fig1 is a schematic , section view of an optical - based sensor (“ sensor ”) 110 , according to an embodiment of the invention , that operates based on the fluorescence of fluorescent indicator molecules 116 . as shown , sensor 110 includes a sensor housing 112 . sensor housing 112 may be formed from a suitable , optically transmissive polymer material . preferred polymer materials include , but are not limited to , acrylic polymers such as polymethylmethacrylate ( pmma ). sensor 110 may further include a matrix layer 114 coated on at least part of 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 ). sensor 110 further includes a radiation source 118 , e . g . a light emitting diode ( led ) or other radiation source , that emits radiation , including radiation over a range of wavelengths which interact with the indicator molecules 116 . for example , in the case of a fluorescence - based sensor , radiation sensor 118 emits radiation at a wavelength which causes the indicator molecules 116 to fluoresce . sensor 110 also includes a photodetector 120 ( e . g . a photodiode , phototransistor , photoresistor or other photosensitive element ) 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 in fig1 to illustrate that sensor 110 may have more than one photodetector . source 118 may be implemented using , for example , led model number eu - u32sb from nichia corporation ( www . nichia . com ). other leds may be used depending on the specific indicator molecules applied to sensor 110 and the specific analytes of interested to be detected . 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 fig1 ), 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 . suitable biocompatible matrix materials , which preferably are 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 . sensor 110 may be wholly self - contained . in other words , the sensor is preferably 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 , sensor 110 may be powered by an external power source ( not shown ), as is well known in the art . for example , the external power source may generate a magnetic field to induce a current in inductive element 142 ( e . g ., a copper coil or other inductive element ). additionally , circuitry 166 may use inductive element 142 to communicate information to an external data reader . circuitry 166 may include discrete circuit elements , an integrated circuit ( e . g ., an application specific integrated circuit ( asic ), and / or other electronic components ). the external power source and data reader may be the same device . in an alterantive embodiment , the sensor 110 may be powered by an internal , self - contained power source , such as , for example , microbatteries , micro generators and / or other power sources . as shown in fig1 , many of the electro - optical components of sensor 110 are secured to a circuit board 170 . circuit board 170 provides communication paths between the various components of sensor 110 . as further illustrated in fig1 , optical filters 134 a and 134 b , such as high pass or band pass filters , may cover a photosensitive side of photodetectors 120 a and 120 b , respectively . filter 134 a may prevent or substantially reduce the amount of radiation generated by the source 118 from impinging on a photosensitive side 135 of the photodetector 120 a . at the same time , filter 134 a allows fluorescent light emitted by fluorescent indicator molecules 116 to pass through to strike photosensitive side 135 of the photodetector 120 a . this significantly reduces “ noise ” in the photodetector signal that is attributable to incident radiation from the source 118 . according to one aspect of the invention , an application for which the sensor 110 was developed — although by no means the only application for which it is suitable — is measuring various biological analytes in the human body . for example , sensor 110 may be used to measure glucose , oxygen , toxins , pharmaceuticals or other drugs , hormones , and other metabolic analytes in the human body . the specific composition of the matrix layer 114 and the indicator molecules 116 may vary depending on the particular analyte the sensor is to be used to detect and / or where the sensor is to be used to detect the analyte ( i . e ., in the blood or in subcutaneous tissues ). preferably , however , matrix layer 114 , if present , should facilitate exposure of the indicator molecules to the analyte . also , it is preferred that the optical characteristics of the indicator molecules ( e . g ., the level of fluorescence of fluorescent indicator molecules ) be a function of the concentration of the specific analyte to which the indicator molecules are exposed . to facilitate use in - situ in the human body , the housing 112 is preferably formed in a smooth , oblong or rounded shape . advantageously , it has the approximate size and shape of a bean or a pharmaceutical gelatin capsule , i . e ., it is on the order of approximately 500 microns to approximately 0 . 85 inches in length l and on the order of approximately 300 microns to approximately 0 . 3 inches in diameter d , with generally smooth , rounded surfaces throughout . this configuration permits the sensor 110 to be implanted into the human body , i . e ., dermally or into underlying tissues ( including into organs or blood vessels ) without the sensor interfering with essential bodily functions or causing excessive pain or discomfort . in some embodiments , a preferred length of the housing is approx . 