High-sensitivity short-wave infrared technology for thermal imaging

Accepted Manuscript High-sensitivity short-wave infrared technology for thermal imaging Maoxing Wen, Liqing Wei, Xiaoqiong ZHuang, Daogang He, Shengwei Wang, Yueming Wang PII: DOI: Reference:

S1350-4495(18)30540-1 https://doi.org/10.1016/j.infrared.2018.10.020 INFPHY 2734

To appear in:

Infrared Physics & Technology

Received Date: Revised Date: Accepted Date:

20 July 2018 16 October 2018 16 October 2018

Please cite this article as: M. Wen, L. Wei, X. ZHuang, D. He, S. Wang, Y. Wang, High-sensitivity short-wave infrared technology for thermal imaging, Infrared Physics & Technology (2018), doi: https://doi.org/10.1016/ j.infrared.2018.10.020

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High-sensitivity short-wave infrared technology for thermal imaging a

Maoxing Wen,a,b Liqing Wei,a,b Xiaoqiong ZHuang,a Daogang He,a Shengwei Wang, and Yueming Wanga,* a

Key Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese

Academy of Sciences, Shanghai 200083, CHINA b

University of Chinese Academy of Sciences, Beijing 100049, CHINA

*[email protected]

Abstract: Thermal imaging technology is widely used in various fields, such as security, defense, medicine and spaceborne applications in the Geo-synchronous Earth Orbit. Most thermal imaging systems operate in the long-wave infrared (LWIR) range or the medium-wave infrared (MWIR) range. However, the longer the wavelength is, the more difficult the imaging system is to provide high resolution due to diffraction limit. Compared to MWIR and LWIR imagers, a short-wave infrared (SWIR) imager achieves higher resolution owing to its higher diffraction limit. However, radiation from an object at 300k in the SWIR band is weaker than that in the MWIR and LWIR bands. Therefore, the sensitivity is low. With the rapid development of detector technology, the sensitivity of SWIR detector is improved by low dark current and noise. It exhibits good performance in thermal imaging applications. In order to validate SWIR thermal imaging key performance, this study analyzes the thermal imaging capability of an SWIR imager compared to MWIR and LWIR. The noise equivalent temperature differences (NETDs) of SWIR, MWIR, and LWIR imagers for thermal imaging applications are calculated. A compact SWIR imager prototype covering a spectral range of 2.0–2.7μm is designed and implemented. The facial thermal imaging experiment proves that the high-sensitivity SWIR imager can obtain high-quality thermal images with an NETD of 24 mK at a temperature of 310 K. In addition, in the continuous integration and multiple read-out (IMRO) imaging mode, the dynamic range of the system is extended by 20 dB. The theoretical analysis and experiment show that the SWIR imager can provide high sensitivity with long integration time. In addition, SWIR thermal imaging can provide higher resolution than MWIR and LWIR imagers. Therefore, SWIR imager has considerable potential for high-sensitivity thermal imaging applications in Geo-synchronous Earth Orbit. Keywords: imaging system; short-wave infrared; thermal imaging; NETD; dark current.

1. Introduction Thermal imaging technology is widely used in various fields, such as security, defense, and medicine. Sensitivity is a key parameter in thermal imaging technology. Most thermal imaging systems operate in the long-wave infrared (LWIR) range or the medium-wave infrared (MWIR) range due to their high performance of sensitivities. SWIR imagers are not common used in thermal imaging application due to low performance of sensitivities and widely used in reflecting detection applications, including military targeting, long-range imaging, and medical imaging [1][2]. However, SWIR imager has the ability to capture the details of a target obscured by haze or smoke. In addition, compared to MWIR and LWIR imagers, an SWIR imager is more compact and has a higher diffraction limit. Therefore, SWIR imager can provide higher resolution than MWIR and LWIR imagers. The rapid development of SWIR imagers has been driven by the requirements of the

