Patent Publication Number: US-11049438-B2

Title: Systems and methods for controlling current in display devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 16/242,907, filed on Jan. 8, 2019 and entitled “SYSTEMS AND METHODS FOR CONTROLLING CURRENT IN DISPLAY DEVICES,” which is a continuation of and claims priority to U.S. patent application Ser. No. 15/606,581, filed on May 26, 2017 and entitled “SYSTEMS AND METHODS FOR CONTROLLING CURRENT IN DISPLAY DEVICES,” which is a continuation of and claims priority to U.S. patent application Ser. No. 13/599,863, filed on Aug. 30, 2012 and entitled “SYSTEMS AND METHODS FOR CONTROLLING CURRENT IN DISPLAY DEVICES,” each of which is incorporated by reference herein in its entirety for all purposes. 
    
    
     BACKGROUND 
     The present disclosure relates generally to power efficient display devices and, more specifically, to automatic current limit (ACL) control that reduces an overall power consumption in organic light emitting diode (OLED) display devices. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Organic light emitting diode (OLED) display devices generate light in response to an electronic signal, such that an OLED display device generates a brighter light in response to a larger electronic signal (e.g., current). As such, the OLED display consumes a high amount of power when rendering bright images on the OLED display. Similarly, the OLED display also consumes a high amount of power when rendering images with a high proportion of white pixels (e.g., mimicking the appearance of a book page or a sheet in a word processing document) or when raising an overall luminance of the OLED display in order to improve viewing in bright environments. In addition to being an inefficient use of power, this use of high power in OLED displays can be detrimental to the performance of the OLED displays. For instance, the high power use reduces battery life and can lead to problems with thermal heating of the electronic device attached to the OLED display. 
     Although the conventional automatic current limit (ACL) circuits may provide some power savings in OLED displays, the resulting image rendered by the display device may be objectionable to a viewer. For example, in a photographic image, or an application that relies on realistic rendering of the colors and luminance levels of an image, the application of the conventional ACL approach may reduce the overall luminance of the displayed image making it difficult to discern subtle differences in colors of the displayed image, and reducing the quality of the image rendered on the OLED display. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     The present disclosure generally relates to a control system that may reduce the drive current provided to each subpixel or to a number of specified subpixels of the display based on various factors related to the image(s) being displayed. In this manner, the control system may provide significant power savings while maintaining the quality of the displayed images. Moreover, the reduction in power can lead to improved lifetime of the displays, and reduce the heat generated by the display during operation. In one embodiment, the control system may receive information that indicate a type of application rendering images on the display, a type image being rendered by the display, an amount of power being consumed by the display, an amount of ambient light level reflecting off the display, or the like. After receiving this information, the control system may determine a degree of current reduction for each subpixel of the display based on these inputs. 
     For instance, in one embodiment, the control system may analyze the application being rendered on the display. If the application displays a large amount of white content (e.g., email, electronic book/reader, word processing, and spreadsheets), the control system may reduce the current available to drive the display uniformly because the overall reduction of white levels in the background should not detract from the quality of the images of text displayed by the application. Alternatively, if the application is designed to display accurate colors (e.g., viewing photographic or video content), the control system may not reduce the current available to drive the display in order to maintain the integrity of images being displayed. 
     In another embodiment, the control system may analyze an image being displayed and identify subpixels in the image that are substantially similar. The control system may then reduce the current available to drive the substantially similar subpixels while maintaining the current available to drive the subpixels that are not substantially similar. 
     In yet another embodiment, the control system may measure a signal representative of the amount of ambient light reflecting off the display. The control system may then modify the extent in which the current being applied to the display is reduced based on the measured ambient light level. For instance, the control system may restrict the current driving the display less in bright environments as compared to in dark environments. By reducing the current available to drive certain pixels, the control system may reduce the luminance or certain aspects of the image such that the rendered image may be more acceptable to a viewer. Accordingly, the control system may be useful for reducing the power consumed by the display in ways that do not render the depicted images objectionable to the viewer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a block diagram of components of an electronic device, in accordance with an embodiment; 
         FIG. 2  is a front view of a handheld electronic device in accordance with an embodiment; 
         FIG. 3  is a view of a computer in accordance with an embodiment; 
         FIG. 4  is a data flow diagram that depicts inputs that an automatic current limit (ACL) controller may use for determining drive currents for a display, in accordance with an embodiment; 
         FIG. 5  is a flow chart that depicts a method for reducing an amount of drive currents sent to a display based on an application being rendered on the display, in accordance with an embodiment; 
         FIG. 6  is a flow chart that depicts a method for reducing an amount of drive currents sent to a display based on an image being rendered on the display, in accordance with an embodiment; 
         FIG. 7  provides two screen shots illustrating an example of an effect of reducing drive currents sent to a display based on an image being displayed, in accordance with an embodiment; 
         FIG. 8  is a flow chart that depicts a method for reducing drive currents sent to a display based on power consumption properties of the display, in accordance with an embodiment; 
         FIG. 9  is a flow chart that depicts a method for reducing drive currents sent to a display based on luminance and color properties of images rendered on the display, in accordance with an embodiment; and 
         FIG. 10  is a flow chart that depicts a method for determining an estimate of luminance of a display using a sampling algorithm, in accordance with an embodiment. 
