CA1319832C - Infrared radiation probe for measuring the temperature of low-emissivity materials in a production line - Google Patents

Abstract

ABSTRACT
The invention is concerned with an infrared radiation probe for measuring the temperature at the sur-face of an object by measurement of the infrared radiation emitted from the surface. The infrared radiation probe according to the invention has a head positionable in spaced relation to the object surface and comprising at least one angularly inclined reflective surface having a predetermined surface area and disposed at an angle relative to the object surface such as to define with the object surface a wedge-shaped reflective cavity for increasing effective emissivity of the object surface by multiple reflections, and a bundle of optical fibers having a predetermined angular aperture at a radiation receiving end connected to the probe head and arranged for collecting the infrared radiation reflected from within the cavity and transmitting the reflected infrared radia-tion to radiation detector means adapted to convert the infrared radiation into a signal representative of the surface temperature of the object. The angular aperture of the optical fiber bundle, the angle defined by the reflec-tive surface and the surface area thereof are such as to substantially prevent stray radiation from being transmit-ted through the optical fibers. The infrared radiation probe according to the invention is particularly useful for measuring the temperature of low-emissivity materials in a production line.

Description

The present invention relates to improvements in the field of radiation thermometry. More particularly, the invention is directed to an infrared radiation probe for measuring the temperature of low-emissivity materials in a production line.
Radiation thermometry is widely used where the measurement by a contact method such as with a thermo-couple is impossible or undesirable, since the surface temperature of an object can be measured quickly and without contact. Non-contact temperature sensors using a fiber optic cable to transmit the infrared radiation sensed to a remote infrared detector have been described, for instance, in U.S. Patent Nos 4,402,790, 4,444,516 and 4,468,771. Only the fiber optic head, which may survive to temperatures of several hundreds of degrees Celsius, will in such a case be introduced in the hostile environment to be probed such as a furnace, while the delicate infrared detector at the other end of the fiber optic cable may be kept in a remote, protected environment.
Although infrared temperature sensors are robust and potentially very sensitive, their reliability is often limited by the incertitude on the value of the thermal emissivity of the inspected surface, as well as by the presence of stray reflections from nearby sources of infrared radiation. Shields are sometimes introduced to eliminate stray radiation which is emitted, e.g. by flames or tubular heating elements in furnaces. Because of sheet wobbling problems encountered in the production of metal sheets, such shields cannot be placed in close proximity to the metal sheet whose temperature is to be probed.