above-mentioned applications, and the principal trend of the development is toward improved sensitivity, spatial resolution, and miniaturization. With the rapid development of detector technology, dark current and readout noise become lower. Thus, the sensitivity of an SWIR detector can be improved considerably. The improved sensitivity of SWIR imaging foster/ promote /facilitates its use in thermal applications, including night vision, plume imaging, diagnostic, monitoring method for various disorders [3-6] and the field of non-destructive defect detection of materials [7, 8]. In addition, SWIR imager can have high dynamic range with continuous integration and multiple read-out (IMRO) technology. Therefore, SWIR imaging has potential ability in spaceborne thermal imaging applications including straw combustion detection and volcano monitoring in Geo-synchronous Earth Orbit. The scientific goal of this paper is to evaluate thermal imaging performance of SWIR imager and potential use in spaceborne remote sensing in Geo-synchronous Earth Orbit. In this study, the thermal imaging capability of an SWIR is analyzed theoretically in section 2. The parameters of the system are discussed to evaluate its performance. The results of the analysis indicate that a high-sensitivity SWIR imager has considerable potential for thermal imaging applications. A comparison of the noise equivalent temperature differences (NETDs) of SWIR, MWIR, and LWIR imagers for thermal imaging applications shows that a high-sensitivity SWIR imager exhibits good temperature sensitivity. In section 3, temperature sensitivity evaluation is conducted using a compact high-sensitivity SWIR imager prototype. In addition, we use a peculiar mode to obtain multiple images of the same scene with different exposure times. The dynamic range of the system can be expanded by fusing these images. The experiments show that sensitivities of SWIR imager can be used in thermal imaging applications. In addition, SWIR imager has higher resolution and dynamic range than MWIR and LWIR imagers.

2. Theoretical analyses of thermal imaging 2.1 Analyses of SWIR imager Infrared thermal imaging system performance is generally measured in terms of temperature resolution or temperature sensitivity. Temperature sensitivity is generally expressed in terms of the NETD. The NETD of a staring imager can be expressed as nFPA NETD   s / T

(1)

where nFPA represents all the noise of a single pixel, given by 2 nFPA  s   BG   Id  nRead

(2)

where ξs, ξBG, and ξId represent the numbers of electrons contributed by the signal, background radiation, and dark current, respectively. Further, nRead represents all the noise in the acquisition system, which includes detector readout noise, electronic noise, and analog-to-digital noise. The differential of the number of electrons contributed by the signal is expressed as 2 c c  ( )  s L A Q A Q ec2 / T   a ( )  0 ( )  m  d 2 E  d 2 E  tint   1 2 2 5  c2 / T d 1 T T 4F 4F  T  (e  1)2

(3) where Ad is the pixel area, 1/F is the system relative aperture, QE is the quantum efficiency of the detector, tint is the integration time, τ(λ) is the system optical efficiency, T is the black-body absolute temperature, and C1 and C2 are constants: C1=1.88365×1023 s-1·m-2·μm3， C2=1.43879×104 μm·K

(4)

According to the expression of the NETD, at a certain temperature, the temperature sensitivity of the system is determined by many system parameters. These parameters are mutually constrained. For instance, the increase in pixel area can improve the system temperature sensitivity but reduces the system spatial resolution, thereby affecting the image quality. To maintain the spatial resolution, the focal length should be increased accordingly, i.e., the F number of the system should be increased such that the NETD of the system remains unchanged. If the spectral response is wider and the cutoff wavelength of the detector is longer, greater radiation is received by the detector, and the differential radiance of the target is higher. Hence, a wider spectral response is beneficial for target detection. However, a wider spectral response results in stronger background radiation, shorter integration time, and larger equivalent bandwidth of the system, which makes detection more difficult. The thermal radiation energy emitted from the earth’s surface at ordinary temperatures is extremely low, i.e., it is in the SWIR band (1.0–2.8 μm). However, the thermal radiation energy received by the SWIR imager can be increased by extending the integration time. If the dark current and system noise are sufficiently low, a high-sensitivity SWIR imager can be realized for thermal imaging. We usually consider that a system can obtain high-quality thermal image data with an NEDT in the range of 50–200 mK. The theoretical analysis of the technical requirements for the dark current and total noise to guarantee that the SWIR imager achieves high-sensitivity (200 mK) thermal imaging is described below. The main parameters of the system are summarized in Table 1. Table 1 Main parameters of the system Item Spectral range (μm)

Value 1.0~2.8

System optical efficiency

0.8

Quantum efficiency

0.7

Pixel size (μm)

25

Saturated electron number (Me-)

2.5

F number

2.0

In thermal imaging applications, the radiation received by the imaging system includes the target thermal radiation and the background radiation. The background radiation of the system can be simplified as radiation from two sources: thermal radiation from the inner wall of the Dewar window and black-body radiation corresponding to the cross angle of the detector Dewar window. A schematic diagram of the background radiation of the instrument is shown in Fig. 1.

