Infrared thermometers for minimizing errors associated with ambient temperature transients

A thermopile detector means for a temperature measuring instrument physically and electrically configured to supply an output signal which indicates target temperature substantially independent of the influence of ambient temperature changes. The detector means includes a first thermopile device exposed to radiation from the target and a transducer means, preferably a second thermopile device, shielded from the target and connected in series opposition to the first.

BACKGROUND OF THE INVENTION 
This invention relates generally to noncontact temperature measuring and 
more particularly to improvements in infrared thermometers for minimizing 
errors associated with ambient temperature transients. 
U.S. Pat. No. 4,456,390 issued to K.G. Junkert and H.P. Vosnick, and 
assigned to the assigned of the present application, discloses a portable 
noncontact temperature measuring instrument incorporating improved 
temperature compensating and signal processing circuitry. More 
specifically, the patent discloses an infrared thermometer which utilizes 
a thermopile detector to develop a temperature indicating output signal 
for driving a display. The thermometer includes a temperature sensor, 
(e.g. a diode) thermally coupled to the thermopile to compensate for 
temperature induced variations in the thermopile responsivity. 
Additionally, the instrument incorporates circuit improvements including 
means for compensating for amplifier drift. 
Although the aforementioned improvements markedly enhance the instrument's 
overall performance by minimizing errors attributable to certain sources, 
nevertheless, errors attributable to rapid ambient temperature transients 
can still occur. Such errors are particularly troublesome in portable 
instruments which are typically used in a variety of industrial and energy 
oriented applications involving relatively harsh environments. More 
particularly, users typically subject such portable instruments to extreme 
and rapid ambient temperature changes as would be experienced when moving 
an instrument from a storage office at 72 degrees F ambient to a location 
proximate to an industrial furnace where the ambient may exceed 100 
degrees F and be characterized by drafts of air flowing over the 
instrument. 
More generally, it is not uncommon for users to subject such portable 
instruments to ambient temperatures varying from 20 degrees F. to 120 
degrees F. and changing at a rate in excess of 1 degree per minute. Such 
ambient temperature changes typically produce an intrusion of heat into 
the sensing area or "hot junction" of the thermopile via thermal paths 
created by external thermopile terminals, housing elements, ambient air 
currents, etc. As a consequence, the output signal developed by the 
thermopile can include a component, attributable to the relatively large 
thermal transients piped into the hot junction via the aforementioned 
thermal paths, which effectively swamps the signal component attributable 
to the relatively low level radiation from the target. 
Efforts have been made to exclude or compensate for the effect of these 
transients. For example, it has been suggested that a relatively large 
heat sink be associated with the thermopile to stabilize it against rapid 
ambient temperature changes (e.g., see, U.S. Pat. No. 4,301,682). The 
problem with this approach is that it slows the response time of the 
instrument and requires relatively long waiting periods to avoid erroneous 
readings. An alternative approach has relied on periodically interrupting 
the incoming radiation or temperature indicating signal to re-zero the 
instrument to null out the influence of ambient temperature. Although 
these approaches can prove helpful, they are generally insufficient to 
avoid the overshoot reading errors generally associated with large and 
rapid ambient temperature changes. 
SUMMARY OF THE INVENTION 
The present invention is directed to improvement in temperature measuring 
instruments for minimizing errors primarily associated with rapid ambient 
temperature changes. 
In accordance with one aspect of the invention, an improved detector means 
is provided which is physically and electrically configured to supply an 
output signal which indicates target temperature substantially independent 
of the influence of ambient temperature changes. 
The present invention is based on the recognition that a thermopile device 
supplies an output voltage typically comprised of a first component 
related to the amount of radiation incident on a sensing area and a second 
component related to ambient temperature transients. Based on this 
recognition, an instrument in accordance with the invention utilizes a 
transducer means, preferably a second thermopile device, which supplies an 
output voltage similarly related to the ambient temperature transients and 
is connected in series opposition to the first thermopile device. 
In accordance with a preferred embodiment, the detector means is comprised 
of first and second thermopile elements connected in series opposition. 
