Abstract:
The present invention provides systems and methods for attenuating the effect of ambient light on optical sensors and for measuring and compensating quantitatively for the ambient light.

Description:
[0001]    The present application is a continuation of U.S. patent application Ser. No. 11/646,266, filed Dec. 28, 2006, which is a divisional of U.S. patent application Ser. No. 10/823,781, filed Apr. 14, 2004, now U.S. Pat. No. 7,157,723, issued Jan. 2, 2008, which claims the benefit of U.S. Provisional Patent Application No. 60/462,695, filed Apr. 15, 2003. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to optical sensors, and, more specifically, to a system and method for attenuating the effect of ambient light on an optical sensor. 
         [0004]    2. Discussion of the Background 
         [0005]    An optical sensor is a device that may be used to detect the concentration of an analyte (e.g., oxygen, glucose, or other analyte). U.S. Pat. No. 6,330,464, the disclosure of which is incorporated herein by reference, describes an optical sensor. 
         [0006]    There may be situations when it is desirable to use an optical sensor in an environment where there is a significant amount of ambient light (e.g., the outdoors on a bright, sunny day). In some circumstances, a significant amount of ambient light may negatively affect the accuracy of an optical sensor. Accordingly, what is desired are systems and methods to attenuate the negative effect of ambient light on the functioning of an optical sensor and/or to measure and compensate quantitatively for the ambient light. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides systems and methods for attenuating the effect of ambient light on optical sensors and for measuring and compensating quantitatively for the ambient light. 
         [0008]    In one aspect, the present invention provides an optical sensor having features that attenuate the amount of ambient light that reaches the optical sensor&#39;s photodetectors. The features can be used together or separately. For example, in some embodiments, the present invention provides an optical sensor wherein the circuit board that is used to electrically connect the electrical components of the sensor is made from an opaque material (e.g., opaque ferrite), as opposed to the conventional aluminum oxide ceramic circuit board. In some embodiments, the photodetectors of the optical sensor are mounted to the bottom side of a circuit board and holes are made in the circuit board to provide a way for light from the indicator molecules to reach the photodetectors. 
         [0009]    In another aspect, the present invention provides methods for using and implanting an optical sensor, which methods, used together or separately, reduce the effect of ambient light on the optical sensor. 
         [0010]    For example, in one aspect the present invention provides a method that includes the following steps: illuminating indicator molecules, thereby causing the indicator molecules to emit light; determining the amount of light reaching a photodetector at a point in time when the indicator molecules are illuminated, thereby determining the sum of the amount of ambient light and the light emitted from the indicator molecules reaching the photodetector; ceasing illuminating the indicator molecules; after ceasing illuminating the indicator molecules, determining the amount of light reaching the photodetector, thereby determining the amount of ambient light reaching the photodetector; and determining the amount of light emitted from the indicator molecules that reached the photodetector by subtracting the second determined amount of light from the first determined amount of light. 
         [0011]    In another aspect, the present invention provides an improved sensor reader and method of operating the sensor reader. For example, in one aspect, the present invention provides a method performed by a sensor reader that includes the steps of: determining the intensity of ambient light; determining whether the intensity of the ambient light is greater than a predetermined threshold intensity; and issuing a warning to the user if it is determined that the intensity of the ambient light is greater than the predetermined threshold intensity. 
         [0012]    The above and other features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying drawings, which are incorporated herein and form part of the specification, help illustrate various embodiments of the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
           [0014]      FIG. 1  shows an optical sensor according to an embodiment of the present invention. 
           [0015]      FIG. 2  shows an optical sensor according to another embodiment of the present invention. 
           [0016]      FIG. 3  shows the top surface of a circuit board according to an embodiment of the present invention. 
           [0017]      FIG. 4  shows the field of view of a photodetector according to an embodiment of the present invention. 
           [0018]      FIG. 5  shows a sensor that has been implanted into a patient according to an embodiment of the present invention. 
