), a logarithm of Y s (T k−1 ), a logarithm of Y i (T k ) and a logarithm of Y i (T k−1 ), respectively. In addition, the denominator (Log(Y s (T k ))−Log(Y s (T k−1 ))) in the right-hand side of Mathematical Expression 2 corresponds to a specific example of “first luminance degradation information” in the invention. Moreover, the numerator (Log(Y i (T k ))−Log(Y i (T k−1 ))) in the right-hand side of Mathematical Expression 2 corresponds to a specific example of “second luminance degradation information” in the invention. 
     Moreover, in Mathematical Expression 2, Y s (T k ) represents a photoreception signal  17 A (luminance information) of the reference pixel at the time T k , and corresponds to latest luminance information in luminance information of the reference pixel. Moreover, Y s (T k−1 ) represents the photoreception signal  17 A (luminance information) of the reference pixel at the time T k−1 (&lt;time T k ), and corresponds to earlier luminance information in the luminance information of the reference pixel. Y i (T k ) represents the photoreception signal  17 A (luminance information) of the ith dummy pixel  16  at the time T k , and corresponds to latest luminance information in luminance information of the ith dummy pixel  16  (a non-reference pixel). Y i (T k−1 ) represents the photoreception signal  17 A (luminance information) of the ith dummy pixel  16  at the time T k−1 , and corresponds to earlier luminance information in the luminance information of the ith dummy pixel  16  (a non-reference pixel). A relationship between the time T k−1  and the time T k  is represented by, for example, the following expression. 
         T   k   =T   k−1   +ΔT    Mathematical Expression 3
 
