Two-Band Infrared Thermographer for
Standoff Temperature Measurements
Julia R. Dupuis*, David Mansur, Robert Vaillancourt, David Carlson,
Elizabeth Schundler, George Genetti
OPTRA, Inc. 461 Boston St., Topsfield, MA 01983
Phone: (978) 887-6600, Fax: (978)887-0022
www.optra.com
ABSTRACT
OPTRA presents a new approach to remote infrared temperature measurements over mid to long standoff ranges in
varying atmospheric conditions. The sensor is intended as a feedback mechanism for use with the Active Denial System
to insure the target’s skin temperature is within a safe range. OPTRA’s sensor employs a small reflector telescope
followed by a series of interference filters which form spatially identical but spectrally separated images on two
miniature uncooled microbolometer focal plane arrays. On a pixel by pixel basis, we algebraically combine and
normalize the two images. By carefully selecting the spectral passbands of the two images, the mathematical process
yields a result that is substantially free of errors caused by humidity, rain, light fog, and atmospheric carbon dioxide.
The package measures six x six x 18-inches and weighs six lbs. The package includes an on-axis miniature visible
imager, and the graphical user interface presents a fused visible/infrared image with user-defined transparency levels.
The visible/infrared combination provides good spatial resolution at large distances and ease of pointing along with the
accurate temperature measurement across the field of view.
Key Words: infrared thermography, remote temperature measurement, standoff, microbolometer
1.0 INTRODUCTION
The evolution of directed energy (DE) in the form of microwave radiation has enabled a new class of highly controllable
non-lethal weaponry. Such weapons direct a microwave source onto the skin of an adversary at a potentially large
standoff, causing a painful burning sensation owing to the heating of water in a shallow layer of the skin. Because of the
very mild penetration, under controlled conditions the result is harmless yet effective at denying the approaching
adversary. The non-lethal and certainly non-injurious aspect of these weapons relies on the ability to control them,
however, which means being able to measure the amount of heating of the skin. In other words, an absolute, remote
temperature measurement is required. A means for accomplishing this measurement is the motivation for our
development work.
2.0 INFRARED THERMOGRAPHY
Infrared (IR) thermography is a well established technique for remotely measuring the temperature of a surface where it
is impractical or impossible to do so by a contact means. The term thermography denotes an imaging capability, but the
concepts are the same for non-imaging sensors. IR thermography exploits the correlation between the temperature of a
surface and the IR energy emitted by the surface. This relationship is described by Stefan’s Law:
R( T) = σ ⋅ T 4
(1)
where σ is the Stefan-Boltzmann constant (= 5.67×10-8 W/(m2 ·K4 )) and T is the temperature of the surface. The
spectrum of the IR light is described by Planck’s blackbody function.
N(λ, T ) ⋅ dλ =
8πhc
1
⋅ hc λkT
⋅ dλ
5
λ
e
−1
(2)
IR thermographic measurements are typically done in one of two spectral bands which exhibit relatively low absorption
of light by water vapor and carbon dioxide (CO2 ); these are the 3 to 5 µm band and the 8 to 12 µm band and are
sometimes called “atmospheric windows”. For this particular measurement where we are interested in measuring human
Copyright 2006 Society of Photo-Optical Instrumentation Engineers.
This paper will be published in The Proceedings of Enabling Technologies and Design of Nonlethal Weapons and is made
available as an electronic preprint with permission of SPIE. One print or electronic copy may be made for personal use only.
or multiple
reproduction, distribution to multiple locations via electronic or other means, duplication of any material in
* Systematic
Contact Author:
[email protected]
this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.
skin temperature over a range of 35 to 55ºC, it makes radiometric sense to work in the 8 to 12 µm band, as this is where
the radiation is concentrated according to equation 2.
In general, this measurement works well as long as all of the radiation from the surface makes it to the detector.
However, as we step outside and attempt to make this measurement over a large standoff between surface and sensor, the
atmosphere has a strong detrimental impact on the accuracy. The key to making an accurate and absolute IR
thermographic temperature measurement is being able to correct for the effects of absorption due to water vapor and CO2
as well as scattering and absorption due to fog, rain, and snow. Thermographers which simply correlate emitted radiance
with surface temperature will report significant temperature errors in the presence of high humidity, rain, and fog, as a
large percent of the light is attenuated before it reaches the sensor as shown in figure 1. Figure 2 illustrates the
associated temperature error of the integrated radiance approach over a 100 m path with humidity, CO2 , and fog.