           [0019]      FIG. 6  shows a sensor having outriggers according to an embodiment of the present invention. 
           [0020]      FIG. 7  shows a functional block diagram of a sensor reader according to an embodiment of the present invention. 
           [0021]      FIG. 8  is a flow chart illustrating a process, according to an embodiment of the present invention, that may be performed by a sensor reader. 
           [0022]      FIG. 9  is a flow chart illustrating a process for attenuating the effect of ambient light on readings provided by an optical sensor. 
           [0023]      FIG. 10  is a flow chart illustrating a process performed by a sensor according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]      FIG. 1  shows an optical sensor (“sensor”)  110 , according to an embodiment of the present invention, that operates based on the fluorescence of fluorescent indicator molecules  116 . The sensor  110  includes a sensor housing  112  (sensor housing  112  may be formed from a suitable, optically transmissive polymer material), a matrix layer  114  coated over the exterior surface of the sensor housing  112 , with fluorescent indicator molecules  116  distributed throughout the layer  114  (layer  114  can cover all or part of the surface of housing  112 ); a radiation source  118 , e.g. an LED, that emits radiation, including radiation over a range of wavelengths which interact with the indicator molecules  116 , i.e., in the case of a fluorescence-based sensor, a wavelength which causes the indicator molecules  116  to fluoresce; and a photodetector  120  (e.g. a photodiode, phototransistor, photoresistor or other photodetector) which, in the case of a fluorescence-based sensor, is sensitive to fluorescent light emitted by the indicator molecules  116  such that a signal is generated by the photodetector  120  in response thereto that is indicative of the level of fluorescence of the indicator molecules. Two photodetectors  120   a  and  120   b  are shown to illustrate that sensor  110  may have more than one photodetector. 
         [0025]    The indicator molecules  116  may be coated on the surface of the sensor body or they may be contained within matrix layer  114  (as shown in  FIG. 1 ), which comprises a biocompatible polymer matrix that is prepared according to methods known in the art and coated on the surface of the sensor housing  112 . 
         [0026]    Suitable biocompatible matrix materials, which must be permeable to the analyte, include some methacrylates (e.g., HEMA) and hydrogels which, advantageously, can be made selectively permeable—particularly to the analyte—i.e., they perform a molecular weight cut-off function. 
         [0027]    Sensor  110  may be wholly self-contained. In other words, the sensor may be constructed in such a way that no electrical leads extend into or out of the sensor housing  112  to supply power to the sensor (e.g., for driving the source  118 ) or to transmit signals from the sensor. Rather, the sensor may include a power source  140  that is wholly embedded or housed within the sensor housing  112  and a transmitter  142  that also is entirely embedded or housed within the sensor housing  112 . 
         [0028]    The power source  140  may be an inductor, as may be the antenna for transmitter  142  as described in U.S. Pat. No. 6,400,974. The transmitter  142  may be configured to wirelessly transmit data to an external reader (see  FIG. 7 ). 
         [0029]    Other self-contained power sources that can be used include microbatteries; piezoelectrics (which generate a voltage when exposed to mechanical energy such as ultrasonic sound; micro generators; acoustically (e.g., ultrasound) driven generators; and photovoltaic cells, which can be powered by light (infrared). 
         [0030]    As shown in  FIG. 1 , many of the electro-optical components of sensor  112 , including a processor  166 , which may include electronic circuitry for controlling, among other components, source  118  and transmitter  142 , are secured to a circuit board  170 . Circuit board  170  provides communication paths between the components. 
         [0031]    As further illustrated in  FIG. 1 , an optical filter  134 , such as a high pass or band pass filter, preferably is provided on a light-sensitive surface of a photodetector  120 . Filter  134  prevents or substantially reduces the amount of radiation generated by the source  118  from impinging on a photosensitive surface of the photodetector  120 . At the same time, the filter allows fluorescent light emitted by fluorescent indicator molecules  116  to pass through to strike a photosensitive region of the detector  120 . This significantly reduces “noise” in the photodetector signal that is attributable to incident radiation from the source  118 . 
