Aural non-detectability of portable HOT infrared imagers
Alexander Veprik
RICOR, Cryogenic & Vacuum Systems, En Harod Ihud, 18960, Israel
ABSTRACT
Further shrinking size, weight, power consumption of the High Operating Temperature (HOT) infrared (IR)
Integrated Detector-Dewar-Cooler Assemblies (IDDCA) eventually calls for development of high-speed cryocoolers. In
case of integral rotary design, the immediate penalty is the more intensive slapping of compression and expansion
pistons along with intensification of micro collisions inherent for the operation of crank-slide linkages featuring ball
bearings. Resulting from this is the generation of impulsive vibration export, the spectrum of which features the driving
frequency along with numerous high-order harmonics covering the entire range of audible frequencies.
In a typical design of an infrared imager, the metal light-weight enclosure accommodates a directly mounted
IDDCA and an optical train, thus serving as an optical bench and heat sink. This usually results in excitation of structural
resonances in the said enclosure and, therefore, in excessive noise generation compromising the aural stealth, especially
during the cooldown (boost) phase of operation.
The author presents the complex approach to a design of aural undetectable infrared imagers in which the IDDCA
is mounted upon the imager enclosure through a silent pad. Special attention is paid to resolving the line of sight
stability and heat sinking issues.
The demonstration imager relying on Ricor K562S based IDDCA meets the most stringent requirement to 10
meters aural non-detectability distance (per MIL-STD 1474D, Level II) even during boost cooldown phase of operation.
Keywords: infrared imager, cryogenic cooler, aural nondetectability, vibration, noise, structural resonance, vibration
mounting, silent pad.
1. INTRODUCTION
In spite of the recent advances and widespread use of uncooled IR technology it is still generally acknowledged that
the “best technology for true IR heat detection is the cooled detectors” [1]. They are superior to the uncooled rivals in
terms of working ranges, resolution and ability to detect/track fast moving objects in dynamic infrared scenes. The
superior performance of such imagers is achieved by using advanced optronic technologies along with maintaining the
IR detector at cryogenic temperatures (77K, typically) using mechanical closed-cycle Stirling cryogenic coolers.
Unfortunately, such imagers appear to be too expensive in buying and use, too bulky, too power thirsty, too noisy for a
massive deployment. Along with these lines, additional preventing factor is awkward batteries supply/recharge logistics.
Over the past few years the industrial progress has led to the development of a new Mercury Cadmium Telluride
(MCT) n/p and p/n technology [2,3] offering an attractive opportunity to operate IR detectors at essentially higher
temperatures (up to 200K) at extremely low rate of defective pixels and dark currents without compromising
performance in a middle wavelength (MW) and long wavelength (LW).
More complex nBn infrared detector architecture is a relatively new concept. It was first introduced by Maimon and
Wicks [4] and has surprised many with both simplicity and level of performance [2,3]. This technology shows good
potential to operate at even higher temperatures. It is not a surprise, therefore, that major market players (DRS
Technologies, Raytheon, Teledyne, Sofradir, Selex Galileo, AIM and SCD) continue exploring existing and future MCT
opportunities.
The direct benefits of using such HOT IR detectors are the lowering the optical, cooling and heat sinking constraints.
This eventually results in simplified and more compact night vision instruments allowing using, in particular, rotary
integral cryocoolers with improved size, weight and power consumption (SWAP) indices. These improvements are
attainable by improving thermodynamic performance, lowering parasitic (conductance and radiation) losses and
increasing operational speed leading to more compact design and improving performance of rotary drivers.
The unfortunate penalty of increasing driving speed during both the boost cooldown and temperature regulation
operational modes is the more intensive slapping of compression and expansion pistons along with intensification of
micro collisions inherent for the operation of crank-slide linkages featuring ball bearings. This results in disproportionate
export of wideband vibration featuring driving frequency and its numerous multiples [5,6].
