Abstract:
The stabilization of a light-emitting diode (LED) calibration standard includes a light-emitting diode (LED), or an array of LEDs; a cylindrical hood surrounding the LED; an interior baffle for keeping the light output of the LED, and ambient light from behind the LED, from escaping to the other side; a photodetector for receiving the light output of the LED and generating a signal proportional to luminous output; and a hood surrounding the photodetector. A variable current source receives the signal and stabilizes the LED light output by adjusting the operating current of the LED to maintain a constant light output from the LED.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) to provisional patent application U.S. Ser. 61/807,659, filed on Apr. 2, 2013. Said application is hereby incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present disclosure relate generally to optical metrology systems and particularly to systems and methods for calibrating metrology systems utilizing light emitting diodes (LED). 
     BACKGROUND 
     A light-emitting diode (LED) standard for calibrating a lighting metrology system must first be calibrated for total luminous flux, chromaticity, or other properties and then kept stable. This is generally done via stabilization of the operating current and temperature of the LED. However, this process cannot guarantee stable light output, which may fluctuate during the necessary seasoning process (typically by 5-10% over the first 1,000-2,000 hours of life) and decline during the normal useful life of the LED (typically by 15-30% over ˜50,000 hours). Such variations far exceed the generally acceptable 1-3% level of uncertainty required for calibration standards. 
     To mitigate these instabilities, users generally season the LED for ˜2,000 hours before using the standard for calibrations. Users may additionally recalibrate the LED after every ˜100 hours of use, which can result in significant development and production delays. Furthermore, the LED generally requires a short period of stabilization after being activated, from several minutes to tens of minutes. While closed-loop active temperature control has attempted to mitigate this problem, the LED is still unusable during this warm-up period, which results in further short-term delays in equipment calibration and general productivity. It may therefore be desirable to accelerate stable operation of the LED in the short term. It may also be desirable to ensure stable long-term operation of the LED while eliminating delays associated with the seasoning and recalibration processes. 
     SUMMARY 
     Embodiments of the present invention concern a method and apparatus for stabilizing at least one light-emitting diode (LED) standard for calibrating a lighting metrology system. In one embodiment, an LED standard may include total luminous flux, total radiant flux, luminous intensity, radiant intensity, chromaticity, or other properties. In one embodiment, an LED assembly includes an LED and a photodetector configured to receive a portion of the luminous output of the LED. In one embodiment, the photodetector may generate an output signal proportional to the luminous output of the LED. In one embodiment, the output signal may then be sent to a controllable current source, and the control loop may adjust the operating current of the LED to maintain a constant output signal from the photodetector. In some embodiments, the LED assembly may include an array of LEDs or LED luminaires controlled by a single control loop, or multiple LEDs each controlled by an individual control loop. 
     In one embodiment, the LED and photodetector are mounted into a hood configured to wholly or partially surround the LED and the photodetector. In one embodiment, the hood is generally cylindrical in shape and equipped with a circular opening at its top basal surface through which a dome lens of the LED may protrude. In one embodiment, the interior of the hood may include a baffle configured to prevent ambient light from behind the LED from entering the front side of the LED and to prevent the light output of the LED from escaping. In preferred embodiments, the interior of the hood and the baffle are colored white for maximum reflectivity without changing the color of visible light. The photodetector may be mounted so as to directly face the LED and receive a portion of its light output through free space; a lens, filter, or aperture may be positioned between the LED and photodetector to manipulate the light output. In some embodiments, the photodetector may be mounted to face in a different direction or in a different location (e.g., behind the baffle) and the light output directed to the photodetector by way of mirrors, prisms, optical fibers, light guides, or other like optical devices. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the invention may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1A  is a diagrammatic illustration of an embodiment of an LED assembly according to the invention; 
         FIG. 1B  is a diagrammatic illustration of an embodiment of an LED assembly according to the invention; 
         FIG. 1C  is a diagrammatic illustration of an embodiment of an LED assembly according to the invention; 
         FIG. 1D  is a diagrammatic illustration of an embodiment of an LED assembly mounted to an integrating sphere; and 
         FIG. 2  is a process flow diagram of a method of stabilizing the output of an LED assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Features of the present invention in its various embodiments are exemplified by the following descriptions with reference to the accompanying drawings, which describe the present invention with further detail. These drawings depict only selected embodiments of the present invention, and should not be considered to limit its scope in any way. 