         FIG. 11  is a flow chart that depicts a method for reducing drive currents sent to a display based on present ambient light conditions, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     The present disclosure is directed to systems, displays, and methods for reducing drive currents provided to an electronic display to improve the power efficiency and/or the appearance of the display. Organic Light Emitting Diode (OLED) displays may use an array of OLEDs to show an image across the display. Each OLED subpixel emits light of a certain color and brightness based on drive currents provided to the OLEDs. In one embodiment, red, green, and blue emitters may be used to display a range of colors. In another embodiment, the OLED display may emit white light, and color filters or fluorescent materials may be used to convert the white light into individual colors. The emitted colors may be red, green, and blue, but an additional white subpixel may also be used. In yet another embodiment, red, green, and blue emitters may be used to emit a range of colors, and these colors may be further refined by passage through a set of color filters such that each emitting color is paired with a particular color of color filter. 
     The drive currents provided to each OLED subpixel may be regulated by an Automatic Current Limit (ACL) controller in a display driver. The ACL controller may reduce the power consumption of the OLED display by reducing the total drive current provided to the OLED display or by restricting the current to all OLED subpixels in a proportional manner. However, instead of uniformly reducing the drive current provided to each OLED irrespective of the image being displayed and/or the viewing conditions, the ACL controller may reduce the drive current provided to each OLED subpixel or to specified OLED subpixels in a manner that provides power savings while maintaining the integrity of images depicted on the OLED display. 
     A variety of electronic devices may incorporate the OLED displays having the ACL controller. An example of a suitable electronic device may include various internal and/or external components, which contribute to the function of the device.  FIG. 1  is a block diagram illustrating the components that may be present in such an electronic device  8  and which may allow the device  8  to function in accordance with the techniques discussed herein. Those of ordinary skill in the art will appreciate that the various functional blocks shown in  FIG. 1  may comprise hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should further be noted that  FIG. 1  is merely one example of a particular implementation and is merely intended to illustrate the types of components that may be present in a device  8 . For example, in the presently illustrated embodiment, these components may include a display  10 , I/O ports  12 , input structures  14 , one or more processors  16 , a memory device  18 , a non-volatile storage  20 , one or more light sensors  22 , a networking device  24 , a power source  26 , and an Automatic Current Limiter (ACL)  28 . 
     With regard to each of these components, the display  10  may be used to display various images generated by the device  8 . In one embodiment, the display  10  may be an organic light emitting diode (OLED) display. An OLED display may include a number of pixels or picture elements that may be used to depict images on the display  10 . In an OLED display, each pixel may be composed of three pixel components, known as subpixels, that may depict red, green, and blue colors, respectively. Alternatively, four pixel components, namely red, green, blue, and white may be employed. Each OLED subpixel may depict is respective color using an emissive electroluminescent layer (i.e., film of organic compound) which emits light in response to an electric current. The color of the light viewed may be the light emitted directly by the OLED subpixels, or the color altered by passage through a color filter containing an absorbing or a fluorescing material. As such, when bright images are rendered on an OLED display, relatively high levels of power may be used by the display  10 . 
     The I/O ports  12  may include ports configured to connect to a variety of external devices, such as a power source, headset or headphones, or other electronic devices  8  (such as handheld devices and/or computers, printers, projectors, external displays, modems, docking stations, and so forth). The input structures  14  may include the various devices, circuitry, and pathways by which user input or feedback is provided to the processor  16 . The input structures  14  may be configured to control a function of the device  8 , applications running on the device  8 , and/or any interfaces or devices connected to or used by the electronic device  8 . 
