Abstract:
A method for calibrating a display screen, the method comprising: displaying an image pattern of a uniform color on the display screen; capturing an image of the display screen with the image pattern displayed thereon by an image capturing device; analyzing the captured image to determine a plurality of image areas of a color diverging from the image pattern; calculating calibration data for each of the plurality of areas of the display screen to compensate the color distortions introduced to the image pattern by the display screen within that area; providing the calibration data to control the display screen.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to calibrating display screens, in particular flat panel OLED displays, in order to provide uniform color distribution over the surface of the display. 
       BACKGROUND 
       [0002]    OLED (Organic Light Emitting Diode) panel displays become more and more popular, especially when unit prices decreases. They have many advantages over LCD (Liquid Crystal Display) panels, in particular with respect to brightness and contrast. However, OLED displays have also some disadvantages. One of them is a poor color stability that manifests as areas that have slightly different color than other areas. These differences can change in time. They are not a major problem when watching movies or photos, but become more visible when the OLED panel is used as a computer or a tablet screen where synthetic pictures are displayed. Differences in color over a plain surface can be easily visible. 
         [0003]    Calibration is a known technique of adjusting the operation of a device to meet predetermined criteria. So far, calibration of brightness of display screens has been performed globally for the whole screen. 
         [0004]    A US patent application US20030053044A1 discloses a system and method for measuring the average light levels from a flat panel display for use in a brightness compensating system. One or more photodiodes are disposed outside of the display area of a flat panel display so as to measure the average light levels directly or via total internal reflection through the transparent substrate of the display. The measured light levels are then used to provide a baseline for calibrating the color balance and brightness of the display using the brightness compensating system. Such system requires a dedicated hardware setup and therefore increases the cost of the display device, is not universal and not applicable for existing devices without the system embedded therein. 
         [0005]    There is a need to provide a system and method for calibrating OLED panels in order to provide uniform color distribution over the surface of the display, which will be universal, applicable to a wide range of displays and will not require dedicated hardware to perform the calibration. 
       SUMMARY 
       [0006]    There is disclosed a method for calibrating a display screen, the method comprising: displaying an image pattern of a uniform color on the display screen; capturing an image of the display screen with the image pattern displayed thereon by an image capturing device; analyzing the captured image to determine a plurality of image areas of a color diverging from the image pattern; calculating calibration data for each of the plurality of areas of the display screen to compensate the color distortions introduced to the image pattern by the display screen within that area; providing the calibration data to control the display screen. 
         [0007]    The method can be iterated for a plurality of image patterns. 
         [0008]    The method may comprise analyzing the captured image by analyzing image histograms to determine areas of dominant color. 
         [0009]    The image areas can be macro-blocks of pixels. 
         [0010]    The image areas can be individual pixels. 
         [0011]    There is also disclosed a system for calibrating a display screen, the system comprising the following modules: a source of an image pattern configured to provide an image pattern for display on the display screen; an image capturing device configured to capture an image of the display screen; an image data analysis module configured to provide calibration data to control the display screen; wherein the system modules are configured to perform the steps of the method as described above. 
         [0012]    The image data analysis module can be separate from the image capturing device. 
         [0013]    The image data analysis module can be accessible as a cloud service via the Internet. 
         [0014]    The image data analysis module can be embedded in the image capturing device. 
         [0015]    The image data analysis module can be a software application operated by a mobile image capturing device. 
         [0016]    The display screen may comprise an embedded controller for receiving the calibration data and calibrating the display screen accordingly. 
         [0017]    The system may comprise comprising a screen controller external to the display screen for receiving the calibration data and applying the calibration data to adjust the signal transmitted to the display screen. 
     
    
     
       BRIEF DESCRIPTION OF FIGURES 
         [0018]    The presented method and system are shown by means of example embodiments on a drawing, wherein: 
           [0019]      FIGS. 1A-1F, 2 and 3  show embodiments of a system for calibrating OLED displays. 
           [0020]      FIG. 4  shows an example of a structure of an image data analysis module. 
           [0021]      FIG. 5  shows an example of a structure of a mobile device. 
           [0022]      FIG. 6  shows in general a method for calibrating an OLED display. 
           [0023]      FIGS. 7A-7B  show details of one embodiment of a calibration procedure. 
           [0024]      FIG. 8  shows details of another embodiment of a calibration procedure. 
       
    
    
     NOTATION AND NOMENCLATURE 
       [0025]    Some portions of the detailed description which follows are presented in terms of data processing procedures, steps or other symbolic representations of operations on data bits that can be performed on computer memory. Therefore, a computer executes such logical steps thus requiring physical manipulations of physical quantities. 
         [0026]    Usually these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. For reasons of common usage, these signals are referred to as bits, packets, messages, values, elements, symbols, characters, terms, numbers, or the like. 
