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Abstract
An infrared scene projector (IRSP) that can operate at an ambient temperature lower than 190 K is developed for the hardware-in-the-loop (HIL) simulation of space-borne IR detection systems. The IRSP is composed of a visible scene generator (VSG), a visible to IR converter, and an IR projection system optimized for cryogenic environments. The core component of the IRSP is a light-driven blackbody micro cavity array (BMCA). The BMCA is a photothermal conversion device. It can transform visible light images into IR images. The BMCA can operate properly in an ultralow temperature environment, and the temperature of the BMCA is the same as the ambient temperature. This property allows the BMCA to generate IR scenes with a pure low temperature background, which is crucial for the ground testing of space-borne IR detection systems. The performance of the IRSP was tested in a vacuum cold chamber. In the cryogenic environment with an ambient temperature of 187.75K, the observed highest temperature of the generated IR scene was 426.15 K, the lowest temperature was 187.75 K, the dynamic range of the IR scene was 38.69 db, and the frame rate of the IR scene reached 76 Hz. The average visible to IR conversion efficiency of BMCA was about 10.6%∼3.1% under different ambient temperatures. The radiation spectrum of the IRSP is close to the standard blackbody radiation spectrum both in the MWIR band and the LWIR band. The IRSP has been applied in a HIL simulation test of a space-borne IR detection system.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, IR detection device has increasingly become one of the essential instruments on spacecraft or satellites [1,2]. Some progress has also been made in the development of IR hardware-in-the-loop (HIL) simulation technology [3–6]. The infrared scene projector (IRSP) is one of the most important components of an IR HIL simulation system. It generates IR scenes that simulate the IR properties of the target and the background. Most conventional IRSPs are developed for room temperature conditions. However, the IRSP in the ground HIL simulation test of a space-borne detection system is usually placed in a vacuum cold chamber, where the internal temperature is cooled to a very low level, e.g., below 200 K. Conventional IRSPs can no longer operate properly at such a low ambient temperature. Therefore, it is necessary to develop an IRSP that can adapt to the cryogenic operating environment.
In a space IR scene, the target temperature is typically higher than 400 K while the background temperature is as low as 100∼200 K [7]. Correspondingly, a qualified cryogenic IRSP should be able to generate IR images with the highest temperature higher than 400 K and the lowest temperature lower than 200 K. At present, the IR image generation devices mainly include the resistor array [8,9], the digital micro-mirror device (DMD) [10–12], the MEMS down-conversion film [13–16], and the blackbody micro cavity array (BMCA) [17]. DMD is a kind of reflective spatial light modulator. It can be used to generate IR images when it is illuminated by an IR light source. The resistor array, the MEMS down-conversion film, and the BMCA are self-radiating devices. They generate IR images by heating the pixels to different temperatures. The difference is that the resistor array is current-driven, whereas the MEMS down-conversion film and the BMCA is light-driven. Both DMD and the resistor array require an electronic driving circuit system. Neither of them can directly work in cryogenic environments since the electronic circuits cannot operate properly at ultralow temperature. Heat preservation measures are required to allow the DMD or the resistor array to work in a cold chamber. However, heat preservation measures will introduce additional thermal noises and make the DMD pixels or the resistor array pixels too hot to generate space IR scenes with sufficiently low temperature backgrounds. The MEMS down-conversion film is a passive device that can operate normally in cryogenic environments. However, the thermal response of the MEMS down-conversion film is relatively slow. It has limitations in the application of dynamic IR scene generation [14].
We present an IRSP for cryogenic environment based on the BMCA in this paper. Similar to the MEMS down-conversion film, the BMCA is also a passive device driven by visible light. It has no electronic circuit and can operate normally in cryogenic environment. The background temperature of the IR scenes generated by the BMCA can be the same as the ambient temperature in the cold chamber. Compared with the MEMS down-conversion film, BMCA shows better time response characteristic and temperature rising characteristic, as well as the mechanical strength. Therefore, the IRSP based on BMCA is a better choice for space IR scene generation in cryogenic environments.
