Sensors and Materials, Vol. 26, No. 4 (2014) 199–214
MYU Tokyo
S & M 0984

Radiometric Calibration of Compact Infrared
Camera (CIRC) for Earth Observation
Eri Kato*, Haruyoshi Katayama, Masataka Naitoh, Masatomo Harada,
Ryoko Nakamura, Michito Sakai, Yasuhiro Nakajima,
Koji Nakau1, Yoshio Tange and Ryota Sato
Satellite Applications Mission Directorate I, Japan Aerospace Exploration Agency,
2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
1Graduate School of Science, Hokkaido University,
Kita 8, Nishi 5, Kita-ku, Sapporo 060-0808, Japan(Received December 2, 2013; accepted February 6, 2014)
Key words:

remote sensing, uncooled infrared detector, wildfire, ALOS-2, JEM-CALET

We have developed a compact infrared camera (CIRC) with an uncooled infrared
array detector (microbolometer) for space application.  CIRC is the first microbolometer
sensor without a calibration function in orbit, like a shutter system or an onboard
blackbody.  The main objective of the CIRC is the technology demonstration of wildfire
detection. Microbolometers are suitable for application in resource-limited sensor
systems or small satellites because no cooling systems are required. Another main
characteristic of the CIRC is its use of athermal optics. The athermal optics systemcompensates for defocus occurring owing to temperature changes. The CIRC achieves asmall size (approximately 200 mm), light mass (approximately 3 kg), and low electrical
power consumption (<20 W) by employing athermal optics and a shutterless system.
The shutterless system suggests the need to devise a method of achieving stray-lightcorrection and temperature calibration without the use of a mechanical shutter forcalibration. We completed the ground calibration test of the CIRC Proto Flight Model (PFM)
in a vacuum environment and at ambient temperature ranging from −15 to 50 °C using
blackbody images at various temperatures (−30 to 50 °C), and accomplished the methodof temperature correction to achieve temperature accuracy of 4 K without a calibration
function. In this paper, we report on the constructed radiometric algorithm and results
relevant to imaging and radiometric quality.

Corresponding author: e-mail: katoh.eri@jaxa.jp

*

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Introduction

Microbolometers are widely used in commercial and military applications. In thelight of recent progress in micro-electromechanical systems (MEMS) technology, highresolution, large-format microbolometers are being developed by many companies.
These devices possess the advantage that they do not require a cooling system suchas a mechanical cooler. Sensors without a cooling system for a detector are small in size, havelight mass, and consume less power. Although the sensitivity of a microbolometer is lessthan that of HgCdTe-based photonic infrared detectors, its advantage of not requiring a
cooling mechanism makes it suitable for use in small satellites or sensor systems whose
resources are limited. For this reason, microbolometers begin to be applied for somespace applications.   In planetary missions, the Longwave IR (LIR) camera onboard
the Venus Climate Orbiter (PLANET-C),(1) which was launched in 2010, also uses a
microbolometer array. In earth-observation missions, the Infrared Spectral Imaging
Radiometer (ISIR)(2) experiment of NASA/GSFC tests the potential of a microbolometer
for application to thermal infrared imaging.  The ISIR was flown as a low-cost shuttle
experiment on the STS-85 space shuttle mission in 1997. Recent missions to measure
the atmosphere vertical profiles such as CALIPSO(3) and EarthCARE(4) also use amicrobolometer as an imaging infrared instrument. JAXA began a research for theapplication of a microbolometer to Earth observation since 2000.
To demonstrate microbolometer technology to thermal infrared imaging from space,
we have developed the compact infrared camera (CIRC).(5–7) CIRC does not have acalibration function in orbit such as a shutter system or an onboard blackbody. The
main objective of the CIRC is to detect wildfires, which are a major and chronic disasterthat affects many countries, particularly those in the Asia-Pacific region.   There areresearch studies suggesting that global warming and climate change may accelerate
the severity of wildfires and other natural disasters.  In the Sentinel Asia project, which
will share disaster information in near real time across the Asia-Pacific region, wildfire
has been chosen as one of the important phenomena to be monitored. Therefore, theirearly detection is important. An effective means of early detection is to increase theirobservation frequency. Our aim is to realize frequent observations by loading CIRCs in
as many satellites as possible by exploiting their small size, light mass, and low power
consumption. Other mission targets of the CIRC include the observation of volcanoes
and urban heat islands. The CIRC will be carried as a technology demonstration payload
on board the Advanced Land Observing Satellite-2 (ALOS-2)(8) and the CALorimetricElectron Telescope (CALET),(9) which will be attached to the Japanese Experiment
Module (JEM-EF) at the International Space Station (ISS).
In this paper, we present the verification results of the athermal characteristics and thecalibration without a calibration function using the CIRC Proto Flight Model (PFM). In § 2,
we provide an overview of the ALOS-2 and CALET.  The specifications of the CIRC aredescribed in § 3. In § 4, data correction algorithm and the ground calibration results of
the CIRC PFM on board the ALOS-2 and CALET are presented.  Airborne observations
made using the CIRC ground test model are presented in § 5. In § 6, we conclude thepaper.

