PATENT DOCUMENT

Publication Number: US-8242707-B2
Application Number: US-84379510-A
Country: US
Kind Code: B2

Title: Ambient light calibration for energy efficiency in display systems

Abstract:
A method, system, and apparatus that can be used to operate a display device in an energy efficient manner. The energy efficient display device can effectively and efficiently compensate for changes in ambient light incident at a display screen of the display device using an internal ambient light sensor to provide control signals to a backlight driver.

Claims:
1. A method, comprising:
 calibrating a light source to a target luminance value, the target luminance value corresponding to an ambient light condition; 
 providing a calibrated light by the light source, the calibrated light having a luminance value within a range of the target luminance value; 
 receiving the calibrated light at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value of the light provided by the calibrated light source; 
 detecting light received at the first part of the optical path by a light detector at a second part of the optical path at a second luminance level, the light detector included in a display system; and 
 calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level, the calibration factor used by a system processor in the display system to modify a control signal sent to a backlight driver unit, the display signal causing the backlight driver unit to output an amount of light in accordance with ambient light detected by the light sensor. 
 
     
     
       2. The method as recited in  claim 1 , wherein the calibrating the light source comprises:
 receiving light from the light source at a light sensor, the light sensor and the light source being optically isolated from external environment; 
 sending an indication of the luminance of the light received at the light sensor to a light meter connected to the light source; 
 generating a luminance value by the light meter based upon the indication provided by the light sensor. 
 
     
     
       3. The method as recited in  claim 2 , comprising:
 forwarding the luminance value by the light meter to a process computer; and 
 comparing the luminance value to the target luminance value. 
 
     
     
       4. The method as recited in  claim 3 , comprising:
 when the luminance value is not within the range of the target luminance value based upon the comparing, 
 forwarding a corrective command to a programmable power supply. 
 
     
     
       5. The method as recited in  claim 4 , comprising:
 modifying a power level of the light source by the programmable power supply based upon the corrective command. 
 
     
     
       6. The method as recited in  claim 1 , further comprising:
 storing the calibration factor in the display system by the system processor. 
 
     
     
       7. The method as recited in  claim 6 , further comprising:
 setting the display system to calibration mode; 
 receiving data by the system processor from the light sensor in accordance with light detected by the light sensor; 
 calculating the calibration factor; 
 storing the calibration factor in a memory device; 
 reporting the calibration factor to an external process computer; and 
 exiting the calibration mode. 
 
     
     
       8. The method as recited in  claim 7 , further comprising:
 when the display system is in normal operating mode, the system processor,
 retrieves the calibration factor from the memory device; 
 receives data from the light sensor, the data in accordance with an ambient light level; 
 determines a calibrated ambient light level; 
 retrieves user settings from the memory device; 
 applies the calibrated ambient light level and the user settings to an ambient brightness control function that modifies a duty cycle and phase of a backlight driver in accordance with the ambient light level. 
 
 
     
     
       9. The method as recited in  claim 1 , further comprising:
 validating the calibration factor comprising:
 determining a power level P provided to the display system by a power source at a user&#39;s typical ambient light level; 
 comparing the power level P to design limits based upon an energy standard; 
 determining an amount of light emitted by the display system; and 
 verifying that the amount of light emitted by the display is within established design limits. 
 
 
     
     
       10. The method as recited in  claim 9 , further comprising:
 wherein the power level P is an average power level Pavg, the average power level being a sum of weighted power levels, the weighted power levels including at least a brighter lighting condition power level and a darker lighting condition power level. 
 
     
     
       11. A non-transitory computer readable medium for storing a computer program for providing an ambient light calibration factor, comprising:
 computer code for calibrating a light source to a target luminance value, the target luminance value corresponding to an ambient light condition; 
 computer code for providing a calibrated light by the light source, the calibrated light having a luminance value within a range of the target luminance value; 
 computer code for receiving the calibrated light at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value of the light provided by the calibrated light source; 
 computer code for detecting light received at the first part of the optical path by a light detector at a second part of the optical path at a second luminance level; and 
 computer code for calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level. 
 
     
     
       12. The computer readable medium as recited in  claim 11 , wherein the calibrating the light source comprises:
 computer code for receiving light from the light source at a light sensor, the light sensor and the light source being optically isolated from external environment; 
 computer code for sending an indication of the luminance of the light received at the light sensor to a light meter connected to the light source; 
 computer code for generating a luminance value by the light meter based upon the indication provided by the light sensor. 
 
