Fiber optic pyrometry with large dynamic range

Fiber optic probe apparatus usable for measuring temperatures with increased dynamic range and frequency domain response and desirable measurement accuracy. A black body signal source, fiber optic signal coupling, and extension of the transducer dynamic range with optical multiplexing are employed; the instrument operates in the range of 1.6 micrometers of optical energy wavelength and preferably employs recently improved indium gallium arsenide photodiode transducer devices and transducer frequency domain compensation. Use of the instrument in measuring combustion flame transient temperatures is disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to the field of multi-channeled fiber optic 
temperature measuring instruments and to the use of fiber optic coupled 
black body transducers in the wide range, rapidly changing measurement of 
temperatures. 
High temperature fiber optic thermometry offers a plausible solution for 
several complex problems in the temperature measurement art. Among these 
problems is the measurement of rapidly fluctuating temperatures, as found 
in a flame; temperature measurement in the presence of strong 
electromagnetic fields, as in electric furnaces or induction furnaces; 
temperature measurement in a nuclear radiation environment, and 
temperature measurement with greater accuracy than afforded by the 
present-day thermocouple measurement. Fiber optic thermometry also 
provides new answers for the problems of temperature probe or thermocouple 
lead heat conduction, temperature probe gas flow disruption, and new 
approaches to laboratory standard temperature measurements. Fiber optic 
thermometry is perhaps most importantly, however, one of the most 
promosing approaches for measuring the temperature transients encountered 
in a turbulent combustion or sooty combustion atmosphere. 
The recent advent of improved solid state phototransducer devices having 
useful properties in the infrared spectral range and intended principally 
for use in the fiber optic communications field has added new dimensions 
to the capability of fiber optic temperature measurement systems. An 
example of such phototransducer devices is found in the indium gallium 
arsenide photodiode device currently manufactured by RCA Corporation, and 
known as the C30980E Photodiode. The C30980E photodiode, which is 
preferred in the described embodiment of the present invention, is 
described technically in a data sheet titled "C30979E, C30979EQC, C30980E, 
C30980EL InGaA.sub.s Photodiodes" which was printed in September 1982 and 
July 1984 by RCA Corporation and in RCA advertisements such as appear at 
page 178 of the journal Laser Focus and Fiberoptic Technology, April 1982. 
Such photodiodes are sold by RCA Corporation from a New Products Division 
Office on New Holland Avenue in Lancaster, PA 17604-3140 and from a 
Photodetector Marketing Office located at Ste. Anne de Bellevue, Quebec, 
Canada H9X 3L3. The RCA and similar solid state photoelectric transducer 
devices which may be or become available from other commercial suppliers 
have favorable response characteristics in a portion of the infrared 
spectrum that is desirable for performing temperature measurements. 
Improvements in optical temperature measurement have also been recently 
achieved at the National Bureau of Standards, by R. R. Dils and others, as 
is exemplified by the Dils articles "High-temperature Optical Thermometer" 
published in the Journal of Applied Physics, Vol. 54, 1983, p. 1198 and "A 
Fiberoptic Probe for Measuring High Frequency Temperature Fluctuations in 
Combustion Gases", Sandia Report 83-8871 published during the first half 
of 1984 by R. R. Dils and D. A. Tichenor. Both of these Dils publications 
are hereby incorporated herein by reference. 
The patent art includes several examples of high temperature measuring 
systems which precede the present invention. Included in this patent art 
is the patent of D. A. Kahn, U.S. Pat. No. 4,326,798, which concerns an 
optical pyrometer system employing spectral segregation of signal 
components received from a workpiece being measured, i.e., from a 
temperature elevated engine turbine blade. The Kahn patent is especially 
concerned with the avoidance of measurement errors resulting from 
transient spurious sources of heat in the measurement field, sources such 
as might be provided by heated particles of carbon in the workpiece 
atmosphere. Although the Kahn patent teaches the use of such herein 
employed elements as a beamsplitter, dual optical-to-electrical 
transducers, dual amplifier channels and the use of a workpiece-inherent 
black body member, the thrust of the Kahn patent is in the direction of 
performing accurate measurements in the presence of transient spurious 
sources of heat and in the elimination of effects from these transient 
spurious heat sources. 
The patent of Folke Lofgren et al, U.S. Pat. No. 4,409,476, concerns a 
fiber optic temperature measurement arrangement which also includes an 
optical beamsplitter and a pair of optical-to-electrical transducer 
elements and additionally includes a photoluminescent black body solid 
material subjected to the temperature being measured. The photoluminescent 
material used in the Lofgren patent is in the nature of a semiconductor 
compound and exhibits the characteristic of responding to excitation by a 
light source, such as a light emitting diode, by emitting light of a 
different wavelength--the emitted light wavelength being dependent on the 
temperature of the semiconductor material. The Lofgren patent contemplates 
use of optical signals of different spectral content at two photodetector 
devices. An optical filter is inserted in the path of one or both 
photodetector devices. The Lofgren patent also contemplates the use of 
time multiplexing in exciting the light emitting diodes--in order to 
segregate signals resulting from the different photoluminescent detector 
devices. The Lofgren patent therefore involves a time multiplexed sensing 
of photoluminescent materials excited by pulses of light emitting diode 
light using a beamsplitting arrangement for achieving different spectral 
responses in the detector unit. 