     In Mathematical Expression 3, ΔT represents a sampling period. In this case, the sampling period ΔT indicates, for example, a period in which the photoreception signal processing circuit  26  derives a value of the denominator and a value of the numerator in the right-hand side of Mathematical Expression 2. The photoreception signal processing circuit  26  consistently keeps the sampling period ΔT constant. 
     For example, as illustrated in  FIG. 6 , in the case where the horizontal axis in  FIG. 6  indicates a ratio (Y i /Y s ) of the initial luminance Y i  of each dummy pixel  16  to the initial luminance Y s  of the reference pixel, an upward-sloping curve indicating an increase in the exponentiation factor n(Y i , Y s ) at the time T k  derived in the above-described manner associated with an increase in the initial luminance Y i  is drawn. It is obvious from Mathematical Expression 2 that the exponentiation factor n(Y i , Y s ) is 1 in Y s /Y s . 
     Next, referring to  FIGS. 7 to 13 , a method of deriving correction information  26 A used for correction of the picture signal  20 A will be described below. 
     In one embodiment consistent with the present invention, the photoreception signal processing circuit  26  selects one pixel from a plurality of dummy pixels  16  as a reference pixel. In the embodiment, the selected dummy pixel  16  is consistently set as the reference pixel without changing the reference pixel to any other dummy pixel  16  (non-reference pixel). 
     Next, the photoreception signal processing circuit  26  obtains the photoreception signals  17 A from the photoreception element group  17  at times T 1  and T 2 . More specifically, at the times T 1  and T 2 , the photoreception signal processing circuit  26  obtains the photoreception signals  17 A (first luminance information) of the reference pixel which is one pixel selected from the plurality of dummy pixels  16 . Moreover, at the times T 1  and T 2  the photoreception signal processing circuit  26  obtains the photoreception signals  17 A (second luminance information) of a plurality of non-reference pixels which are all of the plurality of dummy pixels  16  except for the reference pixel from the photoreception element group  17 . Then, the photoreception signal processing circuit  26  derives luminance degradation information (Log(Y s (T 2 ))−Log(Y s (T 1 ))) of the reference pixel from luminance information of the reference pixel, and derives luminance degradation information (Log(Y i (T 2 ))−Log(Y i (T 1 ))) of each non-reference pixel from luminance information of each non-reference pixel. 
     Next, the photoreception signal processing circuit  26  derives the exponentiation factor n(Y i , Y s ) of the luminance information of each non-reference pixel with respect to the luminance information of the reference pixel at the time T 2  from the luminance degradation information of the reference pixel and the luminance degradation information of each non-reference pixel. Then, the photoreception signal processing circuit  26  derives a luminance degradation function F s (t) (a first luminance degradation function) at the time T 2  representing a temporal change in luminance of the reference pixel from the luminance information of the reference pixel. Moreover, the photoreception signal processing circuit  26  derives a luminance degradation function F i (t) (a second luminance degradation function) at the time T 2  representing a temporal change in luminance of each non-reference pixel from the luminance degradation function F s (t) and the exponentiation factor n(Y i , Y s ). Thus, the photoreception signal processing circuit  26  derives the luminance degradation functions F s (t) and F i (t) at the time T 2  with use of initial luminance information. 
     Next, updating of data will be described below. At the times T k−1  and T k , the photoreception signal processing circuit  26  obtains the photoreception signals  17 A (the first luminance information) of the reference pixel and the photoreception signals  17 A (the second luminance information) of a plurality of non-reference pixels from the photoreception element group  17 . A value (a measured value) of the photoreception signal  17 A of the reference pixel at this time is Y s1  (refer to  FIG. 7 ). Next, the photoreception signal processing circuit  26  estimates luminance information of the reference pixel at the time T k  from the luminance degradation function F s (t) at the time T k−1 . The estimated value at this time is Y s2  (refer to  FIG. 7 ). Then, the photoreception signal processing circuit  26  compares the measured value Y s1  to the estimated value Y s2  to determine whether or not the measured value Y s1  and the estimated value Y s2  are equal to each other. As a result, for example, in the case where the measured value Y s1  is equal to the estimated value Y s2 , the photoreception signal processing circuit  26  considers the luminance degradation function F s (t) at the time T k−1  as the luminance degradation function F s (t) at the time T k . On the other hand, in the case where the photoreception signal processing circuit  26  determines that, for example, the measured value Y s1  is different from the estimated value Y s2  by comparing the measured value Y s1  to the estimated value Y s2 , the photoreception signal processing circuit  26  derives the luminance degradation function F s (t) (the first luminance degradation function) at the time T k  from the luminance information of the reference pixel. 
     Next, the photoreception signal processing circuit  26  derives the luminance degradation information (Log(Y s (T k ))−Log(Y s (T k−1 ))) of the reference pixel from the luminance information of the reference pixel. Moreover, the photoreception signal processing circuit  26  derives the luminance degradation information (Log(Y i (T k ))−Log(Y i (T k−1 ))) of each non-reference pixel from the luminance information of a plurality of non-reference pixels. Then the photoreception signal processing circuit  26  derives the exponentiation factor (Y i , Y s ) at the time T k  from the luminance degradation information of the reference pixel and the luminance degradation information of each non-reference pixel. 
     Next, the photoreception signal processing circuit  26  updates a parameter (for example, p 1 , p 2 , . . . , pm) of the luminance degradation function F s (t) at the time T k−1  to a parameter (for example, p 1 ′, p 2 ′, . . . , pm′) of the luminance degradation function F s (t) at the time T k  (refer to  FIG. 8 ). In other words, the photoreception signal processing circuit  26  updates the parameter of the luminance degradation function F s (t) so as to correspond to the latest luminance information (Y s (T k )) in the luminance information of the reference pixel and earlier luminance information (Ys(T k−1 )) in the luminance information of the reference pixel. The photoreception signal processing circuit  26  stores, for example, a newly determined parameter of the luminance degradation function F s (t) in the memory circuit  27 . 
     Next, the photoreception signal processing circuit  26  derives the luminance degradation function F i (t) (the second luminance degradation function) at the time T k  (refer to  FIG. 11 ) from the luminance degradation function F s (t) at the time T k  (refer to  FIG. 9 ) and the exponentiation factor n(Y i , Y s ) (refer to  FIG. 10 ). More specifically, the photoreception signal processing circuit  26  derives the luminance degradation function F i (t) at the time T k  by the following expression. 
         F   i ( t )= F   s ( t ) n(Yi, Ys)    Mathematical Expression 4
 