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2.

201

ALOS-2 and CALETThe ALOS-2 is a follow-on satellite of the ALOS, which has been contributing tocartography, regional observation, disaster monitoring, and resource surveys since its
launch in 2006. JAXA has been conducting research and development activities toimprove the wide-field and high-resolution observation technologies developed for
ALOS to further address social needs.  These social needs include (1) disaster monitoringof damage areas, both in considerable detail and over large geographies; (2) continuous
updating of data archives related to national land and infrastructure information; (3)effective monitoring of cultivated areas; and (4) global monitoring of tropical rainforests
to identify carbon sinks.   The ALOS-2 will be launched in JFY 2013.   The ALOS2 inherits the L-band synthetic aperture radar (SAR) observations from the ALOSPALSAR with enhanced capabilities.   Table 1 lists the baseline specifications of the
ALOS-2.  The mounting location of the CIRC is shown in Fig. 1.  In essence, the CIRC
will take images of the target area when the SAR is pointing to the right at an off-nadir
angle of 30°.
The CALET will be mounted to a facility on Japan’s Kibo laboratory aboard the ISS
to observe high-energy cosmic rays.  The CALET is expected to reveal various mysteriesof the universe such as: (1) the nature of the sources of high-energy particles andphotons through the high-energy spectrum, (2) the details of particle transportation in thegalaxy, and (3) signatures of dark matter in either high-energy electrons or the gamma
ray spectrum. It will also be capable of monitoring gamma ray transients and solar
modulation.  The CALET will be launched in JFY 2014.  The baseline specifications of
the CALET are listed in Table 1.  The CIRC will be mounted on the side panel of the
CALET, as shown in Fig. 1.

3.

CIRC

The main objective of the CIRC is to demonstrate the technology for detecting
wildfires using a microbolometer.  The CIRC includes athermal optical and shutterlesssystems. The athermal optical system of the CIRC was designed by combining two
infrared materials: germanium and chalcogenide.   The camera’s temperature does
not need to be controlled using a heater. Thus, we realized a CIRC with a small

Table 1Baseline specifications of ALOS-2 and CALET.
Satellite
Size
Mass
Bus power
Communication
Orbit

ALOS-2
9.9 × 16.5 × 3.4 m3<2000 kg
>5200 W
<800 Mbps
628 kmCALET
1.0 × 0.8 × 1.85 m3<0500 kg
>500 W<300 kbps
407 km (nominal)

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CALET

Kibo
(b)

(a)

CIRC
(c)

Fig. 1. Schematic view of (a) ALOS-2, (b) CALET, and (c) mounting location of CIRC.

size, light mass, and low power consumption. Moreover, the CIRC design is basedon a commercial infrared camera and it employs commercial-off-the-shelf (COTS)
components to reduce cost and time for development. It is also possible to increase theobservational frequency of wildfires if CIRCs are carried on various satellites, which can
exploit the camera’s abovementioned key features.