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Consequently, some stray radiation may penetrate between the shield and the wobbling sheet and eventually reach after one or more reflections the sensitive surface of the infrared detector. This results in spurious signals which are erroneously interpreted as temperature variations of the metal sheets.
Emissivity variations are another important source of error for infrared temperature sensing. This problem is particularly felt when trying to evaluate the temperature of low-emissivity materials such as bare metallic surfaces. As an example, the emissivity of an aluminum sheet in a furnace may change from 0.05 to 0.8 because of such difficulty to predict parameters as the surface finish, surface cleanliness and degree of oxida-t1on. As the magnitude of the detected infrared signal is proportional to the emissivity of the surface, it can be understood that it is in general quite difficult to obtain from such measurements an evaluation of the temperature with an accuracy of the order of + 1 percent which is required, for example, in an annealing furnace.
A well-known method (see, for example, U.S.
Pater.~ Nos 3,451,254 and 4,326,798) for making a tempera-ture measurement by infrared radiation sensing in a manner substantially independent of the emissivity of the inspected surface is based on a double-wavelength detec-tion. The infrared radiation is detected within two different spectral ranges using two infrared detectors equipped with optical filters. The ratio of the two signals can be related to the surface temperature by a proper relation independent of the surface emissivity as long as the emissivity is known to be the same in the two spectral ranges. If, as it is often the case for surfaces containing spectrally selective oils or coatings, the emissivity varies across the probed spectral region, the temperature can no longer be reliably measured by this method. It should also be mentioned that the double-wave-length detection method is more affected by the electronic noise of the infrared detectors and of the associated amplifiers and electronic circuity as compared to the single-wavelength detection method. Indeed, the double-wavelength detection method requires that two narrow and well separated spectral ranges be filtered out by narrow optical filters and separately detected by the two detectors. This implies that the amount of radiation within each spectral range is much lower in magnitude as compared to a single-wavelength detector which is not restricted to a narrow spectral range.
Other methods to make temperature measurements less sensitive to the surface emissivity rely on the use of reflective cavities, such as in U.S. Patent No.
3,916,690. A typical implementation of this principle is described by T. Iuchi and R. Kusaka, in "Temperature, its Measurement and Control in Science and Industry", J.F.
Schooley editor, vol. 5, pages 491-503. An elongated cylindrical cavity with a highly reflective inner surface is inserted between the detector and the surface whose temperature is to be measured. Such a cavity has the double function of shielding the detector from stray radiation as well as to surround the detector with a reflective cavity which, when perfectly closed, raises the effective emissivity of the surface to values close to 100 percent. Problems arise, however, if a close proximity of the cavity to the surface cannot be assured, as it is often the case in the presence of substantial wobbling of the probed surface. The precision of this method drops very quickly as the distance between the cavity and the surface increases, making such an approach inappropriate for precise temperature measurements along the production line. Moreover, the requirement for a rotating sector which has a limited lifetime in the hostile sensing environment makes such an approach inappropriate for on-line applications such as temperature sensing within a furnace.
It is therefore an object of the present inven-tion to overcome the above drawbacks and to provide an infrared temperature probe having a reflective cavity which not only substantially increases the effective emis-sivity so that little errors are introduced by variations in the real emissivity of the inspected surface, but also enables to make the temperature measurement less sensitive to variation of the distance between the sensor and the probed surface, while effectively deviating stray light out of the anqular aperture of acceptance of the system.
In accordance with the invention, there is thus provided an infrared radiation probe for measuring the temperature at the surface of an object by measurement of the infrared radiation emitted from the surface, which probe has a head positionable in spaced relation to the object surface and comprising at least one angularly inclined reflective surface having a predetermined surface area and disposed at an angle relative to the object surface such as to define with the object surface a wedge-shaped reflective cavity for increasing effective emissivity of the object surface by multiple reflections, and a bundle of optical fibers having a predetermined angular aperture at a radiation receiving end connected to the probe head and arranged for collecting the infrared radiation reflected from within the cavity and transmit-ting the reflected infrared radiation to radiation detec-tor means adapted to convert the infrared radiation into asignal representative of the surface temperature of the object. The angular aperture of the optical fiber bundle, the angle defined by the reflective surface and the sur-face area are such as to substantially prevent stray radiation from being transmitted through said optical fibers.
