Abstract:
A system for non-invasive measurement of a substance, such as glucose, includes a detector configured to sense radiation and an optical subsystem configured to focus the radiation on a sensitive area of the detector. The system includes one or more temperature sensors attached to one or more of a plurality of elements of the optical subsystem and to the detector and two or more temperature sensors configured to measure two or more respective ambient temperatures. The one or more temperature sensors are configured to measure the temperature of the one or more elements of the optical subsystem and the temperature of the detector. A method of measuring a concentration includes detecting an infrared radiation value, measuring the temperature of the detector, one or more components of the optical system, and two or more ambient temperatures, and correlating the temperatures with calibration parameters to correct the detected infrared radiation value.

Description:
RELATED APPLICATION DATA 
       [0001]    The present application is a continuation-in-part of U.S. application Ser. No. 12/607,903, filed Oct. 28, 2009, entitled APPARATUS AND METHOD FOR NON-INVASIVE MEASUREMENT OF A SUBSTANCE WITHIN A BODY, which is herein incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present application relates generally to the non-invasive measurement of various substances in a body, such as the measurement of the concentration of glucose in the human body and, more specifically, to a infrared detection system to analyze and determine, non-invasively, the concentration of a substance in a body. 
         [0003]    Spectroscopic techniques using infrared (“IR”) radiation are known in the prior art and have been widely used for non-invasive measurement of the concentration of substances of interest in a body. One area of particular interest is the use of these techniques for the non-invasive measurement of the concentration of glucose and other constituents of the human bloodstream. 
         [0004]    The infrared spectra includes the near infrared (approximately 1 to 3 microns), the middle infrared (approximately 3 to 6 microns), the far infrared (approximately 6 to 15 microns), and the extreme infrared (approximately 15 to 100 microns). Known prior art glucose and other non-invasive blood constituent measuring devices operate in the near infrared regions where the absorption of infrared energy by glucose and other blood constituents is relatively low. However, it is known that glucose and other blood constituents have strong and distinguishable absorption spectra in both the middle and far infrared regions. 
         [0005]    It has been found in a far infrared detection system that the resolution of the system should be equivalent to 0.01° C. to provide sufficiently accurate measurements. At this high level of accuracy, the blackbody emission of any component of the system (mirrors, filters, field limiters, detector, for example) can cause perturbations in the measurement. The known solution to such a circumstance is to cool the system to a cryogenic temperature (−180° C., for example), and have the system sealed and filled with dry nitrogen to avoid moisture accumulation. However, for a consumer product, such a solution is impractical and expensive. 
       SUMMARY 
       [0006]    The present application discloses a system to measure, non-invasively, the concentration of a substance in a body. In accordance with one embodiment, an apparatus for the non-invasive measurement of a substance within a body includes a detector for sensing radiation emitted or remitted from a body, a human body, for example. An optical system is provided and aligned to focus IR radiation emitted by the body on a sensitive area of the detector. 
         [0007]    Elements of the system within the field of view of the detector and the detector itself may have a temperature measuring device such as a thermistor attached to it for the purpose of measuring its temperature. At least two additional temperature measuring devices may measure two or more respective ambient temperatures, for example, an exterior ambient temperature and an interior ambient temperature. For the detector to accurately measure the energy radiated by the body, the system is calibrated to compensate for the effect of the temperature of individual elements in the detector field of view. Using a heating or heating/cooling unit for an individual element separately, the temperature of the element can be varied for the purpose of calibration while the temperatures of the other elements of the system remain stable. This process is repeated many times in various ambient temperatures and various body temperatures in order to calibrate the effect of an individual element on the measurement in all ranges of conditions relevant for the measurement. 
         [0008]    This procedure is repeated for individual elements in the field of view of the detector yielding a look-up table (“LUT”) representing the contribution of individual elements to the detector&#39;s measurement. The perturbations due to the temperature of individual system elements are taken into account in measurements, thereby enabling the system to obtain a high level of accuracy. 