         [0032]    However, even though filter  134  may significantly reduce “noise” created by radiation from source  118 , filter  134  may not significantly attenuate “noise” from ambient light sources  198 , particularly because light that passes through skin has a wavelength that may not be filtered by the filter. That is, filter  134  may not significantly prevent ambient light  199  from hitting a photosensitive surface of a photodetector  120 . Accordingly, sensor  110  has other features for dealing with the ambient light. 
         [0033]    For example, substrate  170  of sensor  110  is made of a material that does not propagate stray light or is coated with a finish that prevents it from propagating stray light. Thus, by using such a substrate  170  one can reduce the amount of ambient light reaching the photodetectors  120 . In some embodiments, substrate  170  is a ferrite circuit board  170  while in other embodiments substrate  170  may be a conventional circuit board having a finish that prevents the board from propagating light. 
         [0034]    Additionally, in sensor  110  the photodetectors  120  may be mounted to the underside of circuit board  170 . This may be done by, for example, a technique known as “flip-chip” mounting. This technique of mounting the photodetectors  120  to the underside of the board  170  permits all light-sensitive surfaces except the top surface of the photodetectors  120  to be more easily covered with a light blocking substance  104  (e.g., a black, light blocking epoxy). However, it is contemplated that photodetectors  120  can be mounted on the topside of circuit board  170 , as shown in  FIG. 2 . Like in the embodiment shown in  FIG. 1 , in the embodiment shown in  FIG. 2  all surfaces except the top surface of the photodetector are covered with light blocking substance  104 . 
         [0035]    In embodiments where the photodetectors  120  are mounted to the bottom surface of board  170 , a hole for each photodetector  120  is preferably created through board  170 . This is illustrated in  FIG. 3 , which is a top view of board  170 . As shown in  FIG. 3 , the light source  118  is preferably mounted to the top surface  371  of board  170 . As further shown in  FIG. 3 , two holes  301   a  and  301   b  have been created through board  170 , thereby providing a passageway for light from the indicator molecules to reach the photodetectors  120 . The holes in circuit board  170  may be created by, for example, drilling and the like. Preferably, each photodetector  120  is positioned such that its face is directly beneath and covering a hole, as shown in  FIG. 1 . 
         [0036]    This technique restricts light from entering the photodetectors  120  except from their face and through the hole through the ferrite. As further illustrated in  FIG. 1 , each hole in the ferrite may be filled with an optical pass filter  134  so that light can only reach a photodetector  120  by passing through the filter  134 . 
         [0037]    As mentioned above and illustrated in  FIG. 1 , the bottom surface and all sides of the photodetectors  120  may be covered with black light blocking epoxy  104 . Additionally, to minimize unwanted reflections that might occur from parts on the top surface  371  of the circuit board  170 , a black epoxy may be used as a potting for all components not within the far-field pattern of the optical system. Further, black epoxy may be used to encircle the filters  134  for each photodetector  120 , thereby preventing light leakage from propagating through a glue joint created by the mechanical tolerance between the filters  134  and circuit board holes  301 . 
         [0038]    As further shown in  FIG. 1 , NIR filters  106   a  and  106   b  may be positioned on top of filters  134   a  and  134   b , respectively. Such a configuration would require all light reaching a photodetector  120  to pass through not only filter  134 , but also NIR filter  106 . 
         [0039]    As  FIGS. 1 and 2  make clear, any ambient light that reaches a photodetector  120  must first pass through the matrix  114  containing the indicator molecules and the filters before the light can strike the top surface of the photodetector  120  and, thereby interfere with the optical sensor. Although the matrix  114  is characteristically clear, by increasing the water content of the polymerization reaction, a phase separation occurs which results in a highly porous matrix material  114 . The large size of the pores, along with the differential refractive index of the matrix  114  (versus the surrounding medium), cause substantial light scattering within the matrix  114 . 