Fig. 1 Schematic of background radiation of the instrument

According to the schematic shown in Fig. 1, the photon radiant flux of the Dewar inner wall and structural background can be expressed as [9]

Rdewar   dewar 

2

0

h  2c /  4  R d 2 h 2 d 2 h2 xdx  dx  d  d  r ( x 2  h 2 )2 hc 2 2 2  0 (x  h )  (e  kT  1) 

Rstructure   structure 

2

0

r

2

0

1

 

2c /  4 (e

hc  kT

 1)



d 2h2 xd  dxd ( x 2  h2 )2

(5)

where ξdewar is the surface emissivity of the Dewar interior and ξstructure is the surface emissivity of the structure. The parameters of the detector focal plane assembly (FPA) are

= 0.9, r = 12.25 mm,

and h = 31 mm. r and h are defined in Fig. 1. The detector is cooled by a Stirling cryocooler, and the temperature of the Dewar is 120±1 K. Therefore, the background radiation from the inner wall of the Dewar is negligible, and the background radiation received by the system is equal to the background radiation of the structure. Assuming that the target temperature is 400 K and the ambient temperature is 290 K, the integration time and the electronic flux of the target radiation and background radiation are listed in Table 2. Table 2 Electronic flux received by the system and integration time Item

Value

Electronic flux of background radiation at 290 K (e-/s)

5.814×106

Electronic flux of target thermal radiation at 400 K (e-/s)

4.498×108

Integration time (ms)

4.39

SWIR imager with high temperature sensitivity (200 mK): NETD 

nFPA   s / T

2  s   BG   Id  nRe ad  0.2  s / T

(6)

The curves of relevance between the noise of the information acquisition system and the dark current when the temperature sensitivity of the system is 200 mK are shown in Fig. 2. To achieve an NETD of 200 mK with a target temperature of 300 K, the maximum allowable noise of the information acquisition system is 109 e-, and the corresponding dark current is 8 fA. When the target temperature rises to 310 K, the maximum noise of the information acquisition system is 320 e-, corresponding to the lowest dark current of 22 fA.

Fig. 2 Curves of relevance between noise of the information acquisition system and dark current at different temperatures According to the analysis presented above, the temperature sensitivity of the SWIR imager is better than 200 mK when the target temperature ranges from 320 K to 400 K, and the information acquisition system noise of the high-sensitivity SWIR imager is lower than 320 e-.

2.2 Comparison of SWIR, MWIR, and LWIR imagers The 3–5 μm and 8–12.5 μm bands are two infrared atmospheric windows commonly used in infrared thermal imaging, because most terrains exhibit strong thermal radiation in these two bands. Therefore, MWIR and LWIR imagers are widely employed in thermal imaging. However, according to the theoretical analysis described in Section 2.1, a high-sensitivity SWIR imager exhibits high temperature sensitivity in thermal imaging applications. Compared with MWIR and LWIR imagers, an SWIR imager has several advantages, such as low dark current of detector, low noise level of information acquisition system, low background radiation, and long integration time. Furthermore, in practice, the longer the wavelength of the system response spectrum, the higher is the requirement for uniformity between the pixels of the detector. Moreover, according to the optical diffraction theory, the relationship between the minimum angular resolution of the optical system (φ) and the optical caliber of the system (D) is given by



1.22 D

(7)