According to a preferred embodiment of the invention, the probe head comprises two reflective surfa-ces disposed at an angle relative to one another such as to define with the object surface a reflective cavity having a double wedge configuration. Preferably, the reflective surfaces are gold-plated mirror surfaces.
In a particularly preferred embodiment, the probe head has a body including means for circulating a gas flow therethrough and into the reflective cavity to thereby cool the body and maintain a clean gas atmosphere in the cavity. Where two reflective surfaces are used, the body of the probe head advantageously has a rectangular configuration with opposite ends and a pair of oppositely facing gas chambers are provided one at each end, the 1 31 q832 reflective surfaces being disposed between the gas chambers. The gas flow circulating means preferably com-prise gas inlet means for admitting gas into the chambers and gas outlet means for discharging the gas from the chambers over the reflective surfaces to maintain the surfaces clean. Preferably, the gas chambers are each formed with inner and outer sidewalls and the gas outlet means comprise a pair of opposed, elongated slots defined in the respective inner sidewalls of the gas chambers and extending adjacent to and parallel with the reflective surfaces. A gas deflector is advantageously arranged inside each gas chamber to direct the gas flow through the slots and over the reflective surfaces.
According to another preferred embodiment, the radiation receiving end of the optical fiber bundle is connected to the probe head by means of an elongated connector member secuted to the probe head, and the connector member is provided with an inner optical fiber retention head which is movable along the longitudinal axis of the connector member for adjustably positioning the radiation receiving end of the optical fiber bundle between the reflective surfaces.
The use of an infrared radiation probe with a wedge-shaped reflective cavity produces an effective increase of the emissivity of the inspected surface. Since the effective emissivity is increased with respect to the real emissivity of the material, the detected signal has a correspondingly increased level. This results in a sub-stantially better signal/noise ratio, thus compensating for the relatively high losses at long wavelengths in the 1 3 1 ~832 optical fiber bundle (larger than 1 dB/m at A larger than 2.2 ~m for most silica fibers) and for the low thermal emission at low wavelengths for small values of the tem-perature (the spectral radiant emission at ~ = 1.7 ~m from a surface at 250C is more than two orders of mangitude smaller than the peak emission which takes place at 5 ~um).
In addition to the increased emissivity value, another benefit from the use of a wedge-shaped cavity is a reduced sensitivity to stray light. If a stray-light beam enters the cavity from the outside borders, multiple reflections result in a gradual decrease of the angle of incidence of such a beam so that it will not longer reach the tip end of the optical fiber bundle within the angular aperture. This corresponds to the angle of acceptance outside which light radiation cannot be guided within the fiber through a long distance.
In certain circumstances, the use of an infrared radiation probe with a wedge-shaped reflective cavity may not be required if an infrared sensing probe is properly positioned along the production line, such as in the case where a metal sheet is being reeled after hot rolling or similar processing steps. Such operations take place at displacement speeds of the metal sheet of the order of 20 m/s, and a precise determination of the sheet temperature is required to operate in a temperature range assuring proper softness and grain normalization during air cooling, while retaining most of the structural strength which is produced by mechanical working steps such as rolling. The sheet temperature may in this case be 1 31 ~832 precisely measured by directing the radiation receiving end of a fiber optic cable (normally with a small lens at its end to reduce the angular aperture of the light beam accepted by the optical fiber bundle) toward the wedge formed between the metal sheet and the convex surface of the reeled metal. As both surfaces of this wedge are at essentially the same temperature, there will be no need in this case for a highly reflective mirror-like surface. The average sheet temperature can be evaluated at a safe distance (several meters if the atmosphere is clean) from the reel and alignment is facilitated by scanning the metal-to-reel interface in a plane perpendicular to the reel axis and recording the maximum temperature value which is obtained when the sensor is pointed exactly toward the metal-to-reel wedge.
Accordingly, the present invention also pro-vides in another aspect thereof a method of measuring the temperature at the surface of an object defining with an adjacent reflective surface a wedge-shaped confined space with both surfaces being essentially at the same tempera-ture, which method comprises providing a bundle of opticalfibers having a predetermined angular aperture with one end of the optical fiber bundle being connected to radia-tion detector means and directing the other end of the optical fiber bundle toward the wedge-shaped confined space to collect the infrared radiation emitted from such a space and transmit same to the radiation detector means for conversion into a signal representative of the sur-face temperature of the object.

