Abstract:
A display device includes: first and second photosensors; a reader; a light detector outputting the light amount detected by the photosensors; a first circuit outputting a first signal based on incident light entering the first photosensor; and a second circuit outputting a second signal based on dimmed incident light entering the second photosensor. The reader includes: a coefficient calculator calculating a first measurement ratio of the first signal to the second signal, and a power correction coefficient; a rate calculator deriving modified power coefficients from the power correction coefficient, calculating a second measurement ratio of the power-corrected first and second signals, and calculating a slope correction coefficient; and an output unit deriving modified proportional coefficients from the slope correction coefficient, and correcting the power-corrected first and second signals using the modified proportional coefficients to yield outputted initial light amount signals.

Description:
BACKGROUND 
     1. Technical Field 
     The invention relates to a display device and, more particularly, to a display device that includes a light amount detecting device having a sensitivity correction function in consideration of degradation of a photosensor and may be manufactured in a simple process. 
     2. Related Art 
     An known existing light amount detection circuit utilizes the relationship that a leakage current from a thin film transistor is proportional to the amount of light received, makes a voltage detecting capacitor charge or discharge electric charge by the leakage current, and then monitors a voltage variation between both ends of the capacitor to thereby detect the amount of light (for example, see JP-A-2006-29832). Incidentally, it is generally known that the leakage current from the thin film transistor is proportional to the amount of light received; however, the sensitivity, which is a leakage current value against the amount of light received, decreases due to light exposure. Thus, in the photodetection circuit described in JP-A-2006-29832, because of the decrease in sensitivity, the accuracy of light amount detection decreases. 
     In order to prevent such a decrease in detection accuracy, a known photoelectric conversion element modifies a method of producing a thin film transistor to improve the antidegradation property (for example, see JP-A-9-232620). 
     However, the photoelectric conversion element described in JP-A-9-232620 requires a special manufacturing condition, so manufacturing cost problematically increases. Specifically, when a photosensor is provided inside a display device that uses a thin film transistor or when a display device and a photosensor are manufactured by the same equipment, it is impossible to manufacture the photosensor together with a driving transistor of the display device. Thus, it is necessary to add a manufacturing process or set a complex condition in a manufacturing equipment. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a display device that includes a light amount detecting device that has a sensitivity correction function and may be manufactured in a simple process. 
     An aspect of the invention provides a display device. The display device includes: a substrate; a display area provided on the substrate and includes a switching element in correspondence with each pixel; a photodetection unit having first and second photosensors; a photosensor reader unit; a light amount detecting device that outputs the amount of light detected by the photodetection unit as a light amount signal; a first photodetection circuit that outputs a first output signal based on incident light that enters the first photosensor to the photosensor reader unit; and a second photodetection circuit that outputs a second output signal to the photosensor reader unit based on dimmed incident light, which is dimmed through a light dimming unit as compared with the light that enters the first photosensor and which enters the second photosensor. The photosensor reader unit includes: a photodegradation coefficient calculation unit that calculates a first measurement ratio, which is a ratio of the first output signal to the second output signal, and then calculates a photodegradation power correction coefficient, which is a ratio of the first measurement ratio to an initial ratio that is an initial first measurement ratio measured beforehand; a photodegradation rate calculation unit that derives modified power coefficients on the basis of the photodegradation power correction coefficient, calculates a second measurement ratio, which is a ratio of the power-corrected first and second output signals, using the modified power coefficients, and then calculates a photodegradation slope correction coefficient, which is a ratio of the second measurement ratio to the initial ratio; and an optical signal output unit that derives modified proportional coefficients on the basis of the photodegradation slope correction coefficient, corrects the power-corrected first and second output signals using the modified proportional coefficients so as to be initial light amount signals and then outputs the initial light amount signals. 
     According to the aspect of the invention, it is possible to accurately calculate the initial first or second output signal from the relationship among the first and second output signals, the initial ratio prepared beforehand, the photodegradation power correction coefficient K, the photodegradation slope correction coefficient K″, and the modified proportional coefficients. Thus, it is possible to implement a display device having the function of correcting the sensitivity without adding any modification to the structure of the photosensor. In addition, the manufacturing process for the photosensor may be integrated with the manufacturing process for the driving transistor of the display device. Thus, it is possible to manufacture the photosensor in a simple process. Hence, manufacturing cost may be reduced. 
     The photodegradation rate calculation unit may include a look-up table that associates the photodegradation power correction coefficient with an initial power coefficient correction amount measured beforehand, and the modified power coefficients may be calculated on the basis of the power coefficient correction amount. 
     If the modified power coefficients are expressed as a function of the photodegradation power correction coefficient, when the function becomes a complex expression, the circuit size increases. This causes an increase in manufacturing cost and, in addition, increases power consumption. In place of such a function, the photodegradation rate calculation unit includes the look-up table to eliminate the necessity of a large-size circuit. Thus, it is possible to provide a display device that suppresses manufacturing cost and that reduces power consumption. 
     The photodegradation rate calculation unit, when the photodegradation power correction coefficient is not included in the look-up table, may derive the modified power coefficients through interpolation calculation using the initial power coefficient correction amount measured beforehand in the look-up table. 
     Thus, it is possible to derive modified power coefficients corresponding to a given photodegradation power correction coefficient that is not included in the look-up table. Hence, it is possible to provide a display device that is able to suppress the data size by reducing the look-up table. 
     The optical signal output unit may include a look-up table that associates the photodegradation slope correction coefficient with an initial proportional coefficient correction amount measured beforehand, and modified proportional coefficients may be calculated on the basis of the proportional coefficient correction amount. 
     If the initial proportional coefficient correction amount is expressed as a function of the photodegradation slope correction coefficient, when the function becomes a complex expression, the circuit size increases. This causes an increase in manufacturing cost and, in addition, increases power consumption. In place of such a function, the optical signal output unit includes the look-up table to eliminate the necessity of a large-size circuit. Thus, it is possible to provide a display device that suppresses manufacturing cost and that reduces power consumption. 
     The optical signal output unit, when the photodegradation slope correction coefficient is not included in the look-up table, may derive the modified proportional coefficients through interpolation calculation using the initial proportional coefficient correction amount measured beforehand in the look-up table. 
     Thus, it is possible to derive the initial proportional coefficient correction amount measured beforehand, corresponding to an arbitrary photodegradation slope correction coefficient that is not included in the look-up table. Hence, it is possible to provide a display device that is able to suppress the data size by reducing the look-up table. 
     The first and second photosensors may be thin film transistors, and each may include a capacitor that charges a voltage applied between both ends of the thin film transistor. 
     By so doing, the potentials charged in the capacitors vary in accordance with the amount of incident light that enters the first photosensor and the amount of dimmed incident light that enters the second photosensor. Thus, it is possible to provide a display device that outputs the potentials to the photosensor reader unit as first and second output signals. 
