Compact Infrared Camera (CIRC) for earth observation
adapting athermal optics
Eri. Kato*a, Haruyoshi. Katayamaa, Masataka. Naitoha, Masatomo. Haradaa,
Ryoko. Nakamuraa, Koji. Nakaub, and Ryota. SatoaaJapan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki, Japan, 305-8505;b
Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido, Japan, 060-8628
ABSTRACT
We have developed the compact infrared camera (CIRC) with an uncooled infrared array detector (microbolometer) for
space application. The main mission of the CIRC is the technology demonstration of the wildfire detection using a large
format (640×480) microbolometer. Wildfires are major and chronic disasters affecting numerous countries, especially in
the Asia-Pacific region, and may get worse with global warming and climate change.
Microbolometers have an advantage of not requiring cooling systems such an a mechanical cooler, and is suitable forresource-limited sensor systems or small satellites. Main characteristic of the CIRC is also an athermal optics. The
thermal optics compensates the defocus due to the temperature change by using Germanium and Chalcogenide glass
which have different coefficient of thermal expansion and temperature dependence of refractive index. The CIRC
achieves a small size, light weight, and low electrical power by employing the athermal optics and a shutter-less system.
Two CIRCs will be carried as a technology demonstration payload of ALOS-2 and JEM-CALET, which will belaunched in JFY 2013 and 2014, respectively. We have finished the ground calibration test of the CIRC Proto Flight
Model (PFM). Athermal optical performance of the CIRC have been confirmed by measuring modulation transfer
function (MTF) in a vacuum environment and at environmental temperature from -15 to 50 °C. As a result, MTF wasfound to be effective at capturing clear images across the entire range of operating temperatures. We also provide an
overview of the CIRC and radiometric test results in this presentation.
Keywords: remote sensing, thermal infrared imaging, microbolometer, wildfire, ALOS-2, CALET

1. INTRODUCTION
Microbolometers are widely used in commercial and military applications. Their advantage is that they do not require a
cooling system, such as a mechanical cooler. Sensors without a cooling system for a detector are small in size,
lightweight, and consume less power. Although the sensitivity of a microbolometer is lower than that of an HgCdTebased photonic infrared detectors, its advantage of not requiring a cooling mechanism makes it suitable for small
satellites or sensor systems whose resources are limited.
We have developed the Compact Infrared Camera (CIRC) 1,2,3 as a technology demonstration payload for thermal infrared
imaging from space using a microbolometer. The main mission of the CIRC is to detect wildfires, which are a major andchronic disaster that affects many countries, especially those in the Asia-Pacific region. There are also suggestions that thissituation will get worse with global warming and climate change. In the Sentinel Asia project, which will share disaster
information in almost real time across the Asia-Pacific region, wildfire detection has been chosen as one of the important
activities to be monitored. Therefore, their early detection is important. An effective means of early detection is toincrease their observation frequency. Our aim is to realize frequent observations by loading CIRCs in as many satellites
as possible by taking advantage of their small size, low weight, and low power consumption. Other mission targets of the
CIRC are volcanoes or heat island phenomena in a city. The CIRC will be carried as a technology demonstration payloadof the Advanced Land Observing Satellite-2 (ALOS-2)4, and CALorimetric Electron Telescope (CALET)5, 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 the calibration of the shutter-lesssystem with the CIRC Proto Flight Model (PFM). In section 2, we describe ALOS-2 and CALET. The specifications of*katoh.eri@jaxa.jp; phone +81-50-3362-3121; fax+81-29-868-2961

the CIRC is described in section 3. In section 4, the ground calibration results of the CIRC PFM onboard ALOS-2 and
CALET is presented. The airborne observation with the CIRC ground test model is shown in section 5. In section 6, theconclusion is provided.