     
     
       13. The computer readable medium as recited in  claim 12 , comprising:
 computer code for forwarding the luminance value by the light meter to a process computer; and 
 computer code for comparing the luminance value to the target luminance value. 
 
     
     
       14. The computer readable medium as recited in  claim 13 , comprising:
 computer code for forwarding a corrective command to a programmable power supply when the luminance value is not within the range of the target luminance value based upon the comparing. 
 
     
     
       15. The computer readable medium as recited in  claim 14 , comprising:
 computer code for modifying a power level of the light source by the programmable power supply based upon the corrective command. 
 
     
     
       16. The computer readable medium as recited in  claim 11 , further comprising:
 computer code for storing the calibration factor in the display system by the system processor. 
 
     
     
       17. The computer readable medium as recited in  claim 16 , further comprising:
 computer code for setting the display system to calibration mode; 
 computer code for receiving data by the system processor from the light sensor in accordance with light detected by the light sensor; 
 computer code for calculating the calibration factor; 
 computer code for storing the calibration factor in a memory device; 
 computer code for reporting the calibration factor to an external process computer; and 
 computer code for exiting the calibration mode. 
 
     
     
       18. The computer readable medium as recited in  claim 17 , further comprising:
 when the display system is in normal operating mode, the system processor, 
 retrieves the calibration factor from the memory device; 
 receives data from the light sensor, the data in accordance with an ambient light level; 
 determines a calibrated ambient light level; 
 retrieves user settings from the memory device; 
 applies the calibrated ambient light level and the user settings to an ambient brightness control function that modifies a duty cycle and phase of a backlight driver in accordance with the ambient light level. 
 
     
     
       19. The computer readable medium as recited in  claim 11 , further comprising:
 validating the calibration factor comprising: 
 computer code for determining a power level P provided to the display system by a power source at a user&#39;s typical ambient light level; 
 computer code for comparing the power level P to design limits based upon an energy standard; 
 computer code for determining an amount of light emitted by the display system; and 
 computer code for verifying that the amount of light emitted by the display is within established design limits. 
 
     
     
       20. The computer readable medium as recited in  claim 11 , further comprising:
 wherein the power level P is an average power level Pavg, the average power level being a sum of weighted power levels, the weighted power levels including at least a brighter lighting condition power level and a darker lighting condition power level. 
 
     
     
       21. A system, comprising:
 a calibrated light source arranged to provide calibrated light having a luminance value within a range of the target luminance values, the target luminance value corresponding to an ambient light condition; 
 a light detector arranged to detect the calibrated light received at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value, wherein the detected light is at a second luminance level; and 
 a processor coupled to the light detector for calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level. 
 
     
     
       22. The system as recited in  claim 21 , the light source is calibrated by
 receiving light from the light source at a light sensor, the light sensor and the light source being optically isolated from external environment, 
 sending an indication of the luminance of the light received at the light sensor to a light meter connected to the light source, and 
 generating a luminance value by the light meter based upon the indication provided by the light sensor, 
 forwarding the luminance value by the light meter to a process computer, 
 comparing the luminance value to the target luminance value. 
 
     
     
       23. The system as recited in  claim 22 , wherein when the luminance value is not within the range of the target luminance value based upon the comparing, the process computer forwards a corrective command to a programmable power supply. 
     
     
       24. The system as recited in  claim 23 , comprising:
 wherein the programmable power supply modifies a power level of the light source by the programmable power supply based upon the corrective command. 
 
     
     
       25. An apparatus, comprising:
 means for calibrating a light source to a target luminance value, the target luminance value corresponding to an ambient light condition; 
 means for providing a calibrated light by the light source, the calibrated light having a luminance value within a range of the target luminance value; 
 means for receiving the calibrated light at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value of the light provided by the calibrated light source; 
 means for detecting light received at the first part of the optical path by a light detector at a second part of the optical path at a second luminance level, the light detector included in a display system; and 
 means for calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level, the calibration factor used by a system processor in the display system to modify a control signal sent to a backlight driver unit, the display signal causing the backlight driver unit to output an amount of light in accordance with ambient light detected by the light sensor. 
 
     
     
       26. The apparatus as recited in  claim 25 , wherein the calibrating the light source comprises:
 means for receiving light from the light source at a light sensor, the light sensor and the light source being optically isolated from external environment; 
 means for sending an indication of the luminance of the light received at the light sensor to a light meter connected to the light source; 
 means for generating a luminance value by the light meter based upon the indication provided by the light sensor.

