Abstract:
A method and apparatus are described for driving a modulated radiation source (which can be, for example, an infrared light source). The method affects the power driving a light source in such as way so as to minimize the warm-up time of the source. The apparatus permits feedback control of a light source to specified powers or temperatures. Disclosed embodiments can improve source performance and lifetime and decrease the operating costs of the source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/809,937, filed Jun. 1, 2006, and U.S. Provisional Application No. 60/855,059, filed Oct. 27, 2006. The entirety of each of these applications is hereby incorporated herein by reference and made part of this specification. 
     
     BACKGROUND 
       [0002]    1. Field 
         [0003]    Described embodiments generally relate to radiation sources and to methods and systems for powering light sources. 
         [0004]    2. Description of the Related Art 
         [0005]    Existing radiation sources do not always provide a high enough radiative output. Moreover, existing sources are not always stable, and they do not always operate for a long enough period of time without failing. Improvements are also needed in simplicity and cost of manufacture, as well as compatibility with available radiators. 
         [0006]    Infrared light is absorbed by many types of organic molecules, and thus infrared light sources are used in a variety of spectroscopic applications. One type of infrared light source includes a resistively-heated element, referred to herein, and without limitation, as a heater element, that emits infrared radiation when resistively heated. Heater elements may be free-standing filaments or thin layers deposited on a substrate. An electric current can be provided to the heater element, which heats up due to the dissipation of electric power within the element. 
         [0007]    Heater elements of infrared light sources can be connected to a power source through electrical conductors, such as wires, and may also be in thermal contact with other components of the light source. The connection to the electrical conductors, as well as contact with other components, can act as a conduit for heat (e.g., as a heat sink) to or from the heater element. If the environment is cooler than the peak element temperature, heat flows from the heater element, resulting in a lower element temperature. 
         [0008]    The heater element of a resistively-heated infrared light source radiates at a rate that depends, in part, on the heater element temperature. In general, the radiative output and lifetime of a resistively-heated infrared light source depend on the heater element temperature: a higher temperature results in greater radiative output and, usually, a reduced lifetime. As an example, in some sources the radiative output which increases with the fourth power of heater element temperature and the source lifetime decreases exponentially with peak heater element temperature. 
         [0009]    Infrared light sources can be modulated by being operated continuously at some duty cycle, for example. For example, a source can be driven by a current that is modulated at a 50% duty cycle. The light source is thus powered half the time and is off the other half. In response to the modulated current, the heater element temperature follows the driving current, with an average power that is half the peak power. 
         [0010]    Some light sources are used intermittently—for example, the light is turned on only when needed. The power supplied to the light source cycles between an “on” period of time, when light is needed for some purpose, and an “off” period of time, when no power is provided. The power in the “on” period may be steady or may be a waveform with a repetitive shape, such as a square wave or sine wave. When starting a modulated infrared light source, or when ramping up the power from one average power level to another average power level, the light source output may slowly increase or decrease with time. The unsteady nature of the light source output is problematic when the source is needed for quantitative measurements. 
         [0011]    Thus there is a need for a method and apparatus that provides an infrared source that can operate at high radiative output. The method and apparatus preferably can provide stable light output and provide operation for long periods of time without failure. The apparatus is preferably be simple and inexpensive to manufacture and compatible with currently available infrared radiators. Embodiments disclosed herein fulfill some or all of these needs. 
       SUMMARY 
       [0012]    Certain embodiments provide improved operation of modulated light sources by controlling the power dissipated within the light source during both “on” and “off” periods of time. 
         [0013]    Certain embodiments provide a method for operating a light source including providing power to the light source at a first power during a first time period and a second power during a second time period, where the first power is a constant, non-zero power, and where the second power is a non-steady power. The method may further include obtaining a measurement of the light source, where the measurement has a target value, and providing the power to the light source according to a difference between the measurement and the target value. 
         [0014]    Certain embodiments provide a method for operating a light source comprising providing power to the light source at a first power during a first time period and a second power during a second time period, where the first power is a constant, non-zero power, where the second power is a non-steady power, and obtaining a measurement of the light source, where the providing compares the measurement with a target value. 
         [0015]    Certain embodiments provide an apparatus for operating a light source comprising a circuit to provide power to the light source at a first power during a first time period and a second power during a second time period subsequent to the first time period, where the first power is approximately constant and non-zero, and where the second power is intermittent. The apparatus may further include a sensor to obtain a measurement of the light source; and an electric circuit to control the power according to the measurement. 
