as discussed in relation to in fig6 supra , the relative pathlength varies considerably depending upon the wavelength of light used . fig9 a , 9 b and 9 c illustrate three alternate sampling configurations which optimize the pathlength relative to the wavelength ( s ) selected . as those skilled in the art will appreciate , the relative positioning of the source and detector on finger / thumb 11 will influence the internal volume of tissue sampled spectrophotometrically . in fig9 a the source 21 and detector 23 are placed relatively close to one another . thus , the average optical pathlength 22 travelled by the sampled light ( via partial transmission ) is quite short . dashed lines ( e . g ., 24 ) represent some of the theoretical paths followed by light emitted by a point source placed against finger / thumb 11 . if , in the case of glucose , optical information is desired in the 2000 nm region or longer , then the average optical pathlength 22 should be in range of 0 . 5 - 3 mm . in addition to the 2000 - 2400 region , this optical sampling configuration enables measurement of spectral data over the entire wavelength region from 700 to 2400 nm . in fig9 a the internal volume sampled will be relatively small , consisting of the epidermis 13 and dermis 15 and the outermost portions of the subcutaneous tissue 17 . although the sampling configuration in fig9 a enables sampling of spectral data from 700 to 2400 nm , the spectral data acquired in the 700 - 1400 nm region is not acquired under optimal conditions . given similar signal - to - noise ratios , spectral data in the 700 - 1400 nm range , obtained by the sampling geometries illustrated in fig9 b and 9c , will contain more analyte information than spectra obtained from the geometry illustrated in fig9 a . with similar intensity values at the detector , the information content of the spectra from fig9 a is less , due to the fact that a smaller portion of the internal volume of finger / thumb 11 is sampled by the light . in the configuration shown in fig9 a , 100 % of the light is not forced to transverse the greater distances illustrated by fig9 b and 9c and , thus , does not contain as much information from the deeper subcutaneous tissue . in fig9 b the detector 23 and source 25 are separated by a greater distance and the resulting average partial transmission pathlength 26 is longer . if the average pathlength 26 is assumed to be in the range of 3 - 7 mm , then the amount of light successfully transversing this distance with wavelengths longer than 2000 nm will be negligible in comparison to those which transverse a 0 . 5 - 3 mm pathlength . however , this pathlength represents a reasonable pathlength for light in the region from 1400 to 2000 nm , and also enables measurement of wavelengths below 1400 nm . fig9 c depicts a transmission measurement through finger / thumb 11 . the finger thickness of an average individual is , approximately , 1 cm . the influence of the bone in transmission is poorly understood and , thus , the ray tracings go around the bone for simplicity . the resulting optical information received by detector 23 from source 29 will be in the wavelength region from 700 to 1400 nm . the spectral information above 1400 nm will be negligible due to water absorbance at these longer wavelengths . the geometric configurations shown in fig9 a , 9 b and 9 c enable procurement of optimal spectral data for glucose in each wavelength region ( i . e . 700 to 1400 , 1400 to 2000 , and 2000 to 2400 ) in the best possible manner . for clarification the spectral range of interest is divided into 3 regions : region 1 from 700 - 1400 nm ; region 2 from 1400 - 2000 nm ; and region 3 from 2000 - 2400 nm . fig1 a , 10 b and 10 c are plots showing the spectral regions that can be recorded by each of the three source - detector configurations . fig1 a is the spectral data which can be recorded using the source - detector configuration shown in fig9 a . as previously described , the configuration shown in fig9 a can sample all three regions as shown in fig1 a . fig1 b depicts the spectral data obtained when using the optical configuration illustrated in fig9 b , which spectral data is in regions 1 and 2 , but not in region 3 . fig1 c shows the spectral data generated by using the optical configuration shown in fig9 c . in this case , useful spectral information is available only in region 1 . fig1 is a plot of the resulting spectra from the source detector arrangements illustrated in fig9 a , 9 b and 9 c . the resulting spectra contains the maximal amount of analyte information due to optimization of sampling path in each of the three wavelength regions . the spectrum of fig1 can be further improved by removal of the spectral variations introduced by melanin and other patient variations . specifically , spectra which is indicative of the internal tissue glucose ( independent of the individual , and without such factors as degree of pigmentation , age , skin thickness , and differences in peripheral - skin temperature ) is uniquely useful when doing quantitative measurements . as the intrinsic volumes sampled by the various source - detector geometries illustrated in fig9 a , 9 b and 9 c are different , these differences can be used to remove many undesired influences ( e . g . melanin ). returning to fig9 a , 9 b and 9 c , it can be seen that all source - detector combinations require the light to transverse epidermis 13 and dermis 15 twice . this is true whether the measurement is made through skin only or the fingernail . in the 700 nm to 1100 nm region ( i . e ., region 1 ) the spectral information obtained by using source 21 with detector 23 will correspond to skin information twice , plus a small amount of information on the underlying tissue . in comparison , the spectral information obtained by using source 29 with detector 23 will contain skin information twice , plus a large amount of information on the underlying tissue . the differences in the volumes sampled by the different sampling geometries can be used to cancel out or minimize skin specific differences . as melanin resides solely in the outer layers of skin , the spectra obtained via the two sampling configurations can be processed to yield spectra which minimizes spectral variations which are the result of pigmentation differences . specifically , beer &# 39 ; s law and the relationship of mean optical pathlength with the length of the physical path can be utilized by a ratio and subtraction process to yield the desired spectra . the coefficient values to be used in association with beer &# 39 ; s law are determined by experimental investigation