The patent of Kenya Goto, U.S. Pat. No. 4,367,040, concerns a multichannel 
multiplexed optical temperature measuring system wherein light supplied 
from an external source such as a laser diode, is transmitted 
bi-directionally along optical fiber transmission paths to and from a 
reflection or transmission type of optical sensor. The optical sensors of 
the Goto apparatus modulate the intensity of the supplied light beam in 
response to temperature or other physical quantities being measured. 
The patent of C. E. Everest, U.S. Pat. No. 4,420,265, concerns an infrared 
responsive temperature measuring system intended primarily for 
agricultural applications and having the ability to compensate for sky 
radiation variations included in the measured signal. The patent of G. J. 
Carlson, U.S. Pat. No. 3,486,378, also concerns a temperature measuring 
apparatus wherein two detector cells are multiplexed--with the 
multiplexing of infrared energy in this case being achieved with a 
rotating disk modulator and with one of the detectors being excited by an 
incandescent lamp or other reference light source. 
While each of these patents help identify the state of the fiber optic, 
multiplexed signal, temperature meausurement art preceding the present 
invention, none of the measurement devices taught by these patents achieve 
the advantages of dynamic range multiplexing and the other techniques of 
the present invention. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a fiber optic temperature 
measuring apparatus of unusual temperature range capability. 
Another object of the invention is to provide a fiber optic temperature 
measuring arrangement wherein plural phototransducers are employed in 
order to cover a large dynamic range. 
Another object of the invention is to provide a fiber optic temperature 
measurement arrangement wherein the solid state phototransducer devices re 
reasonably protected from excess optical signal physical damage. 
Another object of the invention is to provide a fiber optic temperature 
measurement arrangement which combines the advantages of a black body 
thermal transducer with operation in a desirable portion of the infrared 
spectrum. 
Another object of the invention is to provide a fiber optic temperature 
measurement arrangement which employs the characteristics of newly 
available photodiode transducer devices to achieve a desirable spectral 
operating region, a wide dynamic operating range and other advantages. 
Another object of the invention is to provide a fiber optic temperature 
measuring system which minimizes the disadvantages of materials such as 
sapphire, which have high cost and relatively poor temperature signal 
conductivity, but are nevertheless needed for desirable high-temperature 
therml properties. 
Another object of the invention is to provide a fiber optic temperature 
measuring system using the inherent filtering properties of optical 
coupling members in combination with optical bandpass filtering elements 
to achieve operation in an unusual and desirable portion of the infrared 
spectrum. 
Another object of the invention is to provide a temperature measuring 
apparatus of unusual frequency domain response capability. 
Another object of the invention is to provide a fiber optic temperature 
measuring arrangement suitable for use in combustion analysis of turbulent 
combustion and sooty combustion work environs--environs affording less 
than ideal temperature measuring conditions. 
Additional objects and features of the invention will be understood from 
the following description and the accompanying drawings. 
These and other objects of the invention are achieved by an apparatus 
providing means for transducing a work environ atmospheric temperature 
into an intensity modulated black body radiant optical signal, means for 
selecting a predetermined spectral band of the black body radiant signal 
as an intensity analog signal representation of the work environ 
temperature band, means for splitting the intensity analog signal into a 
plurality of intensity range segregated optical components, means for 
converting each of the component optical signals into an electrical 
signal, a plurality of electrical signals each corresponding to a 
component optical signal resulting, and means for generating a temperature 
indication signal from selected of the plural electrical signals.

DETAILED DESCRIPTION 
FIG. 1 of the drawings shows a fiber optic temperature measuring apparatus 
capable of temperature measurements over an unusually large dynamic range 
of 1E7 or greater. The principals exhibited in FIG. 1 can be extended to 
achieve dynamic ranges of even larger value. The notation form 1E7 used 
herein will be understood to mean 1 times 10 raised to the seventh power, 
or 1.times.10.sup.7. The notation 1E7 is therefore an abbreviation for one 
times ten raised to the exponent (E) seven and is a notation frequently 
used in computer programming and similar arts. 
The term "dynamic range" in instruments of the FIG. 1 type is understood to 
mean the ratio of maximum usable signal power to power of the minimum 
signal discernible in the inherent noise of the instrument; in other 
words, the ratio of maximum usable signal power to inherent noise signal 
power. Limited dynamic range has been a characteristic of instruments 
preceding the FIG. 1 system. 