     The processor(s)  16  may provide the processing capability to execute the operating system, programs, user and application interfaces, and any other functions of the electronic device  8 . The instructions or data to be processed by the processor(s)  16  may be stored in a computer-readable medium, such as the memory  18 . The memory  18  may be provided as a volatile memory, such as random access memory (RAM), and/or as a non-volatile memory, such as read-only memory (ROM). The components may further include other forms of computer-readable media, such as a non-volatile storage  20 , for persistent storage of data and/or instructions. The non-volatile storage  20  may include flash memory, a hard drive, or any other optical, magnetic, and/or solid-state storage media. The non-volatile storage  20  may be used to store firmware, data files, software, wireless connection information, and any other suitable data. 
     The embodiment illustrated in  FIG. 1  may also include one or more light sensors  22 . The light sensors  22  may include sensors such as photodetectors, photo diodes, photo resistors, photocells, or any other sensor capable of detecting ambient light. In various embodiments, the light sensors  22  may be disposed in the substrate such that they receive light from the direction of the substrate, the direction opposite the substrate, or both. In certain embodiments, a camera may be present in the device and may serve as a light sensor. 
     The components depicted in  FIG. 1  also include a network device  24 , such as a network controller or a network interface card (NIC). The network device  24  may be a Wi-Fi device, a radio frequency device, a Bluetooth® device, a cellular communication device, or the like. The network device  24  may allow the electronic device  8  to communicate over a network, such as a Local Area Network (LAN), Wide Area Network (WAN), or the Internet. Further, the components may also include a power source  26  such a battery or AC power. 
     To prevent excessive power consumption by the display  10 , the electronic device  8  may also include the Automatic Current Limiter (ACL)  28 . The ACL  28  may monitor the overall power or current used by the display  10 , and reduce overall power consumption in the display  10  by controlling the current provided to the display  10 . In one embodiment, the ACL  28  may estimate the power consumption expected for an image frame that is to be displayed on display  10 . The ACL  28  may limit the drive current provided to each subpixel of the display  10  based on various factors. Additional details with regard to the ACL  28  will be discussed below with reference to  FIGS. 4-11 . 
     With the foregoing in mind,  FIG. 2  illustrates an electronic device  8  in the form of a handheld device  30 , here a cellular telephone. It should be noted that while the depicted handheld device  30  is provided in the context of a cellular telephone, other types of handheld devices (such as media players for playing music and/or video, personal data organizers, handheld game platforms, and/or combinations of such devices) may also be suitably provided as the electronic device  8 . As discussed with respect to the general electronic device  8  of  FIG. 1 , the handheld device  30  may allow a user to connect to and communicate through the Internet or through other networks, such as local or wide area networks. The handheld electronic device  30 , may also communicate with other devices using short-range connections, such as Bluetooth and near field communication. By way of example, the handheld device  30  may be a model of an iPod®, iPad®, or iPhone® available from Apple Inc. of Cupertino, Calif. 
     The handheld device  30  includes a display  10  in the form of an OLED display. The display  10  may be used to display a graphical user interface (GUI)  34  that allows a user to interact with the handheld device  30 . The handheld electronic device  30  also may include various input and output (I/O) ports  12  that allow connection of the handheld device  30  to external devices such as a port that allows the transmission and reception of data or commands between the handheld electronic device  30  and another electronic device. 
     In addition to handheld devices  30 , such as the depicted cellular telephone of  FIG. 2 , an electronic device  8  may also take the form of a computer or other type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations, and/or servers). In certain embodiments, the electronic device  8  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, iPad® or Mac Pro® available from Apple Inc. By way of example, an electronic device  8  in the form of a laptop computer  34  is illustrated in  FIG. 3  in accordance with one embodiment. The depicted computer  34  includes a housing  32 , a display  10  (such as an OLED display), input structures  14 , and input/output ports  12 . 
     In one embodiment, the input structures  14  (such as a keyboard and/or touchpad) may be used to interact with the computer  34 , such as to start, control, or operate a GUI or applications running on the computer  34 . For example, a keyboard and/or touchpad may allow a user to navigate a user interface or application interface displayed on the display  10 . 
     As depicted, the electronic device  8  in the form of computer  34  may also include various input and output ports  12  to allow connection of additional devices. For example, the computer  34  may include an I/O port  12 , such as a USB port or other port, suitable for connecting to another electronic device, a projector, a supplemental display, and so forth. In addition, the computer  34  may include network connectivity, memory, and storage capabilities, as described with respect to  FIG. 1 . As a result, the computer  34  may store and execute a GUI and other applications. 
