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BACKGROUND OF THE INVENTION
The invention relates generally to optical temperature measurement, and
more particularly to optical temperature measurement based on the
temperature-dependent absorption and fluorescent emission properties of
selected materials.
Temperature sensing and control are crucial in a variety of situations
arising in medicine, industrial operations, and scientific research. In
many cases, temperature measurements must be conducted remotely because
the processes or machinery to be monitored is inaccessible or involves
hazardous components, such as, high pressures, high radiation levels, high
intensity magnetic or electrical fields, corrosive materials, or the like.
In medicine the need for temperature monitoring arises in several contexts.
Cardiac output and blood flow rates are measured by the thermal dilution
method, wherein a bolus of room temperature solution is injected into a
vein and blood temperature changes are monitored by one or more
temperature sensing devices located down stream. For accurate
measurements, it is desirable to use sensors with frequency responses
greater than the major frequency components of the arterial pressure
pulse, i.e., faster than about 20-30 Hz. In operations involving
artificial hypothermia, such as heart and neurological cases, temperature
monitoring is critical. Accurate temperature monitoring is also critical
where local hyperthermia is induced as a means of cancer therapy. The
differences between normal cells and malignant cells in their
sensitivities to thermal killing is often no more than a few tenths of a
degree. The presence of metallic sensors greatly complicates temperature
control in this mode of therapy as the presence of such sensors can alter
the thermal characteristics of the tissue in which they are imbedded.
Furthermore, electromagnetic heating of tissues causes special
difficulties: electromagnetic interference is induced in the thermometry
electronics, excessive and artifactual heating occurs in sensors
constructed of resistive material (both thermistors and themocouples), and
sensors, especially those contained in highly conductive (and hence
reflective) shields, perturb the electromagnetic fields used for heating.
Thus, application of metallic, electrically-based temperature sensors
presents significant problems: reduced accuracy due to noise and
inadequate frequency response, and direct electrical hazard to the
patient, especially when more than one sensor is employed.
In industrial process control the most common techniques for temperature
measurement utilize thermocouples, thermistors, and resistance
thermometers. These devices generate electrical signals which are
amplified and then converted into temperature readings or employed in
control functions. Frequently, these devices are impractical because the
process is inaccessible, too hazardous, or too corrosive for in situ
placement of sensors. For example, temperature monitoring of nuclear
reactor vessels and coolant systems, underground nuclear waste-disposal
sites, chemical dumping sites, working zones of coal-liquification
reactors, oil refinery processes, geothermal wells, and like processes,
all involve conditions which make the use of standard electrically-based
sensors difficult or impractical.
Many of the above-mentioned difficulties with current information-gathering
technology can be overcome by using remote, in situ optical probes coupled
to a detector by optical waveguides, or fiber optics. Fiber optics are
durable, corrosion-resistant, heat-resistant, impervious to electrical or
magnetic interference, and are available in very small diameters, which
makes them amenable for use with miniature probes.
A large variety of optical temperature sensors have been developed, some of
which are amenable for use with fiber optics, Wickersheim and Alves,
"Recent Advances in Optical Temperature Measurement," Industrial Research
and Development, December 1979; Peterson and Vurek, "Fiber-Optic Sensors
for Biomedical Applications," Science, Vol. 224, pgs. 123-127 (1984).
Optically-based sensors generating fluorescent signals modulated by
ambient conditions are particularly well suited for use with optical
fibers, and several temperature sensors have been developed which are
based on the temperature dependent fluorescent properties of selected
materials, e.g. Quick et al., U.S. Pat. No. 4,223,226, issued September
1980, entitled "Fiber Optic Temperature Sensor,"Samulski, U.S. Pat. No.
4,245,507, issued Jan. 20, 1981, entitled "Temperature Probe," Samulski,
U.S. Pat. No. 4,437,772, issued Mar. 20, 1984, entitled "Luminescent Decay
Time Techniques for Temperature Measurement;" and Hirschfelo, U.S. Pat.
No. 4,542,987, issued Sept. 24, 1985, entitled "Temperature-Sensitive
Optrode;" and Haugen and Hirschfeld, "An Ultrafast Remote Sensor for High
Pressures and Temperatures," Energy and Technology Review, pgs. 78-79
(July 1985). Sensitivity is a major problem with this class of sensors,
particularly when used with fiber optics of significant length. That is,
it is difficult to obtain a strong enough signal from a fluorescent probe
to permit the measurement of small differences in temperature.
