Patent Publication Number: US-9842569-B2

Title: Display device capable of gradually changing luminance and gamma

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of application Ser. No. 13/693,824, filed on Dec. 4, 2012, which claims priority from Korean Patent Application No. 10-2012-0096605 filed on Aug. 31, 2012 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     The disclosed technology relates to a display device, and more particularly, to a display device which can adjust the brightness of a display panel. 
     Description of the Related Technology 
     As portable display devices (such as notebooks, mobile phones and portable media players (PMPs)) as well as display devices for homes (such as TVs and monitors) become lighter and thinner, various flat panel displays are being used. A flat panel display includes a display panel which displays images. Flat panel displays may be classified into liquid crystal displays, organic electroluminescent displays, and electrophoretic displays according to the type of the display panel. 
     A display device can control the luminance of an image displayed on a display panel. The luminance of the display panel can be controlled using various methods. For example, the luminance of the display panel can be controlled by a luminance change command received from outside the display device. Alternatively, a light sensor included in the display device may sense ambient brightness and automatically change the luminance of the display panel based on the sensed ambient brightness. To improve the display quality of an image displayed on the display panel, when the luminance of the display panel is changed, the gamma of the display panel may also be changed. For example, when the luminance of the display panel increases, the gamma of the display panel may also increase, thereby reducing saturation. 
     If the luminance of the display panel sharply changes from a current luminance to a target luminance, flickering can be observed on the display panel, or display quality can be degraded. 
     SUMMARY OF CERTAIN INVENTIVE ASPECTS 
     One inventive aspect is a display device including a display panel whose luminance changes from a start luminance to a target luminance lower than the start luminance and which displays an image. The image includes a start frame having the start luminance and a start gamma, a target frame having the target luminance and a target gamma, and a plurality of intermediate frames sequentially placed between the start frame and the target frame, where a difference in the luminance of the display panel between adjacent frames is successively reduced. 
     Another inventive aspect is a display device including a display panel whose luminance changes from a start luminance to a target luminance higher than the start luminance and which displays an image. The image includes a start frame having the start luminance and a start gamma, a target frame having the target luminance and a target gamma, and a plurality of intermediate frames sequentially placed between the start frame and the target frame, where a difference in the luminance of the display panel between adjacent frames successively increases. 
     Another inventive aspect is a display device including a display panel whose luminance changes from a start luminance to a target luminance and which displays an image. The image includes a start frame having the start luminance and a start gamma, a target frame having the target luminance and a target gamma, and a plurality of intermediate frames sequentially placed between the start frame and the target frame, where a difference in the luminance of the display panel between adjacent frames is successively reduced when the target luminance is less than the start luminance and successively increases when the target luminance is greater than the start luminance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features are described with reference to the attached drawings, in which: 
         FIG. 1  is a block diagram of a display device according to an embodiment; 
         FIG. 2  is a table showing luminances and gammas of frames included in an image according to an embodiment; 
         FIG. 3  is a graph showing frame and luminance in a case where a target luminance is lower than a start luminance according to an embodiment; 
         FIG. 4  is a graph showing frame and gamma in a case where the target luminance is lower than the start luminance according to an embodiment; 
         FIG. 5  is a graph showing frame and luminance in a case where the target luminance is higher than the start luminance according to an embodiment; 
         FIG. 6  is a graph showing frame and gamma in a case where the target luminance is higher than the start luminance according to an embodiment; 
         FIG. 7  is a graph showing frame and luminance in a case where the target luminance is lower than the start luminance according to another embodiment; 
         FIG. 8  is a graph showing frame and luminance in a case where the target luminance is higher than the start luminance according to another embodiment; 
         FIG. 9  is a graph showing frame and gamma in a case where the target luminance is lower than the start luminance according to another embodiment; 
         FIG. 10  is a graph showing frame and gamma in a case where the target luminance is higher than the start luminance according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS 
     Embodiments are described with reference to the attached drawings. 
       FIG. 1  is a block diagram of a display device  1000  according to an embodiment. Referring to  FIG. 1 , the display device  1000  includes a display panel  100 . The display panel  100  may display images. The display panel  100  may be a liquid crystal display panel, an electrophoretic display panel, an organic electroluminescent display panel, or one of various display panels that can display images. 