     Then, the photoreception signal processing circuit  26  updates a parameter of the luminance degradation function F i (t) of each non-reference pixel at the time T k−1  to a parameter of the luminance degradation function F i (t) of each non-reference pixel at the time T k . The photoreception signal processing circuit  26  stores, for example, a newly determined parameter of the luminance degradation function F i (t) in the memory circuit  27 . 
     Next, the photoreception signal processing circuit  26  estimates the luminance degradation rate of each display pixel  13  until the coming of the next sampling period. More specifically, the photoreception signal processing circuit  26  derives an accumulated light emission time T xy  on a reference luminance basis of each display pixel  13  from the luminance degradation function F s (t), the luminance degradation function F i (t) and a history of the picture signal  20 A of each display pixel  13 . The photoreception signal processing circuit  26  determines the accumulated light emission time T xy  on the reference luminance basis of each display pixel  13  by, for example, the following method. 
       FIG. 12  schematically illustrates a process of deriving the accumulated light emission time T xy  on the reference luminance basis of each display pixel  13 . For example, as illustrated in  FIG. 12 , a display pixel  13  emits light with initial luminance Y 1  during a time T=0 to t 1 , and emits light with initial luminance Y 2  during a time T=t 1  to t 2 , and emits light with initial luminance Y n  during a time T=t 2  to t 3 . Strictly speaking, at this time, the luminance of the display pixel  13  is degraded along a degradation curve of the initial luminance Y 1  during the time T=0 to t 1 , and along a degradation curve of the initial luminance Y 2  during the time T=t 1  to t 2 , and along a degradation curve of the initial luminance Y n  during the time t 2  to t 3 . As a result, the luminance of the display pixel  13  is degraded to, for example, 48% as illustrated in  FIG. 12 . Therefore, the accumulated light emission time T xy  on the reference luminance basis of the display pixel  13  is allowed to be determined by determining a time when a degradation rate reaches 48% in a luminance degradation curve (F s (t)) of the reference pixel. Thus, the accumulated light emission time T xy  on the reference luminance basis of each display pixel  13  and a luminance degradation rate of each display pixel  13  are allowed to be determined by tracing a luminance degradation curve in each gradation level according to the magnitude (gradation) of an input signal. 
     Next, the photoreception signal processing circuit  26  derives a correction amount for a picture signal from the determined accumulated light emission time T xy  (or an estimated luminance degradation rate of each display pixel  13 ) and gamma characteristics of the display panel  10 . The photoreception signal processing circuit  26  determines the correction amount for the picture signal by, for example, the following method. 
       FIG. 13  illustrates an example of a relationship between gradation (a value of the picture signal  20 A) at T=0 and T xy  and luminance. Gradation-luminance characteristics at T=0 are so-called gamma characteristics. Gradation-luminance characteristics at T=T xy  are characteristics in which luminance in all gradation levels are attenuated to 48% with respect to the gamma characteristics. In this case, in the case where the value of the picture signal  20 A in a certain display pixel  13  is S xy , it is obvious that the luminance of the display pixel  13  has a value corresponding to a white dot in the drawing at an initial time. In other words, it is estimated that luminance of the display pixel  13  has a value attenuated from initial luminance to 48% after a lapse of the accumulated light emission time T xy  from the initial time. 
     Therefore, the photoreception signal processing circuit  26  derives a correction amount ΔS xy  which is added to the picture signal  20 A (S xy ) so that luminance after a lapse of the accumulated light emission time T xy  from the initial time is equal to the initial luminance. Finally, the photoreception signal processing circuit  26  stores the correction amount ΔS xy  as correction information  26 A in the memory circuit  27 . 