       FIGS. 1A through 1D  illustrate embodiments of an LED assembly  100  according to the present invention. Referring to  FIG. 1A , in one embodiment, LED assembly  100  includes an LED  102  mounted in LED hood  106 . For example, LED hood  106  may include a Φ25 mm cylinder with an open front end. In one embodiment, LED  102  may include dome lens  104 . In one embodiment, LED assembly  100  may include a baffle  108 . In one embodiment, LED  102  may be mounted near flush with the opening of LED hood  106  so that nearly all luminous output  120  in the forward 2π solid angle freely exits LED hood  106 . 
     In one embodiment, a portion of luminous output  120 , at a nearly 90° angle to normal, is blocked by LED hood  106 . A portion of this light output  120  may then be received by photodetector  110 . In one embodiment, photodetector  110  may be mounted below the opening of LED hood  106 , facing LED  102 . In one embodiment, baffle  108  may be configured to divide the interior of LED hood  106  into a front portion, wherein LED  102  and photodetector  110  may be mounted, and a rear portion behind LED  102 . In one embodiment, baffle  108  may be further configured to prevent any ambient light from entering the front portion of the interior of LED hood  106 . In one embodiment, baffle  108  may be further configured to prevent any luminous output  120  from escaping from the front portion of the interior of LED hood  106  into the rear portion. In one embodiment, the interior surfaces of both LED hood  106  and baffle  108  may include a highly reflective coating (e.g., a white coating) to promote reflection of all visible wavelengths of light into the vicinity of LED  102  without changing the color of the light. In one embodiment, photodetector  110  may be configured to output a signal  114  which may control the operating current of LED  102 . In one embodiment, controllable current source  118  of LED assembly  100  may pre-select a target signal value so that the control loop may continually adjust the operating current of LED  102  to maintain a constant photodetector output signal  114 , resulting in a constant luminous output  120 . In one embodiment, LED assembly  100  may additionally include a signal amplifier  116  for receiving signal  114  and passing the amplified signal to controllable current source  118 . In some embodiments of LED assembly  100 , a single control loop may control the operating current for multiple LEDs. In another embodiment, the LED assembly too may include multiple control loops, each controlling the operating current for an individual LED  102 . 
     Referring to  FIG. 1B , in one embodiment, LED assembly too may include at least one aperture or filter  122  mounted in front of the photodetector  110 , configured to attenuate the light output  120  of LED  102  or to alter the spectrum of light output  120  so that photodetector  110  operates in its normal range of luminous intensity and wavelength. In one embodiment, photodetector  110  may be fitted with an optical filter with spectral transmittance, configured to produce a spectral response of the photodetector/filter combination equivalent to that of the human eye. In another embodiment, LED assembly too may include at least one lens configured to manipulate the light emission profile of LED  102  or the light collection profile of photodetector  110 . Referring to  FIG. 1C , in other embodiments photodetector  110  may be configured to face in other directions and mounted in other locations (e.g., beyond the baffle  108 , in the rear portion of the interior of LED hood  106 ), with light output  120  directed from one or more LEDs  102  to the photodetector  110  via one or more mirrors  126 , prisms, optical fibers, light guides or other optical devices. 
     Referring back to  FIG. 1A , in one embodiment LED  102  and photodetector  110  may be mounted to temperature controlled heat sinks  128  to ensure they operate at a constant temperature. In one embodiment, LED  102  and photodetector  110  may share heat sink  128 . In other embodiments, LED  102  and photodetector  110  may be mounted to individual heat sinks. In one embodiment, photodetector  110  may include a photodetector hood  112  configured to prevent any light other than the luminous output  120  of LED  102  from entering the photodetector  110 . In one embodiment, photodetector hood  112  may be cylindrical in shape with an open bottom, and include surfaces coated with reflective material (similarly to LED hood  106  and baffle  108 ). 