         [0027]    Additionally, all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Terms such as “processing” or “creating” or “transferring” or “executing” or “determining” or “detecting” or “obtaining” or “selecting” or “calculating” or “generating” or the like, refer to the action and processes of a computer system that manipulates and transforms data represented as physical (electronic) quantities within the computer&#39;s registers and memories into other data similarly represented as physical quantities within the memories or registers or other such information storage. 
         [0028]    A computer-readable (storage) medium, such as referred to herein, typically may be non-transitory and/or comprise a non-transitory device. In this context, a non-transitory storage medium may include a device that may be tangible, meaning that the device has a concrete physical form, although the device may change its physical state. Thus, for example, non-transitory refers to a device remaining tangible despite a change in state. 
         [0029]    In the present disclosure, the term “video content” is to be understood more broadly as multimedia content comprising video data and associated audio data and associated additional data (such as content description, etc.). The term “video content” is used to distinguish the content from other content types, such as still images or raw data (e.g. files). 
       DETAILED DESCRIPTION 
       [0030]      FIGS. 1A-1F  show embodiments of the system for calibrating OLED displays. 
         [0031]    In one embodiment, as shown in  FIG. 1A , a flat OLED display  110 , i.e. a TV screen or a computer monitor, is configured to display on its screen  111  an image patter  102 . 
         [0032]    A digital camera  120  is used to capture an image  101  of the screen  111 . The image can be captured by the user of the system upon receiving instructions, e.g. on how to setup the lighting (e.g. to capture the image in a daylight or in a dark), what shall be the distance from the display, which area of the display shall be captured by the camera etc. The instructions can be displayed or played (as a video) by the display  110  prior to displaying the image patter  102 . 
         [0033]    The captured image  101  is transmitted (by wire or wirelessly) from the camera  120  to an image data analysis module  130 . The image data analysis module  130  knows the image pattern  102  that has been displayed on the screen  111  while the image  101  was captured. In one embodiment, the image data analysis module  130  may generate the image patter  102  and send it to the OLED display  110  for presentation on the screen  111 . The image data analysis module  130  is configured to compare the captured image  101  with the image patter  102 , determine areas of the screen  111  that need to be calibrated and generate calibration data  103  that are sent (by wire or wirelessly) to the display  110  to calibrate it. 
         [0034]      FIG. 1B  shows an embodiment similar to the one of  FIG. 1A , wherein instead of a digital camera  120  device, a mobile device  140  is used, such as a tablet or a smartphone, with an embedded camera. 
         [0035]      FIG. 1C  shows an embodiment similar to the one of  FIG. 1A , wherein the image data analysis module  160  is a module operating in a cloud. It can operate according to the SaaS (software as a service) model. The image data analysis module  160  is operated by an external server accessible via the Internet  150 . 
         [0036]      FIG. 1D  shows an embodiment similar to the one of  FIG. 1C , wherein instead of a digital camera  120  device, a mobile device  140  is used, such as a tablet or a smartphone, with an embedded camera. 
         [0037]      FIG. 1E  shows an embodiment similar to the one of  FIG. 1D , wherein the mobile device  140  is configured to communicate with the display  110  that is calibrated. The mobile device  140  receives the image pattern  102  from the image data analysis module  160 , sends it to the display  110  to be calibrated, captures the image  101  of the screen  111 , sends it to the image data analysis module  160 , receives calibration data  103  generated by the image data analysis module  160  and sends the calibration data  103  to the display  110 . 
         [0038]      FIG. 1F  shows an embodiment, wherein the mobile device  140  is configured to communicate with the display  110  that is calibrated and to perform the image analysis, i.e. the image data analysis module is embedded in the mobile device. The mobile device  140  sends the image pattern  102  to the display  110  to be calibrated. captures the image  101  of the screen  111 , analyzes the captured image  101  to generate calibration data  103  and sends the calibration data  103  to the display  110 . 
         [0039]      FIG. 2  shows an example of a system such as shown in  FIG. 1F  (which is applicable to other embodiments as well), during use, wherein an image pattern  102 A, preferably of a uniform color plane, is displayed on an uncalibrated screen  111 . If the screen is discalibrated, areas  202 ,  203 ,  205 ,  206  of shades that diverge from the image patter  102 A may appear on screen (along with areas  201 ,  204  which display the correct color). The captured image  101  contains information about the areas  201 - 206  and can be used to calibrate the device by generating calibration data  203  that identifies the areas  202 ,  203 ,  205 ,  206  for which color has to be corrected so that, after calibration, the image displayed on screen  111  is closer to the image pattern  102 A. 