2. ALOS-2 AND CALET
ALOS-2 is a follow-up mission to ALOS, which has been contributing to cartography, regional observation, disastermonitoring, and resource surveys since its launch in 2006. JAXA is conducting research and carrying out development
activities to improve the wide and high-resolution observation technologies developed for ALOS to further fulfill social
needs. These social needs include (1) disaster monitoring of damage areas, both in considerable detail and when theseareas may be large; (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 rain forests to identify carbon sinks.
ALOS-2 will be launched in 2013. ALOS-2 succeeds the L-band synthetic aperture radar (SAR) observation of ALOSPALSAR with enhanced capabilities. Table 1 lists the baseline specifications of ALOS-2. The mounting location of
CIRC is shown in Figure 1. Basically, CIRC will take images of the target area when the SAR is pointing to the right at
an off-nadir angle of 30°.
CALET is an international program of the ISS that will search for signatures of dark matter and provide direct
measurements of the highest energy of the cosmic ray electron spectrum to observe discrete sources of high-energy
particle acceleration in our local region of the galaxy. CALET will address many outstanding questions, including (1) thenature of the sources of high-energy particles and photons through the high-energy spectrum, (2) the details of particle
transportation in the galaxy, and (3) signatures of dark matter in either the high-energy electrons or the gamma ray
spectrum. It will also be capable of monitoring gamma ray transients and solar modulation. CALET will be launched in
2014. The baseline specifications of CALET are listed in Table 1. CIRC will be mounted on the bottom of CALET,
which is shown in Figure 1.
Table 1. Baseline specifications of ALOS-2 and CALET.
Satellite
ALOS-2Size
9.9 m×16.5 m×3.4 m
Mass<2000 kg
Bus power>5200 W
Communication<800 Mbps
Orbit
628 kmCALET
1.0 m×0.8 m×1.85 m<500 kg>500 W<300 kbps
407 km (nominal)

CIRC

Field of View
(9ﾟ×12 ﾟ)Off-nadir Angleof 30ﾟ

Figure 1. Schematic view of ALOS-2 (left) and CALET(right), and the mounting location of CIRC.

3. COMPACT INFRARED CAMERA (CIRC)
The main mission of CIRC is to demonstrate the technology for detecting wildfires using a microbolometer. The CIRC
has the characteristics of athermal optical and shutter-less systems. The athermal optical system of the CIRC wasdesigned by combining two infrared materials: germanium and chalcogenide. The temperature does not need to becontrolled using a heater. Thus, we realized a CIRC with a small size, light weight, and low power consumption.
Moreover, the CIRC design was based on a commercial infrared camera and employs commercial-off-the-shelf (COTS)
parts to reduce cost and time for development. It is also possible to increase the observational frequency of wildfires if
CIRCs are carried on various satellites, which can take advantage of its small size, lightweight, and low power
consumption.
3.1 Specification of the CIRC
The baseline specifications of the CIRC are listed in Table 2. We set the baseline specifications to meet requirements forwildfire detection. The detector has a large format (640 × 480 pixels) to obtain a wide field of view. The 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 altitude of 400 km (CALET). Eliminating the cooling system reduces the size (110 mm ×
180 mm × 230 mm) and electrical power (<20 W). The CIRC PFM onboard ALOS-2 is shown in Figure 2. The CIRC isbased on a commercial infrared camera. We modified the hardware design so that it can be applicable for a space application.
CIRCs have key technologies, i.e., microbolometer, athermal optics, and shutter-less system, for achieving small size, lowweight, and low power consumption.
3.2 Microbolometer
We adopted microbolometers as an infrared (IR) focal plane array (FPA) of the CIRC. Microbolometers are based on theprinciple of detecting infrared energy as minute changes of the IR absorber temperature when an infrared photon enters
it. Their advantage is that they do not require a cooling system, such as a mechanical cooler. Sensors without a detector
cooling system can be made to have a small size, low weight, and consuming less power.
The CIRCs have a 640 × 480 pixel silicon-on-insulator (SOI) diode uncooled IR FPA developed by MELCO. Its pixel
size is 25μm square. The noise equivalent differential temperature (NEDT) is 40mK with f/1 optics. Drive and readout
circuits are almost the same as those of the commercial IR camera. For a space application, we performed a radiation
damage test, and a screening of commercial devices.

Table 2. Specifications of CIRC
Parameter

Specifications

Size

110 mm × 180 mm × 230 mm

Mass

3 kg

Detector

Uncooled infrared detector

Wavelength

8–12 m

Number of pixels

640 × 480
<200 m @600 km (ALOS-2)

Spatial resolution<130 m @400 km (CALET)
Field of view

12° × 9°Exposure

33 ms

Dynamic range

180-400 K

NEDT

0.2 K @ 300 K

FPN

0.3 K @ 300 K

Temperature accuracy

4 K (goal : 2K @ 300 K)

Figure 2. CIRC PFM onboard ALOS-2.