Figure 1 shows the transmission
spectra of water vapor, CO2,
advection fog, and rain for high
humidity, moderate radiation fog,
and light rain over a pathlength of
100 m. We impose the
transmission spectra over the
Planck blackbody profile associated
with a 40ºC surface (given by
equation 2 and read off the right yaxis). The attenuation by these
atmospheric constituents results in
substantially less energy arriving at
the sensor relative to what left the
surface. The effect is a significant
perceived temperature error.
Molecular absorption spectra was
obtained through Hitran PC; fog
and rain attenuation spectra was
obtained using Modtran PC.
Figure
Atmospheric Transmission
Transmission
Figure
1: 2:
Atmospheric
1.0
1.4E-03
0.6
8.0E-04
6.0E-04
0.4
4.0E-04
0.2
2.0E-04
0.0
0.0E+00
7.4
8.4
9.4
10.4
11.4
12.4
13.4
wavelength (microns)
CO2
Fog
Figure 2 shows the projected error
of the integrated radiance
measurement over a pathlength of
100 m. Once the sensor is
calibrated in the laboratory, there is
no easy way for it to compensate
for atmospheric absorbance and
scattering in the path of the
measurement when it is taken
outside. The result, as shown on
the right axis, is a significant
temperature error. This error is
particularly problematic in the DE
application because the target is
always reported colder than it really
is; the microwave source operator
may continue to heat the target to a
desired (but erroneously read)
temperature level, resulting in
potentially significant injury.
Rain
Planck
Figure
3: Projected
Temperature
Error for
Integrated
Figure
2: Projected
Temperature
Error
for
Integrated
Radiance
Radiance
Measurement
Measurement
over 100
over
m 100 m
instrument response (RU)
H2O
6.E-03
22.0
5.E-03
21.5
4.E-03
21.0
3.E-03
20.5
2.E-03
20.0
1.E-03
19.5
0.E+00
305
19.0
310
315
320
325
330
target temperature (K)
unattenuated response
attenuated response
error
temperature error (K)
transmission
1.0E-03
radiance (W/cm2ster um)
1.2E-03
0.8
3.0 OUR TECHNICAL APPROACH
In response to this opportunity, OPTRA has proposed and has demonstrated the feasibility of a novel approach to IR
thermography. We have taken advantage of the quasi-symmetric structure of water vapor absorption and radiation fog
attenuation (figure 1) by spectrally splitting the IR image onto two miniature uncooled microbolometer cameras using
interference filters. The two spectral “channels” approximately balance out the effects as shown in figure 3. In addition,
we filter out the absorption due to CO2 by positioning the cutoff of the longwave filter just short of the 13.4 µm
resonance band. By algebraically combining the images on a per-pixel basis and normalizing the result, we are able to
project a surface temperature measurement in high humidity, fog, and rain with minimal error (figure 4).
Figure
OPTRA's Approach
Figure
3:3:OPTRA’s
Approach
1.0
transmission
0.8
0.6
0.4
0.2
0.0
7.4
9.4
11.4
13.4
wavelength (microns)
CO2
Figure 4 shows the
projected
temperature error for
OPTRA’s
thermographer
measuring skin
temperature over
standoffs of 100,
400, and 700 m.
This figure
illustrates that by
spectrally balancing
the attenuation, we
are able to report the
absolute temperature
of a surface with
minimal error at a
standoff of up to
700m.
Fog
Rain
F1
F2
Figure
Figure
5: Projected
4: Projected
Temperature
Temperature
ErrorError
for OPTRA
for
OPTRA
Measurement
Measurement
(Three
(Three
Standoffs)
Standoffs)
5
4
3
temperature error (K)
H2O
Figure 3 shows the two
effective spectral filters
imposed on the
atmospheric constituent
transmission. By
carefully selecting the
edges of the filters, we
balance out the
attenuation in the two
channels, regardless of the
pathlength or humidity or
fog level. Normalizing
the algebraically
combined images corrects
for spectrally flat
attenuation such as that
caused by rain, snow, and
advection fog as well as
other sources of bulk
attenuation.