and are instrument / configuration dependent . differences in peripheral skin temperature can also be removed by using differences in the volumes sampled by the various sampling configurations . the skin temperature of the hand varies greatly from person to person and is also dependent upon the environment . despite peripheral skin temperature variations , body core temperature remains quite constant . the internal tissue of the hand will be maintained at relatively constant temperature due to constant exposure to core temperature blood . thus , the temperature differences between people &# 39 ; s hands exist in the dermis and epidermis while the underlying tissue remains well thermostated . as temperature differences are present in the outermost layers of the skin , the difference in the volumes sampled can again be used to help minimize spectral variation not associated with analyte concentration . thus , skin temperature differences can be compensated for in a manner similar to melanin . the sampling configurations illustrated in fig9 a , 9 b and 9 c satisfy objectives 1 and 2 of the invention , but do not reduce all possible spectral variations between patients . specifically , the spectral information obtained by transmission through the finger ( i . e . use of source 29 and detector 23 ) will be sensitive to tissue thickness differences . however , such differences in tissue thickness can be minimized by performing the sampling by partial transmission sampling and by sampling from the same side of the tissue , wherein the tissue can be a finger , thumb , or other body part . with reference to fig1 and 13 , finger / thumb holder 33 includes a bottom plate 35 and a pair of guide rails 37 . positioned substantially in the center of plate 35 is a detector 39 and a plurality of sources 41 1 - 4 , 43 1 - 4 and 45 1 - 4 . the average light path from sources 41 1 - 4 to detector 39 is approximated by path 47 ; from sources 43 1 - 4 to detector 39 , by path 49 ; and from sources 45 1 - 4 to detector 39 , by path 51 . as was illustrated in fig9 a , 9 b and 9 c , the optical sampling of the body part must maximize and compensate for the optical propagation characteristics of different wavelengths . in fig1 light path 47 represents an optical pathlength of 0 . 5 - 3 mm , which is the same pathlength as illustrated in fig9 a . light path 49 is similar to that shown in fig9 b , and light path 51 is similar to that shown in fig9 c . in either sampling geometry the relationship between the source and detector determines the length of the optical path and the depth of the internal volume sampled . for each distance a plurality of light sources is used to increase the intensity at detector 39 , and to reduce the total measurement time . a variety of embodiments can be utilized to enable sampling of the finger / thumb in an optimal manner . fig1 illustrates a device 61 using fiber optics to introduce light into the finger / thumb 11 at three different sampling geometries . in the device shown , housing or ring 63 supports six different fiber optic probes 65 a and 65 b , 67 a and 67 b , and 69 a and 69 b which introduce the light into the tissue in three different source - detector configurations . detector probe 71 , also supported by housing 63 receives the light which has transversed the tissue . fiber probes 65 a - 69 b and detector probe 71 are all spring loaded , via springs 73 , to enable repeatable interactions between the tip of each fiber , as indicated at 75 in fig1 , and finger / thumb 11 . each fiber probe is independently spring loaded to enable the sampling device to compensate for the elliptical shape of the finger . as also indicated in fig1 , representative probe 65 a is held by a hollow fiber holding device 77 which includes a hollow stem 79 and a shoulder 81 . stem 79 is held in bore 83 by collar 85 which , in turn , is slidably received in bore 87 in housing 63 . spring 73 is captured between collar 85 and cap 89 which is threaded into housing 63 ( by threads not shown ) or otherwise suitably secured . bore 87 has an internal shoulder 91 to prevent collar 85 from falling out . probes 65 b - 69 b and detector 71 have the same structure as probe 65 a . fig1 a illustrates , on an enlarged scale , the end of a typical probe ( e . g . 65 a ), including metal sheath 93 , surrounding external cladding 95 , which in turn surrounds optical fiber 97 . the angular relationship between fiber optic probes 65 a and 65 b , and detector fiber 71 is , approximately , 30 degrees . this geometrical configuration allows , as discussed above , sampling of the spectral region from 700 to 2400 nm . probes 67 a and 67 b simultaneously introduce light into finger / thumb 11 at 90 degrees relative to detector fiber 71 . the average optical pathlength will be approximately 3 to 7 mm . the configuration composed of probes 67 a and 67 b , and fiber 71 will enable sampling of the spectral region from 700 to 2000 nm . the remaining two probes , 69 a and 69 b , introduce light into finger / thumb 11 on the opposite side of the finger , at approximately 165 degrees relative to fiber 71 . light detected by detector fiber 71 from these 2 probes has propagated through the majority of finger / thumb 11 . for glucose , the spectral region measured by this configuration will be from 700 to 1400 nm . in operation , only one source pair - detector configuration ( e . g . 65 a / 65 b - 71 ) is coupling light into finger / thumb 11 at any one time . the operation of the sources is performed in a manner to determine the optical path transversed by the light . if all sources are active at a given point in time then it would be impossible to determine from which point a given photon of light originated . the introduction of light into the finger from two geometrically similar locations ( e . g . probes 65 a and 65 b ) increases the total light entering finger / thumb 11 , which increases the total amount out of finger / thumb 11 and onto the detector fiber 71 . increasing the intensity at the detector , provided the operation remains linear , is desirable as it reduces total measurement time . a modified version of finger sampling device 61 is illustrated in fig1 . device 661 includes a housing or ring 663 having an outer cylindrical surface 665 and two semi - cylindrical surfaces 667 and 669 , interconnected by plainer surfaces 671 and 673 . as illustrated , the diameter of surface 667 provides a support for finger / thumb 11 . positioned between surfaces 665 and 669 are two fiber optic probes 65 a and 65 b and a detector 71 . probes 65 a and 65 b ( which are angularly positioned relative