     The photodegradation coefficient calculation unit may logarithmically transform the first and second output signals to calculate the photodegradation power correction coefficient, the photodegradation rate calculation unit may acquire logarithms of the modified power coefficients on the basis of the logarithmic photodegradation power correction coefficient and calculate a logarithm of the photodegradation slope correction coefficient, and the optical signal output unit may derive logarithmic modified proportional coefficients on the basis of the logarithmic photodegradation slope correction coefficient, correct the logarithmic first and second output signals to be logarithmic initial light amount signals using the logarithmic modified proportional coefficients, inverse-logarithmically transform the corrected logarithmic initial light amount signals, and then output the initial light amount signals. 
     By so doing, multiplication and division circuits in the photosensor reader unit may be replaced with addition and subtraction circuits. Thus, it is possible to provide a display device that reduce the circuit size and suppresses power consumption. Hence, manufacturing cost may be reduced. 
     The display area may include an electrooptic material layer. 
     By so doing, it is possible to detect the incident light amount in the electrooptic material layer by the photosensors. Thus, it is possible to provide a display device that is able to perform image display with the amount of light emission appropriate in accordance with a usage environment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a plan view of a transflective liquid crystal display device. 
         FIG. 2  is a plan view of one pixel on an array substrate. 
         FIG. 3  is a cross-sectional view that is taken along the line III-III in  FIG. 2 . 
         FIG. 4  is a block diagram that shows the configuration of a light amount detecting device. 
         FIG. 5  is a circuit configuration diagram of a first photodetection circuit and second photodetection circuit. 
         FIG. 6A  and  FIG. 6B  are schematic cross-sectional views of a photodetection unit. 
         FIG. 7  is a view that shows a photoelectric current as a function of an incident light amount. 
         FIG. 8  is a view that shows a photoelectric current as a function of a degraded incident light amount. 
         FIG. 9  is a view that shows the relationship between a photodegradation power correction coefficient and an accumulated illuminance. 
         FIG. 10  is a view that shows the relationship between power coefficients and an accumulated illuminance. 
         FIG. 11  is a view that shows a flowchart in association with correction of a photoelectric current. 
         FIG. 12  is a view that shows light irradiation time and variations in rate of change of sensor output when degradation is not corrected. 
         FIG. 13  is a view that shows light irradiation time and variations in rate of change of sensor output when degradation is corrected in accordance with the aspects of the invention. 
         FIG. 14  is a view that shows a flowchart in association with correction of a photoelectric current according to a second embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, a display device according to embodiments of the invention will be described with reference to the accompanying drawings. The embodiments just illustrate example embodiments of the invention and are not intended to limit the invention, and may be modified at will within the scope of the technical idea of the invention. In the following drawings, for easy understanding of each structure, the scale, number, and the like, of components in each structure are varied from an actual structure. 
     First Embodiment 
       FIG. 1  is a schematic plan view of an array substrate in a transflective liquid crystal display device (display device/electro-optical device) according to a first embodiment of the invention. Note that  FIG. 1  is shown as viewed through a color filter substrate.  FIG. 2  is a plan view of one pixel on the array substrate shown in  FIG. 1 .  FIG. 3  is a cross-sectional view that is taken along the line III-III in  FIG. 2 . 
     As shown in  FIG. 1 , the liquid crystal display device  1000  includes the array substrate AR and the color filter substrate CF, which are arranged so as to face each other. The array substrate AR is formed so that various wires, and the like, are formed on a transparent substrate  1002  made of a rectangular transparent insulating material, such as glass plate. The color filter substrate CF is formed so that various wires, and the like, are formed on a transparent substrate  1010  made of a similar rectangular transparent insulating material. The array substrate AR has a size larger than the color filter substrate CF so as to form an extended portion  1002 A having a predetermined area when arranged so as to face the color filter substrate CF. A seal material (not shown) is adhered around these array substrate AR and color filter substrate CF, and a liquid crystal (electrooptic material)  1014  and a spacer (not shown) are enclosed inside. 
     The array substrate AR has opposite short sides  1002   a  and  1002   b  and opposite long sides  1002   c  and  1002   d . The extended portion  1002 A is formed at one short side  1002   b . A semiconductor chip Dr for source driver and gate driver is mounted on the extended portion  1002 A, and a photodetection unit  10  is arranged at the other short side  1002   a . In addition, a backlight (not shown) is provided on the back surface of the array substrate AR as an illumination unit. The backlight is controlled by an external control circuit (not shown) on the basis of an output from the photodetection unit  10 . 
     The array substrate AR has a plurality of gate lines GW and a plurality of source lines SW on a surface that faces the color filter substrate CF, that is, a surface that contacts the liquid crystal  1014 . The plurality of gate lines GW are arranged at predetermined intervals so as to extend horizontally (X-axis direction) in  FIG. 1 . The plurality of source lines SW are arranged at predetermined intervals so as to extend vertically (Y-axis direction), and insulated from the gate lines GW. These source lines SW and gate lines GW are wired in a matrix. In each area surrounded by the gate lines GW and the source lines SW that intersect with one another, a TFT (see  FIG. 2 ), which serves as a switching element, and a pixel electrode  1026  (see  FIG. 3 ) are formed. The switching element turns on by a scanning signal from the gate line GW. The pixel electrode  1026  is supplied with an image signal from the source line SW through the switching element. 
     Each area surrounded by these gate lines GW and source lines SW forms a so-called pixel, and an area that includes a plurality of these pixels is a display area DA. In addition, the switching element, for example, employs a thin film transistor (TFT). 
     Each gate line GW and each source line SW extend to the outside of the display area DA, that is, to a window-frame area, and are connected to the driver Dr formed of a semiconductor chip such as an LSI. In addition, on the array substrate AR, lead wires L 1  to L 4  are led from first and second photodetection circuits LS 1  and LS 2  of the photodetection unit  10  at the one long side  1002   d  and wired to be connected to terminals T 1  to T 4  that are the contacts with an external control circuit  50 . Note that the lead wire L 1  constitutes a first source line, the lead wire L 2  constitutes a second source line, the lead wire L 3  constitutes a drain line, and the lead wire L 4  constitutes a gate line. 
     The external control circuit  50  includes a photosensor reader unit  20  and a potential control circuit  30 . The photosensor reader unit  20  is connected to the terminals T 1  and T 2 . The potential control circuit  30  is connected to the terminals T 3  and T 4 . The potential control circuit  30  supplies a reference voltage, a gate voltage, and the like, to the photodetection unit  10 , and an output signal is output from the photodetection unit  10  to the photosensor reader unit  20 . Then, the backlight (not shown) is controlled by a light amount signal from the photosensor reader unit  20 . 
     In addition, the driver Dr on the transparent substrate  1002  may be replaced with an IC (Integrated Circuit) chip that includes the driver Dr, the photosensor reader unit  20 , and the like. 
     Next, a specific configuration of each pixel will be mainly described with reference to  FIG. 2  and  FIG. 3 . In the display area DA on the transparent substrate  1002  of the array substrate AR, the gate lines GW are formed parallel to one another at equal intervals, and a gate electrode G of each TFT that constitutes the switching element is extended from the gate line GW. In addition, an auxiliary capacitor line  1016  is formed in substantially the middle between the adjacent gate lines GW so as to be parallel to the gate lines GW, and the auxiliary capacitor line  1016  has an auxiliary capacitor electrode  1017  formed to have an area wider than the auxiliary capacitor line  1016 . 
     In addition, a gate insulating film  1018  made of a transparent insulating material, such as silicon nitride or silicon oxide, is formed all over the entire surface of the transparent substrate  1002  so as to cover the gate lines GW, the auxiliary capacitor line  1016 , the auxiliary capacitor electrode  1017  and the gate electrode G. Then, a semiconductor layer  1019  made of amorphous silicon, and the like, is formed on the gate electrode G through the gate insulating film  1018 . In addition, the plurality of source lines SW are formed on the gate insulating film  1018  so as to intersect with the gate lines GW. A source electrode S of the TFT is extended from the source line SW so as to contact the semiconductor layer  1019 . Furthermore, a drain electrode D made of the same material as those of the source line SW and the source electrode S is provided on the gate insulating film  1018  so as to contact the semiconductor layer  1019 . 