2
1
0
-1
-2
-3
-4
-5
305
310
315
320
325
330
target temperature (K)
100 m error
400 m error
700 m error
4.0 THE HARDWARE
Figure 5a shows a solid model and figure 5b shows the optical layout of the thermographer system. We use a dichroic
beamsplitter which acts as one filter edge, reflecting the shortwave light and transmitting the longwave. The use of the
dichroic insures that all of the light within the spectral bands of interest is used, thereby supporting the radiometric
performance of the system.
Figure 5a: Thermographer Solid Model
Visible Imager
Primary
Telescope Mirror
Vents
Longwave
Microbolometer
Double-Hulled
Housing
Secondary
Telescope Mirror
Shortwave
Microbolometer
Figure 5a shows our opto-mechanical setup. We employ a gold-coated, six-inch f/1.5 Cassegrain telescope to
collect IR light from the target. The light is then quasi-collimated by a germanium relay lens behind the telescope
and directed to the dichroic beamsplitter which passes the longwave portion and reflects downward the shortwave
portion. The longwave and shortwave portions then pass through a bandpass and edge filter, respectively before
being focused by identical germanium doublets onto the two cameras which record the spatially identical but
spectrally separated images. The system also incorporates a visible imager mounted in front of the secondary
telescope mirror so not to contribute to the obscuration; this on-axis placement prevents any bore-site errors. The
visible image provides roughly twice the spatial resolution and five times the field of view relative to the IR
channel, which helps the operator to point the device. The covers are aluminum with iridite coating, which helps
protect the system from EMI. The telescope is covered up to the visible imager, and the back of the system has a
double hull cover which minimized internal thermal gradients caused by external sources such as the sun. The
entire sensor module shown above weighs just six pounds and measures six × six × 18 inches. All of our optics
were fabricated by Spectral Systems; the microbolometers are (FLIR) Indigo Systems’ ThermoVision® Micron
cameras, and the visible imager was made by Point Grey Research.
Figure 5b: Thermographer Optical Layout
Telescope Primary
and Secondary
Mirrors
Longpass
Filter
Shortwave
µb
Fold
Mirror
Field
Stop
Dichroic
Beamsplitter/
Compensator
Collimating
Lens
Ge
Doublets
Bandpass
Filter
Longwave
µb
Figure 5b shows the optical
layout of the thermographer.
IR light from a region of
interest is collected by the
Cassegrain and quasi
collimated by a germanium
lens. The longwave light
transmits a dichroic
beamsplitter, while the
shortwave light reflects.
The longwave portion then
transmits a bandpass filter
before being focused onto
the longwave
microbolometer by a
germanium doublet; the
shortwave portion reflects
off a fold mirror before
transmitting a longpass filter
and being focused onto the
shortwave microbolometer
by an identical germanium
doublet. The result is two
identical IR spatial images
that are separated in spectral
space.
Of extreme importance for any IR temperature measurement is an ability to compensate for internal temperature drift of
the instrument itself. In our case, under size and weight constraints of maintaining a portable platform, this becomes
increasingly important, as active cooling is not an option. To handle this task we have added an internal, passive (i.e. not
temperature controlled) miniature blackbody which we periodically inject into the field stop at the focal plane of the
Cassegrain. A thermistor mounted to the back of the high-emissivity miniature blackbody provides accurate temperature
information which we use to reset the offset of the thermographer’s calibration tables once every minute of operation.
The solenoid moves the blackbody in and out of the field within three frames of the IR imagers, making the whole in situ
calibration process undetectable to the operator. This is analogous to a flat field correction.
All three images are transferred via IEEE1394 (i.e. Firewire). Our system includes a
separate electronics module which houses
the Firewire modules for the two
microbolometer cameras, a Firewire hub to
handle the transfer of the two IR images and
the visible image, a custom PC board with
power supply to drive the solenoid and
receive thermistor readings, and a USB data
acquisition module which digitizes the
thermistor readings and receives solenoid
commands from the software. The final
component of the thermographer system is a
laptop PC from which we operate the system
using LabView software.
