Rapid non-invasive optical analysis using broad bandpass spectral processing

The present invention provides a generally applicable apparatus and method for achieving measurements of a constituent in a sample. This is achieved by employing a detection means having a plurality of detectors responsive to radiation in a selected region of the spectrum, e.g., the infrared. In most embodiments, at least two of the detectors provide broad wavelength bandpass. If narrow bandpass sources or detectors are used, the information generated is processed in a manner similar to broadband information. The broad bandpass response of the detectors can be contrasted with the approach of classical spectrophotometry, in which the spectral response of the detectors is designed to be as narrow as feasible, and substantially narrower than the spectral features of the constituent or constituents of interest. The data is processed such that the contributions of known background constituents and scattering is eliminated prior to further processing, thereby yielding a better result in high background situations. The use of intensity processing rather than absorbance processing also allows the method and apparatus to be used in a variety of non-ideal situations.

BACKGROUND OF THE INVENTION 
The present invention relates to the measurement of the concentration of 
constituents or other properties of interest of a material using 
radiation, preferably near infrared radiation. More particularly, 
apparatus and methods have been developed for measurement of the 
concentration of constituents such as hemoglobin and its variants and 
derivatives, glucose, cholesterol and its combined forms, drugs of abuse, 
and other analytes of clinical and diagnostic significance in a 
non-invasive manner. Because the apparatus developed for use of this 
method does not require the withdrawal of blood in order to perform these 
measurements, it is particularly suitable for testing in the home on a 
chronic basis, such as for glucose levels in diabetics and for kidney 
function, e.g., urea or creatinine testing, in patients undergoing home 
dialysis. 
In addition to home testing, development of clinical testing procedures 
that do not require blood withdrawal has become an important goal, due to 
the spread of AIDS and the associated fears among the public and health 
care personnel. Along with AIDS, other diseases such as hepatitis can be 
spread through the use of invasive procedures without stringent 
precautions to assure sterility. A recent article, "Nosocomial 
transmission of hepatitis B virus associated with the use of a 
spring-loaded finger-stick device," New England Journal of Medicine, 
326(11), 721-725 (1992), disclosed a hepatitis mini-epidemic in a hospital 
caused by the improper use of an instrument for obtaining blood samples. 
The article describes how the hospital personnel unintentionally 
transmitted the virus from patient to patient by misuse of the sampling 
device. Such transfers, potentially hazardous to healthcare personnel as 
well as patients, are eliminated by non-invasive testing such as that 
performed by the subject apparatus and methods. 
Non-invasive testing will become particularly effective in the long-term 
management of diabetes. Improperly controlled glucose levels in diabetes 
patients can result in damage to the circulatory system, the nervous 
system, the retina, and other organs. These damages can be largely 
eliminated by closer control of glucose levels on a daily basis. However, 
this closer control requires measurement of glucose levels four or more 
times a day. With current apparatus and methods, a painful finger prick is 
required for each sample. Furthermore, the part of the apparatus that 
contacts the blood to produce the required chemical change used in the 
measurement is disposed of after each sample. The cost to the user of 
these disposables can reach thousands of dollars per year. The 
inconvenience and discomfort of the sampling adds to the psychic costs of 
the process. Finally, the sampling process, conducted by relatively 
untrained personnel, has been reported to multiply the inherent error in 
the analytical process by a factor of three to five times. Errors in the 
sampling process occur as a result of not obtaining a proper blood sample 
(e.g., the sample may be an admixture of intracellular fluid with the 
blood sample) and as a result of improper application of the sample to the 
disposable part of the apparatus. 
These deficiencies in currently available apparatus and methods have caused 
a number of groups to attempt to develop non-invasive apparatus for 
measurement of concentration of various constituents of blood. 
Commercially, the most successful apparatus for non-invasive chemical 
constituent measurements are those for "pulse oximetry", where the 
apparatus is used to measure relative concentrations of oxyhemoglobin and 
deoxyhemoglobin. These are both strong absorbers in the near infrared, 
with crossing broadband features, so ratioing of intensities of radiation 
at two wavelengths can provide the requisite information. Based in part on 
the success with hemoglobin, much of the current work on non-invasive 
measurements for chemical constituents has also used the near-infrared 
region of the electromagnetic spectrum. Because of the large size of the 
glucose market, most of the additional research is directed to glucose 
although it is only a low concentration material with weak absorbance. The 
region from 700 to 1100 nm in wavelength contains the third overtones of 
the glucose spectrum, theoretically allows minimization of interferences 
due to water absorption, and exhibits good penetration of human tissue. 
Other promising research has used longer wavelengths, from 1100 to about 
2500 nm. 
Substantially all of this work has been carried out using variants on 
classic spectrophotometric methods. Such methods typically use detectors 
which measure the radiation transmitted through or reflected from the 
sample in relatively narrow wavelength bandpasses. The width of the 
passband is kept narrow for several theoretical reasons. First, a narrower 
passband minimizes the practical deviations that can occur relative to the 
theoretical relationships between constituent concentration and 
absorbance. Second, measurement with a narrow detector passband allows a 
better measurement of sharply peaked spectra by providing a measurement 
closer to the radiation peak. This has been believed to improve 
specificity, and for full-spectrum measurements, provide a more faithful 
rendition of the absorbance or reflectance spectrum. 
The wavelength passband within which the detector operates can be a 
property of the source or can be obtained by appropriate filtering means 
placed between source and sample, between sample and detector, or both. 
The width of the passband in classic spectrophotometric apparatus is 
ordinarily chosen to be small with respect to the width of the spectral 
features of the constituent of interest and the sample, if known. 
Typically, a passband halfwidth less than 10% of the spectral halfwidth is 
recommended. 
In some apparatus and methods, the source is designed to scan the spectral 
region of interest, so that the measured wavelength varies over time in a 
controlled manner. In other cases, the source is transformed into a coded 
broadband source, whose interaction with the sample is later decomposed 
into narrow-band responses. 