         [0016]    These features together with the various ancillary provisions and features which will become apparent to those skilled in the art from the following detailed description, can be attained by embodiments shown in the accompanying drawings, by way of example only, wherein: 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic of an infrared light source; 
           [0018]      FIG. 2  is a functional schematic diagram of an embodiment of an infrared light source; 
           [0019]      FIG. 3  is a schematic wiring diagram of a circuit that operates as a light source of  FIG. 1  or  2 ; 
           [0020]      FIG. 4  is one embodiment of a control module of  FIG. 3 ; 
           [0021]      FIG. 5  is a sectional schematic illustrating a thermally driven, infrared radiator; 
           [0022]      FIG. 6  is a graph illustrating one embodiment of power delivered to an infrared light source as a function of time; and 
           [0023]      FIGS. 7 and 8  are graphs illustrating controls to a power supply to provide the power curve of  FIG. 6 . 
       
    
    
       [0024]    Reference symbols are used in the Figures to indicate components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein. 
       DETAILED DESCRIPTION 
       [0025]    Although some preferred embodiments and examples are disclosed below, it will be understood by those skilled in the art that the inventive subject matter extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses and obvious modifications and equivalents thereof. Thus it is intended that the scope of the inventions herein disclosed should not be limited by the particular disclosed embodiments described below. Thus, for example, in any method or process disclosed herein, the acts or operations making up the method/process may be performed in any suitable sequence, and are not necessarily limited to any particular disclosed sequence. Also, for example, various functions may be performed in one or a combination of devices. 
         [0026]      FIG. 1  is one embodiment of a light source  100  that may be operated to generate electromagnetic radiation E. Light source  100  includes a power source  110  that is electrically connected to drive an infrared electromagnetic emitter  120 . Electromagnetic radiation E is generated within emitter  120  in response to power source  110 —that is, emitter  120  accepts electrical signals from power source  110 , and converts the accepted power into electromagnetic radiation E. Electromagnetic radiation E may include, but is not limited to, light in the ultraviolet, visible, and/or infrared portions of the electromagnetic radiation spectrum. In one embodiment, emitter  120  emits electromagnetic radiation E as thermal radiation in proportion to the emitter temperature. Emitter  120  can be, for example, an emitter having a heater element that is resistively heated and which emits according to the instantaneous temperature. 
         [0027]      FIG. 2  depicts another embodiment of the light source  100 , which may be generally similar to the embodiment illustrated in  FIG. 1 , except as further detailed below. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1 and 2 . 
         [0028]    The light source  100  of  FIG. 2  includes a power controller  210 , a power unit  220 , and an emitter diagnostics  240 . Power controller  210 , power unit  220  and diagnostics  240  provide a control system for operating emitter  120 . In some embodiments, diagnostics  240  measure performance parameters of emitter  120 , which may be, but are not limited to, a power dissipated within light source  100  or a temperature measurement of the light source. Diagnostics  240  may include, but is not limited to, circuitry which measures the voltage across and current flowing through emitter  120 , thus permitting the dissipated power to be determined, or a thermocouple in thermal contact with the emitter. Power controller  210  compares the measurements to target values, and provides power to emitter  120  to maintain or approach the targeted values. 
         [0029]    In some embodiments, diagnostics  240  measures one or more properties related to the operation of emitter  120 , and transmits the measures as a control measurement CM to power controller  210 . Power controller  210  accepts control measurement CM, compares the value of the control measurement to a target value, and generates a control error CE that is transmitted to power unit  220 . Power unit  220  in turn provides an electrical power signal P that drives emitter  120 . In some embodiments, light source  100  is controlled to achieve a dissipated power versus time profile. In another embodiment, light source  100  is controlled to achieve a light source temperature. 
         [0030]    In some embodiments, emitter  120  is an electrical resistance-type radiator.  FIG. 5  is a sectional schematic of an electrical resistance-type emitter  520 , which may be generally similar to emitter  120 , except as further detailed below. 
         [0031]    Emitter  520  includes a heater element  521 , a housing  523 , a support  525 , an opening  527 , and electrical leads  501  and  503 . Heater element  521  is the portion of emitter  520  that emits electromagnetic radiation E, and can be for example, a wire filament, or can be a thin film on a backing. Heater element  521  is supported within housing  532  by support  525 , which is either conducting or includes a conducting portion to provide electrical contact with leads  501 ,  503 . Housing  523  provides protection for heater element  521 , a structure for mounting emitter  520 , and opening  527  to direct radiation from emitter  520 . In alternative embodiments, opening  527  has a covering to further protect heater element  521  and which may or may not be shaped to act as a lens to focus electromagnetic radiation E. In use as emitter  120 , leads  501 ,  503  are shown as leads  120   a ,  120   b . Lead  120   a  is connected to power signal P, and lead  120   b  is connected, directly or indirectly, to ground. 
