Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
1999-06-21
2002-04-30
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S126000, C374S009000
Reexamination Certificate
active
06379038
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a radiation pyrometer useful for the measurement of the temperature of a radiating body. More particularly, the present invention relates to a radiation pyrometer that detects and compensates for emissivity that changes with wavelength, as in metals. Additionally, the present invention relates to a radiation pyrometer that enhances the resolution and repeatability of the measured temperature of the radiating body. Additionally, the present invention relates to the technique utilized to enhance the resolution and repeatability of the measured temperature.
BACKGROUND OF THE INVENTION
Radiation pyrometers are known and commercially available. Typically, pyrometers are used to generate a measured temperature of a radiating body. The term “target” is used to indicate the radiating body evaluated for temperature determination, and the term “measured temperature” is used to indicate the value generated by a pyrometer or a pyrometric technique. The measured temperature may, or may not, be the actual temperature of the target.
Pyrometers are particularly useful for measuring target temperatures when the target is positioned in a remote location, or when the temperature or environment near the target is too hostile or severe to permit temperature measurement by other, more conventional, means or when the act of measuring in a contact manner may itself perturb the target temperature. The terms “measuring” and “measure” are use to include all aspects of a pyrometric technique including, but not limited to, energy collection, correlation, data manipulation, the report of the measured temperature, and the like.
Current pyrometers are one of two types: brightness or ratio devices. Brightness and ratio pyrometers both utilize a solution of a form of the Planck Radiation Equation to calculate the target's measured temperature. The Planck Radiation Equation for spectral radiation emitted from an ideal blackbody is
L
λ
=
2
hc
2
λ
5
[
ⅇ
hc
/
λ
K
B
T
-
1
]
-
1
(
Equation
1
)
where L
&lgr;
=radiance in energy per unit area per unit time per steradian per unit wavelength interval,
h=Planck's constant,
c=the speed of light,
&lgr;=the wavelength of the radiation,
k
B
=Boltzmann's constant, and
T=the absolute temperature.
For non-blackbodies,
H
λ
=
ϵ
L
λ
=
ϵ
2
hc
2
λ
5
[
ⅇ
hc
/
λ
K
B
T
-
1
]
-
1
(
Equation
2
)
where H
&lgr;
=the radiation emitted, and
&egr;=emissivity.
In the brightness method of pyrometry, H
&lgr;
and &egr; are measured at a known wavelength, &lgr;, and, therefore, T can be calculated.
Brightness devices rely upon capturing a known fraction of the radiation from a source in a particular solid angle. Brightness pyrometers known in the prior art are dependent upon knowing the emissivity of the target, as required by Equation 2, supra. Emissivity is the ratio of the radiation emitted by the target to the radiation emitted by a perfect blackbody radiator at the same temperature. Typically, emissivity is unknown or estimated to a low degree of accuracy. Additionally, the emissivity is often a function of the target temperature, wavelength of radiation examined, and history of the target. This limits the utility of brightness pyrometry severely.
In practice, it is left to the user of a brightness pyrometer to estimate target emissivity, usually based upon an analysis of the target's composition. The user must then determine if the target's thermal and environmental history have not appreciably altered the target emissivity. The wavelength or group of contiguous wavelengths of radiation examined are determined by the instrument used, and no selection is possible. It is then left to the user to decide whether or not the indicated target temperature is correct.
Brightness pyrometers are also susceptible to effects of the environment. The gases given off by the target or otherwise present in the atmosphere can selectively absorb radiation at various wavelengths, thus altering the energy reaching the pyrometer and hence the measured temperature. Again, current instruments give no guidance or assistance to the user in surmounting this obstacle.
Ratio pyrometers depend upon graybody behavior. A graybody is an energy radiator which has a blackbody energy distribution reduced by a constant, known as the emissivity, throughout the wavelength interval examined. Ratio pyrometers detect the radiation intensity at two known wavelengths and, utilizing Planck's Equation, calculate a temperature that correlates to the radiation intensity at the two specified wavelengths.
One form of the Planck Radiation Equation useful for ratio pyrometry is expressed as
T
=
C
′
(
1
/
λ
1
-
1
/
λ
2
)
ln
R
-
5
ln
(
λ
2
/
λ
1
)
-
ln
(
K
1
/
K
2
)
(
Equation
3
)
where T=absolute temperature;
&lgr;
i
=specific wavelength chosen,
C′=second radiation constant=hc/k
B
;
R=ratio of radiation intensity at &lgr;
1
, to that at &lgr;
2
; and
K
i
=instrument response factor at each wavelength chosen.
Here the low-temperature, short-wavelength approximation has been made; i.e., [e
hc/&lgr;k
B
T
−1] has been replaced with [e
hc/&lgr;k
B
T
].
Tradeoffs must be made in the design of ratio pyrometers, particularly in the wavelengths selected for inspection. Planck's Equation yields higher precision when the selected wavelengths are further apart. However, broadly spaced wavelengths permit extreme errors of indicated temperatures for materials that do not exhibit true graybody behavior. In practice, the two distinct wavelengths are typically chosen close together to minimize target emissivity variations, and the resulting diminution of accuracy accepted as a limitation of the pyrometric device.
Ratio devices are also affected by gaseous absorptions from the workpiece or environment. If a selective absorption occurs for either of the two wavelengths fixed by the instrument, the measured temperature will be incorrect.
Both brightness and ratio devices are therefore critically dependent on target emissivity and atmospheric absorptions in the region under study.
There is another, more subtle error to which both brightness and ratio devices are prone. If the measuring device has a significant bandwidth at the wavelengths utilized, a simple emissivity correction will not suffice for a target with spectral variation of emissivity. The emissivity correction is treated as a variable gain for both classes of devices (brightness and ratio), and is therefore a linear correction. If the bandwidth is large the contribution from neighboring wavelengths of different emissivity will render the resulting radiation intensity variation with temperature non-linear, since the Planck function is non-linear. This implies that there is no single emissivity correction for certain targets if the bandwidth is large. Furthermore, if any element in the optical path has a spectral transmission dependence, the same error applies; no single gain factor can correct for such an optical element (e.g., a gaseous, absorbing atmosphere, a glass window or lens, a mirror, etc.)
Experimenters have investigated multi-wavelength pyrometry for some time. G. A. Hornbeck (Temperature: Its Measurement and Control in Science and Industry, 3 (2), Reinhold, New York, 1962) described a three-wavelength device that could measure temperatures independent of target emissivity if the emissivity variation was linear over the wavelengths examined. The works of Cashdollar and Hertzberg (Temperature: Its Measurement and Control in Science and Industry, 5 453-463, American Institute of Physics, New York, 1982; U.S. Pat. No. 4,142,417) describe temperature measurement of particu
Gonzalez Madeline
Gutierrez Diego
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