Figure 6 is a photo of the thermographer
sensor head.
Figure 6: Thermographer System
5.0 THE USER INTERFACE
The operator is presented with a fused IR-visible image of the scene of interest. The IR temperature information is
conveyed on color blocks where transparent means the temperature is below 35ºC, blue is between 35 and 40ºC, green is
between 40 and 45ºC, and so on. Red denotes that that part of the scene has registered above 55ºC, the temperature at
which damage to the skin is possible. The user interface allows the operator to control the transparency level between
visible and IR images; it also allows the operator to view the entire visible field of view or crop the visible image to the
same field as the IR channel. We’ve also added a zooming capability. Figure 7 shows the graphical user interface
(GUI).
Figure 7: Thermographer Graphical User Interface
6.0 SYSTEM PERFORMANCE SPECIFICATIONS
Table 1 details the thermographer’s performance specifications based on our optical system and microbolometer
parameters. We throughput match the six-inch f/1.5 telescope to an f/1 imaging system which, when coupled with the 50
µm microbolometer pixels, provides a 329 µrad internal field of view (IFOV) of the IR channel. With this we can
resolve 23 cm (roughly the size of a human face) at a 700 m standoff. The f/1 imaging onto the microbolometer at a 15
Hz frame rate supports the = ± 1ºC temperature resolution or NE∆T. Note that this value will depend on radiometric
efficiency which is affected by atmospheric attenuation. The spectral atmospheric correction and in situ calibration
support the = ± 2.5ºC accuracy.
Table 1: System Performance Specifications
SPECIFICATION
VALUE
UNITS
Target Temperature Range
IR Spectral Range
Temperature Accuracy
Temperature Resolution
Standoff Range *
IR IFOV
IR FOV
VIS IFOV
VIS FOV
Measurement Bandwidth
35 – 55
7.8 – 13.2
= ± 2.5
=±1
200 – 700
329
2.29
125
11.5×8.6
15
ºC
µm
ºC
ºC
m
µrad
degrees
µrad
degrees
Hz
* This is the range over which we guarantee the atmospheric
correction and associated temperature accuracy.
7.0 PERFORMANCE EVALUATION
We calibrated the thermographer using an accurate, extended blackbody source made by Infrared Systems Development
(model IR-140 with model IR-301 temperature controller and RS-232 communications kit). We measured the
instrument internal-radiance-corrected response to the blackbody ramping from 35 to 55ºC; the inverse of the linear fit to
this became our instrument transfer function. With the calibrated system, we performed the following measurements.
NE∆T
We measured the RMS value of the thermographer output response to the calibrated blackbody held at a constant
temperature. This value is the temperature precision of the system. Figure 8 shows the NE∆T measurement for a 5x5
coadded superpixel. The single pixel NE∆T is five times this or about 0.76ºC.
∆T
Figure
NENE∆T
Figure24:
8:Thermographer
Thermographer
2
RMS = 0.15ºC
1.5
degrees C
1
0.5
0
-0.5
-1
-1.5
-2
0
10
20
30
40
50
Frame Number
Repeatability
Figure 9 shows the repeatability data. We recorded four sets of data of the thermographer measuring the blackbody
ramping from 35 to 55ºC and subtracted the average linear fit from all four sets. The average standard deviation or RMS
value of this data represents the sensor’s repeatability. The measured values are given in the figure. In general these
values are within the total accuracy of the calibrated blackbody.
Figure 9 : Thermographer Repeatability vs Target Temperature
Spatial Resolution
We verified our internal field of view (IFOV) projection by illuminating the thermographer aperture with a collimated
blackbody and measuring the full-width and half-maximum (in pixels) of the thermographer response at the center, edge,
and corner of the field. Upon the deconvolution of the results, we verified the IFOV to be within the blackbody angularsubtense-uncertainty of the projected value.
Operating Temperature
During the course of our outdoor testing, we operated over an ambient temperature range of 10 to 30ºC and were able to
achieve the performance reported in the following section. Future testing will cover an operating range of 0 to 40ºC.
