Abstract:
A system ( 100 ) for sensing a temperature of a light emitting diode (LED). The system may comprise an LED having a spectral output centered at a first wavelength, a first filter ( 104 ) that transitions from attenuation to transmission at about the first wavelength, and a second filter ( 106 ) that transitions from transmission to attenuation at about the first wavelength. The system may also comprise a first sensor ( 108 ) positioned to sense a first intensity of the LED through the first filter and a second sensor ( 110 ) positioned to sense a second intensity of the LED through the second filter. It will be appreciated that a single sensor may be substituted instead of the first and second sensors, provided that the single sensor is capable of selectively viewing the LED through the first and the second filters. The system may also comprise a computer ( 112 ) configured to derive a temperature of the LED considering the first intensity and the second intensity.

Description:
BACKGROUND 
       [0001]    For many years, Light Emitting Diodes (LED&#39;s) have provided an attractive alternative to traditional incandescent and fluorescent sources because of their small size and energy efficiency. In optical measurement systems, and other applications that benefit from consistent and/or predictable light sources, however, LED&#39;s have been slower to catch on. This is because the spectral output of an LED, both in terms of intensity and wavelength, varies greatly with temperature. Attempts have been made to control this variation, for example, by varying the current and/or voltage of LED&#39;s or even by heating the LED&#39;s prior to use. These methods, however, add additional complexity and expense and, for many applications, still fail to deliver an acceptable level of consistency. 
       SUMMARY 
       [0002]    In one general aspect, the invention is directed to a system for sensing a temperature of a light emitting diode (LED). The system may comprise an LED having a spectral output centered at a first wavelength, a first filter that transitions from attenuation to transmission at about the first wavelength, and a second filter that transitions from transmission to attenuation at about the first wavelength. The system may also comprise a first sensor positioned to sense a first intensity of the LED through the first filter and a second sensor positioned to sense a second intensity of the LED through the second filter. It will be appreciated that a single sensor may be substituted instead of the first and second sensors, provided that the single sensor is capable of selectively viewing the LED through the first and the second filters. The system may also comprise a computer configured to derive a temperature of the LED considering the first intensity and the second intensity. 
         [0003]    In another general aspect, the invention is directed to methods of determining a temperature of an LED. The methods may comprise the steps of sensing a first intensity of the LED through a first filter and sensing a second intensity of the LED through a second filter. The first filter may transition from attenuation to transmission at about a peak wavelength of the LED, and the second filter may transition from transmission to attenuation at about the peak wavelength of the LED. The methods may also comprise the step of calculating a temperature of the LED considering the first intensity and the second intensity. 
         [0004]    In yet another general aspect, the invention is directed to methods of calibrating a system for determining a temperature of a light emitting diode (LED). The methods may comprise the step of activating the LED at a first known temperature. The methods may also comprise the steps of sensing a first intensity of the LED through a first filter, and sensing a second intensity of the LED through a second filter. The first filter may transition from attenuation to transmission at about a peak wavelength of the LED, and the second filter may transition from transmission to attenuation at about the peak wavelength of the LED. The methods may also comprise the step of relating the first intensity and the second intensity to the first temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]    Embodiments of the present invention are described herein, by way of example, in conjunction with the following figures, wherein: 
           [0006]      FIG. 1  shows a diagram of a system for measuring the temperature of a light emitting diode (LED) according to various embodiments; 
           [0007]      FIG. 2A  shows a chart of various response curves of an LED at different temperatures; 
           [0008]      FIG. 2  shows a chart of the response curves of an LED and a pair of sensors according to various embodiments; 
           [0009]      FIG. 3  shows a flowchart of a process flow for calibrating a system for measuring the temperature of an LED according to various embodiments; and 
           [0010]      FIG. 4  shows a flowchart of a process flow for measuring the temperature of an LED according to various embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Embodiments of the present invention are directed to systems and methods for measuring the temperature of a light emitting diode (LED). LED temperatures measured according to various embodiments may be used for any suitable purpose. For example, the spectral output of an LED over a range of temperatures may be generalized based on models of the underlying physics and/or experimental characterizations. Accordingly, the measured temperature of an LED may be used to derive an indication of the LED&#39;s spectral output. Also, for example, the temperature of an LED taken near the time when the LED is first illuminated may indicate the ambient air temperature surrounding the LED. 
