Note: Descriptions are shown in the official language in which they were submitted.
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Description
A Dual Spectra Optical Pyrometer
Having A Serial Array Of Photodetectors
Technical Field
This invention relates to optical pyrometers
and more particularly -to dual spectra optical
pyrometers having a serial array of photodetectors.
Background Art
Optical pyrometers are well known and have
found widespread use in aircraft engine applica-
tions. Dual spectra optical pyrometers have been
used to measure the temperature of turbine blades in
an operating jet engine. By utilizing two spectral
bands the pyrometer can provide the needed dis-
crimination between the optical energy emitted by
the blade and the reflected energy of the combustion
flame.
In the dual spectra optical pyrometer dis-
closed by Gebhart, et al in U.S. Patent 4,222,663
light from a turbine blade is provided to two
pyrometers having different spectral bands whose
outputs are subqe~uently processed to provide an
estimate of the magnitude of the reflected energy.
Typically, silicon photodetectors are used for both
pyrometers and therefore possess the same intrinsic
spectral band. An optical filter whose~passband is
a portion of silicon is positioned in the optical
path of one of the beams to generate a spectral band
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difference between the two pyrometers. In dual
spectra optical pyrometers of the prior art the
spectral bands of the first unfiltered pyrometer
comprises 0.4 to 1.1 microns and comprises 0.4 to
0.85 microns for the second, filtered pyrometer.
The use of two detectors made from similar
spectral band materials also requires additional
optical components to split the received light and
guide the two beams. These additional optical
components must be precisely aligned and remain free
of contamination in the severe environment of a jet
engine. Moreover, the reflection losses from these
optical components further attenuate the weak, short
wavelength light. The limited optical energy
lS available at the short wavelengths results in a poor
signal-to-noise ratio in the electrical signals
provided by the photodetectors. This problem is
exacerbated by the total overlap in spectral bands
which occurs when silicon is used eor both
photodetectors.
~isclosure of Invention
An object of the present invention is to
provide a serial array detector module for use in a
dual spectra optical pyrometer. Another object of
the present invention is to provide a dual spectra
optical pyrometer characterized by a serial array
detector module.
According to the present invention, a serial
array detector module for receiving from a remote
target an optical beam having a spectral width
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comprises a housing that is adapted to receive the
target optical beam, a first detector contained
within the housing that absorbs from the target beam
a first optical component having a first spectral
width selected to be a portion of the target beam
spectral width and providing an electrical signal
equivalent thereof. A remainder optical beam passes
through a first detector to a second detector and
also contained within the housing which provides an
electrical signal equivalent of the remainder
optical beam.
According to another aspect of the present
invention, a dual spectra optical pyrometer having a
serial array detector module used to measure the
temperature of a remote target comprises an optical
guide for receiving from the target an optical heam
that has a spectral width and has an emitted
component from the target and a reflected component
from a fireball having an estimated equivalent blac~
body temperature. ~ housing is adapted to receive
the optical guide. The dual spectra op-tical
pyrometer is characterized by a first detector con-
tained within the housing Eor receiving the target
optical beam and absorbing therefrom a flrst optical
component having a spectral width selected to be a
portion of the target beam spectral width and
passing therethrough an optical beam remainder. The
first detector provides an electrical signal equiv-
alent of the first optical component. ~ second
detector is also contained within the housing and
receives the optical beam remainder and provides a
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second electrical signal equivalent thereof. Also
included in the pyrometer is a signal processor
which receives the first and second detector signals
and also receives signals indica-tive of the first
component spectral width and an estimate of the
equivalent black body fireball temperature. The
signal processor generates a temperature signal
indicative of an equivalent black body temperature
from the first electrical signal and generates from
the second detector signal a temperature qignal
indicative of an equivalent black body temperature.
In addition, the signal processor provides a
temperature correction signal from the difference of
the first and the second temperature signals in
dependence on signals indicative of the estimated
fireball equivalent black body temperature and first
component spectral width. The signal processor
provides a compensated temperature signal from the
difference between the first temperature signal and
the temperature correction signal.
Brief Description of the Drawings
Figure 1 is an illustration of a simplified
block diagram of a dual ~pectra optical pyrometer
having a serial array detector module provided
according to the present invention,
Figure 2 is a sectioned illustration of a
serial array detector module for use in the optical
pyrometer of Figure l; and
Figure 3 is a drawing illustrating the
responsivity characteristics of the photodetector
materials used in the detector module of Figure 2.
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Best Mode for Carrying Out the Invention
Referring first to Figure 1, in an illustra-
tion of a simplified block diagram of a dual spectra
pyrometer having a serial array detector module
provided according to the present invention, a dual
spectra pyrometer 10 includes probe 12 mounted in
casing 14 of a jet engine. The probe should be
positioned to optically view a target such as
rotating turbine blades 16 and 18.
