Keywords

1 Introduction

The long-wave infrared imaging spectrometer is usually composed of telephoto objective, spectroscopic mount, background suppression module, detector, and auxiliary circuit. The telephoto objective images the ground object onto the slit of spectroscopic mount. The slit image reaches the detector after the dispersion and imaging of the spectroscopic mount. The long-wave infrared imaging spectrometer with working band of 8–14 μm has both functions of imaging and spectral analysis. It has more and more extensive application in military field, chemical gas detection, ground object identification, forest fire detection, and drought monitor. The Aerospace Corporation designed the famous airborne LWIR/MWIR Imaging Hyperspectral Sensor abbreviated as SEBASS (Spatially Enhanced Broadband Array Spectrograph System) in 1996 [1]. The spectral image is formed by a spherical prism/conic mirror combination. SEBASS is used as a tool to explore the utility of hyperspectral infrared sensors for remotely identifying solids, liquids, and gases in the 2–14 μm spectral region. University of Hawaii designed Airborne Hyperspectral Imager in 1998 to detect where the land mines were buried through the detection of long-wave infrared bands associated with mine installation from the air [2]. In late 2006, the Jet Propulsion Laboratory applied the quantum well infrared focal plane technology to the Dyson thermal infrared spectrometer and developed a breadboard thermal infrared line spectrometer named the Quantum Well infrared photodetector Earth Science Testbed [3]. In 2008, the Aerospace Corporation designed a new airborne facility instrument for Earth science applications called the Mineral and Gas Identifier. It is an advanced high radiometric sensitivity, moderate spectral resolution, airborne thermal infrared imaging spectrometer as a potential prototype for future space-based TIR instruments [4]. In 2008, the German Aerospace Center proposed for ESA’s deep space mission an imaging spectrometer named MERTIS, which meets the challenges of limited technical resources and a very special operational environment. It is based on modern uncooled microbolometer technology and all-reflective optics design [5]. In 2012, the Jet Propulsion Laboratory reported a novel, completely uncooled, long-wave IR Dyson spectrometer. The F/# of the system is 1.6 and wavelength coverage is 7.5–12 μm [6]. In 2014, the French Aerospace Lab developed an airborne hyperspectral imaging system named SYSIPHE, covering atmospheric bands from the visible to the long-wave infrared. SIELETERS is the infrared component of SYSIPHE. It is composed of two distinct instruments, one for the MWIR and one for the LWIR. They are both imaging static Fourier transform spectrometers and cryogenically cooled to achieve high performance [7]. In 2014, the Shanghai Institute of Technical Physics of The Chinese Academy of Sciences reported a practical thermal infrared hyperspectral imager. The model machine adopts the technical route of “push-broom imaging + thermal infrared focal plane detector + dispersion type of light splitting component + cryogenic optics module + airborne real-time calibration” [8]. Based on the research situation of typical instruments at home and abroad, the self-thermal radiation is severe in the LWIR hyperspectral systems if no measures to suppress are taken.

In this paper, two main methods to suppress the self-thermal radiation in the LWIR hyperspectral systems are introduced and compared. The first method uses a relay lens behind the spectroscopic imaging system to form a real exit pupil, which is matched with the cold stop of focal plane array detector. As a result, its cold shield efficiency is up to 100%. And, the self-thermal radiation outside its field of view can be suppressed. For the second method, the slit and opto-mechanical parts after the slit are all cooled to reduce the background temperature and to lower the self-thermal radiation. The specific comparison results are shown below.

2 System Design

Firstly, the long-wave infrared imaging spectrometer with F/# 2.7 and wavelength ranges of 8–12.5 μm is designed. Its main specification is listed in Table 1. The detector is a HgTeCd infrared detector. The working band of the detector is 8–12.5 μm. The pixel number is 1280 × 256, and the pixel size is 24 μm × 32 μm.

Table 1 Optical design parameters
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The designed optical system is shown in Fig. 1. It is based on the Offner concentric design and consists of the slit, concave mirror, convex grating, and detector. The grating is placed about midway between the slit and the concave mirror. The spectral splitting is accomplished by using a reflective diffraction grating. Different colors of rays represent different wavelengths. The MTF curve and spot diagram are, respectively, shown in Figs. 2 and 3. It can be seen that the value of MTF is near to diffraction limit and the majority of the image spot concentrates in the pixel. The system has a diffraction-limited imaging characteristics.

