Patent Publication Number: US-2010123696-A1

Title: Image display apparatus and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-292977, filed on Nov. 17, 2008; the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for displaying an image by controlling a luminance of the image. 
     BACKGROUND OF THE INVENTION 
     A technique to control a luminance of a display in accordance with a visual environment is well known. For example, a brightness in the visual environment (space illuminance) is acquired using two illuminance sensors (respectively set on the front of the display and the ahead of a remote controller), and a backlight of the display is controlled based on the brightness acquired. This method is disclosed in JP-A 2006-72255 (Kokai). 
     As to this method, the illuminance sensor can acquire only a luminous flux of nomal incident light. In this case, a radiation illuminance emitted by a light source reduces in proportion to a square of a distance. Accordingly, an illumination incident-upon the illuminance sensor largely changes based on the set position or the number of sensors, and a luminance of the backlight cannot be suitably controlled. In general, dependency relationship between the vertical luminous flux and a brightness response of an eye is low. 
     Furthermore, another method is disclosed in JP-A 2008-42472 (Kokai). As to this method, a detection sensor (set on the remote controller) detects a region of the display having a reflected light. By using the region extracted/discriminated from the detection information, a color and a brightness of an image (to be displayed) are corrected. Briefly, the image is corrected using only luminance information of the displaying region. 
     On the other hand, a luminance of a background region adjacent to the display affects on the brightness response of the eye. However, in the prior art, the luminance of the background region is not taken into consideration. Accordingly, the brightness response of the eye adopted to the visual environment cannot be sufficiently estimated. As a result, the luminance of the display cannot be suitably controlled. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and a method for correcting the luminance of the display by estimating a brightness response of a viewer in the environmental illumination. 
     According to an aspect of the present invention, there is provided an image display apparatus comprising: a display unit configured to display an image; a calculation unit configured to calculate a display luminance from an input image; a luminance distribution acquisition unit configured to take a scene image including the display unit from a view position of the display unit, and acquire a luminance distribution as an average of luminances of the scene image; and a correction unit configured to correct the display luminance, so that a corrected display luminance is within an estimated luminance range of perceptible difference of luminances on the display unit; wherein the estimated luminance range is estimated with the average, and wherein the display unit displays the image based on the corrected display luminance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic diagrams of use situation of the image display apparatus. 
         FIGS. 2A and 2B  are graphs showing relationship between a space illuminance and an adaptive luminance, and relationship between a luminance distribution of wide angle and the adaptive response. 
         FIG. 3  is a block diagram of the image display apparatus of a first embodiment. 
         FIG. 4  is a flow chart of processing of the image display apparatus of the first embodiment. 
         FIG. 5  is a graph showing relationship between a brightness response and a display luminance. 
         FIG. 6  is a block diagram of the image display apparatus of a second embodiment. 
         FIGS. 7A and 7B  are schematic diagrams of appearance of the image display apparatus of the second embodiment. 
         FIG. 8  is a flow chart of processing of a remote controller in the image display apparatus of the second embodiment. 
         FIG. 9  is a flow chart of processing of a display unit in the image display apparatus of the second embodiment. 
         FIG. 10  is a block diagram of the image display apparatus of a third embodiment. 
         FIGS. 11A and 11B  are schematic diagrams of appearance of the display unit of the third embodiment. 
         FIG. 12  is a flow chart of processing of the image display apparatus of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be explained by referring to the drawings. The present invention is not limited to the following embodiments. 
     The First Embodiment 
       FIG. 1  shows a space illuminance which a viewer (an observer) views a display (outputting an image) in a predetermined illumination environment, and a scene which the viewer views the display.  FIG. 1A  is an example which an illumination adjacent to the viewer lights and an illumination adjacent to the display also lights.  FIG. 1B  is another example which the illumination adjacent to the viewer lights and the illumination adjacent to the display does not light. 
     In  FIG. 1A , a wall adjacent to the display is bright because the illumination of the display side lights. In  FIG. 1B , a wall adjacent to the display is dark because the illumination of the display side does not light. In the first embodiment, an average luminance (Hereinafter, it is called “luminance distribution of wide angle”) of the display and a circumference region thereof is calculated along a view direction from the viewer to the display, and a display luminance of the display is corrected based on the luminance distribution of wide angle. The luminance distribution is, in case of viewing the display from a view position (apart from the display), an average luminance of the display (panel) and the circumference region (background) thereof. 