The FIG. 1 apparatus includes three major components, a work environ black 
body signal source 100, a signal transmitting optical fiber array 102, and 
a detector assembly 104. Preferred arrangement details of the black body 
signal source 100 are shown in the expanded view 106 and include black 
body cavity-shaped radiating member 110 which is surrounded by a 
protective film layer 112 and received on the end of a high temperature 
optical fiber member 108. 
As indicated in the view 106 in FIG. 1, the black body radiating member 110 
is preferably fabricated as an irridium metal film, while the protective 
film layer 112 is preferably fabricated as an aluminum oxide film, and the 
optical fiber member 108 is preferably made from a bundle of sapphire 
(aluminum oxide) fiber elements. Alternates for these preferred materials 
are of course, possible, and include the use of a platinum metal film for 
the black body radiating member 110, other oxide materials for the 
protective film layer 112, and quartz fibers in limited temperature range 
applications for the sapphire fiber bundle 114. The black body cavity 
radiating member 110 is preferably formed over the end of the optical 
fiber bundle 108 by sputtering metal deposition techniques. Other 
fabrication arrangements including an irridium or tungsten wire disposed 
in front of the fiber bundle terminal or a mechanically deformed metallic 
sheet stock member; or alternately, a direct image of flame radiation 
emission can be used in some applications of the FIG. 1 apparatus. 
Optical signals from the black body radiating member 110 are conducted 
along the sapphire fiber bundle 114 to a low temperature fiber bundle 118. 
The low temperature fiber bundle 118 is used to conduct signal over the 
majority of the distance between the black body signal source 100 and the 
detector assembly 104. Optical signal coupling between the sapphire fiber 
bundle 114 and the low temperature fiber bundle 118 is accomplished by an 
optical coupler 116 of a type known in the fiber optic art. The sapphire 
fiber bundle 114 is of course, employed at the work environ end of the 
path between the black body signal source 100 and the detector assembly 
104 in order to withstand the elevated temperatures existing in the work 
environ, i.e., the temperatures being measured by the FIG. 1 apparatus. 
The sapphire fibers in the bundle 114 are capable of withstanding such 
work environ temperatures, but are undesirably inefficient as conductors 
of the optical signal generated by the black body source 100; in view of 
this conduction limitation and the cost of sapphire conductors, the bundle 
114 is preferably arranged to be as short as possible. 
The optical signal transmitted along the fiber array 102 in FIG. 1 emerges 
from the end terminus 120 located within the detector assembly 104 as the 
divergent optical signal indicated at 122. This divergent signal is 
captured and collimated by the lens L1, 124, which may have a focal length 
in the range of 2.5 cm. The collimated signal 125 emerging from the lens 
L1, 124, is captured by a second lens L2, 128, which is preferably of a 
focal length in the range of 25 cm. The lens L2, 128 serves to focus the 
collimated light signal 125 on the active surface of a pair of photodiode 
transducer elements T1, 140, and T2, 142. A bandpass filter BF, 126 is 
located in the path of the collimated light signal 125 in order to select 
a specific spectral band of optical signal frequencies for use in the 
transducer elements T1, 140 and T2, 142. As indicated subsequently herein, 
the filter 126 together with optical wavelength selective signal 
attenuation characteristics inherent in the optical fibers 102, can be 
used to elect a desirable range of the optical spectrum for operating the 
FIG. 1 apparatus. 
In the FIG. 1 apparatus, optical signal from the lens L2, 128 is received 
on the surface of a pellicle or optical beamsplitting member BS, 130 which 
preferably has the capability of dividing the optical signal into ten 
percent and ninety percent intensity ratio components. These components 
are subsequently converted into electrical signals having different 
intensity weighting factors. The beam splitter reflected optical signal 
component 134 preferably represents ten percent of the optical energy 
incident on the beamsplitting member 130, while the transmitted component 
132 represents ninety percent of the beamsplitter received optical energy. 
Each of the beamsplitter output components 132 and 134 is further 
attenuated by a pair of optical density attenuators OD1, 136, and OD2, 
138, located intermediate the beamsplitting member 130 and the photodiode 
transducer elements 140 and 142. The optical path of the reflected ten 
percent component signal 134 preferably includes an optical density 
attenuator 138 of attenuation factor between 3 and 3.5, while the optical 
path of the transmitted component 132 includes an optical density filter 
of a lesser attenuation factor--between 0.4 and 0.6. (An optical 
attenuator of attenuation factor or neutral density 3 provides attenuation 
of 1E3, or 10.sup.3, while an attenuator of neutral density 4 provides an 
attenuation of 1E4, or 10.sup.4.) As a result of the reflected component 
134 being only ten percent of the beamsplitting member received optical 
energy and the attenuation of the density attenuator 138 falling between 
1,000 and 10,000, the photodiode transducer element T2, 142 is effectively 
energized only upon receipt of high intensity optical signals. 