     Here, an area surrounded by the gate lines GW and the source lines SW corresponds to one pixel. Then, the TFT, which serves as the switching element, is formed of the gate electrode G, the gate insulating film  1018 , the semiconductor layer  1019 , the source electrode S, and the drain electrode D. The TFT is formed in each pixel. In this case, an auxiliary capacitor of each pixel is formed by the drain electrode D and the auxiliary capacitor electrode  1017 . 
     A protection insulating film (also called passivation film)  1020  made of, for example, an inorganic insulating material is laminated all over the entire surface of the transparent substrate  1020  so as to cover these source lines SW, TFT, gate insulating film  1018 . An interlayer film (also called planarization film)  1021  made of acrylic resin, or the like, containing, for example, a negative photosensitive material is laminated all over the entire surface of the transparent substrate  1002  on the protection insulating film  1020 . The surface of the interlayer film  1021  has microscopic asperities (not shown) at a reflection portion  1022  and is flat at a transmission portion  1023 . 
     Then, a reflector  1024  made of, for example, aluminum or aluminum alloy, is formed on the surface of the interlayer film  1021  at the reflection portion  1022  by sputtering. A contact hole  1025  is formed at a portion of the protection insulating film  1020 , interlayer film  1021  and reflector  1024 , which face the drain electrode D of the TFT. 
     Furthermore, in each pixel, a pixel electrode  1026  made of, for example, ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide) is formed on the surface of the reflector  1024 , in the contact hole  1025 , and on the surface of the interlayer film  1021  of the transmission portion  1023 . An alignment layer (not shown) is laminated in a further upper layer with respect to the pixel electrode  1026  so as to cover all the pixels. 
     In addition, in the color filter substrate CF, a light shielding layer (not shown) is formed on the surface of the transparent substrate  1010  made of a glass substrate, or the like, so as to face the gate lines GW and source lines SW of the array substrate AR, and, in correspondence with each pixel surrounded by the light shielding layer, for example, a color filter layer  1027  formed of red (R), green (G) and blue (B) is provided. Furthermore, a topcoat layer  1028  is formed on the surface of the color filter layer  1027  at a position corresponding to the reflection portion  1022 . A common electrode  1029  and an alignment layer (not shown) are laminated on the surface of the topcoat layer  1028  and on the surface of the color filter layer  1027  at a position corresponding to the transmission portion  1023 . Note that the color filter layer  1027  may further employ a color filter layer, such as cyan (C), magenta (M), yellow (Y), or the like, in combination, where appropriate, and may not provide a color filter layer for monochrome display. 
     Then, the thus configured array substrate AR and color filter substrate CF are adhered by the seal material (not shown), and finally the liquid crystal  1014  is enclosed into a space surrounded by both the substrates and the seal material. Thus, the transflective liquid crystal display device  1000  may be obtained. Note that the backlight or a sidelight having a known light source, light guide plate, diffusion sheet, and the like, is arranged below the transparent substrate  1002 . In this case, when the reflector  1024  is provided all over the entire lower portion of each pixel electrode  1026 , a reflective liquid crystal display panel may be obtained, whereas in the case of a reflective liquid crystal display device that uses the reflective liquid crystal display panel, a frontlight is used in place of the backlight or the sidelight. 
       FIG. 4  is a block diagram that shows the configuration of the light amount detecting device  1  formed of the photodetection unit  10  and the photosensor reader unit  20 . The photodetection unit  10  includes a first photodetection circuit LS 1  and a second photodetection circuit LS 2 . A first output signal Sa from the first photodetection circuit LS 1  and a second output signal Sb from the second photodetection circuit LS 2  are output to the photosensor reader unit  20 . 
     The photosensor reader unit  20  includes a photodegradation coefficient calculation unit  21 , a photodegradation rate calculation unit  22 , a memory circuit  23  and an optical signal output unit  24 . 
     The photodegradation coefficient calculation unit  21  is connected to the first photodetection circuit LS 1 , the second photodetection circuit LS 2  and the memory circuit  23 . The photodegradation coefficient calculation unit  21  reads initial power coefficients a and b stored in the memory circuit  23 , and reads the first output signal Sa and the second output signal Sb as a first photoelectric current amount and a second photoelectric current amount, which are leak currents in the photosensor. Then, the photodegradation coefficient calculation unit  21  calculates a first measurement ratio, which is a ratio of the first photoelectric current amount to the second photoelectric current amount, and then calculates a photodegradation power correction coefficient K, which is a ratio of the first measurement ratio to an initial ratio. The initial ratio is a measurement ratio in an initial state and is stored in the memory circuit  23  beforehand. Then, the photodegradation coefficient calculation unit  21  outputs the photodegradation power correction coefficient K and the first photoelectric current amount or second photoelectric current amount to the photodegradation rate calculation unit  22 . 
     The photodegradation rate calculation unit  22  is connected to the photodegradation coefficient calculation unit  21  and the memory circuit  23 . Then, the photodegradation rate calculation unit  22  refers to a look-up table that associates a photodegradation power correction coefficient K with a power coefficient correction amount, and acquires modified power coefficients a′ and b′ corresponding to the photodegradation power correction coefficient K output from the photodegradation coefficient calculation unit  21 . Subsequently, the photodegradation rate calculation unit  22  calculates power-corrected first and second output signals on the basis of the modified power coefficients a′ and b′, calculates a second measurement ratio, which is a ratio of the power-corrected first output signal to the power-corrected second output signal, and then calculates a photodegradation slope correction coefficient K″, which is a ratio of the second measurement ratio to the initial ratio. The initial ratio is the ratio in an initial state measured beforehand. 
     In addition, the optical signal output unit  24  is connected to the photodegradation rate calculation unit  22  and the memory circuit  23 . Then, the optical signal output unit  24  refers to a look-up table that associates the photodegradation slope correction coefficient K″ from the photodegradation rate calculation unit  22  with a proportional coefficient correction amount to thereby calculate a modified proportional coefficient D, corrects the power-corrected first or second output signal to an initial light amount signal on the basis of the modified proportional coefficient D, and then outputs the initial first photoelectric current amount or the initial second photoelectric current amount as the light amount signal S corresponding to the incident light amount. 
       FIG. 5  is a circuit configuration diagram of the photodetection unit  10 . The first photodetection circuit LS 1  of the photodetection unit  10  includes a thin film transistor (photosensor; hereinafter simply referred to as TFT)  100 , a capacitor  110 , and a switching element  120 . The TFT  100  is connected in parallel with the capacitor  110 . That is, a source portion  101  of the TFT  100  is electrically connected to an electrode  111  of the capacitor  110 , and a drain portion  102  of the TFT  100  is electrically connected to an electrode  112  of the capacitor  110 . The source portion  101  and the electrode  111  are connected to an output terminal  140 , and is connected through a switching element  120  to a power supply terminal  130 . Then, the output terminal  140  is electrically connected to the terminal T 1  through the lead wire L 1  shown in  FIG. 1 . 
     In addition, the drain portion  102  of the TFT  100  and the electrode  112  of the capacitor  110  are electrically connected to a drain terminal  191 . The drain terminal  191  is electrically connected to the terminal T 3  through the lead wire L 3  shown in  FIG. 1 . The drain terminal  191  is grounded; however, the drain terminal  191  may be grounded inside the photodetection unit  10  or may be grounded through the terminal T 3 . Then, a gate portion  103  of the TFT  100  is electrically connected to a gate terminal  190 . 
     The second photodetection circuit LS 2  of the photodetection unit  10  includes a thin film transistor (photosensor; hereinafter, simply referred to as TFT)  200 , a capacitor  210 , a switching element  220  and a color filter (light dimmer)  250 . The thin film transistor  200  is connected in parallel with the capacitor  210 . That is, a source portion  201  of the TFT  200  is electrically connected to an electrode  211  of the capacitor  210 , and a drain portion  202  of the TFT  200  is electrically connected to an electrode  212  of the capacitor  210 . The color filter  250  is arranged on the light incident side of the TFT  200 , and the TFT  200  detects light that is dimmed by the color filter  250 . The source portion  201  and the electrode  211  are connected to an output terminal  240 , and is connected through a switching element  220  to a power supply terminal  230 . The output terminal  240  is electrically connected to the terminal T 2  through the lead wire L 2  shown in  FIG. 1 . 