     With regard to Rec.ITU-R BT.709-5 recommended by International Telecommunication Union, a standard view distance of HDTV is three times as high as the display. In case of viewing the display from a position of the standard view distance, a ratio of display (panel) field having 37 inches to the entire visual field is approximately 4%. In this case, if the display-size is changed without changing the view distance, the ratio of display field having 20 inches to the entire visual field is 1.2%, the ratio of display field having 60 inches to the entire visual field is 10.4%, and the ratio of display field having 100 inches is 29.2%. Accordingly, a ratio of display field to an entire visual field including the display and the circumference thereof is desired to be 1%˜30% of the entire visual field. 
       FIGS. 2A and 2   b  are investigation results of an adaptive luminance representing a human eye&#39;s sensitivity to brightness. In general, when the adaptive luminance is larger, the human eye has characteristics to discriminate a difference of brightness on a surface of a brighter object.  FIG. 2A  shows a correlation between the adaptive luminance and a space illuminance. The space illuminance is an illuminance at a viewer&#39;s position. In  FIG. 1 , an illumination adjacent to the viewer largely affects on the space illuminance.  FIG. 2B  shows a correlative relationship between the adaptive luminance and the luminance distribution of wide angle. 
     As shown in  FIGS. 2A and 2B , a correlation between the adaptive luminance and the space illumination is low, but a correlation between the adaptive luminance and the luminance distribution of wide angle is high. Accordingly, in the first embodiment, the adaptive luminance of human eye is estimated from the luminance distribution of wide angle, and an image having a corrected luminance based on the adaptive luminance is displayed. Hereinafter, the image display apparatus  10  of the first embodiment is explained. 
     (1) Component of the Image Display Apparatus  10 : 
       FIG. 3  is a block diagram of the image display apparatus  10  of the first embodiment. The image display apparatus  10  includes a display luminance calculation unit  1 , a luminance distribution acquisition unit  2 , a luminance correction unit  3 , and a display  4 . In the image display apparatus  10 , the display luminance calculation unit  1 , the luminance correction unit  3  and the display  4 , form a main body  11  of display apparatus. In the first embodiment, the luminance distribution acquisition unit  2  can communicate with the main body  11  wirelessly or wirely, which is separate from the main body  11  physically. 
     The display luminance calculation unit  1  inputs a video signal (Hereinafter, it is called “input image”) having digital or analog format from the outside of the image display apparatus  10 , and calculates a luminance (Hereinafter, it is called “display luminance”) of the input image to be actually displayed on the display  4 . The input image has general image information such as a luminance and a chromaticness. 
     The display  4  includes a panel to display the input image based on the video signal, and a control unit to control the panel. Concretely, the display  4  may be any of a CRT display, a liquid crystal display, a plasma display, an organic EL display, and another display. The display  4  desired to display pixels each having the luminance within 0.01 [cd/m 2 ]˜2000 [cd/m 2 ]. 
     The luminance distribution acquisition unit  2  is located at a position to view the display  4 , and acquires a luminance distribution of wide angle along a direction from the position to the display  4 . As shown in  FIG. 3 , the luminance distribution acquisition unit  2  includes an optical lens  201  to focus an incident light, an imaging sensor  202  having at least one light-receiving element (such as CCD or CMOS) arrayed, and a computation unit  203  to compute a luminance value from an output current of the imaging sensor. The optical lens  201  focuses a light incident from a wide angle. The imaging sensor  202  takes a light focused by the optical lens  201 , and photoelectrically transfers the light. The computation unit  203  computes a luminance distribution of wide angle from an output current supplied by the imaging sensor  202 . A spectral sensitivity characteristics of the light-receiving element of the imaging sensor is desired to be near a standard spectral luminous efficiency V(λ) fixed by JIS C7612. However, the spectral sensitivity characteristics may be near a desired standard spectral luminous efficiency by using a color-filter or a response difference. The optical lens may be replaced with a focus means such as a pin-hole structure or a plural lens structure having the focal distance adjusted. 