In most of the classic spectrophotometric apparatus and methods, the 
resultant data initially appear within the apparatus as uncorrected 
intensity versus wavelength data for the sample. The next important step, 
performed within the spectrophotometric apparatus, is a logarithmic 
conversion of the data into absorbance or reflectance units using some 
reference intensity versus wavelength data for normalization. Extensive 
data processing of the transformed data is then employed to attempt to 
isolate the components of the data arising from the constituent(s) of 
interest and the components arising from the background (due to 
constituents that are not of interest and instrumental artifacts). A 
multitude of techniques are employed to perform this isolation, largely 
based on statistical regression techniques. Examples of this general 
approach include the work of Rosenthal et al., U.S. Pat. No. 5,028,787 and 
Clarke, U.S. Pat. No. 5,054,487. 
All of these classic methods essentially search for a unique response or 
pattern of responses due to the constituent of interest at one or more 
specific wavelengths (or narrow wavelength passbands) and then attempt to 
separate these effects from the effects due to background constituents at 
the same narrow wavelength passbands. However, glucose and many other 
constituents of interest possess only weak broadband spectral features in 
the wavelength ranges of interest. Furthermore, the measurement 
environment is generally a mixture containing many constituents with 
overlapping but different broadband spectral structures, several of which, 
including water and hemoglobin, are strong absorbers in the region. In 
non-invasive clinical measurements, the problems are compounded by the 
presence of multiple diffuse radiation scattering centers in the tissue. 
These situations are contrary to the basic assumptions of 
spectrophotometry, and its apparatus and methods are ill-suited to dealing 
with the resultant data. 
Spectra with weak, low resolution features and overlapping backgrounds are, 
however, commonly found in examination of colored objects by reflected, 
emitted, or transmitted light in the visible wavelength range. The human 
eye can distinguish wavelength shifts as small as 2 nm, and can 
distinguish small wavelength shifts even under variable illumination 
conditions. Therefore, the present invention is based on concepts 
analogous to those employed in the human visual system and in colorimetric 
apparatus. 
In apparatus for measuring color (as opposed to concentration), two methods 
are commonly employed. Traditional (tristimulus) colorimetry employs three 
detectors with spectral responses approximating those of the visual cones 
in the human eye (shown in FIG. 1a, the CIE Standard Observer) to create 
an apparatus with spectral sensitivity approximating that of the eye. To 
improve the approximation, newer devices employ sets of narrow-band 
detectors to measure the entire visual spectrum at substantially constant 
sensitivity and then apply software algorithms to simulate the color 
response of the eye. In both cases, resultant outputs may be transformed 
by convolving the Standard Observer response with the known spectra of the 
source to generate data representative of the color of the object being 
measured. 
Data obtained by these colorimetric devices are often presented in 
transformed co-ordinate spaces for easier interpretation. Examples of such 
spaces are shown in FIGS. 1b and c. FIG. 1b is the CIE chromaticity 
coordinate system, while FIG. 1c is the CIE Lab coordinate system. Results 
presented in these systems are interpretable as hues, chromas, 
saturations, brightnesses, and other related terms that are more easily 
related to human perceptions without further mathematical transformation. 
The CIE Lab system attempts to create a coordinate space that is linear 
with perceived color differences. These systems have been used in the 
reflectance or transmittance mode to measure the color of a reflective or 
transmissive sample. None of these systems has been used to directly 
measure the concentration of a constituent or constituents of a sample. 
U.S. Pat. No. 5,321,265, the disclosure of which is incorporated herein by 
reference, discloses a basic concept for creating a system analogous to 
human color perception and to colorimetry using infrared sources and 
appropriate detection means for measuring the concentration of 
constituents of a sample. Briefly, the disclosed methods and apparatus use 
a broadband radiation source to illuminate a sample held in a chamber. 
Radiation from the source is passed through a plurality of spectrally 
overlapping filters before reaching detection means which detect radiation 
transmitted, reflected or emitted from the sample and thereby measure the 
sample's "color" in the region of the spectrum defined by the filter and 
detector responses. U.S. patent applications Ser. Nos. 130,257 and 182,572 
concern modifications to the basic apparatus and methods to achieve better 
results. The present invention concerns further methods and apparatus 
which may be employed toward the same objective. These methods and 
apparatus are all directed to improving the accuracy, sensitivity and 
repeatability of non-invasive measurements of materials such as glucose. 
The present invention, however, is not limited to overlapping detectors, 
although it is preferable that at least some of the detectors overlap. 
Similarly, while broadband detectors are preferred, it is possible to use 
some, or all, narrow band sources or detectors. 
Accordingly, an object of the invention is to provide an apparatus for 
obtaining a non-invasive measure of the concentration of a constituent of 
interest using the infrared portion of the spectrum. 
A further object of the invention is to provide methods for obtaining a 
measure of the concentrations in blood or in tissue of clinically 
important analytes in a non-invasive manner. 
These and other objects and features of the invention are achieved by the 
methods and apparatus described in the Summary of the Invention, the 
Detailed Description and the Drawing. 
SUMMARY OF THE INVENTION 
The present invention features new apparatus and methods for measurement of 
the concentration of a constituent of interest in a sample without 
withdrawing the sample from its normal environment. The apparatus and 
methods also have utility in determining the optical properties of 
objects. The apparatus and methods are generally intended to be used with 
radiation, preferably between 700--2500 nm, transmitted through or 
reflected from the object or sample of interest, particularly in the near 
infrared. 
U.S. Pat. No. 5,321,565 discloses an apparatus and method which employ an 
analog of color vision in the near infrared region of the electromagnetic 
spectrum to provide improved non-invasive measurements of concentrations 
of constituents in a sample. The basic concept is that the use of a 
plurality of broadband spectrally overlapping detectors to measure the 
radiation transmitted, reflected, or emitted in response to broadband 
illumination of a sample would result in an analog of color vision, and 
could generate useful data in situations where other combinations of 
illumination and detectors could not generate useful data. The spectral 
response of the detectors can be defined by physical properties of the 
detectors themselves, or by combinations of detectors with broader 
spectral responses with appropriately chosen filters to create the 
required spectral response. The previously cited patent applications 
provide variations on the methods and apparatus which improve the response 
of the system. 