     With the foregoing discussion in mind, it may be appreciated that an electronic device  8  in the form of either a handheld device  30  or a computer  34  may be provided with an OLED display as the display  10 . Such an OLED display may be utilized to display the respective operating system and application interfaces running on the electronic device  8  and/or to display data, images, or other visual outputs associated with an operation of the electronic device  8 . 
     In embodiments in which the electronic device  8  includes an OLED display, the display  10  may employ inorganic light emitting diodes or organic light emitting diodes (OLEDs) as the display  10 . The OLED display may include a number of pixels that may be composed of red, green, and blue subpixels. The OLED display may generate light in response to an electronic signal. As such, when bright images are shown on the OLED display, relatively high levels of power may be used for displaying images. 
     Keeping the foregoing in mind,  FIG. 4  illustrates a data flow diagram  40  that depicts inputs that the ACL  28  may use to determine drive currents for each subpixel in the display  10  to enable the display  10  to conserve power while maintaining the integrity of the images depicted therein. In one embodiment, the ACL  28  may receive information related to a type of application being rendered by the display  10  (i.e., application type  42 ), an image to be depicted on the display  10  (i.e., image data  44 ), power consumption properties  45  of the display  10 , ambient light measurements  46 , and the like. Based on the application type  42 , the image data  44 , the power consumption properties  45 , and/or the ambient light measurements  46 , the ACL  28  may determine a drive current  48  for each subpixel in the display  10  during each frame of displayed data. As mentioned above, the drive current  48  for each subpixel may be calculated such that the display  10  conserves power while maintaining the quality of the images depicted therein. After determining the drive current  48  for each subpixel in the display  10  during each frame of displayed data, the ACL  28  may provide each respective subpixel in the display  10  with a respective drive current  48 , thereby enabling the display  10  to consume power efficiently. Additional details describing how the ACL  28  may determine the drive current  48  for each subpixel in the display  10  during each frame of the displayed data are provided below with reference to  FIGS. 5-10 . 
     Referring now to  FIG. 5 , the ACL  28  may employ a method  50  to determine the drive current  48  for each subpixel in the display  10  based on the application type  42  being displayed. At block  52 , the ACL  28  may identify the application or program (i.e., application type  42 ) being rendered by the display  10 . In general, the ACL  28  may determine whether the application type  42  corresponds to an application directed towards displaying text for reading, images for viewing, or both. In some embodiments, different applications or programs may be in operation at the same time on a device, visible in different windows on the display. In this case, the ACL  28  may decide whether to apply a different drive current for the images depicted in each displayed window, or whether to apply a relatively uniform reduction in drive current across all of the displayed windows. 
     At block  54 , the ACL  28  may calculate the drive current  48  that may be used to drive each subpixel in the display  10  based on the application identified at block  52  (i.e., application type  42 ). In one embodiment, the calculated drive current may be optimized to conserve power usage with respect to the display  10  while maintaining the integrity and quality of the images being rendered on the display  10 . For example, at block  52 , the ACL  28  may identify an application type  42  that corresponds to an application directed towards displaying text for reading. In this case, at block  54 , the ACL  28  may calculate drive currents  48  that may reduce the power being consumed by the display  10  while maintaining the quality or readability of the text being depicted on the display  10 . Examples of text rendering applications may include a word processing application, a spreadsheet application, an electronic mail (email) application, an electronic reader application, and the like. 
     In general, text-rendering applications may display image data that have black text along a white background. To create a white color for the white background, a high amount of current may be provided to each subpixel in the display  10  that corresponds to the white background. To provide for more energy efficient displays, at block  54 , the ACL  28  may calculate a reduced drive current for each subpixel in the display  10  based on the amount of white background being displayed. In this manner, the overall white level of the white background may be reduced while the black level of the text being displayed in the display  10  may remain relatively the same since achieving black levels in OLED subpixels uses little or no current. Further, the reduction in the overall white level of the background should not detract greatly from the readability of the text being displayed so long as a sufficient amount of contrast exists between the text and the background due to the Bartleson-Breneman effect. The Bartleson-Breneman effect generally states that an image with very high contrast will actually appear brighter than an image of the same maximum luminance, but with lower contrast. In other words, if two displays are displaying the same image such that each image has the same luminance level, the display exhibiting the higher contrast will appear brighter than the image exhibiting the lower contrast. 