Of particular interest are U.S. Pat. Nos. 4,075,493 and 4,215,275 by
Wickersheim, issued 21 Feb. 1978 and 29 July 1980, respectively, and both
entitled "Optical Temperature Measurement Technique Utilizing Phosphors."
Wickersheim discloses a method of measuring temperature by monitoring the
intensity ratio of at least two distinct and optically isolatable
fluorescent emission lines of selected rare earth-doped compounds. His
invention requires at least two photodetection devices, one for each
emission line.
Also of interest is Snitzer et al., U.S. Pat. No. 4,302,970, issued 1 Dec.
1981, entitled "Optical Temperature Probe Employing Rare Earth
Absorption." Snitzer et al. disclose a device which used the temperature
dependent absorption properties of selected rare earths for sensing
temperature. The transmission of light through a rare earth-doped material
is monitored, and temperature is related to the amount of light
transmitted through the material.
SUMMARY OF THE INVENTION
The present invention includes a method and apparatus for optically
monitoring temperature. The invention is based on the discovery of a class
of materials that possess fluorescent emission lines whose intensity
varies directly with temperature over a given temperature range, whenever
excited by light having a first wavelength, and whose intensity varies
inversely with temperature over substantially the same temperature range
whenever excited by light having a second wavelength. Fluorescent emission
lines with such a property are referred to herein as excitation-dependent
emission lines.
Preferably this class of materials includes, but is not necessarily limited
to, solids comprising a host material doped with trivalent rare earth
ions.
It is believed that the above property arises in certain atoms and
molecules because they possess clusters of closely spaced energy levels,
as exemplified by many of the trivalent rare earth ions. Because of the
closely spaced energy levels within a cluster, a population of atoms or
molecules associated with a cluster possesses a plurality of
subpopulations, wherein each subpopulation corresponds to a particular
energy level within the cluster. When the material is in thermal
equilibrium with its surroundings the relative average sizes of the
subpopulations remain constant, but as the surrounding temperature changes
the relative number of atoms or molecules within the subpopulations
changes. By illuminating such materials with appropriate wavelengths of
light, members of each subpopulation can be preferentially excited to an
energy level in a higher energy cluster, the decay from which results in
fluorescent emissions with substantially identical wavelengths, regardless
of the original energy level of the first cluster occupied by the excited
atom or molecule.
This property is exploited for measuring temperature by the following
method. The material is placed in thermal contact with the substance whose
temperature is to be monitored. Next, the material is alternatively
illuminated with a first illumination beam having a first wavelength and a
second illumination beam having a second wavelength, the first and second
wavelengths being chosen, such that the material is caused to fluoresce at
an excitation-dependent emission line. The intensity of the
excitation-dependent emission line is measured for successive periods of
illumination by the first and second illumination beams, and the ratio of
successively measured intensities is computed. Finally, the ratio of
intensities is related to the temperature of the material.
Apparatus of the invention generally comprise means for carrying out the
steps of the method outlined above. Preferably, the apparatus includes a
single white light source and an associated filter wheel for alternatively
generating the first and second illumination beams. Collectively, this
arrangement is one example of an associated light source referred to
below. In further preference, light from the first and second illumination
beams is delivered to the material by way of a single fiber optic, and the
same fiber optic is use to collect fluorescent emissions from the
material. Means are provided for separating the first and second
illumination beams from the fluorescent emission, collected and
transmitted through the fiber optic. The apparatus includes a detector,
and standard electronics for computing and analyzing the measured
intensity ratios. The invention further includes an optrode which, as the
term is used herein, comprises a fiber optic operationally associated with
the temperature probe material in accordance with the disclosure below.
The present invention addresses the problem associated with making
optically-based temperature measurements, particularly those made over
fiber optics and which depend on the generation of fluorescent signals. In
accordance with the invention, closely spaced temperatures are determined
more readily than was heretofore possible with current technology which is
based on the fluorescent properties of a material at a single emission
line. This is accomplished by using temperature probes comprising
materials that possess excitation-dependent fluorescent emission lines,
and by relating to temperature the intensity ratio between successive
emissions at such a line undergoing alternative excitation at its
associated first and second wavelengths.