to detector 71 by , approximately , 30 °) contact fingernail 13 a . as previously explained , fingernail 13 a is basically optically inert in the region from 700 mm to 2400 mm . though only two probes are illustrated , additional ones could be provided . fig1 illustrates a sampling device where all optical sampling is performed through the fingernail . the device could be modified to include small tungsten - halogen sources or other configurations . the end result is a sampling device which makes optimal use of this “ window ” into the body . fig1 illustrates a sampling device like that of fig1 and 15 , except modified for use of small tungsten - halogen light sources such as manufactured by welsh alan . six light sources , such as source 101 a , are arranged on circular housing 103 ( which has the same configuration as housing 63 ) in the same pattern as probes 65 a - 69 b are arranged relative to detector 71 . light source 101 a is secured in socket 105 provided on the end of hollow electrical support stem 107 . stem 107 includes a circular collar 109 fixed thereto , which is slidably received in bore 111 provided in housing 103 . spring 113 is captured between collar 109 and end cap 115 ( suitably secured to housing 103 ), to bias source 101 a into engagement with finger / thumb 11 . lip 117 prevents collar 109 from being pushed out of bore 111 . as described previously , this geometry enables spectral measurement over different regions . in operation , the light sources closest the detector will be turned on and spectral data acquired in the 700 to 2400 nm region . next , the sources at 90 degrees to the detector will be energized and spectral data from 700 to 2000 nm will be recorded . finally , the sources opposite the detector will be turned on and spectral data from 700 to 1400 nm recorded . due to the fact that not all wavelengths need to be recorded and the fact that relatively small number ( e . g . 20 ) will produce good measurement results , a discrete set of light emitting diodes ( leds ) can be utilized instead of broadband sources such as 101 a of fig1 and 17a . currently , leds are commercially manufactured in the wavelength region from 400 to approximately 1550 . however , in the future , diodes will likely become commercially available over the entire spectral region from 400 to 2400 nm . as leds emit wavelengths with a narrow bandwidth ( i . e ., typically less than 10 nm ) further dispersion of the spectra is not necessary . as further dispersion is not necessary a simple inexpensive detector or multiple detectors can be used to record the various wavelength intensities . as illustrated in fig1 - 21 , finger sampling device 121 includes a first set of probes 123 a , 123 b , and a second set of probes 125 a , 125 b , mounted at 9 ° intervals around housing 127 . probes 123 a and 123 b each include a hollow stem portion 129 ( fig1 ) of circular cross - section and a disc shaped head 131 ( fig2 ). secured to head 131 near its perimeter ( by conventional means not illustrated ) are a plurality of leds 133 1 - 12 . secured to the center ( also by conventional means not shown ) is a detector 135 . detector 135 and leds 133 are connected to suitable electrical connectors such as 136 a , 136 b and 136 c . stem portion 129 also includes a circular collar 137 which is slidably received in bore 139 of housing 127 . probe 123 is biased into engagement with finger / thumb 11 by spring 141 which , in turn , is captured between collar 137 and cap 143 ( secured by suitable conventional means , not shown , to housing 127 ). cap 143 has a circular opening 145 dimensioned to slidably receive stem 129 . finally , each bore 139 has a shoulder 147 at its inner end to retain collar 137 . probes 125 a , 125 b are substantially identical to probes 123 , except that they do not include any detectors . thus , each probe 125 includes a hollow stem portion 151 , collar 153 and disc shaped head 155 . collar 153 is slidably received in bore 157 and biased by spring 159 , which is captured between collar 153 and cap 161 . cap 161 has a circular opening 163 for slidably receiving stem 151 . bore 157 includes internal shoulder 165 . as with those on probes 123 , each led on probes 125 represents a narrow bandwidth source predetermined to obtain important spectral information enabling measurement of the concentration of the analyte ( e . g ., glucose ). considering only the region from 400 to 1550 nm , the region over which leds currently operate , the spectral information ( similar to that illustrated in fig1 ) is obtained by three separate operations using the specific regions of 400 to 1100 nm , 1100 to 1400 nm , and 1400 to 1550 nm . the procurement of spectra in the 400 to 1100 nm region requires the measurement of both short pathlength and long pathlength spectral data , which can be measured at the same time , as illustrated in fig2 a . the leds 133 1 - 12 on probe 123 a are energized in a manner such that the intensity associated with each led can be determined . specifically , the leds are energized in accordance with hadamard transform optical coding techniques . this is done because if every led was turned on at the same time it would be impossible to differentiate between specific wavelengths . for each led on probe 123 a , the light propagating through the tissue is measured by both detectors 135 simultaneously ( detector 135 on probe 123 a and detector 135 on probe 123 b ). thus , detector 135 / 123 a will measure those photons having transversed a short partial transmission path 171 , while detector 135 / 123 b will measure those photons having transversed the entire finger . solid line 173 illustrates the average path that the light will travel . dashed lines 171 a and 173 a represent the average short and long paths between a second led on probe 123 a and detectors 135 . the combination of information from these two detectors can be used to remove for skin specific components and result in spectral information ready for analysis , for the reasons discussed above in reference to fig9 a - 11 . the leds on probe 123 b are not energized . spectra in the region from 1100 to 1400 nm does not require compensation for skin differences because the affects of melanin and other skin components are not present at wavelengths above 1100 nm . thus , maximizing the amount of light into the finger while also maximizing the amount of information recorded from the finger is desired and achievable . this can be performed by energizing ( again in accordance with hadamard transform optical coding techniques ) those leds on the 1100 to 1400 nm range on probe 123 b . the light