     Keeping this in mind, the ACL  28  may use the Bartleson-Breneman effect for text-rendering applications and reduce drive currents  48  provided to the subpixels in the display  10 . Since the contrast of black text on a white background in OLED displays will be high due to the high levels of black color that OLEDs are able to provide, the reduction in the overall white level of the white background may not significantly detract from a user&#39;s reading experience. In one embodiment, the ACL  28  may reduce the drive current provided to the subpixels in the display  10  by some percentage or by some overall amount from an amount of current specified by the respective application for the subpixels. For example, if the contrast between black text and a white background on an OLED display is 1000:1, then reducing the white background luminance (i.e., reducing the drive current provided to the white background subpixels) by 20% (to 80% of the original luminance) may simply reduce the contrast between the black text and the white background to 800:1. In this manner, the user&#39;s reading experience may not be significantly affected so long as a sufficient amount of contrast exists between the displayed text and background. By reducing the drive currents  48  provided to the subpixels in the display  10  for text-rendering applications, the ACL  28  may maintain the readability of the displayed text based on the contrast between the displayed black text and the white background while reducing the power being consumed by the display  10 . 
     Instead of reducing the drive current  48  to each subpixel in the display  10 , in one embodiment, the ACL  28  may reduce the drive current  48  provided to the subpixels that correspond to the white background. That is, the ACL  28  may reduce the drive current  48  provided to each subpixel that corresponds to a pixel that displays a white color, while maintaining the drive currents  48  for the subpixels that are not used to display a white color. 
     As mentioned above, when determining the drive current  48 , the ACL  28  may reduce the amount of current provided to the subpixels in the display  10  by some percentage or by some overall amount from an amount of current specified by the respective application. In one embodiment, the ACL  28  may reduce the drive currents  48  provided to subpixels that have a luminance level greater than some luminance level limit. For example, if the luminance level limit is 80% of the maximum luminance value, the ACL  28  may reduce the drive currents  48  to the respective subpixels that have a luminance above 80%. In one embodiment, the ACL  28  may reduce the drive currents  48  provided to those respective subpixels by 20% to 80% or by 60% to 80% while maintaining the drive currents  48  provided to subpixels that have a luminance below 80%. In this manner, the ACL  28  may achieve more significant power savings in the display  10  while maintaining a certain level of quality of the images displayed in the display  10 . 
     Instead of reducing the drive currents  48  to the respective subpixels that have a luminance above the luminance level limit by some percentage value, the ACL  28  may reduce the drive currents  48  provided to each respective subpixel that has a luminance above the luminance level limit such that the respective subpixel has a luminance level that corresponds to the luminance level limit. In either case, after calculating the drive currents  48  for each subpixel in the display  10 , at block  56 , the ACL  28  may send the calculated drive currents  48  to each subpixel in the display  10 . 
     Referring back to block  52 , if the application type  42  is directed towards displaying image data  44  that include colorful photographs or videos, at block  54 , the ACL  28  may not reduce the drive currents  48  in order to preserve the quality of the image data  44  being displayed. As a result, the ACL  28  may provide the drive currents  48  as specified for each subpixel in the display  10  by the respective application. Otherwise, the ACL  28  may reduce the drive currents  48  applied to each subpixel in the display  10  by a small percentage (e.g., less than 10%) such that the image quality of the displayed image is preserved. In this manner, the ACL  28  may limit or eliminate the amount of current reduction being applied to the calculated drive currents  48  in block  54  for applications in which accurate color and luminance are desirable. That is, the ACL  28  may significantly reduce drive currents  48  for applications types  42  that are intrinsically high in power but display images that are not particularly colorful or detailed. Accordingly, the ACL  28  may enable the display  10  to become more power efficient for application types  42  that do not depict particularly colorful or detailed images, while preserving the image quality of the images depicted in the display  10  for those application types  42  that do depict colorful and detailed images. 
     In one embodiment, the ACL  28  may reduce drive currents  48  provided to the display  10  for application types  42  in which images are being displayed according to a method  58  described in  FIG. 6 . Referring to  FIG. 6 , at block  60 , the ACL  28  may receive image data  44  that include one or more images to be displayed on the display  10 . At block  62 , the ACL  28  may analyze the image data  44  and identify one or more portions in the displayed image data  44  that have substantially similar characteristics, such as pixels with substantially similar luminance and color values. For example, portions of the image data  44  that have substantially similar luminance or color values may include portions of the image data  44  that include “white” pixels. White pixels may include pixels that meet or exceed a certain luminance level floor and possess a set of color coordinates within a region defined as “white.” In addition to white pixels, portions of the image data  44  that have substantially similar luminance or color values may include portions of the image data  44  that include the same bright and solid color. 
