Cataloguing and recognition of space targets is becoming one of the important research contents of Space Situational Awareness (SSA). As is known that spectral feature is one important method for spatial targets classification and recognition. Based on the facts that objects have unique spectral distributions, characteristic spectra of objects can be used to classify and recognize objects. In order to acquire data cube of targets in both spatial and spectral dimensions by a snapshot, a hyper-spectral computing imaging technology with double channels was proposed in this paper. The imager can quickly acquire and reconstruct spectral data of space targets and then confirm the type of targets by comparing with prior spectral databases of different space targets. Sensitivity of the imager affects the longest detection distance and the spectral resolution of targets. In order to enhance its sensitivity, SPAD array with detection sensitivity to single-photon level can greatly enhance systems' performances.
Imaging spectrometer can provide both scene image information and spectral information, thus providing in-depth analysis of the composition and characteristics of the scene target. It is an important equipment for observation, analysis and detection. Imaging spectrometers are now emerging as an important market growth point in the field of optoelectronic imaging. This paper presents a compact and lightweight snapshot broadband computational spectral imager, which provides a new approach to VIS-NIR spectral imaging and target identification techniques. Based on the Coded Aperture Snapshot Spectral Imager (CASSI), an imaging method with a shared primary mirror and a dual optical path is proposed. One optical path mainly consists of a coding mask, a relay lens, an Amici prism and a visible near-infrared enhancement detector. Its spatial and spectral resolution is determined by the coding mask and dispersive elements. The optical system finally obtains a blended two-dimensional image on the detector. Another optical path uses a visible NIR-enhanced detector to provide high-resolution spatial information. The high-resolution spectral image information is obtained by a compression-aware reconstruction algorithm. Optical simulations and experimental results show that the system offers significant performance improvements over existing systems, allowing the construction of compact and sensitive spectral imaging systems. We obtained 24 spectral images in the band range 0.44-0.8μm. The new spectral imager introduced in this paper has the advantages of real-time detection, long-range monitoring and high sensitivity. It is especially suitable for Unmanned Aerial Vehicle (UAV) and NanoSat. It can be widely used in the fields of environmental remote sensing, agricultural census, forest survey, vegetation assessment and management, mineral exploration, etc.
The hyperspectral images of airplanes and flying birds are obtained by a xiSpec snapshot mosaic hyperspectral cameras, supported by the Interuniversity Microelectronics Centre (IMEC). The single frame Infrared Patch-Image (IPI) model is used to detect the small targets of airplanes and flying birds under complex cloud background in the hyperspectral images. Based on the non-local autocorrelation property of the background image, the method assumes that the target image is a sparse matrix and the background image is a low-rank matrix. The small target detection is transformed into an optimization problem of recovering the low-rank and sparse matrix. Using stable principal component tracking solution, the decomposed background and target are obtained. The results show that this method can detect bright and dark small targets in complex background at the same time, and the hyperspectral image can effectively improve the detection rate. More importantly, the detection ability is closely related to the intensity difference of the target against the background. Thus, the optimal waveband of different targets can be given by combining the target detection results and the intensity difference curves. This has a guiding significance for the design of specific point target detection payloads.
The conventional diffractive optical imaging spectrometer uses the single-channel scheme, it is mainly aimed at simple targets, or gas targets with known spectral characteristics. The main disadvantage of conventional system is: if the target is a complex scene such as a natural scene, it's very difficult to demodulate spectral images accurately. Because, the focused and defocused spectral information are superimposed on each other. And, the real system has noise, manufacturing error, testing error and calibration error. So, it is difficult to correctly describe the dispersion parameters of the diffractive spectrometer, which will cause large errors of spectral demodulation accuracy. To solve this problem, an efficient system of diffractive spectral imaging is discussed, which includes a reference channel. Based on the conventional single-channel system, a grayscale camera or a color camera is added for imaging. It can provide a priori knowledge of complex scenes for the diffraction imaging channel. The data of the two channels are jointly processed to improve the final demodulation accuracy of the spectral data. The system composition and basic principles are introduced, the performance of the system is analyzed. The virtual simulation experiment of diffractive optic imaging is established. The simulation of diffractive imaging and spectral demodulation of complex scene have been finished. The demodulation output images are almost the same as the original input image. The experiment system of diffractive optic imaging in visible band is also established in the laboratory. Theoretical analysis, imaging simulation and imaging experiment have verified the validity and feasibility of the diffraction imaging system with reference channel. Compared with the single channel system, the spectral demodulation effect is obviously improved, which greatly improves the application potential and application value.
