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Description  |
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FIELD OF THE INVENTION
The present invention relates generally to photometers used for reflectance
and transmittance applications and, more particularly, relates to a
multi-detector photometer readhead which uses light-emitting diodes (LEDs)
to simultaneously detect light from color-developed analytes in a test
sample.
BACKGROUND OF THE INVENTION
Photometer readheads with a plurality of detectors are commonly used for
quantitative chemical analysis, such as analysis of body fluids. A known
quantity of body fluid sample, such as blood or urine, is placed on a test
strip or in a test tube containing reagents which react with one or more
quantitatively unknown body fluid components (analytes) to develop color
in the analytes. Typical analytes of interest for urine include glucose,
blood, bilirubin, urobilinogen, nitrite, protein, and ketone bodies. After
adding color-developing reagents to urine, the foregoing analytes of
interest have the following colors: glucose is bluish green; bilirubin,
urobilinogen, nitrite, and ketone bodies are green; and blood and protein
are red. The color developed in a particular analyte defines the
characteristic discrete spectrum for absorption of light for that
particular analyte. For example, the characteristic absorption spectrum
for color-developed glucose falls within the upper end of the blue
spectrum and the lower end of the green spectrum.
After adding reagents to develop color in the analytes of interest, an
artificial source of controlled, diffuse light having a broad spectral
output illuminates the test sample. The light reflected from or
transmitted through the test sample is detected simultaneously by the
plurality of detectors. The detectors are configured to detect different
bands of wavelengths with each band containing those wavelengths which
would be absorbed by one of the color-developed analytes, if present, in
the test sample. That is, the spectral response of each detector
encompasses the characteristic discrete spectrum of wavelengths for
absorption of light of a particular color-developed analyte. The degree of
absorption of light by that particular analyte is proportional to the
concentration of the particular analyte in the test sample. This means
that the amount of light reflected from or transmitted through the
color-developed analyte to its corresponding detector is inversely
proportional to the concentration of the analyte in the test sample. As a
result, the concentration of the different color-developed analytes is
determined by measuring the intensity of light sensed by the different
detectors.
The detectors are typically silicon photodetectors having a broad band
spectral response covering the range of wavelengths between 300 nm and
1100 nm. To limit the spectral responses of the silicon photodetectors to
different wavelength bands, a different optical filter is positioned in
front of each silicon photodetector. For example, a green optical filter
is positioned in front of a first silicon photodetector, a blue optical
filter is positioned in front of a second photodetector, and a red optical
filter is positioned in front of a third photodetector. Thus, the silicon
photodetectors are accompanied by respective optical filters with each
optical filter transmitting a different band of wavelengths to its
corresponding detector. Furthermore, using a specially designed housing
assembly, the filter and photodetector combinations are optically isolated
from each other to prevent optical crosstalk between the combinations.
Optical crosstalk occurs when light passing through the optical filter and
entering the photodetector of one filter and photodetector combination
also enters the photodetector of another filter and photodetector
combination. Since the concentration of a specific analyte is determined
by the amount of light detected by the filter and photodetector
combination targeting that analyte, optical crosstalk will decrease the
accuracy of that determination. To optically isolate the filter and
photodetector combinations from each other, the housing assembly
containing these combinations includes partitions between these
combinations which are impervious to light.
A drawback of using silicon photodetectors combined with optical filters to
limit the spectral responses of the photodetectors is that the housing
assembly containing the filter and photodetector combinations is
relatively bulky. First, the housing assembly must accommodate the optical
filters by securing the filters in the assembly and locating the filters
in front of their respective photodetectors. Second, the housing assembly
must accommodate the optically impervious partitions for optically
isolating the filter and photodetector combinations from each other. A
related drawback of using filter and photodetector combinations is that
the component and assembly costs are relatively high. Each filter and
photodetector combination is relatively expensive, and it is costly and
difficult to manufacture the housing assembly for mounting these
combinations and preventing optical crosstalk therebetween.