3.1

Specifications of CIRC

The baseline specifications of the CIRC are listed in Table 2.   We set the baseline
specifications such as spatial resolution and dynamic range to meet the requirements forwildfire detection.(10) The detector has a large format (640 × 480 pixels) to obtain a wide
field of view.  Spatial resolution is an important factor for wildfire detection.  The spatial
resolution is 200 m from an altitude of 600 km (ALOS-2) and 130 m from an altitudeof 400 km (CALET).  Eliminating the cooling system reduces the size (110 × 180 × 230mm3) and electrical power consumption (<20 W).  The CIRC PFMs on board the ALOS2 and CALET are shown in Fig. 2.   The performance and functionality are not muchdifferent between both CIRCs. The CIRC is based on a commercial infrared camera.
We modified the hardware design for purposes of space applications.  CIRCs possess key
technologies that are advantageous to space applications, i.e., microbolometer, athermal
optics, and a shutterless system for achieving small size, reduced mass, and low power
consumption.

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Table 2
Specifications of CIRC.
Parameter
Size
Mass
Detector
Wavelength
Number of pixels
Spatial resolution
Field of view
Exposure
Dynamic range
Bit number
NEDT
FPN
Temperature accuracy

Specifications
110 × 180 × 230 mm3
3 kg
Uncooled infrared detector
8–12 µm
640 × 480
<200 m @ 600 km (ALOS-2)<130 m @ 400 km (CALET)
12° × 9°33 ms
180–400 K
14 bits
0.2 K @ 300 K
0.3 K @ 300 K
4 K (goal: 2 K @ 300 K)

(a)

(b)

Fig. 2. CIRC PFM on board (a) ALOS-2 and (b) CALET.

3.2

Microbolometer

We adopted the use of microbolometers along the infrared (IR) focal plane array (FPA)of the CIRC. Microbolometers are based on the principle of detecting infrared energy
as minute changes in the IR absorber temperature when an infrared photon is incident
on the device. Their advantage is that they do not require a cooling system such as amechanical cooler.

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The CIRC has a 640 × 480 pixel silicon-on-insulator (SOI) diode uncooled IR
FPA developed by Mitsubishi Electric Corporation (MELCO),(11) and is the largest
microbolometer ever used for observations of Earth from space. Its pixel size is 25 × 25
μm2. The noise equivalent differential temperature (NEDT) is 40 mK with f/1 optics.
The drive and readout circuits are nearly identical to those of a commercial IR camera.
For purposes of space application, we performed radiation damage tests and screening ofcommercial devices.

3.3

Athermal optics

We employ athermal optics to keep the optical performance in a wide range oftemperature. This is an advantage of the CIRC given the restricted resources ofsatellites, because CIRC does not require active thermal control or a focus mechanism
for the optics. The optics used in the CIRC consist of f/1.2 refractive optics. The focal
length of the optics setup is 78 mm. The temperature of the optics system of the CIRC
changes in orbit. The temperature changes in the optics can lead to defocusing because
the refractive indices of the lens materials are highly dependent on temperature. To
compensate for this defocus, it is necessary to employ a focus mechanism or a heater tokeep the optics temperature constant. However, such mechanisms increase the amountof sensor resources such as size, mass, and electric power consumption. Athermal optics
can compensate for the defocus owing to temperature variation without the use of suchmechanisms.
The optical design of the CIRC is shown in Fig. 3. The athermal optics of the CIRC
compensate for the defocus by utilizing a combination of different lens materials anddiffractive lenses. The optics of the CIRC use a germanium and a chalcogenide glass(GASIR®1).(12) Figure 4 shows the design value of the modulation transfer function (MTF)of the CIRC optics with respect to the ambient temperature range from −15 to 50 °C.  
The designed MTF is constant over a wide range of temperatures.

Fig. 3. (Color online) Optical design of CIRC.

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0.7

0.5
-20

-10

0

10

20

30

40

50

Fig. 4. (Color online) Calculated MTF of CIRC optics in the ambient temperature range from−15 to 50 °C.  These MTF values indicate the design values of only the optical system.  Line styles  represent results at various positions and MTF directions.

3.4

Shutterless system

4.