The simultaneous acquisition of spatial information, spectral information and polarimetric information can obtain more characteristic information to distinguish targets. The conventional spectral polarization imaging system mainly includes the filter/polarization wheel rotation system, the crystal modulation system and multi-path beam splitting system. The disadvantages of these systems are: unsynchronized spectral polarization detection, requiring dynamic modulation, complex system, etc. To solve these problems, a spectral polarization detection technology based on optical fiber image bundle is proposed, which combines optical fiber imaging spectral technology with pixel level polarization detection technology. The input shape of the optical fiber image bundle is plane, and the output shape is linear. Optical fiber image bundle can transform the information of array target into that of linear array. The linear array information is the input of spectral imaging system. The polarization detection uses a micron level polarization array to match the pixel size of the detector. The technology can synchronously acquire the two-dimensional spatial information, the spectral information and linear polarization information of the target. The technology can be used to image the area target in snapshot mode. The experimental device is set up to obtain the spectral image in the visible light range, as well as the polarization degree image and polarization angle image of each spectral segment. The data acquisition ability of the system is verified. With the improvement of optical fiber manufacturing technology, the integration of optical fiber is getting better, and the scale of optical fiber is getting larger. The technology will have a high application value in astronomical observation, atmospheric detection, target recognition and other fields.
High sensitivity is needed for space-based infrared weak target detecting. Pixel-level Digital Integration is an effective method for promoting detection sensitivity. It’s well suited for long integration time or high irradition targets detecting. To meet the application requirements, multistage digital TDI and secondary quantification were proposeed based on pixel-level digital integration. These methods could extend integration time equivalent and restrain quantization noise. The multistage digital TDI could overcome the problem of integral time limited by the dwell time of space-based platform. The secondary quantification could reduce quantization noise to an ignorable level and therefore promote system sensitivity efficiently to photon-noise limit. On this work in progress, detecting systems with milli-kelvin level sensitivity and for infrared weak targets could be carried out.
In this paper, the infrared radiation characteristics of sea background have been studied. First, MODTRAN4.0 was used to calculate the transmittance of mid-infrared and far-infrared, and the solar spectral irradiance, the atmospheric and sea surface radiation. Secondly, according to the JONSWAP sea spectrum model, the different sea conditions grid model based on gravity wave theory was generated. The spectral scattering of the sun and the atmospheric background radiation was studied. The total infrared radiation of the sea surface was calculated. Finally, the infrared radiation of a piece of sea surface was mapped to each pixel of the detector, and the infrared radiation is simulated. The conclusion is that solar radiance has a great influence on the infrared radiance. When the detector angle is close to the sun’s height angle, there will be bright spots on the sea surface.
This paper gives the preprocessing technique research which includes odd-even calibration, bad pixels compensation and non-uniformity calibration for the high sensitivity push-broom long wave infrared camera. The noise equivalent temperature difference (NETD) of the infrared camera has achieved less than 10 mK. The paper analyzes high sensitive imaging effects by using different preprocessing standards according to the test data, also analyzes the effects on imaging quality when using different preprocessing methods. At last the paper gives the suitable image preprocessing methods for the high sensitive infrared camera: the odd-even calibration method considering velocity to height ratio, the bad pixels compensation methods which are unique for temporal and spatial bad pixels and the non-uniformity calibration methods which combines the calibration based on black body testing data and the calibration based on real scene imaging data. It is validated that the residual nonuniformity of the infrared image can be reduced to 0.03% by using the preprocessing methods, which is satisfied for the requirement of high sensitive imaging.
Along with the further application of optical remote sensing, it becomes main trend to realize high spatial resolution, high time resolution, high spectrum resolution and high irradiance sensitivity simultaneously. We present a new satellite-based imaging system that will provide images with these high performances. The structure of the system is compact with small size and light weight. The IR imager, a new generation of high resolution optical remote sensing, is universally acknowledged as the most effective approach to surveil dynamic changes in the environment on the earth. Pushbroom imaging fashion with high efficiency and long-array focal plane detector with passive cooling are adopted to realize area imaging relevant to the flight direction of satellite. The instrument is a dual-optical-path system with long-wave infrared (LWIR) and mid-short-wave infrared (MW-SWIR) bands,which has 4 narrow spectrum bands respectively. An IR dichroic beam-splitter is use to divide wideband incident infrared into LWIR and MW-SWIR. Then two pieces of joint filters, which are integrated in front of detectors and then enveloped by IR Dewars, are used to divide the LWIR and MWIR into 4 spectral bands separately. The focal plane arrays (FPA) are fixed on the optical imaging plane of the lens. The LWIR and MW-SWIR FPA are cooled around 80K or even below. For cooled FPA, optical system must provide a real, accessible exit pupil coupled with a fast f/number refractive component in a Dewar and very close to the FPA. Compared to traditional infrared instruments, high spatial resolution and spectrum resolution can be obtained simultaneously within mass, volume and performance constraints.
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