         [0009]    During the creation and build-up of the LUT, it was found that the temperature effect of a baffle used to limit the field of view of the detector is 10:1 relative to the body reading. Calibration alone cannot compensate for such a significant effect. 
         [0010]    The solution is to reduce the emissivity of the baffle by enhancing its reflectivity. However, enhancing the reflectivity of the baffle creates an additional circumstance of reflecting stray energy to the detector. A spherical baffle was designed with an internal surface, i.e., the surface of the baffle opposite the detector, that is polished and gold-plated to lower the emissivity. The baffle design reduces reflection or multiple reflections from reaching the sensitive area of the detector. 
         [0011]    The base plate that the detector and the baffle are mounted on and the baffle have substantially the same temperature as the detector. The base plate and the outer surface of the baffle are designed as a radiation trap having a dull black surface providing an emissivity of about 97%. 
         [0012]    The design of the system optics creates an image of the detector sensitive area on the surface of the body in order to collect the IR radiation emitted or remitted from the body. The detector averages the IR radiation emitted or remitted from the area on the surface of the body subtended by the image of the detector. 
         [0013]    In accordance with another embodiment, the present optical apparatus comprises two changeable optical filters, a first mirror positioned to a first side of the optical filter, and a second mirror positioned to a second side of the optical filter opposite the first mirror. A detector is positioned to the second side of the optical filter. A baffle partially surrounds a sensitive surface of the detector. Temperature-measuring devices are configured to measure the temperature of the baffle, mirrors and filters. The first mirror is configured to receive IR radiation from a measured surface of the body, collimate the IR radiation to a beam, and reflect the collimated IR beam toward and through the optical filter. One of the optical filters is configured to filter out a portion of the collimated IR beam having wavelengths that fall outside a selected bandwidth, and the second optical filter is configured to filter out a portion of the collimated IR beam having wavelengths that fall within a selected bandwidth. The filters are changeable by a motorized mechanism, and IR radiation measurements may include at least one measurement with one filter and a second measurement with the second filter. The second mirror is configured to receive the collimated and filtered IR beam and reflect it toward the detector. The baffle is configured to block stray IR radiation so that it does not reach the detector sensitive area. 
         [0014]    The two radiation measurements are then corrected individually to reduce the effect of the emission of the system elements on the measurement. The ratio of the two radiation measurements after the correction and normalization for a black body reading is correlated to the concentration of the desired substance in the body, such as the concentration of glucose in the bloodstream of a human body, for example. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The following figures, in which like numerals indicate elements, form part of the present specification and are included to further demonstrate certain features. The methods and apparatuses may be better understood by reference to one or more of these figures in combination with the detailed written description of specific embodiments presented herein. 
           [0016]      FIG. 1  illustrates a block diagram of a system for the non-invasive measurement of the concentration of a substance in a body; 
           [0017]      FIG. 2  is a perspective view of the optical and detector apparatus of  FIG. 1  illustrating the path of travel for electromagnetic rays between the body and the detector; 
           [0018]      FIG. 3  is a perspective view of the detector of  FIGS. 1 and 2 ; 
           [0019]      FIG. 4  is a perspective view of the optical and detector apparatus of  FIG. 2 , showing the locations on the various elements of the optical and detector apparatus of the temperature measurement devices; and 
           [0020]      FIGS. 5 and 6  are cross-sectional views of the detector and baffle of the optical and detector apparatus shown in  FIG. 2 . 
           [0021]      FIG. 7  is a cross-sectional view of another system for the non-invasive measurement of the concentration of a substance in a body showing the position of two thermistors measuring two ambient temperatures. 
       
    
    
       [0022]    These and other embodiments of the present application will be discussed more fully in the description. The features, functions, and benefits can be achieved independently in various embodiments, or may be combined in yet other embodiments. 
       DETAILED DESCRIPTION 
       [0023]    One or more illustrative embodiments are described below. Not all features of an actual implementation are necessarily described or shown for the sake of clarity. 
