1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
356
AN EFFECTIVE MILLI KELVIN THERMAL MANAGEMENT
STRATEGY FOR INFRARED IMAGING SPECTROMETER
Kunal S. Bhatt1
, Rahul Dev2
, A. R. Srinivas3
, Dr. D. P. Vakharia4
1, 4
(Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of
Technology, Surat-395007, India)
2, 3
(SAC, ISRO, Ahmedabad-380015, India)
ABSTRACT
Thermal infrared (TIR) spectroscopy is the subset of infrared spectroscopy that deals
with radiation emitted in the infrared part of the electromagnetic spectrum. Thermal imaging
spectrometer (TIS) is the system that detects the thermal radiation emitted from the
environment.TIS aims to detect very small range of infrared region (7-14 µm). To achieve
this target it is required to maintain temperature of spectrometer detector within few milli
Kelvin accuracy. Achieving precise control of temperature in environment where thermal
dissipation is varying is very challenging aspect. Commercially these types of systems are
controlled by cryocoolers which are very heavy, cumbersome and introduce huge process
time to realize. The work presented in the paper brings out a cost effective and light weight
thermal control strategy to precisely control the detector to 2 to 3 mK. The strategy is
simulated by FEM tools and validated by the experiments.
Keywords: detector, isothermal shield, PID controller, spectrometer, Thermo electric cooler
1. INTRODUCTION
Thermal infrared spectroscopy measures the thermal infrared radiation emitted (as
opposed to being transmitted or reflected) from a volume or surface. This method is
commonly used to identify the composition of surface by analyzing its spectrum and
comparing it to previously measured materials. Data acquired by the spectrometer is
processed and analyzed to map surface composition and mineralogy on the planet. This
typical spectrometer presented here uses the micro bolometer detector as shown in Fig.1 to
capture the thermal radiation in the spectral range of 7-14µm of infrared region. Detector is
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2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March
the device consisting of a photo voltaic layer on charged coupled devices (CCDs). That is
placed at focal plane of the imag
electromagnetic (light) and transforms it in to an electronic charge and finally in digital
format, which is read by the detector electronics. The micro bolometer detector has a vacuum
sealed anti reflection coated germanium (Gr) window to allow the IR radiation within 7
14µm band. The resolution of 580 nm is required with minimum 12 nos. of spectral bands
from 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].
According to Wien’s displacement law the relation between wavelength (
waves and absolute temperature (T) for maximum emissive power is given by [2],
Hence there is requirement of controlling the temperature of Germanium window of the
order of 10mK to achieve the desired radiometric performance.
The detector along with its processing electronics, mechanical mounting and thermal
control system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).
Temperature control of such a system is very complex and challenging task as it consists of
optical interfaces, electrical interfaces, mechanical structure and detector electronics. Typical
model of thermal imaging spectrometer is shown in the Fig.3.
Figure 3 typical configuration of thermal imaging spectrometer
Figure 1 internal structure of
micro bolometer detector
International Journal of Mechanical Engineering and Technology (IJMET), ISSN
6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
357
the device consisting of a photo voltaic layer on charged coupled devices (CCDs). That is
placed at focal plane of the imaging system and it receives useful signals in the form of
electromagnetic (light) and transforms it in to an electronic charge and finally in digital
format, which is read by the detector electronics. The micro bolometer detector has a vacuum
flection coated germanium (Gr) window to allow the IR radiation within 7
m band. The resolution of 580 nm is required with minimum 12 nos. of spectral bands
from 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].
According to Wien’s displacement law the relation between wavelength (λ) of radiation
waves and absolute temperature (T) for maximum emissive power is given by [2],
λmax T = 2900 µm 0
K
Hence there is requirement of controlling the temperature of Germanium window of the
order of 10mK to achieve the desired radiometric performance.
The detector along with its processing electronics, mechanical mounting and thermal
control system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).
such a system is very complex and challenging task as it consists of
optical interfaces, electrical interfaces, mechanical structure and detector electronics. Typical
model of thermal imaging spectrometer is shown in the Fig.3.
configuration of thermal imaging spectrometer
nternal structure of
micro bolometer detector
Figure 2 exploded view of
Detector Head Assembly
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
April (2013) © IAEME
the device consisting of a photo voltaic layer on charged coupled devices (CCDs). That is
ing system and it receives useful signals in the form of
electromagnetic (light) and transforms it in to an electronic charge and finally in digital
format, which is read by the detector electronics. The micro bolometer detector has a vacuum
flection coated germanium (Gr) window to allow the IR radiation within 7-
m band. The resolution of 580 nm is required with minimum 12 nos. of spectral bands
from 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].
According to Wien’s displacement law the relation between wavelength (λ) of radiation
waves and absolute temperature (T) for maximum emissive power is given by [2],
(1)
Hence there is requirement of controlling the temperature of Germanium window of the
The detector along with its processing electronics, mechanical mounting and thermal
control system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).
such a system is very complex and challenging task as it consists of
optical interfaces, electrical interfaces, mechanical structure and detector electronics. Typical
xploded view of
Assembly