emitted from such leds is recorded by detector 135 / 123 a . fig2 b illustrates this concept , with lines 175 and 177 representing a couple of the theoretical pathlengths . as previously discussed , light propagation through tissue at wavelengths longer than 1400 nm becomes heavily influenced by water absorbance . measurement of wavelengths between 1400 and 1550 nm can be performed by energizing the leds in probes 125 a and 125 b . the light emitted from these leds will be simultaneously measured by detectors 135 / 123 a and 135 / 123 b . fig2 c illustrates this arrangement with representative theoretical light paths 179 , 181 , 183 , 185 , 187 , 189 , 191 and 193 . the end result of the preceding process is measurement of the spectral data from 400 to 1550 nm in the least possible time , with the highest possible signal - to - noise ratio and containing the spectral information necessary for analyte measurement . the use of multiple detectors and light sources will improve the signal - to - noise ratio of the recorded data for a given measurement time due to the ability to signal average two measurements simultaneously , and the ability to use hadamard transform techniques . fig2 and 23 , similar to fig1 and 13 , show finger / thumb sampling device 201 using partial transmission with path optimization by separation of the sources and detector . sampling device 201 includes base 203 having a finger support surface 205 , a pair of guide rails 207 ( for positioning finger / thumb 11 ) and a post 209 . device 201 also includes an arm 211 ( which is hinged to post 209 and biased toward surface 205 by spring 213 ), shutter control 215 , and temperature control device 217 . shutter control 215 includes a rotating disc 221 ( having gear teeth 223 on the perimeter thereof ), gear 225 and motor 227 . temperature control 217 includes an electrical heating pad , a temperature sensing device ( e . g ., a thermocouple ) and associated conventional electronics ( all not illustrated ). disc 221 , which is supported by housing 229 in any convenient manner ( not shown ), has a plurality of openings ( typically circular ) 231 , 233 1 - 4 , 235 1 - 4 , and 237 1 - 4 , as best illustrated in fig2 . housing 229 includes a series of openings 241 , 243 a and 243 b , 245 a and 245 b , and 247 a and 247 b , which are always aligned with , respectively , openings ( 251 , 253 a and 253 b , 255 a and 255 b , and 257 a and 257 b ) provided in base 203 . in operation , light is introduced into finger / thumb 11 by fiber optic sources ( not shown ), the ends of which are received in openings 243 a , 243 b , 245 a , 245 b , 247 a , and 247 b . the distances between the fiber optic sources and the fiber optic detector ( also not shown ) which is received in opening 241 is optimized for pathlength for the reasons discussed above . the light introduced into finger / thumb 11 from the fiber optics received in openings 243 a and 243 b is partially transmitted with an average pathlength of 0 . 5 to 3 mm , as illustrated by theoretical paths 261 . when these fibers are emitting light , the detector fiber receives light from 700 and 2400 nm . openings 245 a / 255 a and 245 b / 255 b are separated from aligned openings 241 / 231 / 251 by a greater distance , to optimize the pathlength 263 for recording wavelengths in the 1400 to 2000 nm region . when this second set of fibers are emitting light , the majority of the light received by the detector fiber will be in the wavelength region from 700 and 2000 nm . openings 247 a / 257 a and 247 b / 257 b are at the greatest distance from detector opening 241 / 231 / 251 and , thus , have the longest average pathlength 265 . fibers coupled to these source openings enable the measurement of wavelengths between 700 and 1400 nm . by placement of the source fibers and detector fiber on the same side of the finger / thumb the influence of finger thickness on the resulting spectral data is minimized . by reducing the influence of finger / thumb thickness , between patient differences are minimized and more accurate analyte measurements can be made . in operation only those fibers at a given distance will emit light into finger / thumb 11 at a given time . shutter system 215 controls which fibers are illuminating finger / thumb 11 at any one time . the blocking or passage of light is controlled by disk 221 and the angular orientation of openings 233 1 - 4 , 235 1 - 4 , and 237 1 - 4 relative to each other . central opening 231 allows transmission of light at all times . in operation disk 215 is rotated to position a , b , or c . in position a , disk 215 allows illumination of the finger by fibers connected to openings 243 a and 243 b . in fig2 the disk is shown in position a and shows the complete propagation of light via path 261 . if disk 215 is rotated to position b , the light would follow paths 263 , as shown by the dotted lines . rotation of disk 215 to position c would enable light to travel paths 265 , as shown by the dashed lines . thus , the use of the rotating disk forms a simple reliable shutter system to enable introduction of the light in a easily controlled manner . in addition to pathlength optimization for the various wavelengths , the sampling device shown in fig2 and 23 is thermostated to control finger temperature . control of the sampling devices temperature is performed by temperature control device 217 ( including a heating pad , a temperature sensing device , typically a thermocouple , and associated electronics ). the temperature control unit 217 is attached to plate 203 . with reference to fig2 , the tissue , typically a finger or thumb , is compressed firmly against surface 205 by arm 211 and spring 213 . compression of the finger with approximately 1 kg / cm 2 will , as discussed previously , minimize the influence of arterial pulsation in the optical sampling area . this force is not so extreme as to be painful to the patient . other methods can be used to remove arterial pulsations , such as finger cuffs which are inflated to a pressure which occludes arterial pulsations . however , this cuffing technique has not proven as desirable , as application of force on the finger reduces movement of the tissue relative to the sampling device . previous configurations ( except the embodiment of fig1 - 21 ) have involved the use of one detector , one wavelength separating device ( e . g . aotf , grating , etc .) and multiple sources . fig2 , 25 , 26 and 27 illustrate sampling configurations which utilize one source , multiple detectors , and multiple wavelength separating devices . fig2 and 25 illustrate sample device 701 , which includes a