In a typical design of infrared imager, for the sake of weight saving and compactness, the metal light-weight
enclosure usually serves as the optical bench and heat sink, thus accommodating the directly mounted IDDCA and the
entire optical train. Because of the finite stiffness and low damping, such enclosures normally show numerous undamped
structural resonances over a wide frequency range. The above cooler-induced vibration export is, therefore, easily
transformed into a resonant structural vibration and, consequently, into a wideband audible noise, the spectrum of which
comprises specific resonant peaks correlating closely with the frequencies of the above structural resonances [5,6]. From
experience, even a silent IDDCA may produce quite a lot of aural structure-born noise when mounted upon a wrongly
designed and resonating enclosure of an IR package. The "noisy" thermal imager may be detected from quite a long
distance using special sniper detecting acoustic equipment relying upon a high-sensitive unidirectional microphones or
audibly spotted when used in a close proximity to the opponent force. As a result, aural stealth along with enhanced
imagery, compactness, low power consumption and long life-times becomes a crucial figure of merit characterising the
modern infrared imager [1].
(a)
(b)
Figure 1. Acoustically silent hand-held IR imager with vibration mounted IDDCA
The authors of [5,6] attempted to enable effective use of a camera enclosure as a vibration isolated and damped
acoustic envelope, see Figure 1.a. This objective has been achieved by making use of the vibration mounting of an
IDDCA with the purpose of wideband dynamic damping of the enclosure structural resonances over the typical midfrequency range and vibration isolation over the typical high-frequency range. Based on the theoretical study, the above
vibration protective pad (see Figure 1.b) has been optimally stiffened and damped for achieving the best acoustical
performance without developing excessive cooler–induced line of sight jitter. The vibration protective pad in Figure 1.b
features the top and bottom interface plates which are bonded using the silicon mold with no metal-to-metal contact.
Special technologies were developed for the final accurate machining of the pad, thus assuring precision mounting of the
IDDCA package upon the camera enclosure and reliable alignment of the optical axis. The authors reported on the
successful effort of designing the inaudible at greater then 10 meters cryogenically cooled infrared imager complying
with the stringent MIL-STD-1774D (Level II) requirements in the temperature control operation mode.
Unfortunately, such vibration protective pad appeared to be not efficient enough for use with modern IR imagers
featuring high speed cryocoolers. This called to a development of novel vibration protective pads allowing for
compliance with the requirement of 10m inaudibility in both boosted cooldown and temperature control operational
modes.
2. CRYOCOOLER INDUCED VIBRATION EXPORT
Prior to designing the vibration protective pad it was very instructive to study the nature of cryocooler induced
vibration export and noise. Figure 2 shows the high-speed Ricor model K562S cryocooler mounted on the Kistler type
9272A dynamometer measuring vibration export in three mutual perpendicular directions.
Figure 2. Testing of vibration export.
Figure 3 shows the typical time histories presenting the coolerinduced vibration export in piston (X1) and displacer (X2) axes
during the boost cooldown mode. From Figure 3, the cooler’s
operation is strongly non-smooth – the well-pronounced alldirectional sharp impulses manifest the cyclic (twice per revolution)
side slapping of compression and expander pistons. Figure 4 shows
the appropriate spectra of cooler induced vibration export along the
piston (G1,1) and displacer (G2,2) axes, respectively, during the
boost cooldown. The two spectra in Figure 4 are comprised of the
distinctive harmonics of the fundamental frequency 106.9 Hz, which
appears to be the driving frequency at this operational mode.
Figure 5 shows the typical time histories presenting coolerinduced vibration export along the piston (X1) and displacer (X2)
axes, respectively, during the temperature control mode. From Figure
5, the cooler operation is again non-smooth and accompanied by the
cyclic side slapping of compression and expander pistons manifesting
itself in the form of well-pronounced all-directional sharp impulses.
Figure 3. Time histories of vibration export in boost cooldown mode
Figure 4. Spectral presentation of vibration export in boost cooldown mode
Figure 5. Time histories of vibration export in temperature control mode
Figure 6. Spectral presentation of vibration export in temperature control mode
Figure 6 shows the spectra of cooler induced force export along the piston (G1,1) and displacer (G2,2) axes
during the temperature control mode. The two spectra in Figure 6 are comprised of the distinctive harmonics of the
fundamental frequency 45 Hz – the driving frequency at temperature control mode.
From Figures 3-6, the vibration export during both distinctive operational phases is of impulsive nature;
resulting from this is a typical spectra shape comprising distinctive harmonics of the driving frequency covering the
entire range of audile frequencies. It is worth noticing that during the boost cooldown mode the collisions are more
intensive and frequent, as compared with temperature control mode. This explains difference in the cooler noise
radiation.