The minimum angular resolution of the optical system multiplied by the orbital height of the system gives the ground spatial resolution of the camera. Therefore, to improve the spatial resolution, it is necessary to have a large optical system caliber. When the orbital height and system spatial resolution are set, the optical caliber aperture diameter of the SWIR imager is much smaller than that of an MWIR or LWIR imager. Therefore, in thermal imaging applications, an SWIR imager is beneficial for reducing the volume and weight of the imaging system to meet the requirements of different platforms, and it is thus consistent with the trend of system miniaturization and lightening. As an SWIR imager has many advantages, it is necessary to compare the performances of SWIR, MWIR and LWIR imagers in thermal imaging applications on the basis of their NETDs. The main parameters of the three systems are summarized in Table 3. Suppose that the three systems have no cryogenic optics for background radiation suppression and that the parameters of the three cooled infrared detectors are the same (

= 0.9, r = 12.25 mm, and h = 31 mm). When

the ambient temperature is 290 K, the NETDs of these systems at different temperatures can be determined as shown in Fig. 4(a). The relationship between the NETD and the integration time of the

SWIR imager when the target temperature is 310 K is shown in Fig. 3(b). Table 3 Parameters of three imager systems Item

SWIR

MWIR

LWIR

Spectral range (μm)

1.0~2.8

3~5

8~12.5

Pixel size (μm)

25

25

25

Quantum efficiency

0.7

Dark current (A)

1.0×10

0.7 -14

1.5×10

0.7 -12

1.81×10-10

Saturated electron number (Me-)

2.5

36

36

F number

2.0

2.0

2.0

System optical efficiency

0.8

0.8

0.8

Noise of information acquisition system (e-)

200

1000

1600

Fig. 3(a) NETDs of imagers at different temperatures; (b) NETD of SWIR imager at different integration times with a target temperature of 310 K In Fig. 3(a), the NETD of the LWIR imager varies gradually and smoothly in the temperature range of 270–450 K, with a maximum of 78 mK at 270 K and a minimum of 37 mK at 450 K. Therefore, when temperatures of multiple targets in the imaging scene differ considerably, or when the difference between the temperatures of the target and the background is relatively large, the LWIR imager is the most suitable system for imaging. In the case of the LWIR imager, the temperature sensitivity is high, all the targets and the background are clear in the thermal image, and the NETDs of different objects do not vary significantly. Owing to its precise detection capabilities in complex scenes, the LWIR imager is widely used in a variety of target detection scenarios. In particular, it plays an important role in camouflage recognition and underwater target detection. The NETD curve of the MWIR imager in Fig. 3(a) is steeper than that of the LWIR imager; the maximum NETD is 171 mK and the minimum NETD is 12 mK. The NETDs of the MWIR and LWIR imagers are both equal to 55 mK at a target temperature of 310 K. Because background radiation in the MWIR band is much smaller than that in the LWIR band, when the temperature is higher than 310 K, the NETD of the MWIR imager is better than that of the LWIR imager. On the other hand, the NETD curve of the MWIR imager is smoother than that of the SWIR imager; hence, the MWIR imager can perform better in a wider temperature range compared to the SWIR imager.

The NETD curve of the SWIR imager in Fig. 3(a) is much steeper than that of the LWIR imager. The main reason for this phenomenon is that the thermal radiation of a low-temperature target is not distributed widely in the SWIR band. As the energy received by the SWIR imager is extremely weak when the integration time is short, the noise of the information acquisition system becomes the main noise of the total system. It is difficult to obtain clear images of low-temperature targets in the case of a weak signal, resulting in low temperature sensitivity of such targets; e.g., the NETD is 879 mK at a temperature of 300 K. However, as the temperature of the object increases, the NETD of the SWIR imager decreases rapidly. When the temperature is higher than 400 K, the NETD of the SWIR imager reaches 47 mK, which is close to the NETD of the LWIR imager. The SWIR imager achieves the highest temperature sensitivity with an NETD of 25 mK when the target temperature rises to 450 K. The curve shown in Fig. 3(a) indicates that the high-sensitivity SWIR imager can achieve high temperature sensitivity in thermal imaging applications if the target temperature is in the range of 335–450 K. According to Fig. 3(b), when the target temperature is low, an increase in the integration time leads to considerable improvement in the system sensitivity. For instance, when the integration time increases from 1 ms to 5 ms, the NETD is reduced from 500 mK to 144 mK. The improvement in the NETD slows down, and when the integration time is sufficiently long, the shot noise becomes the main noise of the system.