     Furthermore, when an angle between a view direction (from the viewer to the display  4 ) and a normal line direction of the display  4  (panel) is above ±45 deg along horizontal and vertical directions, a space coordinate of the luminance distribution of wide angle is transferred using a distance between the viewer and the display  4 , and horizontal and vertical visual angles of the display  4 . In this case, the luminance distribution of wide angle can be calculated in the same way as the case that the viewer views the display  4  along the normal line direction. Briefly, a part of function of a general purpose device (a digital still-camera or a cellular-phone) having above-mentioned characteristics may be alternatively used. 
     The luminance correction unit  3  corrects the display luminance (calculated by the display luminance calculation unit  1 ) using the luminance distribution of wide angle (acquired by the luminance distribution acquisition unit  2 ). A method for calculating a corrected display luminance is explained in detail afterwards. A LUT  5  stores a data set necessary to correct the display luminance in correspondence with the luminance distribution of wide angle. The data set is explained in detail afterwards. 
     (2) Operation of the Image Display Apparatus: 
       FIG. 4  is a flow chart of processing of the image display apparatus  10 . First, the display luminance calculation unit  1  calculates a display luminance L t  [cd/m 2 ] (Hereinafter, this unit is omitted) of each pixel from the image (input to the image display apparatus  10 ) (S 1 ). Concretely, the display luminance calculation unit  1  calculates the display luminance L t  of each pixel (to be displayed on the display  4 ) from the input video signal. The display luminance L t  is a luminance value to which gamma-conversion to display the image is performed. In this case, the display luminance L t  of each pixel is previously corrected to be equal to a luminance value actually measured. 
     Next, the luminance distribution acquisition unit  2  takes an image from a set position (able to image the display  4 ) along a direction to the display  4 , and acquires a luminance distribution A of wide angle (S 2 ). In the first embodiment, a light amount via the lens is detected at a predetermined interval, and an absolute luminance of each pixel is calculated from the light amount acquired by each light-receiving element of the imaging sensor  202 . The luminance distribution A of wide angle may be acquired by the viewer&#39;s indication. 
     Hereinafter, a method for calculating an absolute luminance of a surface of the object from the light amount (incident on the imaging sensor  202 ) is explained. First, in case of imaging mechanism having auto exposure function, under a condition excluding influence of gamma-conversion, a relationship between the light amount (incident on the imaging sensor  202 ) and the luminance of the surface of the object is represented as an equation (1) by APEX (Additive system of Photographic Exposure, which was proposed in the 1960 American Standards Association) standard value. 
       log 2( B/ 3.42)=2×log 2 F −log 2 T −log 2( S/ 3.125)  (1) 
     In the equation (1), “B” is a coefficient to correct the luminance, “F” is a stop value (F value), “T” is a shutter speed (sec), and “S” is ISO sensitivity. In general, “F, T, S” are determined by Through-the-lens type exposure meter inside the camera, and preset so that a metering point has 18% gray. Accordingly, each pixel value (x, y) of the image is converted by an equation (2). 
       Labs( x,y )=100× B×Y ( x,y )/(18 ×Y max)  (2) 
     In the equation (2), “Lab (x, y)” is an absolute luminance [cd/m 2 ] of light received by the light-receiving element (x, y), “Y (x, y)” is a gradation value of the image, and “Ymax” is a maximum value (100 as a regular value) in CIE1931XYZ color system. By above-mentioned luminance value, a gradation value Y (x, y) of each pixel is converted. Furthermore, by averaging the absolute luminance Labs (x, y) acquired by all light-receiving elements, a luminance distribution A [cd/m 2 ] of wide angle along a direction from the viewer to the display can be acquired. 
     In the first embodiment, in case of viewing from the view position to the display  4  while fixing “F, T, S”, an entire viewing field including the display  4  and a circumference thereof is taken as a scene image. After that, a luminance value output from each light-receiving element is corrected by a point spread function (PSF) of the lens, the luminance distribution A of wide angle is calculated by averaging the absolute luminance Labs (x, y) (acquired by each light-receiving element) and temporally stored. When the luminance distribution A of wide angle from the absolute luminance Labs (x, y), in addition to PSF, by setting a weight to Labs (x, y), the luminance distribution A of wide angle may be calculated as a weighted average. 
