Xiaopeng Shao

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.

Simulation on light refocusing through a highly scattering turbid medium using circular Gaussian distribution model

Abstract. We present a simulation method for studying turbid media in the optical field based on circular Gaussian distribution (CGD) model, by which the transmission matrix, representing the modulation of a turbid medium on the amplitude and the phase of incident light, can be generated directly and efficiently. As an application example, light refocusing through a turbid medium is realized employing the CGD model approach, combining with wavefront–phase modulation technique and Fresnel diffraction theory, which is applied to describe the light propagation between optical elements of the entire system. Simulation results based on this approach agree well with theoretical analysis for light refocusing, which can validate the feasibility of CGD model. This work can be used for exploring the potential applications of turbid media in the optical field further, especially for developing new microscopic imaging technologies beyond the diffraction limit.

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.

Simulation and experimental verification for imaging of gray-scale objects through scattering layers

We analyze the imaging of gray-scale objects through highly scattering layers. The theoretical investigation with numerical simulations shows that the contrast of the speckle autocorrelations varies regularly with the change of the gray scale of the object. Therefore, gray information is well contained in the autocorrelations of the speckle patterns, and gray-scale objects are able to be exacted from these autocorrelations via speckle correlation technology. Combined with phase retrieval via the generalized approximate message passing algorithm, recovery of the objects is realized and accurate gray-scale reconstruction is demonstrated via numerical simulations. Experiment results further demonstrate the good performance of the scheme in the imaging of gray-scale objects through scattering layers. Particularly, this work will be beneficial for applications of imaging through turbid media in biomedical and biophotonics imaging.

Spherical aberration and modulation transfer function

The fundamental problem of the modulation transfer function(MTF) from the viewpoint of the lens designer is to find relation between the MTF and the geometrical aberrations. Let it be required to develop the spherical aberration into a polynomial expansion. The incoherent point spread function(PSF) of the optical imaging system is derived from the diffraction integral in the presence of aberrations. The optical transfer function(OTF) is the Fourier transform of the PSF, and the modulus of the OTF is the MTF. The relation between the spherical aberration and the MTF is denoted by numerical integration method. The normalized MTF is numerically calculated for various amounts of spherical aberration. A comparison is made between the MTF of the corrected spherical aberration using the optimum design for the minimum root mean square(RMS) wavefront aberration and those for the minimum peak-to-valley(P-V) wave front aberration.

How to deal with color in super resolution reconstruction of images

Super resolution (SR) reconstruction is a profitable technology to acquire high resolution images from low resolution images without replacing devices. This study was concentrated on searching strategies of dealing with color information in the SR reconstruction process. Based on an algorithm with dictionary learning, different algorithms were designed to test which color coordinate systems could obtain better image reconstruction quality, involving color spaces of RGB, YIQ, YCbCr, HSI, HSV, and CIELAB. Their results were compared via typical numerical measures, and the recommended strategies are to adopt merely L* coordinate in CIELAB space or merely Y coordinate of YIQ 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.

Design of a wide-field imaging optical system with super-resolution reconstruction

The need for a portable image acquiring system has become as strong as the extension of digital imaging technology, for this, a new mono-centric wide-field optical system is proposed. Recently, some high-resolution and wide-field imaging systems have been raised already, with which fairly clear and wide field of view (FOV) images could be easily obtained, however, their sizes are comparatively too large to be conveniently carried . With ZEMAX, a new optical design is emulated by scaling the structure of current wide-field optical systems and introducing the proposed lens-let arrays, the size of the whole system is comparatively smaller with the structure consisting of a two-glass mono-centric lens, lens-let array (the lenses in the array can be different), and a specific detector. Lens-let array is used to make the image plane from curve to almost flat. This hardware is small enough to apply to helmets and computers and the FOV of which is wide. Verified by a series of merit function, this optical design is found to have an acceptable imaging resolution and the computational imaging method is applied to this system to acquire a higher imaging resolution. From each lens-let a series of low resolution images are obtained and in this system a high-resolution image can be retrieved from multiple low-resolution images with super-resolution reconstruction method. Compared from the size and the imaging resolution, this new optical design is much smaller and has a higher imaging resolution.