2. System composition and working principles
2.1 System composition
The IRSP consists of a visible scene generator (VSG), a visible to IR converter, and a cryogenic IR projection system designed for cryogenic environments, as shown in Fig. 1.
The VSG generates an optical scene composed of visible light images. Each visible light image is an 8-bit grayscale image with 256 grayscales. The visible light images are then imaged into the visible to IR converter in the vacuum cold chamber. The core component of the visible to IR converter is a BMCA. The pixels of the BMCA are heated to different temperatures by absorbing different visible light energies. An IR image corresponding to the visible light image is then generated on the BMCA surface. The IR image is projected to the IR imaging device outside the chamber by the cryogenic IR projection system. The synchronization module is used to generate trigger signals to adjust the timing of the visible light image generation and the IR image acquisition, so that the IR imaging device can capture IR images with the maximum dynamic range.
2.2 Visible to IR conversion based on BMCA
The BMCA is a kind of photothermal conversion device. It has a substrate made of silicon (Si) and an absorption layer made of metal black, as shown in Fig. 2(a,b). Many micro square cavities are etched onto the Si substrate. The top of each micro cavity is covered with a layer of metal black, which has been etched into a “double S” shaped structure. The visible light incident on the surface of the BMCA is partly absorbed by the metal black directly. The rest of the visible light enters the micro cavity through the gap of the “double S” structure, and is reflected by the inner surface of the micro cavity several times and finally absorbed by the micro cavity. Metal black has high absorptivity (>91%) in the visible band and high emissivity (≈0.88) in the IR band. This structure makes each BMCA pixel have a very low thermal inertia, and can quickly heat up after absorbing visible light. Thereby achieving an efficient conversion from visible light to IR radiation. The micrograph of BMCA is shown in Fig. 2(c,d).
Fig. 2. Schematic diagram of BMCA. (a) the structure of BMCA. (b) illustration of the micro cavity trapping visible light. (c) micrograph of BMCA (top view). (d) micrograph of BMCA (cross sectional view).
The thermodynamic behavior of a BMCA pixel can be describe as:
Table 1. Coefficients in Eq. (1)
The BMCA is sealed in a stainless steel chamber which is vacuumed inside to eliminate thermal convection between the BMCA and the air. The Si substrate of the BMCA is backed by a copper heat sink, which is cooled with liquid nitrogen flowing behind it, as shown in Fig. 3. The purpose of this active cooling design is to dissipate the heat accumulated on the BMCA in time to keep the surface temperature of the BMCA at a low level, thereby generating an IR scene with a sufficiently low background temperature.
2.3 Light-driven mechanism of BMCA
The BMCA is driven (or heated) by a sequence of visible light-driven images generated by the VSG. A DMD is adopted as the image generation device of the VSG. The DMD is illuminated by a modulated laser. The visible light-driven images generated by the DMD is imaged onto the BMCA through a visible imaging lens. The BMCA then converts the visible light-driven image sequence into IR images. The IR images are then projected into the IR imaging device by the IR projection system. The IR projection system is a Cassegrain system containing a primary mirror and a secondary mirror. The exit pupil of the projection system coincides with the entrance pupil of the IR imaging device. The optical design of the IRSP is shown in Fig. 4.
A visible scene generator (VSG) is essential for producing the light-driven images for the BMCA. There are two main VSG types including the amplitude modulation type like the liquid crystal display (LCD), and the time modulation type like the DMD. A LCD-based VSG will cause “tailing” problem in dynamic IR scene generation [17]. Therefore, we adopt a DMD-based VSG to drive the BMCA. The VSG consists of a modulated laser, a beam homogenizer, a DMD, and a visible imaging lens. The appearance of the VSG is shown in Fig. 5.
BMCA is discrete in space. Therefore, BMCA will spatially samples the visible light image projected onto its surface. As shown in Fig. 6.
Fig. 6. Principle of the spatial sampling effect of BMCA. (a) ideal image. (b) spatially sampled image.