         [0040]    This scatter is beneficial in helping to attenuate any ambient light arriving from an external source before it can enter the sensor housing. Accordingly, in some embodiments of the invention, the process of making the matrix  114  is altered so that the matrix  114  will be highly porous. 
         [0041]    For example, in some embodiments, matrix  114  is produced by (a) combining 400 mLs HEMA with 600 mLs distilled water (a 40:60 ratio), (b) swirling to mix, (c) adding 50 uL 10% ammonium persulfate (APS) (aqueous solution) and 10 uL 50% TEMED (aqueous solution), and (d) polymerizing at room temperature 30 minutes to one hour. This process will produce a highly porous matrix (or “white gel” matrix). Polymerization at higher or lower temperatures can also be used to form a white gel matrix. An example is the formation of a 30:70 gel using 175 uL distilled water+75 uL HEMA+8.44 uL VA-044 (2,2′-Azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride) (other free radical initiators such as AIBN (2,2′-Azobisisobutyronitrile) might also be used). 
         [0042]    Another feature of sensor  110  is that at least part of the housing  112  may be doped with organic or inorganic dopants that will cause the doped part of the housing  112  to function as an optical filter. For example, it is contemplated to dope a part of housing  112  with savinyl black, which is an organic light blocking material. If necessary, under certain propagation vectors of ambient light, it is possible to selectively dope the housing  112  in such a way so as to only permit the region directly within the photodetectors&#39;  120  field of view to propagate light. This mechanism would use a “saddle” graft architecture fabricated by the pre-machined encasement procedure. 
         [0043]    By use of the non-transparent material  104  and the non-light propagating circuit board  170 , the optical field of view of the photodetectors  120  is controlled and restricted to the region of the indicator matrix installation on the surface of the sensor housing  112 . The optical field of view for one photodetector  120 ( a ) of the embodiment shown in  FIG. 1  is illustrated in  FIG. 4 . 
         [0044]    Because light cannot pass through the circuitry from the backside, the sensor  110  can be surgically installed in-vivo so as to orient the optical view of the photodetectors  120  in the most favorable placement to minimize light passing through the skin. For example, in some embodiments, orienting the sensor optical field of view inward toward body core tissue may be most favorable. This is illustrated in  FIG. 5 . As shown in  FIG. 5 , the one surface of the photodetector not covered by the non-transparent material  104  (i.e., surface  590 ) faces inward toward body core tissue  501  and away from the skin  520  to which it is the closest. Because it is possible that this orientation may not be maintained in-vivo following installation (e.g., the sensor might roll during normal limb movement), it is contemplated that in some embodiments it will be advantageous to incorporate anti-roll “outriggers” on the sensor housing  212 .  FIG. 6  is a front view of sensor  110  with outriggers  610  and  611  attached to sensor housing  212  to prevent rolling. 
         [0045]    In addition to providing an improved optical sensor design that significantly attenuates the effect of ambient light on the proper functioning of the optical sensor  110 , the present invention also provides improvements to the external signal reader that receives the output data transmitted from the optical sensor  110 . As discussed above, this output data, which carries information concerning the concentration of the analyte in question, may be transmitted wirelessly from sensor  110 . 
         [0046]      FIG. 7  illustrates an example of an external reader  701 . In the embodiment shown in  FIG. 7 , the optical sensor  110  is implanted near a patient&#39;s wrist and the reader  701  is worn like a watch on the patients arm. That is, reader  701  is attached to a wrist band  790 . In some embodiments, reader  701  may be combined with a conventional watch. Preferably, wrist band  790  is an opaque wrist band. By wearing an opaque wrist band  790 , the patient will reduce the amount of ambient light reaching the optical sensor. 
         [0047]    As shown in  FIG. 7 , reader  701  includes a receiver  716 , a processor  710 , and a user interface  711 . The user interface  711  may include a display, such as, for example, a liquid crystal display (LCD) or other type of display. The receiver  716  receives data transmitted from the sensor. The processor  710  may process the received data to produce output data (e.g., a numeric value) that represents the concentration of the analyte being monitored by the sensor. 
