Infrared thermometer with fiber optic remote pickup

An infrared thermometer (10) with a fixed aperture (99), focusable remote pickup head (12) conveys infrared light (16) from a target (14) to a photosensor (52) of a defocused sensor head (20). The remote pickup head (12) has a reimaging lens (84) and means for selectively adjusting the focus of the lens (84) relative to the target (14) after the pickup head has been fixedly mounted. The aperture setting is kept fixed during the focus adjustment to eliminate the need to recalibrate after the focus adjustment. The defocused sensor head (20) has a defocused relay lens (50) which is selectively out of focus to convey a slightly blurred, out of focus image of the output end of the fiber optic cable (18) to the photosensor (52) to reduce noise, average the light signal and reduce calibration difficulties due to high intensity spots in the image field and to make the unit less susceptible to misfocusing. Precision is further enhanced by provision of a field stop member (86) with a narrow slot, for instance, for passing light only to selected parts of the input end of the fiber optic cable (18) which makes up the optical field.

BACKGROUND OF THE INVENTION 
Infrared thermometers are well known instruments for remotely determining 
temperature when it is neither possible nor practical to use a direct 
temperature sensing probe. The infrared light emitted by a target is 
transmitted to a photosensor which converts the light to electrical 
signals that are then processed or analyzed for conversion into a visual 
indication of temperature. 
Reference can be made to U.S. Pat. Nos. 2,968,946 of Goldberg et al.; 
4,527,869 of Irani et al. and patents and other references cited therein 
for a more detailed description of such infrared thermometers. 
More recently, infrared thermometers have been developed which employ fiber 
optic cables to transmit light from a remote pickup head containing a 
fixed focus reimaging lens system to a photosensor of a sensing head which 
converts the light to electrical signals. These signals are then passed to 
a processor, or analyzer, which converts the electrical signals to a 
visual indication of temperature. 
These fiber optic infrared thermometers have some disadvantages which 
hinder their use or interfere with their accuracy. Known remote pickup 
heads have employed fixed focus lens systems, such that if it is desired 
to focus on a different target, the entire remote pickup head must be 
moved relative to the target. This requires relative crude adjustment of 
the mounting mechanism which holds the remote pickup in place each time it 
is desired to change focus and renders set up of the instrument difficult. 
Alternatively, remote pickup heads have been provided which allow for 
adjustment of the focus but only at the expense of destroying calibration 
during the adjustment process. 
Another problem has been due to the fact that at the sensor head, the light 
from the end of the fiber optic cable from the target is passed to the 
photosensor by means of a lens system focused directly on the photosensor. 
This results in sensitivity and calibration difficulties due to high 
intensity spots at the end of the fiber optic cable and resultant 
calibration problems due to improper focusing. 
Known fiber optic infrared temperature sensors also lack provision of a 
selectable field stop, such as an elongated slot shaped field stop. 
Accordingly, in these sensors the target's selectivity cannot be improved 
by arranging to have the light from only a selected portion of the entire 
field of focus impinge on the fiber optic cable and, thus, on the 
photosensor. While it is known to use such field stops in lens systems 
fixedly attached to the sensor head, in known fiber optic thermometers the 
effective fields have been established by the size and shape of the fiber 
optic cable, itself, and no means have been provided of selectively 
changing the field of view. 
SUMMARY OF THE INVENTION 
It is therefore the principal object of the present invention to provide an 
infrared thermometer with a fiber optic remote pickup and method which 
overcomes the disadvantages and problems of known infrared thermometers. 
In the present invention, a remote pickup assembly is provided in a remote 
infrared thermometer having a sensing head for converting light signals 
applied to a light input port thereof to corresponding electrical output 
signals representative of temperature and an analyzer for converting the 
electrical signals to an indication of temperature, comprising a fiber 
optic cable assembly and a remote, reimaging lens assembly which is 
focusable. The fiber optic cable assembly includes a flexible fiber optic 
cable with an input end and an output end thereof for transmitting light 
therebetween, a first connector for mechanically connecting the output end 
of the fiber optic cable to the light input port of the sensing head, and 
a second connector for mechanically connecting the input end of the fiber 
optic cable to a light output port of the focusable reimaging lens 
assembly. The focusable reimaging lens assembly includes a protective lens 
housing having an input end for gathering light from the target and a 
lens, an output port housing having an output port for receipt of light 
focused thereon by the lens, means mounted within the output port housing 
for defining a fixed aperture intermediate the lens and the outlet port 
and focusing means. The focusing means comprises means for mounting said 
output port to said protective lens housing for movement relative to said 
lens to change the focus of light thereon and on the input end of fiber 
optic cable connected thereto. 