         [0012]      FIG. 1  shows a block diagram of a system  100  for measuring the temperature of a light emitting diode (LED)  102 . In addition to the LED  102 , the system  100  may include one or more sensors  108 ,  110 , with a pair of optical filters  104 ,  106  positioned between the LED  102  and the sensors  108 ,  106 . The sensors  108 ,  110  may sense the output of the LED  102  through the respective filters  104 ,  106 . Two sensors  108 ,  110  are shown, however, it will be appreciated that one of the sensors  108 ,  110  may be omitted, for example, if the remaining sensor is able to selectively view the output of the LED  102  through both of the filters  104 ,  106 . The system  100  may also comprise a computer  112  or other suitable processing device to store and analyze signals from the sensors  108 ,  110 . 
         [0013]    It will be appreciated that the spectral output of the LED  102  may vary with temperature in a predictable way. For example, the materials used to produce LED  102 , such as GaAs, GaN, etc., have inherent dispersive properties (e.g., dielectric constant, complex refractive index, etc.) that vary with both wavelength and temperature (dn/dT and dn/dλ). As a result, the LED  102  may exhibit behaviors that vary proportionately to λ 2 /Δλ, as the temperature, forward current and/or forward voltage change. Also, as temperature increases, the physical dimensions of LED&#39;s  105  dies may change. This, together with the changes in dispersion, may result in a net shift of the peak wavelength, λ, output, a decrease in light output, and a change in the bandwidth, Δλ, as the temperature of the LED&#39;s  105  change. 
         [0014]    For example,  FIG. 2A  shows a general shape of a model  250  of the spectral output of an LED, according to various embodiments. Three curves  252 ,  254  and  256  are shown representing the output of the LED at three different temperatures. The curve  252  shows the spectral output of the LED at a first temperature. The curve  254  shows the spectral output of the LED at a second temperature higher than the first. Finally, the curve  256  shows the LED at a third temperature lower than the first temperature. It can be seen that, generally, as temperature increases, the LED&#39;s spectral output may generally increase in bandwidth and decrease in intensity. 
         [0015]      FIG. 2  shows a diagram  200  of the spectral responses of the LED  102  and filters  104 ,  106  in conjunction with the corresponding sensor or sensors  108 ,  110 . Curve  202  represents the spectral output of the LED  102 , while curves  204  and  206  represent the spectral responses of the filters  104  and  106 , respectively. The filters  104 ,  106 , as shown by filter curves  204  and  206 , may be chosen to have adjacent or roughly overlapping attenuation bands at about the peak wavelength of the LED  102 , shown by LED curve  202 . In various embodiments, the LED  102  may be chosen with a nominal peak wavelength of 590 nm, while the filter  104  (curve  204 ) may be a green band-pass filter and the filter  106  (curve  206 ) may be a red band-pass filter. It will be appreciated, however, that the system  100  may include any suitable LED and filter combination. For example, a 505 nm LED and/or a 525 nm LED could be used in conjunction with blue and green filters. 
         [0016]    As described above, it will be appreciated that as the temperature of the LED  102  changes, the position and/or shape of the curve  202  will also change in a predictable way. For example, as the temperature of the LED  102  increases, the LED spectral output  202  may be shifted to a longer wavelength (to the right in the diagram  200 ). When this occurs, more of the LED&#39;s total output may be attenuated by filter  104  (curve  204 ), and more of the LED&#39;s total output may be passed by the filter  106  (curve  206 ). As the temperature of the LED  102  decreases, the opposite may occur. Accordingly, the peak wavelength of the LED  102 , and therefore its temperature, may be sensed by comparing the intensity of the LED  102  as viewed through filter  104  to the intensity of the LED  102  as viewed through filter  106 . 
