Camouflage technology refers to the technology of hiding or changing the optical characteristics of the object, which can effectively reduce the probability of the object being detected and thus improve its survival probability.The development of multi-band detection technology brings serious challenges to the traditional single-band camouflage technology, which makes the research of multi-band camouflage technology very important and urgent.
Radiant heat dissipation is the dissipation of waste heat generated by the target through the radiation channel to achieve the purpose of reducing the target heat load.At the same time, because the intensity of the object's thermal radiation signal is proportional to the quadrant of its temperature, it is also beneficial to reduce the intensity of the target's thermal radiation signal in the detection band.
In the visible and infrared bands, the target exposes its information mainly by two types of signals: its reflected signal to an external light source and its own thermal radiation signal (Figure 1).On the one hand, the object in the natural environment is illuminated by an external light source, and the reflected signal of the object is exposed to its presence and information.Of all natural sources, solar radiation is undoubtedly the most important, its energy is mainly concentrated in the spectral range of 0.15-4 μm (top right of Fig. 1) and has a critical impact on camouflage in the visible (VIS, 400-780 nm), near-infrared (NIR, 0.78-1.4 μm) and short-wave infrared (SWIR, 1.4-2.5 μm).On the other hand, the object itself radiates infrared signals outwards and can be detected by infrared detectors operating in atmospheric transparent windows.The working bands of infrared detectors are medium-wave infrared (MWIR, 3~5μm) and long-wave infrared (LWIR, 8~14μm), but the peak wavelength of thermal radiation will move in the short wave direction.
Complex signal sources and different camouflage requirements for each band pose a huge challenge to the design of broadband camouflage devices covering both visible and full infrared bands.Previous studies have focused on the camouflage of medium- and long-wave infrared radiation signal, visible light and near-infrared band reflection signal.In addition, while satisfying the camouflage requirements of each band, how to use the non-detecting band for radiant heat dissipation as much as possible also puts forward higher requirements for the spectral control capability of the device.
Recently, Professor Li Qiang's team of Zhejiang University achieved the camouflage of all infrared bands (NIR, short-wave infrared, medium-wave infrared and long-wave infrared) and visible light bands through thin film structure.
该研究成果以“Whole-infrared-band camouflage with dual-band radiative heat dissipation”为题发表在Light: Science & Applications。
principle of all-infrared band camouflage
According to the different signal sources of each detection band, the team put forward the camouflage requirements of each infrared band and visible band:
(1) For short-wave infrared bands, the effects of solar radiation and thermal radiation need to be considered simultaneously.Under ideal meteorological conditions, the intensity of solar radiation in the short-wave infrared band is equivalent to that of blackbody radiation at 330 ℃.When the target is lower than this temperature, the solar radiation intensity is greater than its own thermal radiation intensity, and the absorption rate (i.e., emissivity) needs to be increased to reduce the reflected signal.However, in practical applications, the intensity of solar radiation is generally weaker than the ideal situation, so low emissivity has a wider application.
(2) For medium-wave infrared and long-wave infrared bands, the thermal radiation signal dominates, and the solar radiation energy is weak, so its emissivity should be reduced to suppress the thermal radiation signal;
(3) For visible and near-infrared bands, the main signal source is the reflected external light source (such as solar radiation) signal, and the thermal radiation signal can be ignored, so its reflectance should be reduced as much as possible to reduce the reflected signal;
Infrared Camouflage and Radiant Heat Dissipation Verification
Accordingly, the team designed the Al2O3/Ge/Al2O3/Ge/ZnS/GST/Ni thin film structure (Fig. 2), which satisfies: (i) low reflectivity (0.129 and 0.281) in visible and near-infrared bands; (ii) low emissivity (0.270, 0.042 and 0.218) in long-infrared bands.
The camouflage device was heated to 200 ℃ and observed by medium/long wave infrared thermal imaging camera. The radiation temperature was only 86.3 ℃/94.7 ℃.The signal intensity of short-wave infrared camera is 39.3% lower than that of reference blackbody.
In particular, the team experimentally verified the device's short-wave infrared camouflage capability under solar irradiation.At lower temperatures, the solar radiation intensity reflected by the mirror is greater than the thermal radiation intensity, so that the total signal strength of the device is greater than that of the chromium film.At higher temperatures, because the intensity of thermal radiation has surpassed that of solar radiation, the device exhibits weaker total signal strength in both mirror and diffuse reflection directions.
In addition, in the experiment of constant input heating power, the team proved that the thermal equilibrium temperature was 14.4 ℃ lower than that of chromium film with high radiation in non-detecting wavelength band.