3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
358
2. THERMAL REQUIREMENTS OF SPECTROMETER
TIS uses bolometer detector which dissipates 150mW heat and detector card dissipates
80mW in operation; the detector is required to be maintained at 20±5 0
C during operation
with accuracy of 10mK.
TIS Electro Opto Mechanical (EOM) module should be maintained at 20±50
C during
operation.
The electronic card components dissipate 1.2 watts of heat the design should ensure
proper heat transfer from pcb components to heat sink and should not allow the
temperature to rise more than 400
C.
Conventionally Thermo electric coolers (TECs) are used to control the temperatures of such
detector. Commercially available TECs with PID controller have accuracy of the order of ±0.10
C
[3]. Customized TECs may give the control of temperature up to ±0.010
C but that again increase
the cost of the system.
3. PRESENT TIS DHA CONFIGURATION
Figure 4 present TIS-DHA configuration
As shown in the Fig.2&4 the detector package seats on the detector mount. The detector
mount, package and electronic card are fixed to the DHA frame (heat sink). Thermal design
ensures that there are no hot spots in the vicinity of detector. Thermal radiations pass through
various optical components and they are focused on germanium window by focusing optics.
Detector collects dispersed wavelengths allowed by germanium window in 7-14µm range.
The simulations show that the present design allows 130mk over one degree variation in the
ambient temperature on germanium (Gr) window. Results and temperature profiles of the same
are shown in Fig.5 and Fig.6 respectively.
Figure 5 temperatures on Gr window, package and TEC sink vs. ambient temperature for present
configuration

4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
359
Boundary conditions
Initial temperature: 200
C
Emissivity of aluminum black anodized surface : 0.8
Ambient temperature: 16 to 190
C
TEC temperature : 200
C
TEC Dissipation(including detector dissipation of 150mW) : 280 to 320 mW
Temperature profiles
4. THERMAL DESIGN OPTIMIZATION
The design strategies shown in table1are analyzed to resolve thermal control. In all of
the strategies the germanium window temperature and sink temperature which form
background for the detector are monitored and controlled.
Table 1 Various thermal design strategies
(a) (b)
(c) (d)
Figure 6 temperature profile of (a) Gr window (b) package (c) detector mount
(d) DHA frame for existing TIS configuration

5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
360
4.1 Thermal simulation of TIS detector
Here the results of thermal analysis performed for different strategies are shown.
Standard properties are taken for the thermal simulations [5].
4.1.1 Strategy-1
In this strategy entire structure is controlled at higher temperature without any design
modifications in present configuration.
The maximum achieved variation on Gr window is 2mK for ± 1.50
C variation in ambient
as shown in Fig.7. The temperature profiles are shown in the Fig.8.
Figure 7 temperatures on Gr window, package and TEC sink vs. ambient temperature for
strategy 1
4.1.2 Strategy-2 (Designing isothermal shield encompassing whole PCB)
In this strategy a shield encompassing the whole PCB along with detector is designed
as shown in Fig.8. The maximum achieved variation on Gr window is 4mK for ± 10
C
variation in ambient as shown in Fig.9.
Figure 8 TIS DHA configuration with isothermal shield encompassing whole PCB

6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March
4.1.4 Strategy-3 (Designing mini i
In this strategy an isothermal shield encompassing only the detector package is
designed as shown in Fig.10 and it is controlled along with DHA
Figure 10 TIS DHA configuration with i
The maximum achieved variation on Gr
as shown Fig.11. The temperature profiles
Figure 11 temperatures on Gr window, p
Figure 9 temperatures on G
International Journal of Mechanical Engineering and Technology (IJMET), ISSN
6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
361
3 (Designing mini isothermal shield extended to detector mount)
an isothermal shield encompassing only the detector package is
and it is controlled along with DHA frame (heat sink).
TIS DHA configuration with isothermal shield extended to detector mount
maximum achieved variation on Gr window is 3.9 mK for ± 1.50
C variation in ambient
. The temperature profiles are shown in the Fig.12.
window, package and TEC sink vs. ambient temperature for
strategy 3
emperatures on Gr window, package and TEC sink vs. ambient
temperature for strategy 2
International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
April (2013) © IAEME
sothermal shield extended to detector mount)
an isothermal shield encompassing only the detector package is
frame (heat sink).
sothermal shield extended to detector mount
C variation in ambient
ackage and TEC sink vs. ambient temperature for
ackage and TEC sink vs. ambient