The photodiode transducer element T1, 140 by way of receiving the 
beamsplitter ninety percent component and having relatively small optical 
attenuation in the optical density attenuator OD1, 136 is responsive to 
low-level optical signals. During receipt of optical signals of sufficient 
intensity to activate the transducer element T2, 142, the low level 
photodiode transducer element T1, 140 is however, protected from physical 
damage by the presence of the attenuator 136 and its characteristic of 
attenuating the transmitted optical signal component 132 by a factor of 
1E4 to 1E6. 
Transducer elements of the preferred solid state type are found generally 
to have a physical damage threshold in the range of 500 milliwatts of 
incident optical energy; the optical density filter OD1, 136 is therefore 
provided with the capability of attenuating the largest expected 
transmitted signal component 132 to this or a lower level and is 
principally incorporated in the FIG. 1 apparatus to achieve such 
protection. Optical-to-electrical transducers of the solid state 
photodiode type are preferred for use in the FIG. 1 apparatus for inter 
alia reasons relating to transducer physical damage since the most 
plausible alternative transducer device, the photomultiplier vacuum tube 
is found to readily incur photocathode physical damage from incident 
optical energy in a FIG. 1 type apparatus. 
Each of the transducers T1, 140 and T2, 142 in the FIG. 1 apparatus is 
preferably a type C30980E photodiode supplied by RCA Corporation, or 
alternately, is a device of similar characteristics from another source. 
Devices of this type are sold by RCA Corporation in a variety of sizes and 
configurations, including some arrangements which incorporate light pipes, 
optical fibers, or preamplifiers. Such devices inherently have good speed 
of response and quantum efficiencies, however tradeoff among these and 
other characteristics are indicated in the RCA published device 
characteristics. Such devices are capable of providing the spectral 
response indicated in FIG. 3 of the drawings. Transducers or detectors of 
this type are preferably fabricated from alloys of indium, gallium, and 
arsenic or indium gallium arsenic/indium phosphide and are described in 
the above-referenced RCA Corporation data sheet. A series of published 
articles also describes transducers of this type. Several articles of this 
nature are to be found in the Institute of Electrical and Electronic 
Engineers (IEEE) Transactions on Electron Devices and notably in special 
issues on optoelectronic devices and light emitting diodes and long 
wavelength photodetectors found at Volume ED-29, Number 9, September 1982 
and Volume ED-30, Number 4, April 1983. The paper "Long-Wavelength (1.3 to 
1.6 .mu.m) Detectors for Fiber Optical Communications" by G. E. Stillman, 
L. W. Cook, G. E. Bulman, N. Tabatabaie, R. Chin, and P. D. Dapkus in the 
September 1982 transactions publication and the papers "InGaAsP 
Photodiodes" by E. E. Stillman, L. W. Cook, N. Tabatabaie, G. E. Bulman, 
and V. M. Robbins found at page 364, and "Large-Area and Visible Response 
VPE InGaAs Photodiodes" by P. P. Webb and G. H. Olsen found at page 395 
of the April 1983 transactions publication are of special interest with 
respect to such devices. The text of these IEEE special issues is hereby 
incorporated by reference herein. Detectors of this type can be operated 
either as a photovoltaic device--an EMF generator, or as a photoconductive 
device--a variable impedance element. Since the noise related leakage 
current (and also the transducer output signal response) is lower in the 
photoconductive mode of operation, this mode of transducer operation is 
preferred in FIG. 1. 
Transducers of this type have principally evolved in response to a need in 
the fiber optics communications field where signals in the infrared 
spectral region are coupled between a light emitting diode and a receptor 
by way of long lengths of fiber optic media. Photodiode transducer 
elements of this type are capable of operating over a linear dynamic range 
in the order of 1E4. By way of the 10/90 beamsplitting member 130 and the 
optical density attenuators 136 and 138 in FIG. 1, two individual linear 
ranges of 1E4 are stacked or added to provide an overall detector assembly 
dynamic range in the order of 1E8 in the FIG. 1 apparatus. The 
beamsplitting member, neutral density attenuator, and multiple transducer 
element arrangement of the detector 104 in FIG. 1 can be extended to 
include additional transducer elements and additional operating dynamic 
range with the inclusion of adequate protection for the most sensitive of 
the transducer elements. T1, 140 in FIG. 1, for example requires 
protection from physical damage during receipt of optical signals of the 
highest contemplated intensity. Generally, however, some difficulty is to 
be expected in extending the dynamic range of a FIG. 1 type apparatus 
significantly beyond the 1E8 region in view of the physical damage 
potential for the low-level or most sensitive of the transducer elements. 