Figures 7.a,b show narrowband spectra of sound pressure (SP) and 1/3 octave band spectra of sound pressure
level (SPL), respectively, produced by the freely hanged IDCCA during the above cooldown boost and temperature
control modes. The measurements were taken in the semi-anechoic chamber, measurement distance was 1m. From
Figure 7, the noise radiated by the cooler in the boost cooldown mode is essentially higher; this outcome follows from
the increased intensity and frequency of side piston slaps. The structure on the narrowband spectrum follows this of
vibration export – it is comprised of the distinctive harmonics of the driving frequency. From Figure 7, the detached
cooler itself radiates essential level of aural noise making it aural detectable from 30 meters per MIL-STD 1474D.
It is important to notice that the information on the noise produced by the separated IDDCA can be used for
assessing the technical condition, but cannot be used for predicting noise radiation by the entire IR imager.
1.E-03
Background
Detached, boost
Detached, temperature control
8.E-04
SP, Pa
6.E-04
4.E-04
2.E-04
10000
9000
8000
7000
6000
5000
4000
3000
2000
0
1000
0.E+00
Frequency, Hz
(a)
50
SPL, dB re 20 µPa
Background
Detached, boost
Detached, temperature control
40
30
20
10
10000
8000
6300
5000
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3150
2500
2000
1600
1250
1000
800
630
500
400
315
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125
0
1/3 octave band central frequency, Hz
(b)
Figure 7. Acoustic noise produced by the detached IDDCA in the boost cooldown and temperature control modes
In a typical design, the said IDDCA is rigidly mounted upon the usually lightweight metal enclosure
accommodating the entire optical train and serving, therefore, as the optical bench and heatsink. The above explained
wideband vibration export may result in an excitation of usually undamped structural resonances in the said enclosure;
this in turn results in excessive noise radiation and, therefore, increased range of aural detectability. From the author’s
experience, some IR imagers working in the temperature control mode are aural detectable from a distance of up to
200m; the aural stealth in the boost cooldown mode is sometimes provided by running the imager in a special
acoustically treated bag.
3. VIBRATION MOUNTING AND
ATTENUATION OF CRYOCOOLER
INDUCED AURAL NOISE
Figure 8 shows the schematics of vibration
mounted IDDCA incorporating the heat sinking
arrangement. In Figure 8, the IDDCA is mounted
upon the vibration protective pad formed by the
upper and bottom plates interconnected by the
silicone mold. The two heat conductive bosses are
connected to the above plates and face each other
with the small clearance (app. 0.3mm); they are
located in the cavity formed inside the said silicone
mold, which is filled by the heat conductive semiflowable thermal paste Lambda Gel DP-100, see
Figure 8. Schematics of vibration mounted IDDCA
http://www.taica-sh.com.cn/product/pro_08-1.htm). Such an arrangement provides for sufficient heat conductance from
upper plate to the bottom plate which is in turn connected to the system enclosure.
Vibration
protective
pad
IDDCA
Front
support
IDDCA
Front
support Vibration
protective
pad
Figure 9. Design of technology demonstrator
Figure 10. Vibration protective pad
The excessive line of sight jitter resulting from torque-wise vibration export about the motor axis is closely
controlled by the front support made of the plate and Silicone O-ring supporting the front portion of the evacuated
envelope. In the final design, the front support may incorporate the two-axis adjustment arrangement.
The acoustic testing has been performed in the semi-anechoic chamber using the thin walled aluminum dummy
box mimicking the enclosure of the IR imager and allowing for direct and vibration mounting of the IDDCA. Figure 9
shows CAD images and picture explaining the position and mounting of the IDDCA inside the box; for the direct
mounting the all-metal replica of vibration protective pad has been used. Figure 10 shows the vibration protective pad.
Due to the improved technology, the sufficiently high – 0.05mm –parallelism of mounting surfaces has been attained
practically without machining the compliant parts.
Noise radiation by enclosure
It will be very instructive now to compare the acoustic noise produced by the detached cooler and this by the
metal enclosure subjected to the action of the above vibration export (case of rigid mounting). Figure 11, for example,
compares narrowband sound pressure (a) and 1/3 octave band (b) sound pressure level spectra produced by the detached
cooler and this of enclosure mock-up during the cooldown boost mode. In Figure 11.a, in particular, we observe the
excitation of numerous structural resonances over the frequency range up to 8kHz. This results in essential increase of
sound pressure levels over the entire range of audible frequencies as clearly seen in Figure 11.b.
0.004
0.0035
Background
Direct mounting, boost
Detached, boost
0.003
SP, Pa
0.0025
0.002
0.0015
0.001
0.0005
0
0
1000
2000
3000
4000
5000
Frequency, Hz
6000
7000
8000
9000
10000
(a)
50
SPL, dB re 20 µPa
Background
Detached, boost
Direct mount, boost
40
30
20
10
10000
8000
6300
5000
4000
3150
2500
2000
1600
1250
1000
800
630
500
400
315
250
200
160
125
0
1/3 octave band central frequency, Hz
(b)
Figure 11. Noise at cooldown boost mode
Similar situation takes place during the temperature control mode. Figure 12 compares narrowband sound pressure
(a) and 1/3 octave band (b) sound pressure level spectra produced by the detached cooler and the enclosure mock-up
carrying the directly mounted cooler during. In Figure 12.a we observe excitation of numerous structural resonances over
the frequency range up to 5kHz. This results in essential increase of sound pressure levels over the entire range of
audible frequencies, as seen in Figure 12,b.
0.004
Background
0.0035
Direct mount, temperature control
Detached, temperature control
0.003
SP, Pa
0.0025
0.002
0.0015
0.001
0.0005
0
0
1000
2000
3000
4000
5000
Frequency, Hz
6000
7000
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9000
10000
(a)
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6300
5000
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3150
2500
2000
Detached, temperature control
1600
1250
1000
800
630
500
400
315
Direct mount, temperature control
250
200
160
Background
125
SPL, dB re 20 µPa
45
40
35
30
25
20
15
10
5
0
1/3 octave band central frequency, Hz
(b)
Figure 12. Noise at temperature control mode
Efficiency of vibration protective pad
In order to facilitate the use of the system enclosure as a vibration isolated and damped acoustic envelope we
use a compliant vibration mount for isolating the high-frequency portion of the vibration export and for a dynamic
damping the low-frequency structural resonances, utilizing the inertia of the relatively heavy cryogenic engine. Figure 13
compares narrowband sound pressure (a) and 1/3 octave band (b) sound pressure level spectra produced by the directly
and vibration mounted cooler operating in the boost cooldown mode. In Figure 13.a we observe essential attenuation of
structural resonances over the entire frequency range, practically down to the background level with the exception of
2000-2500Hz and 3200-3800Hz bands showing some residual noise. Use of vibration mount results in essential
attenuation of sound pressure levels over the entire range of audible frequencies, as seen in Figure 13.b, comparing
appropriate SPL spectra with the most stringent requirement of 10 meters aural non-detectability (Level II) per MIL-STD
1474D (recalculated to 1 meter measurement distance).
0.004
SP, Pa
Background
0.003
Vibration mount, boost
Direct mount, boost
0.002
0.001
0
0
1000
2000
3000
4000
5000
6000
Frequency, Hz
(a)
7000
8000
9000
10000
60
Background
Direct mount, boost
Vibration mount, boost
MIL-STD 1474: 10m@Level II@1m
SPL, dB re 20 µPa
50
40
30
20
10
1/3 octave band central frequency, Hz
10000
8000
6300
5000
4000
3150
2500
2000
1600
1250
1000
800
630
500
400
315
250
200
160
125
0
(b)
Figure 13. Noise at boost cooldown mode
From Figure 13.b, the system with vibration mounted IDDCA meets this requirement with tiny margin at
2500Hz. From dynamic testing, this is the area of resonance in the pad support structure. The said residual noise may be
further supressed by modifying the support feature while designing actual IR imager.
0.004
Background
0.003
SP, Pa
Direct mount, temperature control
0.002
Vibration mount, temperature control
0.001
0
0
1000
2000
3000
4000
5000
Frequency, Hz
6000
7000
8000
9000
10000
(a)
60
SPL, dB re 20 µPa
50
Background
MIL-STD 1474: 10m@Level II@1m
Direct mount, temperature control
Vibration mount, temperature control
40
30
20
10
8000
6300
5000
4000
3150
10000
1/3 octave band central frequency, Hz
2500
2000
1600
1250
1000
800
630
500
400
315
250
200
160
125
0
(b)
Figure 14. Noise at temperature control mode
Figure 14 compares narrowband sound pressure (a) and 1/3 octave band (b) sound pressure level spectra
produced by the directly and vibration mounted cooler operating in the temperature stabilization mode. In Figure 14.a we
observe essential attenuation of structural resonances over the entire frequency range practically down to the background
level with the exception of 500Hz area showing some residual noise.