base 703 , a finger support surface 705 , and a pair of guide rails 707 for positioning finger / thumb 11 . like sampling device 201 , but not illustrated , device 701 includes an arm , secured to a post and biased into engagement with the finger / thumb 11 by a spring . positioned within base 703 is a single broadband light source 709 and 24 band - pass optical filters , 6 of which are illustrated in fig2 ( i . e ., 711 1 , 711 5 , 713 1 , 713 5 , 715 1 , and 715 5 ). these band - pass optical filters are constructed of specially coated glass which permits only the preselected wavelengths ( either a single wavelength or a band of contiguous wavelengths ) to pass . the other wavelengths are attenuated or not permitted to pass through . each of the band - pass filters is coupled to a detector ( i . e ., 717 1 - 8 , 719 1 - 8 , and 721 1 - 8 ). the detectors are electrically connected an analog to digital converter via wires such as indicated by 723 in fig2 . in operation , broadband source 709 is energized with some of the light partially transmitted through finger / thumb 11 , as illustrated , by traces 725 , 727 and 729 . the light then passes through the band - pass optical filters which reduce the broadband light into preselected wavelengths ( as indicated above ). these discrete wavelengths are then detected on the detectors 717 1 - 721 8 . with the use of 24 detectors , 24 wavelengths are measured . the actual measurement of these intensity values could be one at a time , through standard sample and hold electronics or by hadamard transform optical coding . although 24 detectors are illustrated , the number could be increased or decreased . this design is based on the fact that accurate analyte measurement can be obtained with a certain number of preselected wavelengths . see fig8 . sampling device 731 , illustrated in fig2 and 27 , includes a base 733 , having a finger support surface 735 , a pair of guide rails 737 , and a post 739 . device 731 also includes a temperature control device 740 , and an arm 741 , which is hinged ( not shown ) to post 739 and biased toward surface 735 by spring 743 . filter wheel assembly 745 , which is secured to surface 747 of base 733 , includes a rotating filter wheel 749 ( having gear teeth 751 on the perimeter thereof ), gear 753 , and stepper motor 755 . filter wheel 749 , which is supported by housing 757 in any convenient manner ( not shown ) is provided with 24 band - pass filters 759 1 - 24 , each of which passes wavelength subsets ( λ 1 , λ 2 , λ 3 . . . λ 24 ). housing 757 also includes openings 761 , 763 and 765 ( which are aligned with , respectively , openings 771 , 773 and 775 in base 735 ) and central opening 767 ( which is aligned with opening 777 in base 735 ). as illustrated , detectors 781 , 783 and 785 are positioned in the lower end of openings 771 , 773 and 775 . light source 787 is positioned relative to aligned openings 767 , 777 . detectors 781 , 783 and 785 are connected to an analog to digital converter via signal line 789 . in operation , light is introduced into finger / thumb 11 via broadband source 787 via a light pipe , a portion of which is partially transmitted as indicated by paths 791 , 793 and 795 . light with a wavelength λ , which traverses path 791 passes through filter 759 1 , and is detected by detector 781 . similarly , light with a wavelength λ 2 , which traverses path 793 , passes through band - pass filter 795 2 to detector 783 . finally , light with a wavelength λ 3 , which traverses path 795 , passes through band - pass filter 795 3 to detector 785 . as filter wheel 749 has eight discrete positions ( i . e ., a - h ), once the intensities of wavelengths λ 1 , λ 2 , and λ 3 have been measured , stepper motor rotates wheel 749 from the position illustrated in fig2 to the position where position b is aligned with detectors 781 , 783 and 785 . in this position the intensities of wavelengths λ 4 , λ 5 and λ 6 are then measured . wheel 749 is rotated through the remaining positions until all 24 wavelengths are measured . regardless of the exact finger sampling device used , each enables optimization of the path used for optical sampling vis - a - vis the light propagation characteristics of the measured wavelengths . the associated instrumentation needed to generate and subsequently measure these “ optimized ” wavelengths can take a variety of forms . fig2 illustrates in a box diagram the general configurations that such instrumentation can take . the three major categories involve dispersion options followed by source options , and finally those options available for detectors . the ten configurations are discussed below . fig2 schematically illustrates configurations 1 through 4 , where the light is dispersed or separated before it interacts with the finger . in all cases the light sources generates light with a band width broader than desired for the noninvasive measurement . in most cases the light source will be a broadband light source such as a tungsten halogen lamp . the broadband light is subsequently separated or dispersed and only the wavelengths of interest interact with the finger . the dispersion of the light can be performed by a number of devices . dispersion devices in common use are aotfs , fourier transform interferometers , and filter wheels . fig3 and 37 illustrate the use of an aotf to disperse the light before it interacts with the tissue . the light of fig3 and 37 then propagates through the tissue and is subsequently detected by multiple detectors . this corresponds to configuration 2 in fig2 . fig3 schematically illustrates configurations 5 and 6 , where the light source is capable of emitting light of a narrow bandwidth and subsequent dispersion of the light is not necessary . some light sources having these characteristics are light emitting diodes , lasers of all types , and tunable lasers . although a tunable laser is a single unit , it is considered as multiple sources in this description . fig1 and 34 illustrate configuration 6 and describe a noninvasive measurement device incorporating multiple sources and multiple detectors . fig3 schematically illustrates configurations 7 through 10 , where the light is dispersed following its interaction with the finger . the light source emits light of a bandwidth greater than desired for noninvasive measurement and subsequent separation of the light is required . the separation of the light can be performed by numerous commercially available components . fig2 - 27 illustrate configuration 8 wherein the dispersion of light is performed by selective optical filtering . fig3 illustrates configuration 9 wherein