Multiwavelength pyrometer for gray and non-gray surfaces in the presence of interfering radiation

A method and apparatus for detecting the temperature of gray and non-gray bodies in the presence of interfering radiation. A gray body has a constant emissivity less than 1 and a non-gray body has an emissivity which varies with wavelength. The emissivity and reflectivity of the surface is determined over a range of wavelengths. Spectra are also measured of the extraneous interference radiation source and the surface of the object to be measured in the presence of the extraneous interference radiation source. An auxiliary radiation source is used to determine the reflectivity of the surface and also the emissivity. The measured spectrum of the surfaces in the presence of the extraneous interference radiation source is set equal to the emissivity of the surface multiplied by a Planck function containing a temperature term T plus the surface reflectivity multiplied by the spectrum of the extraneous interference radiation source. The equation is then solved for T to determine the temperature of the surface.

TECHNICAL FIELD 
This invention generally relates to multiwave pyrometry for gray and 
non-gray surfaces. The invention particularly relates to multiwavelength 
pyrometry for gray and non-gray surfaces which takes into account 
extraneous interfering radiation to accurately determine the temperature 
of a surface. 
BACKGROUND ART 
In pyrometry, the temperature of a surface can be determined by measuring 
the radiation emanating from the surface. The emanating radiation will 
usually contain two components: the radiation emitted from the surface and 
radiation from other sources which is reflected off the surface. 
Kirchoff's law states that emissivity of a surface plus reflectivity of a 
surface is equal to 1. For a theoretical black body, the emissivity is 
equal to 1 and the reflectivity is equal to 0. A black body is a 
theoretical object, that emits energy with complete efficiency at all 
Wavelength, and absorbs all energy directed at it at all wavelengths and 
therefore has no reflectivity. 
A black body cannot exist in reality, because no surface radiates or 
absorbs all energy. A gray body is defined as an object which has a 
constant emissivity and therefore constant reflectivity at all 
wavelengths. As the emissivity of a gray body remains constant over 
various wavelengths, the emissivity of a gray body object is usually known 
or can be determined. From the emissivity, the temperature of the object 
can be determined by solving a Planck formula for temperature. 
A nongray body is a body which has its emissivity vary depending on the 
wavelength measured. Therefore, to accurately determine the temperature of 
a nongray body using conventional pyrometry techniques, the emissivity of 
the nongray body must be known at the wavelength at which the pyrometer is 
operating. 
A conventional single color pyrometer measures the radiation emitted from 
an object to be measured. As the radiation contains both an emission 
component from the object being measured and a reflection component 
containing radiation reflected off the object being measured, the 
emissivity of the measured object must be known to obtain an accurate 
temperature measurement. However, a single color pyrometer might be able 
to ignore the reflectivity of a surface to be measured if its reflectivity 
is small when compared to its emissivity. 
Two color pyrometers measured the emitted radiation at two wavelengths and 
assumes that the ratio of emissivity at the two wavelengths is known. 
Therefore, the temperature is determined without knowing the emissivity of 
the object to be measured. In general, as long as the emissivity of the 
object to be measured does not change rapidly with wavelength, a fairly 
accurate temperature measurement might be obtained using the two color 
method. 
However, both the one color and two color pyrometry methods are susceptible 
to errors caused by reflected radiation. Reflected radiation originates 
from sources other than the surface under consideration and reaches the 
pyrometer through reflection from the surface. Ceramics are a class of 
material which typically have low emissivity, and hence there is a high 
reflectivity in the band where conventional pyrometers operate. 
Consequently, a potential for large error exists when conventional 
pyrometry is employed to measure the temperature of ceramics. 
DISCLOSURE OF THE INVENTION 
It is therefore an object of this invention to provide a noncontact 
temperature measurement system which can accurately determine the 
temperature of a gray or nongray surfaces. 
It is another object of this invention to provide a noncontact temperature 
measurement system which accurately measures the temperature of a gray or 
nongray surface by taking into account the reflectivity of the surface and 
the component in the measured radiation which corresponds to energy from 
an extraneous interference source reflected off the surface. 
These and other objects are achieved according to the present invention by 
providing a method and apparatus for determining the temperature of a gray 
and nongray object by measuring the radiation emanating from the object 
which contains an emissive component and a reflective component due to an 
extraneous interference source in the environment of the object to be 
measured. 