The present invention provides a more generally applicable apparatus and 
method for achieving measurements of a constituent in a sample. This is 
achieved by employing a detection means having a plurality of detectors 
responsive to radiation in a selected region of the spectrum, e.g., the 
infrared. In most embodiments, at least two of the detectors provide broad 
wavelength bandpass. If narrow bandpass sources or detectors are used, the 
information generated is processed in a manner similar to broadband 
information. The broad bandpass response of the detectors can be 
contrasted with the approach of classical spectrophotometry, in which the 
spectral response of the detectors is designed to be as narrow as 
feasible, and substantially narrower than the spectral features of the 
constituent or constituents of interest. For most constituents of interest 
such as glucose, the detectors bandpass width is broader than the 
individual identifying spectral characteristics of the constituent of 
interest. 
In classic spectrophotometry, the width of the passband is kept narrow for 
several reasons. First, a narrower passband minimizes the practical 
deviations that can occur relative to the theoretical relationships 
between constituent concentration and absorbance. Beer's law, which 
linearly relates absorbance and concentration, is inaccurate if too broad 
a bandwidth is selected. Second, measurement with a narrow detector 
passband allows a better measurement of sharply peaked spectra by 
providing a measurement closer to the radiation peak. This usually 
improves specificity, and for full absorbance spectrum measurements, 
provides a more faithful rendition of the spectrum. In return for these 
advantages, however, narrow passband detection suffers from lower signal, 
so the signal to noise ratios for a given apparatus configuration can be 
too low to provide meaningful data on low concentration constituents. In 
the in vivo measuring environment, where the analyte, sample, and 
background spectra are broad, and the absorption due to trace analytes is 
low relative to the background, the advantages of narrow bandpass 
detectors become less important, while the suboptimal S/N behavior becomes 
critical. In fact, Beer's law does not apply to scattering media such as 
is found in an in vivo environment, since it was developed for solutions 
without scattering centers. In contrast, the present invention preferably 
uses broad bandpass detectors, combined with precomputational processing 
techniques to maintain S/N ratios. The term "broad bandpass" can be 
expressed in terms of the halfwidths of relevant sample, constituent, or 
background spectra, as well as a variety of other factors. Narrow bandpass 
detectors and/or sources are commonly thought to be required in 
spectrophotometry for purposes of maintaining adequate linearity and/or 
specificity. Generally, spectrophotometric systems employ bandpasses under 
20 nm in order to achieve these objectives. The present invention relies, 
rather, on the improved signal obtained from broad bandpass detectors and 
sources to provide more optimum signal to noise ratios. If narrow bandpass 
filters are used in the present invention, their use is permitted if the 
features being observed are broad, if no sharp anomaly in the spectrum is 
observed in the area under investigation, and if the S/N ratio for the 
associated detector remains adequate. In such cases, the narrow bandpass 
detector outputs are treated as if they were generated by broad bandpass 
detectors. 
FIG. 2 compares the spectral response from the various embodiments of the 
present invention, the disclosure in U.S. Pat. 5,321,265, and the prior 
art systems. The prior patent required that the apparatus and methods use 
only overlapping broad bandpass detectors, while the prior art used 
neither overlapping nor broadband detectors. The present invention permits 
the use of a combination of broad and narrow bandpass detectors, as well 
as the use of both overlapping and non-overlapping sources and detectors; 
thus, the plurality of detection means may be varied over a greater range 
of possible combinations and permutations. The increased range of possible 
detection means allows the sensitivity and specificity of measurements of 
a given constituent made with this apparatus and method to be greatly 
enhanced. 
The apparatus of the present invention comprises a radiation source, 
detection means including a plurality of detectors with spectral responses 
as delineated above, each of which generates an output, processing means 
which convert some of these outputs into processing input signals, and 
analysis means generating a signal from these data. The output signal from 
the analysis means is indicative of the concentration of the constituent 
of interest. In some embodiments of the invention, the radiation source 
may be in the form of at least two radiation sources, normally in the near 
infrared region. In those embodiments, the individual radiation sources 
are preferably arranged such that each of the radiation sources 
congruently illuminates the sample. Congruent illumination means that all 
sources illuminating the sample are completely superimposable in all 
respects as viewed from the illuminated point (or area) on the sample. 
After passing through (or being reflected or emitted from) the sample, the 
radiation is transmitted to the detection means. Preferably, the detectors 
in the detection means are arranged in a manner such that the detection 
means provide congruent measurement of the sample or samples at all the 
detectors. Congruent measurement indicates that the sample and the 
detectors are arranged such that the detectors are completely 
superimposable in all respects as viewed from the observation point (or 
area) on the sample. The combination of congruent illumination and 
congruent measurement of the sample insures that all of the detectors 
examine the same region of the sample through the same solid angle, and 
thereby minimizes variations due to inhomogeneities in the sample. In a 
preferred embodiment, the two forms of congruency are used to position the 
detectors so that the sample is illuminated and viewed through narrowly 
restricted solid angles, minimizing the effect of wide-angle scattering of 
the radiation in the tissue on the detector responses. 
In some modes of practice of the invention, for example when more than one 
source is used, a fiber optic bundle may be used to combine the multiple 
sources for illumination of the sample. Similarly, a fiber optic bundle 
could be used to provide a path from the sample for transmitted, reflected 
or emitted radiation to the plurality of detectors. The fibers in the 
bundle are uniformly distributed in order to assure congruency of the 
illumination of (or detection from) the sample. Other embodiments of the 
invention employ beam splitting means to divide or combine the radiation 
to congruently illuminate or measure the response from the sample or 
samples. In other embodiments, combinations of beam splitting means and 
fiber optic means may be employed to assure the congruency of both 
illumination and detection. 