the dispersion of the light is performed by an aotf following interactions with the finger . fig3 illustrates a variation of configuration 10 wherein the light is separated by an aotf . two detectors are employed in order to record both the light having propagated through the finger as well as the intensities of the light source via a background fiber . as one skilled in the art will recognize an infinite number of instrument configurations can be realized for measurement of the appropriate spectral information . for clarity four instrument examples are illustrated and their operation described . fig3 illustrates a noninvasive analyte monitor 301 using multiple small tungsten - halogen light sources . again , partial transmission with separation of the detector and sources for optimization of the various paths within the finger is employed . monitor 301 includes a finger sampling device 303 which , like sampling device 201 , includes a base 305 , finger support surface 307 , a pair of guide rails 309 ( for positioning finger or thumb 11 ), arm 311 , support arm post 313 , spring 315 for biasing finger / thumb 11 into engagement with surface 307 , and temperature control 317 . the light sources 321 a and 321 b , 323 a and 323 b , and 325 a and 325 b and detector light pipe 327 are received in base 305 to make contact with finger / thumb 11 in a repeatable manner , as illustrated . light sources 321 a - 325 b are connected to ( via signal line 330 ) and controlled by conventional electronics in housing 331 . as those skilled in the art will appreciate , those light sources at a given distance from the detector light pipe 327 will be energized simultaneously . thus , for measuring long path wavelengths , sources 321 a and 321 b will be on at the same time . the light enters the tissue of finger / thumb 11 and propagates through , with a portion exiting into light pipe 327 . light pipe 327 is composed of fused silica and serves to transport the light from the finger / thumb 11 to imaging optics 333 which , in turn , directs the light 335 onto the aperture of aotf crystal 336 . the specific wavelength or wavelengths transmitted by aotf 336 is determined by the rf signals introduced onto the crystal by tunable rf source 338 via signal line 339 . as those skilled in the art will appreciate , a piezoelectric crystal ( linbo 3 ) is bonded to the teo 2 crystal on a specific crystal face to inject an acoustic wave in the required direction . the light 341 exiting aotf 336 is detected by detector 343 which , in the preferred embodiment , is a thermoelectrically cooled indium gallium arsenide detector . other suitable detectors ( such as insb , lead sulfide and germanium / silica ) may also be used . thermoelectric cooling is performed by cooler 345 . the resulting analog signal from detector 343 is communicated to a / d converter 347 by signal line 349 . central processing unit 351 ensures that the intensity seen by detector 343 is within its linear dynamic range , via pre - established intensity limits . if the intensity is not within range then a signal is sent to rf source 338 , via signal line 353 , to increase or decrease the rf power to aotf crystal 336 until the intensity observed by detector 343 is within its linear operating range . with reference to fig8 some wavelengths need to be measured at relatively high resolution ( i . e . narrow bandwidth ), while some can be measured at low resolution . by introduction of one or several rf frequencies the bandwidth of the light transmitted by aotf 336 can be altered . the aotf driver electronics 354 , connected between central processing unit 351 and rf source 338 , will be instructed by central processing unit 351 to allow measurement of the exact frequencies needed , at the resolution needed and at the appropriate signal - to - noise ratio . the wavelength transmitted and subsequently recorded is controlled by the frequency of the rf signal applied to aotf crystal 336 . the resolution can be decreased by simultaneous introduction of several rf frequencies onto aotf crystal 336 . the signal - to - noise ratio can be controlled by the length of time a specific wavelength is recorded . after a given wavelength or wavelengths are recorded , central processing unit 351 generates a signal to cause tunable rf source 338 to change the frequency being generated , and the next wavelength ( s ) is ( are ) recorded . following measurement of all wavelengths using a given source - detector configuration ( e . g ., 321 a and 321 b / 327 ), central processing unit 351 signals , via signal line 355 , source driver electronics 331 to switch off the current to sources 321 a and 321 b , and to turn on sources 323 a and 323 b . the process is then repeated , and repeated again for sources 325 a and 325 b . the wavelength intensity values from a / d converter 347 are communicated to central processing unit 351 and then transmitted via signal line 359 for storage in memory storage unit 361 . following measurement of all necessary wavelength intensity values in the manner set forth above , all these intensity values are processed by spectral processing algorithms stored in module 363 to produce a processed spectra , such as shown in fig1 . the resulting processed spectra is devoid of or has minimal patient to patient differences and is ready for quantitative analysis . quantitative analysis of the processed spectra is preformed by central processing unit 351 in conjunction with the multivariate calibration model and algorithms stored in module 365 and the processed spectra stored in memory storage unit 361 . the analysis process determines the concentration of the analyte . the multivariate methodology used is disclosed in u . s . pat . no . 4 , 975 , 851 , the disclosure of which is incorporated herein by reference . the concentration value is subsequently transmitted via signal line 376 for display by unit 375 . for example , glucose concentration would be displayed in mg / dl units on screen 377 . concurrent with the concentration determination , processed spectra is examined to determine if it is an outlier . outliers are spectra not representative of the calibration samples . the outlier detection methods used are also disclosed in u . s . pat . no . 