Long Range Outdoor Temperature Measurements
Figures 10a and 10b show the outdoor data taken at the standoff ranges denoted in the legend. We present both the
OPTRA measurement as well as the sum response calculated with the same data. The sum response is equivalent to the
current integrated radiance technique where we simply sum the short and long channels and correlate measured radiance
with target temperature from the calibration data set. Note that both channels have been individually corrected for
instrument radiance prior to calculating both the OPTRA measurement and the sum responses, so this is a fair
comparison. The data clearly show that the OPTRA technique offers significantly better accuracy than the sum
technique. Table 2 summarizes the ambient temperature and relative humidity for each measurement.
Figure
Temperature Error
Figure
10a:26a:
Thermographer
Error vs
vsTarget
TargetTemperature
Temperature
OPTRA
Normalized
Measurement
Difference
70
Temperature Error (C)
60
50
40
30
20
10
0
-10
35
40
cal
Temperature Error (C)
60
56 yards
45
Target Temperature (C)
93 yards
124 yards
50
55
160 yards
184 yards
Figure
10b:
Thermographer
ErrorvsvsTarget
Target
Temperature
Figure
26b:
Temperature Error
Temperature
Sum Response
Measurement
Sum Response
50
40
30
20
10
0
-10
35
40
cal
56 yards
45
Target Temperature (C)
93 yards
124 yards
50
55
160 yards
184 yards
Table 2: Data Climatic Conditions
DATA SET
TEMP (F/C)
RH
PWV
56 yards
93 yards
124 yards
160 yards
184 yards
45/7.2
45/7.2
45/7.2
56/13.3
54/12.2
60%
60%
60%
75%
82%
6,013 ppm
6,013 ppm
6,013 ppm
11,300 ppm
11,500 ppm
We are able to report the target temperature (of a blackbody of absolute accuracy, ±1.5ºC) within an error of ±5ºC over
most of the 35 to 55ºC temperature range. The associated error attributed to the thermographer then is ±4.77ºC. The
integrated radiance measurement, in contrast, reported a 55ºC temperature error at the largest range which is over a
factor of 10x worse than the OPTRA system. With some room for improvement to meet the ±2.5ºC accuracy goal, our
system is showing considerable promis e.
Extraneous to the thermographer sources of error are mostly related to the blackbody. As some of our images show
(figure 7), the blackbody surface may not be very uniform. In addition, outdoor operation of the blackbody is difficult in
the presence of sun and wind which have respective heating and cooling effects of the blackbody’s calibrated surface. It
is possible that the effective blackbody accuracy was not within the specified ±1.5ºC particularly for the outdoor
measurements (keeping in mind that the initial calibration was done indoors under controlled environmental conditions).
Measures to correct for these uncertainties may include mounting a number of thermocouples directly to the blackbody
surface which will allow us to measure its temp erature at several places.
8.0 CONCLUSIONS AND FUTURE PLANS
We have shown a promising approach to remote infrared absolute measurement of skin temperature. We executed a
series of outdoor long range measurements, in which we were able to report the temperature of a calibrated blackbody
source to within about ±5ºC over a temperature range of 35 to 55ºC at a standoff of up to 184 yards (168 m). At each
standoff we also produced an integrated radiance measurement which exhibited a 55ºC temperature error at 184 yards.
Our approach showed a significant improvement in accuracy relative to the integrated radiance measurement. Future
improvements will include tighter control over the reference blackbody during outdoor measurements, as we suspect
external conditions such as wind and sun may have contributed errors. Other tests included NE∆T, repeatability,
operating temperature, and spatial resolution. We also produced a graphical user interface with a user-controlled fused
visible/IR image. The temperature information is conveyed by false color which is fused with the visible image. The
visible image also has user-defined electrical zoom capabilities.
Future work will include outdoor field testing with the Active Denial System.
ACKNOWLEDGEMENTS
This research is being conducted under an SBIR Phase II contract funded by the U.S. Marine Corps Systems Command.
Technical Monitors: Mr. Carlton Land, Joint Non-Lethal Weapons Directorate, Quantico, VA and Dr. Patrick A. Mason,
Air Force Research Laboratory, AFRL/HEDR, Brooks AFB, TX. This SBIR was funded to develop the thermographer
as a feedback mechanism for Directed Energy non-lethal weapons where the goal is to make an accurate and absolute
skin temperature measurement at the standoff ranges described in this paper.