Further features and advantages of the inven-tion will become more readily apparent from the following description of preferred embodiments as illustrated by way of example in the accompanying drawings, in which:
Fig. 1 is a schematic representation of an infrared radiation probe according to a preferred embodi-ment of the invention;
Fig. 2 schematically illustrates the light path within the reflective cavity of the probe shown in Fig. l;
Fig. 3 schematically illustrates the path of stray-light infrared beams within the reflective cavity of the probe shown in Fig. l;
Fig. 4 is a sectional view of the probe head, taken along line 4-4 of Fig. S;
~ Fig. 5 is a sectional view taken along line 5-5 of Fig. 4;
Fig. 6 is another sectional view taken along line 6-6 of Fig. 4;
Fig. 7 which is on the same sheet as Fig. 1 schematically illustrates an alternative embodiment for temperature sensing of a metal sheet being reeled where the we&ge is provided by two adjacent metal sheets;
Fig. 8 is a plot of effective emissivity vs.
cavity angle ~ for different orientations of the optical fiber bundle; and Fig. 9 shows an experimental curve of the tem-perature of an aluminum sheet vs. the ratio R between the output signals from the detectors of the detection and processing unit illustrated in Fig. 1.

g _ Turning first to Fig. 1, there is illustrated an infrared radiation probe generally designated by reference numeral 10 and having a head 12 positionable in spaced relation to the surface 14 of a material 16, such as a metal sheet, whose temperature is to be measured. A
fiber optic cable 18 interconnects the probe head 12 with a detection and processing unit 20 for transmitting the infrared radiation emitted from the surface 14 of the metal sheet 16 to the unit 20. The probe head 12 comprises two flat mirrors 22 whose reflective surfaces 24 are disposed at an angle relative to one another such as to form with the surface 14 of the metal sheet 16 a reflec-tive cavity 26 of double wedge configuration. Alternative-ly, the probe head 12 can comprise a single reflective surface of conical or~pyramidal configuration, with a wide angle cross-section similar to that shown in Fig. 1.
The detection and processing unit 20 illus-trated in broken lines comprises two infrared detectors 28 with narrow-band optical filters 30 in order to select two appropriate spectral detection regions for a double-wave-length temperature evaluation. A converging lens 32 and a beam splitter 34 are arranged for dividing the infrared beam emerging from the fiber optic cable 18 into two beam portions and directing same through the filters 30 and onto the detectors 28. A chopper 36 is also provided to modulate the optical signal at a predetermined frequency so as to enable subsequent electronic filtering by the signal processor 38. The detectors 28 are preferably pho-todiodes such as PbS, PbSe, HgCdTe or Si devices, depend-ing on the spectral region to select for the required 1 31 q832 temperature range. As an example, two PbSe photodiodes with optical filters 30 centered in the 1.7 ~m and 2.2 jum spectral regions may be used to monitor surface temperatu-res from 250C to 500C avoiding the high-absorption spectral regions of silica optical fibers and of the moist-air and CO2 bands of the atmosphere within the cavity 26. A wider temperature range can be obtained using different spectral regions.
The delicate detectors and associated electro-nics can thus be kept at a safe distance from the high-temperature environment through the use of a fiber optic cable 18 made of silica optical fibers which can withstand temperatures of the order of 500C. The length of the cable is normally limited by the attenuation of the silica fibers to 5 to 10 meters. The body 40 of the probe head 12 may as well be made to withstand temperatures of the order of 500C, particularly if some air coolinq is provided by the circulation of an air-purge flow 42 as shown in Fig.
1. The air-purge has the double purpose of cooling the probe head 12 and of keeping the reflective surfaces 24 clean by keeping an atmosphere of filtered air or of inert gas in the cavity 26. Such a shielding gas which is admitted via the gas inlet 44 is preferably injected through an elongated slot 46 oriented so as to force the gas circulation over the reflective surfaces 24 in order to avoid contact of -the outside contaminated atmosphere with the highly reflective, usually gold-plated, mirror surfaces 24.