Ground Calibration Test of CIRC PFM On Board ALOS-2 and CALET

We eliminated the mechanical shutter from the CIRC, and this enabled us todownsize the CIRC. A mechanical shutter is more commonly used as a calibration
source. Therefore, we devised a method of achieving temperature calibration and straylight correction from within the CIRC. We obtained images of a blackbody at various
temperatures for different CIRC temperatures to perform stray-light correction using thetemperature of the CIRC. The details are presented in § 4.

We carried out a ground calibration test of the CIRC PFMs on board the ALOS2 and CALET at the Tsukuba Space Center at JAXA.  CIRCs do not have a calibration
function, which is challenging unlike in the case of previous space sensors. Therefore,
we need to construct correction algorithm using only calibration data during the groundcalibration test and CIRC’s temperatures.   The objectives of this calibration are (i)to construct a data correction algorithm and (ii) to confirm the imaging quality andradiometric quality of the CIRC with the calibrated images.

4.1

Construction of data correction algorithm

The stray light (i.e., the overall radiation except that from the target) had to be filteredout, because a background image cannot be taken without a calibration function. The
stray-light intensity changes depending on the temperature of the CIRC. Therefore,
we generated a calibration data table (stray-light and gain correction coefficients) andalgorithm using blackbody images at various temperatures (−30 to 50 °C) obtained with
the CIRC PFM at various environmental temperatures (−15 to 50 °C).   The flowchart

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used for the data correction is shown in Fig. 5, and each process in the flowchart isdescribed below.
(I)		 Bad pixel correction is required to correct the brightness of a bad pixel by
substituting the mean brightness of the normal surrounding pixels. A “bad” pixel
is defined as a pixel that has more sensitivity than ±19% of the average value ofsurrounding 11 × 11 pixels after estimation of sensitivity using blackbody images
at 15 and 40 °C.  Pixels whose sensitivity changes within 30 s beyond the abovedefined value are also included as bad pixels.
(II)		 Dummy correction is carried out to subtract the electrical background noise using
dummy pixels, namely, optical black pixels. Dummy pixels of the CIRC do notinclude a thermally insulating structure and an infrared ray absorbing structure.
These pixels exhibit no sensitivity to incident infrared rays, and have electricalnoise the same as effective pixels. Dummy correction technique is widely usedin CMOS and CCD, and the optical black signal is subtracted from such an image
signal to generate a corrected image signal.(13) The CIRC has twenty dummy pixels
per line. In the case of the CIRC, eighteen of them are used in this correction, andthe average is subtracted from each pixel in the line.
(III) Stray-light correction is performed for each pixel using the stray-light correction
coefficients, which represent the amount of stray light change with changes in the
CIRC temperature.  The procedure for calculating this coefficient is described later.  
(IV) Gain correction represents the sensitivity correction performed for each pixel using
the gain correction coefficients.   By means of this gain correction, data of the
CIRC are simultaneously converted from the digital number (DN) to the equivalentradiance or temperature.
Next, we describe the methods for stray-light and gain correction.  These coefficientsare calculated on the basis of the data table generated via the ground calibration test,
which is described in § 4.2.2.  The stray-light correction coefficient (β) is defined using

Fig. 5. Flow chart indicating procedure for data correction.

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eq. (1).  This coefficient is measured in units of DN per K, and it has the dimension of a
640 × 480 array.
β=

ImageB − ImageA
T ref b − T ref a(1)

Then, ImageA and ImageB represent the corrected data obtained after dummy correction
with the same blackbody temperatures (Tbb) and different environmental temperatures (Fig.
6). The parameters Tref_a and Tref_b denote reference temperatures that correspond to theaverage of the lens and detector temperature of the CIRC under the tentative assumptionthat these temperatures mainly equally contribute to the stray light. The stray-lightcorrection coefficients, depending on the reference temperature, indicate the amountof stray-light change with changes in the temperature of the CIRC. If the reference
temperature of the CIRC is changed by ΔT, the corresponding increase in stray light isexpressed as β × ΔT.
Next, we calculate the gain correction coefficient (α). Data for calibration wereobtained at different blackbody temperatures (−30, −10, 10, 30, and 50 °C).  The firststep in the calculation is to carry out stray-light correction for the calibration data tocorrespond to the reference temperature (Tref_sd) of one observational data set. It resultsin the uniformity of stray light. Five data sets corrected using the above procedurefor various blackbody temperatures (−30, −10, 10, 30, and 50 °C) are obtained in thismanner.   The gain correction coefficient is derived using the five data sets, and it ismeasured in units of W/µm/m2/sr/DN, as seen from eq. (2).
α=(RadT bb c ) − (RadT bb d )ImageC − ImageD(2)