The first and second thermopiles are physically arranged so as to be 
exposed to the same ambient temperature but only one of the thermopiles is 
exposed to the target whereas the other is shielded from the target. 
In accordance with another aspect of the preferred embodiment, the first 
and second thermopiles are selected such that the thermopile shielded from 
the target, i.e. "inactive", supplies a higher amplitude output signal 
than the thermopile exposed to the target: i.e. "active". A trim resistor, 
connected in parallel with the inactive thermopile, is selected to null 
the temperature indicating signal supplied by the two series opposed 
thermopiles over a broad range of ambient temperatures. 
In accordance with a further aspect of the preferred embodiment, the active 
and inactive thermopiles are mounted in the same housing so as to 
experience the same ambient temperature. Moreover, an infrared radiation 
barrier is incorporated in the housing to shield the inactive thermopile 
from infrared radiation. 
In accordance with a still further aspect of the preferred embodiment, the 
lead-in conductors for each thermopile are physically configured to 
minimize the intrusion of thermal transients to the thermopile's hot 
junction. More specifically, in accordance with the preferred embodiment, 
the thermopile lead-in conductors are formed by relatively thin narrow 
paths of conductive material laid down in serpentine fashion so as to 
accommodate the maximum length in the available space and thus introduce a 
large thermal resistance.

DETAILED DESCRIPTION 
Attention is initially directed to FIG. 1 which depicts a typical hand held 
battery operated noncontact temperature measuring instrument 7. The 
instrument 7 is intended to be used to measure the temperature of a remote 
body or target 9 which radiates infrared energy. The instrument 7 is 
generally comprised of a handle portion 11 and a barrel portion 12. The 
barrel portion 12 contains detector means which responds to the infrared 
radiation from target 9 to produce a temperature indicating signal for 
driving a temperature display 14. An instrument as depicted in FIG. 1, 
frequently called an infrared thermometer, is disclosed in the 
aforementioned U.S. Pat. No. 4,456,390. That patent discloses circuit 
improvements for compensating for temperature induced variations in 
detector responsivity and additionally for compensating for amplifier 
drift. 
The improvement introduced in the aforementioned patent considerably 
enhance the instrument's overall performance but nevertheless fail to 
eliminate errors attributable to rapid and large ambient temperature 
transients. These errors are particularly noticeable and troublesome when 
infrared thermometers as depicted in FIG. 1 are used in industrial 
applications where they may be moved relatively quickly from a storage 
temperature at 72 degrees F to the vicinity of an industrial furnace, for 
example, where temperatures can readily exceed 100 degrees F. These large 
and rapid ambient temperature changes typically pipe small amounts of heat 
into the detector along various thermal paths and result in overshoots of 
the temperature reading in the direction of temperature change. 
The present invention is directed primarily to an improved infrared 
detector means useful in an infrared thermometer of the type depicted in 
FIG. 1 for minimizing temperature reading errors attributable to ambient 
temperature changes. Although the detector means improvements to be 
discussed hereinafter are applicable to a broad range of infrared 
thermometers, including that depicted in detail in the aforementioned U.S. 
Pat. No. 4,456,390, they will be disclosed herein in association with a 
microcomputer based signal processing system as depicted in FIG. 2. 
FIG. 2 is a block diagram depicting electronic circuitry of an infrared 
thermometer 7 incorporating a detector means in accordance with the 
present invention. Basically, the electronic circuitry is comprised of a 
detector means 20 which supplies an output voltage on terminal 22 
indicating the temperature of the target 9. The details of the detector 
means 20 will be discussed in greater detail hereinafter. Suffice it to 
understand at this point that output terminal 22 is coupled to a switching 
circuit 24, schematically depicted as including single pole single throw 
switch 26 and single pole single switch 28. Switch 26 connects detector 
means output terminal 22 to the input 30 of amplifier 32. Switch 28 
connects the amplifier input 30 to ground through resistor 34. The output 
of amplifier 32 is represented as voltage V1 and is supplied to 
multiplexer 36. 