         [0040]      FIG. 3  shows an example of another system in use, wherein the OLED display  110  is driven by a screen controller  301  with calibration data storage. The screen controller  301  may be a dedicated device or may form part of a device that provides a signal to the OLED display, such as a personal computer, a video disc player, a game console, a television set-top box, a video interface etc. The image pattern  102 A and the calibration data  103  are sent to the screen controller  301 . The screen controller  301  is responsible for applying the calibration to the signal transmitted to the display  110  such as to compensate the color distortions introduced by the display. 
         [0041]      FIG. 4  shows an example of a structure of the image data analysis module  130 ,  160 . The module comprises wired interfaces  402 , such as USB, Ethernet, serial port (such as RS-232) and/or wireless interfaces  442 , such as Wi-Fi or Bluetooth. The interfaces  402 ,  442  are used to communicate with external devices, such as the camera  120 , the mobile device  140 , the OLED display  110 , the screen controller  301 , directly or via a network such as the Internet  150 . 
         [0042]    The module operates utilizing memory blocks  430 , including RAM  431  and Flash  432  memory blocks. 
         [0043]    Data can be processed using a controller  410  or a digital signal processor  420 . 
         [0044]    A clock  450  is used to synchronize the timing of generating the image pattern and capturing of the image of screen, if such timing synchronization is necessary. 
         [0045]    The module is controlled by a controller  410  comprising a plurality of units configured to provide the functionality of the method as described below. An image processing manager  411  is configured to manage image processing operations. An image decomposition unit is configured to decompose the image, as in steps  604 ,  705 ,  807  of the methods presented below. A histogram analysis unit is configured to analyze histograms of the captured image, as in steps  605 ,  706 ,  808 . A calibration unit is configured to generate calibration data  103 , as in steps  606 ,  713 ,  809 . An image pattern generator is configured to generate image patterns  102 , as in steps  601 ,  702 ,  803 . 
         [0046]      FIG. 5  shows an example of a structure of the mobile device  140  for capturing the screen image. The mobile device may comprise data transmitting/receiving blocks  541  and external interfaces  542  for communicating with other devices, such as the image data analysis module  130 ,  160 , the display  110 , the screen controller  301 , directly or via a network such as the Internet  150 . 
         [0047]    The module operates utilizing memory blocks  530 , including RAM  531  and Flash  532  memory blocks. 
         [0048]    A clock  550  is used to generate a time stamp for the captured image, if necessary. 
         [0049]    A camera  560  is used to capture the image of the screen. 
         [0050]    A media player  580  can be used to display the captured image on the display  570 . 
         [0051]    Data can be processed using a controller  510  or an image signal processor  520 . 
         [0052]    The module is controlled by a controller  510  comprising a plurality of units configured to provide the functionality of the method as described below. Units  511 - 515  correspond to the units  411 - 415 . 
         [0053]      FIG. 6  shows in general a method for calibrating the OLED display. First, in step  601 , an image pattern is selected. The image pattern is preferably a uniform color plane, such as white, red, green, blue or yellow plane. Next, the selected image pattern is displayed on screen  602 . Image is captured in step  603  using a camera (stand-alone or embedded in a mobile device). Next, the captured image is processed in step  604  to identify the areas of shades that differ from the image pattern and decompose the captured image into these areas. The image is then interpreted in step  605  to determine the amount of correction that need to be applied to the areas  202 ,  203 ,  205 ,  206  diverging from the image pattern such as to improve the correspondence of the area with the image pattern. The calibration data is calculated in step  606  based on the determined amount of correction. Next, depending on the system configuration and modules and devices used, the calibration data is applied to calibrate the display. In one embodiment, such as shown in  FIGS. 1A-1D and 2 , the calibration data is sent in step  607  directly to the remotely calibrated display  110 . In other embodiments, such as shown in  FIGS. 1E-1F , the calibration data can be stored in step  608  in the non volatile memory of the mobile device that controls the calibrated display  110 . In other embodiments, such as shown in  FIG. 3 , the calibration data can be sent in step  609  to the screen controlled  301  connected to the calibrated display. 