     In one embodiment, the ACL  28  may identify the portions of the image data  44  that have substantially similar characteristics by comparing the luminance and/or color coordinates of a respective pixel with its neighboring pixels. Pixels that are immediately adjacent to the respective pixel may be categorized as part of a first level of proximate pixels. Similarly, pixels that are immediately adjacent to the first level pixels may be categorized as part of a second level of proximate pixels. The ACL  28  may identify the portion of the image data  44  that have substantially similar characteristics based on whether the portion of the image data  44  includes some number of pixels or levels of proximate pixels that have substantially similar luminance and/or color coordinates. For example, the ACL  28  may identify portions of the image data  44  for areas of the image data  44  that include pixels in which the luminance and color coordinates of pixels up to four levels away are substantially the same as the respective pixel. 
     After identifying the portions of the image data  44  that have substantially similar characteristics, at block  64 , the ACL  28  may reduce the drive currents  48  provided to the subpixels that correspond to the portions of the image data  44  identified at block  60 . In this manner, the ACL  28  may reduce the luminance in portions of the image data  44  that may be used for background purposes while maintaining the luminance of the images depicted in the image data  44 . An example of the effects of reducing the luminance in the portions of image data  44  that are part of the background of the image data  44  is illustrated in  FIG. 7 . 
     Referring to  FIG. 7 , image  63  depicts the results of using a conventional ACL controller to reduce the overall power of the image data  44  uniformly by dimming both the white portions and the color portions of the image data  44 . From a power saving viewpoint, reducing the white luminance provides substantial power benefits, but reducing the image luminance provides only marginal power benefits. Moreover, reducing the image luminance decreases the quality of the colors displayed in the image. In general, users may not be concerned with the luminance of the background or frame, but they will be very sensitive to a decrease in the luminance of the colored image. 
     Keeping this in mind, the ACL  28  may achieve significant power savings while simultaneously providing for accurate luminance and color coordinates for the displayed images by reducing the luminance in just the background portion of the image data  44 , as illustrated in image  65  of  FIG. 7 . Referring back to  FIG. 5 , after determining the drive currents  48  for the identified portions of the image data  44 , at block  56 , the ACL  28  may send the calculated drive currents to the display  10 . 
     In addition to modifying the drive currents  48  based on the application type  42  or the image data  44  rendered on the display  10 , the ACL  28  may also modify the drive currents  48  provided to the display  10  based on the power consumption properties  45  of the display  10 , as depicted in method  66  of  FIG. 8 . Referring now to  FIG. 8 , at block  68 , the ACL  28  may determine power consumption properties  45  for the display  10 . At block  70 , the ACL  28  may determine whether the power consumption properties  45  are greater than some limit. If the power consumption properties  45  are greater than the limit, the ACL  28  may proceed to block  72  and reduce the drive currents  48  to be provided to the display  10 . If, however, the power consumption properties  45  are not greater than the limit, the ACL  28  may proceed to block  74  and maintain the drive currents  48  to be provided to the display  10 . 
     In one embodiment, the power consumption properties  45  may be determined based on the luminance and color properties displayed in each pixel in the display  10 . In certain devices such as an OLED display, the power consumption properties  45  in generating different colors vary for each color because each individual pixel in an OLED display displays its own color. For example, a blue pixel in an OLED display is generally less power efficient than a green pixel, even if both of these pixels have the same luminance. The difference in efficiency for each color generally depends on an exact material composition and structure of the OLED subpixels (i.e., OLED layer). Similarly, the relative efficiency for white OLEDs with color filters generally depends on color subpixel, due to the OLED material, the OLED design properties, and the optical properties of the color filter. As such, by accounting for both the luminance and color properties of each pixel in the display  10 , the ACL  28  may more accurately determine the power consumption properties  45  for the display  10 . A method  75  depicting how the power consumption properties  45  may be determined using both the luminance and color properties of each pixel in the display  10  is described in greater detail below with reference to  FIG. 9 . 
     Referring to  FIG. 9 , at block  76 , the ACL  28  may receive red, green, and blue color data (RGB data) for each pixel in the display  10 . At block  78 , the ACL  28  may transform the RGB data into International Commission on Illumination (CIE)  1976  (L*, u*, v*) color space or L*u*v* coordinates. After transforming the RGB data for each pixel into L*u*v* coordinates, at block  80 , the ACL  28  may scale the luminance (L*) value by a factor (P u*v* ) that depends on the corresponding u*v* value. The scaling factor may be used to more accurately characterize the amount of power being consumed by the respective pixel based on the color that the respective pixel is displaying. 