The parameters RadTbb_c and RadTbb_d denote the radiances corresponding to each
blackbody temperature (Fig. 7). ImageC and ImageD denote two of the five data setswith different blackbody temperatures.   Four gain correction coefficients (α1 to α4)
are obtained every 20 °C (Fig. 8).  The gain correction coefficients relate the radianceand DN. An observational data set (DNobs) is calibrated using the appropriate gain
coefficients using eq. (3),Fig. 6. Assumed parameters of images A and B.

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Fig. 7. Assumed parameters of images C and D.

Fig. 8. Four gain correction coefficients corresponding to a set of reference temperatures.

Radobs = RadTbb + α × (DNobs − DNTbb),(3)

where DNobs is equal to the DN in the case of DNTbb < DNobs < DNTbb+20. These DNTbb and
DNTbb+20 values indicate the DN at the blackbody temperatures of Tbb and Tbb + 20 °C,
respectively. The radiance at Tbb is RadTbb. Radobs denotes the data converted to radiancefrom DNobs. In this manner, the observed data can be converted to the corresponding
values of radiance.

4.2 Performances
4.2.1 Imaging quality

The MTF was measured to evaluate the imaging quality of the CIRC at various
operating temperatures (−15 to 50 °C).  The experimental setup for MTF measurement
is shown in Fig. 9. Collimated infrared rays were made to pass through a germanium
window located on the side of the vacuum chamber. The CIRC was able to capture
images of a four-bar target. Figure 10 shows the measurement points of the four-bar
target for MTF, at the center, ±5° along the cross-track (CT) direction, and ±3.5° along
the along-track (AT) direction.

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209

Fig. 9. Experimental setup of MTF measurement.

Fig. 10. Measurement points of MTF on image acquired by CIRC. The nine positions are marked
Nos. 1 to 9. Dummy pixels (20 × 480) are optical black pixels, which exhibit non-photosensitivity
and have the same electrical properties as standard effective pixels.

Figure 11 shows the MTF results of the CIRC PFMs for the ALOS-2 and CALET.  
The MTF along the CT direction for the ALOS-2 [Fig. 11(a)] is constant with a value
in the range from 0.2 to 0.39, which is independent of environmental temperature. In
contrast, the MTF along the AT direction [Fig. 11(b)] is slightly lower in low-temperatureenvironments.   In the case of the CIRC PFM for the CALET, the MTF values range
between 0.24 and 0.33 along the CT direction [Fig. 11(c)] and from 0.34 to 0.54 along
the AT direction [Fig. 11(d)].  The trend of the environmental temperature dependence isthe same as that observed in the results of the CIRC PFM for ALOS-2.  As a result, the
MTF of the CIRC was found to be effective in capturing clear images across the entire
range of operating temperatures for both PFMs.  Thus, we confirmed the athermal optical
performance of the CIRC.

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ALOS-2

ALOS-2

(a)

(b)

CALET

CALET

(c)

(d)

Fig. 11. (Color online) Measurement results of MTF on board ALOS-2 [(a) and (b)] and CALET [(c)and (d)].  (a) and (c) display MTF values along the CT direction, while (b) and (d) represent thosealong the AT direction.