     Next, an operation and effects of the display  1  according to one embodiment consistent with the present invention will be described below. The picture signal  20 A and the synchronization signal  20 B are inputted into the display  1 . Thereby, each display pixel  13  is driven by the signal line drive circuit  23  and the scanning line drive circuit  24  so as to display a picture based on the picture signal  20 A of each display pixel  13  on the display region  12 . Moreover, each dummy pixel  16  is driven by the dummy pixel-photoreception element group drive circuit  25 , and at the same time, the photoreception element group  17  is driven by the dummy pixel-photoreception element group drive circuit  25 . Thereby, constant currents with different magnitudes flow through the dummy pixels  16 , and each of the dummy pixels  16  emits light with luminance according to the magnitude of the constant current, and emission light from each of the dummy pixels  16  is detected by the photoreception element group  17 . As a result, the photoreception signal  17 A corresponding to emission light from each of the dummy pixels  16  is outputted. Next, the following process is performed by the photoreception signal processing circuit  26 . That is, the exponentiation factor n(Y i , Y s ) of the photoreception signal  17 A (luminance information) of a non-reference pixel with respect to the photoreception signal  17 A (luminance information) of the reference pixel is derived from the photoreception signal  17 A. Next, the luminance degradation function F s (t) of the reference pixel is derived from the luminance information of the reference pixel, and the luminance degradation function F i (t) of the non-reference pixel is derived from the luminance degradation function F s (t) and the exponentiation factor n(Y i , Y s ). Then, the accumulated light emission time T xy  on the reference luminance basis of each display pixel  13  and the luminance degradation rate of each display pixel  13  are estimated with use of the luminance degradation function F s (t), the luminance degradation function F i (t) and the history of the picture signal  20 A of each display pixel  13 . Next, the correction amount ΔS xy  is added to the picture signal  20 A (S xy ) of each display pixel  13  so that luminance after a lapse of the accumulated light emission time T xy  from the initial time is equal to the initial luminance. Thereby, the luminance of each display pixel  13  becomes initial luminance. 
     Thus, in the embodiment, the luminance degradation rate of each display pixel  13  is estimated with use of the luminance degradation function F s (t), the luminance degradation function F i (t) obtained from the luminance degradation function F s (t) and the exponentiation factor n(Y i , Y s ), and the history of the picture signal  20 A of each display pixel  13 . Thereby, luminance degradation in each display pixel  13  is allowed to be estimated at high accuracy, so an accurate correction amount ΔS xy  is allowed to be added to the picture signal  20 A (S xy ) of each display pixel  13  so that the luminance of each display pixel  13  becomes the initial luminance. As a result, burn-in is accurately preventable. 
     As one of techniques of estimating the luminance degradation rate of each display pixel  13 , for example, a method using an acceleration factor α is used. In this method, first, for example, as illustrated by a broken line in  FIG. 14 , a time T when the luminance degradation rate of the dummy pixel  16  with initial luminance Y i  becomes equal to the luminance degradation rate of the dummy pixel  16  with initial luminance Y s  is determined. Next, for example, as illustrated in  FIG. 15 , in the case where a horizontal axis indicates Log(Y i /Y s ) and a vertical axis indicates Log(T), the time T is plotted, and dots of each luminance degradation rate are connected with a straight line, and then a gradient of the straight line of each luminance degradation rate is determined. The gradient is the above-described acceleration factor α. Next, for example, as illustrated in  FIG. 16 , in the case where a horizontal axis indicates a luminance degradation rate D and a vertical axis indicates the acceleration factor α, the acceleration factor α is plotted. Then, in this technique, the luminance degradation rate of each display pixel  13  is estimated from black dots in  FIG. 16  in which the accelerated factor α is plotted. More specifically, the luminance degradation rate of each display pixel  13  is estimated by the following expression. 
     