     At block  82 , the ACL  28  may sum the scaled luminance value (L*×P u*v* ) for each pixel in the display  10 . Referring back to block  70  in  FIG. 8 , the ACL  28  may then compare the sum (i.e., power consumption value) to some limit. If the sum is greater than the limit, the ACL  28  may proceed to block  72  and reduce the drive currents  48  provided to each subpixel in the display  10 , as described above. Alternatively, if the sum is not greater than the limit, the ACL  28  may proceed to block  74  and maintain the drive currents  48  as specified by the corresponding application. 
     In one embodiment, the ACL  28  may forego block  78  and apply scaling factors for each pixel at block  80  to each corresponding subpixel. That is, the individual RGB values for each pixel may be multiplied by an appropriate scaling factor (e.g., P R , P G , P B ), which may be stored in a lookup table, and the resulting products may be summed together to determine the power consumption properties  45  of the display  10 . As such, the power consumption properties  45  for the display  10  may be calculated by summing the values of R×P R , G×P G , and B×P B  for all of the subpixels in the display  10 . The scaling factor (P R , P G , P B ) may represent a value that is proportional to the amount of power that would be consumed in driving a respective subpixel to its respective red, green, or blue value. After summing the values of R×P R , G×P G , and B×P B  for all of the subpixels in the display  10 , the ACL  28  may proceed to block  70  of method  66  and determine whether the sum is greater than the limit. 
     If the sum is greater than the limit, at block  72 , the ACL  28  may reduce the drive currents  48  provided to each respective pixel such that each respective pixel may have RGB values at some threshold. For instance, the ACL  28  may compare the red, green, and blue digital levels (e.g., 0 to 255 for an 8-bit subpixel) for corresponding red, green, and blue subpixels in each pixel in the portion of the image data  44  to the threshold. If the red, green, or blue subpixel in each pixel of the portion of the image data  44  has a digital level above the threshold, the ACL  28  may reduce the drive current  48  provided to each of the corresponding subpixels to the threshold. In one embodiment, the ACL  28  may reduce the drive currents  48  as described above only if each of the three subpixels in the respective pixel is below the threshold to prevent any change to occur in tinted background colors. 
     In certain situations, a change in color in a portion of the display  10  may cause the sum to exceed the limit at block  70  and may cause the ACL  28  to reduce the drive currents  48  provided to the display  10  at block  72 . For instance, if a large portion of the display  10  changes from green to blue, and since blue emission uses more power than green emission, then the power consumption properties  45  for the display  10  will increase due to the increased current consumption that corresponds to blue pixels in OLED displays. In this case, if a different portion of the same display  10  is held constant while the other portion changes color from green to blue, then the change in color could lead to an overall reduction in the drive currents  48  applied to all of the display  10 , which will change the portion of the display  10  intended to remain constant. As a result, a user viewing the images depicted on the display  10  may be disappointed in the quality of the images depicted in the display  10 . For example, if most of the content depicted in the display  10  changes from a dark image to a light image, then a user will likely not notice a reduction in the brightness of the light image as a power-saving measure. However, if only part of an image changes in brightness and other portions of the image are unchanged, then the user may object to any significant change in the brightness of the portion of the image that is intended to remain constant. In this case, the ACL  28  may override the method  66  described above and keep the applied current at a previous level until there is a significant change in the displayed content. Alternatively, the ACL  28  may implement the current reduction gradually over a period of time, so that the user does not notice a distinct change in the image brightness. For instance, the current reduction may occur in a series of small steps over a period of one to ten seconds, so that the change is barely noticeable to the viewer. 
     At block  71 , the ACL  28  may perform an optional process that determines whether a change in the colors or color intensities of the images depicted in the display  10  exceeds a certain threshold. If the colors of the images do indeed change such that the amount of change exceeds the threshold, the ACL  28  may proceed to block  74  and maintain the drive currents  48  as specified. However, if the colors of the images do not change such that the amount of change does not exceed the threshold, the ACL  28  may proceed to block  72  and reduce the drive currents  48  as described above. In this manner, the ACL  28  may avoid changing the drive current  48  provided to each subpixel in the display  10  when the power consumption value becomes greater than the limit due to a change the color of a portion of the display  10  but not due to a change in the luminance of the display  10 . 
     Although method  75  has been described for OLED displays equipped with RGB color filters, it should be noted that in certain embodiments method  75  may also be performed for OLED displays equipped with RGBW color filters. In this case, after the ACL  28  receives the RGB data for each pixel in the display  10  at block  76 , the ACL  28  may convert the RGB data into a RGBW data and the remaining steps of method  75  may be performed based on the RGBW data. 