4.2.2 Radiometric quality

The experimental setup for generating the calibration data table is shown in Fig. 12.
The CIRC was installed in a vacuum chamber and enclosed by a shroud to control its
ambient temperature with a heater and cooler. The blackbody was set up in the frontof the lens.  Figure 13 depicts the data table of the blackbody temperatures (−30 to 50°C, every 20 °C) obtained with the CIRC PFM at various environmental temperatures(−15 to 50 °C, every 1 °C).   Radiometric calibration is performed by interpolating thecoefficients within the range of the data table.
The NEDT and fixed pattern noise (FPN) values were less than 0.2 and 0.3 K,
respectively (Table 3). These results are consistent with the design values of the CIRC,
and they satisfy its specifications.  Figure 14 shows a comparison between a raw image
and a corrected image obtained using the CIRC PFM for the ALOS-2.   Performing
data correction makes it possible to capture smooth images without a calibration
function. The temperature accuracy is less than 2 K upon a comparison of the brightness
temperature of the corrected image and the actual blackbody temperature. This is within
the specification for the temperature correction accuracy.  In the case of the CIRC PFM
for the CALET, the temperature accuracy is from 0.5 to 2.8 K, which is also within thespecification.

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Fig. 12. Experimental setup for generating calibration data table.

Fig. 13. Calibration data table for radiometric correction. The data at various blackbody
temperatures (−30 to 50 °C, every 20 °C) were obtained at various environmental temperatures (−15to 50 °C, every 1 °C).

Table 3Results of NEDT and FPN.
for ALOS-2
for CALETNEDT (K)
0.19
0.18

FPN (K)
0.27
0.21

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(a)

(b)

Fig. 14. Blackbody image obtained by CIRC on board ALOS-2.  (a) and (b) show the raw image
and corrected image, respectively.

5.

Airborne Observation with CIRC Ground Test ModelWe carried out airborne observations using the ground test model (GTM) of
the CIRC, which is a roughly similar to the PFM and has optical and radiometricperformances equivalent to the corresponding PFM. The CIRC GTM will be used forvicarious calibration to compare with on-board data of the PFM. Airborne observationis one of the vicarious calibration experiments, and it is thought to be necessary becauseof the CIRC without a calibration function.  This section shows results of first airborne
experiments using the CIRC GTM to check the data correction algorithm as described in § 4.
Observational flights were carried out on March 22 and 28, 2012.  The aircraft used
was a Cessna172 Skyhawk made by the Cessna Aircraft Company. The observation
area was Tsukuba City, which is south of Ibaraki Prefecture and Narita City in Chiba
Prefecture, Japan.  The flight altitude ranged from 300 to 750 m.  The ground sample
distance (GSD) at these altitudes ranged from about 10 to 25 cm.
Figure 15 shows an aerial image of the Tsukuba Space Center, which is one of the
facilities in JAXA, captured from an altitude of 750 m with a compact digital camera.
The yellow rectangle represents the field of view of the CIRC.   Figure 16 shows acomparison between the raw image and corrected image captured by an airborne
observation from an altitude of 750 m.   We confirmed that the data were correctedwithout a calibration function and that the actual images could be corrected using the
ground calibration data.
The temperature correction accuracy was verified using the image of the pond area
shown in Fig. 16(b), indicated by the white box. The temperature of the pond measured
using a water temperature gauge was about 11 °C.  The corrected data obtained using the
CIRC GTM yielded a temperature of 11.5 °C, with the assumption that the emissivity ofwater was 0.98.  The temperature correction accuracy was about 0.5 °C, which is within
the specification of 2 K for an environmental temperature of ~300 K.

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Fig. 15. Aerial image of Tsukuba Space Center from an altitude of 750 m.

(a)

(b)

Fig. 16. Images captured by airborne observation: (a) raw image and (b) corrected image in which
the white box indicates the pond area.

6.

Conclusions

The CIRC is the first microbolometer sensor without a calibration function for space
application, and achieves a small size, light mass, and low electrical power consumption
by employing athermal optics and a shutterless system. We accomplished the method of
1
temperature correction to achieve the temperatureaccuracy of 4 K without a calibration
function in orbit.   We have also confirmed that other performances (MTF and NEDT)
are satisfied, and these devices are launch-ready.  In addition, the data obtained throughairborne observations could be corrected by the method of temperature correction, andthe temperature correction accuracy was within the specification of 4 K.   This study
represents an important first step in the application of a microbolometer for Earthobservations.

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Acknowledgements
The authors would like to thank the CIRC team members in this work.

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13 J. C. Ahn and S. Si: United States Patent US7839438 (2010).