     In addition, the drain portion  202  of the TFT  200  and the electrode  112  of the capacitor  210  are electrically connected to the drain terminal  191 . The drain terminal  191  is shared with the TFT  100 , and is electrically connected to the terminal T 3  through the lead wire L 3  shown in  FIG. 1 . Then, a gate portion  203  of the TFT  200  is electrically connected to the gate terminal  190  that is shared with the TFT  100 . 
     The output terminal  240  is electrically connected to the terminal T 2  through the lead wire L 2  shown in  FIG. 1 . The drain terminal  191  is electrically connected to the terminal T 3  through the lead wire L 3  shown in  FIG. 1 . The gate terminal  190  is electrically connected to the terminal T 4  through the lead wire L 4  shown in  FIG. 1 . 
       FIG. 6A  and  FIG. 6B  are schematic cross-sectional views of the photodetection unit  10 .  FIG. 6A  shows the first photodetection circuit LS 1 .  FIG. 6B  shows the second photodetection circuit LS 2 . First, the first photodetection circuit LS 1  will be described with reference to  FIG. 6A . The TFT  100  that constitutes the first photodetection circuit LS 1 , the capacitor  110  and the switching element  120  are formed on the transparent substrate  1002 . The gate portion  103  of the TFT  100 , the electrode  112  of the capacitor  110 , the gate portion  123  of the thin film transistor, which is the switching element  120 , are formed on the transparent substrate  1002 . A gate insulating film  72  is laminated so as to cover the gate portion  103 , the electrode  112  and the gate portion  123 . 
     On the gate insulating film  72 , a semiconductor layer  104  is formed above the gate portion  103 , and a semiconductor layer  124  is formed above the gate portion  123 . A conductive film  173  connected to the drain portion  102  of the semiconductor layer  104 , a conductive film  174  connected to the source portion  101  and the drain portion  122  of the semiconductor layer  124  and a conductive film  175  connected to the source portion  121  are formed on the gate insulating film  72 . The conductive film  174  constitutes the electrode  111  of the capacitor  110  in an area above the electrode  112 . 
     The protection insulating film  76  is laminated so as to cover these conductive films  173 ,  174  and  175 . A black matrix  125  is formed on the protection insulating film  76  so as to cover the semiconductor layer  124  of the switching element  120  in plan view. 
     The first photodetection circuit LS 1  is formed on the same substrate with the display area DA, and may be partially manufactured in the same process with the array substrate AR. For example, the gate insulating film  72  of the first photodetection circuit LS 1  may be manufactured together with the gate insulating film  1018  of the array substrate AR, the gate insulating film  76  of the first photodetection circuit LS 1  together with the gate insulating film  1020  of the array substrate AR, the conductive films  173 ,  174  and  175  of the first photodetection circuit LS 1  together with the source electrode S and drain electrode D of the array substrate AR, and the semiconductor layers  104  and  124  of the first photodetection circuit LS 1  together with the semiconductor layer  1019  of the array substrate AR, and the like. 
     Subsequently, the second photodetection circuit will be described with reference to  FIG. 6B . The TFT  200  that constitutes the second photodetection circuit LS 2 , the capacitor  210 , and the switching element  220  are formed on the transparent substrate  1002 . The gate portion  203  of the TFT  200 , the electrode  212  of the capacitor  210 , the gate portion  223  of the switching element  220 , which is the thin film transistor, are formed on the transparent substrate  1002 . The gate insulating film  72  is laminated so as to cover the gate portion  203 , the electrode  212  and the gate portion  223 . 
     On the gate insulating film  72 , a semiconductor layer  204  is formed above the gate portion  203 , and a semiconductor layer  224  is formed above the gate portion  223 . A conductive film  273  connected to the drain portion  202  of the semiconductor layer  204 , a conductive film  274  connected to the source portion  201  and the drain portion  222  of the semiconductor layer  224  and a conductive film  275  connected to the source portion  221  are formed on the gate insulating film  72 . The conductive film  274  constitutes the electrode  211  of the capacitor  210  in an area above the electrode  212 . 
     The protection insulating film  76  is laminated so as to cover these conductive films  273 ,  274  and  275 . A black matrix  225  is formed on the protection insulating film  76  so as to cover the semiconductor layer  224  of the switching element  220  in plan view. Then, in the TFT  200 , the color filter  250  is formed on the protection insulating film  76  The color filter  250  dims incident light that enters the second photodetection circuit LS 2  by 1/n (n&gt;1) as compared with that of the first photodetection circuit LS 1 . 
     The second photodetection circuit LS 2  is formed on the same substrate with the display area DA, and may be partially manufactured in the same process with the array substrate AR. For example, the gate insulating film  72  of the second photodetection circuit LS 2  may be manufactured together with the gate insulating film  1018  of the array substrate AR, the gate insulating film  76  of the second photodetection circuit LS 2  together with the gate insulating film  1020  of the array substrate AR, the conductive films  273 ,  274  and  275  of the second photodetection circuit LS 2  together with the source electrode S and drain electrode D of the array substrate AR, and the semiconductor layers  204  and  224  of the first photodetection circuit LS 2  together with the semiconductor layer  1019  of the array substrate AR, and the like. 
     The light amount detecting device  1  of the display device  1000  according to the aspects of the invention has the function of correcting sensitivity of the photosensor, which decreases due to photodegradation. Hereinafter, the principle of correcting sensitivity of the photosensor will be described. First, light is irradiated to the photodetection unit  10  of which the capacitors  110  and  120  are charged to predetermined potentials. Then, because leakage current occurs in the TFTs  100  and  200 , the potentials of the capacitors  120  and  220  decrease over time. At this time, the potentials of the electrodes  111  and  211  of the capacitors  110  and  210  are output from the photodetection unit  10  as a first signal Sa and a second signal Sb. Then, the photosensor reader unit  20  reads information corresponding to a photoelectric current from signals of the potentials output from the photodetection unit  10 , executes correction on the information, and then outputs the corrected information as a light amount signal. Thus, a calculation method using the photoelectric current will be described below, and the photoelectric current used in calculation may be replaced with a value read by the photosensor reader unit  20 . 
     For correcting the sensitivity of the photosensor, first, a photodegradation power correction coefficient K is calculated. The photodegradation power correction coefficient K is a ratio of a first measurement ratio to an initial measurement ratio. The first measurement ratio is a ratio of a first photoelectric current in consideration of an initial power coefficient a of a measured (degraded) first photodetection circuit LS 1  to a second photoelectric current in consideration of an initial power coefficient b of the second photodetection circuit LS 2 . Next, modified power coefficients a′ and b′ are calculated on the basis of the calculated photodegradation power correction coefficient K. Then, using the modified power coefficients a′ and b′, a second measurement ratio, which is a ratio of the power-corrected first output signal to the power-corrected second output signal. After that, a photodegradation slope correction coefficient K″, which is a ratio of the second measurement ratio to the initial ratio, is calculated. Thereafter, modified proportional coefficients are derived on the basis of the photodegradation slope correction coefficient K″, and the power-corrected first and second output signals are corrected to be the initial light amount signal using the modified proportional coefficients and output as the light amount signals S of incident light. 
     Here, a calculation method for the photodegradation power correction coefficient K will be described.  FIG. 7  is a view that shows a photoelectric current I as a function of an incident light amount L.  FIG. 7  shows a first photoelectric current of the first photodetection circuit LS 1  as a function Ia(L 1 ) of an incident light amount L 1  and shows a second photoelectric current of the second photodetection circuit LS 2  as a function Ib(L 1 ) of an incident light amount L 1 . From these, an initial ratio, which is a ratio of the first photoelectric current Ia(L 1 ) to the second photoelectric current Ib(L 1 ) before degradation (initial state), may be obtained. 
     Because the photoelectric current I increases in proportion to the incident light amount L, when the initial sensitivity in the first photodetection circuit LS 1  is Xa 0 ^ (a 0 ) and the initial sensitivity in the second photodetection circuit LS 2  is Xb 0 ^ (b 0 ), the first photoelectric current Ia(L) in the first photodetection circuit LS 1  and the second photoelectric current Ib(L) in the second photodetection circuit LS 2  may be expressed as follows (where “^” denotes power, and a and b are respectively called power coefficients).
 