         [0032]    Heater element  521  can have a low thermal mass (that is, it can heat up rapidly). Housing  523  is in thermal contact with heater element  521  and may affect the operation of emitter  520  by providing a thermal mass and acting as a heat sink for heater element  521 . Thus, heat from heater element  521  is transported to housing  523  at a rate that depends on the housing and radiator base temperature. Housing  523  may also affect the operation of emitter  520  by radiating at a temperature that is both different from, and which responds at a different time response than, heater element  521 . 
         [0033]    Examples of emitter  520  include, but are not limited to, the IR Source manufactured by Axetris, the microsystems division of Leister (Leister Technologies, LLC, Itasca, Ill.) or the pulse IR Emitter manufactured by Boston Electronics Corporation (Brookline, Mass.). 
         [0034]      FIG. 3  is a schematic wiring diagram  300  of a circuit that operates as a light source which may be generally similar to light source  100  of  FIG. 1  or  2 , except as further detailed below. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1 ,  2 , and  3 . 
         [0035]    Power controller  210  includes control module  311  and a differential amplifier  313 . Power controller  210  accepts a control measurement CM, which is a measure of the electric power P(t) dissipated in emitter  120 , which may be an emitter  520 , and produces a control error CE, which is a signal e(t) that is used to power the emitter. More specifically, control module  311  generates a signal Ps(t) that is a target for the measured dissipated electric power P(t). The signals Ps(t) and P(t) are provided to differential amplifier  313 , which has an output e(t) that is proportional to the difference between the target and measured power, that is: 
         [0000]        e ( t )= K 1( Ps ( t )− P ( t )),  (1) 
         [0000]    where K 1  is a constant of the differential amplifier  313 . 
         [0036]      FIG. 4  is one embodiment of a control module  311 , which may be generally similar to the embodiment illustrated in  FIG. 1 ,  2 , or  3 , except as further detailed below. Where possible, similar elements are identified with identical reference numerals in the depiction of the embodiments of  FIGS. 1 ,  2 ,  3 , and  4 . 
         [0037]    Control module  311  includes a modulation control module  401 , a lamp power set point module  403 , and a switch  405 . Switch  405  is responsive to a control input  405   a  that connects one of two inputs  405   b  and  405   c  with an output  405   d . Thus, for example, if input  405   b  is provided to output  405   d  for a control input  405   a  greater than or equal to a voltage Vset, and if input  405   c  is provided to output  405   d  for a control input  405   a  less than a voltage Vset, then the voltage Ps(t) is given by: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0038]    With reference to  FIG. 3 , the control error CE, which may be, for example, the error signal of Equation 1, is provided to power unit  220 , which generates a power signal P, such as a time varying voltage V(t). In the embodiment of  FIG. 3 , power unit  220  includes a power supply  321  and a control element  323 . Control element  323  is, but is not limited to, a linear control element, a linear pass transistor, or a modulated switch. Control element  323  accepts the output from power supply  321  and preferably generates a power signal that is directly proportional to the error signal, as V(t)=K 2 e(t), where K 2  is a constant of control element  323 . Substituting in Equation 1 gives 
         [0000]        V ( t )= K 1* K 2*( Ps ( t )− P ( t )),  (3) 
         [0000]    that is, the power signal is proportional to the difference between a desired set point and a measured set point value. Emitter  120  is connected to power signal P and emits electromagnetic radiation E in response the time varying signal V(t). 
         [0039]    The embodiment of diagnostics  240  shown in  FIG. 3  includes a resistive element  341  having a first end  341   a  and a second end  341   b , operational amplifiers  343  and  345 , and a multiplying circuit  347 . Diagnostics  240  measures the voltage and current across emitter  120  and multiplies the voltage and current to generate a signal P(t) that is a measure of the power consumption in the emitter, which is the power dissipated in the light source. More specifically, the voltage difference across emitter  120  is provided as input to amplifier  343 , generating a signal proportional to the voltage difference. The non-powered lead of emitter  120  is connected to ground through resistive element  341 , and the voltage across ends  341   a  and  341   b , provided as input to amplifier  345  generates a signal proportional to the current through emitter  120 . 