The neutral density values of 0.4-0.6 and 3.0-3.5 in FIG. 1 together with 
the 10% and 90% attenuations of the beamsplitting member 130 effectively 
divide the optical signal 122 into two components of dynamic range 1E4 
each. The optical density attenuators 136 and 138 can be adjusted or 
selected in order to provide location adjustment of these ranges and to 
provide the slight degree of range overlap desired between the transducer 
elements 140 and 142 during receipt of optical signals in the 
mid-intensity range. 
The use of a black body transducer for generating optical signals of large 
intensity variation, that is, signals affording good temperature 
resolution characteristics is described in the above-identified 1983 
article published by Ray R. Dils. The 1983 Dils article is hereby 
incorporated by reference into the present specification. The Dils article 
also indicates that temperature measurement has been based on use of an 
optical fiber as a signal generator in addition to the herein described 
use of optical fiber as a signal transmission medium. The use of an 
optical fiber signal generator in the FIG. 1 apparatus is, of course, 
within the spirit of the present invention. 
By way of explanation, it should be noted that the black body signal source 
100 in FIG. 1 may be thought of in the nature of a transducer device, that 
is, a device which is heated in the work environ and as a result of this 
heating emits an intensity varying signal usable for measuring the work 
environ temperature. For the sake of terminology consistency however, the 
word "transducer" is herein, preferred for use in connection with the 
photodiode elements 140 and 142 in FIG. 1. 
The optical passband of the spectrally selective elements in the FIG. 1 
apparatus relates to a notable aspect of the present invention. The 
desired properties for the bandpass filter 126 and the optical fiber 
array, the principal other spectrally selective element in the FIG. 1 
apparatus, can be appreciated with the assistance of FIGS. 2, 3 and 4 in 
the drawings. The curves in FIG. 2, for example, show the relationship 
between spectral radiance or optical signal intensity and optical spectrum 
wavelength for a black body signal source operated at a series of 
different temperatures; the FIG. 2 temperatures 201, 203, etc. are 
measured in degrees Kelvin. FIG. 3 as indicated above shows the 
relationship between electrical output signal and optical input signal 
wavelength for the indium gallium arsenide transducer element preferred 
for use in the transducer element P1, 140 and P2, 142 in FIG. 1. The 
curves in FIG. 4 of the drawings show the relationship between optical 
signal wavelength and optical signal attenuation for one type of sapphire 
opticl fiber usable in the fiber bundle 114 in FIG. 1. The spectral scales 
200, 300 and 400 in each of FIGS. 2, 3, and 4 include a desirable portion 
of the optical spectrum for use in the FIG. 1 apparatus. 
The spectral radiance or signal output scale 202 in FIG. 2 extends 
logarithmically over a dynamic range of 1E12. Actually, however, some 
signal falling outside the range of FIG. 2 are of interest in the present 
invention, one representation of such signals is included in Table I at 
the end of this specification. The optical signal wavelength scale 200 in 
FIG. 2 shows portions of the near and far infrared spectrum also on a 
logarithmic scale. The black body temperatures in FIG. 2 extend between 
the 200.degree. K. indicated at 201 and the 6000.degree. K. indicated at 
203. An operating band of choice for the FIG. 1 instrument is shown by the 
dotted lines 204, 208, and 212 in FIG. 2. 
One aspect of the FIG. 2 curves concerns the change in optical signal 
dynamic range which results from operating a black body temperature 
transducer at different wavelengths along the scale 200--i.e., from 
limiting the spectral band of interest within the scale 200. At a 
wavelength of 50 micrometers, for example, the temperatures between 
200.degree. K. ajnd 6000.degree. K. involve a signal dynamic range of 1E2, 
while at a wavelength of 10 micrometers a signal dynamic range slightly 
under 1E4 is generated. At a wavelength of 3 micrometers moreover, a 
dynamic range in the neighborhood of 1E9 is required to cover the 
temperatures between 200.degree. and 6000.degree. K. 
In the past, wavelengths in the range of 0.6 to 0.8 micrometers have been 
employed for fiber optic probe temperature measuring instruments. At these 
wavelengths, as can be observed in FIG. 2, an optical signal dynamic range 
capability in the order of 10.sup.16 and 10.sup.13, respectively, is 
needed in order to measure temperatures between 300.degree. and 
2300.degree. K., the temperatures found in a possible turbulent combustion 
flame. A problem has therefore existed previously in that no known 
detector was possessed of such large dynamic range capability. In the 
dynamic range sense, therefore, the large output signal of a black body 
probe is desirable for achieving good temperature sensitivity and 
accuracy, but is somewhat troublesome when the need to cover wide 
temperature ranges is present. 
Optical multiplexing as represented by the beamsplitting member 130 in FIG. 
1 makes it feasible to overcome the dynamic range problem of a wide 
temperature range black body transducer signal through splitting the 
signal into multiple component signals each covering a 1E3-1E4 dynamic 
range. 