     The display panel  100  may include a plurality of pixels PX arranged in a matrix and may display an image by controlling gray levels of the pixels PX. The display panel  100  may receive first through m th  data signals D 1  through Dm and first through i th  scan signals G 1  through GI. The first through m th  data signals D 1  through Dm may include information about the gray levels of the pixels PX. The pixels PX may determine whether to receive the first through m th  data signals D 1  through Dm based on the first through i th  scan signals G 1  through GI. 
     The display panel  100  may have a variable luminance. The luminance of the display panel  100  may be changed by an external manipulation or may be automatically changed according to the setting of the display device  1000 . This will be described in more detail later. The luminance of the display panel may generally refer to a global brightness of the display panel and may or may not be related to a particular image being displayed. The luminance of the display panel may correspond, for example, to a brightness of an image displayed with a maximum gray level. The luminance of the display pane can be changed automatically or manually. 
     The display device  100  may further include a timing controller  400 , a scan driver  500 , a gamma voltage generator  300 , and a data driver  200 . 
     The timing controller  400  may receive image data R, G and B and a panel luminance control signal PLC. The image data R, G and B may include information about a gray level of an image to be displayed on the display panel  100 , and the panel luminance control signal PLC may include information about the luminance of the display panel  100 . The timing controller  400  may generate a scan control signal SCS, a data control signal DCS and a gamma control signal GCS corresponding to the image data R, G and B and the panel luminance control signal PLC. 
     The scan driver  500  may receive the scan control signal SCS and generate the first through i th  scan signals G 1  through GI corresponding to the scan control signal SCS. 
     A gamma curve is a function which outputs a gray level of an image that is displayed on the panel corresponding to a gray level of image data R, G, and B. The gamma curve may, for example, be used in the equation: Y=M*(x/M) g , where Y is the gray level displayed, M is a maximum gray level, x is the image data R, G, or B, and g is gamma. 
     The gamma voltage generator  300  may receive the gamma control signal GCS and generate a gamma voltage GV corresponding to the gamma control signal GCS. The gamma control signal GCS may include information about a gamma value and the luminance of the display panel  100 . The gamma voltage GV may be a set of voltages corresponding to gray values that an image can have. 
     The data driver  200  may receive the data control signal DCS and the gamma voltage GV. The data control signal DCS may include information about a gray level of an image. The data driver  200  may generate the first through m th  data signals D 1  through Dm from the data control signal DCS by referring to the gamma voltage GV. 
     Luminance change of the display panel  100  is described in more detail with reference to  FIG. 2 .  FIG. 2  is a table showing luminances and gammas of frames according to an embodiment. 
     Referring to  FIG. 2 , the luminance of the display panel  100  may change from a start luminance Ls to a target luminance Lt. An image displayed on the display panel  100  may include a start frame Fs and a target frame Ft that follows the start frame Fs. The luminance of the display panel  100  in the start frame Fs may be the start luminance Ls, and the luminance of the display panel  100  in the target frame Ft may be the target luminance Ft. The image displayed on the display panel  100  may further include first through n th  intermediate frames F 1  through Fn. The first through n th  intermediate frames F 1  through Fn may be located between the start frame Fs and the target frame Ft. The display panel  100  may have first through n th  intermediate luminances L 1  through Ln in the first through n th  intermediate frames F 1  through Fn, respectively. Each of the first through n th  intermediate luminances L 1  through Ln may have a value between the start luminance Ls and the target luminance Lt. If an image includes the first through n th  intermediate frames F 1  through Fn, whose respective luminances are between the start luminance Ls and the target luminance Lt, between the start frame Fs and the target frame Ft, a sharp change in the luminance of the display panel  100  can be avoided, thus preventing the degradation of display quality. 
     When the luminance of the display panel  100  is changed, a gamma value may be changed accordingly. For example, as the luminance of the display panel  100  increases, the gamma of the display panel  100  may also increase. Conversely, as the luminance of the display panel  100  decreases, the gamma of the display panel  100  may also decrease. The gamma of the display panel  100  in the start frame Gt may be a target gamma Gt. In addition, the display panel  100  may have first through n th  intermediate gammas G 1  through Gn in the first through n th  intermediate frames F 1  through Fn, respectively. Each of the first through n th  intermediate gammas G 1  through Gn may have a value between the start gamma Gs and the target gamma Gt. 