Figs. 2 and 3 show the basic principle of the wedge-shaped reflective cavity 26. Fig. 2 shows how such a cavity results in a higher effective emissivity for the infrared probe. The path of a light beam leaving the fiber optic cable 18 is shown to suffer multiple reflections in the cavity 26 formed by the gold-plated surfaces 24 and the surface 14 of the metallic material 16 to be probed, assumed to be specularly reflecting in this case. It can be seen that the light beam is trapped within the cavity 26, and that it is finally absorbed almost exclusively by the sheet-metal surface 14 if the gold-plated surfaces 24 have a very high reflectivity (typically 99~ for the infrared). As the directional spectral emissivity and absorptivity are equal, one can reverse the direction of the light path and conclude that the effective emissivity of the cavity 26 is near to 100 percent and that it is mainly determined by the temperature of the sheet metal 16, the gold-plated surfaces 24 playing the role of a reflector which recycles the infrared radiation emitted by the hot sheet metal surface 14. The wedge angle c~ is function of the space defined between the probe head 12 and the inspected surface 14, the angular aperture of the fiber optic cable 18, the dimensions (i.e. surface area) of the mirror surfaces 24 and the specular-diffuse character of the reflections at the inspected surface 14;
such a wedge angle typically ranges from about 6 to about 12 and is preferably about 10.
In addition to producing an effective increase of the effective emissivity of the inspected surface 14, the use of the wedge-shaped reflective cavity 26 also 1 3~ 9832 enables to provide a reduced sensitivity to stray light.
This can be understood with reference to Fig. 3. If a stray-light beam enters the cavity 26 from the outside borders, multiple reflections result in a gradual decrease of the angle of incidence of such a beam so that it will no longer reach the tip end of the fiber optic cable 18 within the angular aperture 0 which typically ranges from about 15 to about 30 and is preferably about 23. This corresponds to the angle of acceptance outside which light radiation cannot be guided within the optical fibers through a long distance. Out-of-aperture light beams which are partially guided through a small length of the fiber optic cable 18 may eventually be eliminated by coating a short length of the fiber cable with black paint or index-matching epoxy anywhere between the probe head 12 and the detection and processing unit 20. Such a stray-light problem cannot be eliminated by conventional shielded detectors.
The angle c~ of the wedge, as well as the orientation of the optical fiber bundle, should be appro-priately chosen to maximize the number of multiple reflec-tions (reducing the probability of light losses through the center gap defined between the mirrors 22) while avoiding light losses at the ends of the cavity 26, and this should be assured for a suitable range of mirror-to-sheet distances, typically a few centimeters, depending on sheet wobbling. Fig. 8 shows the results of a numerical simulation showing the value of the effective emissivity vs the wedge angle ~ for a cavity 26 formed of two mirrors of size 12.3 cm x 16.7 cm with a center gap of 0.6 cm, situated 5 cm above a specularly-reflective aluminum surface, with an angular aperture 0 of the collecting fiber optic cable 18 equal to 23. The real emissivity assumed for the aluminum surface was 0.2, while the reflectivity of the gold-plated mirrors was assumed to be 98 percent.
A reflectivity value of 98 percent for the gold-plated mirrors 22 is difficult to be maintained in the long run because of the presence of impurities, such as ammonia sulphite or other combustion residues in the furnace, which tend to adhere to the reflective surfaces 24 if the air-purge system is not operating perfectly.
Such a problem may be attenuated by using back-metallized glass mirrors whose front surface is chemically inert and may be periodically cleaned more easily than a gold-plated surface. Nevertheless, an important practical advantage of the invention is that the presence of contaminants on both the metal sheet 16 and the reflective surfaces 24 will normally not affect the temperature reading.
A further advantage of an increase in the emissivity is a reduced dependence of the calibration curve on the variations of the spectral emissivity and thus on the variable surface properties of the material. A
bare aluminum surface may have an emissivity varying between nearly 0.05 to 0.8 depending on the cleanliness and oxidation of its surface, a variation by a factor of 16. If the effective emissivity is raised by the reflec-tive cavity 26 above 0.5 for example, the range of possi-ble emissivity variations is reduced from 0.5 to 1, a factor of 2 only. Taking advantage both of the effect of the cavity 26 and of the double-wavelength detection, a precise evaluation of the temperature of a low-emissivity surface is possible with little error introduced by emis-sivity variation. Fig. 9 shows an experimental curve of the temperature (measured by a thermocouple) of an alumi-num sheet vs. the ratio R of the signals detected by the two detectors 28 shown in Fig. 1. Also shown is the fitting curve T = aR3 + bR2 + cR + d with a = -120.267, b = 675.916, c = -1372.66 and d = 1309.71 which is used as the empirical calibration curve in the signal processor 38 shown in Fig. 1. It can be seen that the electronic noise for such a curve is limited to + 3C, or nearly + 1 per-cent of the measured temperature.
Turning now to Figs. 4, S and 6 which show details of the probe head 12, the body 40 of the probe head has a rectangular configuration with two pairs of opposite sidewalls 48 and 50. The fiber optic cable 18 which comprises a bundle of optical fibers 52 is connected to the probe head 12 by means of an elongated connector member 54 secured to the body 40 with screws 56. The connector member 54 is provided with an inner optical fiber retention head 58 which is movable along the longi-tudinal axis of the connector member 54 for adjustably positioning the tip of the optical fiber bundle in the center gap 60 defined between the mirrors 22 so that the tip is flush with the mirror surfaces 24, as best shown in Fig. 6. The optical fiber reten-tion head 58 can be locked into position by means of screws 62. As also shown in Fig.