Description:
BACKGROUND 
     1. Field of the Described Embodiments 
     The described embodiments relate generally to display devices. In particular, apparatus, method and system for providing an ambient light calibration factor used in a transmissive display are described. 
     2. Description of the Related Art 
     Solid state displays that use solid state elements such as liquid crystal, or LC, for presenting visual content have become ubiquitous. In a particular type of solid state display, a light source, referred to as a backlight, provides illumination that is used to form an image on a viewable display panel. For example, in those solid state displays that utilize liquid crystal image elements (referred to as a liquid crystal display, or LCD), the backlight can take the form of a discrete light source. In some cases, the backlight can take the form of a plurality of light emitting diodes, or LEDs, that can provide a substantially white light. The white light, in turn, that can be projected through an image forming layer having a plurality of image elements. The plurality of image elements can include a liquid crystal material that can be selectively rendered almost fully transparent to almost fully opaque based upon an image signal applied to control elements. When combined with color filters (usually three color filters are used representing the primary colors, red (R), blue (B), and green (G)), the plurality of image elements can form an array of pixels that can be used to create an image that can be viewed on a display panel that is typically covered by a protective layer formed of glass or plastic. 
     However, in order to provide a viewer with an acceptable (or in some cases, exceptional) viewing experience, the viewable image should appear bright and not washed out under all ambient light conditions. For example, in a viewing area that is brightly lit (naturally by sunlight or artificially using, for example, incandescent lighting), the image presented on the display panel can appear washed out due to the high ambient light level reducing the overall contrast between the displayed image and the surrounding area. Therefore, a number of displays attempt to maintain an acceptable viewing experience by using an ambient light sensor to detect an ambient light level. The ambient light level is then used to adjust the light output of the backlight. For example, the ambient light sensor compensates for ambient light by making the display bright enough for an acceptable viewing experience. Therefore, it is important for optimal viewing and power consumption that any change in ambient light level detected by the ambient light sensor be effectively compensated by modifying the amount of light provided by the backlight. This is particularly true for energy efficient display systems since it is the backlight that consumes a substantial amount of the power required to operate the display. Unfortunately, however, the optical path of a display system can include several optically active layers through with ambient light must pass before being detected by the ambient light sensor. Each optically active layer can contribute to an overall optical path tolerance, or variation. This variation can be on the order of ±80% indicating that an ambient light level L 1  detected by the ambient light sensor can only be correlated to an actual ambient light level in the range of 0.2 L 1  to 1.8 L 1  making efficient backlight control difficult. Moreover, this large variance can result in a concomitantly large variance in display screen luminance. 
     In order to qualify as energy efficient (Energy Star, for example), a consumer product, such as a display, must meet certain requirements for power use and efficiency. Since the backlight typically accounts for most of the energy used by the display, it is important to be able to efficiently and effectively control the power used by the backlight in order to meet a specific energy standard. Unfortunately, since the optical path tolerance makes effective and efficient ambient light control of the backlight difficult to achieve, display manufacturers compensate by reducing the overall light output of the backlight for all ambient light conditions. This reduction in overall light output typically results in an inferior image presented by the display. 
     In view of the foregoing, there is a need for providing an energy efficient display that provides a viewer with a desirable viewing experience under most if not all ambient light conditions. 
     SUMMARY OF THE EMBODIMENTS 
     A method for providing an ambient light calibration factor can be performed by carrying out at least the following operations. Calibrating a light source to a target luminance value where the target luminance value corresponds to an ambient light condition, providing a calibrated light by the light source, the calibrated light having a luminance value within a range of the target luminance value, receiving the calibrated light at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value of the light provided by the calibrated light source, detecting light received at the first part of the optical path by a light detector at a second part of the optical path at a second luminance level, the light detector included in a display system, and calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level, the calibration factor used by a system processor in the display system to modify a control signal sent to a backlight driver unit, the control signal causing the backlight driver unit to output an amount of light in accordance with ambient light detected by the light sensor. 
     A non-transitory computer readable medium for storing a computer program for providing an ambient light calibration factor is described. The computer program includes computer code for calibrating a light source to a target luminance value, the target luminance value corresponding to an ambient light condition, computer code for providing a calibrated light by the light source, the calibrated light having a luminance value within a range of the target luminance value, computer code for receiving the calibrated light at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value of the light provided by the calibrated light source, computer code for detecting light received at the first part of the optical path by a light detector at a second part of the optical path at a second luminance level, and computer code for calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level. 
     A system includes a calibrated light source arranged to provide calibrated light having a luminance value within a range of the target luminance values, the target luminance value corresponding to an ambient light condition, a light detector arranged to detect the calibrated light received at a first part of an optical path at the target luminance level, the optical path having a plurality of elements each of which cause an associated variance from the target luminance value, wherein the detected light is at a second luminance level, and a processor coupled to the light detector for calculating a calibration factor based upon the relationship between the target luminance level and the second luminance level. 
     Other apparatuses, methods, features and advantages of the described embodiments will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional apparatuses, methods, features and advantages be included within this description be within the scope of and protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
         FIG. 1  graphically illustrates the data presented in Table 1 showing representative Lambertian angular response curve and representative non-Lambertian angular response curve typical of a less costly light sensor. 
         FIG. 2  shows representative display undergoing calibration where calibration system 
         FIG. 3  shows representative calibration system in accordance with the described embodiments. 
         FIG. 4  shows a calibration factor CF stored in a display device in accordance with the described embodiments. 
         FIG. 5  shows a flowchart detailing a process for generating an ambient light calibration factor in accordance with the described embodiments. 
         FIG. 6  shows a flowchart describing a process for storing an ambient light calibration factor CF in accordance with the described embodiments. 
         FIG. 7  shows a flowchart describing a process for utilizing an ambient light calibration factor CF in a display system in accordance with the described embodiments. 
         FIG. 8  shows a flowchart describing a process for validating a calibration coefficient in accordance with an embodiment of the invention. 
         FIG. 9  is an exploded perspective view of liquid crystal display (LCD) in accordance with an embodiment of the invention. 
         FIG. 10  is a cross-sectional view showing one side of the LCD shown in  FIG. 9  in an assembly state. 
     