The invention also advantageously overcomes the problem of low
signal-to-noise ratio in remote measurements made over long distance fiber
optics: As temperature changes the change in the intensity ratio of
successive emissions of excitation-dependent lines is greater than the
changes in the absolute intensity of either emission alone.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 diagrammatically illustrates a preferred implementation of the
invention;
FIG. 2 diagrammatically illustrate a preferred implementation of the
invention utilizing a fiber optic; and
FIG. 3 illustrates the temperature dependent responses of the 611 nm
excitation-dependent emission line of europium in zirconolite when excited
with a first wavelength of 488 nm and a second wavelength of 457 nm.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is directed to making optically based measurements of
temperature. It accomplishes this broad object by utilizing temperature
probe materials that possess at least one excitation-dependent emission
line. Sensitivity of temperature measurements is enhanced by relating to
temperature the intensity ratios of sequential emissions of such an
emission line undergoing alternative excitation by a first and a second
illumination beam.
The preferred temperature probe materials are solid host materials doped
with trivalent rare earth ions. Preferably the trivalent rare earth dopant
is europium or neodymium; more preferably the dopant is europium at a
molar concentration of between about 4-6 percent.
The solid host material can be any solid in which trivalent rare ions can
be inserted in appropriate concentrations, and through which optical
signals can be transmitted. The preferred choice of host material depends
in part on the physical conditions to be encountered in a particular
application. The host materials include crystals, such as, calcium
fluoride, barium fluoride, sodium chloride, sodium sulfate, strontium
sulfate, lead sulfate, lead arsenate, barium arsenate, or the like. The
host and dopants in such systems can be combined by coprecipitation
followed by calcination. Host materials also include glasses and ceramics.
In particular, ceramic and glass-ceramic materials developed for storage
of high level radioactive waste are the preferred host materials for high
temperature applications in corrosive environments. Some such materials
can be prepared from melts in accordance with the disclosures of U.S. Pat.
Nos. 4,274,976; 4,314,909; 4,329,248; or 4,383,855, which patents are
incorporated by reference. The preferred ceramics can also be prepared by
the sol-gel process using standard techniques, e.g., formation of gels by
mixing Ti and Zr alcoxides with Ca(NO.sub.3).sub.2 and Eu(NO.sub.3).sub.3
and hydrolyzing.
By way of example, the following temperature probe material was prepared
both from melts and from sol-gels. The material consisted of zirconolite
as the host material and trivalent europium at a 5% (molar) concentration
as the dopant. The probe material was obtained from melts of
stoichiometric mixtures of ZrO(NO.sub.3).sub.2 2H.sub.2 O,
Ca(NO.sub.3).sub.2 4H.sub.2 O, and TiO.sub.2, with small amounts of
Eu(N0.sub.3).sub.3 added. The mixtures were fired to at least 1200.degree.
C. Sol-gels of the material were prepared from two isopropanol solutions,
the first containing Ca(N0.sub.3).sub.2 and Eu(N0.sub.3).sub.3, and the
second containing Ti(OCH.sub.2 CH.sub.3).sub.4 and Zr(OCH.sub.2
CH.sub.3).sub.4. The first solution was cooled to approximately
-30.degree. C., then mixed with the second solution. Small amounts of
water and NH.sub.4 OH were added to the mixture of the first and second
solutions to promote gelation. For one embodiment of the invention, the
resulting gel was made in capillary tubes. The gels were removed from the
capillary tube via extrusion with air pressure to form fibers of so-called
alcogels. The fibers were mineralized by first preheating overnight at
about 600.degree.-700.degree. C., followed by a final heating overnight at
about 1200.degree. C., to give white fibers. Alternatively the sol-gels
can be molded in other forms, such as sheets or layers.
FIG. 3 illustrates the temperature dependency of the 611 nm
excitation-dependent emission line of europium in zirconolite produced by
the sol-gel process just described. The molar concentration of europium
was 5 percent. The temperature probe material was illuminated by 488 nm
(curve A) and 457 nm (curve B) light over the temperature range of
20.degree.-160.degree. C.