The system for determining the surface temperature of the object to be 
measured contains a spectral radiometer connected to a computer. The 
spectral radiometer can have attached to its optical input a fiber optic 
cable which can withstand harsher environments than the spectral 
radiometer can withstand. The system also contains an auxiliary radiation 
source which can be an infrared source which is aimed at the surface and 
whose reflection is detected by the spectral radiometer so that the 
reflective properties of the surface to be measured can be taken into 
account. The measurement system will often include an extraneous 
interference source which cannot be removed from the environment which 
causes radiation to be reflected off the surface and detected by the 
spectral radiometer. 
The temperature of the surface is calculated by measuring four spectra each 
including a plurality of wavelengths which can be, for example, from 2.5 
.mu.m to 14.5 .mu.m, using the spectral radiometer. The four measured 
spectra are: 
(1) a spectrum, S.sub.O, the direct spectrum of the auxiliary radiation 
source; 
(2) a spectrum S.sub.I, the direct spectrum of the extraneous interference 
course; 
(3) a spectrum S.sub.II, of the surface in the presence of the extraneous 
interference source with the auxiliary radiation source turned off; and 
(4) a spectrum S.sub.III, which is the spectrum of the surface in the 
presence of the extraneous interference source with the auxiliary 
radiation source turned on. 
The spectrum S.sub.II which has been measured can be represented 
mathematically by the equation: 
##EQU1## 
with the known .lambda. corresponding to the wavelength, z(.lambda.) 
corresponding to the measured reflectivity which is the quantity S.sub.II 
(.lambda.)-S.sub.II (.lambda.) divided by S.sub.o (.lambda.). C.sub.1 and 
C.sub.2 are known constants and S.sub.I (.lambda.) has been measured. The 
unknown parameters are T, the temperature of the surface, and f and h 
which are parameters relating to the beam geometry of the system. 
The spectra are measured within a predetermined wavelength range and 
therefore the three unknowns T, f, and h can be determined using a 
least-squares curve fitting computer program on the system computer. 
Therefore, an accurate temperature of the surface can be obtained which 
takes into account the measured emissivity and reflectivity of the surface 
and the radiation from an extraneous interference source which is 
reflected off the surface.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, there is illustrated a pyrometer system 
constructed in accordance with the present invention. A spectral 
radiometer 8 measures various spectra at a plurality of wavelengths within 
a range through fiber optics 10. The fiber optics allows a spectral 
measurement to be performed without subjecting the spectral radiometer to 
a possibly harsh environment where the surface to be measured exists, 
although the fiber optics is not absolutely necessary. A commercially 
available spectral radiometer such as model SR-500 from CI Corporation can 
be used. The spectral radiometer is connected to a computer 4 through a 
bus 6. The computer 4 can be any computer capable of interfacing with the 
radiometer and can be an IBM.RTM. compatible personal computer, for 
example. The bus 6 can be implemented according to the IEEE 488 standard 
and contain a data bus and a control bus. 
The extraneous interference source 12 can be any radiation source found in 
the environment of the pyrometry system. A heat lamp used to raise the 
temperature of the surface might be an extraneous interference source. The 
extraneous interference source can be simulated in a laboratory by a 40 W, 
nominal-temperature 644.degree. K. soldering iron which has a temperature 
controller of unknown temporal stability, for example. It can be placed in 
front of the surface 2 in approximately the same angular relationship as 
an auxiliary radiation source 16 and should be well shielded from the 
direct view of the spectral radiometer. 
The auxiliary radiation source 16 can be a 20 W infrared lamp, for example. 
The lamp can be regulated, for example, by a constant current power supply 
such that the typical light output rms ripple of the auxiliary radiation 
source is 0.05%. The auxiliary radiation source can be Model 6575 from the 
Oriel Corporation, for example. In the pyrometry system, spectral 
measurements must be taken both with and without the auxiliary radiation 
source so there should be a convenient way to turn the auxiliary radiation 
source on and off. The auxiliary radiation source 16 can manually be 
turned on and off or can be turned on and off automatically using a 
chopper 14. 