       
         
           
             
               
                 
                   
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     In Mathematical Expression 5, T(D x , Y i ) represents a time (a reach time) until the dummy pixel  16  with the initial luminance Y i  reaches the luminance degradation rate D x . T(D x , Y i ) represents a time (a reach time) until the dummy pixel  16  with the initial luminance Y s  reaches the luminance degradation rate D x . Further, α(D x ) represents an acceleration factor α in the luminance degradation rate D x . 
     However, in the above-described technique, the following issue arises. For example, as illustrated in  FIG. 14 , it is assumed that the luminance degradation rate of the dummy pixel  16  with the initial luminance Y i  is determined until a time T x  and at this time, the luminance degradation rate of the dummy pixel  16  with the initial luminance Y 1  is 80%. The luminance degradation rate of the dummy pixel  16  with initial luminance Y i  except for the initial luminance Y 1  is typically smaller than 80%. at the time T x . For example, the luminance degradation rate of the dummy pixel  16  with initial luminance Y s  is 65% at the time T x , and the luminance degradation rate of the dummy pixel  16  with initial luminance Y n  is 35% at the time T x . The acceleration factor α is derived by determining a time necessary to reach a certain degradation rate in all dummy pixels  16  with the initial luminance Y 1  to Y n . Therefore, only an acceleration factor α when the luminance degradation rate is 100% to 85% is determined from data of the luminance degradation rate of each dummy pixel  16  obtained until the time T x . As a result, the acceleration factor α when the luminance degradation rate is smaller than 85% is only estimated. Therefore, for example, as illustrated in  FIG. 16 , it may be uncertain that a relationship between the acceleration factor α and the luminance degradation rate establishes a curve A or a curve B. Therefore, in the method using the acceleration factor α, estimation accuracy of the luminance degradation rate of each display pixel  13  varies depending on a progress degree of luminance degradation in the dummy pixel  16  with the initial luminance Y 1 . When luminance degradation in the dummy pixel  16  with the initial luminance Y 1  progresses, a relationship between the acceleration factor α and the luminance degradation rate is clear. However, the luminance degradation in the dummy pixel  16  with the initial luminance Y 1  is generally very moderate, so to obtain a necessary relationship between the acceleration factor α and the luminance degradation rate for estimation, observation for a very long period is necessary. Therefore, the method using the acceleration factor α is not realistic. 
     On the other hand, in the embodiment, the luminance degradation rate of each display pixel  13  is allowed to be estimated from data (Y s (T k ), Y s (T k−1 )) at the time of observation. Thereby, luminance degradation in each display pixel is allowed to be estimated at high accuracy without observation for a long time. Therefore, an estimating method in the embodiment is extremely realistic. Moreover, in the embodiment, the luminance degradation rate of each display pixel  13  is allowed to be estimated from data (Y s (T k ), Y s (T k−1 )) at the time of observation, so a memory amount and a calculation amount which are necessary for updating are allowed to be reduced. 
     In the above-described embodiment, each of the dummy pixels  16  with initial luminance Y 1  to Y n  is configured of a single pixel including a combination of organic EL elements  14 R,  14 G and  14 B, but each dummy pixel  16  (a low-luminance pixel) with low initial luminance Y i  may be configured of a plurality of dummy pixels (second dummy pixels) (not illustrated). In such a case, the photoreception signal processing circuit  26  is allowed to derive the denominator or the numerator in the right-hand side of Mathematical Expression 2 from an average value of luminance of the plurality of second dummy pixels. Thereby, a measurement error in the dummy pixel  16  with low luminance is allowed to be reduced, so luminance degradation in the display pixel  13  with low luminance is allowed to be estimated with high accuracy. As a result, burn-in is preventable more accurately. 
     Moreover, in the above-described embodiment, a specific dummy pixel  16  is consistently the reference pixel, but a dummy pixel  16  which has been a non-reference pixel may become the reference pixel. For example, when the photoreception signal processing circuit  26  detects that the luminance of the reference pixel reaches a predetermined value or less, the photoreception signal processing circuit  26  excludes the dummy pixel  16  which has been set as the reference pixel, and sets one pixel selected from a plurality of non-reference pixels as a new reference pixel. After that, the photoreception signal processing circuit  26  derives the denominator and the numerator in the right-hand side of Mathematical Expression 2 in the same manner. In such a case, even if a failure occurs in the reference pixel, luminance degradation is allowed to be estimated continuously. Thereby, reliability in estimation of luminance degradation is allowed to be improved. 
     Further, in the above-described embodiment, the sampling period ΔT is consistently constant, but the sampling period ΔT may be variable. For example, the photoreception signal processing circuit  26  may change the sampling period ΔT depending on an accumulated light emission time of the plurality of dummy pixels  16 . In such a case, for example, when the accumulated light emission time T xy  reaches a long time, and luminance degradation hardly occurs, the sampling period ΔT is allowed to be extended. Thereby, a calculation amount necessary for updating is allowed to be reduced. 
     Moreover, in the above-described embodiment, the exponentiation factor n(Y i , Y s ) is derived with use of Mathematical Expression 2. However, for example, the exponentiation factor n(Y i , Y s ) may be derived with use of the following expressions. 
     