With this infrared thermometer, the object of the invention is achieved, in 
part, through provision of a method of using a remote infrared thermometer 
to determine the temperature of an target comprising the steps of mounting 
an analyzer at a first location remote from the target to provide and 
indication of temperature based on electrical signals from a photosensor 
head electrically connected therewith, mounting the photosensor head at a 
location remote from the target, mounting a focusable lens assembly with a 
preselected aperture at a fixed location with respect to the target but 
within focusable range of the target, adjusting the focusable lens 
assembly while keeping said aperture fixed after the assembly is mounted 
at said fixed location to focus the lens assembly on a selected target, 
and passing the light from the lens assembly to a remote photosensor of 
the photosensor head via an elongated fiber optic cable for conversion 
thereby to corresponding electrical signals representative of the light 
from the target on which the lens assembly is focused. 
The objective of the invention is also achieved in part through provision, 
in a remote infrared thermometer having a sensing head for connecting 
light signals applied to a light input port thereof to corresponding 
electrical output signals representative of temperature and an analyzer 
for converting the electrical signals to an indication of temperature, a 
remote pickup assembly having a selected field stop for blocking light to 
the input end of the fiber optic cable. This pickup assembly with selected 
field stop capability preferably comprises a remote reimaging lens 
assembly including a protective housing having an input end for gathering 
light from a target and an output end, a lens within the housing, and a 
fiber optic cable for conveying light from the remote reimaging lens 
assembly to the light input port of the sensor head and a field stop 
assembly. The field stop assembly includes a field stop and means for 
mounting the field stop between the reimaging lens assembly and the fiber 
optic cable to block the light to all but selected parts of the end of the 
fiber optic cable. 
The objective of the invention is also obtained in part by providing, in a 
remote infrared thermometer having a sensing head with a housing and an 
electronic photosensor within the housing for converting light received 
thereby to corresponding electrical signals representative of temperature, 
a remote pickup assembly for transmitting light from a remote target to 
the photosensor with a relay lens intermediate the end of a fiber optic 
cable and the photosensor which is defocused with respect to the 
photosensor. The remote pickup assembly preferably comprises an elongated, 
fiber optic cable having an input end connected with a lens system for 
receipt of light from a target and an output end connectable with the 
housing, means associated with the housing for connecting the fiber optic 
cable thereto with the output end located at a preselected focal plane 
within the housing, a relay lens with a focal point mounted within the 
housing for relaying light received from the light output end of the fiber 
optic cable to the photosensor, and means for mounting the relay lens 
within the housing with its focal point spaced from the electrical 
photosensor to slightly blur the light image of the end of the fiber optic 
cable which is relayed to the photosensor. 
Thus, the objective is also achieved through provision of a method of using 
a remote infrared thermometer to determine the temperature of a target, 
comprising the steps of transmitting light from a lens system focused on a 
target through an input end of a fiber optic cable, receiving the focused 
light from an output end of the fiber optic cable at a sensor location 
remote from the target and lens system, passing the light from the output 
end of the fiber optic cable to a photosensor at the sensor location 
through a relay lens which is slightly unfocused with respect to the 
photosensor to slightly blur the image of the output end of the fiber 
optic cable relayed to the photosensor, and converting electrical signals 
produced by the photosensor in response to the blurred image to an 
indication of temperature.