One apparatus for carrying out the method of the invention is illustrated
in FIG. 1. White light source 10 emits illumination beam 11 which is
collimated by lens 12 and directed through filter wheel 14. Filter wheel
14, driven by motor 16, contains two different kinds of band pass filters
arranged in a alternating sequence so that temperature probe material 26
is alternatively illuminated with light having a first wavelength and
light having a second wavelength. One filter passes light of the first
wavelength of the excitation dependent emission line and the other passes
light of the second wavelength of the excitation dependent emission line.
Light passed by filter wheel 14 is directed to beam splitter 18; a portion
of the light from beam splitter 18 is directed to lens 20 which focuses
that portion of the beam onto photomultiplier tube 22. Photomultiplier
tube 22 generates a gating signal used by signal processor 24. The gating
signal provides a time base for determining when temperature probe
material 26 is being illuminated. The portion of the illumination beam not
directed to photomultiplier 22 by beam splitter 18 is directed to
temperature probe material 26 which is in contact with the substance 30
whose temperature is to be monitored. Preferably, temperature probe
material 26 has a reflective backing 28 to redirect light through the
temperature probe material 26. Reflective backing 28 is selected so that
heat flow between temperature probe material 26 and substance 30 is
maximum, i.e., preferably reflective backing 28 is as thin as possible and
comprises a good thermal conductor. A portion of the light emitted by
temperature probe material 26 is collected and focussed onto
photomultiplier tube 36 by lenses 32 and 34. The signal generated by
photomultiplier tube 36 is directed to signal processor 24. Signal
processor 24 includes standard sample and hold circuitry which allows the
ratio of the intensities of the excitation dependent emission line to be
computed for successive periods of illumination by the first and second
illumination beams. The computed ratio can be read out directly or can be
converted to an actual temperature by way of a calibration table. The
ratio or temperature is displayed by readout device 38.
FIG. 2 illustrates another apparatus for carrying out the method of the
invention which employs the temperature probe material attached to a fiber
optic. Light source 10 emits illumination beam 11 which is collimated by
lens 12 and directed through filter wheel 14. Filter wheel 14 is driven by
motor 16 and the filter wheel motor combination is the same as that
described for the apparatus of FIG. 1. The beam passed by filter wheel 14
is split into two portions by beam splitter 18. One portion is directed to
lens 20 which focuses the beam onto photomultiplier 22. The function of
photomultiplier tube 22 is the same as that described for the
corresponding element in FIG. 1; namely, it generates a gating signal
which is used by signal processor 24 to compute the ratio of successive
intensities of fluorescent emissions emitted by temperature probe material
26. The portion of the illumination beam 11 passed by beam splitter 18 is
directed to lens 40 which focuses illumination beam 11 onto the first end,
46, of fiber optic 44. Fiber optic 44 transmits illumination beam 11 from
its first end 46 to its second end 48. Temperature probe material 26 is
attached to second end 48 of fiber optic 44. Light from the illumination
beam emanating from second end 48 impinges on temperature probe material
26 causing it to fluoresce. A portion of the fluorescent emissions are
collected by second end 48 of fiber optic 44 and transmitted to its first
end 46. Fluorescent emissions 47 emanate from first end 46 of fiber optic
44 and are directed by beam splitter 42 to lens 50 which collimates them
and directs them through filter 52. Filter 52 is a band pass filter which
only passes light having wavelengths substantially the same as that of the
excitation dependent emission line of temperature probe material 26. Light
passed by filter 52 is directed to lens 54 which focuses the fluorescent
emissions onto photomultiplier tube 36. Photomultiplier tube 36 generates
a signal which is directed to signal processor 24. Signal processor 24 and
readout 38 operate substantially the same as the corresponding components
of the apparatus of FIG. 1.
The descriptions of the foregoing embodiments of the invention have been
presented for purposes of illustration and description. They are not
intended to be exhaustive or to limit the invention to the precise form
disclosed, and obviously many modifications and variations are possible in
light of the above teaching. The embodiments were chosen and described in
order to best explain the principles of the invention and its practical
application to thereby enable others skilled in the art to best utilize
the invention in various embodiments and with various modifications as are
suited to the particular use contemplated. It is intended that the scope
of the invention be defined by the claims appended hereto.
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