Assuming the conversion efficiency of BMCA pixels is uniform, then the response function of a BMCA pixel is:
where h and v are the size of the BMCA pixel in the horizontal and vertical directions, respectively, and rect() is the rectangular function.The response function of the BMCA is:
where V and H are the number of pixels in the horizontal and vertical directions, respectively.The sampling function of a BMCA pixel is:
where δ() is the impulse function, Δx and Δy are the horizontal and vertical spacing between adjencent BMCA pixels, mΔx and nΔy are the horizontal and vertical position of the pixel in the mth column and the nth row.Combining Eq. (2), (3), and (4), we have the spatial sampling model of BMCA as:
We take a high-resolution image as the input image, and then use the 1/4 and 1/8 spatial resolution models to sample the input image, respectively. The simulation results are shown in Fig. 7.
Fig. 7. Simulation of the spatial sampling effect of BMCA. (a) input image. (b) sampled image using 1/4 resolution model. (c) sampled image using 1/8 resolution model.
The resolution of BMCA is designed as 1280×800. And the resolution of the DMD used in VSG is 2560×1600. Therefore, BMCA can be regarded as a 1/4 resolution spatial sampling model for the visible light image that is projected on the BMCA surface.
To generate a visible light-driven image sequence, we disassemble an 8-bit decimal grayscale image into eight binary images. The transform equation between the decimal grayscale and the binary grayscale is:
where gd (x,y) is the decimal grayscale of the pixel with coordinates of (x,y), and gb,j (x,y) is the number of the j-th bit of the binary grayscale of the pixel with coordinates of (x,y). The value range of gd (x,y) is [0,255], and the value of gb,j (x,y) is 0 or 1. We first convert the decimal grayscale of all the pixels of the image into the binary form. Then respectively extract the number of the j-th bit of the binary grayscale of all the pixels to assemble eight binary images, as shown in Fig. 8.The modulation waveform of the laser that illuminates the DMD is shown in Fig. 9. The duration of each modulation period is tmp. Each modulation period is equally divided into 8 sub-periods. The duration of each sub-period is tsp, that is:
Fig. 9. The timing relationship between the binary image display and the laser modulation sub-periods.
The laser power in the i-th sub-period is expressed as:
The DMD displays one of the eight binary images in a sub-period. Easy to know that different binary images are illuminated by different laser powers. Therefore, the role of DMD can be understood as a selection of laser power combination that can enter the visible imaging lens. The DMD goes through the operations of reset, stabilization, and image display in sequence when display a binary image. Suppose the image display time is tdp. Then the average laser power that is reflected by the DMD and enters the visible imaging lens during a laser modulation period is:
Substituting Eq. (7) and Eq. (8) into Eq. (9), we get:
The average laser power at the pixel coordinates (x,y) represents the grayscale of the pixel. From Eq. (10), we know that Pave(x,y) is proportional to the decimal grayscale gd(x,y) . Therefore, the average grayscale of the light-driven image sequence in a complete laser modulation period can reach 256 gray levels.
The thermal response of a BMCA pixel depends on the thermodynamic parameters of the pixel (see Eq. (1) and Table 1) and the waveform of the laser incident on the pixel. The waveform of the laser incident on the pixel is determined by both the modulated waveform of the laser source and the binary image sequence displayed by the DMD in a modulation period (see Fig. 9).
Let Pmax=5×104W/m2, tdp=49.5μs, tsp=62.5μs, tmp=500μs, and gd (x, y) =1, 150, 255. Then the laser waveforms corresponding to different grayscales and the thermal response curves of the BMCA pixel excited by the laser waveforms are shown in Fig. 10. The laser waveforms are shown on the left side of Fig. 10, and the corresponding pixel thermal response curves are shown on the right side of Fig. 10.
Fig. 10. Thermal response curves of a BMCA pixel with different average grayscales. (a) gd (x, y) =1. (b) gd (x, y) =150. (c) gd (x, y) =255.
2.4 IR scene projection system
The function of the IR projection system is to project the IR images that are generated on the BMCA surface to the entrance pupil of the IR imaging device. The main parameters of the IR projection system are shown in Table 2.