 Ia ( L )= Xa   0 ^( a   0 )· L  
 
 Ib ( L )= Xb   0 ^( b   0 )· L  
 
     Thus, when a light amount L 0  enters as incident light, the amount of dimmed incident light in the second photodetection circuit LS 2  is L 0 /n. Thus, at the light amount L 0 , the first photoelectric current Ia(L 0 ) in the first photodetection circuit LS 1  and the second photoelectric current Ib(L 0 /n) in the second photodetection circuit LS 2  are expressed as follows.
 
 Ia ( L   0 )= Xa   0 ^( a   0 )· L   0  
 
 Ib ( L   0   /n )= Xb   0 ^( b   0 )·( L   0   /n )
 
Thus, the initial ratio is Ia(L 0 )/Ib(L 0 /n)=n·(Xa 0 ^(a 0 )/Xb 0 ^(b 0 )). The initial ratio is not dependent on the light amount L 0  but is obtained as a function of the initial sensitivities Xa 0 ^(a 0 ) and Xb 0 ^(b 0 ) and n. Thus, a measurement ratio at a given incident light amount L may be set to the initial ratio.
 
     Next, a degraded measurement ratio (first measurement ratio) is calculated.  FIG. 8  is a view that shows a photoelectric current I as a function of a degraded incident light amount L.  FIG. 8  shows initial first and second photoelectric currents as functions Ia(L) and Ib(L), a degraded first photoelectric current of the first photodetection circuit LS 1  as a function Ia′(L), and a degraded second photoelectric current of the second photodetection circuit LS 2  as a function Ib′(L). 
     The photosensor degrades due to photoexposure to decrease luminous sensitivity. Thus, a photoelectric current decreases as compared with that of the initial state. Such a decrease in luminous sensitivity may be obtained as a function R(p) (note that R(p)&lt;1) of an accumulated light amount p, which is an accumulation of the amount of irradiated light from the initial state. That is, when the accumulated light amount in the first photodetection circuit LS 1  after a certain period of time has elapsed is p, the accumulated light amount in the second photodetection circuit LS 2  is p/n. Thus, when the sensitivity of the first photodetection circuit LS 1  after photoexposure of the accumulated light amount p is Xa′ and the sensitivity of the second photodetection circuit LS 2  after photoexposure of the accumulated light amount p/n is Xb′, Xa′ and Xb′ may be expressed as follows.
 
 Xa′=R ( p )· Xa   0 ^( a )
 
 Xb′=R ( p/n )· Xb   0 ^( b )
 
     Note that the power coefficients a and b also vary due to photoexposure; the variations in power coefficients a and b may be obtained as a function Q(p) (note that Q(p)&lt;1) of the accumulated light amount p, which is an accumulation of the amount of irradiated light from the initial state. Thus, when the modified power coefficient of the first photodetection circuit LS 1  after receiving photoexposure of the accumulated light amount p is a′ and the modified power coefficient of the second photodetection circuit LS 2  after receiving photoexposure of the accumulated light amount p/n is b′, a′ and b′ may be expressed as follows.
 
 a′=Q ( p )· a   0  
 
 b′=Q ( p/n )· b   0  
 
     Thus, the first photoelectric current Ia′(L) of the degraded first photodetection circuit LS 1  and the second photoelectric current Ib′ (L) of the degraded second photodetection circuit LS 2  may be expressed as follows.
 
 Ia ′( L )= Xa′·L=R ( p )· Xa   0 ^( a ′)· L=R ( p )· Xa   0 ^( Q ( p )· a   0 )· L  
 
 Ib ′( L )= Xb′·L=R ( p )· Xb   0 ^( b ′)· L=R ( p )· Xb   0 ^( Q ( p/n )· b   0 )· L  
 
On the other hand, because the first photodetection circuit LS 1  has no light dimmer, such as the color filter  250 , the accumulated light amount of the first photodetection circuit LS 1  is larger than that of the second photodetection circuit LS 2 . Thus, the TFT  100 , which is the photosensor, degrades early, and a reduction rate of the first photoelectric current Ia′ (L) is larger.
 
     Thus, when a certain light amount L 1  enters as incident light, the amount of dimmed incident light in the second photodetection circuit LS 2  is L 1 /n. Thus, at the light amount L 1 , the first photoelectric current Ia′ (L 1 ) of the first photodetection circuit LS 1  and the second photoelectric current Ib′ (L 1 /n) of the second photodetection circuit LS 2  are expressed as follows.
 
 Ia ′( L   1 )= Xa′·L   1   =R ( p )· Xa   0 ^( a ′)· L   1   =R ( p )· Xa   0 ^( Q ( p )· a   0 )· L   1  
 
 Ib ′( L   1   /n )= Xb ′·( L   1   /n )= R ( p/n )· Xb   0 ^( b ′)·( L   1   /n )= R ( p/n )· Xb   0 ^( Q ( p/n )· b   0 )· L   1   /n )
 
     Thus, the degraded first measurement ratio is expressed as follows. 
                         Ia   ′     ⁡     (     L   1     )       /       Ib   ′     ⁡     (       L   1     /   n     )         =     n   ·     (       R   ⁡     (   p   )       /     R   ⁡     (     p   /   n     )         )     ·     (         Xa   0   ⋀     ⁡     (       Q   ⁡     (   p   )       ·     a   0       )       /     (       Xb   0   ⋀     ⁡     (       Q   ⁡     (     p   /   n     )       ·     b   0       )       )                   [     Expression   ⁢           ⁢   1     ]               
Because the degraded first measurement ratio is not dependent on the incident light amount L 1 , it is possible to obtain the same measurement ratio even when obtained by a given incident light amount L.
 
     From the thus obtained degraded first measurement ratio and the initial ratio, the photodegradation power correction coefficient K is obtained as follows. 
                               K   =       ⁢       (         Ia   ′     ⁡     (     L   1     )       /       Ib   ′     ⁡     (       L   1     /   n     )         )     /     (       Ia   ⁡     (     L   0     )       /     Ib   ⁡     (       L   0     /   n     )         )                   =       ⁢             n   ·     (       R   ⁡     (   p   )       /     R   ⁡     (     p   /   n     )         )     ·     (         Xa   0   ⋀     ⁡     (       Q   ⁡     (   p   )       ·     a   0       )       /                   (       Xb   0   ⋀     ⁡     (       Q   ⁡     (     p   /   n     )       ·     b   0       )       )             n   ·     (         Xa   0   ⋀     ⁡     (     a   0     )       /       Xb   0     ⁡     (     b   0     )         )                     =       ⁢         R   ⁡     (   p   )         R   ⁡     (     p   /   n     )         ·         Xb   0   ⋀     ⁡     (     b   0     )         (       Xb   0   ⋀     ⁡     (       Q   ⁡     (     p   /   n     )       ·     b   0       )           ·       (       Xa   0   ⋀     ⁡     (       Q   ⁡     (   p   )       ·     a   0       )             Xa   0   ⋀     ⁡     (     a   0     )                         [     Expression   ⁢           ⁢   2     ]                 
Thus, the photodegradation power correction coefficient K is derived as a function of the accumulated light amount p. Note that the initial ration Ia(L 0 )/Ib(L 0 /n)=n·(Xa 0 ^(a 0 )/Xb 0 ^(b 0 )) needs to be recorded beforehand in a data storage unit, such as a memory.
 