Another drawback of using filter and photodetector combinations is that the
optical filters still transmit wavelengths, albeit attenuated wavelengths,
outside the passband. If the spectral responses of the combinations
overlap with each other, the accuracy of the measured concentrations of
the different analytes targeted by the combinations is reduced. For most
accurate calculations of analyte concentration, it is preferable to
strictly confine the spectral responses of the photodetectors to the
respective characteristic absorption bands of the color-developed analytes
without the spectral responses overlapping each other.
A need therefore exists for a multi-detector readhead of a photometer for
reflectance and transmittance applications which overcomes the
aforementioned shortcomings associated with existing photometers using
filter and silicon photodetector combinations for light detection.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a
multi-detector readhead of a photometer for reflectance and transmittance
applications which enhances optical isolation between the detectors and,
at the same time, is relatively compact in design.
Another object of the present invention is to provide a multi-detector
readhead which is cost-effective and easy to manufacture.
In a particular embodiment the foregoing objects are realized by providing
a multi-detector readhead for measuring concentrations within a test
sample of preselected color-developed analytes having different
characteristic absorption bands. The readhead comprises an artificial
light source for illuminating the test sample, a plurality of
light-emitting diodes for detecting light reflected from or transmitted
through the sample, and a housing assembly for supporting the plurality of
light-emitting diodes. The light-emitting diodes are configured to
function as spectrally selective detectors with different spectral
responses. The spectral response of one of the light-emitting diodes is
preferably outside the characteristic absorption bands of the reaction
products within the sample, while the spectral responses of the remaining
light-emitting diodes encompass respective ones of the characteristic
absorption bands of the color-developed analytes. In a reflectance
photometer, the plurality of light-emitting diodes are disposed on the
same side of the test sample as the light source so that the
light-emitting diodes detect light reflected from the test sample. In a
transmittance photometer, the plurality of light-emitting diodes are
disposed on the opposite side of the test sample relative to the light
source so that the light-emitting diodes detect light transmitted through
the test sample.
The light-emitting diodes have different spectral responses with virtually
no overlap so as to inherently provide optical isolation between the
light-emitting diodes. Therefore, no optical filters are necessary to
limit the spectral responses of the light-emitting diodes, and the housing
assembly need not be specially designed with partitions to prevent optical
crosstalk between the light-emitting diodes. Without the need for optical
filters or a specially designed housing assembly, the photometer readhead
of the present invention is less expensive than the prior art readhead and
is easier to manufacture. In addition, since the housing assembly need not
accommodate optical filters or optically impervious partitions, the
housing assembly of the present invention is more compact than the housing
assembly employed with the prior art readhead.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the invention will become apparent upon
reading the following detailed description and upon reference to the
drawings in which:
FIG. 1 is a diagrammatic side view of a readhead for a transmittance
photometer in accordance with the present invention;
FIG. 2 is a diagrammatic side view of a readhead for a reflectance
photometer in accordance with present invention;
FIG. 3 is a plan view of a detector assembly employed with the readheads in
FIGS. 1-2;
FIG. 4 is a section taken generally along the line 4--4 in FIG. 3; and
FIG. 5 is a perspective view of the detector assembly in FIGS. 3-4.
While the invention is susceptible to various modifications and alternative
forms, a specific embodiment thereof has been shown by way of example in
the drawings and will herein be described in detail. It should be
understood, however, that it is not intended to limit the invention to the
particular forms disclosed, but on the contrary, the intention is to cover
all modifications, equivalents, and alternatives falling within the spirit
and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawings, FIG. 1 illustrates a readhead 10 for a
transmittance photometer having an artificial light source 12, a detector
assembly 14, and processing circuitry 16. The light source 12 preferably
emits controlled, diffuse light having a broad spectral output. Examples
of such artificial light sources meeting these criteria include
incandescent, halogen, or fluorescent lamps. Alternatively, the light
source 12 may be a plurality of light-emitting diodes having different
spectral emission bands. The light produced by the light source 12
impinges on a test tube or vial 18 containing a body fluid sample.