Table 2. Main parameters of the IR projection system
We adopt a coaxial Cassegrain structure as the structural form of the IR projection system, as shown in Fig. 11. The expression for the surface shape of the primary mirror and the secondary mirror is:
Table 3. Surface parameters of the primary mirror and the secondary mirror
The challenge of designing the IR projection system is to ensure the imaging quality over a wide temperature range (100∼300 K). The mirror material and the optomechanical structure material have different thermal expansion coefficients. As the temperature changes, the mirror and the optomechanical structure will deform to different degrees, thus creating thermal stress between the mirror and the optomechanical structure [18–20]. This will lead to the deformation of the mirror surface, thereby reducing the imaging quality of the projection system. The IR projection system designed above is a reflective optical system with rotary symmetry. Compared with the transmissive projection system, the reflective optical system can avoid the change of lens refractive index with temperature. In addition, the thermal stress generated on each mirror in the coaxial system has rotary symmetry, which is conducive to the thermal stress unloading design of the optomechanical structure.
The initial parameters (see Table 3) of the IR projection system are designed for the ambient temperature of 300 K. However, the operating temperature range requirement of the projection system ranges from 300 K to 100 K. Therefore, we have carried out a heatless design on the optomechanical structure to reduce the influence of temperature changes on the imaging quality. In order to make the thermal stress between the mirror and the mirror frame as small as possible. We choose glass-ceramics with a small thermal expansion coefficient (7×10−7∼11×10−7 K-1) as the mirror material and choose Invar which has a thermal expansion coefficient (9×10−7 K-1) close to that of glass-ceramics as the material of the optomechanical structure. In addition, we used elastic pressing sheets to fix the mirror, thereby introducing an elastic buffer between the mirror and the mirror frame to weaken the deformation of the mirror, as shown in Fig. 12.
Fig. 12. Optomechanical structure of the IR projection system. (a) Overall assembly diagram. (b) Mounting structure of the primary mirror. (c) Mounting structure of the secondary mirror.
The thermal deformation of the primary mirror and the secondary mirror as the ambient temperature drops from 300 K to 100 K are simulated and the results are shown in Fig. 13. The maximum deformation of the primary mirror and the secondary mirror is 0.00426 mm and 0.00398 mm, respectively.
In addition to the mirror surface deformation, the spacing change between the primary mirror and the secondary mirror due to the thermal expansion and contraction of the Invar positioning screw can also have a significant impact on the imaging quality of the projection system. The spacing change between the primary mirror and the secondary mirror as the temperature drops from 300 K to 100 K is analyzed, as shown in Fig. 14.
Fig. 14. The spacing change between the primary mirror and the secondary mirror as the temperature changes from 300 K to 100 K.
From Fig. 13 and Fig. 14, we know that the surface shapes of the primary mirror and the secondary mirror as well as the spacing between them change significantly when the ambient temperature drops from 300 K to 100 K. The MTF of the IR projection system at ambient temperatures of 300 K and 100 K were analyzed using Zemax, respectively, the results are shown in Fig. 15. We can see that the MTF of the projection system decreases when the ambient temperature drops from 300K to 100K. But the MTF at a spatial frequency of 3.4 lp/mm is still greater than 0.3 at 100 K, which is able to meet the requirements for IR scene projection.
Fig. 15. MTF of the IR projection system under different ambient temperatures. (a) Tamb=300 K. (b) Tamb=100 K.
2.5 Appearance of the IRSP system
The space occupied by the entire IRSP system is approximately: 1200×900×360 mm. The assembly diagram of the IRSP system is shown in Fig. 16.
3. Performance test of the IRSP in a cryogenic environment
3.1 Dynamic range
The temperature of a BMCA pixel will first rise and then stabilize when it is heated by the visible laser. And the temperature of the BMCA pixel will gradually drop to the ambient temperature after the laser is removed. Suppose the maximum BMCA temperature is Tmax, the ambient temperature is Tamb, and the maximum temperature rise is ΔT = Tmax-Tamb. Here we define three time parameters for the thermal response of BMCA. The first time parameter, thermal rising time τrise, is defined as the time for the BMCA temperature to rise from Tamb+0.1ΔT to Tamb+0.9ΔT. The second time parameter, thermal keeping time τkeep, is defined as the time during which the BMCA temperature keeps above Tamb+0.9ΔT. The third time parameter, thermal decaying time τdrop, is defined as the time required for the BMCA temperature to drop from Tamb+0.9ΔT to Tamb+0.1ΔT after the laser is removed, as shown in Fig. 17.