       
         
           
             
               
                 
                   
                       
                   
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     In Mathematical Expression 6, the denominator of the second term in the right-hand side of Mathematical Expression 6 represents degradation speed of the reference pixel at the time Tk. The numerator of the second term in the right-hand side of Mathematical Expression 6 represents degradation speed of the non-reference pixel at the time Tk. The second term in the right-hand side of Mathematical Expression 7 is obtained by dividing the degradation speed of the reference pixel at the time Tk by the degradation speed of the non-reference pixel at the time Tk. 
     In the case where the exponentiation factor n(Y i , Y s ) is derived with use of Mathematical Expression 6 or 7, the exponentiation factor n(Y i , Y s ) is allowed to be derived only by four arithmetic operations, and logarithm calculation which is performed when Mathematical Expression 2 is used is not necessary. Therefore, in the modification, a calculation amount is allowed to be reduced to smaller than a calculation amount when the exponentiation factor n(Y i , Y s ) is derived with use of Mathematical Expression 2. 
     Next, application examples of the display  1  described in the above-described embodiment and the above-described modifications will be described below. The display  1  according to at least one embodiment consistent with the present invention are applicable to displays of electronic devices in any field which display a picture signal inputted from outside or a picture signal produced inside as an image or a picture, such as televisions, digital cameras, notebook personal computers, portable terminal devices such as cellular phones, and video cameras. 
       FIG. 17  illustrates a television to which a display unit consistent with the present invention is utilized. The television has, for example, a picture display screen section  300  including a front panel  310  and a filter glass  320 . The picture display screen section  300  is configured of the display  1  according to the above-described embodiment or the like. 
       FIGS. 18A and 18B  illustrate appearances of a digital camera to which a display  1  unit consistent with the present invention is utilized. The digital camera has, for example, a light-emitting section for a flash  410 , a display section  420 , a menu switch  430 , and a shutter button  440 . The display section  420  is configured of the display  1  according to the above-described embodiment or the like.  FIG. 19  illustrates an appearance of a notebook personal computer to which a display  1  unit consistent with the present invention is utilized. The notebook personal computer has, for example, a main body  510 , a keyboard  520  for operation of inputting characters and the like, and a display section  530  for displaying an image. The display section  530  is configured of the display  1  according to the above-described embodiment or the like. 
       FIG. 20  illustrates an appearance of a video camera to which the display  1  unit consistent with the present invention is utilized. The video camera has, for example, a main body  610 , a lens for shooting an object  620  arranged on a front surface of the main body  610 , a shooting start/stop switch  630 , and a display section  640 . The display section  640  is configured of the display  1  according to the above-described embodiment or the like. 
       FIGS. 21A to 21G  illustrate appearances of a cellular phone to which the display  1  unit consistent with the present invention is utilized. The cellular phone is formed by connecting, for example, a top-side enclosure  710  and a bottom-side enclosure  720  to each other by a connection section (hinge section)  730 . The cellular phone has a display  740 , a sub-display  750 , a picture light  760 , and a camera  770 . The display  740  or the sub-display  750  is configured of the display  1  according to the above-described embodiment or the like. 
     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.