    
    
     DESCRIBED EMBODIMENTS 
     In the following paper, numerous specific details are set forth to provide a thorough understanding of the concepts underlying the described embodiments. It will be apparent, however, to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the underlying concepts. 
     This paper discusses a method, system, and apparatus that can be used to operate a display device in an energy efficient manner. In one embodiment, the energy efficient display device can effectively and efficiently compensate for changes in ambient light incident at a display screen of the display device using an internal ambient light sensor to provide control signals to a backlight driver. The internal ambient light sensor can be part of a stack of optical elements included in an optical path through which the ambient light must pass in order to be detected. In the described embodiment, the optical elements can include a protective display layer, and a plurality of apertures and/or openings in either or both the protective display layer and a masking layer (such as ink). The optical elements can also include material used form a light pipe and any angular variations in the light pipe used for directing the ambient light to the ambient light detector. Variations can also be caused by the ambient light detector itself. For example, variations due to light sensor material as well as angular variations due to mechanical tolerances of the display can all add to the overall optical tolerance. As described above, for conventional display systems, the optical path associated with the ambient light detector can have an overall optical tolerance on the order to about ±80%. However, in the described embodiments, the overall optical tolerance of the ambient light detector optical stack can be reduced to about ±5% using at least a calibration factor (CF) to modify a signal used to control a backlight driver unit. In some embodiments, an alignment factor (AF) can also be used to modify the backlight driver unit control signal. 
     In the described embodiments, the calibration factor (CF) can compensate for the overall luminance variation caused by the elements in the optical path that the ambient light must follow in order to reach the ambient light detector. In this way, the correlation between the luminance value of the ambient light detected at the ambient light sensor and the actual luminance value is greatly improved. Using the example above, with the overall optical stack tolerance reduced to ±5%, the ambient light level detected by an internal ambient light sensor can be correlated to the actual ambient light level in the range of 0.95 L 1  to 1.05 L 1  which is a substantial improvement over the prior art range of 0.2 L 1  to 1.8 L 1 . 
     An ideal light sensor will exhibit what is referred to as a Lambertian angular response in which the output of the light sensor is proportional to the cosine of the angle of incidence where an angle of incidence of about zero (0°) degrees is normal to the display screen. However, less costly light sensors typically utilize photo-detectors that do not exhibit the Lambertian response. On the contrary, the typical angular response of the less costly light sensors is generally not well correlated to a cosine curve and is typically determined experimentally as shown in Table 1 enumerating and contrasting a Lambertian angular response and a non-Lambertian angular response. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Lambertian 
                 Non-Lambertian 
               