     For high pixel count displays, performing method  75  may involve a significant amount of processing time and power. To alleviate the amount of processing time and power used to perform method  75 , the ACL  28  may randomly sample a subset of all the pixels in the display  10  and determine an estimate of the luminance of the overall display  10  based on the sample. For instance,  FIG. 10  illustrates a method  84  for determining an estimate of luminance of the display  10  using a sampling algorithm. To improve accuracy, the ACL  28  may divide the display  10  into a number of fixed areas across the display area. The ACL  28  may then randomly sample one or more pixels in each fixed area to better insure that the current reduction is representative of images shown across the entire screen. For example, the display  10  may be divided into 64 rectangles of uniform height and an equal or a different uniform width, spaced uniformly across the display. The ACL  28  may then perform the pixel sampling within each of these designated rectangles. 
     Referring now to  FIG. 10 , at block  86 , the ACL  28  may sample a fraction or subset of the image data  44  to be depicted on the display  10 . At block  88 , the ACL  28  may convert the sampled image data to a linear intensity scale by, for example, applying a degamma function. Using the linear intensity scale, at block  90 , the ACL  28  may determine statistics for the relative intensity of each subpixel in the sampled image data. At block  92 , the ACL  28  may then use the statistics to calculate an amount of power being consumed by the display  10 . The ACL  28  may then compare this calculated power value to the limit as described in block  70  and proceed to block  72 , block  71 , or block  74  depending on whether the calculated power value is greater than the limit. 
     For each of the methods described above (i.e., method  50 ,  58 ,  75 , or  84 ), if a portion of the display  10  changes rapidly between frames of data, the ACL  28  may provide rapidly fluctuating drive currents  48  to the pixels of the display  10 , thereby causing a flicker effect or other visual artifacts to be depicted on the display  10 . To prevent these types of visual artifacts, the methods described above may be modified such that the ACL  28  may not be allowed to change the drive currents  48  more than once during some period of time. For example, in method  66 , the ACL  28  may not be allowed to change the drive currents  48  at block  72  more than once in a five second period. 
     Referring back to  FIG. 4 , in addition to the application type  42 , the image data  44 , and the power consumption properties  45 , the ACL  28  may use ambient light measurements  46  to determine the drive currents  48  for each subpixel in the display  10 . The ambient light measurements  46  may be acquired from the light sensors  22 , described above, and may indicate the overall illumination level impinging on the light sensors  22 . In general, the ambient light measurements  46  may indicate whether the device is outdoors or indoors. In one embodiment, the ACL  28  may adjust the drive currents  48  provided to the display  10  based on the ambient light measurements  46  according to a method  96  described below with reference to  FIG. 11 . 
     At block  98 , the ACL  28  may receive ambient light measurements  46  from the light sensors  22 . At block  100 , the ACL  28  may receive data pertaining to images that are to be rendered on the display  10 . At block  102 , the ACL  28  may calculate drive currents  48  for each subpixel in the display  10  based on the ambient light measurements  46 . In one embodiment, if the ambient light measurements  46  are greater than some threshold, the ACL  28  may reduce the drive currents  48  provided to the display  10 . In this manner, for high ambient light measurements  46 , the ACL  28  may implement a different set of drive currents  48  as compared to lower ambient light measurements  46 . 
     In one embodiment, the ACL  28  may calculate the drive currents  48  based on the application type  42 , the image data  44 , the power consumption properties  45 , the ambient light measurements  46 , or any combination of these inputs. For example, if the ACL  28  receives ambient light measurements  46  that are greater than the threshold (e.g., outdoor usage) and an application type  42  that corresponds to a text-rendering application, the ACL  28  may increase the luminance of the entire display  10  to enable a user to more easily view the depicted text in the display  10 . If, however, the ACL  28  receives ambient light measurements  46  that are greater than the threshold (e.g., outdoor usage) and an application type  42  that corresponds to an image-rendering application, the ACL  28  may provide drive currents  46  to the display  10  based on the amount of white color being depicted in the display  10 . Here, the ACL  28  may reduce the drive currents  48  a greater amount for images that have a large portion of white depicted in the display  10  as compared to the images that have a small portion of white depicted in the display  10 . 
     By employing the methods described herein, the ACL  28  may provide greater power savings for the display  10  and avoid generating high levels of heat in the display  10 , which may damage various components in the display  10 . Further, a user may experience a more satisfactory viewing experience on the display  10  while the display  10  employs various power consumption savings techniques. 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.