6, screws 64 are provided in the sidewalls 50 for adjust-ably positioning the mirrors 22 such that their edges tightly fit against the optical fiber retention head 58.
The probe head 12 is provided with a pair of oppositely facing gas chambers 66 in fluid flow communica-tion with the gas inlets 44 for the circulation of an air-purge flow, the air or other gas being discharged from the chambers 66 through the slots 46 which are defined in the inner sidewalls 68 of the chambers and which extend adjacent to and parallel with the mirrors 22. A gas deflector 70 is arranged inside each gas chamber 66 so as to direct the gas flow through the slots 46 and over the mirror surfaces 24.
Slightly different configurations can be used for the reflective cavity 26, such as a reflective cavity with slightly arcuate mirrors to obtain a particular light path within the cavity, thus minimizing losses through the center gap 60, or a lens-capped fiber optic tip to further reduce the angular aperture of the fiber optic cable.
In certain circumstances, the use of an infrared radiation probe with a wedge-shaped reflective cav~ty may not be required if an infrared radiation sens-ing probe is properly positioned along the production line, such as shown in Fig. 7. A metal sheet 72 is being reeled after hot rolling or similar processing steps. As previously mentioned, such operations take place at dis-placement speeds of the metal sheet of the order of 20 m/s, and a precise determination of the sheet temperature is required to operate in a temperature range assuring proper softness and grain normalization during air cooling, while retainin~ most of the structural strength which is produced by mechanical working steps such as rolling. The sheet temperature may in this case be preci-sely measured by directing the tip of a fiber optic cable 18' (normally with a small lens 74 at its end to reduce the angular aperture of the light beam accepted by the optical fiber bundle) toward the wedge 76 formed between the metal sheet 72 and the convex surface of the reeled metal 72', the other end of the fiber optic cable 18' being connected to a detection and processing unit such as the unit 20 shown in Fig. 1. As both surfaces of this wedge are at essentially the same temperature, there will be no need in this case for a highly reflective mirror-like surface. The average sheet temperature can be evaluated at a safe distance (several meters if the atmosphere is clean) from the reel and alignment is faci-litated by scanning the metal-to-reel interface in a plane perpendicular to the reel axis and recording the maximum temperature value which is obtained when the sensor is pointed exactly toward the metal-to-reel wedge.

Claims (20)

1. An infrared radiation probe for measuring the temperature at the surface of an object by measurement of the infrared radiation emitted from said surface, said probe having a head positionable in spaced relation to the object surface and comprising at least one angularly inclined reflective surface having a predetermined surface area and disposed at an angle relative to the object surface such as to define with the object surface a wedge-shaped reflective cavity for increasing effective emissivity of the object surface by multiple reflections, and a bundle of optical fibers having a predetermined angular aperture at a radiation receiving end connected to the probe head and arranged for collecting the infrared radiation reflected from within said cavity and transmit-ting the reflected infrared radiation to radiation detec-tor means adapted to convert said infrared radiation into a signal representative of the surface temperature of said object, the angular aperture of said optical fiber bundle, the angle defined by said reflective surface and the surface area thereof being such as to substantially prevent stray radiation from being transmitted through said optical fibers.