         [0048]    For example, in some embodiments, sensor  110  may transmit two sets of data to reader  701 . The first set of data may correspond to the output of the photodetectors  120  when the light source  118  is on and the second set of data may correspond to the output of the photodetectors  120  when the light source  118  is off. 
         [0049]    Processor  710  processes these two data sets to produce output data that can be used to determine the concentration of the analyte being monitored by the sensor. For instance, the first set of data may be processed to produce a first result corresponding to the sum of (1) the total amount of light from the indicator molecules that reached the photodetectors  120  and (2) the total amount of ambient light that reached the photodetectors  120 . The second set of data may be processed to produce a second result corresponding to the total amount of ambient light that reached the photodetectors  120 . The processor  710  may then subtract the second result from the first result, thereby obtaining a final result that corresponds to the total amount of light from the indicator molecules that reached the photodetectors  120 . The processor  710  may then use the final result to calculate the concentration of the analyte and cause the user interface  711  to display a value representing the concentration so that the patient can read it. 
         [0050]    Advantageously, reader  701  may include a small photodetector  714 . By including photodetector  714  in the reader  701 , the reader may monitor the amount of ambient light. Further, the processor can be programmed to output a warning to the patient if the amount of ambient light detected by photodetector  714  is above a pre-determined threshold. For example, if the output of photodetector  714 , which may be input into processor  710 , indicates that there is a relatively high amount of ambient light, processor  710  may display an alert message on user interface  711  to alert the patient that the sensor may be non-functional due to the high amount of ambient light. The patient can then take the appropriate action. For example, the patient can move to an area where there is less ambient light or shroud the sensor so that less ambient light will reach the sensor. 
         [0051]      FIG. 8  is a flow chart illustrating a process  800  that may be performed by processor  710 . Process  800  may begin in step  802 , where processor  710  receives an input indicating that a user of reader  701  has requested to obtain a reading from the sensor or where processor  710  automatically determines that it is time to obtain data from the sensor. 
         [0052]    In step  804 , processor  710  obtains from photodetector  714  information regarding the intensity of the ambient light. In step  806 , processor  710  determines, based on the information obtained in step  804 , whether the intensity of the ambient light is such that it is likely the sensor will not be able to function properly. For example, processor  710  may determine whether the intensity of the ambient light is greater than some pre-determined threshold. If the intensity of the ambient light is such that it is likely the sensor will not be able to function properly, then processor  710  proceeds to step  890 , otherwise processor  710  proceeds to step  808 . 
         [0053]    In step  890 , processor  710  issues a warning to the user. For example, processor  710  may display a message on user interface  711  or communicate to the user that there is too much ambient light. 
         [0054]    In step  808 , processor  710  activates the sensor. For example, processor  710  may wirelessly provide power to the sensor, send an activation signal to the sensor, or otherwise activate the sensor. 
         [0055]    In step  810 , processor  710  obtains data from the sensor. For example, as discussed above, the data received from the sensor may include data corresponding to the output of photodetectors  120  when light source  118  is on and data corresponding to the output of photodetectors  120  when light source  118  is off. Sensor  110  may wirelessly transmit the data to receiver  716 , which then provides the data to processor  710 . 
         [0056]    In step  812 , processor  710  processes the received data to produce a result that, if sensor is operating correctly (e.g., there is not too much ambient light), can be used to calculate the concentration of the analyte being monitored by the sensor. For example, as discussed above, processor  710  may subtract the data corresponding to the output of photodetectors  120  when light source  118  is off from the data corresponding to the output of photodetectors  120  when light source  118  is on to produce a result that can be used to determine the concentration of the analyte being monitored by the sensor. 
         [0057]    In step  814 , processor  710  causes information or a message regarding the analyte being sensed by the sensor to be displayed to the user, wherein the information or message is based on the result produced in step  812 . 