     In an image estimated as a surface of the display  4 , a smaller weight is set to the luminance value output from light-receiving elements corresponding to a center region of the image, a larger weight is set to the luminance value output from light-receiving elements corresponding to a circumference region of the image, and the luminance distribution A of wide angle may be calculated using the weighted average. By above-mentioned weighting, the luminance distribution A of wide angle having the large weight set to luminances corresponding to a background of the display  4  is calculated. 
     Next, by using the luminance distribution A acquired at S 2 , the luminance correction unit  3  respectively calculates an upper limit luminance and a lower limit luminance of a perception range of eye&#39;s brightness response (S 3 ). Relationship among a brightness response, the display luminance and the luminance distribution A of wide angle, is represented as an equation (3). 
         R=R   m   ×L   t   n /( L   t   n   +A   n )  (3) 
     In the equation (3), “n” is a constant within “0.7˜2.0”, which becomes a larger value when the luminance distribution A of wide angle is larger. “R m ” is a coefficient of brightness response as a constant based on the luminance distribution A of wide angle. In the first embodiment, a data set of values of “R m  and n” in correspondence with the luminance distribution A of wide angle is stored in the LUT  5  to be referred. 
       FIG. 5  shows an example of a brightness response curve of the equation (3). In  FIG. 5 , a horizontal axis represents the display luminance L t , and a vertical axis represents the brightness response R normalized by each R m . The brightness response curve is V 1  in case that the luminance distribution A of wide angle is 10 [cd/m 2 ], and the brightness response curve is V 2  in case that the luminance distribution A of wide angle is 100 [cd/m 2 ]. In this case, an upper limit R max /R m  of the brightness response is 86%, and a lower limit R min /R m  of the brightness response is 17%. A range “17%˜86%” of the brightness response is a brightness perception range D. In other words, if the image is displayed with a luminance over the brightness perception range D, the viewer cannot perceive a difference of the luminance of the image displayed. For example, as to the brightness response curve V 1 , pixels displayed with the luminance below 1 [cd/m 2 ] are perceived as black pixels by the viewer. In the same way, pixels displayed with the luminance above 100 [cd/m 2 ] are perceived as white pixels by the viewer. 
     The upper limit luminance L 1  and the lower limit luminance L 0 , which corresponds to an upper limit R max  and a lower limit R min  of the brightness response R respectively, are calculated by equations (4) and (5) each transformed from the equation (3). In the first embodiment, the upper limit R max /R m  and the lower limit R min /R m  of the brightness response R is previously determined. 
     
       
         
           
             
               
                 
                   
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     Next, the luminance correction unit  3  corrects a luminance value L t  of the input image so that a corrected luminance value is within a range between the upper limit L 1  and the lower limit L 0 , and generates the corrected display luminance L c  (S 4 ). The corrected display luminance L c  is calculated by an equation (6). 
         L   c   =L   1 ×( L   t   −L   0 )/( L   1   −L   0 )  (6) 
     In this case, the corrected display luminance L c  of all pixels in the input image is calculated. A method for calculating the corrected display luminance is not limited to the equation (6). Briefly, the luminance value is corrected to be within the perception range. Last, the display  4  displays the image based on the corrected display luminance L c  (calculated at S 4 ) and a chromaticness of the input video signal (S 5 ). 
     As mentioned-above, by displaying the image having the corrected luminance within the perception range of eye&#39;s brightness response at S 5 , a contrast of the luminance perceived by the viewer rises. Furthermore, the image having smooth gradient without jump and clipping is displayed. Briefly, according to the image display apparatus of the first embodiment, a luminance range corresponding to the perception range of eye&#39;s brightness response is estimated using the luminance distribution of wide angle having high correlation with the eye&#39;s brightness response. Accordingly, the eye&#39;s brightness response is adaptively estimated, and the display luminance is suitably controlled for the human vision. 
     The Second Embodiment 
     An image display system  20  of the second embodiment is explained. 
     (1) Component of the Image Display System  20 : 
       FIG. 6  is a block diagram of the image display system  20 . The image display system  20  includes a main body  21  of display apparatus, and a remote controller  22 . The main body  21  of display apparatus wirelessly communicates with the remote controller  22 . The main body  21  of display apparatus includes the display luminance calculation unit  1 , the luminance correction unit  3 , the display  4 , and a signal receiving unit  15 . 