In a particularly useful embodiment of the invention, the sources used to 
illuminate the sample are coded using electrical or mechanical means, 
creating, for example, sources with different temporal patterns. These 
coding patterns appear at the various detectors, attenuated in intensity 
by their passage through (or reflection from) the sample, but with pattern 
unchanged. Analysis means employing techniques such as electrical narrow 
bandpass filters may then be employed as decoding means, permitting the 
interpretation of the differential response of the various detectors to 
the various spectral patterns of the sources. This differentiation aids in 
the interpretation of the overall response of the sample to the 
illuminating sources. Preferably, at least some of the sources are 
broadband sources. 
The apparatus can further include a sample chamber or similar means for 
containing or restraining the sample to be measured. The containment means 
is designed to hold the sample stationary, if necessary, over the time 
period of the measurement. In a particularly useful embodiment of the 
invention, the apparatus is provided with at least two sample chambers, 
holding samples of similar but not identical nature. The same source or 
combination of sources is used to congruently illuminate each of the 
samples by the use of beam splitting or fiber optic means, and separate 
sets of detectors receive the radiation passing through each of the 
samples. In certain instances, incongruent illumination is used in order 
to determine perturbations in background. The spectral sensitivity of the 
separate sets of detectors used may be the same or may be different. In 
some of the embodiments of the invention, the partial spectral overlap of 
some of the detectors, along with the spectral non-overlap of at least one 
detector, is utilized. This spectral pattern is maintained by appropriate 
choices of the detectors and any associated filtering means. Constituents 
of the samples to be measured, present in the samples at the same 
concentration, will appear different depending on the illumination 
spectrum presented to the samples because of differences in the background 
properties of the samples. Appropriate analyses of the radiation patterns 
reaching the separate sets of detection means permit the response of the 
apparatus to the constituent of interest to be enhanced relative to the 
response of the apparatus to the background in the samples. 
The invention is applicable to transmittance, reflectance, or emittance 
modes of operation. The term "reflectance", as used herein, means not only 
classical reflectance measurements but also transflectance or diffuse 
reflectance measurements where there is some limited surface penetration. 
Similarly, the term "emittance", as used herein, is used in its broadest 
aspects and includes fluorescence and any similar form of excited 
emission. "Transmittance", which is a more accurate term than absorbance 
in a scattering media, should also be given the broadest possible 
interpretation. 
The methods and apparatus disclosed in the invention are particularly 
well-suited for the non-invasive measurement of constituents found in 
human or animal tissues under normal or clinically significant 
circumstances. Examples of such constituents include but are not limited 
to glucose, hemoglobin and its variants, pharmaceutical drugs and drugs of 
abuse, alcohol, and other species. Sample chambers may allow sampling at 
various sites, including but not limited to the fingers. 
In another preferred mode of practice of the invention, the radiation 
leaving a single measurement site is split among at least two sets of 
detection means, preferably viewing the measurement site congruently. 
These detection means have different sets of spectral sensitivities, 
determined, in a preferred embodiment, by the use of different sets of 
filtering means in association with the different sets of detectors. In 
such an embodiment, at least some of the filtering means are individually 
chosen within each set in order to maintain appropriately broad but 
differing spectral sensitivities for the detectors. The detector 
sensitivities may be all overlapping, all non-overlapping, or some 
combination thereof. When properly chosen, the differing spectral 
sensitivities of the detector sets will produce different data on the 
background constituents of the sample and substantially similar data on 
the constituents of interest. Appropriate analysis of the data generated 
by the two sets of detection means permit the response of the apparatus to 
the constituent of interest to be enhanced relative to the response of the 
apparatus to the background in the sample. In some cases, a black/white 
luminosity detector, which is responsive to and overlaps the spectral 
response of all the detectors, is used to provide a reading on the total 
illumination. A variety of detection means may be used to practice the 
disclosures of this invention. Preferred detectors having sensitivities 
appropriate to the invention include detectors based on silicon, on lead 
selenide, on indium gallium arsenide, germanium, and lead sulfide but 
other detectors may also be employed. 
Outputs from the detectors are processed using techniques which represent 
another part of the invention. In the simplest form of the invention, the 
processing of the detector outputs is performed by following the standard 
methods established for tristimulus colorimetry, in which each output is 
normalized by dividing it by the sum of all the other outputs. It is to be 
noted that in the present invention, the processing and combination of the 
individual detector outputs to provide the detector response is performed, 
preferably in the hardware, on the direct measurements of radiation 
intensity received by the individual detectors. This is to be contrasted 
with the standard methods of spectrophotometry, in which signals are 
logarithmically transformed into absorbances in accordance with Beer's law 
prior to subsequent analysis to obtain constituent concentrations. The 
problem with the latter procedure is that as Beer's law breaks down, i.e., 
the absorbance is not linearly related to concentration, the information 
generated by the device degenerates. Note further that the processing used 
in the present invention provides an improved normalization of the 
responses, which will aid in the elimination of background interferences. 
A further advantage accrues to the mode of data processing disclosed in 
this invention. The response of the detectors to a "pure" sample, e.g., a 
sample only containing the constituent of interest, may be described by a 
multi-dimensional analog of a "color" in the infrared. Analogous to the 
CIE color description, each constituent will have a specific description 
in the multi-dimensional coordinate system chosen. The dimensionality of 
the coordinate system is equal to, or less than, the number of detectors, 
and the description changes from coordinate system to coordinate system 
depending on the range and sensitivity of the detectors. 
Assuming that all of the signal from all the background including 
interfering constituents and scattering effects could be segregated into a 
single background "color", then the signal from a sample containing the 
constituent of interest in such a background could be described in terms 
of a specific vector analogous to the hue and chroma values it would have 
in the visual CIE color description. In a mixture or solution, increasing 
concentrations of the constituent of interest could be described as having 
increased brightness (the L-coordinate in CIE Lab space) with no change in 
the direction of the vector from hue and chroma. If such a segregation 
were accomplished, the concentration of the constituent of interest in 
such a mixture or solution could be expressed as a function of a single 
measured variable in the appropriate coordinate system, i.e., C=k.sub.0 
+k.sub.1 f(x). 