Reagents have been added to the body fluid sample to develop color in the
analytes of interest. As the light travels through the test sample, the
light is absorbed to varying degrees and transmitted in a scattered
pattern. Some of the transmitted light is detected by the detector
assembly 14, which is disposed on the opposite side of the fluid sample
relative to the light source 12. The detector assembly 14 includes a
plurality of detectors with different spectral responses for detecting the
light transmitted through the mixture. One of the detectors preferably has
a spectral response outside the characteristic absorption bands of the
color-developed analytes within the test sample so that the detector
senses light without absorption. The amount of light detected by this
detector represents a reference value against which the light detected by
the other detectors may be compared to determine the degree of absorption.
These other detectors sense different bands of wavelengths with each band
covering the characteristic absorption band of a corresponding
color-developed analyte. Each detector of the detector assembly 14
converts the light (photons) detected by that detector into an electrical
output signal which is transmitted to the processing circuitry 16. The
processing circuitry 16 determines the concentrations of the
color-developed analytes based on the amount of light detected by each
detector.
FIG. 2 illustrates a typical geometrical arrangement for a readhead 20 of a
reflectance photometer. The readhead 20 includes an artificial light
source 22, a detector assembly 24, and processing circuitry 26. Like the
light source 12, the light source 22 preferably generates controlled,
diffuse light having a broad spectral output. Alternatively, the light
source 22 is a plurality of light-emitting diodes having different
spectral emission bands. The light source 22 illuminates a body fluid
sample on a test strip or pad impregnated with reagents. The reagents
react with analytes of interest within the body fluid sample to develop
color in the analytes. Light striking the test strip 28 is a absorbed to
varying degrees and reflected in a scattered pattern. Some of the
reflected light is detected by the detector assembly 24. Like the detector
assembly 14; the detector assembly 24 includes a plurality of detectors
for detecting different wavelength bands. One of the wavelength bands is
outside the characteristic absorption bands of the
color-developed-analytes to generate a reference value. The remaining
wavelength bands each encompass the characteristic absorption band of a
corresponding color-developed analyte so that the amount of light sensed
by the detectors with these remaining wavelength bands is proportional to
the concentrations of the analytes of interest. These concentrations are
calculated by transmitting the electrical output signals produced by the
different detectors of the detector assembly 24 to the processing
circuitry 26.
Since the readhead 20 is used for reflectance applications, the detector
assembly 24 is disposed on the same side of the test strip 28 as the light
source 22. In the preferred embodiment, the light source 22 is mounted in
the readhead 20 perpendicular to the test strip 28. The detector assembly
24 is mounted at a scattering angle of 45 degrees as measured between a
line representing the direction of travel of the incoming light from the
light source 22 to the test strip 28 and a line representing the direction
of travel of the reflected light from the test strip 28 to the detector
assembly 24. While other scattering angles may be used, it is well-known
in the art that a scattering angle of 45 degrees is most efficient.
The detector assemblies 14, 24 in FIGS. 1-2 are preferably constructed as
depicted in FIGS. 3-5. Each detector assembly includes a plurality of
light-emitting diodes 30, 32, 34, and 36 mounted to a printed circuit
board 38. The cathodes of these light-emitting diodes are disposed
adjacent the printed circuit board 38 and are connected a ground plane on
the circuit board 38 using solder, conductive paste, or the like. To
ground the cathodes a grounding wire 39 extends through the printed
circuit board 38 and is connected to the ground plane. The anodes of the
light-emitting diodes 30, 32, 34, and 36 are disposed opposite the
cathodes and are coupled to respective lead wires 40, 42, 44, and 46 by
means of respective bond wires 50, 52, 54, and 56. At one end, the lead
wires 40, 42, 44, and 46 extend through the printed circuit board 38 and
are electrically insulated from the ground plane formed thereon. At the
other end, the lead wires 40, 42, 44, and 46 are connected to processing
circuitry (see FIGS. 1-2) disposed on a motherboard. In the foregoing
manner, the light-emitting diodes 30, 32, 34, and 36 are configured to
function as detectors, instead of emitters. In an alternative embodiment,
the printed circuit board 38 is replaced with a metal plate (e.g., brass
plate) so that the grounding wire 39 and the cathodes of the
light-emitting diodes 30, 32, 34, and 36 are simply connected to any part
of the plate for grounding. To prevent grounding of the lead wires 40, 42,
44, and 46 as they pass through the plate, the lead wires are insulated
from the plate at those locations where they pass through the plate.
