Abstract:
LED brightness compensation system and method to account for aging and/or temperature effects on LED brightness. The system includes one or more LEDs and a circuit coupled to the LEDs to maintain substantially constant LED brightness based on determined operating characteristics of the LEDs. The circuit includes an LED brightness controller for controlling the current running through the LEDs and a brightness compensation controller for directing the LED brightness controller to compensate for aging and/or temperature. The method includes: storing adjustment information in a memory unit; energizing one or more LEDs with an electric current; accumulating the operating time; sensing the operating temperature of the LEDs; and adjusting the current supplied to the LEDs based on the stored adjustment information, the accumulated time the LEDs have been energized, and the operating temperature of the LEDs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The brightness of Light Emitting Diodes (LEDs) in LED based lights decreases over their operating time. The operating temperature of the LEDs also affects LED brightness. In many systems, it is desirable to maintain a relatively constant LED brightness level. Some previous attempts at compensating for factors affecting LED brightness have relied on a simple linear model for LED brightness changes over time. Additionally, they have not accounted for temperature effects on LED brightness. Other previous attempts at compensating for LED brightness changes involve adjusting the light output to be greater than the minimum required when the LEDs are first used and deactivating the LEDs after a calculated period of time when the light output has been predicted to have decreased to below an acceptable level. This has the unfortunate side effect of having to provide higher energy levels to the LEDs, thus increasing their operating temperature, increasing the brightness decay rate, and shortening the operating life of the LEDs. Still other previous attempts have used optical brightness sensors to provide feedback in maintaining constant LED brightness. However, using optical brightness sensors is disadvantageous because it is a complex and expensive solution. Accordingly, there is a need for an LED brightness compensation system and method that more accurately reflects the characteristics of LED brightness changes than a simple linear model and that does not involve the expense and complexity of using optical brightness sensing. There is a further need for an LED brightness compensation system and method to account for the effects of temperature changes on LED brightness. 
       SUMMARY OF THE INVENTION 
       [0002]    The present invention provides an LED brightness compensation system and method that accounts for the effects of operating time and/or temperature on LED brightness. An example system includes one or more LEDs and a circuit electrically coupled to the LEDs for maintaining a substantially constant brightness of the LEDs over an extended period of time based on previously determined operating characteristics of the LEDs. 
         [0003]    In one aspect of the invention, the circuit includes an LED brightness controller for controlling the current running through the LEDs and a brightness compensation controller for directing the LED brightness controller to compensate for LED brightness changes over time. In another aspect of the invention, the brightness compensation controller includes: a memory unit for storing adjustment information to compensate for factors affecting LED brightness; a time keeping component for accumulating the period of time the LEDs have been energized; and a logic component for determining an adjustment output based on the stored adjustment information and the accumulated period of time the LEDs have been energized. In other embodiments, these components and/or functions are included in a microcontroller. 
         [0004]    In an additional aspect of the invention, the circuit includes a configuration interface that allows changes to be made to the adjustment information stored in the memory unit. In a still further aspect of the invention, the circuit includes a temperature sensor for producing a signal related to the operating temperature of the LEDs, and the brightness compensation controller includes an input for the signal produced by the temperature sensor. 
         [0005]    In other aspects, the invention includes a method for adjusting a current through one or more Light Emitting Diodes (LEDs) to compensate for factors affecting LED brightness. The method includes: storing adjustment information to compensate for factors affecting LED brightness in a memory unit; energizing one or more LEDs with an electric current; accumulating the time the LEDs have been energized; sensing the operating temperature of the LEDs; and adjusting the current supplied to the LEDs based on the stored adjustment information, the accumulated time the LEDs have been energized, and the operating temperature. In some embodiments, the current is adjusted by changing the level of a constantly applied current. In other embodiments, the current is adjusted by pulse width modulating (PWM) a pre-determined current level. 