4 , 975 , 851 . in simple terms , if the spectra is unique or dissimilar from those used to develop the model then the accuracy of the measurement is not well defined . the determination of measurement accuracy is performed by central processing unit 351 while using the processed spectra stored in memory storage unit 361 and the outlier detection algorithms stored in module 367 . the result of the analysis can be displayed by unit 375 as a bar graph 379 indicating accuracy . memory storage unit 361 and modules 363 , 365 and 367 are interconnected by signal line 381 . fig3 illustrates the major components of a robust noninvasive glucose monitor 401 employing a broadband light source and fiber optics . the optical sampling of finger / thumb 11 is performed with the same structure and in the same manner as previously discussed in reference to fig2 and 23 . the optical illumination is performed by a broadband light source 403 , typically a tungsten halogen source , which is coupled by any suitable conventional method to a group of source fibers 405 a and 405 b , 407 a and 407 b , and 409 a and 409 b . source 403 is also coupled to background fiber 411 for the reasons explained below . the filament used in source 403 is elongated , so the distance from the filament to each fiber is constant . illumination of source 403 is controlled by electronics 412 . the source fibers are connected from source 403 to shutter box 215 , as previously described in connection with fig2 and 23 . in operation , shutter box 215 allows light from fibers located at the same distance from the detector to be simultaneously transmitted into finger / thumb 11 . as shown , illumination of finger / thumb 11 is with those fibers closest to the detector fiber 413 . as before , rotation of disk 215 is controlled by motor 227 which , in turn , is coupled to shutter driver electronics 415 via signal line 417 . the light having propagated through finger / thumb 11 is collected by detector fiber 413 which may be a single fiber or a fiber optic bundle . detector fiber 413 is connected , by fiber coupler 421 , to imaging optics 423 , which focuses the light 425 onto a portion of the aperture of aotf crystal 427 . aotf crystal 427 is , preferably , made of teo 2 and has an aperture of , approximately , 0 . 5 cm .× 0 . 5 cm . background fiber 411 is coupled to light source 403 in a conventional manner ( not shown ), such as used for source fibers 405 a - 409 b . at its opposite end , fiber 411 is connected onto imaging optics 429 by coupler 431 . the light from both detector fiber 413 and background fiber 411 are imaged simultaneously onto the aperture of aotf 427 . the optical transmission properties of aotf 427 are controlled by the rf signals incident to the crystal , which are produced by radio frequency source 435 coupled to the piezoelectric crystal on aotf 427 by signal line 437 . rf source 435 is , in turn , controlled by driver electronics 439 via signal line 441 . electronics 439 are controlled by central processing unit 443 via signal line 445 . the desired wavelengths of light are transmitted through aotf 427 and are incident upon two detectors 451 and 453 , which are matched so as to have similar response curves . in the preferred embodiment the detectors are composed of indium gallium arsenide and are thermoelectrically cooled by thermoelectric cooler 455 to improve performance . the two detectors receive the light from aotf 427 and convert the light intensity into a series of electrical signals indicative of the light transmitted by , respectively , background 411 and detector 413 fibers . the electrical signals which correspond to the intensity values at the detector are transmitted to electronics 457 via signal lines 459 and 461 . within electronics 457 is an a / d converter and computational hardware that ensures that both detectors are functioning within their respective operational range . if the intensity of the light received from aotf 427 is not within the established linear operating range of the detectors , the rf power incident onto aotf 427 is changed until the response is within such range . for each wavelength recorded ( for both background 411 and detector fiber 413 ), rf source 435 generates a different rf frequency . the digital numbers corresponding to the intensity values at each wavelength from both detectors 451 and 453 are communicated from electronics 457 to central processing unit 443 via signal line 463 . the digital intensity values are subsequently stored in memory module 465 until all wavelength intensities have been recorded . following measurement of all necessary wavelength intensity values , these values are processed by spectral processing algorithms stored in module 467 . the result is a final processed spectra , such as previously illustrated in fig1 . the intensity values from the proposed spectra are also stored in memory module 465 for subsequent processing . the final processed spectra is the spectral data which has been processed to minimize between patient differences and is now ready for quantitative analyte measurement . quantitative analysis of the processed spectra is preformed by central processing unit 443 in conjunction with the multivariate calibration model and algorithms stored in module 469 and the stored processed spectra stored in module 465 . the analysis process , carried out in the manner set forth in u . s . pat . no . 4 , 975 , 581 , determines the analytes concentration . the concentration value is subsequently displayed by unit 471 , connected to central processing unit 443 via signal line 472 . for example , glucose concentration would be displayed in mg / dl units by display 473 . concurrent with the concentration determination , the processed spectra is examined to determine if it is similar to those used to generate the calibration model . if the spectra is unique or dissimilar from those used to develop the model then the accuracy of the measurement is poorly defined . the determination of measurement accuracy is performed by central processing unit 443 while using the processed spectra stored in module 465 and outlier detection algorithms stored in module 475 . the result of this analysis is displayed on accuracy bar graph 479 . central processing unit 443 , and modules 465 , 467 , 469 and 473 are interconnected by signal lines 481 . robust noninvasive monitor 501 , fig3 , is based on finger sampling device 121 , illustrated in fig1 - 20 . the leds and detectors on finger sampling device 121 are controlled in the manner as described in reference to fig2 a , 21 b and 21 c . the activation of the leds is controlled by led driver electronics 511 via signal lines 512 a , b , c , d , and e . the electrical signals from the detectors are transmitted ( via signal lines 512 b , c , d , e and f ) to and processed by detector electronics 513 . the resulting intensity values are communicated to central processing unit 515 , via signal line 517 , and subsequently stored in memory unit 519 . following completion of