7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
362
Temperature profiles
Figure 12 Temperature profiles of (a) Gr window (b) package (c) DHA frame-sink (d) DPE
card for strategy 3
5. SENSITIVITY ANALYSIS
Sensitivity analysis is performed to observe the effect of variation of controlling
temperatures on germanium window.
5.1 Variation of isothermal shield temperature by ± 0.010
C
The maximum variation on Germanium window is 12mK for 20mK variation in
isothermal shield temperature. Results are shown in the Fig. 13.
(a) (b)
(c) (d)
Figure 13 temperatures on Gr window, package and TEC sink vs. isothermal shield
temperature

8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
363
5.2 Variation of DHA frame temperature by ± 0.010
C
The maximum variation on Gr window is 10mK for 20mK variation in DHA frame
temperature as shown in Fig.14.
6 EXPERIMENTAL VALIDATIONS
The Fig.15 shows the experimental test set up of the spectrometer system.
Fig.15 experimental test set up for TIS-DHA thermal control
The TIS structure along with all mounted components is taken for experimentation.
The heat is supplied to the DHA frame and isothermal shield with thermo foil heaters. The
temperature on the shield and DHA frame is controlled by the PID controller which maintains
the temperature on the system by controlling the heat supply from the heaters. The other
single foil heater is mounted on the PCB to simulate the heat dissipated by electronic
components. This heater is given the power supply of 1.2 watts with the help of variac. The
PT 100 temperature sensors are mounted on the spectrometer system at required locations.
The temperatures of various subsystems are monitored, recorded and plotted through the
software interface of temperature data acquisition system [6]. The environmental temperature
of spectrometer system is varied by varying environmental chamber temperature. Fig.16
shows the realized hardware mounted with sensor & heaters and entire system is wrapped
with MLI blankets.
Figure 14 temperatures on Gr window, package and TEC sink vs. DHA frame
temperature

9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
364
Fig.16 integrated thermal imaging-infrared imaging spectrometer system
6.2 Measurements of the temperatures for the spectrometer system with thermal control
strategy
Table 2 Temperatures on alumina package, detector mount and focusing optics lens for
ambient temperature variation
The table 2 shows the ambient temperatures, controlling temperatures, temperatures on
detector mount and package. The control temperature is set to 32 0
C and ambient temperature
is varied from 31.10
C to 27.6 0
C temperature. The Fig.17 shows the temperature measured
and corresponding plots obtained by data logger system.
The maximum achieved temperature variations on alumina package and detector mount
are 22 mK and 26 mK for 3.5 0
C variation in ambient temperature
Figure 17 experimental readings of two extreme temperatures 31.1 0
C and 27.6 0
C

10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
365
6.3 Measurements of sensitive parameters
The change of the temperature on the alumina package is measured in relation to the
change in controlling temperatures. The set temperature value is kept offset by ± 100 to 200
mK intentionally and maintained constant by the PID controller.
Table 3 Temperatures on alumina package, detector mount and focusing optics lens for
controlled temperature variation
The maximum achieved variation on package is 134 mK for 203 mK variation in isothermal
shield temperature and maximum achieved variation on detector mount is 113 mK for 222
mK variation in isothermal shield temperature. Results are shown in table 3.
7. CONCLUSIONS
Thermal infrared imaging spectrometer system has been rigorously analyzed to yield
milli Kelvin thermal control. The simulated results are validated by experimentation. The
measurements performed on the developed hardware for the simulated strategies show the
followings.
• For a change in the ambient temperature of the order of 1.2 0
C causes a change of 3
mK on the package thus meeting the set target of 10 mK.
• Similarly a change of 1.2 0
C in ambient causes 3 mK on detector mount, 15 mK on
heat sink of detector and 6mK on the isothermal shield.
• In order to meet above target it is required that the control temperature targets of heat
sink and isothermal shield are to be kept within a range of ± 0.01 0
C.
• Mini isothermal shield extended up to detector mount (strategy-3) with temperature
control on DHA frame and isothermal shield is a viable solution to be adopted for
thermal control of TIS detector.
REFERENCES
[1] Michael R. Holt, Thermal management strategy for the hyper spectral imager for the
coastal ocean, master diss., Utah state university, Logan, Utah, 2007.
[2] S.P. Sukhatme, text book on heat transfer fourth edition (University Press (India) Pvt.
Ltd., Hyderabad, 2005).

11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME
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[3] G Johnson, Thermal management for CCD performance on the advanced camera for
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[4] David G. Gilmore, precision temperature control, Handbook of space craft thermal
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