IL103296A - Method for accurate measurement of temperature and a pyrometer device therefor - Google Patents

Description

"METHOD FOR AN ACCURATE MEASUREMENT OF TEMPERATURE AND A PYROMETER DEVICE THEREOF" >ipon»i> ipiun MWWDD *ro npmna ίπ»τ»ί> n©»¾>" "IT men> THE APPLICAHTS: tnppaan YECHNXON RESEARCH & DEVELOPMENT imu»2»i Tpna!> t»÷3»n tow FOUNDATION LTD, TECHNION CITY, HAIFA 32000. .32000 na»n , »3aoa-n»i THE MVENTORt SR. ELI T. TALMORE •no$o .t> &« "i 15/5 ELIEZER ALTER ST. 15 5 ma* ητ?»*κ aim RAMAT ALON, HAIFA. .na»n t)i†ni ran The present invention relates to a method and device for the measuring of temperature. More particularly, the invention relates to a method and device for the measurement of temperature, without any physical contact with the object in question.

BACKGROUND OF THE INVENTION.

Temperature is one of the most basic and important parameters in the analysis and control of various phenomena* The accurate and rapid measurements of temperature is of great importance in most materials manufacturing and processing applications. The main technology, can provide accurate measurements only if a physical contact with the subject, such as thermocoupling, exists or extensive calibration was performed. An accurate knowledge of temperature determines the quality of material manufacturing,' suh as metals, semiconductors, glass, plastics and ceramics. Also, temperature monitoring is most important in a safe and efficient operation of power stations, nuclear reactors and aircraft engines.

The main disadvantages of contact measurements of temperature are: - Thermocouples are not capable to withstand the extreme temperature ranges and advert conditions.

- A thermocouple will disturb the uniformity and purity of the subject to be measured.

The non-contact temperature measurements devices which were developed in the last sixty years, known by the term pyrometers, are measuring incoming infra-red radiation emitted from a heated surface and subsequently conversion its value to a temperature. The amount of radiant energy emitted by a heated body is known to be proportional to the temperature of the body and may be calculated using the Stefan-Boltzman law or Plank radiation formula. Existing pyrometers employ different signal processing techniques ,such as: brightness,ratio or multiwave length.

However, the accuracy of these devices has been limited because they have employed estimated emittance factors in their radiation formula calculation. Large errors in the range of 20°K to 50° are obtained using commercial pyrometers. Flame pyrometry using commercial sensors produces errors* in the order of 150° to 250° . This will cause significant increase in energy consumption and lowers the efficiency of energy generation of power stations.

Brightness pyrometers, which allow the operator to match the appearance of a heated, calibrated standard to the appearance of the subject, attempt to solve the emlssivi-ty problem by making standard from the same material as the object, but this solution restricts severely the use of the device because it may not be possible to get a standard of the same material and condition. Furthermore* the emissivity of the object can change quite rapidly as the environment and surface conditions change.

Some improvements over the above pyrometer, were achieved using the total radiation pyrometer. This device uses the Stefan-Boltzmann relation and measures the radiance electronically using a photodetector. However, even this device still suffers from inaccuracies caused by uncertainties in the emissivity determination.

The main reasons connected with the complications by the optical pyrometers, result from the emissivity of the surface, both in absolute value and directional characteristic, its dependency ,^ wavelength and the extent of extraneous radiation on the surface. The often large inaccuracies are attributed to the fact that they require unrealistic assumptions.

The pyrometers that incorporate means for emissivity estimate through reflectivity measurements are als known as active pyrometers. The procedure was described in a paper by J.L.Gardner and T.P.Jones (J.Phys.E.Sci .Instr. Vol .13, 1980) , pointing out that directional emissivity of opaque materials is related to hemispherical reflectance. This reflectance is measured in the direction of emitted radiation while incident on the surface uniformly over the total hemisphere. As mentioned in this review, the hemispherical reflectance is difficult to measure in a system where the main goal is a non-contact measurement of surface temperature.

Based on the above technique, a method was described in the EP patent number 294,747, wherein it is suggested to perform the measurements simultaneously at three wavelengths with an angle of incidence and angle of reflection normal to the surface. The ratio between the hemispherical and directional reflectance is measured as described in the Gardner et al procedure.

In a recent U.S. patent number 5,029,117, an apparatus Is described for the measurement of temperature semiconductor wafer, using the bi-directional reflectance measurement at large angle at single wavelength. The wafer is suggested to be illuminated by fiber optics and the radiation will be reflected on the detector. Nothing is mentioned therein on the question of geometry.

It is an object of the present invention to provide a method for a non-contact measurement of temperature. It is another object of the present invention to provide a method and apparatus for the measurement of temperature by a non-contact principle which results in a more accurate determination. than the previous used techniques. It is yet another object of the present invention.to provide a method and device for the measurement of temperature by a non-contact technique, in which the emittance factor associated with the body whose temperature is being measured is accurately derived.