               
                 Angle 
                 Response 
                 Response 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 −90 
                 0.00 
                 0.00 
               
               
                 −80 
                 0.17 
                 0.00 
               
               
                 −70 
                 0.34 
                 0.02 
               
               
                 −60 
                 0.50 
                 0.10 
               
               
                 −50 
                 0.64 
                 0.25 
               
               
                 −40 
                 0.77 
                 0.43 
               
               
                 −30 
                 0.87 
                 0.65 
               
               
                 −20 
                 0.94 
                 0.83 
               
               
                 −10 
                 0.98 
                 0.95 
               
               
                 0 
                 1.00 
                 1.00 
               
               
                 10 
                 0.98 
                 0.95 
               
               
                 20 
                 0.94 
                 0.83 
               
               
                 30 
                 0.87 
                 0.65 
               
               
                 40 
                 0.77 
                 0.43 
               
               
                 50 
                 0.64 
                 0.25 
               
               
                 60 
                 0.50 
                 0.10 
               
               
                 70 
                 0.34 
                 0.02 
               
               
                 80 
                 0.17 
                 0.00 
               
               
                 90 
                 0.00 
                 0.00 
               
               
                   
               
            
           
         
       
     
       FIG. 1  graphically illustrates the data presented in Table 1 showing representative Lambertian angular response curve  102  and representative non-Lambertian angular response curve  104  typical of a less costly light sensor. During calibration and characterization of the display device, external ambient light sensors that exhibit a Lambertian (or essentially Lambertian) angular response can be used to detect an ambient light level. For example,  FIG. 2  shows representative display  200  undergoing calibration where calibration system ambient light sensor  202  having a Lambertian response can be oriented to have an angle of incidence of about 90° relative to normal N of display screen  204 . In this orientation, sensor  202  can capture an optimal amount of diffuse ambient light provided by light sources  206 . However, display system ambient light sensor  208  is one that generally is not expected to exhibit the Lambertian angular response curve  102  but more likely to have an angular response more like that of non-Lambertian angular response curve  104 . In the described embodiment, angular calibration factor AF can be used to account for the differences in angular response between the calibration data provided by calibration system ambient light sensor  202  and display system light sensor  208 . In this way, angular calibration factor AF can be used to modify the operation of the backlight driver unit separately or in combination with calibration factor CF. 
     These and other embodiments are discussed below with reference to  FIGS. 1-10 . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes only and should not be construed as limiting. 
       FIG. 3  shows representative calibration system  300  in accordance with the described embodiments. Calibration system  300  can be used to determine validated calibration factors (CF) that can be used to modify a control signal. The control signal can be used to control an amount of light output from an illumination source such as a backlight. The modification of light provided by the backlight can be in accordance with a change in an ambient light level detected by an ambient light detector. Calibration system  300  can be used in a laboratory environment or in a manufacturing environment to accurately determine consistent and validated calibration factors for a particular display system under a variety of ambient light conditions. Calibration system  300  can include at least system under test (SUT)  302 , light source  304  and light sensor  306  electrically connected to and part of light meter  308 . SUT  302  can take the form of a solid state display along the lines of a liquid crystal display, or LCD. Light source  304  can take the form of an incandescent light, a CCFL or a plurality of light emitting diodes, or LEDs. Light sensor  306  can include a photo detector unit and associated circuitry. Enclosure  310  can optically isolate SUT  302 , light source  304  and light sensor  306  from the external environment. In this way, the calibration process can be unaffected by any extraneous light not originating from light source  304 . Enclosure  310  can take the form of a shroud formed of opaque material such as black cloth or other appropriate materials. 
     Light meter  308  can receive electrical signals from light sensor  306  indicative of an amount of light detected by a photo-detector included in light sensor  306 . In the described embodiment, light sensor  306  can be placed in close proximity to SUT  302  in order to accurately simulate the amount and intensity of light from light source  304  that reaches SUT  302 . By placing light sensor  306  in close proximity to SUT  302 , any attenuation of light from light source  304  can be taken into account providing a more accurate calibration of light source  304  and ultimately calibration factor CF for SUT  302 . For example, when light source  304  provides light having luminance level L source , then any attenuation can result in light received at SUT  302  having a reduced luminance value L SUT  that is less than L source . Light sensor  306  can be placed in close proximity to SUT  302  having luminance value L sense  that is essentially the same as that of the light received at SUT  302 , namely L sense  is proportional to L SUT . 
     Light meter  308  can be electrically connected to process computer  312 . Process computer  312  can be a standalone unit or be incorporated into a separate calibration unit either of which can be coupled directly to a data port of SUT  302 . In any case, process computer  312  can provide control signals to programmable power supply  314  in response to input signal  316  received from light meter  308 . Input signal  316  can, in turn, be directly related to the luminance L sense  of light from light source  304  received at light sensor  306 . In this way, control loop  318  can be used by process computer  312  to calibrate light source  304 . In one embodiment, light source  304  can be calibrated to simulate a user&#39;s expected ambient light level at SUT  302 . For example, light source  304  can be calibrated to provide an ambient light level having a luminance value of about 300 lux (lx) where 1 lx is equal to 1 lumen (lm) per square meter (m 2 ). 
     In one embodiment, control loop  318  can operate as follows. Based upon a target luminance value provided to process computer  312 , process computer  312  can provide control signal  320  to programmable power supply  314 . Programmable power supply  314  can respond to control signal  320  by sending power signal  322  to light source  304 . Power signal  322  can cause light source  304  to either increase or decrease an amount of light detected at light sensor  306 . Light sensor  306 , in turn, generate signal  324  that can be passed to light meter  308 . Light meter  308  can pass signal  316  indicative of the amount of light from light source  304  detected at light sensor  306 . Process computer  312  can evaluate information provided by signal  316  in order to determine if light source  304  is providing light within an acceptable range of a target luminance value. Based upon the evaluation, process computer  312  determines that light source  304  is providing light within the acceptable range of the target luminance value, then the control loop ends, otherwise, process computer  312  updates control signal  320  in accordance with the evaluation of the light output of light source  304 . 
     SUT  302  can include internal light sensor  326 . Light from light source  304  reaching SUT  302  as calibrated ambient light L SUT  can reach internal light sensor  326  by following optical path  328 . As described above, optical path  328  can present a number of elements each of which can affect the detection of ambient light L SUT  by internal light sensor  326 . Since light source  304  has been calibrated to provide light in the acceptable range of the target luminance value, the luminance of ambient light L SUT  can be provided to SUT  302  by process computer  312  as a corrected light meter reading (LC≈L SUT ). In this way, the light level (LS) detected by internal sensor  326  can be used to determine calibration factor CF according to equation (1): 
     