     The photodegradation power correction coefficient K varies as shown in  FIG. 9  in accordance with the accumulated illuminance. Note that  FIG. 9  is a view in which the photodegradation power correction coefficient K in regard to the light amount detecting device  1  of the display device  1000  of the aspects of the invention and the measured data of the accumulated light amounts are plotted. The relationship of  FIG. 9  is obtained empirically beforehand. Then, when the relationship between the photodegradation power correction coefficient K and the accumulated illuminance is stored in a look-up table, the accumulated illuminance may be obtained on the basis of the photodegradation power correction coefficient K input from the photodegradation coefficient calculation unit  21 . In addition, the power coefficient a of the first photodetection circuit LS 1  and the power coefficient b of the second photodetection circuit LS 2  vary as shown in  FIG. 10  in accordance with the accumulated illuminance. Note that  FIG. 10  is a view in which the accumulated light amount and the measured data of the power coefficients a and b are plotted. The relationship of  FIG. 10  is obtained empirically beforehand. Thus, when the relationship between the accumulated illuminance and the power coefficients a and b is stored in a look-up table, the power coefficients a and b are obtained from the accumulated illuminance. As a result, the modified power coefficients a′ and b′ are obtained from the photodegradation power correction coefficient K, which is an output from the photodegradation coefficient calculation unit  21 . Then, it is possible to correct the sensitivity in regard to the photosensor with a light dimmer and the photosensor without a light dimmer from the modified power coefficients a′ and b′. 
     Here, when the power-corrected first and second photoelectric currents are Ia″(L 1 ) and Ib″(L 1 ), Ia″(L 1 ) and Ib″(L 1 ) may be expressed as follows.
 
 Ia ″( L   1 )= Xa ′^( a ′)· L   1  
 
 Ib ″( L   1 )= Xb ′^( b ′)· L   1  
 
In addition, the second measurement ratio, which is a ratio of the power-corrected first output signal to the power-corrected second output signal is Ia″(L 1 )/Ib″(L 1 /n). Furthermore, when the photodegradation slope correction coefficient with respect to the power-corrected photoelectric current ratio is K″, the photodegradation slope correction coefficient K″ is expressed as a ratio of the second measurement ratio to the initial ratio, that is, K″=(Ia″(L 1 )/Ib″(L 1 /n))/(Ia(L 0 )/Ib(L 0 /n)).
 