         [0006]    In additional aspects of the invention, the method includes testing a first test LED to determine the current required to keep the first test LED at a relatively constant brightness over an extended period of time as a function of time to determine adjustment information. In other aspects of the invention, the method also includes testing a second test LED to determine the current required to keep the second test LED at a relatively constant brightness for operation at different operating temperatures as a function of temperature. 
         [0007]    As will be readily appreciated from the foregoing summary, the invention provides an LED brightness compensation system and method that more accurately reflects the characteristics of LED brightness changes than a simple linear model and that does not involve the expense and complexity of using optical brightness sensing. The invention also provides an LED brightness compensation system and method that accounts for the effects of temperature changes on LED brightness. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0009]      FIG. 1A  is a diagram of an LED brightness compensation system in accordance with an embodiment of the invention; 
           [0010]      FIG. 1B  is a diagram of an LED brightness compensation system that includes operating temperature compensation in accordance with an embodiment of the invention; 
           [0011]      FIG. 2  is a diagram showing additional detail for an example embodiment of a brightness compensation controller shown in  FIG. 1B ; 
           [0012]      FIG. 3A  is a chart showing LED energy adjustment data for the effect of operating time on LED brightness in accordance with an example embodiment of the invention; 
           [0013]      FIG. 3B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 3A  that is used in configuring an example embodiment of the invention; 
           [0014]      FIG. 4A  is a chart showing the LED energy adjustment data for the effect of operating temperature on LED aging in accordance with an example embodiment of the invention; 
           [0015]      FIG. 4B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 4A  that is used in configuring an example embodiment of the invention; 
           [0016]      FIG. 5A  is a chart showing LED energy adjustment data for the instantaneous effect of operating temperature on LED brightness in accordance with an example embodiment of the invention; 
           [0017]      FIG. 5B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 5A  that is used in configuring an example embodiment of the invention; and 
           [0018]      FIG. 6  is a flowchart of a method of compensating for the effects of operating time and operating temperature on LED brightness in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1A  is a diagram of a Light Emitting Diode (LED) brightness compensation system  20   a  in accordance with an embodiment of the invention. The LED brightness compensation system  20   a  includes one or more LEDs  22   a . The LED brightness compensation system  20   a  also includes an LED brightness controller  24   a  electrically connected to the one or more LEDs  22   a  for controlling the energy supplied to the one or more LEDs  22   a  based on a signal received from a brightness compensation controller  26   a . In one embodiment, a switch  27   a  is connected between the LED brightness controller  24   a  and the anode of the first LED of the one or more LEDs  22   a . The switch  27   a  is also connected to the brightness compensation controller  26   a  to provide ON/OFF state information for the one or more LEDs  22   a . However, in other embodiments, the switch  27   a  is located in a different location and/or the brightness controller  26   a  receives ON/OFF state information for the one or more LEDs  22   a  in a different manner. The brightness compensation system  20   a  is also connected to a power supply (not shown). The brightness compensation controller  26   a  is configured to send the signal to the LED brightness controller  24   a  based on the accumulated operating time of the one or more LEDs  22   a  which is determined by monitoring the ON/OFF state information provided by the switch  27   a  in this example. In this example embodiment, the signal is a digital signal, but in other embodiments the signal is an analog signal. As examples, the brightness controller  24   a  uses pulse width modulation in some embodiments and direct current control in other embodiments of the invention. In other embodiments of the invention, the brightness compensation controller  26   a  and the LED brightness controller  24   a  are combined. 