To isolate the light-emitting diodes 30, 32, 34, and 36 from environmental
effects such as humidity and to prevent contamination of the die
composition forming the light-emitting diodes 30, 32, 34, and 36, the
light-emitting diodes are enclosed within a cylindrical housing having a
transparent cover or window 58, a base corresponding to the printed
circuit board 38, and a cylindrical side wall 60 bridging the window 58
and the printed circuit board 38. To permit light energy from the test
sample to freely enter the housing, the window 58 is preferably composed
of glass or plastic. To concentrate the light energy as it enters the
housing, the window 58 may be configured in the shape of a lens. The
cylindrical side wall 60 of the housing is preferably composed of metal or
plastic and, if composed of metal, may be connected to the circular edge
of the printed ,circuit board 38 by spot welding.
Serving as light detectors, each of the light-emitting diodes 30, 32, 34,
and 36 has a narrow spectral response which permits the diode to detect
only a narrow band of wavelengths. The detected wavelength band is
determined by the die composition of the light-emitting diode. In
accordance with tests performed by the present inventor, Table 1 lists the
typical spectral response characteristics of light-emitting diodes having
different die compositions:
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Response Bandwidth
Die Composition
Peak Wavelength
at FWHM
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GaAs 910 nm 170 nm
GaAlAs 840 nm 50 nm
GaAlAs 680 nm 150
GaAlAs 670 nm 165
GaP 520 nm 90
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The "Peak Wavelength" is the wavelength corresponding to the maximum
response (in amperes per watt) of the light-emitting diode. The "Response
Bandwidth at FWHM" is the full width, in terms of wavelength, of a
spectral response curve at half the maximum response. In other words, the
"Response Bandwidth at FWHM" is the difference between the wavelengths on
both sides of a spectral response curve at which the response quantity
reaches half its maximum intensity.
It can be seen from Table 1 that the spectral responses of the first and
second entries substantially fall within the infrared spectrum, the
spectral responses of the third and fourth entries substantially fall
within the red spectrum, and the spectral response of the fifth entry
substantially falls within the green spectrum. Furthermore, the response
bandwidths of the tested light-emitting diodes range from approximately 50
nm to approximately 170 nm. In addition to the tested light-emitting
diodes, light-emitting diodes with spectral responses falling within other
portions of the light spectrum or with response bandwidths smaller than 50
nm may be employed for the present invention.
The three light-emitting diodes 30, 32, and 34 in FIGS. 3-5 are selected
such that their spectral responses are substantially distinct from each
other. Moreover, the light-emitting diodes 30, 32, and 34 are selected
such that their spectral responses encompass the characteristic discrete
spectrums for absorption of light of three different color-developed
analytes (formed from analytes of interest) having three non-overlapping
characteristic absorption bands. For any given test, the concentrations of
three different color-developed analytes having substantially
non-overlapping absorption bands may be determined. For example, if
reagents develop the color red for analyte A, blue for analyte B, and
green for analyte C, the characteristic absorption bands of analytes A, B,
and C are located within the respective red, blue, and green portions of
the visible light spectrum. Therefore, the light-emitting diodes 30, 32,
34 are selected such that their spectral responses do not overlap with
each other and such that their spectral responses encompass the respective
characteristic absorption bands of analytes A, B, and C. In this example,
the light-emitting diode 30 would be "red" light-emitting diode, the
light-emitting diode 32 would be a "blue" light-emitting diode, and the
light-emitting diode 34 would be a "green" light-emitting diode.