         [0024]    Referring now to  FIG. 1 , a block diagram of a system  10  for the non-invasive measurement of the concentration of a substance in a body is shown. Infrared (“IR”) radiation emitted or reflected from the surface of a body  11  is collected and collimated by optics subsystem  13  and focused on IR detector assembly  15 . The body  11  is the source of the IR radiation being measured by the system  10 . The body  11  is often a portion of a surface of a body of interest, such as a human body, for example. The optical subsystem  13  includes at least two changeable filters  33 ,  35 , as shown in  FIG. 2 , that allow two different wavelength bandwidth signals, the first including a characteristic wavelength of a desired substance, such as glucose, for example, to be measured, the second being a portion of the emitted radiation not including the substance characteristic wavelength to be used as a reference signal. 
         [0025]    The detector assembly  15  senses both signals and provides an output voltage that is proportional to the intensity of each of the two signal measurements to the microprocessor  17 . Temperature sensors, as shown in  FIG. 4 , provide the temperatures of the various optical subsystem and detector assembly components and the ambient temperature to the microprocessor  17  via lines  2 ,  6 , and  8  and a look up table (“LUT”)  21 . In a calibration process, the temperature of the optical subsystem and detector assembly individual components is varied while the temperature of the remaining system components is held stable to provide a set of calibration parameters that are stored in the LUT  21 . 
         [0026]    The microprocessor  17  uses the set of predetermined calibration parameters to correct each of the two radiation measurements to reduce the effect of the emission of the system elements on the measurement. The ratio of the two radiation measurements after the correction and normalization for a black body reading is correlated to the concentration of the desired substance in the body, such as the concentration of glucose in the bloodstream of a human body, for example. The result is then provided to an output device  19 , such as an LCD or LED video monitor, for example. 
         [0027]    Referring now also to  FIG. 2 , a schematic perspective view of the configuration of the optical and detector components of the system  10  shown in  FIG. 1 , illustrating the path of travel for IR rays between the body  11  and the detector  15  is shown. The detector  15  includes the detector element  23 , detector base  25  and a baffle  27 . The configuration of the optical and detector components is designed such that an image  12  of the sensitive or active area  47  of the detector  15  (as shown in  FIG. 3 ) is created at the body  11  on the focal plane of mirror  31 . 
         [0028]    The area of image  12  may have a diameter approximately 6 mm. IR radiation emitted from or reflected by the body  11  at image  12  in beam  41  is collected and collimated by mirror  31 . The IR radiation is reflected by mirror  31  and propagated to mirror  29  in a collimated beam  43  of parallel rays via filter  33  or filter  35 . The focal plane of mirror  29  is located at the surface of the sensitive area  47  of the detector assembly  15 . The beam  43  reaching mirror  29  is reflected and propagated as beam  45  and focused at the focal plane of mirror  29  incident on the detector assembly  15  sensitive area  47 . 
         [0029]    The detector assembly  15  is partially surrounded by a baffle  27  on the side facing the mirror  29 . The baffle  27  insures that substantially only beam  45  is incident only on the sensitive area  47 . Baffle  27  also blocks any stray radiation from reaching the sensitive area  47  of detector assembly  15 . Thus, the optical subsystem  13  is aligned such that the image  12  is positioned at the surface of body  11  and the beam  41  of IR radiation is incident on the sensitive area  47  of detector assembly  15  via mirror  31 , filter  33  or filter  35  and mirror  29 . 
         [0030]    In one embodiment, mirrors  29  and  31  are ninety-degree (90°) off-axis parabolic mirrors coated with gold or other suitable reflective material. Mirror  29  may have a focal length of about one (1) inch and mirror  31  may have a focal length of about three (3) inches. Other suitably designed reflective mirrors may be used for the optical subsystem  13  such as ellipsoid mirrors or a combination of ellipsoid and hyperbolic mirrors, for example. 