         [0054]      FIGS. 7A-7B  show details of one embodiment of the calibration procedure. First, in step  701 , the parameters of the image pattern are determined, for example the brightness. Next, in step  702 , the image pattern  102  is generated and displayed on the remote calibrated display screen in step  703 . Next, the image  101  is captured by the camera in step  704 . The image is then divided into macro-blocks and/or pixels, according to the resolution of the calibrated screen and the camera  705 . This step may include cropping the captured image such as to disregard the areas that are outside the screen  111  (if a user has captured an image larger than the screen which contains the surrounding of the screen as well) and/or to rotate the image (if the axes of the image do not correspond to the axes of the screen). Next, regions of dominant color are determined, based on image histograms in step  706 . The regions are then analyzed consecutively. A first region of a dominant color is selected in step  707  and information about color disturbances for each macro-block and/or pixel of that region is obtained in step  709 . That information is recorded in the temporary memory in step  709 . The procedure then proceeds to other regions in steps  710 ,  711 , until all regions are analyzed. Next, in step  712  the obtained information about color disturbances for all macro-blocks and/or pixels is read from the temporary memory and calibration data is determined in step  713  for these macro-blocks and/or pixels. For each region of dominant color, static white balance (i.e. balance for low intensities) and dynamic white balance (i.e. balance for high intensities) is setup. For example, if the displayed image is darker than the image patter in one block, that block is calibrated to display a brighter image. In contrast, if the displayed image is brighter than the image pattern in one block, that block is calibrated to display a darker image. Next, in step  715  the calibration data is recorded in the non volatile memory of the calibrated display screen  110  or screen controller  301 . The procedure may then proceed to generating a next image pattern in step  716 , which can be the same as the previous pattern (to check whether the calibration was successful for that image pattern and the image quality is improved) or another pattern (to calibrate the display for other conditions). For example, the following patterns may be consecutively applied:
       white, white   white, red, green, blue   white, white, red, red, green, green, blue, blue   white, red, green, blue, white, red, green, blue   low brightness white, high brightness white   medium brightness white, low brightness white, high brightness white       
 
         [0061]    In a particular embodiment, a camera having a higher resolution than the resolution of the display screen is to be used, such that a plurality of pixels of the image captured by the camera are approximated to a single pixel of the display screen. 
         [0062]    Preferably, the calibrated display is a flat display panel. In more elaborate embodiments, the method and system can be configured to calibrate curved display panels as well, for example by processing the captured image such as to take into account the differences in distance from the camera eye to the region of the display. Alternatively, a plurality of images of the display with the image pattern may be captured, for example an image of the left portion of the display taken from the right, an image of the central portion of the display taken in front of the display and an image of the right portion of the display taken from the left. Furthermore, a panoramic image can be captured by moving the camera window across the length of the display, from one side to the other. 
         [0063]      FIG. 8  shows details of another embodiment of the calibration procedure. After the calibration process is invoked in step  801  (e.g. by the user launching a calibration application, or automatically by the system in a periodic manner for example once a month or once a quarter), the number of calibration steps are specified in step  802  (for example, by the user selecting a desired level of calibration, such as rough calibration (e.g. 1 step) or detailed calibration (e.g. 6 steps)). Next, in step  803  a test image pattern  102  is sent to the calibrated display screen. For example, if a plurality of calibration steps are to be performed, and the system is setup to generate image patterns of a white screen only, the consecutive image patterns may differ by the level of brightness. In step  804  an application run at the mobile device, such as a smartphone, may analyze whether the image of the display  110  that is seen by the camera of that device corresponds with predetermined requirements (e.g. such that the display  110  occupies 90% of the field of view of the camera and the edges of the display are substantially parallel to the camera axes)—if not, the user is ordered to redirect the camera in step  812 . Once the camera is correctly aligned, the picture of the screen is captured in step  805 . The captured image  101  is sent to the image data analysis module in step  806 , wherein it is segmented into a number of blocks depending on the resolution of the calibrated screen and the camera in step  807 . Next, a color histogram is calculated for each block in step  808 . The calibration data values are next determined based on the histograms in step  809 . The calibration is repeated for the required number of iterations in step  810 . Next, the calculated calibration data is sent to the remote calibrated display screen or to the screen controller in step  811 . 
         [0064]    Preferably, the step of image acquisition  603 ,  704 ,  805  may include image correction to compensate for image distortions introduced by the lens of the camera, which is particularly useful for mobile devices with lower-quality camera lens. The image correction may be particularly aimed to compensate for distortions of brightness levels, such as vignetting. 
         [0065]    It can be easily recognized, by one skilled in the art, that the aforementioned method for calibration of a display screen may be performed and/or controlled by one or more computer programs. Such computer programs are typically executed by utilizing the computing resources in a computing device. Applications are stored on a non-transitory medium. An example of a non-transitory medium is a non-volatile memory, for example a flash memory while an example of a volatile memory is RAM. The computer instructions are executed by a processor. These memories are exemplary recording media for storing computer programs comprising computer-executable instructions performing all the steps of the computer-implemented method according the technical concept presented herein. 
         [0066]    While the invention presented herein has been depicted, described, and has been defined with reference to particular preferred embodiments, such references and examples of implementation in the foregoing specification do not imply any limitation on the invention. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the technical concept. The presented preferred embodiments are exemplary only, and are not exhaustive of the scope of the technical concept presented herein. 
         [0067]    Accordingly, the scope of protection is not limited to the preferred embodiments described in the specification, but is only limited by the claims that follow.