Pingli Han

Polarimetric dehazing utilizing spatial frequency segregation of images.

A procedure for the detection and removal of haze from dense hazy images has been proposed. It involves the analysis on the content of low-spatial-frequency information of a scene. The image contaminated by haze is decomposed into different spatial frequency layers by the wavelet transform, by which the hazy parts of the image are focused on the low-frequency components. A dehazing method combining both the airlight and direct transmission is employed to specially dehaze the low-frequency parts. The high-frequency parts are processed by a transfer function to enhance the clarity of the hazy image. Finally, a dehazed image with high clarity is obtained by image construction which employs the low- and high-frequency components. Experiments and analyses demonstrate the good performance of the scheme in terms of improving the contrast and clarity of hazy images. Particularly, it works well in improving the visual range of images captured in hazy weather conditions.

Detection of infrared stealth aircraft through their multispectral signatures

Abstract. A concise band selection method employing multispectral signatures of stealth aircraft whose infrared radiation was remarkably reduced was proposed for precise target detection. The key step was to select two or more optimal bands which could clearly signify the radiation difference between the target and its background. The principle of preliminary selection was based on the differences of radiation characteristics for the two main constituents of the aircraft’s plume gas, i.e., CO2 and H2O. Two narrow bands of 2.86 to 3.3 and 4.17 to 4.55  μm were finally selected after detailed analyses on contrast characteristics between the target and background. Also, the stability of the selected bands was tested under varying environments. Further simulations and calculations demonstrated that the multispectral detection method utilizing the two selected narrow bands could markedly improve the essential performances of target detection systems and increase their achievable detection distance. The stability of the aircraft’s multispectral signatures enabled this target detection method to achieve excellent results.

Active underwater descattering and image recovery

Underwater imaging is a promising but challenging topic due to the scattering particles in water, which result in serious light attenuation. Therefore, underwater images suffer from low-contrast and low-resolution issues. In this study, in order to recover high-quality underwater images, the point spread functions (PSFs) are estimated by a slant-edge method. The experiment modulates the illumination source to deal with backscattering and the imager to take two images in orthogonally polarized states. This imaging method benefits the satisfactory edge extraction. The PSF estimation is performed based on the extracted slant edge to enable recovery of the image. In addition, the modulation transfer function (MTF) is introduced to evaluate the resolution variation with the spatial frequencies. It manifests considerable resolution enhancement in the recovered images. Moreover, the proposed underwater image recovery method also reduces the effect from the scattering as an effective compensation to the polarization imaging approach.

Design of a circular polarization imager for contrast enhancement in rainy conditions

We present the design of a circular polarization imager for imaging in rainy conditions, which is free from image calibration and correction before obtaining the orthogonal-state contrast image. The system employed a quarter wave plate in front of two Wollaston Prisms (WPs) to capture circularly polarized information and to acquire two orthogonally polarized images simultaneously on the charge coupled device (CCD). Along with the WPs, a reimaging part with multiaperture structure composed of two separate specialized reimaging modules, were implemented to make sure the two orthogonally polarized intensity images are exactly indicating the same scene. Exploiting circularly polarized information provides advantages over a linear polarization imaging system when considering the turbulence of media and illumination. Substantial data have demonstrated the effects of the novel designed polarization imaging system.

Polarization characteristics of objects in long-wave infrared range.

Research on polarization characteristics of objects has become indispensable in the field of target detection. Though widespread studies on applying polarization to target detection and material identification exist, theoretical descriptions have varied widely in accuracy and completeness. Incomplete descriptions of polarization characteristics invariably result in poor demonstration of changes caused by macroscopic influence factors. For objects that are of finite surface, a comprehensive model is built to analyze the polarization characteristics of their thermal emission. With the Stokes theory and the superposition principle of light waves, the relation between the degree of linear polarization and the spatial geometrical parameters, such as the detection distance and the shape of objects, is discussed in the long-wave infrared range in detail. This model can be applied to analyze the linear polarization characteristics among different materials.

Pixel-level image reconstruction method of polarization images acquired by multi-aperture imaging systems

To acquire high-resolution IR polarization images, a pixel-level image reconstruction method was introduced. It was aimed at IR polarization imaging systems employing multi-aperture principle. The geometric mapping relation between images was firstly studied and was basis of this method. Parameters of the mapping relation were calculated, and then pixels of each image obtained were mapped to a virtual digital plane at which precise and resolution enhanced polarization images could be obtained by taking advantage of the pixel deviation and rearranging the pixels. Experimental results demonstrated that the algorithm could assist the multi-aperture imaging system in rendering easily precise and high-resolution polarization images.

A novel IR polarization imaging system designed by a four-camera array

A novel IR polarization staring imaging system employing a four-camera-array is designed for target detection and recognition, especially man-made targets hidden in complex battle field. The design bases on the existence of the difference in infrared radiation’s polarization characteristics, which is particularly remarkable between artificial objects and the natural environment. The system designed employs four cameras simultaneously to capture the00 polarization difference to replace the commonly used systems engaging only one camera. Since both types of systems have to obtain intensity images in four different directions (I0 , I45 , I90 , I-45 ), the four-camera design allows better real-time capability and lower error without the mechanical rotating parts which is essential to one-camera systems. Information extraction and detailed analysis demonstrate that the caught polarization images include valuable polarization information which can effectively increase the images’ contrast and make it easier to segment the target even the hidden target from various scenes.

A normal laser warning system model based on Hadamard Transform

It is very important to extract the useful information from weak laser signal which is obtained in complex battlefield environment as laser warning taking an increasingly important role in laser countermeasures. The weak signal merging in noise becomes difficult to detect since the signal to noise ratio (SNR) of the signal received by the laser warning system is very low in real battlefield. Traditional signal detection methods, in which only mean filter or wiener filter are used; perform poorly in improving the SNR of the signals. A modified matrix of Hadamard Transform based on the Weighting Theory, overcame the disadvantages of matrices that are commonly used to cope with the low SNR signal. The modified matrix generating method of Hadamard Transform is introduced in detail, and then theory analysis, calculations and simulations on the modified matrix Hadamard Transform are presented. The results showed that this kind of Hadamard Transform performs excellently in increasing detection probability and decreasing False Alarm Ratio (FAR) of the laser warning system.

Research on enhancing performance of laser warning systems utilizing microlens array

An improved optical structure of a laser warning system based on microlens array is proposed aiming at the high precision imaging warning system with a smaller size. Microlens array owning the advantage of high efficiency of energy use, motion sensitivity, etc. as a multi-aperture optical element, is applied in many optical systems. It is a tough task to obtain satisfied images with a curved base microlens array because the widely used plane detectors are not fit for these kinds of microlens array with poor imaging quality though it achieves the goal of wide field of view (FOV). We address to design a model by combining the curved base microlens array with the aspheric converging lens to solve the poor imaging quality caused by cured base microlens array. This method will make it possible to enlarge the FOV with better image quality. The ray tracing results show that the image quality acquired from plane detectors is improved using curved base microlens array, but with more simple fabricated structure than that of fisheye lens, which is widely used to get a wider FOV.