Under certain conditions such as higher temperatures, the surface emission 
will greatly overwhelm the radiation reflected from the surface from the 
20 W auxiliary radiation source. A reflected spectrum may not be 
accurately calculated by subtracting two spectra of a surface measured 
with and without the radiation source. However, the difference in spectra 
can be readily measured by chopping the auxiliary radiation source with a 
chopper 14 or using phase detection schemes using a lock-in amplifier. As 
most spectral radiometers have an external signal chopping capability, 
subtraction of the two spectra can be automatically performed by the 
spectral radiometer in a single operation. The chopper 14 accomplishes 
this function by selectively allowing the radiation from the auxiliary 
radiation source to pass. The chopping can be performed in a conventional 
manner by a mechanical device which periodically blocks the auxiliary 
radiation source. The on-off signal of the chopper is communicated to the 
phase detecting lock-in amplifier (internal or external to the 
spectrometer) to produce via a lead 9 a spectrum which is the difference 
of the signals when the auxiliary beam is on and when the auxiliary beam 
is not irradiating on the surface. 
The spectral radiometer 8 can produce a spectrum either in voltage or in 
radiation energy units. When the spectrum produced is in volts, the 
voltage signal is a function of the wavelength of the received radiation, 
the intensity of the radiation, and the spectral radiometer responsivity. 
An obtained voltage spectrum can be converted into a radiation energy 
spectrum in radiation energy units by dividing the voltage spectrum by the 
radiometer's response function which can be obtained during routine 
calibration operations. 
To obtain the temperature of the surface 2 illustrated in FIG. 1, four 
spectra are measured by the pyrometry system. The spectra are measured at 
a plurality of wavelengths within a range of wavelengths from 2.5 .mu.m to 
14.5 .mu.m, for example. The first spectrum required is the spectrum of 
the auxiliary radiation source, S.sub.O (.lambda.), and can be obtained 
prior to measurement of the surface spectra. The spectrum S.sub.O 
(.lambda.) is obtained by directing the input of the radiometer directly 
at the auxiliary radiation source. 
The second measured spectrum, S.sub.I (.lambda.), is the spectrum of the 
extraneous interference source. The spectrum of the extraneous 
interference source is obtained by directing the input of the spectral 
radiometer directly at the extraneous interference source. 
The next spectrum measured is S.sub.II (.lambda.) which is the spectrum of 
the surface to be measured when the interfering radiation source is 
present. The spectrum S.sub.II (.lambda.) contains two components; 
S.sub.II.sup.1 (.lambda.) which is the spectrum of the surface emitted 
components; and S.sub.II.sup.2 (.lambda.) which is the spectrum of the 
surface reflected radiation and includes the extraneous interference 
radiation source reflecting off of the surface. The auxiliary radiation 
source is turned off during the measurement of S.sub.II (.lambda.). 
The last measured spectrum, S.sub.III (.lambda.), is the radiation spectrum 
of the surface in the presence of the extraneous interference radiation 
source and the auxiliary radiation source. 
Once the spectra S.sub.O (.lambda.), S.sub.I (.lambda.), S.sub.II 
(.lambda.), and S.sub.III (.lambda.) are obtained, calculations can be 
performed using the computer attached to the spectral radiometer to 
determine the temperature of the surface. While the spectra are in a 
digital representation in the computer, the numbers which are in the 
computer are representative of the radiation spectra which are physical 
signals representing physical characteristics of the surface to be 
measured. 
The spectrum S.sub.II (.lambda.) has been determined through measurement 
and can be mathematically expressed using an equation which contains the 
parameter T, corresponding to the temperature of the surface. By choosing 
a value T=T.sub.e in the equation representing the spectrum S.sub.II 
(.lambda.) to yield calculated values equal to the measured value of the 
spectrum S.sub.II (.lambda.), the unknown temperature T of the surface can 
be determined. 
A mathematical expression for the spectrum S.sub.I (.lambda.) is written as 
the sum of two terms: 
EQU S.sub.II (.lambda.)=S.sub.II.sup.1 (.lambda.)+S.sub.II.sup.2 (.lambda.)(1) 
The component S.sub.II.sup.1 (.lambda.) describes the spectrum of the 
surface emission radiation. It is a Planck function modified by the 
wavelength-dependent emissivity of the non-gray or gray surface and is 
known to be: 
##EQU2## 
In equation 2, e(.lambda.) is the emissivity of the surface, C.sub.1, 
C.sub.2 are radiation constants having the values 
3.741832.times.10.sup.-16 W.multidot.m.sup.2 and 1.43879.times.10.sup.-2 
m.multidot.K respectively, .lambda. is the wavelength, and T is the 
temperature of the surface C.sub.1 is 2.pi.c.sup.2 h and C.sub.2 is ch/k 
where c=3.times.10.sup.8 m/sec is the velocity of light, 
h=6.6.times.10.sup.-34 J-sec is the Planck constant, 
k=1.38.times.10.sup.-23 J/K is Boltzman's constant. 