       FIG. 1D  illustrates an embodiment of an optical radiation metrology system  130  including an LED  102  with dome lens  104  mounted to the interior of integrating sphere  131 . In one embodiment, LED hood  106  may be used as a coupler between LED  102  and a port  132  of an integrating sphere  131  when stabilizing the output of LED  102  prior to testing. In one embodiment, LED hood  106  may include a key and stopper (not shown) for ensuring constant orientation and amount of insertion into the port of the integrating sphere. In one embodiment, integrating sphere  131  may include baffles  134   a  and  134   b  and auxiliary lamp  138 . In one embodiment, a spectrometer, or other photometric device  136  may be connected to the integrating sphere  131  to measure the light output of a light source under test after calibration of the system  130 . The target light source to be measured may include an LED, an arc lamp, a plasma light source, an incandescent light source, a fluorescent light source, a laser, or another light source mounted to port  132  inside integrating sphere  131 . 
     In one embodiment, the long-term stability of LED assembly too may be made dependent on the long-term stability of photodetector  110  rather than on the long term stability of LED  102 . As the long-term stability of photodetector  110  is generally better than 1%, as opposed to the long-term stability of LED  102  operating under constant current and temperature (as discussed above), in one embodiment the long-term stability of LED assembly  100  may be improved by at least a factor of ten. In one embodiment, in addition to longer operational lifespan, the improved stability of the LED assembly too may prevent the need for frequent recalibration, reducing both ownership/operation cost and downtime. In one embodiment, LED assembly too may stabilize faster in the short term once activated, particularly where LED  102  and photodetector  110  are independently temperature controlled, and the luminous output  120  of the LED  102  may be stabilized before thermal equilibrium of LED  102  is established. In one embodiment the ˜2,000 hour (nearly three months) seasoning process may be eliminated, along with corresponding delays in productivity. 
       FIG. 2  illustrates a process flow diagram of a method  200  for stabilizing the output of at least one light-emitting diode (LED) mounted in an assembly according to an embodiment of the present invention. It is noted herein that the method  200  may be carried out utilizing any of the embodiments described previously. It is further noted, however, that method  200  is not limited to the components or configurations described previously as multiple components and/or configurations may be suitable for executing method  200 . 
     At step  210 , the method  200  directs a portion of the output of the at least one LED to at least one photodetector mounted in the LED assembly. In one embodiment, the output of the at least one LED is directed to the at least one photodetector through free space. In another embodiment, the output of the at least one LED is directed to the at least one photodetector through at least one of a lens, an aperture, and a filter. In another embodiment, the output of the at least one LED is directed to the at least one photodetector through an optical filter configured to convert the spectral response of the photodetector to substantially the spectral response of a human eye. In another embodiment, the output of the at least one LED is directed to at least one photodetector mounted in the assembly through at least one of a mirror, a prism, an optical fiber, and a light guide. 
     At step  220 , the method  200  generates at least one signal corresponding to the output of the at least one LED via the at least one photodetector. At step  230 , the method  200  receives the at least one signal via a variable current source operably coupled to the at least one LED. At step  240 , the method  200  adjusts the current source to maintain a constant signal from the at least one photodetector. In one embodiment, the method  200  receives a preselected target value for the signal and maintains the received signal at the target value by adjusting the operating current of the at least one LED via a controllable current source. In another embodiment, the method  200  manually sets a preselected target value for the signal. 
     Those having skill in the art will appreciate that there are various vehicles by which processes and/or systems and/or other technologies described herein may be effected (e.g., hardware, software, and/or firmware), and that the preferred vehicle will vary with the context in which the processes and/or systems and/or other technologies are deployed. For example, if an implementer determines that speed and accuracy are paramount, the implementer may opt for a mainly hardware and/or firmware vehicle; alternatively, if flexibility is paramount, the implementer may opt for a mainly software implementation; or, yet again alternatively, the implementer may opt for some combination of hardware, software, and/or firmware. Hence, there are several possible vehicles by which the processes and/or devices and/or other technologies described herein may be effected, none of which is inherently superior to the other in that any vehicle to be utilized is a choice dependent upon the context in which the vehicle will be deployed and the specific concerns (e.g., speed, flexibility, or predictability) of the implementer, any of which may vary. Those skilled in the art will recognize that optical aspects of implementations will typically employ optically-oriented hardware, software, and or firmware. 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, different other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures may be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality may be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “connected”, or “coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “couplable”, to each other to achieve the desired functionality. Specific examples of couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components. 
     While particular aspects of the present subject matter described herein have been shown and described, it will be apparent to those skilled in the art that, based upon the teachings herein, changes and modifications may be made without departing from the subject matter described herein and its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as are within the true spirit and scope of the subject matter described herein.