Fig. 1
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System structure

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Fig. 2
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MTF curve of the designed system, the upper: 8.00 μm, the middle: 10.25 μm, the bottom: 12.5 μm

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Fig. 3
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Spot diagram of the designed system, the upper: 8.00 μm, the middle: 10.25 μm, the bottom: 12.5 μm

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3 Stray Light Analysis

Considering that target energy is weak and background thermal radiation is serious in the long-wave infrared hyperspectral imaging system, effective measures must be taken to suppress the self-thermal radiation stray light. Firstly, the average irradiance of the target at focal plane can be calculated by the following equation:

$$ E=\frac{\varepsilon \tau \eta M(T)}{4{NF}^2} $$
(1)

In Eq. (1), ε is the emissivity of the target, τ is the transmittance of the system, η is the grating diffraction efficiency, N is the number of the spectral channel, and F is the F number of the system. M(T) is the total radiation emission of detecting band and related to wavelength. It is defined by the following formula:

$$ M(T)={\int}_{\lambda_1}^{\lambda_2}\varepsilon \frac{c_1}{\lambda^5}\frac{1}{{\mathrm{e}}^{C_2/\lambda T}-1} d\lambda $$
(2)

In Eq. (2), c1, c2 is the first and second Planck constants, c1 = 3.7415 × 10−16 W ⋅ m2, c2 = 1.4388 × 10−2 m ⋅ K. The self-thermal radiation stray light in the long-wave infrared hyperspectral imaging system is analyzed based on the above equations.

The long-wave infrared imaging spectrometers are commonly used to detect the normal temperature events. The temperature of the target is taken as 300 K, and the emissivity of the target is taken as 0.9. After computation, the average irradiance of image surface is 1.393E − 7 W/mm2. When no measures are taken to suppress background radiation, the operating temperature of the instrument is 300 K. For an element, its radiation exitance is calculated through its working temperature. Then, we can get its radiant power combined its surface area. We set this element as the light source in the LightTools software, and set the corresponding power. Then the effect of the self-thermal radiation of the element on the focal plane can be obtained by tracing the light. Figure 4 is the stray light analysis model constructed by the software SolidWorks. Table 2 shows the influence of self-thermal radiation of the main components and the corresponding stray light contribution. All mechanical components are temporarily blackened.

Fig. 4
figure 4

The stray light analysis model

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Table 2 Optical properties of various components of the system
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From Table 3, it can be seen that the total irradiance of various components at image plane is 1.30E − 4 W/mm2 when the working temperature is 300 K. The largest contribution of stray radiation is the shell. At this time, the magnitude of self-thermal radiation irradiance at image plane is three orders of magnitude higher than that of signal energy under normal temperature. As thus, the signal is completely drowned out by noise and the spectrometer cannot work properly. Effective measures must be taken to suppress background radiation.

Table 3 Thermal radiation of main components of the system (working at room temperature 300 K)
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3.1 Method I: The Introduction of the Cold Stop

We use a relay lens behind the Offner spectroscopic imaging system to form a real exit pupil, which is matched with the cold stop of focal plane array detector. As a result, its cold shield efficiency is up to 100%. Figure 5 is the system with a relay lens connected.

Fig. 5
figure 5

The system with a relay lens connected

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After the relay lens is connected, the transmittance of the system is reduced due to the introduction of lens. The temperature of the target is still 300 K, and the emissivity is 0.9. At this moment, average irradiance of the image surface is 1.23E − 7 W/mm2. Figure 6 is the stray light analysis model of the system with a relay lens connected. Table 4 shows the influence of self-thermal radiation of the main components and the corresponding stray light contribution. The temperature of the Dewar is assumed 50 K.

Fig. 6
figure 6

Stray light analysis model of the system with a relay lens connected

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Table 4 Thermal radiation of main components of the system with a relay lens connected
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From Table 4, we can see that the introduction of the cold stop can suppress background radiation to a certain extent. At this time, the irradiance of background radiation at focal plane is about 14% of the situation in which no suppression measures are taken. However, the irradiance of background radiation at focal plane is still two orders of magnitude higher than that of the signal.

3.2 Method II: The Whole Cooling Opto-Mechanical Parts

From Method I, it can be seen that the introduction of the cold stop can suppress background radiation to a certain extent, but the signal is overwhelmed by noise so that the spectrometer cannot work properly. Therefore, we think about the second method: the whole cooling opto-mechanical parts namely that the slit and opto-mechanical parts after the slit are all cooled in the Dewar to reduce the background temperature and to lower the self-thermal radiation. The stray light analysis model is still shown in Fig. 4. Table 5 shows the influence of self-thermal radiation of the main components and the corresponding stray light contribution when the Dewar is 150 K. Table 6 shows the background radiation irradiance at focal plane and the corresponding stray light coefficient under different cooling temperatures.

Table 5 Thermal radiation of main components of the system (when the cooling temperature is 150 K)
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Table 6 Background radiation irradiance at focal plane and corresponding stray light coefficient under different cooling temperatures
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From the Table 5, it can be seen that background radiation irradiance at focal plane is 1.34E − 6 W/mm2 when the cooling temperature is 150 K almost 1/100 of that when the temperature is 300 K. From the Table 6, it can be seen that temperature has a great influence on background radiation. The effect of background radiation is very small when the cooling temperature is 90 K.

4 Conclusion

Comparing two methods, we find that the introduction of the cold stop can suppress background radiation to a certain extent. However, it is still not enough for the long-wave infrared imaging spectrometers. Therefore, we need to take the second method: the whole cooling opto-mechanical parts. It can suppress background radiation better than the introduction of the cold stop.