     In order to control a display luminance, if the luminance is automatically controlled at a predetermined interval or a random interval, a viewer is apt to become aware of change of the display luminance, and this change gives unnatural impression to the viewer. As to the image display apparatus of the second embodiment, by correcting the display luminance in synchronization with active control operation (such as a channel operation, timing of power-on or power-off), the display luminance is changed at a timing hard for the viewer to perceive the change of luminance. 
     The signal receiving unit  15  includes wireless communication means such as infrared rays or Bluetooth®, which is set on the front of the display  4  of the main body  21 . The remote controller  22  includes a luminance distribution acquisition unit  200 , an operation unit  13 , and a signal transmission unit  12 . The signal transmission unit  12  has the same wireless communication means as the signal receiving unit  15 , which is set on the front of the remote controller  22 . The operation unit  13  has an interface for the viewer to input a control signal of the display apparatus. For example, the operation unit  13  has a channel and a switch to control a volume or a power, as the interface for the viewer to operate the display apparatus. 
     In the same way as the first embodiment, the luminance distribution acquisition unit  200  has an optical lens  201 , an imaging sensor  202 , and a computation unit  203 . The luminance distribution unit  200  is set on the front of the remote controller  22 , i.e., on the same face where the signal transmission unit  12  is set. When the viewer operates (to display, output speech, or broadcast data) the main body  21  of display apparatus by the remote controller  22  in his/her hand, the signal transmission unit  12  often turns to a direction toward the display  4 . Accordingly, the luminance distribution acquisition unit  200  is located on the same front where the signal transmission unit  12 . In this case, the luminance distribution acquisition unit  200  takes an image of a scene which the viewer views along a direction toward the display  4 , and acquires a luminance distribution of wide angle by averaging luminance of the image. Furthermore, the viewer&#39;s intentional operation (of the operation unit  22 ) to acquire the luminance distribution of wide angle greatly reduces. As a result, brightness data of a background of the display  4  from the viewer&#39;s position can be accurately acquired. 
       FIGS. 7A and 7B  show an example of appearance of the image display system  20 .  FIG. 7A  shows appearance of the remote controller  22 , and  FIG. 7B  shows appearance of the main body  21  of display apparatus. As shown in  FIGS. 7A and 7B , the luminance distribution acquisition unit  200  and the signal transmission unit  12  are located on the same face of the remote controller  22 , and the signal receiving unit  15  and the display  4  are located on the same face of the main body  21  of display apparatus. 
     When the viewer operates the remote controller  22 , a face where the signal transmission unit  12  is set in the remote controller  22  is turned to the signal receiving unit  15  of the main body  21  of display apparatus. Accordingly, the luminance distribution acquisition unit  200  takes an image of a scene along a view direction from the viewer to the display  4  at a timing to operate the operation unit  13 , and calculates a luminance distribution A of wide angle by averaging the luminance of the image. 
     (2) Operation of the Image Display System: 
       FIG. 8  is a flow chart of processing of the remote controller  22 . First, the remote controller  22  detects the viewer&#39;s operation of the operation unit  13  (S 31 ). The operation unit  13  notifies the luminance distribution unit  200  of the viewer&#39;s operation. 
     Next, the imaging sensor  202  of the luminance distribution acquisition unit  200  takes a scene image in synchronization with the timing for the viewer to operate (input by a button) the operation unit  13  (S 32 ). For example, by acquiring the luminance distribution A of wide angle immediately after pushing a button of the remote controller  22 , a scene along a view direction from the viewer to the display  4  corresponds to the luminance distribution A of wide angle acquired from a scene image which the remote controller  22  is turned to the display  4 . 
     Next, the computation unit  203  calculates a luminance distribution A of wide angle from the luminance of the scene image taken by the imaging sensor  202  (S 33 ). In the second embodiment, the computation unit  203  of the remote controller  22  calculates the luminance distribution A of wide angle. However, by transmitting the scene image (two-dimensional information) having luminance values taken from the imaging sensor  202  to the main body  21  of display apparatus, the main body  21  may calculate an average of the luminance values. As a result, a circuit-scale of a micro computer of the luminance distribution acquisition unit  200  is reduced, and a price unit for manufacture can be lowered. In this case, data amount transmitted from the remote controller  22  to the main body  21  increases. 