In general, unexpected or unknown interfering constituents may act to 
prevent complete segregation or discrimination of the detector response to 
the constituent to be measured from the response to the interfering 
constituents. However, appropriate choice of the number and spectral 
sensitivity of the detectors and use of other discriminatory techniques 
disclosed herein can eliminate effects due to known background components, 
such as water, hemoglobin, and tissue scattering. When measuring trace 
constituents such as glucose, the effects of these known background 
components may be on the order of 10.sup.4 or more higher than the effect 
of the desired constituent itself. This segregation can be accomplished by 
interpolation from one set of detectors to another, because the effects of 
these dominating background constituents vary smoothly, albeit possibly 
non-linearly, from wavelength to wavelength. The effects of the trace 
constituents such as glucose can be seen as offsets on the more global 
effects of these background components. As a result, the residual signal, 
e.g., the signal after the effects of the backgroumd components are 
removed, will have a much higher component due to the trace constituent of 
interest than if these corrections were not performed. The portion of the 
residual signal due to the constituent of interest may then be accurately 
estimated by the analysis means. This is accomplished by characterizing 
the residual signal as an analog of a location in an n-dimensional space, 
where n is equal to, or less than, the total number of detectors composing 
the residual signal. 
In most situations, the residual signal is much smaller than the signal due 
to the known background components. In vector analysis terms, most of the 
components parallel to the vector direction (or effects) of the 
constituent of interest are related to concentration. The small signal 
range implies that the signal due to the constituent of interest can be 
expected to be substantially linear with concentration. 
Elimination of the effects due to known background constituents and 
scattering reduces the number of calibration steps. Calibration will thus 
require the estimation of fewer calibration constants than in standard 
spectrophotometric methods, since spectrophotometric methods must account 
for known background interference signals in their calibration processes. 
In the optimal case, the residual signal will, by proper choice of 
detector number and characteristics, have a "color" that "matches" that of 
the pure constituent of interest, resulting in the simplest form of 
calibration. 
In another embodiment of the invention, one may employ narrow bandpass 
detectors such as are used in spectrophotometric apparatus to generate a 
measurement of radiation intensity as a function of wavelength. This 
information may then be processed by appropriate processing means to 
generate similar colorimetric-like information representative of broad 
features of the sample spectrum and thus provides a method by which 
standard spectrophotometric apparatus may be adapted to employ the 
beneficial features of this invention. For example, the output from a 
plurality of narrow bandpass detectors may be combined to simulate the 
broadband detection one obtains from the broad bandpass detectors 
preferred in the present invention and the calorimetric approach can then 
be used to obtain similar results. This broadband detection is based on 
the combined properties of the source, the sample and the detectors. 
Other aspects of the present invention provide further powerful assistance 
in the removal of background interferences and the normalization of 
signals at the detectors. As noted, a black/white luminosity detector, 
whose spectral response completely overlaps all of the individual 
detectors, may be included in the apparatus to provide another level of 
signal normalization. 
Similar benefits are obtained from another aspect of the invention, in 
which a portion of the illuminating radiation is transmitted through a 
reference material to an additional detector. In one embodiment, the 
reference material may be placed in the apparatus between the sample and 
the additional detector by splitting the output from the sample so that a 
portion of the output goes to the additional detector. This reference 
material can be a liquid solution containing a high concentration of the 
constituent of interest, or a particular interfering substance, or be a 
selected solid material, and thereby reduces the effects of variation of 
the constituent of interest on the measurement of that constituent in the 
sample. Inclusion of the reference material in this manner provides an 
estimate of the response of the detection means to a sample with a zero 
concentration of the constituent of interest. In a second embodiment, the 
reference material is chosen to simulate the spectral response of a sample 
with a zero concentration of the constituent of interest, but with 
substantially all of the background constituents included. Use of the 
reference detector in this manner minimizes the effect of many background 
constituents. 
A further highly preferred embodiment of the invention relates to the time 
scale of the individual measurements. Many spectrophotometric apparatus 
require as much as one to two minutes to generate all the individual 
narrow bandpass wavelength measurements required to approximate the entire 
spectrum of interest. This can be a problem if there are changes in the 
sample over the time of obtaining the spectrum. By contrast, the present 
system obtains all of its data substantially simultaneously, generating 
many individual measurements per second. This permits quantification of 
changes in the sample measurement due to physiological effects at a slower 
time scale, such as the change in volume of the sample produced by the 
arterial pressure pulse, occurring at a frequency of 0.5-3 hertz. By 
measuring the sample throughout one or a short series of arterial pulses, 
the apparatus is able to discriminate the signal provided by effects due 
to the pulse of blood volume introduced from the effects of the 
steady-state blood volume and those due to the surrounding tissue. This 
data treatment reduces interference from scattering and interferences from 
the tissue and provides further opportunities for signal normalization and 
data discrimination. Additionally, measurements can also be made at a rate 
that is substantially simultaneous with respect to any time varying 
instabilities, drifts, or other variations in the characteristics of the 
apparatus. The effects of such instabilities are thereby minimized. 
In another aspect of the invention, the signals are input into an analysis 
means some of whose components simulate the functions of a neural network. 
Such a simulation may be performed using electronic hardware or using 
computer software. The simulation provides a closer approximation to the 
functions of human color vision. Among the advantages accruing from this 
aspect of the invention is an increased capability of the apparatus to 
compensate appropriately for variations in illumination spectra and 
intensity. 
The Drawing and the Detailed Description will more clearly describe the 
nature and scope of the disclosed invention.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides methods and apparatus for non-invasive 
measurement of the concentration of a constituent of interest that 
interacts with radiation in a selected region of the electromagnetic 
spectrum, preferably 700-2500 nm. The mode of interaction occurring may, 
for example, be absorption, reflection, or emission of radiation. 