         [0017]      FIG. 3  shows a process flow  300 , according to various embodiments, illustrating a method for calibrating the temperature measuring system  100 . It will be appreciated that the steps of the process flow  300  may be performed in any suitable order, and that some or all of the steps may be performed simultaneously. At step  302 , the LED  102  may be activated at a first known temperature. The intensity of the LED  102  through the filter  104  may be measured at step  304 . The intensity of the LED  102  through the filter  106  may be measured at step  306 . At step  308 , the computer  112  may create or supplement a model relating the temperature of the LED  102 , the intensity of the LED  102  through the filter  104  and the intensity of the LED  102  through the filter  106  (e.g., the model shown above at  FIG. 2A ). In various embodiments, creating or supplementing the model may involve calculating one or more coefficients matching the observed intensities to the model. In various embodiments, the computer  112  may also explicitly solve for the peak wavelength of the LED  102 . 
         [0018]    At decision step  310 , the computer may determine whether additional measurements will be taken to further supplement the model. If an additional measurement is desired, the temperature of the LED  102  may be changed at step  312 . For example, the temperature of the LED  102  may be varied by allowing it to be activated for a given period of time, activating additional LED&#39;s near the LED  102 , etc. The process may then continue with step  304  as described above. It will be appreciated that one measurement may be sufficient to develop the model, however, additional measurements may improve the accuracy of the model. Also, taking measurements over a broad range of temperatures or other operating conditions may allow the model to compensate for nonlinearities in LED heating behavior, the effects of additional LED&#39;s (not shown) near the LED  102 , etc. 
         [0019]      FIG. 4  shows a process flow  400 , according to various embodiments, for measuring the temperature of the LED  102  using the system  100 . At step  402 , the LED  102  may be activated. The intensity of the LED  102  through the filter  104  may be measured at step  404 , and the intensity of the LED  102  through filter  106  may be measured at step  406 . It will be appreciated that the respective intensities of the LED  102  through filters  104  and  106  may be measured near the time that the LED  102  is activated, or at any time thereafter. At step  408 , the first and second intensities of the LED  102  may be used to calculate a temperature of the LED  102 , for example, according to a model generated as described above. The temperature of the LED  102  may then be used in any suitable way, for example, as described above. In various embodiments, the LED  102  may be part of an array of LED&#39;s positioned in close proximity to one another. It will be appreciated that, in this case, other LED&#39;s included in the array may be assumed to have the same temperature as the LED  102 . This assumption is likely to be more accurate where all of the LED&#39;s in the array are activated for similar amounts of time under similar conditions. 
         [0020]    It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating, for purposes of clarity, other elements, such as, for example, details of various physical models of LED&#39;s, etc. Those of ordinary skill in the art will recognize that these and other elements may be desirable. However, because such elements are well known in the art and because they do not facilitate a better understanding of the present invention, a discussion of such elements is not provided herein. 
         [0021]    As used herein, a “computer” or “computer system” may be, for example and without limitation, either alone or in combination, a personal computer (PC), server-based computer, main frame, server, microcomputer, minicomputer, laptop, personal data assistant (PDA), cellular phone, pager, processor, including wireless and/or wireline varieties thereof, and/or any other computerized device capable of configuration for processing data for standalone application and/or over a networked medium or media. Computers and computer systems disclosed herein may include operatively associated memory for storing certain software applications used in obtaining, processing, storing and/or communicating data. It can be appreciated that such memory can be internal, external, remote or local with respect to its operatively associated computer or computer system. Memory may also include any means for storing software or other instructions including, for example and without limitation, a hard disk, an optical disk, floppy disk, ROM (read only memory), RAM (random access memory), PROM (programmable ROM), EEPROM (extended erasable PROM), and/or other like computer-readable media. 
         [0022]    The computer  112  may operate according to software code to be executed by a processor(s) of the computer  112  or any other computer system using any type of suitable computer instruction type. The software code may be stored as a series of instructions or commands on a computer readable medium. The term “computer-readable medium” as used herein may include, for example, magnetic and optical memory devices such as diskettes, compact discs of both read-only and writeable varieties, optical disk drives, and hard disk drives. A computer-readable medium may also include memory storage that can be physical, virtual, permanent, temporary, semi-permanent and/or semi-temporary. A computer-readable medium may further include one or more data signals transmitted on one or more carrier waves. 
         [0023]    While several embodiments of the invention have been described, it should be apparent that various modifications, alterations and adaptations to those embodiments may occur to persons skilled in the art with the attainment of some or all of the advantages of the present invention. It is therefore intended to cover all such modifications, alterations and adaptations without departing from the scope and spirit of the present invention as defined by the appended claims.