     Here, when the modified proportional coefficient of the output value of the target photosensor (here, the photosensor with a light dimmer) to the initial value is D, D=Ib″(L 1 )/Ib. Thus, when the relationship between a photodegradation slope correction coefficient K″ and an initial proportional coefficient correction amount measured beforehand is stored in a look-up table, the modified proportional coefficient D is obtained from the photodegradation slope correction coefficient K″, so the power-corrected second photoelectric current Ib″(L 1 ) may be corrected to the initial state before degradation using Ib=Ib″(L 1 )/D. Through the above described steps, it is possible to correct the power-corrected second photoelectric current Ib″(L 1 ) into the initial second photoelectric current Ib and then output the initial second photoelectric current Ib. 
     Next, the operation when such correction of the photoelectric current is performed in the light amount detecting device  1  of the display device  1000  according to the aspects of the invention will be described. 
       FIG. 11  is a view that shows a flowchart in association with correction of a photoelectric current.  FIG. 11  shows step S 1  in which first and second output signals, which are voltage outputs, are converted into photoelectric current amounts; step S 2  in which a first measurement ratio, which is a ratio of the converted first and second photoelectric current amounts, is calculated; step S 3  in which a power correction coefficient K, which is a ratio of the first measurement ratio to an initial ratio, is calculated; step S 4  in which modified power coefficients a′ and b′ are calculated; step S 5  in which power-corrected first and second output signals are calculated; step S 6  in which a second measurement ratio, which is a ratio of the power-corrected first output signal to the power-corrected second output signal, is calculated; step S 7  in which a photodegradation slope correction coefficient K″, which is a ratio of the second measurement ratio to the initial ratio, is calculated; step S 8  in which a modified proportional coefficient D is calculated from the photodegradation slope correction coefficient K″; and step S 9  in which a photoelectric current derived through calculation is output as a light amount signal S of incident light. 
     First, in the photodetection unit  10 , the capacitors  110  and  210  are charged to a potential Vs. Then, incident light of the light amount L 1  irradiated to the TFT  100 , and dimmed incident light of the light amount L 1 /n is irradiated to the TFT  200 . Thus, photoelectric currents (leakage currents) are generated in the TFTs  100  and  200 . Then, the potentials of the capacitors  110  and  210  decrease. The photodetection unit  10  outputs the potentials of the capacitors  110  and  210  at that time as a first output signal Sa and a second output signal Sb. 
     Then, in the photodegradation coefficient calculation unit  21 , initial power coefficients a and b are read from the memory circuit  23 , the potential signals of the first output signal Sa and second output signal Sb, output from the photodetection unit  10 , are read as photoelectric currents in the TFTs  100  and  200 . The potentials charged in the capacitors  110  and  210  are equivalent to potential differences between the source portions  101  and  201  and the drain portions  102  and  202  in the TFTs  100  and  200 , respectively. As the amount of incident light increases, the photoelectric current increases. Thus, the potentials of the capacitors  110  and  210  decrease by a large amount. In contrast, as the amount of incident light reduces, the photoelectric current reduces. Thus, the potentials of the capacitors  110  and  210  decrease by a small amount. Thus, by acquiring the potential signals after a predetermined period of time has elapsed from initiation of irradiation of incident light, it is possible to read as signals of the photoelectric currents. That is, as the potentials of the capacitors  110  and  210 , which are potential signals, decrease, the photoelectric currents increase, while as the potentials of the capacitors  110  and  210  increase, the photoelectric currents reduce. In the photodegradation coefficient calculation unit  21 , the potential signal is associated with the photoelectric current, and a signal of a degraded first photoelectric current Ia(L 1 ) and a signal of a degraded second photoelectric current Ib(L 1 /n) are acquired from the potential signals. 
     Then, in step S 2 , from the thus acquired degraded first photoelectric current Ia(L 1 ) and second photoelectric current Ib(L 1 /n), the first measurement ratio (Ia(L 1 )/Ib(L 1 /n)) is calculated. 
     Then, in step S 3 , the initial ratio (Ia(L 0 )/Ib(L 0 /n)), which is stored beforehand in the memory circuit  23 , is read to the photodegradation coefficient calculation unit  21 , and the photodegradation power correction coefficient K (=(Ia(L 1 )/Ib(L 1 /n))/(Ia(L 0 )/Ib(L 0 /n))) is calculated as a ratio of the first measurement ratio to the initial ratio. At this time, the above described initial first photoelectric current Ia(L 0 ) and the initial second photoelectric current Ib(L 0 /n) may be stored beforehand in the memory circuit  23  in place of the initial ratio, and in step S 2 , the initial ratio may be calculated. 
     After that, the process proceeds to step S 4 . In step S 4 , the photodegradation power correction coefficient K calculated in step S 3  is output to the photodegradation rate calculation unit  22 . Then, in the photodegradation rate calculation unit  22 , first, the power coefficient correction amount stored in the memory circuit  23  is called, and the look-up table that associates the photodegradation power correction coefficient K with the power coefficient correction amount is referred to. By so doing, the modified power coefficients a′ and b′ corresponding to the photodegradation power correction coefficient K are acquired. 
     Here, the look-up table will be described.  FIG. 9  is a view in which the photodegradation power correction coefficient K in regard to the light amount detecting device  1  of the display device  1000  of the aspects of the invention and the measured data of the accumulated light amounts are plotted.  FIG. 10  is a view in which the accumulated light amount and the measured data of the power coefficients a and b are plotted. Thus, the accumulated light amount (illuminance×time) irradiated to the photosensor is obtained from the value of the photodegradation power correction coefficient K shown in  FIG. 9 . In addition, it is possible to correct a power coefficient for a photosensor with a light dimmer and a power coefficient for a photosensor without a light dimmer from  FIG. 10 . As the degradation proceeds, the photodegradation power correction coefficient K and the power coefficients all decrease. 
     Then, the function curve shown in  FIG. 9  shows the accumulated light amount as a function of the photodegradation power correction coefficient K as a variable based on the measured data. In addition, the function curve shown in  FIG. 10  shows the power coefficient a or b as a function of the accumulated light amount as a variable. As long as a circuit that implements the above functions may be configured in the photodegradation rate calculation unit  22 , it is possible to calculate the power coefficients a and b in association with a photodegradation power correction coefficient K. However, if such an irregular function is intended to be implemented by a circuit configuration, the circuit configuration becomes complex. Then, in the present embodiment, the look-up table that associates the photodegradation power correction coefficient K with the power coefficient correction amount based on the two function curves shown in  FIG. 9  and  FIG. 10  is created, and stored in the memory circuit  23 . By so doing, it is not necessary to provide a complex circuit that is necessary to calculate the modified power coefficients a′ and b′, so it is possible to reduce the circuit size. 
     When the data size of the look-up table stored in the memory circuit  23  needs to be reduced, for example, it is only necessary that the values of the photodegradation power correction coefficient K are stored in units of 0.02 as the look-up table. Then, when the value of the photodegradation power correction coefficient K is not included in the look-up table, interpolation calculation is performed using adjacent data. Thus, even when the value is not included in the look-up table, it is possible to derive the modified power coefficient a′ or b′ from the photodegradation power correction coefficient K. For example, two points corresponding to the two photodegradation power correction coefficients K that place a certain photodegradation power correction coefficient K in between are selected from the look-up table, and these points are connected with a straight line. Thus, the power coefficients a and b corresponding to the photodegradation power correction coefficient K that is not included in the look-up table is determined. Specifically, when the photodegradation power correction coefficient K is 0.03, the modified power coefficient a′ or b′ may be derived from the average of power coefficients a′ or b′ corresponding to the photodegradation power correction coefficients K of 0.02 and 0.04. 
     Referring back to the description of  FIG. 11 , in step S 5 , in the photodegradation rate calculation unit  22 , the first and second output signals are converted into the power-corrected first and second output signals on the basis of the modified power coefficients a′ and b′. In step S 6 , the second measurement ratio, which is a ratio of the first and second output signals, is calculated. In step S 7 , the photodegradation slope correction coefficient K″, which is a ratio of the second measurement ratio to the initial ratio read from the memory circuit  23 , is calculated. Furthermore, in step S 8 , in the optical signal output unit  24 , the modified proportional coefficient D is calculated on the basis of the look-up table that associates the photodegradation slope correction coefficient K″ with the proportional coefficient correction amount. Then, in step S 9 , the power-corrected second photoelectric current Ib″(L 1 /n) is corrected to calculate the initial second photoelectric current Ib(L 1 /n). Then, in step S 9 , the initial second photoelectric current Ib(L 1 /n) is output as the light amount signal S of incident light. 
     According to the display device that includes the thus configured light amount detecting device  1 , the following advantageous effects may be obtained. That is, the light amount detecting device has the function of correcting the sensitivity so that the degraded second photoelectric current Ib′(L 1 ) is corrected on the basis of the photodegradation power correction coefficient K and the modified power coefficient a′ or b′ to obtain the initial second photoelectric current Ib(L 1 ). Thus, even when degradation due to photoexposure occurs, the light amount detecting device outputs an accurate light amount signal S. In addition, the photodetection unit  10  does not use a photoelectric conversion element that improves the antidegradation property, so it is possible to manufacture both the photosensor and the driving transistor of the display device in the same process. Thus, it is possible to manufacture the photosensor in a simple process and, therefore, manufacturing cost may be reduced. 