         [0020]      FIG. 1B  is a diagram of an LED brightness compensation system  20   b  that is similar to the system  20   a , but that also includes operating temperature compensation in accordance with an embodiment of the invention. In addition to having an LED brightness controller  24   b  in common with the system  20   a , the LED brightness compensation system  20   b  also includes a temperature sensor  28  for sensing the operating temperature of one or more LEDs  22   b . The temperature sensor  28  is in signal communication with a brightness compensation controller  26   b , which is similar to the brightness compensation controller  26   a  except that it is also configured to send a signal to control the LED brightness controller  24   b  based on the temperature sensed by the temperature sensor  28  in addition to the accumulated operating time of the one or more LEDs  22   b . In one embodiment, a switch  27   b  is connected between the LED brightness controller  24   b  and the anode of the first LED of the one or more LEDs  22   b . The switch  27   b  is also connected to the brightness compensation controller  26   b  to provide ON/OFF state information for the one or more LEDs  22   b . However, in other embodiments, the switch  27   b  is located in a different location and/or the brightness controller  26   b  receives ON/OFF state information for the one or more LEDs  22   b  in a different manner. The brightness compensation system  20   b  is also connected to a power supply (not shown). As for the system  20   a , in other embodiments of the invention, the brightness compensation controller  26   b  and the LED brightness controller  24   b  are combined. In one embodiment, the LED brightness compensation system  20   b  is used on an aircraft in association with exterior and/or interior LED lighting. 
         [0021]      FIG. 2  is a diagram showing additional detail for an example embodiment of the brightness compensation controller  26   b  shown in  FIG. 1B . The brightness compensation controller  26   b  includes a non-volatile memory unit  30  in data communication with a configuration interface  32  and a processor  36 . The processor is in signal communication with the LED brightness controller  24   b , the switch  27   b , and the temperature sensor  28 . The brightness compensation controller  26   b  also includes a clock  34  in communication with the processor  36 . The processor  36  includes volatile memory in some embodiments for temporary storage. Other embodiments also include a volatile memory unit (not shown) external to the processor  36 . The brightness compensation controller  26   b  is also connected to a power supply (not shown). In other embodiments, a programmable logic device having the required functionality is used. In still other embodiments, individual components are used to implement the brightness compensation controller  26   b . Although the configuration interface  32  is shown directly connected to the memory unit  30  in this embodiment, the configuration interface  32  is connected to the processor  36  in other embodiments such that the processor  36  can coordinate storage of information in the memory unit  30 . 
         [0022]    The configuration interface  32  is used to store correction factors (adjustment information) in the memory unit  30  based on previously conducted testing and/or simulation of one or more LEDs similar to the one or more LEDs  22   b  being used in the system  20   b . In some embodiments, the configuration interface  32  may be used to update the stored adjustment information as desired if improved LED data becomes available. In some embodiments, the configuration interface  32  may also be used to update algorithm control information stored in the memory unit  30  as well to be used by the processor  36  to change LED brightness models employed in the adjustment algorithm. As an example, the algorithm control information contains instructions for interpolating between stored adjustment information values in some embodiments. For example, the instructions contain information for a linear interpolation and/or a curve-fitting algorithm to be used by the processor  36 . The processor  36  generates a control signal that is presented at an output connected to the LED brightness controller  24   b . The control signal may be an analog voltage, a digital pulse width modulated signal, or a digital data signal, for example. The processor  36  generates the control signal based on the correction factors stored in the memory unit  30 , the signal from the temperature sensor  28 , and the signal from the clock  34 . In some embodiments, the clock  34  accumulates the operating time of the one or more LEDs  22   b  and presents the accumulated time to the processor  36 . In other embodiments, the clock  34  presents a signal containing time information without regard to the operating time of the one or more LEDs  22   b  and the processor  36  calculates and stores the accumulated operating time of the one or more LEDs  22   b  based on the time information contained in the signal from the clock  34  and ON/OFF state information for the one or more LEDs  22   b  received from the switch  27   b . 
         [0023]      FIG. 3A  is an example compensation chart showing LED energy adjustment data for the effect of operating time on LED brightness in accordance with an example embodiment of the invention. The values shown in the compensation chart would be determined by characterizing the response of a specific type of LED to be used. As an example, an LED could be characterized by placing the LED in a testing system that measures the change in energy required to be supplied to the LED to maintain constant brightness over the course of many hours of operation.  FIG. 3B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 3A  that is used in configuring an example embodiment of the invention. This information is loaded using the configuration interface  32  and stored in the non-volatile memory unit  30 . 