In an operating jet engine the blades of the
turbine reach an elevated temperature. ~s such they
emit radiation, the spectral distribution of which
is a function of temperature and can usually be
approximated by the well known "black body"
approximation or "grey body" approximation if
emissivity correction is included. In addition,
light from the jet engine combustion Elame or
fireball is reflected off the turbine blade and also
comprises part of the target optical beam. The
temperature of the fireball is substantially higher
than that of the turbine blade, and as a result, the
sum of the two beams produces an equivalent black
body spectral energy distribution which yields a
temperature much higher than the actual -temperature
2S of the turbine blade.
The radiation from the turbine blades
comprises a target optical beam having a spectral
width which is collected by the probe. The probe
may include lenses and other conventional optical
components. In other aspects, the probe is of a
conventional design and includes such elements as a
housing for the optical fiber, and provisions for
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purge gas flow through the probe housing. The
components of the probe described hereinabove are
used in a typical diagnostic pyrometer. Those
skilled in the art will recognize that substitutions
and modifications can be made depending upon the
pyrometer' 5 use as a diagnostic or in-flight
pyrometer, and depending on each engine type.
The target optical beam is received by an
optical guide 20 secured inside the probe by
conventional techniques. The guide is conventional
and typically comprises a fused fiber optic bundle
or conventional wide band quartz or fused silica
type, suc~ as an Ensign Bickford Optics HC-414-lu
fiber. As detailed hereinaEter with respect to
Figure 2 the target optical beam is provided to
detection module 22. In the best mode embodiment
the detection module comprises a irst photodetector
absorbing a Eirst component of the target beam
having a spectral width selected to be a portion of
the target optical beam and transmitting there-
through the remainder of the target optical beam.
Also included is a second photodetector seria]ly
positioned thereafter, absorbing the remainder
optical beam. The serial positioning of the second
photodetector relative to the first produces
spectral band division between the two as the first
photodetector filters the radiation provided to the
second photodetector. With a detection module
provided according to the present invention,
coupling efficiencies into the two photodetectors
are almost 100% of the entire respective spectra]
ranges.
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The first photodetector provides signals on
lines 24 which are indicative of the received energy
of first component beam and comprises a first signal
channel whose spectral band corresponds to that of
first component beam. Similarly, the second
photodetector comprises a second signal channel
whose spectral band is limited to that of the
remainder optical beam and provides signals on lines
26 indicative thereof.
These signals are received by signal processor
28 which is a type known in the art and in the best
mode embodiment includes appropriate conventional
analog electrical circuits. In addition, the signal
processor receives signals on lines 30 and lines 32
from external processing apparatus 34 not shown and
not a part of the present invention that are
indicative of the spectral range of the Eirst
photodetector signals and an estimate of equivalent
black body temperature of the Eireball or combustion
flame.
The signals from the first photodetector are
processed to provide a signal inAicative of an
equivalent black body temperature or the first
signal channel. The signal processor performs the
same function for the second photodetector signals,
yielding signals indicative of an equivalent black
body temperature for the second signal channel.
The signal processor computes temperature
correction signals (Tc) and provides compensated
temperature signals (Tt) on lines 36 to external
signal processor 38 by relating the temperature
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correction signals to the second temperature signals
such that
Tt = Tu ~ Tc (1)
In order to accurately compute temperature
correction signals, the signal processor must
receive (1) the first and second temperature
signals, ~2) signals indicative of -the spectral
width of the first signal channel, and (3) signals
indicative of an estimate of the equivalent black
body temperature of the fireball. In addition,
temperature correction signals are a functlon of the
percent of reflected energy in the target optical
beam, a parameter often expressed as the difference
between the first temperature (Tl) and the second
temperature (T2) or (Tl - T2).
As is well known in the art, the percent
reflected energy in the target beam can vary from 0
to 50% before rendering the pyrometer output signals
unacceptable, given only an estimate of the equiva- -
lent black body temperature of the combustion flame.A~ a result, there is a funct.Lonal relationship
between the magnitude of the temperature correction
(Tc) signal and the percent of reflected energy
(Tl - T2), for a given second photodetector
temperature signal magn.itude.
With only an estimated Eireball equivalent
black body temperature, the error in the temperature
correction signal is small when the percent
reflected energy is less than 50%. For example,
given an estimated fireball temperature of 4500F,
and a 50~ reflected energy component, the magnitude
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of error is less than approYimately 30F, even
though the estimated fireball temperature can be off
by 300F. Continuing wi-th the example, if the
percent reflected energy is less than 50%, the
family of curves (Tc v. (Tl - T2)) resulting from
multiple values of second photodetector temperature
signals can be approximated by a single equation
using conventional curve fitting techniques,
yielding
Tt = T2 ~ [(-3T2 -150/(3500 -TU~(T2 -Tl) (2)
where, Tt is the compensated temperature, and Tl, T2
are as indicated hereinabove.
Those skilled in the art will note that other
empirically derived equations can be obtained for
other fireball equivalent black body temperatures.
~ oreover, it is also apparent to those skilled
in the art that alternative algorithms employing
either analog or digital means can be substituted.
Specifically, a digital embodiment of the signal
processor includes a high speed computer with
conventional computer-memory and analog-to-digital
convention, which generates and stores in memory
temperature correction signals similar to those
described hereinabove in conventional lookup table
format, with compensated temperature signals
obtainable therefrom by conventional techniques.