The detector means 20, as depicted in FIG. 2, is comprised of a first 
thermopile device TP1 and a transducer or second thermopile device TP2. As 
will be discussed in further detail hereinafter, thermopile TP1 is exposed 
to infrared energy radiating from the target 9 to be measured. On the 
other hand, thermopile TP2 is shielded from the infrared radiation. 
However, the thermopile devices TP1 and TP2 are mounted proximate to one 
another so as to be exposed to the same ambient temperature. As is well 
known, thermopile devices are voltage generators which produce a DC output 
voltage related to the infrared energy incident thereon. In accordance 
with the present invention, the devices TP1 and TP2 are connected in 
series opposition between ground terminal 40 and the aforementioned output 
terminal 22. 
In the absence of infrared energy incident on thermopile TP1, the devices 
TP1 and TP2, if perfectly matched, should respond identically to ambient 
temperature conditions and thus generate the same DC output voltages. 
Since the devices are connected in series opposition, this should produce 
a zero output voltage at terminal 22. Although it is theoretically 
desirable to prefectly match the thermopile devices TP1 and TP2, in 
actuality, it is impossible to perfectly match these devices. Accordingly, 
in accordance with the invention, at the time of initially assembling an 
infrared thermometer, the transfer characteristics of a pair of thermopile 
devices are measured. The thermopile device which generates a higher 
output voltage over the desired range of ambient temperatures is then used 
as the inactive shielded device TP2. The larger output of device TP2 is 
then trimmed down with an external loading resistor 44 to equal the 
transient output of the active thermopile device TP1 exposed to the 
radiation. The determination of the appropriate value for trim resistor 44 
is performed at a subassembly fabrication stage by placing the detector 
means 20, while preferably already mounted in the barrel portion 12, in an 
oven and monitoring the voltage output of the thermopile devices at 
terminals 22 and 46 with a voltmeter. Trimming preferably should be 
performed during the temperature transient cycle when the output voltage 
from the series connected thermopiles peaks. At this time a resistance is 
introduced in parallel with thermopile TP2 to zero the output on terminal 
22. The magnitude of the trimming resistor determined while the detector 
means is in the oven is then noted and the closest available standard 
resistor is subsequently installed at 44 during a subsequent fabrication 
stage. Typically, the characteristics of the thermopile devices remain 
constant over their life and thus no further adjustment of resistor 44 
should be necessary over the life of the instrument. 
FIG. 2 depicts a temperature sensor in the form of diode D1 incorporated in 
the detector means 20. Whereas the function of thermopile TP2 is to 
develop a signal to compensate for ambient temperature transients, i.e. 
rapid temperature changes, the function of diode D1 is to develop a 
reference signal indicative of the stable, i.e. nontransient, ambient 
temperature level. The anode and cathode terminals of diode D1 are 
connected to input terminals 50 and 52 of a diode signal conditioning 
circuit 54. Circuit 54 is analogous to circuit 26 in the aforementioned 
U.S. Pat. No. 4,456,390 and provides an output signal V2 which is 
representative of the temperature measured by diode D1. Voltage V2 is also 
coupled to the input of multiplexer 36 and is used to compensate for 
temperature induced variations in the responsivity of detector means 20. A 
third voltage, V3, is also applied to the input of multiplexer 36 and is 
derived from an emisivity control circuit 60 comprised of a fixed resistor 
62 and a variable resistor 64 connected in series. Resistor 64 is intended 
to be manually controlled by a user of the infrared thermometer to adjust 
the instrument for different emisivities of the target. More specifically, 
the emissivity control effectively comprises a gain control and is 
provided to enable the user to adjust the instrument depending upon the 
perceived emisivity of the target. For example, targets comprising perfect 
black bodies have an emisivity of 1.0, by definition, indicating that for 
a given temperature, they radiate as much infrared energy as possible. On 
the other hand, a grey body may typically have an emissivity of 0.8 
meaning that a grey body yields only about 80% of the radiation of a black 
body at a given temperature. Proper use of the infrared thermometer 
requires that the user compensate for the difference in emisivity of a 
body. Accordingly, when the user notes that the target comprises a grey 
body, he will crank the control knob on resistor 64 to effectively 
increase the magnitude of voltage V3 by 1.0/0.8. 