     A change in luminance and gamma with respect to frame in a case where the target luminance Lt is lower than the start luminance Ls is described in more detail with reference to  FIGS. 3 and 4 .  FIG. 3  is a graph showing frame and luminance in a case where the target luminance Lt is lower than the start luminance Ls according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the luminance (Ls, L 1 , L 2 , . . . , Ln, Lt) of the display panel  100  may be successively reduced from the start frame Fs to the target frame Ft. More specifically, a reduction (LD 0 , LD 1 , . . . , LDn) in the luminance of the display panel  100  between adjacent frames may be gradually reduced. When the target luminance Lt is lower than the start luminance Ls, if the reduction (LD 0 , LD 1 , . . . , LDn) in the luminance of the display panel  100  between adjacent frames is gradually reduced, the luminance of the display panel  100  may change relatively gently at low luminance. Since human eyes are more sensitive to brightness changes at low luminance than at high luminance, if the reduction (LD 0 , LD 1 , . . . , LDn) in the luminance of the display panel  100  between adjacent frames is gradual in the case where the target luminance Lt is lower than the start luminance Ls, the perception of the change in the luminance of the display panel  100  by human eyes may be reduced. Accordingly, this can prevent the degradation of image quality due to luminance changes. According to some embodiments, the luminance of the display panel  100  may be reduced exponentially. However, the present invention is not limited thereto. 
       FIG. 4  is a graph showing frame and gamma in a case where the target luminance Lt is lower than the start luminance Ls according to an embodiment of the present invention. Referring to  FIG. 4 , the gamma of the display panel  100  may be reduced at an equal rate in the first through n th  intermediate frames F 1  through Fn between the start frame Fs and the target frame Ft. That is, a difference between the start gamma Gs and the first intermediate gamma G 1 , a difference between adjacent gammas of the first through n th  intermediate gammas G 1  through Gn, and a difference between the n th  intermediate gamma Gn and the target gamma Gt may all be equal. If x is a natural number in a range of 1 to n, an x th  intermediate gamma Gx may be given by Gx=Gs+x*((Gt−Gs)/(n+1)). 
     The change in luminance and gamma with respect to frame in a case where the target luminance Lt is higher than the start luminance Ls will now be described in more detail with reference to  FIGS. 5 and 6 .  FIG. 5  is a graph showing frame and luminance in a case where the target luminance Lt is higher than the start luminance Ls according to an embodiment. 
     Referring to  FIG. 5 , the luminance (Ls, L 1 , L 2 , . . . , Ln, Lt) of the display panel  100  may successively increase from the start frame Fs to the target frame Ft. More specifically, an increase (LD 0 , LD 1 , . . . , LDn) in the luminance of the display panel  100  between adjacent frames may gradually increase. When the target luminance Lt is higher than the start luminance Ls, if the increase (LD 0 , LD 1 , . . . , LDn) in the luminance of the display panel  100  between adjacent frames gradually increases, the luminance of the display panel  100  may change relatively gently at low luminance. If the increase (LD 0 , LD 1 , LDn) in the luminance of the display panel  100  between adjacent frames gradually increases in the case where the target luminance Lt is higher than the start luminance Ls, perception of a change in the luminance of the display panel  100  by human eyes may be reduced. Accordingly, this can prevent the degradation of image quality due to luminance changes. According to some embodiments, the luminance of the display panel  100  may be reduced exponentially. However, the present invention is not limited thereto. 
       FIG. 6  is a graph showing frame and gamma in a case where the target luminance Lt is higher than the start luminance Ls according to an embodiment. Referring to  FIG. 6 , the gamma of the display panel  100  may increase at an equal rate in the first through n th  intermediate frames F 1  through Fn between the start frame Fs and the target frame Ft. That is, a difference between the start gamma Gs and the first intermediate gamma G 1 , a difference between adjacent gammas of the first through n th  intermediate gammas G 1  through Gn, and a difference between the n th  intermediate gamma Gn and the target gamma Gt may all be equal. If x is a natural number in a range of 1 to n, an x th  intermediate gamma Gx may be given by Gx=Gs+x*((Gt−Gs)/(n+1)). 
     A method of setting the number of intermediate frames in a case where the target luminance Lt is lower than the start luminance Ls is described with reference to FIG.  7 .  FIG. 7  is a graph showing frame and luminance in a case where the target luminance Lt is lower than the start luminance Ls according to an embodiment. 