Additionally, since such interactions also define the optical properties 
of objects, this invention in its various embodiments may also be used for 
measurement of such optical properties. 
The apparatus and methods disclosed herein provide improved capabilities 
for non-invasive concentration measurement under conditions where the 
classical spectrophotometric systems are inadequate or useless. These 
capabilities arise from the recognition that such experimental conditions 
can cause the information obtained by a spectrophotometric system to be 
unreliably related to the concentration of the constituents of interest. 
Complex analysis techniques to decompose the information derived from 
spectrophotometric methods have been attempted to increase the utility of 
the data obtained, but these methods usually introduce noise in the data 
processing and lose information in unexamined regions of the spectrum. 
Instead, by applying an analog of color perception to concentration 
measurements, significantly better information relating to concentration 
can be obtained. The technique is particularly useful in the near infrared 
region of the spectrum. 
The application of these colorimetric principles to the measurement of 
concentrations may be understood by referring to FIGS. 1a through 1c, 
which show different schemes used for interpreting data in classic 
instrumental colorimetry. FIG. 1a shows the CIE 1931 Standard Observer, 
which approximates the spectral response of the three types of cones in 
the human retina. FIG. 1b, called a chromaticity plot, is a convenient 
two-dimensional representation of the systematic variation of the response 
of the standard observer of FIG. 1a to monochromatic light of different 
wavelengths. Each point on the continuous curve in FIG. 1b is plotted as a 
normalized (X,Y) pair. The pairs of values are obtained from the three 
response curves in FIG. 1a by dividing the individual response by the sum 
of the three responses: 
EQU D=x'+y'+z' X=x'/D Y=y'/D Z=z'/D 
This normalization procedure completely defines X, Y, and Z. Specification 
of X and Y values on the two dimensional plot of FIG. 1b specifies Z as 
well, since the normalization assures that X+Y+Z=1. Monochromatic light 
produces a response falling along the horseshoe shaped curve in FIG. 1b. 
Using this normalization procedure, monochromatic light falls at the same 
point along the curve independent of its brightness or intensity, so that 
the intensity, nominally D, must be specified separately. White light of 
any intensity falls at the point X=0.307, Y=0.314, the point designated WL 
on FIG. 1b. 
If the light is not monochromatic, as is the case with light from real 
objects, the perceived color is described by points within the interior of 
the curve. The hue or dominant "color" of the object is defined as the 
perceived color of the monochromatic light which lies at the intersection 
of the outer curve with a line from the white light point (WL) through the 
object's location on the chromaticity plot. Line WL-R on FIG. 1b is an 
example of this type of line and point R indicates the hue. The 
saturation, also called chroma, of the light from the object is given by 
the relative distance of the object's location on the plot between the 
white light point and the horseshoe-shaped curve. The saturation measures 
the purity of the object color by indicating the required proportion of 
white light to be mixed with the monochromatic color (corresponding to R 
on the Figure) to generate the perceived color. 
The co-ordinate transformation shown in FIG. 1b makes the Standard Observer 
curves easier to understand but in practice leaves users with 
non-linearities in attempting to use it to quantitate perceived colors. 
Further transformations, such as that shown in FIG. 1c, the so-called CIE 
Lab space, both minimize those non-linearities and are more easily 
interpreted by users and have supplanted the chromaticity coordinates in 
practical usage. For the purposes of this invention, it is important to 
note that many non-linear transformations of colorimetric data can be 
applied successfully in the treatment of measurement data intended for 
different applications. 
In the present invention, an analog of colorimetric space in the near 
infrared region of the spectrum is employed. The individual cones of the 
retina are replaced by a detection means (or several detection means) each 
formed of a set of n radiation detectors. As in the retina, these 
detectors preferably have broad, overlapping spectral responses, which are 
defined by the response of the base detector combined with that of any 
optical filtering means which may be associated with that detector. 
However, while the present invention is preferentially practiced by broad 
bandpass detectors with spectral overlap, non-overlapping detectors, 
either broadband or narrowband, can be used to replace or supplement as 
required provided their signal to noise behavior is adequate for the 
measurement. The breadth of response of the broad bandpass detectors is 
broad in contrast with spectrophotometric practice, in which detector 
spectral bandpass is chosen to be as narrow as feasible. For the purposes 
of this invention, broad bandpass may be taken as broad relative to the 
halfwidths of one or more relevant sample, constituent, or background 
spectral features, as that relative breadth is thought to affect linearity 
and/or specificity in spectrophotometry. Broad bandpass detectors useful 
in the present invention normally have a bandwidth of at least 50 nm, 
although in some circumstances, bandwidths as small as 20 nm may be 
considered broad. FIG. 2a shows the spectral response of a set of all 
narrow bandwidth detectors, FIG. 2b shows the response of three broadband 
overlapping detectors such as is shown in U.S. Pat. No. 5,321,265, FIG. 2c 
shows the response of three broad non-overlapping detectors, FIG. 2d shows 
the response of three broadband detectors where two overlap and one does 
not overlap, FIG. 2e shows the response of two overlapping broadband 
detectors and one non-overlapping narrowband detector, and FIG. 2f shows 
two broadband detectors and one narrowband detector, with no overlap. As 
is well understood in the art, many possible detector and filter 
configurations are feasible that maintain the configurations indicated in 
FIG. 2 while achieving other purposes insofar as limitation of noise or 
enhancement of signal is concerned. The constraint on the detector 
spectral responses shown in FIG. 2 provides substantially greater choice 
in the configurations of detectors available, and therefore allows 
improved optimization of the apparatus. 
Possible arrangements for the spectral sensitivities of the detectors may 
be understood by referring to FIG. 3, which presents approximate 
transmission spectra for water and aqueous glucose solutions in the 
infrared region. The difficulty in obtaining reliable spectra for glucose 
solutions is well-known and is caused by the small magnitude of the 
absorbance by glucose in this spectral region and from the non-specific 
effects on the absorbances and volume as the glucose is combined in 
solution with the water. The observed changes in the spectrum indicated in 
FIG. 3 are a combination of all these effects, and serve as only an 
indication of those changes that might be expected in a non-invasive 
biological measurement. A rough estimate based upon data in the literature 
(see, e.g., Koashi et al., U.S. Pat. No. 4,883,953; and Rosenthal, U.S. 