The fourth light-emitting diode 36 is preferably selected such that its
spectral response does not contain the characteristic absorption spectrums
of the color-developed analytes. In the preferred embodiment, this
light-emitting diode 36 has a spectral response in the infrared spectrum
because none of the color-developed analytes has a characteristic
absorption band in this portion of the light spectrum. The amount of light
received by the light-emitting diode 36 is not affected by the
concentration of the analytes of interest within the body fluid sample.
Therefore, the amount of light detected by the infrared light-emitting
diode 36 establishes a reference value corresponding to negligible
absorption of light. Using processing circuitry (see the processing
circuitry 16, 26 in FIGS. 1-2), the amount of light detected by the other
three light-emitting diodes 30, 32, and 34 is compared to this reference
value to determine the amount of light absorbed by the respective
color-developed analytes, thereby determining the concentrations of the
analytes.
A significant advantage of using the light-emitting diodes as detectors, as
opposed to the filter and photodetector combinations of the prior art, is
that the light-emitting diodes inherently provide optical isolation
between each other because they reject wavelengths outside their spectral
responses. This inherent optical isolation means not only that optical
filters are not necessary to limit the spectral responses of the
light-emitting diodes, but also that the housing need not include
partitions to optically isolate the light-emitting diodes from each other.
Without the optical filters and the housing partitions, the detector
assembly (and the photometer readhead) of the present invention is more
compact, less costly, and easier to manufacture than the prior art
detector assembly and readhead. The compactness of the detector assembly
also results from the light-emitting diodes being much smaller in width
than the filter and photodetector combinations used in the prior art
readhead. In the preferred embodiment, the diameter of the cylindrical
housing is approximately 0.04 inches and the width of each light-emitting
diode is approximately 0.01 inches. In contrast, the housing assembly for
housing the filter and photodetector combinations of the prior art has an
approximate width of 0.40 inches and the width of each combination is 0.08
inches or more. It should be apparent that the detector assembly
illustrated in FIGS. 3-5 is substantially more compact than the detector
assembly employed with the prior an filter and photodetector combinations.
Due to the simple construction of the housing and the fact that the
light-emitting diodes are significantly less expensive than the filter and
photodetector combinations of the prior art, the detector assembly in
FIGS. 3-5 is more cost-effective than the detector assembly used in the
prior art readhead.
Another important advantage of using the light-emitting diodes, as opposed
to the filter and photodetector combinations, is that the spectral
responses of the light-emitting diodes are more discrete than the spectral
responses of the filter and photodetector combinations of the prior art.
As a result, the spectral response of each light-emitting diode is
strictly confined to the characteristic absorption band of its
corresponding color-developed analyte. Since the spectral responses of the
light-emitting diodes do not overlap with each other, the concentrations
of the analytes of interest in the test sample are measured with a high
degree of accuracy.
In addition to implementing the detector assemblies with the light-emitting
diodes 30, 32, 34, and 36, the light sources 12, 22 may also be
implemented with light-emitting diodes to further reduce the cost of the
readhead and reduce power consumption of the light sources 22. The number
of light-emitting diodes in the light source is equal to the number of
light-emitting diodes in the detector assembly. Therefore, if the detector
assembly includes the four light-emitting diodes 30, 32, 34, and 36, the
light source would include four light-emitting diodes. The light-emitting
diodes in the light source are selected so that their spectral emission
bands are associated with the spectral responses of the light-emitting
diodes 30, 32, 34, and 36 of the detector assembly. This results in the
creation of several unique spectral bandpasses.
While the present invention has been described with reference to one or
more particular embodiments, those skilled in the art will recognize that
many changes may be made thereto without departing from the spirit and
scope of the present invention. For example, the detector assembly in
FIGS. 3-5 may be modified to include more or less than four light-emitting
diodes. Furthermore, the infrared light-emitting diode 36 used for
establishing a reference value indicative of negligible light absorption
may be removed from the detector assembly. In this case, the reference
value is established using another detector assembly which receives light
directly from the light source, without the light first being reflected
from or transmitted though the test sample. Each of these embodiments and
obvious variations thereof is contemplated as falling within the spirit
and scope of the claimed invention, which is set forth in the following
claims.
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