Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1319832
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.
- 1 - ~F
1 3 1 9832
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
1 3 1 9832
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
1319832
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
-- 4
1 31 9832
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.
1 31 9832
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 _
1319832
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
-- 10 --
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.
1319832
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,
1319832
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
- 14 -
1319832
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.
1 31 9832
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
- 16 -
1 31 9832
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.