     Next, the signal transmission unit  12  wirelessly transmits a control signal (from the operation unit  13  in response to the viewer&#39;s operation) and a signal of the luminance distribution A of wide angle (acquired by the luminance distribution acquisition unit  200 ) to the main body  21  of display apparatus. In this case, any of the control signal and the luminance distribution signal may be preferentially transmitted in a transmission time. However, by comparing an operation period to execute the control signal with an operation period to calculate a corrected display luminance L c  based on the luminance distribution signal, one signal having a longer operation period is desired to be transmitted first. 
       FIG. 9  is a flow chart of processing of the main body  21  of display apparatus. In  FIG. 9 , S 11 , S 13  and S 14  are respectively same as S 1 , S 3  and S 4  of  FIG. 4 . Accordingly, explanation of S 11 , S 13  and S 14  is omitted. 
     First, the signal receiving unit  15  decides whether a luminance distribution signal is received from the remote controller  22  (S 12 ). If it is decided that the luminance distribution signal is received (Yes at S 12 ), processing is forwarded to S 13 . 
     Next, when a control unit (not shown in  FIG. 6 ) of the display  4  executes the control signal, it is decided whether a display change of the display  4  is above a threshold (predetermined standard) (S 15 ). If the display change is above the threshold (Yes at S 15 ), processing is forwarded to S 16 . Briefly, by controlling the display luminance in synchronization with a timing when the display change on the display  4  is large, it is hard for the viewer to perceive change of the display luminance. As a control operation to largely change the display contents, a switching operation of channel, a start of transmission/receiving of data broadcast, and power on/off of the main body  21  of display apparatus, are considered. For example, control of the volume is not an operation to change the display. In this case, the display luminance may not be changed at the timing to control the volume. 
     Last, a timing to execute the control signal is synchronized with a timing to display the image based on the corrected display luminance L c  (S 16 ). In the second embodiment, after calculating the corrected display luminance L c , display change of the display  4  is decided at S 15 . However, in order to reduce the computation amount, processing of S 15  may be inserted between S 12  and S 13 . Furthermore, only when an input using a predetermined button is operated in the operation unit  13  of the remote controller  22 , the imaging sensor  202  may take the scene image. 
     The Third Embodiment 
     An image display system  30  of the third embodiment is explained. 
     (1) Component of the Image Display System  30 : 
       FIG. 10  is a block diagram of the image display system  30  of the third embodiment. In comparison with the image display system  20  of  FIG. 6 , the image display system  30  further includes an incident illuminance detection unit  26 . The incident illuminance detection unit  26  includes an illuminance sensor and various kinds of control circuit, and detects an illuminance of an environmental light incident from the outside of the image display system  30 . Furthermore, the incident illuminance detection unit  26  calculates a surface reflection luminance of the display  4  from the illuminance. 
     A luminance correction unit  27  calculates a corrected display luminance using the display luminance (calculated by the display luminance calculation unit  1 ), the luminance distribution A of wide angle (received by the signal receiving unit  15 ) and an incident illuminance signal (detected by the incident illuminance detection unit  26 ). Concretely, the luminance correction unit  27  prepares a program to calculate the corrected display luminance and another computation unit. 
       FIGS. 11A and 11B  show appearance of the main body  31  of display apparatus.  FIG. 11A  shows appearance of a front view of the main body  31 .  FIG. 11B  shows appearance of a side view of the main body  31 . The incident illuminance detection unit  26  is located on the same surface as a panel (screen) of the display  4 . In order to exactly calculate the surface reflection luminance, the illuminance sensor is desired to be located so that a normal line direction of the display  4  is equal to a normal line direction of a light-receiving surface of the illuminance sensor of the incident illuminance detection unit  26 . 
     (2) Operation of the Image Display System  30 : 
     Next, operation of the image display system  30  of the third embodiment is explained by referring to  FIG. 12 .  FIG. 12  is a flow chart of processing of the image display system  30 . In  FIG. 12 , S 21 , S 22 , S 26 , S 27  and S 28  are respectively same as S 11 , S 12 ,  514 , S 15  and S 16  of  FIG. 9 . Accordingly, explanation of S 21 , S 22 , S 26 , S 27  and S 28  is omitted. 