In addition to the aforementioned voltages V1, V2, and V3, a voltage V4 
indicative of battery level is also applied to multiplexer 36. The output 
of multiplexer 36 is applied to an analog to digital converter 68. Thus, 
the particular voltage supplied by multiplexer 36 to the converter 68 will 
result in the converter providing a multibit digital signal output to 
microcomputer 70. The microcomputer 70 adjusts the temperature to produce 
an output signal on line 72 for driving the aforementioned display 14. 
Additionally, the microcomputer 70 is schematically depicted as 
controlling various elements of the circuitry depicted in FIG. 2. For 
example, the microcomputer 70 controls the multiplexer 36 so that the 
voltage V1, V2, V3 and V4 applied thereto are sampled in a desired 
sequence. The microcomputer also controls the switching circuit 24 to 
alternatively define first and second states. During the first or 
operational state, switch 28 is open and switch 26 is closed so that the 
output of detector means 20 is applied from terminal 22 to the input 30 of 
amplifier 32. Periodically, the microcomputer 70 opens switch 26 and 
closes switch 28 to perform a zeroing operation. The performance of 
automatic zeroing is described in the aforementioned U.S. Pat. No. 
4,456,390 which depicts an auto zero circuit 34 which assures 
substantially driftless operation of the temperature indicating amplier by 
periodically reducing the amplifier output signal to zero under no input 
signal conditions. During this time, the bias currents associated with the 
amplifier (e.g. amplifier 32 in FIG. 2) are adjusted to zero the output of 
amplifier 32. 
From the foregoing description of FIG. 2, the overall operation of an 
infrared thermometer in accordance with the present should be understood. 
It is again pointed out that the improvements in accordance with the 
present invention relate to the detector means 20 which provide a 
temperature indicating output signal on terminal 22. The electronic 
circuitry shown in FIG. 2 responsive to the output signal on terminal 22 
have been shown only schematically because various known arrangements are 
suitable for responding to the temperature indicating signal supplied by 
detector means 20. 
FIG. 3 is a sectional view illustrating a first implementation 84 of the 
detector means 20 utilizing two separately housed thermopiles supported in 
the barrel portion 12 of the infrared thermometer 7. Basically, the barrel 
portion 12 comprises a cylindrical housing 80 having an open front 
entrance support web 82. The cylindrical housing 80 is closed at the rear 
by a mirror element 86 which focuses the radiant energy incident thereon 
into horn 88 leading to thermopile device 90. The thermopile device 90 in 
FIG. 3 thus corresponds to the aforementioned thermopile device TP1 
depicted schematically in FIG. 2. A second thermopile device 92, 
corresponding to the aforementioned device TP2, is depicted as extending 
from the support web 82 of the cylindrical housing 80. The casing of 
thermopile device 90 is depicted as having a window 94 for permitting the 
radiation to pass therethrough to the thermopile sensing area 96. In 
contrast, the casing 98 of thermopile device 92 is closed to prevent any 
infrared energy from falling onto the sensing area of the thermopile 92. 
The embodiment of FIG. 3 assumes that the thermopile device 90 and 92 are 
identically constructed except that the casing of thermopile 90 has a 
window therein for passing infrared energy therethrough. 
Although FIG. 3 assumes that radiation first passes through open web 82 and 
is then reflected by mirror element 86 to thermopile sensing area 96, it 
is pointed out that the unit can be alternatively constructed by making 
element 86 a lens and web 82 a closed wall. With such an alternative 
configuration, radiation would enter the housing 80 through lens element 
86 and be focused onto thermopile sensing area 96. 