It can be therefore observed from FIG. 2 that because of dynamic range 
considerations as well as because of spectral attenuation characteristics 
discernible in FIG. 4 of the drawings a shift to somewhat longer 
wavelengths than the previous 0.6 to 0.8 micrometer wavelengths is 
desirable for a wide range fiber optic optical pyrometer apparatus. 
From the combination of optical fiber attenuation characteristics shown in 
FIG. 4, photodiode transducer characteristics shown in FIG. 3, and 
blackbody characteristics shown in FIG. 2, an instrument operating in the 
spectral wavelength region iof 1.6 micrometers can be observed to be 
desirable. At a 1.6 micrometer spectral wavelength, a dynamic range of 1E8 
will cover a temperature range on the order of 400.degree.-2300.degree. K. 
with some accuracy falloff below 500.degree. K. and will provide 
measurement feasibility for the variations observed in a turbulent flame 
combustion atmosphere. The dotted line 206 in FIG. 2 and the arrow 204, 
indicate the preferred 1.6 micrometer spectral operating wavelength in 
FIG. 2. The dotted lines 214 and 210, the arrows 212 and 208 show the 
extent of the signal dynamic range for a 1.6 micrometer spectral 
wavelength operating point in FIG. 2. 
Actual photon flux values in watts for a black body device operated at 
temperatures between 300.degree. K. and 2300.degree. K. are shown in Table 
1 at the end of this specification. As indicated by these photon flux 
values, a FIG. 1 instrument operated with a center optical spectrum 
wavelength of 1.6 micrometers involves a signal dynamic range of 1E11 to 
cover the temperature band of 300.degree. K. to 2300.degree. K. A large 
part of this dynamic range is, however, devoted to the low, or room 
temperature, end of this temperature band so that an instrument covering 
the temperature band of 500.degree. K. to 2300.degree. K. involves a 
signal dynamic range of 1E6. The herein provided dynamic range of 1E8 
therefore allows a lower temperature intermediate these 300.degree. K. and 
500.degree. K. values. An instrument operating in the 0.7 micrometer 
wavelength region in the visible area of the spectrum would require a 
dynamic range of 1E13 to cover the 300.degree.-2300.degree. K. temperature 
band; clearly the selected 1.6 micrometer operating frequency and 1E8 
dynamic range are preferable for an instrument of moderate sensitivity 
that is to be capable of measuring over a large range of temperatures. 
The scales 300 and 302 in FIG. 3 indicate value of spectral wavelength and 
relative signal amplitude, respectively, for a preferred indium gallium 
arsenide photodiode transducer device. The line 306 indicates the location 
of the preferred 1.6 micrometer spectral operating wavelength and the 
point 308 in FIG. 3 indicates location of this desired 1.6 micrometer 
wavelength on the response curve 304 of the preferred indium gallium 
arsenide photodiode transducer device. Other transducer devices having 
usable output signal magnitudes in the 1.6 micrometer wavelength region 
could, of course, be used in the FIG. 1 apparatus as an alternate to the 
preferred indium gallium arsenide photodiode transducer. 
The attenuation response of sapphire fiber elements usable in the high 
temperature opticl fiber bundle 114 in FIG. 1 is shown in FIG. 4 of the 
drawings. In FIG. 4, spectral wavelengths are indicated along the scale 
400 and attenuation values along the scale 402. In FIG. 4, the attenuation 
is shown to include two curves 404 and 406 lying on either side of an 
attenuation peak 405. The attenuation value in decibels is indicated along 
the scale 402 in FIG. 4 for a specified fiber length, near 1000 meters, of 
a specific sapphire fiber material. Fibropsil QSF200, which is 
manufactured by the Saphikon Division of Tyco Laboratories. The curve 406 
portion of FIG. 4 and especially the point 412 and the lines 408 and 410 
are of interest with respect to the operating point for the FIG. 1 
apparatus--these lines and points relate to the 1.6 micrometer preferred 
wavelength. As indicated by the attenuation values along the scale 402 in 
FIG. 4, the sapphire fiber bundle portion 114 of the signal transmitting 
optical fibers 102 in FIG. 1 contributes significantly to attenuation of 
the optical signal; indeed only the indicated desirable characteristics of 
the sapphire fibers make use of such fibers attractive in the FIG. 1 
apparatus. 