The DMD trigger signal is used to alert the DMD driver board to refresh the binary image displayed by the DMD. Each DMD trigger signal is synchronized with the falling edge of the sub-period of the laser modulation signal. The binary image sequence displayed by the DMD is imaged onto the BMCA surface. Therefore, each BMCA pixel will be heated by the laser power combination selected by the binary image sequence. Its temperature gradually rises and then stabilizes. The highest BMCA temperature can be measured by adjusting the delay between the exposure starting time of the IR imaging device and the synchronization signal so that the exposure interval falls within the thermal keeping interval of the BMCA.
The highest BMCA temperatures Tmax excited by different laser power densities were measured under different ambient temperatures, as shown in Fig. 18. It can be seen from Fig. 18 that the highest BMCA temperature is basically linear with the incident laser power density. When the ambient temperature is 187.75 K and the laser power density is 5.16 W/cm2, the highest BMCA temperature is 426.15 K, the corresponding temperature rise is 238.4 K.
Fig. 18. Highest BMCA temperatures excited by different laser power densities under different ambient temperatures.
Define the dynamic range of the IR scene as:
Substitute Tmax=426.15 K, Tmin=Tamb=187.75 K into Eq. (12) and Eq. (13), we get that the dynamic range of the IR scene in cryogenic environment is D=38.69 db.
3.2 Conversion efficiency
Define the visible to IR conversion efficiency of BMCA as the ratio of the radiant exitance of BMCA to the power density of the input visible laser:
where σ=5.67×10−8 W/(m2·K4) is the Stefan-Boltzmann constant. The conversion efficiency of BMCA under different ambient temperatures is shown in Fig. 19.It can be seen from Fig. 19 that the conversion efficiency of BMCA decreases as the ambient temperature decreases. When the ambient temperature is 292.45K, the average conversion efficiency of BMCA is about 10.6%. When the ambient temperature is 187.75K, the average conversion efficiency of BMCA decreases to about 3.1%. The possible reason for this phenomenon is that the aluminum black absorption layer of BMCA shrinks in volume after the temperature is reduced. The loose and porous structure of aluminum black becomes denser, resulting in a lower absorption rate of incident visible light, thereby causing a decrease in the conversion efficiency of BMCA.
3.3 Frame rate
We keep the power density of the input visible laser constant at Pave=5.16 W/cm2. The BMCA temperature change curve with respect to time was acquired by adjusting the delay time between the exposure interval of the thermal imager with the synchronization signal (see Fig. 17). The BMCA temperature change curves were measured at ambient temperatures of 235.15 K, 273.15 K and 292.45 K, respectively, as shown in Fig. 20. Notice that we did not measure the temperature change curve of the BMCA at the ambient temperature of 187.75 K. This is because the thermal imager we used has a lower limit of 233.15 K for temperature measurement.
The time response parameters of the BMCA at different ambient temperatures are listed in Table 4.
Table 4. Time response parameters of the BMCA at different ambient temperatures
The maximum frame rate of the IRSP is calculated by:
The temperature keeping time of the BMCA τkeep should be longer than the exposure interval of the IR imaging device to ensure that the IR imaging device can capture the complete 256 grayscales. Generally, the exposure interval of the IR imaging device is 0.5∼5ms. Let τkeep=5ms, τrise=3.46ms, τdrop=4.58ms. Then the frame rate of the IRSP is about F≈76 Hz.
3.4 Radiation spectrum
A spectro-radiometer was used to measure the radiation spectrums of BMCA with different temperatures. The radiation spectrums of a standard blackbody with the same temperatures are also measured using the same spectro-radiometer. The results are shown in Fig. 21.