BRIEF DESCRIPTION OF THE INVENTION.

The invention relates to a method for an accurate measurement of the temperature of an object using an active pyrometer system providing a nearly hemispheric reflectance, which comprises means of wide angular coverage of said object,with an overlapping wide angular illumination by multi-spectral lamp, wherein the reflectance measurement is Independent on directivity pattern of the object, thus producing the correct emlssivity value^ from which an accurate temperature of said object is determined.According to the invention, the incident and back-reflected radiation have the same divergence angle and thus the radiation emitted by said object is measured simultaneously with the reflected radiation.

The invention also describes a novel active pyrometer device which possesses means of wide angular coverage of the object to be measured with an overlapping wide angular illumination by a multi-spectral lamp which emits light energy through a fiber optic light which serves also as means for collecting the radiation over the same large angle. In this manner, the incident and back-reflected radiation has the same divergence angle. As a result, the total reflective energy will be equal to the m β m reflected energy in the normal direction for both specular and diffuse surface, which means that the power obtained in both cases will be the same, irrespecive on the surface reflection directivity pattern.

BRIEF DESCRIPTION OF THE DRAWINGS.

Figure 1, illustrates schematically the errors of different types of pyrometers measuring temperatures of melted steel baths.

Figure 2, illustrates a preferred scheme of the device according to the present invention, which provides a true temperature pyrometer.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE INVENTION The method according to the present invention provides means of wide angular coverage of the surface to be inspected with overlapping wide angular illumination by a multi-spectral lamp. A preferred lamp Is a Xe-lamp and the most preferred one is a Xe-flash lamp. One may also conceive to use either electronically or mechanically modulated lamp.

In Figure 1, data are given on measurements of the temperature in baths of molten steel with four different pyrometers. As appears, great errors in the measured temperature exist. As can be noticed, the pyrometers 1, 2 and 3, without an accurate emissivity correction, can produce errors reaching even 350°K. Only the pyrometer 4, which is based on a quasllinear emissivity model, produces accurate and consistent results. However, the accuracy of such pyrometers have been limited because they have employed estimated emittance factors in their radiation formula calculation.

As shown in Figure 2, the device according to a most preferred embodiment of the present invention comprises a Xe flash lamp (1) which sends short pulses of light energy, in the range of 10 to 30 microseconds, through the fiber optic light guide (2), while item. (3) is a pyrometer. The fiber optic light guide comprises a bundle of fiber optic elements, made of a common material such as: glass, quartz, liquid light guide, etc. The energy exits from the fiber optic with a large exit half-angle . in the range of 10° to 40° and preferably in the range of 15° to 25°. Thus, in case of a typical distance of 1 meter from the object, an illuminated spot of between 50 to 80 cm on the object will prevail. Using a large area of illumination will reduce significantly the problem of object heating by the measuring device. On the other hand, the use of large spots averaging reduces the problem of non-uniformity of the temperature.

The lamp pulses from the flash lamp. after passing through the fiber optic light guide are emitted (4) to the object (7) and back-reflected (5) to the fiber optic light guide and further collected over the same large angle. From the object (7) thermal emitted radiation (6) Cemitted 1 ' .Ί' , Actual surfaces have bi-directional reflections with large peak in the direction of specular angle typically of 10° to 20°f j ide. Therefore, the measurement procedure has to Include the specular direction and the beam width has to be larger than the bi-directional reflectance peak width in order to ensure accurate measurements . In case of fiber optics possessing a full angle of 30°, most of the bi-directional reflectance peaks will be covered. Therefore, the method according to the present invention, enables a much better approximation of the hemispherical reflectance than the other existing bidirectional reflectance measurements with a narrow angular coverage.

One of the advantages of the method according to the present invention is the fact that the quantity required for the pyro etric calculations, is only a hemispherical reflectance and not a bi-directional reflectance as in the known methods of to-day. It is well-known that the reflectance measurements of bi-directional reflectance of the surface are most inaccurate. This does explain the significant decrease in the error of the measurement obtained with the method according to the present invention.

According to an optional embodiment, in order to increase the overall accuracy, it is suggested to eliminate the "noise effect". This is achieved, by incorporating multi- wavelengths re lectance-radiance measurements. In this manner,the random measurement noise will be significantly lower than systematic errors introduced by reflectance measurements.

The present invention also provides a novel pyrometer device for the measurement of temperature which overcomes the basic problems known in the art of pyrometry, i.e. emisslvity. The principle of the device is the fact that it measures the radiation emitted by the object, as in the known passive pyrometry. and simultaneously it will measure the radiation reflected by the object, i.e. active pyrometry, using in both cases the same optic fiber element. According to another embodiment, it is possible that the radiation emitted and reflected are using two angular overlapping optical fibers. Once the object transparency over the wavelength of interest is known, then its emisslvity can be easily calculated from the formula known for the reflectance measurements: Ε(λ .T) o j .t. $(λ ,T) wherein: E « emisslvity λ * wavelength X = transparency S · reflectance in order to obtain accurate reflectance data, it is required to differentiate between the two sources of radiation: reflected and emitted. This differentiation will be provided using a high-energy pulsed source. With the same technique, the dark noise of the detector will be measured.