         [0058]    In addition to providing an improved optical sensor design and an improved reader, the present invention provides an improved method for operating an optical sensor, which method also attenuates the negative effect of ambient light. The method may be used with a conventional optical sensor or with optical sensors according to the present invention.  FIG. 9  is a flow chart illustrating a process  900  for attenuating the effect of ambient light on readings provided by an optical sensor. 
         [0059]    Process  900  may begin in step  901 , where a determination of the amount of ambient light reaching the photodetector is made. For example, in step  901  a signal produced by one or more photodetectors is obtained during a period of time when the indicator molecules are not in a fluorescent state. In step  902 , a determination is made as to whether the amount of ambient light reaching the photodetector is such that it is likely the sensor will not be able to provide an accurate reading. If the amount of ambient light reaching the photodetector is such that it is likely the sensor will not be able to provide an accurate reading, then the process proceeds to step  990 , otherwise the process proceeds to step  903 . 
         [0060]    In step  990 , information indicating that there is too much ambient light is transmitted to a sensor reader. After step  990 , the process may end or proceed back to step  902 . 
         [0061]    In step  903 , the indicator molecules are illuminated for about x amount of time (e.g., 50 or 100 milliseconds). For example, in step  903 , the light source  118  may be activated for 100 milliseconds to illuminate the indicator molecules. In one embodiment, the light source is activated using about a 2 milliamp drive current. Next, while the indicator molecules are illuminated, the signal produced by a photodetector  120  is read (step  904 ). 
         [0062]    Next (step  908 ), the signal obtained in step  901  is subtracted from the signal obtained in step  904  to produce a new signal, which new signal should better correspond to the concentration of the analyte than the signal read in step  904  because the signal read in step  904  includes not only the light emitted by the indicator molecules but also the ambient light that has reached the photodetector. Next (step  910 ), the new signal is transmitted to an external reader. After step  910 , the process may proceed back to step  901 . 
         [0063]    Process  900  may be performed by processor  266 . That is, in some embodiments, processor  266  may have software, hardware or a combination of both for performing one or more steps of process  900 . For example, processor  266  may include an application specific integrated circuit (ASIC) that is designed to carry out one or more of the steps of process  900 . 
         [0064]      FIG. 10  is a flow chart illustrating another process  1000  according to an embodiment of the invention. Process  1000  may begin in step  1002  where light source  118  is turned on for about x amount of time (e.g., 50 or 100 milliseconds). For example, in step  1002 , the light source  118  may be activated for 100 milliseconds to illuminate the indicator molecules. 
         [0065]    In step  1004 , data corresponding to the outputs produced by photodetectors  120   a  and  120   b  while light source  118  is on is transmitted to reader  701 . In step  1006 , reader  701  receives the data. The data may include a reading from photodetector  120   a  and a reading from photodetector  120   b , which is referred to as the reference photodetector. In step  1008 , reader  701  processes the received data to produce a first value. For example, the value may be produced by dividing the reading from photodetector  120   a  by the reading from photodetector  120   b.    
         [0066]    Next, light source  118  is turned off (step  1010 ). In step  1012 , data corresponding to the outputs produced by photodetectors  120   a  and  120   b  while light source  118  is off is transmitted to reader  701 . In step  1014 , reader  701  receives the data. The data may include a reading from photodetector  120   a  and a reading from photodetector  120   b.    
         [0067]    In step  1016 , reader  701  processes the received data to produce a second value. For example, the second value may be produced by dividing the reading from photodetector  120   a  by the reading from photodetector  120   b . In step  1018 , reader  701  subtracts the second value from the first value to obtain a result that can be used to determine the concentration of the analyte being monitored by the sensor. In step  1020 , reader  701  displays information concerning the concentration of the analyte (e.g., it displays a value representing the determined concentration). 
         [0068]    Although the above described processes are illustrated as a sequence of steps, it should be understood by one skilled in the art that at least some of the steps need not be performed in the order shown, and, furthermore, some steps may be omitted and additional steps added. 
         [0069]    While various embodiments/variations of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.