EQU Kirchhoff's law states: emissivity+reflectivity=1 (3) 
As emissivity is a function of wavelength .lambda., .beta.and .theta. which 
refer to the polar angular coordinate angles specifying a direction with 
reference to a suitably chosen coordinate system, and T.sub.e, the 
temperature of the surface, Kirchhoff's law can be written as: 
EQU e(.lambda.,.beta.,.theta.,T.sub.e)+r(.lambda.,.beta..theta.,T.sub.e)=1(5) 
Solving Kirchhoff's law for emissivity gives: 
EQU e(.lambda.,.beta.,.theta.,T.sub.e)=1-r(.lambda.,.beta..theta.,T.sub.e)(4) 
In pyrometry applications, the angular dependence disappears because the 
optics of a pyrometers detector selects signals in a narrowly defined 
direction. As a result, only the wavelength dependence needs to be 
considered at a particular temperature. The emissivity of the surface is 
therefore given by: 
EQU e(.lambda.)=1-r(.lambda.) (6) 
For the pyrometer system of the present invention, the measured 
reflectivity, z(.lambda.), at any wavelength is the ratio: 
##EQU3## 
where S.sub.IV (.lambda.) is the reflected radiation due to the auxiliary 
radiation source and S.sub.O (.lambda.) is the unobstructed spectrum of 
the auxiliary radiation source. S.sub.IV (.lambda.) is equal to the 
spectrum S.sub.III (.lambda.)-S.sub.II (.lambda.) Which substituted into 
equation 7 yields: 
##EQU4## 
As all terms of equation 8 have been measured, the value z(.lambda.) can 
be calculated using the computer 4. 
Because S.sub.O (.lambda.) was measured on a direct path; that is it was 
not reflected off of the surface, whereas S.sub.III (.lambda.) and 
S.sub.II (.lambda.) were measured on a reflected path off of the surface, 
the ratio z(.lambda.) does not represent the true reflectivity but is the 
true reflectivity modified by a constant f. Therefore, the true 
reflectivity r can be expressed as: 
##EQU5## 
where f is an unknown and must be determined. In principal, the constant f 
can be determined from the geometry of the experimental set up. However, 
it can more easily be determined by conventional curve fitting as will be 
explained later, and for out of laboratory situations, the constant f may 
not be able to be determined from the geometry of the set up. 
Substituting the value of r(.lambda.) of equation 9 into equation 6 gives 
the emissivity as: 
##EQU6## 
Substituting the value of the emissivity from equation 10 into equation 2 
results in: 
##EQU7## 
S.sub.II.sup.2 (.lambda.) describes the reflected interfering radiation 
from the extraneous interference radiation source. S.sub.II.sup.2 
(.lambda.) is proportional to the product of the surface reflectivity 
r(.lambda.), S.sub.I (.lambda.) which is the spectrum of the extraneous 
interference radiation source, and a constant of proportionality g due to 
beam geometry. Even though g and f are both constants which result because 
of beam geometry, g is a different constant than f. 
Therefore, S.sub.II.sup.2 (.lambda.) can be represented as: 
EQU S.sub.II.sup.2 (.lambda.)+gr(.lambda.)S.sub.I (.lambda.) (12) 
Substituting for the reflectivity from equation 9 gives: 
##EQU8## 
If the ratio g/f of equation 13 is replaced by: 
##EQU9## 
where h is a constant, the equation: 
EQU S.sub.II.sup.2 (.lambda.)=hz(.lambda.)S.sub.I (.lambda.) (15) 
is obtained. In equation 15, z(.lambda.) is obtained by calculating 
equation 8. 
Substituting S.sub.II.sup.1 (.lambda.) from equation II and S.sub.II.sup.2 
(.lambda.) from equation 15 into equation 1 gives: 
##EQU10## 
where C.sub.1 and C.sub.2 are first and second radiation constants, 
.lambda. is the wavelength, z(.lambda.) is the measured reflectivity and 
calculated according to equation 8 and S.sub.I (.lambda.) is the measured 
direct spectrum of the extraneous interference source. The adjustable 
parameters f, h (or g), and T, the temperature are unknowns. The constants 
f and h are included in equation 16 to account for beam geometry. These 
constants may be equal to 1 under certain conditions. However, if they are 
considered to be 1, inaccurate temperature determinations will probably 
result. 