         [0031]    Filter  33  and filter  35  are mounted in frame  37 , frame  37  being positioned between mirror  29  and mirror  31 . The filters  33 ,  35  are switched between positions intercepting the beam  43  using a suitable driving mechanism, such as a motor or pneumatic pressure, for example, coupled to frame  37 . In one embodiment, motor  39  is coupled to the frame  37  and positions the frame  37  between the mirror  29  and mirror  31  such that the desired filter  33 ,  35  intercepts the beam  43 . One of the filters, filter  33 , for example, may be a narrow band filter passing the wavelengths of the spectral characteristic of the substance being measured. The other filter, filter  35 , for example, may be a narrow band filter passing those wavelengths of a spectral characteristic not sensitive to the substance being measured. For example, in some embodiments, filter  33  may limit the bandwidth to that region of the spectrum where there is no emission for the substance being measured (for glucose, for example, the bandwidth would be 10.5 g-15 g), while filter  35  would have a bandwidth characteristic of the emission of the substance being measured (for glucose, the bandwidth would be 8.5 g-10.5 g). 
         [0032]    Referring now also to  FIG. 3 , a perspective view of the detector element  23  shown in  FIGS. 1 and 2  is illustrated. Any suitable IR detector responsive to the desired wavelengths of interest may be used. The detector element  23  includes a chip providing the IR sensitive material forming the detector sensitive area  47 . The chip, or sensitive area  47 , is enclosed in a case  51  and mounted to a base  53 . The case  51  has an appropriately-sized opening forming a window  49  in its top surface to allow the IR radiation to reach the sensitive area  47 . The window  49  is covered by a material transparent to the radiation of interest, such as silicon or other suitable material. Leads  55  connect the detector element  23  to the microprocessor  17  and other circuitry. In one embodiment, a passive IR sensor known as a thermopile detector is used. Thermopile detectors respond to IR power emitted by an object in its field of view by producing a voltage that is proportional to incident power. One suitable thermopile detector is manufactured by Dexter Research Corporation (part number ST150). The thermopile detector used in one embodiment has a sensitive area  47  with dimensions of 1.5 mm×1.5 mm and a window  49  of silicon. 
         [0033]    Referring now also to  FIG. 4 , a perspective view of the optical subsystem  13  and detector assembly  15  of  FIG. 2  is shown, illustrating suitable locations on the various elements of the optical subsystem and detector assembly where temperature measurement devices may be located. An individual element of the optical subsystem or detector assembly may emit electromagnetic radiation including IR radiation as a function of its temperature. In order to achieve the resolution used to produce an accurate measurement of the desired substance, the emission of individual elements in the system may be taken into account. 
         [0034]    Elements of the optical subsystem  13  within the field of view of the detector assembly  15 , as well as the detector assembly  15 , include one or more suitable temperature sensing devices mounted at suitable locations on the element to accurately measure the temperature of the element. In one embodiment, thermistors are used as the temperature measuring devices. A thermistor is a temperature dependent resistor often composed of a semiconductor material. The resistance of a thermistor is inversely proportional to temperature, i.e., as the temperature increases, its resistance decreases. While other suitable temperature sensors can be used, thermocouples, for example, often a thermistor provides a greater output voltage. 
         [0035]    In the embodiment shown in  FIG. 4 , thermistor  61  is located internally to the detector assembly  15  to measure the temperature of the cold junction where a thermopile detector is used. Thermistor  63  measures the temperature of the baffle  27 . Thermistors  65  and  67  measure the temperature of mirror  29 , and thermistors  71  and  73  measure the temperature of mirror  31 . Two thermistors are used for each mirror due to the size and mass of the mirrors. Thermistor  69  measures the temperature of the filters  33 ,  35  and of frame  37  assembly. Thermistor  75  measures the ambient room temperature. The temperature of an individual element is matched with a set of predetermined calibration parameters stored in LUT  21  together with the temperature of detector  15 , ambient temperature, and the temperature of body  11 , to compensate for any perturbations in a substance concentration measurement due to the temperatures of the various optical subsystem and detector assembly elements. 