Figure 2 is a sectioned illustration of a
serial array photodetector module for use wi-th the
dual spectra optical pyrometer of Figure 1. The
serial array photodetector module 40 includes
housing 42 which is of a conventional type such as a
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TO-5 or TO-8 including meta:L header 46 and case 44
that have been appropriately modified. The housing
also includes waveguide connector 48 which is
attached by conventional techniques to the case. In
the best mode embodiment the case has been modified
by drilling a hole through the center so that the
optical fiber (20, Fig. 1) is attached to the
waveguide connector and can pass therethrough,
defining optic axis 50.
Positioned on the header is a serial
detector array comprising photodetectors 52 and 54.
Photodetector 54 is mounted by conventional
techniques to the header. 'rO enhance long wavelength
responsivity, a metal layer 55 is fabricated
underneath photodetector 54. Electrodes 56 and 58
are only partially shown and provide electrical
contact with photodetector 54 mounted a]ong the optic
axis. In addition, the case has provisi,on for
insulated standoff electrodes 60 and 62 which pass
through -the header allowi,ng ce~ramic washer 64 to be
positioned coaxially with thc opt,ic axis, immediately
above photodetector 54. Irhe ceramic washer is of a
conventional type and has an annular openlng.
Photodetector 52 ,is mounted on ~he ceramic washer
directl,y over the opening by conventional techniques
approxima-tely parallel to photodetector 54.
Electrlcal con-tact is provided to photode-tector 52
through electrodes 60 and 62 by conventional
techniques, including in the best mode embodiment,
metallic contacts fabricated on the top surface of
the ceramic washer.
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The metal case is slid over the header posi-
tioning the optical fiber in substantial contact
with photodetector 52 such that the target optical
beam exits the optical fiber, and is provided
thereto absorbing from the target optical beam a
Eirst component thereof. The remainder of the
target optical beam passes through photodetector 52
and is provided -to photodetector 54. A conventional
encapsulant 66 such as a silicone epoxy is used in
the gap between -the header and case and the optical
fiber thereby sending out moisture and other contam-
inants. In the best mode embodiment photodetector
52 comprises silicon, while photodetector 54
comprises indium gallium arsenide.
Figure 3 is a drawing illustrating the
responsivity characteristics of silicon and indium
gallium arsenide which are used as photodetector
materials in the best mode embodiment of the
detector module o Figure 2. Axes 67 and 68
correspond to logarithmic output current per
incident power versus wavelength respectively.
~ ual spectra optical pyrometers of the prior
art usually have photodetectors comprised oE
silicon. Silicon photodetectors are reliable as
well as inexpensive, and display little drift in
performance as a function of temperature.
Overlap in the spectral bands which results by
using the same material for both photodetectors
creates inherent performance limitations. The
signal-to-noise ratio is limited because most of the
energy in the target optical beam is at wavelengths
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longer than those absorbed by silicon. Moreover,
the component of the target beam reflected off of
the turbine blade has a very weak intensity; the
overlap between spectral bands further reduces
available signal-to-noise ratios.
The serial array photodetector module provided
according to the present inve~tion not only has the
advantages of mechanical stability and fewer optical
components, but the negligible overlap between the
responsivity of the photodetector materials ensures
inherently greater signal-to-noise ratios.
In the best mode embodiment, the first photo-
detector in the serial array comprises silicon. As
indicated hereinabove with respect to Figure 2, when
~he target optical beam is incident on -the first
photodetector (52, Fig. 2), the silicon will
substantially absorb that portion thereof having
wavelengths between 0.4 and 1.05 microns as indi-
cated by curve 70, intercept points 72 and 74
corresponding to 0.4 and 1.05 microns respectively.
The remainder of the target optical beam will pass
through the silicon and be provided to the second
photodetector (54, Fig. 2) which is responsive to
light having wavelengths between 1.05 and 1.8
microns shown by curve 76, with in-tercept points 78
and 80 corresponding to 0.8 and 1.8 micro~s respec-
tively. In the best mode embodiment photodetector
54 comprises an indium gallium arsenide photodiode
which is selected because of its responsivity and
high frequency response plus a relatively low dar~
current and noise at moderate ambient temperatures.
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Those skilled in the art will note that other
infrared detecting materials, such as germanium, may
be substituted.
In addition, the serial array photodetecting
module provided hereinabove has improved high
frequency response because it po~sesses efficient
radiant power signal transfer between the optical
fiber and the photodetectors. Photodetector 52
receives a total radiant optical power exiting the
waveguide except for a small coupling loss at the
optical fiber detector interface. Photodetector 54
receives approximately all of the incident power
from approximately 1.05 microns to the upper limit
of its responsivity range, approximately 1.8 microns
in the best mode embodiment. This includes over 75
of the highest output portion of its response range,
enhancing the pyrometer's overall signal-to-noise
performance.
Similarly, although the invention has been
shown and described with respect to a best mode
embodiment thereof, it should be understood by those
skilled in the art that various other changes,
omissions and additions thereto may be made therein
without departing from the spirit and scope of the
invention.