Equation 16 for the spectrum S.sub.II (.lambda.) contains three unknowns, 
one of them being temperature. Accordingly, the equation for the spectrum 
S.sub.II (.lambda.) cannot be algebraically solved for temperature but 
must be solved in some other way. One way to solve for temperature is 
using least-squares curve fitting with .lambda., z(.lambda.) and S.sub.I 
(.lambda.) treated as independent variables and solving for the parameters 
f, h and T. Even though there are three unknowns and one equation, the 
spectra have been measured over a spectral range, e.g. at a plurality of 
wavelengths, and therefore, it is possible to accurately determine the 
three unknowns. The computer 4 can quickly determine the temperature T and 
f and h using a commercially available least-squares software package. 
Examples of commercially available software which can perform the least 
squares function are RSI by BBN Software Product Corporation of 
Massachusetts, or SAS by the SAS Institute, Inc. of Cary, N.C. 
Each of the spectrum is measured at a plurality of wavelengths within a 
wavelength range of 2.5 .mu.m to 14 .mu.m, for example. The least-squares 
method for solving equation 16 for the unknowns operates as follows. A 
guess is made for the value of the temperature T and the other unknowns f 
and h. At each wavelength, the spectrum is calculated using these guesses 
in equation 16. Because these values are only guesses, there will probably 
be a difference between the value of the left side of equation 16 and the 
value of the right side of equation 16. The squares of these differences 
at each wavelength are added together. Different guesses are then made for 
each of the three unknown parameters and substituted into equation 16 and 
compared with the experimentally determined data. The difference between 
the experimentally measured spectrum and the calculated spectrum are 
subtracted for each wavelength and these differences are squared and added 
together. The combination of T,f, and h that gives the least squares is 
the correct combination. The least-squares computer program can perform 
many guesses to get accurate values for the parameters T, f and h which 
therefore gives an accurate determination of the temperature of the 
surface. 
After the spectra are measured over a range of wavelengths, not only is the 
temperature T determined but the parameters f and h are also determined. 
If the geometry of the system is not changed, the parameters f and h will 
not change and therefore, subsequent temperature measurements with the 
same geometry do not have to be performed over a range of wavelengths and 
the least squares curve fitting program does not have to be used. The 
spectra S.sub.II and S.sub.III must be measured again but the measurement 
needs only be performed at one wavelength. This wavelength should be at a 
location where the fitted curved determined by least-squares fitting is 
close to the measured curve and therefore has a small error. It is not 
necessary to again determine the spectrum S.sub.O of the auxiliary 
radiation source as this spectrum should remain constant. If the spectrum 
S.sub.I of the extraneous interference source changes over time, the 
spectrum of the extraneous interference source should be measured again. 
If there is little possibility of the spectrum of the extraneous 
interference source changing, it may not be necessary to again measure the 
spectrum of the extraneous interference source. Therefore, once the four 
spectra are measured over a wavelength range and curve fitting has been 
performed to determine the parameters f and h, subsequent temperature 
measurements which use the same system geometry can be very quickly made 
as only two or three spectra need to be measured at one wavelength with no 
need to perform subsequent curve-fitting analysis. 
An experiment was performed to examine the accuracy of the invention. A 
non-gray silicon carbide (SiC) wafer sample measuring 3 by 25 by 50 mm 
nominally polished to a 10 .mu.m finish was used as surface 2. The SiC 
surface was heated and allowed to equilibrate in front of a black body 
furnace. The SiC sample completely covered the black body cavity opening 
of the furnace. The black body furnace temperature was regulated by a 
temperature controller within plus or minus 0.5.degree. C. The auxiliary 
radiation source was regulated by a constant-current power supply such 
that its typical light output RMS ripple was 0.05%. The spectrum, 
illustrated in FIGS. 2-5 were then measured at a plurality of wavelengths 
within a range from 2.5 .mu.m to 14.5 .mu.m and contain 323 measurements 
or channels. Between 2.5 .mu.m and 4.39 .mu.m, the channel spacing was 
0.018 .mu.m, between 4.39 and 7.985 .mu.m, the channel spacing was 0.034 
.mu.m, and between 7.985 and 14.5 .mu.m, the channel spacing was 0.06 
.mu.m. The first measured spectrum was the spectrum of the auxiliary 
radiation source, S.sub.O (.lambda.). In the experiment, the auxiliary 
radiation source was a 20 watt infrared lamp regulated by constant current 
supply. The spectrum produced is illustrated in FIG. 2 in volts and was 
directly determined by the spectral radiometer. 