         [0024]      FIG. 4A  is a chart showing the LED energy adjustment data for the effect of operating temperature on LED aging in accordance with an example embodiment of the invention. The values in the chart indicate age acceleration and deceleration factors as they relate to operating temperature. For example, at 20 degrees Celsius, the aging adjustment is zero resulting in one hour of adjusted aging for every actual clock hour of operation. However, at 40 degrees Celsius, the aging will be 10% faster resulting in 66 minutes of adjusted aging for every actual clock hour of operation.  FIG. 4B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 4A  that is used in configuring an example embodiment of the invention. This information is loaded using the configuration interface  32  and stored in the non-volatile memory unit  30 . 
         [0025]      FIG. 5A  is a chart showing LED energy adjustment data for the instantaneous effect of operating temperature on LED brightness in accordance with an example embodiment of the invention. The values in the chart indicate immediate adjustments to the energy supplied to the LEDs to adjust for temperature dependent brightness changes. In an example embodiment, the operating temperature is monitored approximately once per second and adjustments are made accordingly. These adjustments help maintain constant LED brightness levels because LEDs get brighter at lower temperatures given the same energy input. Reducing current to LEDs at lower temperatures and increasing current at higher temperatures is useful to maintain constant LED brightness.  FIG. 5B  is a table showing LED energy adjustment data corresponding to the chart shown in  FIG. 5A  that is used in configuring an example embodiment of the invention. Based on this example table, at −20 degrees Celsius, a 20% reduction in energy would produce the same brightness that would exist at 20 degrees Celsius with constant energy. The information in the table is loaded using the configuration interface  32  and stored in the non-volatile memory unit  30 . 
         [0026]      FIG. 6  is a flowchart of a method  50  of compensating for the effects of operating time and operating temperature on LED brightness in accordance with an embodiment of the invention. The method  50  begins at a block  52  where an LED with characteristics similar to those of the one or more LEDs  22   b  is tested for the effect of operating time (aging) on brightness to obtain adjustment information correlated with operating time. Next, at a block  54  an LED with characteristics similar to the one or more LEDs  22   b  is tested for the effect of operating temperature on brightness to obtain adjustment information correlated with operating temperature. This testing includes the instantaneous effect of operating temperature on LED brightness as well as the effect of operating temperature on the rate of LED aging in some embodiments. Then, at a block  56 , the adjustment information correlated with operating time and/or temperature obtained in the blocks  52  and/or  54  is stored in a memory unit. Next, at a block  58 , one or more LEDs are energized with an electric current. Then, at a block  60 , the time the one or more LEDs have been energized is accumulated. This accumulating of energized time continues throughout the following steps of the method  50 . In this example embodiment, energized time is accumulated once per second. However, in other embodiments, time is accumulated at different rates such as once per minute, once per hour, once per millisecond, or at some other frequency for example. Next, at a block  62 , the operating temperature of the one or more LEDs is sensed. In this example embodiment, the operating temperature is sensed approximately once per second. However, in other embodiments the operating temperature is sensed at other frequencies. Then, at a block  64 , a control signal is determined and output based on the stored adjustment information, the accumulated time the one or more LEDs have been energized, and/or the sensed LED operating temperature. Following this, at a block  66 , the current supplied to the one or more LEDs is adjusted based on the control signal. In an example embodiment, the current is adjusted by changing the level of a constantly applied current. However, in other embodiments, the current is adjusted by pulse width modulation of a pre-determined current level. The method  50  then loops back to the block  60 . 
         [0027]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, the brightness compensation controller may be implemented using a microcontroller, a programmable logic device, or by using individual components. Additionally, different algorithms may be used in the brightness compensation controller to determine control signals for adjustment factors that occur between stored data values. For example, interpolations may be made using a linear fit based on the two closest data values, a curve could be fit using a commonly known statistical curve fitting formula based on the stored data values, or a constant correction may be applied in a step-wise fashion until the next data value is reached. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.