       
         
           
             
               
                 
                   CF 
                   = 
                   
                     LC 
                     LS 
                   
                 
               
               
                 
                   Eq 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
     
     In order to validate calibration factor CF, SUT  302  can report calibration factor CF to process computer  312  for validation. By validating calibration factor CF, process computer  312  can verify that calibration factor CF is within an allowable range of calibration factors. This allowable range of calibration factors can be based upon, for example, tolerances of the various optical elements included in the optical path. Such elements can include, for example, light pipes, light sensor angle, the light sensor, and so on as described above. 
     In the described embodiment, process computer  312  can validate calibration factor CF as follows. Process computer  312  can determine power level P provided by, power source  330  by reading power meter  332  at, for example, a user&#39;s typical ambient light level L typical  as detected by screen luminance meter  334 . Power level P can then be compared to design limits based upon energy standards (such as those provided by the Environmental Protection Agency, or EPA, as determined by the EnergyStar standard) and any power consumption tolerance of SUT  302 . In some cases, process computer  312  can also verify that light emitted by the display of SUT  302  is within established design limits. 
     As part of the validation of the calibration factor, process computer  312  can determine power level P L  corresponding to a condition of low ambient light level and power level P H  corresponding to a condition of high ambient light level. Process computer  312  use the determined values of P L  and P H  to calculate average weighted power Pavg based upon equation (2)
 
 P avg= WH×PH+WL×PL   Eq (2)
         where:
           Pavg is weighted average power;   WH is brighter (higher) lighting condition weight factor;   PH is brighter (higher) lighting condition power level;   WL is darker (lower) lighting condition weight factor; and   PL is darker (lower) lighting condition power level.   
               