Pat. No. 5,028,787) suggests that physiological levels of glucose will 
absorb radiation in this region at approximately 1/2000 the magnitude of 
the absorbance of physiological concentrations of water. In addition, the 
effects of other absorbing species, such as proteins, lipoproteins and 
lipids, as well as scattering, must be accounted for in order to obtain 
clinically significant data. 
Thus, to measure analytes such as glucose in the presence of large amounts 
of background, the detection means must be configured so as to maximize 
those changes caused by glucose, while minimizing the effects of changes 
in other constituents. The apparatus disclosed herein aids in this 
configuration, compared to the prior art, by allowing more possible 
configurations of the detector sets. It does so while maintaining the 
advantages of the calorimetric analog previously disclosed, in that a 
response (equivalent to a "color") due to glucose throughout the entire 
spectrum, measured with an optimized weighting function (the chosen 
co-ordinate space defined by the detector sensitivities), is still made 
available to the analysis means. In a preferred embodiment, operating on 
intensity levels, specifically by combining the intensities before any 
additional mathematical processing occurs, minimizes the problems caused 
by the deviations from Beer's law. This type of processing and analysis 
allows the comparison of data from several detection means which can 
generate information indicative of the concentration of the constituent of 
interest while rendering the interfering features of the backgrounds from 
each of the detectors to be less distinct than it is in the individual 
detectors. Further improvements in accuracy are accomplished by 
eliminating effects generated by known background components and 
interfering substances prior to further processing. In a preferred 
embodiment, the outputs from several detectors are keyed to background 
effects such as scattering and known interferents such as water and/or 
hemoglobin, so these can be used to correct for background offsets which 
leads to more linear results using the background corrected data. 
One configuration of apparatus that is particularly useful for the 
non-invasive measurement of analytes such as glucose in human or animal 
tissues is illustrated in FIG. 4. This apparatus achieves congruent 
detection, thereby gaining its advantages. This apparatus has a source of 
illuminating radiation 10, which preferably generates infrared radiation 
in the 700-2500 nm range. In some embodiments, source 10 can be replaced 
by several sources arranged so as to provide congruent illumination. If 
several sources are used, they can be identical, generate a range of 
wavelengths, or some combination thereof. Congruent illumination is 
achieved from multiple sources by the use of reversed beam splitters 
serving as beam combiners, fiber optic bundles, or the like. If fiber 
optic bundles are used, they accept the radiation from the different paths 
and are combined into a fiber optic bundle having randomly spaced fibers 
which results in a substantially uniform mixture of the radiation from the 
various sources. Sources 10 are located so that the illumination emitted 
from the sources and reaching the sample chamber 30 is congruent 
illumination. The radiation from source 10 is focused by a launch lens 20 
through an aperture 30. Aperture 30 leads to sample chamber 40, which is 
shown having a finger portion 45 therein. If fiber optic bundles are used, 
they are arranged as is shown in U.S. patent application Ser. No. 130,257. 
Sample chamber 40 may, in some advantageous embodiments, be replaced by a 
plurality of devices for holding similar but not identical sampling sites 
(such as multiple fingers). In such a case, beam splitting means are 
employed to insure separately congruent illumination of each of the 
sampling sites using the same set of sources. 
Radiation transmitted or reflected from the sampling site 42 in sampling 
chamber 40 is directed to the detectors 72, 74 and 76 by beam splitting 
means 60,62 and 64, or lenses or fiber optic bundles as required. Before 
reaching the detectors 72, 74 and 76, the radiation is first passed 
through any required filters 82, 84 and 86. Filters 82-86 are used with 
detectors 72-76 to provide the requisite wavelength sensitivity. As 
described previously, any combination of overlapping or non-overlapping, 
narrowband or broadband, filters can be used. Preferably, at least some of 
the filters are overlapping broadband filters, which are advantageous in 
segregating signal components from scattering and known interferents such 
as hemoglobin and its variants, lipids, and proteins. Although three 
detectors and filter sets are shown, the number of detectors is not 
limited and in certain circumstances, having many more detectors may be 
useful. Detectors having sensitivity to infrared radiation, may be, but 
are not limited to, silicon, lead selenide, indium gallium arsenide, 
germanium, and lead sulfide cells. 
The spectral properties of filters 82-86, along with the spectral 
sensitivities of detectors 72-76, combine to produce the spectral response 
of the detector set or sets. In particularly advantageous embodiments, 
multiple detector sets and filters are employed to create sets with 
different spectral responses, optimized for different characteristics of 
the constituent or constituents of the sample to be measured. These 
different sets may then be used to detect radiation transmitted through 
the same sample 30 or through similar but not identical samples. 
Appropriate comparison of the data generated by these analyses serves to 
minimize the effect of background relative to the signal produced by the 
constituent of interest. Using the colorimetric analogy, it may be stated 
that the apparent color of the constituent of interest will be maintained 
in the two detector sets, while the apparent colors of background 
constituents will be different, so that comparison of the data will 
accentuate the color of interest. 
FIG. 5 shows a different variation on the apparatus of the invention. 
Sources 10a, 10b, 10c, and 10d with emissions including all or part of the 
700-2500 nm region, produce the required near infrared radiation. Coding 
devices 12a, 12b, 12c, and 12d which may be mechanical apertures or 
electrical means for interrupting the radiation following paths toward the 
sample and detector, impose distinct temporal patterns on each of the 
radiation paths employed. This temporal coding allows discrimination of 
radiation reaching the detector from the intended source from that 
originating external to the apparatus, as well as allowing the 
discrimination of illumination from one source relative to that from 
another source. The radiation from each of radiation sources 10 preferably 
has an associated filter 16a-d associated therewith to limit the radiation 
from each source to a particular section of the spectrum. While it is 
preferable to use at least some broadband overlapping sources, the sources 
may be broadband or narrowband, overlapping or not. 