The curves in FIG. 4 also indicate that the sapphire fibers in the bundle 
114 contribute desirably to the optical wavelength selection achieved in 
the FIG. 1 apparatus. The curve portion 406 in FIG. 4, for example, 
indicates sharply increasing optical attenuation in the fiber bundle 114 
at wavelengths of 1.5 micrometers and below; this curve actually therefore 
indicates the sapphire bundle to be a wavelength filter which together 
with the bandpass filter 126 in FIG. 1 defines the operating spectral 
region for the FIG. 1 apparatus. The bandpass filter 126 can be of such 
characteristics as to attenuate optical wavelengths lying outside the 
sharp attenuations shown in FIG. 4, wavelengths below 1.5 micrometers and 
above 1.8 micrometers, thereby assuring that only a small spectral band 
round the 1.6 micrometers is active in the FIG. 1 apparatus. The bandpass 
filter 126 can also be used to accommodate the presence of undesirably 
short lengths in the fiber bundle 114--lengths which are too short to 
achieve the desired optical signal attenuation. 
FIG. 5 in the drawings shows a block diagram representation of an 
electronic system usable for processing the electrical signals generated 
by the photodiode transducer elements T1, 140 and T2, 142 in FIG. 1. These 
photo transducer elements are represented in electrical form at 504 and 
506 in FIG. 5, and are shown symbolically to receive photons of optical 
energy 500 and 502. The FIG. 5 electronic system includes a pair of 
electronic signal preamplifiers 508 and 510 which feed the amplifier, 
sample and hold, and analog-to-digital converter modules 509, 511. The 
preamplifiers 508 and 510 are preferably of the junction field effect 
transistor (JFET) type having an input JFET device operated in the 
trans-impedance circuit configuration with high input impedance and low 
output impedance. The modules 509 and 511 which include the amplifiers 512 
and 512, the sample and hold circuits 516 and 518, and the 
analog-to-digital converters 520 and 522 are available commercially as 
integrated circuit devices from suppliers such as Analog Devices, Inc. The 
modules 509 and 511 can, for example, be data acquisition modules 
identified as Analog Devices Incorporated part numbers DAS 1156. The 
sample and hold and analog-to-digital converter combination preferably 
provides 15 bit resolution with throughput rates in the range of 20 
kilohertz for the FIG. 1 apparatus. 
Digital signals provided by the analog-to-digital converters 520 and 522 
are time sampled by the multiplexer 524 in FIG. 5, and transmitted to a 
digital computer 528 by way of a data bus 526. The output of the digital 
computer 528 is transmitted by a second data bus 530 to a display 532 
which may include a cathode ray tube 534, or other display arrangements. 
The display 532 may also include a keyboard for entering operator 
instructions and display selection information. 
Since the two photodiode transducer elements T1, 140 and T2, 142 or their 
electrical representations at 504 and 506 in FIG. 5 are each responsive to 
only a predetermined intensity range of signal from the black body source 
100, only one of the electrical signals received from these photodiode 
transducer elements is of interest in making a given temperature 
measurement. A high temperature work environ measurement producing large 
intensity signals, for example, will provide a usable, linear operating 
range, signal from the high temperature photodiode transducer element T2, 
142 and also provide a larger but non-linear operating range signal from 
the low temperature photodiode transducer element T1, 140. 
Selection between the signal generated by the photodiode element 506 and 
the signal generated by the photodiode element 504 is accomplished in the 
computer 528 in FIG. 5 using a signal amplitude discriminating algorithm. 
The discriminating algorithm in the computer 528 should provide the 
following capabilities: 
Selection of the signal representing transducer operation within the 
transducer linear range 
Exclusion of transducer signals falling above a predetermined linear range 
of transducer operation 
Selection of either one or an average of both transducer signals for 
intermediate intensity signals 
Exclusion of a transducer signal residing within the transducer noise 
background level. 
A signal amplitude responsive algorithm is satisfactory for performing 
these selections. 
Implementation of a computer algorithm performing these functions is of 
course, dependent upon the type of computer used at 528 and the operating 
system and programming language employed with the computer, a program of 
this type is, in any event within the capability of persons skilled in the 
computer art. 
Dedicated hardware could, of course, be used to replace the computer 528 
and the display 532, such hardware would include amplitude discriminating 
circuitry, data storage devices, and a display in the form of a light 
emitting diode alphanumeric character matrix or a liquid crystal matrix of 
the type commonly used in electronic wristwatches. A cathode ray tube 
display device could also be employed with such a dedicated hardware 
system. A significant portion of such hardware could also be adapted from 
commercially available temperature indicating instruments which operate 
with a thermocouple or other sensing devices. 