2.3 Extending the dynamic range of an SWIR imager Fig. 3(a) shows that the NETD of the SWIR imager decreases sharply as the temperature increases, and Fig. 3(b) indicates that the NETD can be improved considerably by prolonging the integration time. The integration time for a high-temperature target to achieve high-sensitivity detection is short. As a low-temperature target produces extremely weak thermal energy, it exhibits poor temperature sensitivity. When extending the integration time to ensure that the imager receives adequate energy in the form of thermal radiation from a low-temperature target, high-temperature targets will be saturated and cannot be detected. Owing to the limited dynamic range of the system, it is difficult to consider high-sensitivity detection of both high-temperature targets and low-temperature targets. Thus, an increasing number of methods have emerged to solve this problem by extending the dynamic range of the imager system. Image fusion based on multiple exposures is the most common digital processing method for not only extending the dynamic range of the system but also improving the SNR of the images. This method images the same scene at various exposure times, where short exposure times are mainly used to collect image information of high luminance areas before the sensor is saturated and long exposure times are used is to acquire image information of low luminance areas. Thus, we can obtain an image with a high dynamic range and low time-domain noise, which preserves the original details of the scene as far as possible. The conventional image fusion method based on multiple exposures is as follows. The imager collects multi-frame image data with different integration times. Then, these images are transformed into a high-dynamic-range image. The integration time in each imaging process is set; thus, the imager needs to image the same scene many times to obtain data with different integration times. The detector Dewar assembly used in this study has a peculiar working mode called the continuous integration and multiple read-out (IMRO) mode, which allows the exposure time to accumulate step by step with lossless read-out. When the detector works in the IMRO mode, it can be controlled by changing the configuration parameter to reset its integral capacitor. When this parameter is set as “1” to start the IMRO mode, the integration time of the detector and charge on the integral

capacitor are constantly accumulated until the parameter is configured as “0” to stop the IMRO mode. Then, the charge on the integral capacitor is reset to zero. Suppose that the frame frequency of the system is 100 Hz. The IMRO mode starts in the first frame and stops in the 20th frame; hence, the IMRO series is 20. Then, the integration time of the first frame image is 10 ms, that of the second frame image is 20 ms, that of the third frame image is 30 ms, and that of the 20th frame image is 200 ms. When the SWIR imager works in the IMRO mode, it can rapidly obtain multi-frame images of the same scene with different exposure times. The spacing interval between imagers of different exposure times is extremely short. Thus, the radiance of the targets of the imagers can be considered as invariable. Therefore, the imaging method with multiple exposure times used in this study is considerably different from the conventional method; multiple images with different exposure times can be obtained rapidly and efficiently.

3 Experiments and results 3.1 Thermal imaging using SWIR imager An SWIR imager prototype is designed and implemented to verify the feasibility of the SWIR imager for thermal imaging. The imaging experiment system includes an optical lens, an SWIR HgCdTe detector, an optical board, and an operating computer. The main parameters of the SWIR imager are summarized in Table 4. Table 4 Main parameters of the SWIR imager Item

Value

Spectral range (μm)

2.0~2.7

System optical efficiency

0.8

Quantum efficiency

0.7

Pixel size (μm)

25 -

Saturated electron number (Me ) F number Dark current (A) Noise of information acquisition system (e-)

2.5 (low gain) 2.0 1.0×10-14 200

A facial thermal imaging experiment was conducted in a laboratory without illumination. The temperature in the laboratory was 290 K, and the imager was operated at low gain with an integration time of 100 ms. The thermal image obtained by the SWIR imager is shown in Fig. 4, and a photograph of the same scene captured by a Nikon digital camera is provided for comparison. The settings of the camera are as follows: F number = 4 and exposure time = 1/25 s.

(a) Thermal image by SWIR imager

(b) Photograph captured by digital camera

Fig. 4 Comparison of thermal image obtained by SWIR imager and photograph captured by digital camera The photograph captured by the digital camera shows that the main energy received by the SWIR imager is the thermal radiation from the target. The color of the thermal image corresponds to the thermal radiation energy of the target, i.e., the surface temperature of the target. The color of the obtained image is clear, demonstrating that the temperature sensitivity of the SWIR imager is relatively high and that the SWIR imager can detect relatively small temperature differences. After the thermal image was obtained, calibration of the NETD was carried out to quantitatively measure the temperature sensitivity of the system. An extended source black body is generally used to calibrate NETD in the laboratory. The temperatures of the black body are set as T1 and T2 (T2 > T1), and the radiation responses of the SWIR imager to the black body are DN1 and DN2, corresponding to the SNR of the system (SNR1 and SNR2). Then, the NETD of system at temperature T1 is given by

NETD 

T2  T1 SNR2  SNR1

(6)

The theoretical and measured values of the NETD at different temperatures are listed in Table 5. Table 5 Theoretical and measured values of NETD Blackbody temperature (K)

290

300

310

Theoretical NETD (mK)

65

42

29

Measured NETD (mK)

53

40

24

The measured NETD values in Table 5 prove that the high-sensitivity SWIR imager can provide high temperature sensitivity in thermal imaging applications, owing to the extremely low system noise and abundant integration time. There is a deviation between the theoretical and measured NETD values, the reasons for which are as follows: environmental temperature changes lead to errors in radiation energy measurement, the actual value of the information acquisition system noise is smaller than 200 e-, and the black body is susceptible to environmental effects. In addition, the method of NETD measurement leads to errors in the result.

3.2 Imaging with extended dynamic range An experiment was carried out in a completely black laboratory to verify the expansion of the dynamic range when the imager operates in the IMRO mode. The SWIR imager operated at a low gain

with 18 series of IMRO, i.e., the integration time of the image increased from 10 ms to 180 ms successively. The imaging targets included a 413 K soldering iron, containers filled with water at 353 K and 313 K, and a 293 K square box. The images obtained by the SWIR imager in IMRO mode are shown in Fig. 5; the parts highlighted in red in the figure have reached the saturation state.

(a) Tint=10ms

(b) Tint=20ms

(d) Tint=100ms

(e) Tint=150ms

(c) Tint=90ms

(f) Tint=180ms

Fig. 5Images obtained by SWIR imager in IMRO mode From Fig. 5, when the integration time is 10 ms, the soldering iron is nearly saturated, the 353 K water barely appears, and the signals of the 313 K water and 293 K square box are too weak to be detected. The soldering iron in the image is saturated when the integration time is 20 ms. As the integration time increases, the signals of the water and box area increase in succession. The 353 K water reaches the maximum value before saturation at an integration time of 100 ms, and the 313 K water and 293K square box did not reach the saturation state even at the longest integration time of 180 ms in the IMRO mode. In the 18 images obtained by the IMRO mode, each target in the scene could obtain a clear image, so we could fuse the 18 images to get a high-dynamic-range image (HDRI). The integration time of provided 18 images are from 10 ms to 180 ms. Then, non-uniformity of images are calibrated and each position has 18 values. The unsaturated max value pixel of same position in 18 calibrated images was selected and placed in the same position in a new image. Select and calculate all the positions and the fused image is acquired. The false color fused image is shown in Fig. 6(a).

(a) image after fused

(b) image after transformed

Fig. 6. Fusion image and transformed image of IMRO mode. As the dynamic range of the fused image is much larger than that of the display device, the details of the image cannot be clearly displayed by a normal gray-scale image. To preserve the details of the HDRI as far as possible, a common method is to compress the image range to match the range of the display device by logarithmic transformation. The fused image after transformation is shown in Fig. 6(b). After measurement, when the SWIR imager operates in the IMRO mode, the system noise at different integration times is summarized in Table 6. Table 6 System noise at different integration times Integration time (ms)

10

50

100

180

Digital value of noise

4.8

6.0

7.2

8.5

In this SWIR imager system, the target in the image has a saturated digital value as 16000. Hence, when the integration time is fixed in the range of 10 ms to 180 ms, the maximum (DRmax) and minimum (DRmin) values of the system dynamic range are

DRmax  20log

16000  70.458(dB) 4.8

DRmin  20log

16000  65.494(dB) 8.5

(8)

After fusion of the images in Fig. 6 with integration time from 10 ms to 180 ms, the dynamic range of the system becomes DR fuse  20 log

16000 18  90.599(dB) 8.5

(9)

According to Eqs. (8) and (9), the dynamic range of the system has been greatly expanded by image fusion based on multiple exposures. Owing to the peculiar IMRO mode, the high-sensitivity SWIR imager can rapidly obtain multiple images with different exposure times to ensure that all the targets in the scene have a clear and high-quality image. In addition, as the IMRO series increases, the dynamic range of the system will be further expanded. When the dynamic range of the system is sufficiently high, both high-temperature and low-temperature objects will obtain high temperature sensitivity. Hence, the NETD curve of the SWIR imager will change gradually and smoothly at different temperatures. The IMRO mode helps the SWIR imager to expand its dynamic range and operate in a wider temperature range in thermal imaging applications.