DETAILED DESCRIPTION 
Referring now to FIG. 1, the remote infrared thermometer 10 of the present 
invention is seen to include a focusable remote pickup head 12 for 
collecting light emitted by a light emitting target 14 of a temperature 
responsive process such as a steel or glass manufacturing process. The 
target is customarily a particular part of a mass of heated material which 
is emitting light because of its relatively high temperature. A 
significant portion of this light is in the portion of the light spectrum 
known as the infrared spectrum. The infrared light 16 is transmitted 
through a lens system of the focusable remote pickup head 12 to an input 
end of a flexible fiber optic cable 18 of approximately three to six feet 
in length or longer to a defocused sensor head 20. The defocused sensor 
head contains a photosensor circuit which responds to the infrared light 
received from the end of the fiber optic cable 18 to provide corresponding 
electrical signals representative of temperature. The electrical 
temperature signals are conveyed by an electrical data cable to a 
processing, analyzing indicator 24 which both analyzes the temperature 
signals to convert them to temperature and then selectively shows the 
temperature on a temperature display 26. Control switches 27 provide 
control over various indication parameters, power, etc. Alternatively, 
some of the processing is performed by circuitry associated with the 
sensor head before signals are conveyed to the indicator 24. The 
processing or analyzing circuitry which perform the functions of the 
indicator 24 forms no part of the present invention and is conventional. 
Referring to FIGS. 2A, 2B, 2C and 2D, the defocused sensor head 20 has a 
housing 28 to which is mounted the output end 30, FIG. 2D, of the fiber 
optic cable 18 by means of a suitable threaded, releasible cable connector 
31. The input end 77, FIG. 3B, of the fiber optic cable 18 is connected to 
the focusable remote pickup head 12 by a similar releasible, threaded 
connector 33. On the interior of a forward portion 32 of the housing 28 is 
located a relay lens 50 which conveys light to a photosensor element 52 
protectively contained within the housing 28. Referring to FIG. 2D, in 
keeping with one aspect of this invention, the light from the output end 
30 of the fiber optic cable 18 gathered by relay lens 50 is focused not at 
the electronic photosensor element 52, but, instead, is focused at a focal 
point 54 slightly in front of the photosensor element 52 to transmit a 
slightly blurred image to the photosensor element 52. This advantageously 
reduces noise sensitivity, averages the light signal received by the 
photosensor and makes the thermometer less sensitive to dimensional 
tolerances of the location of the relay lens 50 relative to the output end 
of the fiber optic cable 18. A dichroic mirror 55 reflects part of the 
light to the focal point 54, so light from a site 56 aligned with the 
focal axis of lens 50 can be passed through lens 50 and the remainder of 
the system for alignment purposes. 
The output end 30 of the fiber optic cable 18 is held in a plane spaced 
from the relay lens 50 by a preselected distance to achieve the desired 
out of focus, or defocusing, function by means of connector 31. However, 
it is desired to locate the center of the output end 30 of the fiber optic 
cable 18 in direct alignment with the focal axis 36 of the relay lens 50. 
This is achieved during set up by attaching the output end 30 to the 
housing 28 by means of an optical aligner 34. Referring also to FIG. 2B, 
the aligner 34 can be adjusted radially relative to the focal axis 36 by 
means of three identical fasteners 57, FIG. 2D, screwed into the front end 
of housing 28 through radial alignment slots 38. After adjustment is made 
for an optimum signal, the fasteners 57 are tightened to hold the optical 
aligner 34 in place. 
Referring also to FIG. 2C, the back 40 of housing 28 has a multipin 
connector receptacle 42 for connecting the photosensor circuitry to the 
electrical data cable 22. If the housing 28 contains some signal 
processing circuitry, it is also provided with an adjustment screw 44 for 
changing emissivity related circuit parameters of the photosensor circuit 
and an invalid alarm indicator 46. 
Referring now to FIGS. 3A and 3B, the remote, focusable pickup head 12 is 
seen to include a two part tubular housing having a relatively enlarged 
protective lens housing section 58 with an objective lens 84 and 
relatively narrow outlet port, or output port, housing section 60 received 
within a bore 62 in the protective lens housing section. The protective 
lens housing section 58 is fixedly mounted to an L-bracket 64 by means of 
a nut 66 secured to a forward threaded part 68 of protective lens housing 
section 58. Alternatively, the protective lens housing section 58 is 
secured by means of a strap 72 around its outer surface. A screw 76 is 
used to tighten the strap 72 after the housing section 58 has been placed 
within focusable range of the target 14. Then housing section 60 can be 
moved in and out to focus on the target. Also, attached to the protective 
lens housing section 58 is an air purge fitting 70 for passage of 
pressurized clean air into the protective lens housing 58 to protect the 
lens 84 against dirt laden ambient air. 
