February 26, 2024

Full infrared camouflage with dual-band radiative heat dissipation

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Principle for full infrared band and visible band camouflage compatible with radiative heat dissipation. a Typical detection bands ranging from visible to long-wave infrared and two primary signal sources: reflection of solar radiation and thermal emission from the object. b The absorptivity/emissivity spectrum (black line) of an ideal wavelength selective emitter designed to neutralize multiband detectors. The red and blue areas represent the solar irradiance spectrum and the atmosphere transmittance spectrum, respectively. c – e The band-integrated irradiance of solar radiation and blackbody radiation at various temperatures of objects in the NIR, SWIR, and MWIR bands. The total detected signal intensity of objects with different average emissivity (ε = 0.25, 0.5, 0.75) is plotted in solid lines. Credit: Light: Science and Applications (2023). DOI: 10.1038/s41377-023-01287-z

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Principle for full infrared band and visible band camouflage compatible with radiative heat dissipation. a Typical detection bands ranging from visible to long-wave infrared and two primary signal sources: reflection of solar radiation and thermal emission from the object. b The absorptivity/emissivity spectrum (black line) of an ideal wavelength selective emitter designed to neutralize multiband detectors. The red and blue areas represent the solar irradiance spectrum and the atmosphere transmittance spectrum, respectively. c – e The band-integrated irradiance of solar radiation and blackbody radiation at various temperatures of objects in the NIR, SWIR, and MWIR bands. The total detected signal intensity of objects with different average emissivity (ε = 0.25, 0.5, 0.75) is plotted in solid lines. Credit: Light: Science and Applications (2023). DOI: 10.1038/s41377-023-01287-z

Camouflage refers to the ability to reduce the signal captured by detectors, thus improving survival rates. However, combining detectors operating in multiple spectral bands represents a significant challenge, necessitating the development of multiband cloaking technologies. Furthermore, camouflage often conflicts with the demands of radiative heat dissipation, which have significant contributions to the thermal management of objects.

Objects typically reveal their presence through two types of signals: reflected signals from external light sources and thermal emission signals from the objects themselves. On the one hand, objects in nature are illuminated by external light sources, among which solar radiation is the most significant. Solar radiation emits its energy mainly in the spectral range of 0.15–4 μm and plays a crucial role in the visible (VIS, 400–780 nm), near-infrared (NIR, 0.78–1.4 μm) and infrared ranges. shortwave (SWIR, 1.4–2.5 μm) detection bands.

Conversely, objects radiate energy through thermal emission, which can be detected by detectors operating in atmospheric transmission windows (medium-wave infrared (MWIR, 3–5 μm) and long-wave infrared (LWIR, 8– 14μm) ). With increasing temperature, the peak wavelength of thermal emission shifts toward the shortwave direction, making the radiative signal in the SWIR band non-negligible.

In a new article published in Light: Science and Applications, a team of scientists, led by Professor Qiang Li from the State Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, China, and co-workers have developed a full-infrared band cloaking device, which is compatible with radiative heat dissipation in two undetected bands (2.5–3 μm and 5–8 μm). Based on their understanding of signal sources, they proposed the spectral characteristics of cloaking devices:

  • In the SWIR band, low emissivity has a wider scope of application. The highest irradiance of sunlight is similar to that of a blackbody at 330°C. However, in practical scenarios, where solar irradiance is generally lower than its highest level, inhibition of thermal emission contributes more to the reduction of total signal intensity.
  • In the MWIR and LWIR bands, low emissivity is more suitable, as thermal emission generally dominates the detected signal and sunlight intensity is weak enough to be negligible.
  • In the VIS and NIR bands, low reflectivity is preferable since the predominant source of the detected signal is reflected solar radiation and thermal emission is generally negligible.

AltwoO3/Ge/AltwoO3The /Ge/ZnS/GST/Ni multilayer structure is employed to modulate the ultra-wide spectrum from visible to LWIR range. The unique architecture of this structure allows it to meet the diverse demands of the entire infrared and visible range while achieving efficient radiative heat dissipation within two undetected bands.

Appearing grayish blue, the fabricated sample exhibits low average reflectivity in the VIS/NIR bands (0.129/0.281). When heating to 200°C, the radiative (apparent) temperatures of the sample under the MWIR/LWIR cameras are only 86.3°C/94.7°C. Compared to the reference blackbody, the signal intensity of the sample is 39.3% smaller under the SWIR camera. Particularly, the performance of SWIR camouflage is demonstrated under solar radiation. At higher temperatures, the sample exhibits lower signal intensity than the Cr reference in all observation directions. While at lower temperatures, the sample maintains the edge except in the direction of specular reflection of solar radiation.

The effectiveness of radiative heat dissipation is demonstrated by placing the Cr sample and reference subjected to the same input electrical heating power. With input power of 20 W (equivalent to a power density of 2,000 Wm−2), the surface temperature of the sample is 174.5°C, which is 14.4°C lower than that of the Cr reference.

These lower surface temperatures help reduce thermal load and improve MWIR and LWIR camouflage performance.

“This work provides a comprehensive guideline for developing cloaking technologies compatible with radiative heat dissipation, against complicated signal sources, and multispectral detection technologies,” the scientists noted.

“This full-band infrared cloaking device can facilitate applications that require sophisticated spectrum manipulation and stimulate innovative pathways for modern thermal management technologies and contribute to an energy-efficient future,” they say.

More information:
Bing Qin et al, Full-band infrared camouflage with dual-band radiative heat dissipation, Light: Science and Applications (2023). DOI: 10.1038/s41377-023-01287-z

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