         [0040]      FIG. 6  is a graph of one embodiment of a targeted modulated power for operating a light source  120 . The power curve of  FIG. 6  may, for example, be obtained by the operation of light source  100 , and is a measure, for example, of the power dissipated within light source  120 . The power set point Ps(t) fluctuates between an “on” period and a “stand by” period. During the “on” period, Ps(t) is an oscillating, square wave power pulse having amplitude that varies from a minimum value of 0 to a maximum value of Pmax, with a frequency f. In an alternative embodiment, the minimum value is Pmin, which is greater than zero. During the “stand by” period, Ps(t) is a constant value of Psb. The power curve of  FIG. 6  can be achieved using light source  100  and the values of Pset(t) shown in  FIG. 7  and the values of Sw(t) shown in  FIG. 8 . 
         [0041]    In some embodiments, the effect of the value of Psb can be described as follows. As the value of Psb is increased from zero, fluctuations in the housing temperature will diminish. At a value of Psb that approximately corresponds to an average power dissipation during the “on” period and “stand by” period, the temperature during these two periods will be approximately constant, greatly minimizing temperature variation. Thus, providing power according to  FIG. 6  can result in a light source housing temperature that changes little with time. In addition, this results in a heater element that is only operated at peak temperature when needed, and thus also increases the heater element lifetime. 
         [0042]    In some embodiments, the values of Psb, Pmax, Pmin, and the “on” and “stand by” periods of time are predetermined such that the average power dissipation within the light source is the same during the “on” and “stand by” time periods, resulting in an approximately constant light source temperature. In another embodiment, the values of Pmax, Pmin, and the “on” and “stand by” periods of time are determined by required energy E, and the value of Psb is selected to minimize the temperature variation of the light source. In yet another embodiment, the value Psb is modified based on temperature measurement of the light source. 
         [0043]    Thus, for example, if infrared light is needed for 5 minutes every 15 minutes, then it would be expected that the source lifetime should be at least 3 times longer than if driven continuously at some duty cycle. It has been found that, in practice, the increase in source lifetime greatly exceeds this estimate. One likely explanation is that periodically operating the heater element at a moderate temperature anneals the element or other light source components, reversing damage done during high temperature operation and resulting in a much improved lifetime. 
         [0044]    The operation of light source  100  may further be modified to heat the light source from a cold start. Thus, for example, during instrument warm-up, the source can be driven at perhaps 80% of the peak power level with 100% duty cycle to accelerate heating of the source heat sink. The timing of this would have to be determined by experiment. 
         [0045]    In addition, if there are one or two wavelengths which are especially sensitive to signal-to-noise, the source may be overdriven during the measurement of those wavelengths as long as the total overdrive time is small compared to the measurement cycle. 
         [0046]    In an alternative embodiment, diagnostics  240  includes a thermocouple and associated circuitry for measuring the temperature of a part of emitter  120 , and a power controller  210  includes circuitry for maintaining a constant temperature. Thus, for example, in some embodiments, the power controller  210  adjusts Psb so that the “on” and “standby” temperatures are approximately constant. 
         [0047]    In yet another alternative embodiment, the light source power is controlled such that an average light source housing temperature including, but not limited to the temperature of housing  523 , in an “on” period is the same or approximately equal to the average light source temperature in a “stand by” period. In either case, control can be obtained by measuring either the light source power or housing temperature. 
         [0048]    The systems, methods, and devices described above can be used to drive a radiation source in a spectroscopic device, which can be incorporated into a medical device, for example. Thus, in some embodiments, the systems, methods, and devices described above can be used with the devices, systems, and methods described in the context of analyte detection and/or quantification in: U.S. Patent Publication No. 2007/0103678, published May 10, 2007 (Atty. Docket No. OPTIS.150A); U.S. patent application Ser. No. 11/734,261, filed Apr. 11, 2007 (Atty. Docket No. OPTIS.165A); and U.S. Provisional Patent Application No. 60/939,023, filed May 18, 2007 (Atty. Docket No. OPTIS.184PR). The entirety of each of the documents listed in this paragraph is hereby incorporated herein and made part of this specification. 
         [0049]    It will be understood that the steps of methods discussed are performed in some embodiments by an appropriate processor (or processors) of a processing (i.e., computer) system executing instructions (code segments) stored in appropriate storage. It will also be understood that the disclosed methods and apparatus are not limited to any particular implementation or programming technique and that the methods and apparatus may be implemented using any appropriate techniques for implementing the functionality described herein. The methods and apparatus are not limited to any particular programming language or operating system. In addition, the various components of the apparatus may be included in a single housing or in multiple housings that communication by wire or wireless communication. 
         [0050]    Reference throughout this specification to “one embodiment,” “an embodiment,” or “some embodiments,” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments. 
         [0051]    Similarly, in the above description of exemplary embodiments, various features of the inventions are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that any claim require more features than are expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment.