Abstract:
A system to compensate for luminance degradation of an emissive display is provided. The system employs a combination of a consumption rate limit and temperature to regulate and control display luminance for emissive displays (such as organic light emitting diodes). A consumption rate limit is employed to determine a display luminance ceiling at which to drive the display. By employing the aspects disclosed herein, the display systems may achieve a longer lifetime.

Description:
BACKGROUND 
       [0001]    Organic light emitting diode (OLED) displays are becoming more commonplace for their numerous advantages. However, emissive display technologies, such as OLED displays, suffer from differential aging (or luminance degradation), and must be carefully analyzed and used to ensure that lifetime expectations are met. Differential aging is where portions or colors of the display used more frequently emit a lower luminance than portions used less frequently. Light valve technology such as liquid crystal, interferometric modulator, LCOS, micro-minor, and electrophoretic displays do not suffer from differential aging because they depend on a general light source that decays independent of localized screen use. Since emissive technology displays suffer from differential aging, screen saver functions are required if the same data is displayed over long periods of time. Although OLED displays have many benefits, their major disadvantage is aging. In addition, aging of OLED displays is accelerated substantially at elevated temperatures, commonly associated with automotive environments. 
         [0002]    For example, an implementer of a display may configure or request OLEDs to be driven at levels greater than 600 cd/m 2  in order to maintain display visibility, the amount of permanent luminance consumption (or amount that the luminance decreases) increases dramatically as the OLED operating temperature is increased. The amount of compensation that may be applied at the pixel level to minimize burned-in image effects is limited which necessitates the use other methods to minimize the OLED consumption rate under infrequent adverse conditions. 
         [0003]    Another technique has been proposed, thermal derating, to address the problems associated with differential aging. A thermal derating method allows for control of an overall temperature associated with a display technologies operation, in order to maximize the lifetime of an OLED. 
         [0004]    However, each of the above proposed methods may not be robust enough to handle the issues associated with luminance degradation. 
       SUMMARY 
       [0005]    A system to compensate for luminance degradation of a display is described herein. The system includes a luminance control system coupled to the display and configured to provide power to the display thereby controlling the display luminance, the luminance control system including an input to receive a consumption rate (CR) limit; a temperature sensor proximate the display and in electrical communication with the controller, wherein the luminance control system is configured to vary the display luminance, based on a temperature measured by the temperature sensor and the CR limit. 
         [0006]    In another example of the system, the display luminance is determined by a relationship between the consumption rate, maximum luminance of the display, and temperature of the display. 
         [0007]    In another example of the system, the display luminance is maximized so as to be under CR limit by employing the following relationship: 
         [0000]        L   OP   =f ( CR,K °).
 
         [0008]    In another example of the system, the temperature sensor is a thermistor. 
         [0009]    In another example of the system, the luminance control system retrieves a luminance based on correlating the measured temperature with a value in a predefined lookup table. 
         [0010]    In another example of the system, the luminance control system is implemented with a circuit to perform the relationship. 
         [0011]    In another example of the system, the luminance control system employs a PID control loop. 
         [0012]    Also provided is a display integrated with a luminance control system. The display includes a consumption rate (CR) limit; a temperature sensor proximate the display and in electrical communication with the controller, wherein the luminance control system coupled to the display and configured to provide power to the display thereby controlling the display luminance, and the luminance control system is configured to vary the display luminance, based on a temperature measured by the temperature sensor and the CR limit. 
         [0013]    Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram of a system to compensate for luminance degradation according to the aspects disclosed herein; 
           [0015]      FIG. 2  is a graph plotting the relationship of luminance and temperature variances versus the consumption rate (CR); 
           [0016]      FIG. 3  is a graph plotting the relationship between luminance and temperature when constrained by a CR limit; and 
           [0017]      FIG. 4  is a graph illustrating differences between using the system of  FIG. 1  versus not using the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to  FIG. 1 , a system embodying the principles of the present invention is illustrated therein and designated at  10 . As its primary components, the system  10  includes a control circuit  12  (luminance control system), an emissive display  14 , and a temperature sensor  16  (for example, via a thermistor). A desired luminance signal  18  is provided to the control circuit  12 , the desired luminance signal  18  is often generated from a display brightness control (not shown). 
         [0019]    The control circuit  12  generates a display drive signal  20  based on the desired luminance signal  18 . The display drive signal  20  is provided to the emissive display  14 , causing the emissive display  14  to operate at a specific display luminance level. The temperature sensor  16  is located proximate the emissive display  14  and configured to monitor a temperature of the emissive display  14 . The temperature sensor  16  generates a feedback signal  22  which is received by the control circuit  12 . The feedback signal  22  is based on a calculated luminance. The calculated luminance, otherwise known as L op , will be explained later, and is partially derived by the known temperature measured by the temperature sensor  16  and the CR limit  23 . 
         [0020]    Also shown in a CR Limit  23 . The CR limit  23  is set by a user via the relationships discussed in the experimentally derived formulas/graphs disclosed herein. The CR limit  23  may be manually set, set by a lookup table, set by an electronic circuit employed to calculate a relationship based on the temperature, and the like. 
         [0021]    The control circuit  12  includes logical circuits configured to receive the measured temperature, apply the measured temperature to the derived relationship for establishing a CR limit  23 , and communicate a L op  to the emissive display  14  (via the feedback signal  22 , which causes the display drive signal  20  to modify the emissive display  14 ). 
         [0022]    In one such example, experimental testing may be employed to determine a relationship to determine a consumption rate for a specific OLED. As shown below, in one example of testing a sample OLED emissive display  14 , the following value for determining a consumption rate (CR) is obtained (nits is a measure of luminance): 
         [0000]    
       