irradiation / measurement phase , the stored wavelength intensity values are processed by central processing unit 515 . the spectral processing is performed as described in reference to fig3 and 33 . the processing uses memory module 519 , spectral processing module 521 , multivariate model and algorithm module 523 and outlier detection module 525 . the results of the spectral analysis are displayed on screen 531 and accuracy bar graph 533 of display unit 355 . modules 519 , 521 , 523 and 525 are connected to central processing unit 515 via signal lines 537 . display 535 is connected to central processing unit 515 via signal line 539 . driver electronics 511 is coupled to central processing unit 515 by signal line 541 . all components are located in housing 543 . whereas this preferred embodiment has focused on the use of leds , those skilled in the art will recognize that any single or selected wavelength emitting device could be used in a similar manner . for example , the leds could be replaced by a combination of a tungsten light source with a selective filter on the output side . it is also recognized that small diode lasers or other lasers could be used in place of the leds . thus , the apparatus and associated methodology described in fig3 is applicable to any light sources generating a discrete number of wavelengths . fig3 is an illustration of a finger sampling device which utilizes a single broadband source which is transmitted through a wavelength separating device . similar to previous ones , sampling device 801 includes a base 803 , finger support surface 805 , guide rails 807 , post 809 , hinged pressure arm 811 , bias spring 813 , and temperature control 815 . device 801 also includes a wavelength separating device 817 , coupled to base 803 via light pipe 819 . preferably device 817 is an aotf . however , a filter wheel or other device which has the ability to separate broadband light into specific wavelengths could be used . the specific wavelength that is emitted from device 817 is then partially transmitted through finger / thumb 11 as illustrated by traces 821 , 823 and 825 . after partial transmission through finger / thumb 11 the light at the selected wavelength is then detected by detector rings 831 , 833 and 835 supported ( by means not shown ) on disc 837 . fig3 shows the equi - distant nature of the detector rings . thereafter , the wavelength is changed and another specific wavelength is partially transmitted through finger / thumb 11 . the process is repeated until all desired wavelengths are transmitted . fig3 illustrates the major components of a robust noninvasive glucose monitor 841 employing a single broadband light source and the sampling device of fig3 and 32 . monitor also includes broadband light source 843 coupled to source electronics 847 which are controlled by central processing unit 845 via electrical connections 849 and 851 . aotf 817 is , as with the embodiment of fig2 , coupled to central processing unit 845 via tunable rf source 853 , aotf driver electronics 855 and signal lines 857 , 859 and 861 . also , as with the embodiment of fig2 , monitor 841 includes memory storage unit 863 , module 865 ( in which are stored spectral processing algorithms ), module 867 ( in which is stored the multivariate calibration model and spectral processing algorithms and outlier detection module 869 . memory unit 863 and modules 865 , 867 and 869 are interconnected via signal line 871 . signals from detectors 831 , 833 and 835 are transmitted to analog - to - digital converter 877 . the digital values from converter 877 are transmitted to central processing unit 845 via electronic bug 875 and processed in the manner disclosed with monitor 301 ( fig3 ). the result of the analysis is transmitted via signal line 879 for display by unit 881 as a specific value on display 883 and a bar graph 885 indicating accuracy . whereas this specification has focused on the noninvasive measurement of glucose , those skilled in the art will appreciate that changes can be made to the preferred embodiment to measure other analytes . it is recognized that the wavelength region used for measurement will vary between the different blood analytes of interest . for example , acceptable accurate results for bilirubin and hemoglobin are possible through use of the 300 - 1000 nm region . specifically , bilirubin has a significant absorption peak at approximately 454 nm and oxygenated hemoglobin has a peak at approximately 410 nm . alcohol , another analyte of significant interest , has a sharp spectral absorbance at 1190 nm . thus , the method of sampling and the associated optical instrumentation may be changed to optimize measurement accuracy for any number of analytes without affecting the scope of this invention . whereas the drawings and accompanying description have shown and described the preferred embodiment of the present invention , it should be apparent to those skilled in the art that various changes may be made in the form of the invention without affecting the scope thereof .
an improved method and apparatus for determining noninvasively and in vivo one or more unknown values of a known characteristic , particularly the concentration of an analyte in human tissue . the method includes : irradiating the tissue with infrared energy having at least several wavelengths in a given range of wavelengths so that there is differential absorption of at least some of the wavelengths by the tissue as a function of the wavelengths and the known characteristic , the differential absorption causeing intensity variations of the wavelengths incident from the tissue ; providing a first path through the tissue ; optimizing the first path for a first sub - region of the range of wavelengths to maximize the differential absorption by at least some of the wavelengths in the first sub - region ; providing a second path through the tissue ; and optimizing the second path for a second sub - region of the range , to maximize the differential absorption by at least some of the wavelengths in the second sub - region . in the preferred embodiment a third path through the tissue is provided for , which path is optimized for a third sub - region of the range . with this arrangement , spectral variations which are the result of tissue differences can be reduced . at least one of the paths represents a partial transmission path through the tissue . this partial transmission path may pass through the nail of a finger once and , preferably , twice . also included are apparatus for : reducing the arterial pulsations within the tissue ; and maximizing the blood content in the tissue .