         [0036]      FIG. 7  is a cross-sectional view of a housing  743  containing a detector (not shown) that may be like detector  15  described herein and optical subsystem  713  that may be like optical subsystem  13  described herein. Housing  743  also contains a microprocessor  717  that may be like microprocessor  13  described herein and an output device  719  that may be like output device  19  described herein. An optical chamber  741  within housing  743  contains at least a portion of optical subsystem  713 . A thermistor  777  measures an interior ambient temperature in optical chamber  741 . The interior ambient temperature may be used to calculate directly the effect of the interior ambient temperature on the substance concentration measurement. Instead, or in addition, the interior ambient temperature may be used to calculate the effect of the interior ambient temperature on elements of optical subsystem  713  in accordance with their thermal capacity. Then, the effect of individual element temperature on the substance concentration measurement may be calculated. A thermistor  775  is mounted in an exterior facing cavity of housing  743  sealed from the interior of housing  743  and measures an exterior ambient temperature outside housing  743 . Thermistor  775  may be used with a lookup table to calculate the effect of the exterior ambient temperature on a temperature of the measured surface. Thermistor  775  measures exterior ambient temperature at a point as close as possible to the measured surface without being so close as to be influenced by heat transfer from the measured surface itself. 
         [0037]    Referring now also to  FIG. 5 , a cross-sectional view of the detector assembly  15  and baffle  27  of the optical and detector apparatus of  FIG. 2  is shown. In the illustrated embodiment, detector element  23  is held by a retainer ring  81  in thermal contact with detector base  25 . Baffle  27  is attached to the detector base  25  with fasteners  26 , establishing good thermal contact between the detector element  23 , ring  81 , detector base  25 , and baffle  27 . The inner surface  83  of baffle  27  may be gold-coated and polished to create a mirror. The inner surface  83  of baffle  27  is designed to have a very low emissivity and high reflectivity. The shape of the inner surface  83  of baffle  27  is designed to reduce reflection or multi-reflection of radiation incident on the sensitive area  47  of the detector element  23 . 
         [0038]    In one embodiment, the inner surface  83  of baffle  27  forms a spherical surface, the center of the sphere coinciding with the center of the detector sensitive area  47 , enclosing the detector element  23 . An opening  95  is formed in the portion of the sphere over and opposite the sensitive area  47 . The dimensions of the opening  95  are sufficient to allow the beam  45  (as shown in  FIG. 2 ) to be incident on the sensitive area  47  and reduce stray radiation reaching the detector sensitive area  47 . The front surface  89  of the detector element  23 , the exposed surface  87  of retainer ring  81  and the exposed portion  85  of detector base  25  within the sphere are coated with a suitable material, such as a suitable black coating, for example, to create a radiation trap for any stray radiation. Thermistor  63  measures the temperature of the baffle  27  to enable compensation for its emission effects on the substance concentration measurements. 
         [0039]    Referring now also to  FIG. 6 , a cross-sectional view of the detector assembly  15  and baffle  27  of the optical and detector apparatus of  FIG. 2  according to another embodiment is shown. In this embodiment, as described above with reference to paragraph 0028 and  FIG. 2 , mirrors  29  and  31  may be ninety-degree (90°) off-axis parabolic mirrors coated with gold or other suitable reflective material. The inner surface  83  of baffle  27  may form a spherical surface having the center  99  of the sphere positioned off center with respect to the center  97  of the detector sensitive area  47 . An opening  95  is formed in the portion of the sphere over and opposite the sensitive area  47 . Since the maximum of the IR energy distribution of an off-axis mirror is off center, the position of the center  99  of the baffle opening  95  is also offset from the center  97  of the detector sensitive area  27  to provide increased IR energy collection. The dimensions of the opening  95  are sufficient to allow the beam  45  (as shown in  FIG. 2 ) to be incident on the detector sensitive area  47  and reduce stray radiation reaching the detector sensitive area  47 . 
         [0040]    Although the methods and apparatuses have been described in terms of certain embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments which do not provide all of the features and benefits set forth herein, are also considered included.