The spectral angle metric (SAM) is used to analyze the similarity between the BMCA radiation spectrum and the standard blackbody radiation spectrum [21]. Suppose the standard blackbody radiation spectrum is represented by spectral vector Ssb, and the BMCA radiation spectrum is represented by spectral vectors SBMCA:
SAM ranges from 0 to 180°. The smaller the SAM, the more similar the shape of the two spectral curves. The SAM between the BMCA radiation spectrum and the standard blackbody radiation spectrum with different temperatures are listed in Table 5. It can be seen that the radiation spectrum of BMCA matches better with that of the standard blackbody in the MWIR band. And in the LWIR band, although the deviation of SAM is larger than that of MWIR band, it is still within the acceptable range for practical applications.
Table 5. SAM between the BMCA radiation spectrum and the standard blackbody radiation spectrum
3.5 IR image generation
The IRSP presented in this paper has been applied in a HIL simulation test of a space-borne exploration system. Clear IR images were captured using a tailored space-borne IR imaging device at an ambient temperature of 187.75 K, as shown in Fig. 22.
Fig. 22. IR images generated by the IRSP in cryogenic environment. (a) Input digital decimal grayscale image. (b) IR image captured at an ambient temperature of 187.75 K.
From Fig. 22, we can see that the IR image captured by the IR imaging device is degraded compared with the input digital image. This image degradation is mainly caused by the spatial sampling effects of the BMCA and IR detector array. The input digital image has the same resolution as the DMD used by the VSG, which is 2560×1600. The resolution of BMCA is 1280×800. The resolution of the IR imaging device is 640×480. As discussed in Section 2.3, BMCA will spatially sample the input visible light image once. And then the IR detector array will spatially sample the generated IR image again. In addition, the optical systems inside the IRSP including the visible imaging lens of VSG and the IR projection system will also affect the image quality. Combining the above factors, a relatively fuzzy IR image as shown in Fig. 22(b) is captured. However, the actual IR image generated by BMCA is more clear than Fig. 22(b), but it can only be observed with higher resolution IR imaging device.
4. Conclusion
The design of an IRSP that can operate under cryogenic environments is demonstrated in this paper. The system composition and the working principles are described in details. The IRSP is composed of a VSG, a BMCA-based visible to IR converter, and an IR projection system optimized for cryogenic environments. A vacuum sealing structure with active cooling function for BMCA is presented. The spatial quantification and the grayscale quantification effects of BMCA are analyzed. An IR projection system with thermal stress unloading ability is designed. The projection system can guarantee imaging quality over a wide temperature range from 100K to 300K. The performance of the IRSP under different anbient temperatures (187.75 K, 235.15 K, 273.15 K and 292.45 K) are tested in a vacuum cold chamber. When the ambient temperature is cooled to 187.75 K, the observed highest temperature of the generated IR image is 426.15 K. The dynamic range of the IR scene was 38.69 db. The average visible to IR conversion efficiency of BMCA is about 3.1%. The frame rate of the IR scene can reach 76 Hz. The radiation spectrum of the IRSP is close to the standard blackbody radiation spectrum.
The core component of the IRSP is BMCA. It is a passive device and can be placed directly in cryogenic environments. The temperature of the BMCA is the same as the ambient temperature. Therefore, the BMCA is very suitable for generating IR scenes with pure low temperature background, which is important for the ground HIL simulation test for space-borne IR detection system. The limitation of BMCA is that its conversion efficiency is relatively low. The required input visible laser power has to be quite high to generate a high enough target temperature. In addition, since PI is used as the attachment material for aluminum black in BMCA pixels and PI will melt at a temperature above 673.15 K, the highest target temperature that BMCA can generate will not exceed 650 K. For most LWIR scenes, the maximum temperature of 650K is sufficient. But for some MWIR scenes, the target temperature may exceed 1200K, and BMCA may not be the best choice at that case.
This paper provides a comprehensive and practical simulator solution for the ground test and HIL simulation test of space-borne IR detection system. IR scenes with pure cryogenic background were experimentally observed. Future work will focus on improving the spatial resolution and the visible to IR conversion efficiency of BMCA.
Funding
China Postdoctoral Science Foundation (2020TQ0036).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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