         
           
             
               
                 
                   CR 
                   = 
                   
                     
                       
                         L 
                         op 
                         2 
                       
                       
                         L 
                         max 
                       
                     
                      
                     
                       0.1 
                       
                         
                           
                             
                               ( 
                               
                                 6.42 
                                  
                                 
                                   10 
                                   10 
                                 
                               
                               ) 
                             
                              
                             ° 
                           
                            
                           
                               
                           
                            
                           
                             K 
                             
                               - 
                               3 
                             
                           
                         
                         - 
                         1075.5 
                       
                     
                      
                     
                         
                     
                      
                     
                       nits 
                       hour 
                     
                   
                 
               
               
                 
                   FORMULA 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0023]    wherein: 
         [0024]    L op  is Operating Luminance of emissive display  14 ; 
         [0025]    L max =Maximum Display Luminance for emissive display  14 ; and 
         [0026]    ° K=Temperature in Kelvin as measured by the temperature sensor  16   
         [0027]    FORMULA 1 may be used to calculate the CR limit when the luminance is known as well as the temperature. 
         [0028]      FIG. 2  illustrates a graph  200  of FORMULA 1. Graph  200  includes three axis, a temperature axis  210  (in Celsius), a L op    220  (in Luminance), and a CR axis  230  (in candelas (cd)/meter(m) 2 /Hour). As shown, the various CR limits are calculated per luminance and temperature, and plotted in a three-dimensional graph. 
         [0029]    The plotted value  240  shows a three-dimensional graph of the calculated CR values/limits when the x-axis  220  and y-axis is varied. The various inputs (i.e. the two axis shown), generate a different range of CR limits, as shown by values on the z-axis  230 . The ranges shown are respectively  241 - 244 . 
         [0030]    The calculated CR illustrates the benefit of operating an emissive display  14  at a lower L op . CR 1  and CR 2 , using FORMULA 1, are examples of consumption rates for two test cases respectively. The two test cases are: 
         [0031]    1. Operating at a level of 600 cd/m2 at 45° C. (318° K). 
         [0032]    2. Operating at a level of 300 cd/m2 at 35° C. (308° K). Note that the lower temperature is estimated due to the lower operating luminance compared to the 600 cd/m2 case 1. 
         [0000]    
       
         
           
             
               CR 
               1 
             
             = 
             
               
                 
                   600 
                   2 
                 
                 300 
               
                
               
                 0.1 
                 
                   
                     
                       
                         ( 
                         
                           6.42 
                            
                           
                             10 
                             10 
                           
                         
                         ) 
                       
                        
                       318 
                     
                      
                     
                       ° 
                       
                         - 
                         3 
                       
                     
                   
                   - 
                   1075.5 
                 
               
             
           
         
       
       