     Referring to  FIG. 7 , the luminance of the display panel  100  may be reduced from a first start luminance Ls 1  to a first target luminance Lt 1  or from a second start luminance Ls 2  to a second target luminance Lt 2 . The second start luminance Ls 2  may be lower than the first start luminance Ls 1 . A difference Lg 1  between the first start luminance Ls 1  and the first target luminance Lt 1  may be equal to a difference Lg 2  between the second start luminance Ls 2  and the second target luminance Lt 2 . The luminance of the display panel  100  in a first start frame Fs 1  may be the first start luminance Ls 1 , the luminance of the display panel  100  in a first target frame Ft 1  may be the first target luminance Lt 1 , the luminance of the display panel  100  in a second start frame Fs 2  may be the second start luminance Ls 2 , and the luminance of the display panel  100  in a second target frame Ft 2  may be the second target luminance Lt 2 . 
     The graph of luminance with respect to frame shows a gradual reduction in luminance. For example, the graph of luminance with respect to frame may be, but is not limited to, a graph showing an exponential reduction in luminance. If the graph of luminance with respect to frame shows a gradual reduction in luminance, a difference Fg 1  between the first start frame Fs 1  and the first target frame Ft 1  may be less than a difference Fg 2  between the second start frame Fs 2  and the second target frame Ft 2 . The number of intermediate frames placed between a start frame and a target frame may be determined by a difference between the start frame and the target frame in  FIG. 7 . Therefore, a greater number of intermediate frames may be placed between the start frame and the target frame when the luminance of the display panel  100  changes from the second start luminance Ls 2  lower than the first start luminance Ls 1  to the second target luminance Lt 2  than when the luminance of the display panel  100  changes from the first start luminance Ls 1  to the first target luminance Lt 1 . If a greater number of intermediate frames are placed between the start frame and the target frame when the luminance of the display panel  100  changes from the second start luminance Ls 2  lower than the first start luminance Ls 1  to the second target luminance Lt 2  than when the luminance of the display panel  100  changes from the first start luminance Ls 1  to the first target luminance Lt 1 , an image may include more intermediate frames when the luminance of the display panel  100  changes at relatively low luminance. This ensures gentle changes in luminance, thereby preventing the degradation of display quality due to the luminance changes. 
     In  FIG. 7 , the first start frame Fs 1  precedes the second start frame Fs 2 , and the first target frame Ft 1  precedes the second target frame Ft 2 . However, the order of frames shown in  FIG. 7  may be valid only between the first start frame Fs 1  and the first target frame Ft 1  and between the second start frame Fs 2  and the second target frame Ft 2 . That is, the second start frame Fs 2  can precede the first start frame Fs 1 , and the second target frame Ft 2  can precede the first target frame Ft 1 . 
     A method of setting the number of intermediate frames in a case where the target luminance Lt is higher than the start luminance Ls is described with reference to  FIG. 8 .  FIG. 8  is a graph showing frame and luminance in a case where the target luminance Lt is higher than the start luminance Ls according to an embodiment. 
     Referring to  FIG. 8 , the luminance of the display panel  100  may increase from a third start luminance Ls 3  to a third target luminance Lt 3  or from a fourth start luminance Ls 4  to a fourth target luminance Lt 4 . The fourth start luminance Ls 4  may be lower than the third start luminance Ls 3 . A difference Lg 3  between the third start luminance Ls 3  and the third target luminance Lt 3  may be equal to a difference Lg 4  between the fourth start luminance Ls 4  and the fourth target luminance Lt 4 . The luminance of the display panel  100  in a third start frame Fs 3  may be the third start luminance Ls 3 , the luminance of the display panel  100  in a third target frame Ft 3  may be the third target luminance Lt 3 , the luminance of the display panel  100  in a fourth start frame Fs 4  may be the fourth start luminance Ls 4 , and the luminance of the display panel  100  in a fourth target frame Ft 4  may be the fourth target luminance Lt 4 . 