The frequency domain response of the temperature measuring apparatus in 
FIG. 1 and FIG. 5 can extend into the kilohertz region and thereby enable 
use of the apparatus in the measurement of rapidly fluctuating or 
transient temperature conditions such as may exist in engine combustion, 
turbulent combustion, or sooty combustion environs. Thermal inertia in the 
black body signal source 100 and limited thermal conductivity in the 
optical fiber bundle 114 are the most limiting considerations in 
determining the frequency domain response of the FIG. 1 and FIG. 5 
apparatus. Through the use of a frequency rolloff compensation algorithm 
in the computer 528, these limitations can be accommodated to a useful 
degree and enable higher frequency temperature measurement capability than 
has heretofore been available. Sampling frequency rates up to 20 kilohertz 
are, for example, achievable with the FIG. 1 and FIG. 5 apparatus. Since 
the maximum frequency requirement for studying events in a turbulent flame 
has been found to lie in the region of 10 kilohertz, a 20 kilohertz 
response is more than adequate for such studies. Notwithstanding this 
range of overall system response, however, it should be realized that the 
naked frequency domain response of the black body signal source 100 can 
have a 250 millisecond time constant; the extension of responses in this 
range of time constants to the 20 kilohertz region is achieved with the 
use of frequency domain compensation in the computer 528. 
Heretofore, the available photon transducers for the preferred 1.5 to 1.8 
micrometers spectral wavelength region have been comprised of germanium, 
doped germanium, indium antimonide, and lead selinide. The addition of the 
preferred indium gallium arsenide transducer provides a noise equivalent 
power output from the transducer which is superior to previous detector 
types by more than an order of magnitude when measured in the non-cooled 
or 300.degree. K. temperature environment. The quantum efficiency of 
indium gallium arsenide is on the order of 60% at the preferred 1.6 
micrometer operating wavelength. The indium gallium arsenide photodiode 
transducer provides junction capacitances comparable to that of silicon 
PIN detectors and is therefore suitably low for high frequency domain 
temperature measurement operation. 
Calibration of the FIG. 1 and FIG. 5 apparatus to provide absolute 
temperature measurement may be achieved by calibrating the electro-optical 
response function with the aid of a black body radiation source. Single 
point calibration can be used to allow computation of the absolute 
temperature using a transfer function relationship since the emissivity of 
irridium or one of the alternate black body materials is substantially 
temperature independent, as can be seen from the explanation for photon 
flux vs. temperature below. The FIG. 1 and FIG. 5 apparatus is capable of 
temperature measurements with resolution and accuracy exceeding 0.5 
percent over the selected temperature range. 
The use of a black body transducer in order to obtain increased temperature 
measurement sensitivity is based on a convolution of the optical fiber 
bandpass characteristics with Planck's black body radiation equation, such 
a convolution enables the measurement of temperature from a measurement of 
photon flux arriving at a photodiode transducer element. The applicable 
Planck equation has the form of 
##EQU1## 
where a=area of the cavity exit (m.sup.2) 
.epsilon..sub.o =apparent emittance of the cavity 
c.sub.1 =first radiation constant (3.7418.times.10.sup.-16 W M.sup.2) 
c.sub.2 =second radiation constant (1.43879.times.10.sup.-2 m.K) 
.lambda..sub.o =wavelength in vacuum (m) 
T=temperature (K) 
.lambda..sub.o is herein selected to provide maximum temperature change 
sensitivity and low optical signal absorption in sapphire at elevated 
temperatures. 
While the apparatus and method herein described constitute a preferred 
embodiment of the invention, it is to be understood that the invention is 
not limited to this precise form of apparatus or method, and that changes 
may be made therein without departing from the scope of the invention, 
which is defined in the appended claims. 
TABLE I 
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Black Body Probe Intensity Variation as a Function of Temper- 
ature with a Central Wavelength of 1600 nm and Halfwidth of 
100 nm; W = Watts. 
T (.degree.K.) 
Intensity (W) T (.degree.K.) 
Intensity (W) 
______________________________________ 
300.0 0.32867E-10 350.0 0.19043E-08 
400.0 0.41009E-07 450.0 0.45345E-06 
500.0 0.31322E-05 550.0 0.15331E-04 
600.0 0.57866E-04 650.0 0.17868E-03 
700.0 0.47087E-03 750.0 0.10926E-02 
800.0 0.22857E-02 850.0 0.43890E-02 
900.0 0.78459E-02 950.0 0.13204E-01 
1000.0 0.21107E-01 1050.0 0.32282E-01 
1100.0 0.47525E-01 1150.0 0.67673E-01 
1200.0 0.93596E-01 1250.0 0.12617E+00 
1300.0 0.16624E+00 1350.0 0.21466E+00 
1400.0 0.27221E+00 1450.0 0.33963E+00 
1500.0 0.41759E+00 1550.0 0.50670E+00 
1600.0 0.60750E+00 1650.0 0.72045E+00 
1700.0 0.84593E+00 1750.0 0.98426E+00 
1800.0 0.11357E+01 1850.0 0.13004E+01 
1900.0 0.14784E+01 1950.0 0.16699E+01 
2000.0 0.18748E+01 2050.0 0.20931E+01 
2100.0 0.23247E+01 2150.0 0.25694E+01 
2200.0 0.26270E+01 2250.0 0.30973E+01 
2300.0 0.33802E+01 
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