Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
2002-06-04
2004-05-18
Shah, Kamini (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C374S121000, C374S127000
Reexamination Certificate
active
06738724
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to thermal radiation analysis. More specifically this invention relates to radiometric determination of temperature, emissivity, and stray light.
2. General Background and Description of Related Art
Optical pyrometers are of three general types: brightness, ratio, or multiwavelength (i.e. MW). Brightness and ratio pyrometers require prior knowledge of surface emissivity and environmental interference. In addition to wavelength, emissivity, which is the ratio of the emitted radiation of a real radiator to that of an ideal one, can depend on composition, surface finish, and temperature. Environmental interference in the form of radiation absorption or scattering within the transmission medium can also be a problem.
Brightness devices rely on capturing a known fraction of the energy emitted by the target; the user must know the emissivity to get the correct temperature value. For many circumstances this may not be possible.
Ratio pyrometry attempts to circumvent the emissivity issue by utilizing the ratio of the intensities measured at two different wavelengths instead of the intensity magnitude. The resulting representative equation is solved for temperature with the assumption that the division has canceled out the emissivity. This method works if the emissivity is the same at both wavelengths, but this is only certain in an ideal or semi-ideal (gray-body) radiator. Concern over emissivity cancellation affects the design of ratio pyrometers: the closer together the wavelengths are chosen, the more likely the emissivities are to cancel, but the greater the degradation of actual performance. As a result, accurate temperature measurements with this approach is not possible in many instances. One way to minimize the errors is to average the results of many ratio pairs (Felice, U.S. Pat. No. 5,772,323); however, this approach cannot calculate accurate source temperatures for functional forms of the emissivity that systematically increase or decrease the color temperature.
Multiwavelength (MW) pyrometry was developed to simultaneously calculate the temperature and spectral emissivity of a thermal radiator from spectral intensity measurements made at several wavelengths. Originally, this involved assuming a specific parameterized wavelength dependence of the spectral emissivity, and utilizing spectral intensity measurement data to determine the adjustable parameters by solving simultaneous equations involving the Plank distribution for the parameters and the temperature. However, this method is highly sensitive to radiation intensity measurement errors and to differences between the actual and assumed emissivity functional forms, which increase the temperature calculation errors as the number of parameters increase.
A subsequent MW approach provides an improvement to the original approach by determining the temperature and emissivity parameters using best-fit least-squares fitting of numerous spectral intensity data points (Kahn, et al., U.S. Pat. No. 5,132,922). This reduces the sensitivity to measurement errors by removing the emphasis from an exact functional fitting of the experimental data. Instead, the regression fit utilizes the statistical averaging of a large data set to more accurately determine temperature. An additional improvement is to preprocess the measurement data to minimize the effects of noise and external influences before utilizing the curve fitting technique. However, even these improvements do not prevent significant errors for a variety of functional forms of the emissivity.
The main problems with current MW approaches include the following:
1. The assumed functional forms of the emissivity distribution may not adequately describe the wavelength dependence and lead to significant errors in the temperature computation.
2. Unaccounted for spectral components of reflected or transmitted stray light can severely limit measurement accuracy.
3. Measurements within media that absorb and radiate can significantly degrade calculations.
SUMMARY OF THE INVENTION
The invention provides new types of thermal radiation analysis and analyzers that determine temperature and can be used to determine spectral emissivity, the spectral distribution of extraneous radiation and atmospheric absorptivity as well. It comprises a two-stage, passive MW measurement approach, each stage of which is novel. Neither stage requires prior knowledge or independent assessment of the emissivity, and the final calculated temperature and emissivity are insensitive to the functional form of the emissivity. Stage-1 provides an emissivity compensating methodology that typically provides accuracies of less than 1%, and Stage-2 provides a multi-temperature simultaneous calculation that utilizes the results of Stage-1 to provide typical accuracies of less than 0.1 K.
For a thermal radiation source at temperature T, the following approximate relationship can be written for the measured spectral intensity W(&lgr;,T), and the emissivity &egr;(&lgr;,T),
Ln[W
(&lgr;,
T
)&lgr;
5
&agr;]−Ln
[&egr;(&lgr;,
T
)]=−
a
0
/(&lgr;
T
) (1)
&agr; is a constant that depends on the solid angle of light intercepted (&agr;=37415, for &lgr; in units of microns and W in units of Watts/m
2
, when all the radiated light is intercepted), &lgr; is the wavelength, Ln is the natural log, and a
0
is a constant equal to 14388 &mgr;mK (&lgr; in units of &mgr;m and T in units of K). The idea is provide a best fit of the right-hand-side (RHS) of Eqn. (1) to the left-hand-side (LHS) to determine T. Unfortunately, &egr;(&lgr;,T) is generally unknown; however, the spectral variation of the emissivity term is much less than that of the spectral intensity term, and a rough estimate of the emissivity term is enough to provide a fairly accurate estimate of T. Initially, the emissivity term is assumed to be constant, and a spectral least squares best-fit of the RHS and the emissivity term to the intensity data in Eqn. (1) is used to determine T and the constant emissivity within various spectral sub-regions (approximately 10 contiguous sub-regions) of W(&lgr;,T) such that the LHS best fits the RHS, thereby determining if the color temperature is generally increasing, decreasing, or remaining constant with wavelength. Then, an emissivity of a predetermined functional form that yields a similar increase, decrease or constant color temperature is determined, and a new value for T is calculated using Eqn. (1). More explicitly, Ln[&egr;(&lgr;,T)] can be written as,
Ln
[&egr;(&lgr;,
T
)]=
x
0
(
T
)+
x
1
(&lgr;,
T
) (2)
The simplest approximation for x
1
is,
x
1
(&lgr;,
T
)=
s
x
(
T
)(&lgr;−&lgr;
L
) (3)
&lgr;
L
is the shortest wavelength of the measured spectrum. s
x
is determined by choosing the value that best reproduces the changes in color temperature observed. More explicitly, by inserting Eqn. (2) and Eqn. (3) into Eqn. (1) and solving for s
x
if there is an equal number of wavelengths in the different sub-regions, then for the j
th
wavelength in the i
th
subregion, the i
th
s
x
is given by,
s
xi
:=
[
Ln
[
(
λ
i
+
1
,
j
)
5
·
W
(
λ
i
+
1
,
j
,
T
)
-
(
λ
i
,
j
)
5
·
W
(
λ
i
,
j
,
T
)
]
+
a
0
(
λ
i
+
1
,
j
T
i
+
1
)
-
a
0
(
λ
i
,
j
T
i
)
]
·
(
λ
i
+
1
,
j
-
λ
i
,
j
)
-
1
•
(
4
)
s
x
is obtained by taking the average of s
xi
. Eqn. (1) can now be rewritten as,
Ln[W
(&lgr;,
T
)&lgr;
5
/&agr;]−s
x
(
T
)(&lgr;−&lgr;
L
)=
x
0
(
T
)−
a
0
/(&lgr;
T
) (5)
T and x
0
are determined by a least-squares best-fit of the RHS of Eqn. (5) to the LHS. This value of T is then used with Eqn. (1) to consistently determine the emissivity.
To minimize sensitivity to noise (from atmospheric absorptivity, electrical noise, etc.), standard signal preprocessing of the
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