0 . 5 inches to 0 . 85 inches and a preferred diameter is approx . 0 . 1 inches to 0 . 11 inches . in the embodiment shown in fig1 , source 118 is elevated with respect to a top side 171 of circuit board 170 . more specifically , in the embodiment shown , source 118 is fixed to a support member 174 , which functions to elevate source 118 above side 171 and to electrically connect source 118 to circuitry on board 170 so that power can be delivered to source 118 . the distance ( d ) between source 118 and side 171 generally ranges between 0 and 0 . 030 inches . preferably , the distance ( d ) ranges between 0 . 010 and 0 . 020 inches . support member 174 may be a circuit board . circuit board 170 may have a groove 180 for receiving a proximal end 173 of member 174 . this feature is further illustrated in fig3 , which is a perspective , top view of board 170 . in some embodiments , support member 174 may include an electrical contact 158 ( e . g ., a conductive pad or other device for conducting electricity ) disposed on a surface thereof and electrically connected to source 118 . the contact 158 electrically connects to a corresponding electrical contact 157 that may be disposed in groove 180 through an electrical interconnect 159 ( e . g ., a circuit trace or other transmission line ). contact 157 may be electrically connected to circuit 166 or other circuit on circuit board 170 . accordingly , in some embodiments , there is an electrical path from circuit 166 to source 118 . as further shown in fig1 , a reflector 176 may be attached to board 170 at an end thereof . preferably , reflector 176 is attached to board 170 so that a face portion 177 of reflector 176 is generally perpendicular to side 171 and faces source 118 . preferably , face 177 reflects radiation emitted by source 118 . for example , face 177 may have a reflective coating disposed thereon or face 177 may be constructed from a reflective material . referring now to photodetectors 120 , photodetectors 120 are preferably disposed below a region of side 171 located between source 118 and reflector 176 . for example , in some embodiments , photodetectors 120 are mounted to a bottom side 175 of board 170 at a location that is below a region between source 118 and reflector 176 . in embodiments where the photodetectors 120 are mounted to bottom side 175 of board 170 , a hole for each photodetector 120 is preferably created through board 170 . this is illustrated in fig3 . as shown in fig3 , two holes 301 a and 301 b have been created in board 170 , thereby providing a passageway for light from indicator molecules 116 to reach photodetectors 120 . the holes in circuit board 170 may be created by , for example , drilling , laser machining and the like . preferably , each photodetector 120 is positioned such that light entering the hole is likely to strike a photosensitive side of the photodetector 120 , as shown in fig1 . this technique also diminishes the amount of ambient light striking photodetector 120 . as further illustrated in fig1 , each hole in board 170 may be contain a filter 134 so that light can only reach a photodetector 120 by passing through the corresponding filter 134 . the bottom side and all sides of the photodetectors 120 may be covered with black light blocking epoxy 190 to further diminish the amount of ambient light striking photodetector 120 . in one embodiment , photodetector 120 a is used to produce a signal corresponding to the light emitted or adsorbed by indicator molecules 116 and photodetector 120 b is used to produce a reference signal . in this embodiment , a fluorescent element 154 may be positioned on top of filter 134 b . preferably , fluorescent element 154 fluoresces at a predetermined wavelength . element 154 may be made from terbium or other fluorescent element that fluoresces at the predetermined wavelength . in this embodiment , filter 134 a and filter 134 b filter different wavelengths of light . for example , filter 134 a may filter wavelengths below 400 nm and filter 134 b may filter wavelengths below 500 nm . referring now to fig2 , fig2 illustrates a sensor 210 according to another embodiment of the invention . as shown in fig2 , sensor 210 is similar to sensor 110 . a primary difference being that reflector 176 is replaced by a support member 202 , which is connected to end 194 of board 170 and to which source 118 is fixed . in this embodiment , and support member 174 is replaced with a reflector 209 . like reflector 176 , reflector 209 has a reflective face 211 that faces source 118 . additionally , so that photodetector 120 a remains closer to source 118 , photodetector 120 a may switch places with photodetector 120 b and filter 134 a may switch places with filter 134 b . fluorescent element 154 may also be re - positioned so that it remains on top of filter 134 b . as shown in fig1 and 2 , in some embodiments , indicator molecules 116 may be positioned only in a region that is above a region 193 , which region is between source 118 and reflector 176 . referring now to fig4 , fig4 is a schematic , section view of an optical - based sensor 410 , according to another embodiment of the invention . sensor 410 includes many of the same components as sensor 110 . however , the positioning of source 118 , photodetector 120 a and filter 134 a in sensor 410 is different