The radiation from sources 10a-d is transmitted through a sample held in 
sample chamber 30 and is sent to detector 40. In some circumstances, 
multiple detectors may be used. Detector 40 normally has a decoder 44 
associated therewith which decodes the coded signals and identifies the 
source of radiation. Decoder 44 typically employs electrical narrow 
bandpass filters, and allows the signals reaching the detectors to be 
decoded and transduced for transfer to the processing means. FIG. 5 shows 
one means of achieving this congruent illumination, using reversed beam 
splitters, although fiber optics could be used as well. 
In another aspect of the invention, an additional beam splitter directs a 
portion of the source or combined sources to a reference detector, with 
spectral response overlapping that of all the other detection means. This 
element of the apparatus aids in the normalization of the signal and 
calibration of the output by accounting for temporal variations in the 
source or sources. Another useful embodiment interposes a reference 
element between beam splitter and reference detector. The reference 
element may be chosen to have a transmission spectrum similar to that of 
the constituent of interest at a concentration that is very high compared 
to that anticipated in operation of the apparatus. The signal from the 
detector under those circumstances will serve as a stable calibration 
point useful in calibrating the response of the apparatus. In another 
useful embodiment, the reference element is chosen to have a transmission 
spectrum similar to that of the expected background constituents, and the 
response of the detector to illumination through this reference element 
can be used to compensate for interferences due to such background 
constituents. 
The basic optics and principles involved using the general approaches 
described herein are described in further detail in U.S. patent 
application Ser. No. 130,257 and U.S. patent application Ser. No. 182,572. 
One major difference between the aforementioned patent applications and 
the present invention is the possible use of narrowband and 
non-overlapping detectors. While such detectors are not excluded by the 
prior applications, the present invention, including the processing 
techniques described herein, allows optimization of their use. 
The outputs from the detectors in any embodiment are then transmitted to 
processing means 90. Processing means 90 has the capability to process the 
outputs from the detectors into data streams for analysis by analysis 
means 100 using a variety of techniques. In one preferred embodiment, some 
of the outputs are combined before any other processing is performed, 
according to the model of tristimulus colorimetry, as in the equations 
presented above. Note that contrary to the typical methodology of 
spectrophotometry, detector outputs are not logarithmically transformed 
from intensity to absorbance before this processing step. As discussed 
earlier, this processing step also serves as a primary mode of data 
normalization, providing an initial element of normalization and 
background processing. 
In another advantageous embodiment, the outputs from the detectors are 
processed by processing means 90 at a rate much faster than significant 
physiological processes affecting the detector outputs and occurring in 
the sample or samples. Of such processes that affect the detector outputs, 
the most significant is the change in the blood volume of the tissue 
occasioned by the arterial blood pressure pulse, also known as the 
plethysmographic wave. Depending on a variety of physiological factors, 
these waves can occur at frequencies from 30 to 180 per minute, or 0.5 to 
3 cycles per second. Hence, processing means 90 must process the data from 
the detectors at a rate faster than approximately 20 cycles per second. 
When this is accomplished, the apparatus is able to effectively separate 
the signal due to pulsatile changes in blood volume from that due to the 
steady-state blood volume and from the effectively stationary tissue 
contributions to the signal. This separation greatly facilitates the 
measurement of the constituent of interest by reducing background 
contributions due to averaging of unwanted components. 
In still another advantageous embodiment, processing means 90 is 
constructed in such a manner as to allow compensation for known background 
constituents prior to further processing. Such compensation may be 
performed by certain types of hardware or software based neural networks. 
For example, certain detectors having responses with minimal correlation 
to the concentration of the constituent of interest may give known outputs 
which can be strongly correlated to known background and interferent 
levels. Using the outputs from these detectors, the effects of these 
background constituents on the outputs of other detectors can be 
minimized. With such compensation methods, the uncertainty in the response 
of the apparatus to the constituent of interest is markedly reduced in 
comparison with apparatus functioning without such compensation. Then, 
mathematical and statistical techniques known to those skilled in the art 
such as partial least squares and principal component analysis may 
function more effectively on the remaining data to separate the response 
of the apparatus to the constituent of interest from the response to the 
remaining background constituents, by identifying a projection axis which 
minimizes the analyte concentration error. 
In another preferred embodiment, processing means 90 may be used to combine 
detector outputs from detectors whose radiation sensitivities have been 
designed to approximate the effects of the background constituents, 
including the effects of scattering sites, present within the sample. The 
resultant data from processing means 90 is then compared with a second set 
of data generated from a different set of detectors before analysis by 
analysis means 100. This second set of data is representative of the total 
signal reaching the detectors. By this method, the effects of scattering 
and tissue background constituents on the detector responses can be 
largely compensated for before the analysis means is employed to separate 
out the effects of the constituent of interest in the blood from those of 
background constituents and scattering within the blood. 
As a further advantage of the invention, the processing means processes the 
outputs from the detectors in such a manner as to generate parameters 
representative of broad spectral features of the sample. These broad 
spectral features may, in preferred embodiments, be analogous to those 
obtained by tristimulus colorimetry in the visual region of the spectrum. 
Processing of the data in this manner allows the overall "color" in this 
portion of the spectrum produced by the interaction of the sources with 
the sample to be decomposed into its component colors, the colors of the 
background constituents and the color of the constituent of interest. It 
is further disclosed that, in the proper coordinate representation of said 
color, the analogous brightness of said color may be proportional to the 
concentration of the constituent of interest, in a manner analogous with 
that occurring in the visual spectrum. This proportionality may provide a 
great advantage to users of the apparatus in terms of ease and expense of 
calibration and recalibration of the apparatus. 
The foregoing description of the invention is meant to be only exemplary 
and is not intended to limit the scope of the invention. The invention is 
defined by the following claims.