     In the described embodiment, weighting factor WH is typically greater than weighting factor WL in order to provide a more conservative (power wise) estimate of the power consumption of SUT  302 . For example, weighting factor WH can be on the order of 0.8 whereas weighting factor WL can be on the order of 0.2. 
     As further shown in  FIG. 4  calibration factor CF can be stored in SUT  302  in the form of display device  400 . Display device  400  can include light sensor  402 , system processor  404 , and memory device  406  that can take the form of non-volatile memory such as EEPROM. Display device  400  can also include backlight driver  408  configured to provide control signals to a backlight unit (not shown) that provides illumination used to provide a displayable image on a display panel. Calibration factor CF can be stored in display system  400  in one embodiment as follows. Process computer  312  can be connected to system  400  by way of an input/output data port such as a USB data port. Process computer  312  can cause display device  400  to enter a calibration mode by process computer  312  sending trigger signal  410  to system processor  404 . 
     In one embodiment, trigger signal  410  can include information such as corrected light meter reading LC. In calibration mode, system processor  404  can sample light sensor  402  for an indication a luminance value of light received through optical path  412  corresponding to ambient light  414  provided by light source  304 . System processor  404  can then calculate calibration factor CF based upon the sampled light reading LS and light meter reading LC according to equation (1). Once calculated, calibration factor CF can be stored in memory device  406 . Once calibration factor CF is stored in memory device  406 , system processor  404  can cause display device  400  to exit the calibration mode. In one embodiment, display device  400  exits the calibration mode after system processor  404  has reported calibration factor CF to process computer  312 . 
     Once calibration factor CF has been stored in memory device  406  and display device  400  is no longer in calibration mode, system processor  404  can retrieve calibration factor CF from memory device  406  as well as any user settings  416  (such as a most recent brightness) from memory device  406 . During normal operation of display device  400 , system processor  404  can sample light received at light sensor  402  and determine calibrated ambient light level LA as equation 3:
 
 LA=CF×LS   eq. (3)
 