2. An infrared radiation probe according to claim 1, wherein said object surface is such as to reflect the infrared radiation reflected thereonto by said reflective surface and to provide reflections of specular-diffuse character, and wherein said wedge-shaped cavity has a wedge angle which is function of the space defined between said probe head and said object surface, the surface area of said reflective surface, the angular aperture of said optical fiber bundle and the specular-diffuse character of the reflections at said object surface.

3. An infrared radiation probe according to claim 2, wherein said wedge angle ranges from about 6° to about 12°

4. An infrared radiation probe according to claim 3, wherein said wedge angle is about 10°.

5. An infrared radiation probe according to claims 1, 3 or 4, wherein the angular aperture of said optical fiber bundle ranges from about 15° to about 30°.

6. An infrared radiation probe according to claims 1, 3 or 4, wherein the angular aperture of said optical fiber bundle is about 23°.

7. An infrared radiation probe according to claim 1, wherein said probe head comprises a reflective surface of conical or pyramidal configuration.

8. An infrared radiation probe according to claim 1, wherein said probe head comprises two reflective surfa-ces disposed at an angle relative to one another such as to define with the object surface a reflective cavity having a double wedge configuration.

9. An infrared radiation probe according to claim 8, wherein said reflective surfaces are mirror surfaces.

10. An infrared radiation probe according to claim 9, wherein said reflective mirror surfaces are gold-plated.

11. An infrared radiation probe according to claims 8, 9 or 10, wherein said reflective surfaces are substan-tially flat.

12. An infrared radiation probe according to claims 8, 9 or 10, wherein said reflective surfaces are slightly arcuate.

13. An infrared radiation probe according to claim 1, wherein said probe head has a body including means for circulating a gas flow therethrough and into said cavity to thereby cool said body and maintain a clean gas atmo-sphere in said cavity.

14. An infrared radiation probe according to claim 8, wherein said probe head has a body including means for circulating a gas flow therethrough and into said cavity to thereby cool said body and maintain a clean gas atmo-sphere in said cavity.

15. An infrared radiation probe according to claim 14, wherein said body has a rectangular configuration with opposite ends and a pair of oppositely facing gas chambers are provided one at each end, said reflective surfaces being disposed between said gas chambers, and wherein said gas flow circulating means comprise gas inlet means for admitting gas into said chambers and gas outlet means for discharging said gas from said chambers over said reflec-tive surfaces to maintain said surfaces clean.

16. An infrared radiation probe according to claim 15, wherein said gas chambers are each formed with inner and outer sidewalls, and wherein said gas outlet means comprise a pair of opposed, elongated slots defined in the respective inner sidewalls of said gas chambers and extending adjacent to and parallel with said reflective surfaces.

17. An infrared radiation probe according to claim 16, wherein a gas deflector is arranged inside each said gas chamber to direct the gas flow through said slots and over said reflective surfaces.

18. An infrared radiation probe according to claim 8, wherein the radiation receiving end of said optical fiber bundle is connected to said probe head by means of an elongated connector member secured to said probe head, and wherein said connector member is provided with an inner optical fiber retention head which is movable along the longitudinal axis of said connector member for adjust-ably positioning the radiation receiving end of said optical fiber bundle between said reflective surfaces.

19. A method of measuring the temperature at the surface of an object defining with an adjacent reflective surface a wedge-shaped confined space with both surfaces being essentially at the same temperature, which comprises providing a bundle of optical fibers having a predeter-mined angular aperture with one end of the optical fiber bundle being connected to radiation detector means and directing the other end of the optical fiber bundle toward said confined space to collect the infrared radiation emitted from said confined space and transmit same to said radiation detector means for conversion into a signal representative of the surface temperature of said object.

20. A method according to claim 19, wherein said other end of said optical fiber bundle is provided with a lens to reduce the angular aperture of said optical fiber bundle.