3.3 Athermal Optics
The optics of the CIRC are f/1.2 refractive optics. The focal length of the optics is 78 mm. The temperature of the CIRC
is changed on orbit. The temperature change of the optics will cause a defocus because refractive indices of lensmaterials are highly dependent on temperature. In order to compensate for this defocus, we have to employ a focus
mechanism or a heater to keep the optics temperature constant. However, such mechanisms increase sensor resources.
Athermal optics can compensate for the defocus due to the temperature change without such mechanisms.
Figure 3 shows the optical design of the CIRC. The athermal optics of the CIRC compensate for the defocus by utilizing
a combination of different lens materials and diffractive lenses. The optics of the CIRC use a germanium and achalcogenide glass (GASIR®1). Figure 4 shows the calculated MTF of the CIRC optics versus the ambient temperaturefrom -15ºC to 50ºC. The MTF is constant over the wide range of temperatures.

Figure 3. Optical Design of the CIRC.

Figure 4. Calculated MTF of the CIRC Optics in the Ambient Temperature from -15ºC to 50ºC.

3.4 Shutterless system
We eliminated the mechanical shutter from the CIRC. This enables us to downsize the CIRC. A mechanical shutter ismore commonly used as a calibration source. Therefore, we devised a way to achieve temperature calibration and straylight correction from the inside the CIRC. We obtained images of various temperature blackbody with different CIRC
temperatures in order to perform stray-light correction by temperature of the CIRC. The details are show in section 4.

4. GROUND CALIBRATION OF THE CIRC PFM ONBOARD ALOS-2 AND CALET
We carried out a ground calibration test of the CIRC PFM onboard ALOS-2 and CALET at the facility in Tsukuba Space
Center in JAXA. The aims of this calibration were (1) to construct a data correction algorithm and (ii) to confirm theimaging quality and the radiometric quality.
4.1 Constructing data correction algorithm
The stray light (i.e., the overall radiation except that from the target) had to be removed, because a dark image cannot betaken as a result of the shutter-less system. We constructed a calibration data table (stray light correction coefficient andgain coefficient) and algorithm using blackbody images at various temperatures (–10°C to 50°C) obtained with CIRC
PFM at various environmental temperatures (–15°C to 50°C). A flow chart of the data correction is shown in Figure 5.
① Bad pixel correction
A bad pixel is a pixel that has a brightness lower or higher than the surrounding pixels. We correct the brightnessof a bad pixel by substituting the mean brightness of the normal surrounding pixels.
② Dummy correction
The electrical background is subtracted using dummy pixels, which have no sensitivity to incident infrared rays.
③ Stray light correction
Stray light correction is performed for each pixel by using the stray light correction coefficient.
④ Gain correction
Sensitivity correction is performed for each pixel by using the gain coefficient.
The experimental setup for the construction of the calibration data table is shown in Figure 6. The CIRC was installed ina vacuum chamber and enclosed a shroud to control its ambient temperature with a heater and cooler. The blackbody was
set up in the front of the lens.

Figure 5. Flow chart of data correction.

Figure 6. Experimental setup for construction of the calibration data table.

4.2 Confirming performance
We confirmed the imaging quality and radiometric quality of image after the data correction discussed in section 4.1.
4.2.1 Imaging quality
The modulation transfer function (MTF) was measured in order to evaluate the imaging quality of CIRC at various
operating temperatures (–15°C to 50°C). The experimental setup for MTF measurement is shown in Figure 7. The
collimated infrared rays passed through a germanium window on the side of the vacuum chamber. The CIRC was able tocapture images of the four-bar target. Figure 8 shows the measurement points of the four-bar target for MTF, at thecenter, ±5° in the cross-track (CT) direction, and ±3.5° in the along-track (AT) direction.

Figure. 7 Experimental setup of MTF measurement.