         
           
             
               CR 
               1 
             
             = 
             
               0.13 
                
               
                   
               
                
               
                 nits 
                 hour 
               
             
           
         
       
       
         
           
             
               CR 
               2 
             
             = 
             
               
                 
                   300 
                   2 
                 
                 300 
               
                
               
                 0.1 
                 
                   
                     
                       
                         ( 
                         
                           6.42 
                            
                           
                             10 
                             10 
                           
                         
                         ) 
                       
                        
                       308 
                     
                      
                     
                       ° 
                       
                         - 
                         3 
                       
                     
                   
                   - 
                   1075.5 
                 
               
             
           
         
       
       
         
           
             
               CR 
               2 
             
             = 
             
               0.0267 
                
               
                   
               
                
               
                 nits 
                 hour 
               
             
           
         
       
     
         [0033]    Thus, using the experimentally derived FORMULA 1, the CR an OLED experiences when being provided a lower luminance operating condition may drastically decreases. The examples/graph  200  shown above illustrates the important experimentally derived interplay between temperature, luminance level, and an experimentally derived CR. 
         [0034]    Based on the above, an operator of an emissive display  14  can implement a control circuit  12  with an input to enter in a predefined CR limit  23 . Thus, the implementer of system  10  may provide a specific CR limit  23 , and with that, derive a drive signal  20  to drive an emissive display  14  (see FORMULA 2). 
         [0035]      FIG. 3  illustrates a graph  300  for relating a L op    310  with a measured temperature  320 . The various values  301 - 306  are shown plotted on graph  300 . Thus, for a desired consumption rate (which may be set by the implementer of system  10 , or by methods explained below), the plotted L op  should be used to drive the emissive display  14 . 
         [0036]    The following relationship is derived from FORMULA 1, and solved for L op : 
         [0000]    
       
         
           
             
               
                 
                   
                     L 
                     op 
                   
                   = 
                   
                     
                       
                         
                           CR 
                            
                           
                             L 
                             max 
                           
                         
                          
                         
                           ⌊ 
                           
                             
                               
                                 6.42 
                                  
                                 
                                   10 
                                   10 
                                 
                                  
                                 ° 
                               
                                
                               
                                   
                               
                                
                               
                                 K 
                                 
                                   - 
                                   3 
                                 
                               
                             
                             - 
                             1075.5 
                           
                           ⌋ 
                         
                       
                       0.1 
                     
                   
                 
               
               
                 
                   FORMULA 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0037]    The importance of knowing or setting a CR limit is exemplified by a vehicle situation. When a driver/passenger enters into a vehicle, the internal temperature may be hot due to solar or other conditions (known as a “hot start condition”). If the CR limit is set based on a desire to protect an OLED from degradation, and the temperature is known—the luminance may be altered to adhere to the CR limit shown in graph  300 . Thus, as the air conditioning turns on, and the vehicle&#39;s cabin lowers, the luminance may increase and provide the user with a brighter display. 
         [0038]      FIG. 4  illustrates a graph  400  explaining the implementation of the CR limit based on the hot start situation described above. On the left side of the y-axis  410 , the CR is shown, as a function of time, and specifically with plotted values  401  and  402 . Value  401  represents the generated CR (normal CR) without limiting the CR. Value  402  represents a condition in which the CR is limited to 0.15 as a maximum (as shown in the graph, as the temperature  403  decreases, the value  402  reduces based on the relationship shown in FORMULA 2). 
         [0039]    The x-axis  420  is the time (in hours). Also shown is time period  421  and time period  422 . In time period  421 , the temperature  403  is initially at a higher value (approximately 85 degrees), while the temperature  403  settles to an equilibrium at time period  422 . 
         [0040]    Values  404  and  405  represent the luminance value in the cases where the CR limit is not used and is used, respectively. Essentially, the luminance value is controlled to be lower based on a CR limit value  402  being implemented. As shown, because the luminance is lower for at least the period  421 , the CR is heavily decreased (as shown by the value difference between 401 and 402). 
         [0041]    Assuming in the example above, with a drop in temperature of about 85 degrees to 45 degrees, the comparison between when luminance is not CR limited, and when it is can be calculated by the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   CR 
                   = 
                   
                     
                       