Although the detector means 20 of FIG. 2 can be implemented as shown in 
FIG. 3 utilizing two structurally separate thermopile devices, it is 
preferable to more intimately arrange the thermopiles to better assure 
their exposure to a common ambient temperature. Accordingly, attention is 
now directed to FIG. 4 which depicts a preferred structural arrangement of 
the detector means 20 in which the two thermopile devices TP1 and TP2 are 
mounted with a common housing or casing 100, preferably comprising a 
standard hermetically scaled TO-5 package. 
The housing 100 comprises an essentially closed wall 104 enveloping a 
cavity 106 and is preferably comprised of a cap member 108 and a bottom 
wall member 110. A window opening 112 is formed in the cap member 108 to 
permit radiation from the target to pass into the cavity 106. A piece of 
optical filter material 114 is mounted immediately adjacent the window 
opening 112. The filter material 114 is preferably selected to define an 
infrared pass band of interest; e.g. 8-14 micrometers or 2.0-2.4 
micrometers. 
In accordance with the present invention, two thermopile devices are 
supported within the cavity 106, being respectively formed on first and 
second insulative sheets 118,120, preferably formed of mylar. The mylar 
sheet 118 is stretched across and supported on a thermally conductive 
substrate 122, preferably of berylia. Similarly the mylar sheet 120 is 
attached to a berylia substrate 124. The substrates 122 and 124 are 
toroidally shaped, having respective central apertures 126 and 128. 
The substrates 122 and 124 are stacked against opposite surfaces of an 
infrared radiation barrier 130. The barrier 130 preferably comprises a 
sheet of mylar having a gold plated surface adjacent to the substrate 122, 
i.e. toward the window opening 112. 
The stacked thermopile mylar layers 118,120, thermally conductive berylia 
substrate 122,124, and mylar barrier layer 130 are supported on 
electrically conductive posts 140, 142 which extend into the cavity 106 
through openings 141, 143, in the bottom wall member 110. The conductive 
posts 140, 142 not only support the aforementioned stack but also provide 
electrical connection to the thermopile devices formed on the surfaces of 
mylar layers 118, 120. This will be described further in connection with 
FIG. 5. 
With further reference to FIG. 4, it is pointed that a plate 150 is 
supported parallel to and spaced from the mylar layer 118 by spacer 
elements 152. Plate 150 includes a central aperture 154 which functions to 
define the boundary of the radiation field passing through to the sensing 
area of the thermopile formed on sheet 118. FIG. 4 does not depict the 
details of the thermopiles devices formed on mylar sheets 118, 120. 
Rather, FIG. 4 merely shows a deposit of smoke black 160 on mylar layer 
118 positioned in alignment with and immediately beneath the field 
defining aperture 154 of plate 150. The smoke black 160 is deposited over 
the hot junction of the thermopile formed on the upper surface of mylar 
sheet 118, as will be discussed in greater detail in connection with FIG. 
5. Smoke black 162 on the bottom surface of mylar sheet 120 covers the 
sensing area or hot junction of the thermopile formed thereon. 
Other aspects of FIG. 4 to be noted are the utilization of a third 
conductive post 166 which is electrically connected to the conductive wall 
104 of the housing 102. Potting material 168 is deposited around posts 
140, 142 and 166. The potting material 168 insulates the posts 140 and 142 
from the conductive wall 104 of the housing 102 as the posts pass through 
openings 141 and 143 in the bottom wall member 110. Additionally, the 
potting material 168 contributes to the hermetic sealing of the housing 
100 to retain an inert gas therein, typically xenon, argon, or nitrogen. 
FIG. 4 further depicts a diode 174 mounted on the lower surface of the 
mylar sheet 120. The diode 174 corresponds to the diode D1 previously 
discussed in connection with FIG. 2. The thermopile element formed on 
mylar sheet 118 corresponds to thermopile device TP1 depicted in FIG. 2 
and the thermopile element formed on mylar sheet 120 corresponds to the 
aforediscussed thermopile device TP2. 
Attention is now directed to FIG. 5 which comprises a sectional view taken 
substantially along the plane 5--5 of FIG. 4 depicting the thermopile 
element formed on the surface of mylar sheet 118 beneath the smoke black 
deposition 160. The configuration of the thermopile formed on the mylar 
sheet 120 is identical to that depicted in FIG. 5. 