(0, 0)No.1

No.2+3.5° No.3

No.4

No.5

No.6

-5°

No.7

0°

No.8

+5° Cross
Track
(CT)

-3.5° No.9

Along Track(AT)(639, 479)

Figure 8. Measurement points of MTF on image acquired by CIRC

Figure 9 shows the MTF results of CIRC PFM for ALOS-2 and CALET. The MTF in the CT direction for ALOS-2(Figure 9 (a)) is constant with the value of 0.2 ~ 0.39, which is regardless of the environmental temperatures. In contrast,
the MTF in the AT direction (Figure 9 (b)) is slightly lower in low-temperature environments. In the case of CIRC PFM
for CALET, the MTF are 0.24 ~ 0.33 in the CT direction (Figure 9 (c)) and 0.34 ~ 0.54 in the AT direction (Figure 9 (d)),
respectively. The trend of the environmental temperature dependence is the same as the results of CIRC PFM for
ALOS-2. MTF was found to be effective at capturing clear images across the entire range of operating temperatures forboth CIRC PFM. We confirmed the athermal optical performance of the CIRC.
4.2.2 Radiometric quality
The noise equivalent differential temperature (NEDT) and flat pattern noise (FPN) were estimated, shown in Table 3.
These are the results expected from the design of the CIRC, and satisfied its specifications.
Figure 10 shows a comparison between a raw image and corrected image obtained by PFM for ALOS-2. Performing data
correction makes it possible to capture smooth images without using a shutter system. The temperature accuracy is lessthan 2 K in 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 CIRC PFM for CALET, thetemperature accuracy is 0.5 ~ 2.8K, which is also within the specification.

(a)ALOS-2

(c)

CALET

(b)

ALOS-2(d)

CALET

Figure. 9 Measurement results of MTF onboard ALOS-2 (left) and CALET (right).
(top) MTF in the CT direction, (bottom) MTF in the AT direction.

Table 3. Results of NEdT and FPN
for ALOS-2
for CALET

NEdT [K]0.19
0.18

Figure 10. Blackbody image obtained by CIRC onboard ALOS-2.
(top) raw image and (bottom) corrected image.

FPN [K]
0.27
0.21

5. AIRBORNE OBSERVATION WITH THE CIRC GROUND TEST MODEL
We carried out airborne observations using the ground test model (GTM) of CIRC, which has optical and radiometricperformances equivalent to the corresponding PFM. Observational flight was carried out on March, 22 and 28, 2012. The
aircraft was a “Cessna172 Sky hawk” made by the Cessna Aircraft Company. The observation area was Tsukuba City,
Tsuchiura City in the south of Ibaraki Prefecture, and Narita City in Chiba Prefecture, all in Japan. The flight altitude
ranged from 300 m to 750 m. The ground sample distance (GSD) at these altitudes ranged from about 10 cm to 25 cm.
Figure 11 shows the aerial image of Tsukuba Space Center, which is a facility in JAXA, captured from an altitude 750 m
with a compact digital camera. The yellow rectangle is field of view of the CIRC. Figure 12 shows a comparison
between the raw image and corrected image captured by an airborne observation from an altitude of 750 m. We
confirmed that the data were corrected in the shutter-less system and that the actual images could be corrected by using
the ground calibration data.
The temperature correction accuracy was checked using the image of the pond area shown in Figure 12 with the whitebox. The temperature of the pond measured using a water temperature gauge was about 11°C. The corrected dataobtained by CIRC GTM revealed a temperature of 11.5°C, with the assumption that the emissivity of the water was 0.98.
The temperature correction accuracy was about 0.5°C, which is within the specification of 2 K with the environmental
temperature of ~ 300K.

Figure 11. Aerial image of Tsukuba Space center from 750m above.

Figure 12. Image captured by airborne observation:(left) raw image and (right) data corrected image, where white box shows pond area.

6. CONCLUSIONCIRC is an instrument equipped with an uncooled infrared array detector, which will be launched in JFY 2013 onboard
ALOS-2, and in JFY 2014 onboard JEM/CALET. We have finished the ground calibration test of the CIRC onboard
ALOS-2 and CALET, and have confirmed that its performance is as expected and sufficient for launch. We have also
performed data correction using a shutter-less system by analyzing the data obtained through airborne observations. The
temperature correction accuracy was about 0.5°C, which is within the specification of 2 K.

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