This can be easily obtained, by incorporating a multiwave length reflectance-radiance measurement noise.

The method according to the present invention approaches the ideal hemisphere configuration. In case of a diffuse surface, the bi-directional reflectance is isotropic and the calculation will be based only upon individual ray-tracing. By an integration over all the incident rays, the intensity back-reflected into the cone will be equal to the total intensity reflected in the normal direction or any other specific direction. In this manner, the reflected energy for the diffused or specular surface will be equal to that of the existing device for the same incident cone.

Although the optic fiber has a large angular subtense, it will collect only small fractions, such as back-reflected photons. Instead of collecting N photons which are reflected in the normal direction, it will collect N photons each back-reflected.

Due to the fact that the reflectance measurements are generally less-accurate then the radiance measurements, it is necessary to eliminate some of the noise effect. This can be easily obtained by incorporating a multiwave length reflectance-radiance measurement noise.

Since the method used, requires a number of narrow band pass wavelengths in the spectral region of 0.90 to 1 urn, some wavelengths filters are preferred to be inserted, the number thereof depending on. the accuracy required. The results using the device according to the present invention show that, in case of a temperature of 1000°K where the emissivity measurements were performed with 10% accuracy at three wavelengths after 1 second,the temperature measured had a relative error of only 0.25%.

Summing up, using the method and the device according to the present invention, the true temperature measured by the pyrometric system, in the range of 500°K to 2500° , will provide an accuracy of better than 1%.

The device can be easily manufactured and provides most accurate measurements, approaching an ideal hemispheric reflectance. Assuming a fiber optics with half-angle of 50°, the ratio between the measurements of the specular and diffuse surface will be 1.7, while the ideal result will be 1.

While a preferred embodiment of the present invention has been shown and described, it should be understood by persons skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects as covered by the appended Claims.

Claims (18)

1. C LA I MS :- 1. Method for an accurate measurement of temperature of an object using an active pyrometer system providing a hemisphere reflectance, which comprises means of wide angular coverage of said object, with an overlapping wide angular illumination by a multispectral lamp, wherein the reflectance measurement is independent on directivity pattern of the object, thus producing the correct emissivlty value from which an accurate temperature of said object is determined.

2. The method according to Claim 1, wherein the incident and back-reflected radiation have the same divergence angle.

3. The method according to Claims 1 or 2, wherein the wide angular coverage of the object is carried out by a multi-spectral lamp.

4. The method according to Claim 3, wherein said lamp is Xe-flash lamp.

5. The method according to Claim 4, wherein said Xe-flash lamp Is sending short pulses of light energy through a fiber optic element.

6. The method according to Claim 5, wherein said short pulses of light energy are in the range of 10 to 30 microseconds.

7. The method according to Claims 5 or 6, wherein the energy exists from the fiber optic with a large exit half angle o in the range of 10° to 40°,

8. The method according to Claims 5 to 7, wherein the lamp pulses from the flash lamp are back-reflected to the fiber optic light guide, being collected over the same large angle.

9. The method according to Claims 5 to 8, wherein said fiber optic light guide element is made from glass,quartz and liquid light guide,

10. The method according to Claims 1 to 9, wherein the quantity required for the pyrontetric calculations is a hemispherical reflectance,

11. The method according to Claims 1 to 10, wherein the "noise effect" is eliminated by incorporating ultiwave length reflectance-radiance measurements.

12. A pyrometer device for the measurement of temperature, which comprises a light source which sends short pulses of light energy through a fiber optics light guide.

13. The pyrometer according to Claim 12, wherein said light source is a Xe-fiash lamp.

14. 4. The pyrometer according to Claim 12, wherein said light source is an electronically or mechanically modulated lamp.

15. The pyrometer according to Claims 12 to 14, wherein the radiation emmltted and reflected are using the same optic fiber element. 16. The pyrometer according to Claims 12 to 14, wherein radiation emitted and reflected are using two angular overlapping optical fibers. 17. The pyrometer device according to Claims 12 to 16, wherein said energy has large exit, half-wangle o in a preferred range of 15° to 25°. 1,8, The device according to Claim ,17, wherein the differentiation between the two sources of radiation is obtained by a high-energy pulsed source.

16. The device according to Claim 12 to 18, wherein said device approaches the ideal hemisphere configuration.

17. A method for an accurate measurement of the tempera-ture of an object substantially as described in the specification and claimed In any one of Claims 1 to 11.

18. A pyrometer device for an accurate measurement of temperature, substantially as described in the specification and claimed in any one of Claims 12 to 16. For the Applicants, Simon Lavie Patent Attorney