Deposited thermopile detectors are well known in the art and are discussed, 
for example, in the aforementioned U.S. Pat. No. 4,456,390. That patent 
references a conventional thermopile detector, such as the Model 1M 
manufactured by Dexter Research Center of Dexter, Mich. Such thermopile 
detectors include a sensing area or hot junction 180 (FIG. 5) and a 
plurality of cold junctions 182. Each cold junction can be considered as 
forming a voltage generating cell with respect to the hot junction 180. 
The multiple voltage generating cells are connected in series to thus 
produce an output voltage across points 184 and 186 which comprises the 
sum of the voltages generated between the hot junction 180 and each of the 
cold junctions 182. To this extent, the thermopile device depicted in FIG. 
5 is conventional. An improved feature in accordance with the invention 
involves the manner of interconnecting the points 184 and 186 to the 
aforementioned conductive posts 140, 142. 
As previously alluded to, as the performance of existing infrared 
thermometers has been improved and users have expressed a desire to use 
the instruments in increasingly harsh environments, errors attributable to 
ambient temperature changes have become a limiting factor on instrument 
performance. The improvements disclosed in this application are intended 
to minimize the errors, e.g. temperature overshoots, attributable to 
ambient temperature changes. It has been recognized in accordance with the 
invention that these errors are, in part, attributable to heat which flows 
from the outside world via thermal paths, e.g. conductive posts 140, 142 
to the hot junction 180. In conventional thermopile detectors, e.g. see 
FIG. 3 of aforementioned U.S. Pat. No. 4,456,390, a relatively wide low 
electrical and thermal resistance path is used to interconnect the posts 
to the voltage generating points 184 and 186. In accordance with the 
present invention, in order to minimize the transfer of thermal transients 
from the outside world along the posts 140, 142 to the hot junction, a 
relatively narrow and thin conductive pattern 190 is deposited on the 
mylar sheet 118 to define the lead-in connections between the posts 140, 
142 and the voltage output points 184, 186. Most significantly, the 
conductive path 190 is formed in an essentially serpentine fashion to 
accommodate a very long length of lead-in connector, within the available 
space on mylar sheet 118. The utilization of a thin, very long lead-in 
conductor pattern yields a high thermal resistance path between the posts 
140, 142 and the thermopile voltage thermal output points 184, 186 
respectively. As a consequence, heating of the hot junction 180 by heat 
piped along the posts 140, 142 is minimized. 
From the foregoing, it should now be apparent that an improved detector 
means particularly useful in a portable infrared thermometer has been 
disclosed herein for minimizing temperature reading errors attributable to 
ambient temperature changes. Significantly, the improved detector means 
utilizes first and second thermopile devices which are respectively 
exposed to and shielded from the target being measured. The two thermopile 
devices are mounted proximate to one another so as to experience the same 
ambient temperature changes and they are electrically connected in series 
opposition so as to hull the effect of ambient temperature changes. In 
accordance with a preferred aspect, the shielded thermopile device is 
selected to have a higher voltage output than the exposed thermopile 
device and a trim resistor is utilized to assist in identically matching 
the effective transfer characteristics of the two thermopile devices. In 
accordance with further aspects of the preferred embodiment, the two 
thermopile devices are closely mounted with the same housing to achieve 
optimum proximity. They are separated by a radiation barrier so as to more 
efficiently direct radiation from the target onto the active thermopile 
device TP1 and better shield the inactive thermopile device TP2. 
Additionally, each of the thermopile devices is preferably constructed in 
a manner to have high thermal resistance lead-in conductor paths 
connecting the detector external posts to the voltage output points of the 
thermopiles. The introduction of high thermal resistance lead-in paths, as 
by utilizing the serpentine pattern, minimizes the introduction of heat 
from the environment to the hot junction. 
Although a preferred embodiment of the invention has been disclosed herein, 
it is recognized that various equivalent modifications may now become 
obvious to those skilled in the art and it is accordingly intended that 
the appended claims be interpreted to include such modifications.