                         
                           L 
                           op 
                           2 
                         
                         
                           L 
                           max 
                         
                       
                        
                       
                         [ 
                         
                           
                             1.08 
                              
                             
                               10 
                               
                                 - 
                                 4 
                               
                             
                           
                           + 
                           
                             2.01 
                              
                             
                               10 
                               
                                 - 
                                 4 
                               
                             
                              
                             
                                
                               
                                 
                                   - 
                                   t 
                                 
                                 / 
                                 0.1 
                               
                             
                           
                         
                         ] 
                       
                     
                      
                     
                         
                     
                      
                     
                       nits 
                       hour 
                     
                   
                 
               
               
                 
                   FORMULA 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0042]    FORMULA 3 may be integrated to calculate a specific luminance degradation (LD): 
         [0000]    
       
         
           
             
               
                 
                   
                     LD 
                     = 
                     
                       
                         ∫ 
                         0 
                         t 
                       
                        
                       
                         CR 
                          
                         
                            
                           t 
                         
                       
                     
                   
                    
                   
                     
 
                   
                    
                   LD 
                   = 
                   
                     
                       
                         
                           L 
                           op 
                           2 
                         
                         
                           L 
                           max 
                         
                       
                        
                       
                         [ 
                         
                           
                             1.08 
                              
                             
                               E 
                               
                                 - 
                                 4 
                               
                             
                              
                             t 
                           
                           + 
                           
                             2.01 
                              
                             
                               
                                 E 
                                 
                                   - 
                                   5 
                                 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                      
                                     
                                       
                                         - 
                                         t 
                                       
                                       / 
                                       0.1 
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         ] 
                       
                     
                      
                     
                         
                     
                      
                     nits 
                   
                 
               
               
                 
                   FORMULA 
                    
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0043]    Thus, using the sample values in graph  400 , the case where CR is not limited produces the following amount of delta LD: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               LD 
             
             = 
             
               
                 
                   L 
                   op 
                   2 
                 
                 
                   L 
                   max 
                 
               
                
               
                 [ 
                 
                   2.01 
                    
                   
                     
                       E 
                       
                         - 
                         5 
                       
                     
                      
                     
                       ( 
                       
                         1 
                         - 
                         
                            
                           
                             
                               - 
                               ∞ 
                             
                             / 
                             0.1 
                           
                         
                       
                       ) 
                     
                   
                 
                 ] 
               
             
           
         
       
       
         
           
             
               Δ 
                
               
                   
               
                
               LD 
             
             = 
             
               
                 
                   
                     600 
                     2 
                   
                   300 
                 
                  
                 2.01 
                  
                 
                   E 
                   
                     - 
                     5 
                   
                 
               
               = 
               
                 0.02412 
                  
                 
                     
                 
                  
                 
                   nits 
                   start 
                 
               
             
           
         
       
     
         [0044]    And the case where CR is limited produces the following amount of delta LD: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               LD 
             
             = 
             
               
                 
                   ( 
                   
                     0.15 
                     - 
                     0.1296 
                   
                   ) 
                 
                  
                 0.247 
               
               + 
               
                 0.02412 
                  
                 
                    
                   
                     
                       - 
                       0.247 
                     
                     / 
                     0.1 
                   
                 
               
             
           
         
       
       
         
           
             
               Δ 
                
               
                   
               
                
               LD 
             
             = 
             
               0.00704 
                
               
                   
               
                
               
                 nits 
                 start 
               
             
           
         
       
     
         [0045]    where start is the amount of nits (a unit of luminance) used per the start period. 
         [0046]    As shown, by implementing an experimentally found CR limit, the reduction in nits per a start period is drastic (0.00704 versus 0.024). By limiting the luminance to an experimentally calculated CR limit, an OLED life may be improved and the degradation may be avoided. 
         [0047]    Various techniques may be employed to utilize a CR limit—with each of the techniques being implemented in a control system provided along with an OLED display: 
         [0048]    a lookup table to determine the operating luminance as a function of measured OLED temperature; 
         [0049]    a relation between the operating luminance as a function measured OLED temperature; and 
         [0050]    a proportional, integral, derivative (PID) control loop to control the operating luminance as a function of the measured OLED temperature. 
         [0051]    As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims.