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 mid-

frequency 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 3 shows the typical time histories presenting the cooler-

induced 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 all-

directional 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 cooler-

induced 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 2. Testing of vibration export.

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.

(a)

(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 semi-

flowable thermal paste Lambda Gel DP-100, see

0.E+00

2.E-04

4.E-04

6.E-04

8.E-04

1.E-03

0

10

00

20

00

30

00

40

00

50

00

60

00

70

00

80

00

90

00

10

00

0

SP

, P

a

Frequency, Hz

Background Detached, boost Detached, temperature control

0

10

20

30

40

50

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

SP

L, dB

re 2

0 µ

Pa

1/3 octave band central frequency, Hz

Background Detached, boost Detached, temperature control

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.

Figure 9. Design of technology demonstrator

Figure 10. Vibration protective pad

IDDCA

Vibration protective pad

Front support

Front support

IDDCA

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.

(a)

(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

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP

, P

a

Frequency, Hz

Background Direct mounting, boost Detached, boost

0

10

20

30

40

50

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

SP

L, d

B r

e 2

0 µ

Pa

1/3 octave band central frequency, Hz

Background Detached, boost Direct mount, boost

(a)

(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).

(a)

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

0.004

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP

, P

a

Frequency, Hz

Background Direct mount, temperature control Detached, temperature control

0

5

10

15

20

25

30

35

40

45

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

SP

L, d

B r

e 2

0 µ

Pa

1/3 octave band central frequency, Hz

Background Direct mount, temperature control Detached, temperature control

0

0.001

0.002

0.003

0.004

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP

, P

a

Frequency, Hz

Background

Vibration mount, boost

Direct mount, boost

(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.

(a)

(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.

0

10

20

30

40

50

60

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

SP

L, d

B r

e 2

0 µ

Pa

1/3 octave band central frequency, Hz

Background Direct mount, boost Vibration mount, boost MIL-STD 1474: 10m@Level II@1m

0

0.001

0.002

0.003

0.004

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

SP

, P

a

Frequency, Hz

Background

Direct mount, temperature control

Vibration mount, temperature control

0

10

20

30

40

50

60

125

160

200

250

315

400

500

630

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

SP

L, d

B r

e 2

0 µ

Pa

1/3 octave band central frequency, Hz

Background MIL-STD 1474: 10m@Level II@1m

Direct mount, temperature control Vibration mount, temperature control

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

6.5W

m K⋅, low outgasing and persistence in mechanical and thermal properties over the wide range of temperatures and

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