than the positioning in sensor 110 . as shown in fig4 , a base 412 is mounted to an end 413 of circuit board 170 . a top side 414 and bottom side 416 of base 412 each may lie in a plane that is generally perpendicular to a plane in which side 171 of board 170 lies . bottom side 416 may have a groove 418 therein that receives end 413 of board 170 . groove 418 facilitates fixing base 412 to board 170 . photodetector 120 a may be mounted on top side 414 of base 412 . preferably , photodetector 120 a is mounted on base 412 so that photosensitive side 135 of photodetector 120 a lies in a plane that is generally perpendicular to the plane in which side 171 of board 170 lies and faces in the same direction as top side 414 . filter 134 a is preferably disposed above side 135 of photodetector 120 a so that most , if not all , light that strikes side 135 must first pass through filter 134 a . filter 134 a may be fixedly mounted to photodetector 120 a . for example , a refractive index ( ri ) matching epoxy 501 ( see fig5 ) may be used to fix filter 134 a to photodetector 120 a . in some embodiments , base 412 may include at least two electrical contacts disposed thereon ( e . g ., on side 414 ). for example , as shown in fig4 , a first electrical contact 471 and a second electrical contact 472 are disposed on side 414 of base 412 . a wire 473 ( or other electrical connector ) preferably electrically connects photodetector 120 a to electrical contact 471 and a wire 474 ( or other electrical connector ) preferably electrically connects source 118 to electrical contact 472 . contact 471 electrically connects to a corresponding contact 475 via an electrical interconnect 476 . similarly , contact 472 electrically connects to a corresponding contact 477 via an electrical interconnect 478 . contacts 475 , 477 are preferably disposed on the end of board 170 that is inserted into groove 418 . contacts 475 , 477 may be electrically connected to circuit 166 or other circuit on circuit board 170 . accordingly , in some embodiments , base 412 provides a portion of an electrical path from circuit 166 to source 118 and / or photodetector 120 a . referring now to fig5 , fig5 further illustrates the arrangement of photodetector 120 a , filter 134 a and source 118 . as shown in fig4 and 5 , source 118 is mounted on a top side 467 of filter 134 a . accordingly , as shown in fig4 and 5 , photodetector 120 a , filter 134 a and source 118 are aligned . that is , as shown in fig5 , both filter 134 a and source 118 are each disposed in an area that is over at least a portion of photosensitive side 135 of photodetector 120 a . preferably , a non - transparent , non - translucent base 431 is disposed between source 118 and filter 134 . opaque base 431 functions to prevent light emitted from source 118 from striking side 467 of filter 134 a . base 431 may be a gold - clad - molybdenum tab ( molytab ) or other opaque structure . epoxy 555 may be used to fix source 118 to base 431 and base 431 to filter 134 a . preferably , in this embodiment , source 118 is configured and oriented so that most of the light transmitted therefrom is transmitted in a direction away from side 467 , as shown in fig4 and 5 . for example , in the embodiment shown , the light is primarily directed towards an end 491 of housing 102 . preferably , indicator molecules 116 are located on end 491 so that they will receive the radiation emitted from source 118 . as discussed above , indicator molecules 116 will respond to the received radiation , and the response will be a function of the concentration of the analyte being measured in the region of the indicator molecules 116 . photodetector 120 a detects the response . referring now to fig6 , fig6 is a schematic , section view of an optical - based sensor 610 , according to another embodiment of the invention . sensor 610 includes many of the same components as sensor 110 . also , sensor 610 is similar to sensor 410 in that , in sensor 610 , photodetector 120 a , filter 134 a and source 118 are preferably aligned . further , like in sensor 410 , in sensor 610 filter 134 a may be fixedly mounted on side 135 of photodetector 120 a and source 118 may be fixedly mounted on side 467 of filter 134 a , and the photodetector 120 a , filter 134 a , source 118 assembly may be located adjacent an end 491 of housing 102 , as illustrated in fig6 . however , the orientation of source 118 , photodetector 120 a and filter 134 a in sensor 610 is different than the orientation in sensor 410 . for example , in sensor 610 , side 135 of photodetector 120 a faces in a direction that is substantially perpendicular to the longitudinal axis of housing 102 . additionally , in sensor 610 , filter 134 a and / or photodetector 120 a are directly fixed to board 170 such that base 412 may be removed . in the embodiment shown , filter 134 a and / or photodetector 120 a are directly fixed to end 413 of board 170 . in one or more of the above described embodiments , housing 102 may be filled with a material to keep the components housed in housing 102 from being able to move around . for example , housing 102 may be filled with an optical epoxy either before or after board 170 and the components attached thereto are inserted into housing 120 . epo - tek 301 - 2 epoxy from epoxy technology of billerica , mass . and / or other epoxies may be used . 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 .