     The incident illuminance detection unit  26  detects an incident illuminance E [1×] of the environmental light incident upon the display  4  (S 23 ). In this case, the incident illuminance is desired to be detected at a timing to operate the display  4  by the remote controller. As a result, when the channel of the remote controller is operated, a light amount of the environmental light incident upon the display  4  can be detected at once. The incident illuminance may be detected at a predetermined interval or a random interval. In this case, an illuminance detected at a time near the operation timing (by the remote controller  32 ) is desired to be used as the incident illuminance E. 
     The luminance correction unit  27  calculates a surface reflection luminance O [cd/m 2 ] from the incident illuminance E (acquired at S 23 ) and a surface reflection ratio S of the display  4  by an equation (7) (S 24 ). 
         O=E×S/π   (7) 
     In the equation (7), “S” is a variable changed within “0˜1” by depending on surface treatment of the display  4 . In the third embodiment, “S” is set as a known constant value. In general, the display is defined as a luminous object and a reflection object. By reflecting the environmental light from a surface of the display, the surface reflection luminance occurs on the display. Accordingly, change of a surface luminance of the display  4  by incident environmental light can be correctly estimated. 
     The luminance correction unit  27  calculates a brightness response curve using the luminance distribution A of wide angle (acquired at S 23 ), the display luminance L t  of each pixel (extracted at S 21 ) and the surface reflection luminance O [cd/m 2 ] (acquired at S 24 ) (S 25 ). In the third embodiment, the brightness response curve of an equation (8) is used. 
         R=R   m ×( L   t   +O ) n /(( L   t   +O ) n   +A   n )  (8) 
     As to the third embodiment, in the same way as the first embodiment, the upper limit luminance L 1  and the lower limit luminance L 0 , which corresponds to an upper limit R max  and a lower limit R min  of the brightness response R respectively, are calculated by equations (9) and (10) each transformed from the equation (8). The upper limit R max /R m  and the lower limit R min /R m  of the brightness response R is previously determined. 
     
       
         
           
             
               
                 
                   
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     In general, the surface reflection luminance of the display additionally acts upon the display luminance L t  of each pixel extracted from the video signal. Furthermore, maximum of the surface reflection luminance to be added is approximately 10 [cd/m 2 ] in a bright visual environment (e.g. E=10000 [1×] and S=0.0033 [%]). Accordingly, as to a dark region (such as L t &lt;10 [cd/m 2 ]) on the image, the display luminance L t  of each pixel acquired at S 21  is largely different from an actual luminance on a surface of the display  4 . 
     In the third embodiment, a brightness response curve suitable for the visual environment can be calculated using a display luminance (as a sum of L t  and the surface reflection luminance O) and the luminance distribution A of wide angle. As to a light-receiving element of the luminance distribution acquisition unit  200 , a detectable luminance range is desired to be above 0.01 [cd/m 2 ] and below 10000 [cd/m 2 ]. However, by shifting the detectable luminance range based on the illuminance of the environmental light, the detectable luminance range may be desirably extended. 
     In this case, in synchronization with the incident illuminance E (detected by the incident illuminance detection unit  26 ) of the environmental light incident upon the display  4 , by adjusting at least one or all of the stop value F, the shutter speed T [sec] and the ISO sensitivity S, the absolute luminance Labs (x, y) is desired to be calculated. 
     Especially, when the incident illuminance E is larger than a predetermined regular value, “F, T and S” are set as a smaller value. When the incident illuminance E is smaller than the predetermined regular value, “F, T and S” are set as a larger value. As a result, the luminance within a wider range can be acquired using a cheap light-receiving element (having a narrow response range), and the manufacture cost can be lowered. 
     As mentioned-above, as to the third embodiment, by using a surface reflection of the environmental light incident upon the display  4 , the upper limit luminance and the lower limit luminance corresponding to the brightness perception range can be accurately calculated. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and embodiments of the invention disclosed herein. It is intended that the specification and embodiments be considered as exemplary only, with the scope and spirit of the invention being indicated by the claims.