3.3 SWIR thermal detection from Geo-synchronous Earth Orbit SWIR imager can provide higher resolution than MWIR and LWIR in Geo-synchronous Earth Orbit. The parameters of SWIR, MWIR and LWIR are shown in Table. 7. Table. 7. The parameters of SWIR, MWIR and LWIR imagers in Geo-synchronous Earth Orbit Item

SWIR

MWIR

LWIR

Spectral range (μm)

1.0~2.8

3.7~4.8

7.7~9.5

System optical efficiency

0.8

0.8

0.8

Quantum efficiency

0.7

0.7

0.7

Pixel size (μm)

15

30

30

Diameter (m)

1

1

1

83(@1.9 μm)

175.7(@4 μm)

373 (@8 μm)

Focal length (m)

6.5

6.85

2.89

Integration time (ms)

200

5

0.06

System noise (e )

50

1000

1000

NETD (mK)

20

22

22

Diffraction limited GSD (m)

- *

*

System noise level are referenced with Sofradir detectors.

The Table. 7 shows that SWIR can provide GSD with 83 m in Geo-synchronous Earth Orbit. The sensitivity of SWIR is 20 mK with 200 ms integration time. It is as the same level as MWIR and LWIR imagers. The frame rate of SWIR imager is low due to long integration time. However, it does not like moving target detection [10], high frame rate is not necessary in thermal detection applications like straw combustion and volcano detection. High resolution can provide us more details than high frame rate.

4. Conclusion SWIR imagers have witnessed rapid development over the past decade, and the principal trend of the development is toward improved sensitivity, spatial resolution, and miniaturization. This paper focused on a high-sensitivity SWIR imager for use in thermal imaging applications. The results of the theoretical analysis showed that the high-sensitivity SWIR imager can obtain high-quality thermal images, with advantages of low dark current and low information acquisition system noise. The performances of SWIR, MWIR, and LWIR imagers in thermal imaging applications were also compared on the basis of their NETDs. Compared with MWIR and LWIR imagers, an SWIR imager has several advantages, such as lower dark current and system noise, higher spectral detectivity, smaller background radiation, and a more compact optical system. Owing to the limited dynamic range of the system, the temperature range of the SWIR imager with high temperature sensitivity is quite small. A peculiar image fusion mode (IMRO mode) was discussed to extend the dynamic range of the system. With the IMRO mode, images with different exposure times can be obtained rapidly and efficiently; the distribution of the integration time is more detailed in a wider range. In addition, all the images are obtained as a single image. Finally, a prototype SWIR imager was designed to verify the NETD of the system. The results of facial thermal imaging showed that the high-sensitivity SWIR imager can achieve an NETD of 24 mK at a temperature of 310 K. The IMRO mode helps to extend the dynamic range of the system from 70.458 dB to 90.599 dB in 18 stages. In summary, the results of simulation analysis and experiment demonstrated that a high-sensitivity SWIR imager has considerable potential for thermal target detection from Geo-synchronous Earth Orbit. SWIR imager can have higher resolution than MWIR and LWIR imagers due to higher diffraction limit. In addition, SWIR imager has higher dynamic range, compact integration.

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Conflict of interest The author declares that there is no conflict of interest.

Highlights: SWIR thermal imaging can has the same level sensitivity as MWIR and LWIR. IMRO imaging mode extended the dynamic range of SWIR imager to 90 dB. High sensitive SWIR imager prototype with IMRO imaging mode is designed and the experiment shows SWIR imaging can be used in spaceborne thermal imaging applications in Geo-synchronous Earth Orbit.