     System  400  can apply calibrated ambient light level LA and any user settings to ambient light control function  418  executed by system processor  404 . Ambient light control function  418  can issue command  420  to backlight driver  408  that can respond by, for example, changing a backlight duty cycle and/or a backlight phase. 
       FIG. 5  shows a flowchart detailing process  500  for generating a calibration factor for modifying a control signal used by a backlight driver to compensate for an ambient light condition in accordance with the described embodiments. Process  500  can begin at  502  by calibrating a light source. The light source can be calibrated to a target luminance value. The target luminance value can correspond to an expected ambient light condition experienced by a display device. Next at  504 , a calibration factor CF is determined based upon, in part, the light provided by the calibrated light source. An ambient light sensor internal to a display device detects the light provided by the calibrated light source having a known target luminance. The luminance value of the light detected by the internal ambient light sensor is then compared to the light provided by the calibrated light source at the target luminance. The calibration factor CF is that ratio of the detected luminance value and the target luminance value. The calibration factor CF can be used to compensate for variations caused by elements in an optical path that the light from the light source must travel to reach the internal ambient light detector. Next, at  506 , the calibration factor CF is validated. By validation, it is meant that the energy usage and screen luminance values are evaluated for compliance to both system design standard and energy efficiency standard. 
       FIG. 6  shows a flowchart describing process  600  for storing an ambient light calibration factor CF in accordance with the described embodiments. Process  600  can begin at  602  by triggering a system processor to enter a calibration mode. In the calibration mode, the system processor can receive data from an internal light sensor at  604 . The data received from the internal light sensor can correspond to ambient light provided by a calibrated light source. At  606 , the system processor can then calculate a calibration factor CF based upon the data received from the internal light sensor and data received from an external circuit such as a process computer. The data received from the process computer can include a corrected light meter reading. At  608 , the calibration factor CF can be stored in a memory device and reported to the process computer at  610  at which point, the system processor can exit the calibration mode at  612 . 
       FIG. 7  shows a flowchart describing process  700  for utilizing an ambient light calibration factor CF in a display system in accordance with the described embodiments. Process  700  can begin at  702  by the system controller retrieving the calibration factor CF from the memory device. At  704 , during normal operation, the system processor can receive data from the internal ambient light sensor. At  706 , a calibrated ambient light level is determined based upon the calibration factor CF and the data received from the internal sensor. At  708 , the calibrated ambient light level and any user settings are retrieved from the memory device. They are applied to an ambient light brightness control function at  710 . In one embodiment, the ambient light brightness control function can be executed by the system processor. At  712 , the ambient light brightness control function can modify the output of a backlight driver. In one embodiment, a duty cycle and phase of backlight driver can be modified. 
       FIG. 8  shows a flowchart detailing process  800  for validating a calibration coefficient in accordance with the described embodiments. Process  800  can being at  802  by determining a power level P provided to a display system by a power source at a user&#39;s typical ambient light level. Next at  804 , the power level P is compared to design limit power levels based in part upon an energy standard. For example, the energy standard includes power limits defining what is considered to be an energy efficient display. At  806 , if the power level P does not meet the standard, then at  808  the calibration factor is re-calculated and control is passed back to  802 . On the other hand, if the power level does meet the standard, then at  810  a determination of an amount of light emitted by the display is determined. At  812 , the amount of light emitted by the display is then compared to design limits for the display. If the light emitted by the display does not meet the design limits, then at  808 , the calibration factor is recalculated and control is passed back to  802 , otherwise, the calibration coefficient is acceptable at  814 . 
       FIG. 9  is an exploded perspective view of liquid crystal display (LCD)  900  in accordance with an embodiment of the invention.  FIG. 10  is a cross-sectional view showing one side of LCD  900  shown in  FIG. 9  in an assembly state. Referring to  FIGS. 9 and 10 , LCD  900  includes support main  914 , backlight unit  950 , and liquid crystal display panel  906  stacked within the support main  914 , and top casing  902  for surrounding the edges of liquid crystal display (LCD) panel  906  and lateral portions of the support main  914 . LCD panel  906  includes liquid crystal intervened between front substrate  905  and rear substrate  903 , and spacers for maintaining a gap between the front substrate  905  and rear substrate  903 . A color filter and a black matrix are formed in the front substrate  905  of the LCD panel  906 . Signal lines, such as data lines and gate lines, are formed in the rear substrate  903  of LCD panel  906 . A thin film transistor (hereinafter referred to as a “TFT”) is formed at crossings of the data lines and the gate lines. The TFT switches a data signal to transmit the data signal from the data line to a liquid crystal cell, in response to a scan signal gate pulse transmitted from the gate line. A pixel electrode is formed in a pixel area between the data line and the gate line. Further, pad regions to which the data lines and the gate lines are respectively coupled are formed in one side of rear substrate  903 . A driver integrated circuit (not shown) for applying a driving signal to the TFT is mounted is attached to each pad region. The data signal, transmitted from the driver integrated circuit, is sent to the data lines and also supplies a scan signal to the gate lines. An upper polarization sheet is attached to front substrate  905  of LCD panel  906 , and a lower polarization sheet is attached to rear substrate  903  of rear substrate  903 . 
     Backlight unit  950  includes plurality of light sources for providing light to LCD panel  906 . The light sources can be LED devices. The duty ratio of the output signal of the inverter is T on ×100/(T on +T off ), where ‘T on ’ denotes a turn-on period of the light source and ‘T off ’ denotes a turn-off period of the light source. The duty ratio of the output signal determines the luminance of the light source. 
     The plurality of optical sheets  908  stacked over the diffusion sheet  910  redirects light incident from the diffusion sheet  910  to be incident perpendicular to the liquid crystal display panel  906 , thus improving optical efficiency. To this end, the optical sheets  908  include two sheets of prism sheets and two sheets of spreading sheets. The two sheets of prism sheets stand a travel angle of spreading light, emitted from the diffusion sheet  910 , in a direction vertical to the liquid crystal display panel  906 . The two sheets of spreading sheets spread the vertically incident light again. The top casing  902  is formed in a rectangular belt having a plan portion and a lateral portion, which are curved at a right angle to each other and surrounds the corners of the LCD panel  906  and the sides of the support main  914 . 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a computer readable medium for controlling manufacturing operations or as computer readable code on a computer readable medium for controlling a manufacturing line. The computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the computer readable medium include read-only memory, random-access memory, CD-ROMs, DVDs, magnetic tape, and optical data storage devices. The computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     While the embodiments have been described in terms of several particular embodiments, there are alterations, permutations, and equivalents, which fall within the scope of these general concepts. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present embodiments. For example, although an extrusion process is preferred method of manufacturing the integral tube, it should be noted that this is not a limitation and that other manufacturing methods can be used (e.g., injection molding). It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the described embodiments.

Metadata:
Filing Date: 20100726
Publication Date: 20120814
Grant Date: 20120814
Priority Date: 20100726
Inventors: LUM DAVID W.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05B47/11", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05B47/11", "inventive": true, "first": true, "tree": "[]"}, {"code": "Y02B20/40", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02B20/40", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 45493058