Use of vibration mount results in essential attenuation of sound pressure levels over the entire range of audible
frequencies, as seen in Figure 14.b, comparing appropriate SPL spectra with the most stringent requirement of 10 meters
aural non-detectability (Level II) per MIL-STD 1474D recalculated to 1 meter measurement distance.
This recalculation was needed since from 2 meters it was quite different to distinguish between the actual noise
radiated by the demonstrator and background noise. From Figure 14.b, the system with vibration mounted IDDCA meets
the above requirement with essential margin.
4. HEAT MANAGEMENT
As is known, the vibration isolator relying on the silicon rubber is a poor heat conductor. Once the cryogenic
cooler operates in a closed space without sufficient thermal contact with the camera enclosure and with no means for the
heat sinking through forced convection, there appears an issue of rejecting the heat radiated by the cooler (e.g. 2.5W at
23ºC ambient) to the environment. The problem was successfully resolved by the above explained pad’s design and
applying a very soft semi-flowable thermal paste DP-100 series λ-gel showing outstanding heat conductivity of
W
6.5
, low outgasing and persistence in mechanical and thermal properties over the wide range of temperatures and
m⋅K
time.
The above thermal paste was applied in such a manner as to fill the above explained 0.3mm gap between the
faces of heat conductive bosses, as explained in Figure 8. In doing so, the entire amount of gel was effectively
encapsulated inside the pad, thus preventing it from getting into the sensitive areas of the camera interior and staining the
optics. This arrangement allows the entire camera enclosure to serve as a heat sink. Experimentation has shown that the
temperature gradient between the cooler and enclosure skin under standard ambient conditions (23ºC, 2.5W DC, 8 hours
of steady-state operation) reaches only 2.5 ºC, which is considered as quite satisfactory.
CONCLUSIONS
The aural stealth of the sophisticated cryogenically cooled portable IR imager may be achieved by mounting the
cryogenic engine upon the imager enclosure using the silent pads. The properties of such a pad need to be chosen so as
to:
use the cryocooler lumped mass as a wideband dynamic absorber suppressing the structural resonances in
the cooler enclosure over the mid-frequency span
provide for essential vibration isolation over the high-frequency span
allow using the imager enclosure as the vibration isolated and damped acoustic envelope
not to affect the line of sight stability
provide for sufficient heat sinking
The 10 meters aural non-detectability distance per MIL-STD 1474D (Level II) has been achieved during the both boost
cooldown and temperature regulation operational modes.
REFERENCES
[1] Gething, M.J., “Seeking the Heat in the Night”, Jane’s International Defense Review, Vol. 38, (2005), pp. 42-47
[2] Cowan, V.M., Morath, C.P., Swift S.M., LeVan P.D., Myers, S., Plis E., S Krishna, “Electrical and Optical Characterization of
InAs/GaSb-based nBn IR Detector”, Proc. of SPIE, 7780, (2010), 778006
[3] Vuillermet, M., Rubaldo, L., Chabuel, F., Pautet, C., Terme, J.C., Mollard, L., Rothman, J. and Baier N., “HOT Infrared
Detectors using MCT Technology”, Proc. SPIE 8012, (2011), 80122W.
[4] Maimon, S. and Wicks, G.W., “nBn Detector, an Infrared Detector with Reduced Dark Current and Higher Operating
Temperature,” Applied Physics Letters 89 (2006), 151109
[5] Veprik, A., Vilenchik, H., Broyde, R., Pundak, N., “Aural stealth of portable cryogenically cooled infrared imagers”,
Proc. SPIE 6206, Infrared Technology and Applications XXXII, (2006), 620625
[6] Veprik, A., Vilenchik, H., Broyde, R., Pundak, N., Struckhoff A., “Portable cryogenically cooled infrared imager: how silent it
might be?”, Proc. SPIE 6542, Infrared Technology and Applications XXXIII, (2007), 65422P