     The graph of luminance with respect to frame may be show a gradual increase in luminance. For example, the graph of luminance with respect to frame may be, but is not limited to, showing an exponential increase in luminance. If the graph of luminance with respect to frame shows a gradual increase in luminance, a difference Fg 3  between the third start frame Fs 3  and the third target frame Ft 3  may be less than a difference Fg 4  between the fourth start frame Fs 4  and the fourth target frame Ft 4 . The number of intermediate frames placed between a start frame and a target frame may be determined by a difference between the start frame and the target frame in  FIG. 8 . Therefore, a greater number of intermediate frames may be placed between the start frame and the target frame when the luminance of the display panel  100  changes from the fourth start luminance Ls 4  lower than the third start luminance Ls 3  to the fourth target luminance Lt 4  than when the luminance of the display panel  100  changes from the third start luminance Ls 3  to the third target luminance Lt 3 . If a greater number of intermediate frames are placed between the start frame and the target frame when the luminance of the display panel  100  changes from the fourth start luminance Ls 4  lower than the third start luminance Ls 3  to the fourth target luminance Lt 4  than when the luminance of the display panel  100  changes from the third start luminance Ls 3  to the third target luminance Lt 3 , an image may include more intermediate frames when the luminance of the display panel  100  changes at relatively low luminance. This ensures gentle changes in luminance, thereby preventing the degradation of display quality due to the luminance changes. 
     In  FIG. 8 , the fourth start frame Fs 4  precedes the third start frame Fs 3 , and the fourth target frame Ft 4  precedes the third target frame Ft 3 . However, the order of frames shown in  FIG. 8  may be valid only between the third start frame Fs 3  and the third target frame Ft 3  and between the fourth start frame Fs 4  and the fourth target frame Ft 4 . That is, the third start frame Fs 3  can precede the fourth start frame Fs 4 , and the third target frame Ft 3  can precede the fourth target frame Ft 4 . 
     Gamma changes in intermediate frames according to another embodiment is described with reference to  FIGS. 9 and 10 .  FIG. 9  is a graph showing frame and gamma in a case where the target luminance Lt is lower than the start luminance Ls according to another embodiment. 
     Referring to  FIG. 9 , a reduction in gamma between adjacent frames from the start frame Fs to the target frame Ft may be gradually reduced. In the current embodiment, a graph of the change in luminance with respect to frame in the case where the target luminance Lt is lower than the start luminance Ls may be substantially identical to the graph of  FIG. 3 . Therefore, if the reduction in gamma between adjacent frames from the start frame Fs to the target frame Ft is gradually reduced, the gamma of the display panel  100  may change more gently at low luminance than at high luminance. Since human eyes are more sensitive to changes in an image at low luminance than at high luminance, if the gamma of the display panel  100  changes more gradually at low luminance, perception of changes in the image caused by gamma changes by human eyes may be less. Accordingly, this can prevent the degradation of display quality due to luminance and gamma changes. According to some embodiments, if x is a natural number in a range of 1 to n, an x th  intermediate gamma Gx may be given by Gx=Gs+x*((Gt−Gs)/(n+1))*(log (n+1) x). 
       FIG. 10  is a graph showing frame and gamma in a case where the target luminance Lt is higher than the start luminance Ls according to another embodiment. 
     Referring to  FIG. 10 , an increase in gamma between adjacent frames from the start frame Fs to the target frame Ft may gradually increase. In the current embodiment, a graph of the change in luminance with respect to frame in the case where the target luminance Lt is higher than the start luminance Ls may be substantially identical to the graph of  FIG. 5 . Therefore, if the increase in gamma between adjacent frames from the start frame Fs to the target frame Ft gradually increases, the gamma of the display panel  100  may change more gradually at low luminance than at high luminance. Since human eyes are more sensitive to changes in an image at low luminance than at high luminance, if the gamma of the display panel  100  changes more gradually at low luminance, perception of changes in the image caused by gamma changes by human eyes may be less. Accordingly, this can prevent the degradation of display quality due to luminance and gamma changes. According to some embodiments, if x is a natural number in a range of 1 to n, an x th  intermediate gamma Gx may be given by Gx=Gs+x*((Gt−Gs)/(n+1))*(log (n+1) x). 
     Embodiments of the present invention provide various advantages, such as those that follow. A display device which can maintain display quality even when the luminance of a display panel changes can be provided. In addition, the luminance of the display panel is changed more gently at low luminance, thereby preventing the degradation of display quality. 
     The effects of the present invention are not restricted to those set forth herein. It should be understood that the exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for use as or with other similar features or aspects in other embodiments.