Referring particularly to FIG. 3B, the outlet port housing has a 
cylindrical bore 78 which has a light outlet port 82. The outlet port 82 
is defined by a hole in the bottom of one of two cylinders 92 and 94 which 
are received within the bore 78 in end to end relationship. A baffle 
opening 96 in the closed bottom of cylinder 94 allows passage of light 
focused by lens 84 to be directly conveyed to the outlet port 82 at the 
bottom of cylinder 92. The peripheral edge of cylinder 92 abuts against 
the bottom of cylinder 94. A retainer 98 which is threadably fastened 
within the open end of bore 78 abuts against the open end of cylinder 94 
to properly align the baffle opening between the light output port 86 and 
the lens 84. Advantageously, both cylinders 92 and 94 are substantially 
identical to reduce cost. 
An important feature of the invention is that the retainer 98 defines a 
fixed aperture, or aperture stop 99 which is mounted within the outlet 
port housing section 60 at a fixed distance from the outlet port 82. 
Accordingly, the aperture 99 and not the distance between the lens 84 and 
the outlet port 82 determines the quantity of light being received by the 
fiber optic cable 18 for a given temperature of the target 14. Since the 
aperture remains fixed during focusing, the relationship between the light 
quantity and the temperature remains fixed. Accordingly, all calibration 
can be accomplished at the factory before focusing and the need for 
recalibration for different focus settings is eliminated. 
In keeping with another aspect of the invention, a field stop member 86 
overlies the outlet port 82 to block the transmission of light from the 
lens 84 to the input end 77 of the fiber optic cable 18 except for that 
permitted to pass through a slot formed in the field stop member 86. The 
field stop member alternatively has a circular field stop opening smaller 
than the input end of the fiber optic cable 18 to increase selectivity. In 
either event, the field stop member 86 is releasably held in place against 
the outlet port by the inlet end of the fiber optic cable 18 against which 
it abuts. 
After the fiber optic cable 18 has been interconnected through means of 
connector 33 with outlet port 82 and field stop member 86 and the 
protective lens housing section 58 has been fixedly secured by means of 
strap 76 and possibly by connection with air purge fitting 70 within 
focusable range of the target 14, the focusable remote pickup head 12 is 
focused on a selected target 14. The outlet port housing 60 is slid within 
bore 62 to a selected position at which the light from a target 14 at a 
selected distance from the lens 84 and inlet end 80 is focused through 
outlet port 82 and field stop member 96 onto the input end of the fiber 
optic cable 18. Decreasing the distance between the input end of the fiber 
optic cable 18 and the lens 84 proportionately increases the distance 
between the focusable remote pickup head 12 and the target 14 and vice 
versa. 
After focusing on the selected target 14 has been accomplished, the 
position of the outlet port housing 60 relative to the protective lens 
housing 58 is locked in place by means of a set screw 100. Measurements 
without recalibration are then possible because of the presence of the 
fixed aperture 99. The set screw 100 is threaded through a screw hole in 
the side of the outlet port and tightened down onto the bottom of the 
groove 101 extending along the side of the outlet port housing 60. 
Advantageously, this focusing feature enables fine focus adjustments which 
would not be possible by moving the mounting of the entire remote pickup 
head 12 relative to the target 14. 
Referring to FIG. 4, another embodiment of the sensor head is shown in 
which the air purge fitting 70 has been eliminated and the pickup head 12 
has a mounting nut 104 fastened to the threaded part 68 of the protective 
lens housing 58. Otherwise, it is functionally structured identically to 
that shown in FIG. 3B. 
While a particular embodiment has been shown and described, it should be 
appreciated that the breadth of the invention is not limited to this 
detailed description but, instead, is defined by the following claims.