     In addition, by storing the initial power coefficient correction amount and the initial proportional coefficient correction amount that are necessary for creating the look-up table in the memory circuit  23 , a complex circuit configuration in association with calculation of the modified power coefficient a′ or b′ is not necessary. Thus, power consumption is suppressed, the area of the circuit is reduced, and, as a result, manufacturing cost may be suppressed. 
     In addition, when the calculated photodegradation power correction coefficient K is not included in the look-up table, by performing interpolation calculation using the power coefficients a or b corresponding to the two photodegradation power correction coefficients K that place the intended photodegradation power correction coefficient K in between, it is possible to derive the modified power coefficient a′ or b′. Thus, the look-up table is reduced to suppress the data size. 
       FIG. 12  is a view that shows light irradiation time and variations in rate of change of sensor output when degradation is not corrected.  FIG. 13  is a view that shows light irradiation time and variations in rate of change of sensor output when degradation is corrected in accordance with the aspects of the invention. When  FIG. 12  and  FIG. 13  are compared, it appears that, when degradation correction is performed in accordance with the present embodiment, degradation correction is performed in a wide range of light amounts. 
     In the present embodiment, the initial second photoelectric current Ib(L 1 ) of the second photodetection circuit LS 2  is calculated as the light amount signal S. Instead, the initial first photoelectric current Ia(L 1 ) of the first photodetection circuit LS 1  may be obtained as the light amount signal S. 
     Measurement of the incident light amount L in the light amount detecting device  1  of the present embodiment may be continuously performed at predetermined intervals. Then, when the following measurement is performed, by applying a potential Vg to the gate terminal  190 , the TFTs  100  and  200  are turned on to discharge the potentials of the capacitors  110  and  210 . Then, an electric potential Vs is charged again to the capacitors  110  and  210  to perform measurement. 
     The light amount detecting device  1  is connected to the backlight (not shown), and outputs the light amount signal of external ambient light, measured by the light amount detecting device  1 , to the backlight. In the backlight, the amount of light emission is adjusted on the basis of the light amount signal from the light amount detecting device  1 . Specifically, when ambient light is bright like natural light during the daytime, it is set to increase the amount of light emission of the backlight. On the other hand, when used in a dark environment like during the night, it is set to reduce the amount of light emission of the backlight. Thus, it is possible to perform image display with the amount of light emission appropriate in accordance with an environment used. 
     Note that here, the liquid crystal display device is described; the display area may be applied to a display device, such as an organic EL device, a twisting ball display panel that uses a twisting ball painted into different colors for respective areas having different polarities as an electrooptic material, a toner display panel that uses a black toner as an electrooptic material, or a plasma display panel that uses high-pressure gas such as helium or neon as an electrooptic material. 
     Second Embodiment 
     Next, a second embodiment will be described. In the second embodiment, potential signals output from the photodetection unit  10  to the photosensor reader unit  20  are read as photoelectric currents, and the photoelectric currents are logarithmically transformed and then calculated. 
     First, a calculation method through logarithmical transformation will be described. When the photodegradation power correction coefficient K in the first embodiment is logarithmically transformed, Log 2 K=Log 2 {(Ia′(L 1 )/Ib′(L 1 /n))/(Ia(L 0 )/Ib(L 0 /n))}=(Log 2 (Ia′(L 1 ))−Log 2 (Ib′(L 1 /n)))−(Log 2 (Ia(L 0 ))−Log 2 (Ib(L 0 /n))). Then, when the photodegradation slope correction coefficient K″ is logarithmically transformed, Log 2 K″=Log 2 (Ia″(L 1 )/Ib″(L 1 ))/(Ia(L 1 )/Ib(L 1 ))=Log 2 (Ia″(L 1 ))−Log 2 (Ib″(L 1 ))−(Log 2 (Ia(L 1 ))−Log 2 (Ib(L 1 ))). Thus, through logarithmical transformation, multiplication and division are replaced with addition and subtraction. 
     By so doing, from the logarithmically transformed power correction coefficient Log 2 K and the logarithmically transformed photodegradation power correction coefficient Log 2 K″, the initial logarithmically transformed photoelectric current Log 2 (Ib(L 1 )) is calculated by Log 2 (Ib(L 1 ))=Log 2 (Ib″(L 1 ))−Log 2 D. Then, the logarithmically transformed photoelectric current Log 2 (Ib(L 1 )) is inverse-logarithmically transformed, and the initial second photoelectric current Ib(L 1 )=Ib″(L 1 )/D is calculated. The thus obtained initial second photoelectric current Ib is output as the light amount signal S of incident light. 
     Next, the operation of the light amount detecting device  1  of the display device  1000  according to the second embodiment will be described.  FIG. 14  is a view that shows a flowchart in association with correction of a photoelectric current according to the second embodiment.  FIG. 14  shows step S 11  in which a first output signal Sa and second output signal Sb output from the photodetection unit  10  are read as a degraded first photoelectric current Ia′(L 1 ) and second photoelectric current Ib′(L 1 ), and are then logarithmically transformed; step S 12  in which a logarithmically transformed first measurement ratio is calculated; step S 13  in which the logarithmically transformed initial ratio is read from the memory circuit  23  and a logarithmically transformed power correction coefficient Log 2 K is calculated; step S 14  in which modified logarithmically transformed power coefficients Log 2 a′ and Log 2 b′ corresponding to the calculated logarithmically transformed power correction coefficient Log 2 K are acquired from the look-up table, and a logarithmically transformed photodegradation slope correction coefficient Log 2 K″ is calculated from the modified power coefficients Log 2 a′ and Log 2 b′; step S 15  in which the logarithmically transformed initial photoelectric current Log 2 (Ib(L 1 )) is calculated; step S 16  in which the logarithmically transformed initial photoelectric current Log 2 (Ib) is inverse-logarithmically transformed; and step S 17  in which the inverse-logarithmically transformed second photoelectric current Ib is output as a light amount signal S. 
     The memory circuit  23  according to the second embodiment stores the logarithmically transformed initial power coefficients Log 2 a and Log 2 b, the logarithmically transformed initial ratio Log 2 (Ia(L 0 ))−Log 2 (Ib(L 0 /n)), the logarithmically transformed power coefficient correction amount and the proportional coefficient correction amount. 
     First, in step S 11 , in the photodegradation coefficient calculation unit  21 , a degraded first photoelectric current Ia′(L 1 ) and a degraded second photoelectric current Ib′(L 1 /n) at a certain incident light amount L 1  are acquired from the first output signal Sa and the second output signal Sb output from the photodetection unit  10 , and these first photoelectric current Ia′(L 1 ) and second photoelectric current Ib′(L 1 /n) are logarithmically transformed to calculate Log 2 (Ia′(L 1 )) and Log 2 (Ib′(L 1 /n)). 
     Then, in step S 12 , in the photodegradation coefficient calculation unit  21 , a logarithmically transformed first measurement ratio Log 2 (Ia′(L 1 ))− Log2 (Ib′(L 1 /n)) is calculated. 
     After that in step S 13 , in the photodegradation coefficient calculation unit  21 , the logarithmically transformed initial ratio Log 2 (Ia(L 0 ))−Log 2 (Ib(L 0 /n)) is read from the memory circuit  23 , and a logarithmically transformed photodegradation power correction coefficient Log 2 K=Log 2 (Ia′(L 1 ))−Log 2 (Ib′(L 1 /n))−((Log 2 (Ia(L 0 ))−Log 2 (Ib(L 0 /n))) is calculated. 
     In step S 14 , the logarithmically transformed photodegradation power correction coefficient Log 2 K calculated in step S 13  is output from the photodegradation coefficient calculation unit  21  to the photodegradation rate calculation unit  22 . Then, in the photodegradation rate calculation unit  22 , using the look-up table that associates the logarithmically transformed photodegradation power correction coefficient Log 2 K output from the photodegradation coefficient calculation unit  21  with the logarithmically transformed initial power coefficient correction amount supplied from the memory circuit  23 , modified logarithmically transformed power coefficients Log 2 a′ and Log 2 b′ are obtained. On the basis of these modified logarithmically transformed power coefficients Log 2 a′ and Log 2 b′, logarithmically transformed power-corrected photoelectric currents Ia″(L 1 ) and Ib″(L 1 ) are calculated. Then, a logarithmically transformed photodegradation slope correction coefficient log 2 K″=Log 2 (Ia″(L 1 ))−Log 2 (Ib″(L 1 ))−(Log 2 (Ia(L 1 ))−Log 2 (Ib(L 1 ))) is calculated. 
     In step S 15 , in the optical signal output unit  24 , using the look-up table that associates the logarithmically transformed photodegradation slope correction coefficient Log 2 K″ with the logarithmically transformed initial proportional coefficient correction amount supplied from the memory circuit  23 , a modified logarithmically transformed proportional coefficient log 2 D is calculated. Then, the logarithmically transformed modified proportional coefficient log 2 D=log 2 (Ib″(L 1 ))−log 2 (Ib(L 1 )) of the second photoelectric current is calculated. After that, a logarithmically transformed initial second photoelectric current Log 2 (Ib(L 1 ))=Log 2 (Ib″(L 1 ))−Log 2 D is calculated. 
     Subsequently, in step S 16 , in the optical signal output unit  24 , the logarithmically transformed initial second photoelectric current Log 2 (Ib(L 1 )) is inverse-logarithmically transformed to calculate an initial second photoelectric current Ib(L 1 ). 
     Then, in step S 17 , the initial second photoelectric current Ib(L 1 ) calculated in step S 16  is output as a light amount signal S of the incident light amount L 1  of incident light. 
     According to the second embodiment, the following advantageous effects may be obtained. Through calculation of logarithmic transformation, multiplication and division are replaced with addition and subtraction, so it is possible to reduce the circuit configuration. Thus, the area of the circuit is reduced, and, as a result, manufacturing cost may be reduced. Hence, power consumption is suppressed. 
     As described in the first embodiment, the first output signal Sa and the second output signal Sb input to the photosensor reader unit  20  are read as time required to decrease the potentials of the capacitors  110  and  210  from Vs to Vc and then logarithmically transformed, thus making it possible to calculate and output the light amount signal S. 
     In the present embodiment as well, measurement of the incident light amount L in the light amount detecting device  1  is performed at predetermined intervals. Then, when the following measurement is performed, by applying a potential Vg to the gate terminal  190 , the TFTs  100  and  200  are turned on to discharge the potentials of the capacitors  110  and  210 . Then, an electric potential Vs is charged again to the capacitors  110  and  210  to perform measurement. 
     The entire disclosure of Japanese Patent Application No. 2008-070789, filed Mar. 19, 2008 is expressly incorporated by reference herein.