The second measured spectrum, S.sub.I of the extraneous interference source 
is illustrated in FIG. 3 showing the relative intensity at different 
wavelengths in energy units. The relative intensity in arbitrary energy 
units is obtained by dividing the spectrum of the extraneous interference 
source in volts by the response function of the radiometer which is easily 
determined through calibration of the radiometer. 
The third measured spectrum is the spectrum of the SiC surface in the 
presence of the extraneous interference radiation source with the 
auxiliary radiation source turned off. The spectrum is illustrated in FIG. 
4 in volts and in two components, the emitted component and the reflected 
component corresponding to S.sub.II.sup.1 and S.sub.II.sup.2 respectively. 
FIG. 5 also contains the spectrum of S.sub.II.sup.1 by itself. This 
spectrum is not used for any measurements but has been illustrated to show 
the effect of the extraneous interference source on the spectrum of the 
surface. FIG. 5 illustrates that at short wavelengths, the interference 
component of spectrum S.sub.II is over 50% of the spectrum; at other 
wavelengths, the effect is not as great. However, FIG. 5 illustrates the 
importance of taking into account the effect on the spectrum due to the 
extraneous interference source. 
The last measured spectrum S.sub.III, represents the spectrum of the 
surface with the auxiliary radiation source turned on in the presence of 
the extraneous interfering source. FIG. 4 illustrates spectrum S.sub.III 
in volts and FIG. 5 illustrates the spectrum S.sub.III in energy units. 
Equation 6 has two terms, z(.lambda.) and S.sub.I (.lambda.) which relate 
to the measured spectra. As z(.lambda.) is a unitless number between 0 and 
1 corresponding to the reflectivity of the surface, there is no need to 
convert the spectra used to define z(.lambda.) from volts to energy units. 
This is because z(.lambda.) is the ratio of two spectra and the units 
divide out of the reflectivity term. However, the spectrum S.sub.I 
(.lambda.) of equation 16 must be in the proper units and therefore the 
spectrum S.sub.I (.lambda.) must be divided by the response of the 
spectral radiometer so that the spectrum S.sub.I (.lambda.) has the proper 
energy units. 
FIG. 6 illustrates the spectrum S.sub.IV and corresponds to the difference 
of the spectra S.sub.III and the spectra S.sub.II. The spectrum S.sub.IV 
of FIG. 6 can be obtained by mathematically subtracting the spectrum 
S.sub.III from S.sub.II or can be directly determined by the spectral 
radiometer when the radiometer has an input in which the auxiliary 
radiation source is chopped. 
FIG. 7 illustrates the measured reflectivity, z(.lambda.) which corresponds 
to the quantity S.sub.III (.lambda.)-S.sub.II (.lambda.) divided by the 
spectrum S.sub.O (.lambda.), as expressed in equation 8. 
From the measured and calculated spectra, the curve fitting program yielded 
the surface temperature of 341.3.degree. K., f=0.375 and h=3.49. The 
fitted curve, corresponding to equation 16 is illustrated in FIG. 8 
together with the measured surface spectrum emission and the 341.3.degree. 
K. Planck curve. The 341.3.degree. K. Planck curve is the theoretical 
spectrum of a black surface at 341.3.degree. K. without taking into 
account the radiation from the extraneous interference source which has 
been reflected off the surface. 
The actual temperature of the surface was determined to be 339.7.degree. K. 
using a 0.125 mm (5 mil) type K (Chromel-Alumel) thermocouple which is in 
agreement with the multiwavelength pyrometry measurement of 341.3.degree. 
K. to within 1%. Analysis was also performed without taking into account 
the extraneous interference radiation source and the temperature was 
determined to be 373.4.degree. K. Therefore, neglecting the effect of the 
interference would introduce an error of about 10% as compared with an 
error of about 1% when the extraneous interference radiation source was 
taken into account. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.