Spatial Frequency Filtering Research Articles

Modelling the operation of spatial-frequency filters during the perception of complex dynamic scenes

This paper discusses the process of perceiving dynamic images subjected to processing with spatial-frequency filters that simulate the characteristics of the receptive fields of the neurons of the primary visual cortex. A technique was used that makes it possible to give a quantitative estimate of how subjects perceive the emotional state of people’s faces on images presented to the subjects. It was shown that, besides the vertical and horizontal components of the spatial-frequency spectrum, a substantial role is played by the diagonal components in the process of perceiving the images of faces. Even though the visual system is less sensitive to the diagonal components than to the vertical and horizontal ones, the information contained in them makes it possible to distinguish the individual features and emotional state of a person’s face.

Highly angular dependent high-contrast grating mirror and its application for transverse-mode control of VCSELs

We report on the design and fabrication of a highly angular dependent high contrast grating (HCG) mirror. The modeling and experiment on amorphous-Si/SiO2 HCG clearly show the large angular dependence of reflectivity, which enables single transverse-mode operations of large-area VCSELs. We fabricate 980 nm VCSELs with the angular dependent HCG functioning as a spatial frequency filter. We obtained the single transverse mode operation of the fabricated device in contrast to conventional VCSELs with semiconductor multilayer mirrors.

Effects of spatial frequency filtering in natural scenes: Evidence from eye movements and computational modeling

Effects of spatial frequency filtering in natural scenes: Evidence from eye movements and computational modeling

Effects of spatial frequency filtering in natural scenes: Evidence from eye movements and computational modeling

Effects of spatial frequency filtering in natural scenes: Evidence from eye movements and computational modeling

Combination of Transverse Mode Selection and Active Longitudinal Mode-Locking of Broad Area Semiconductor Lasers

Experimental results of the combination of transverse mode selection and active mode-locking with anti-reflection-coated broad area lasers (BALs) are presented. The BALs are subject to feedback from a free-space external Fourier-optical 4f-setup with a reflective spatial frequency filter in the Fourier-plane for transverse mode selection. Driving the BALs with a high frequency modulated pump current above threshold active longitudinal mode-locking is achieved. Pulse durations as low as 88 ps are obtained, while the Gaussian-like fundamental or a higher order transverse mode up to mode number 5 is selected on purpose. Pulse duration and shape are nearly independent of the selected transverse mode.

Synthesis of stochastic images with given fractal dimension

Synthesis methods based on spatial-frequency filtering have been developed for fractal images for which the structure functions are power laws over a wide range of increments which is about one-quarter of the image size. It is shown that, using the Weierstrass function, only stationary processes (images) can be simulated. An algorithm for modeling fractional Brownian images is proposed and studied. It is established that the integral characteristic of the spectrum of a Brownian image with the Hurst parameter α is well approximated by a power function with an exponent 2(α+1) for α ∈ (0, 1/2] and an exponent of 3 for α ∈ (1/2, 1). Using as an examples modifications of images of samples of amorphous alloys, it is shown that statistically self-similarity images little different from the original image can be simulated. This allows one to determine to what extent the structures visualized in the images exhibit fractal properties.

Image Enhancement for Sub-Harmonic Phased Array by Removing Surface Wave Interference with Spatial Frequency Filter

Closed cracks are difficult to detect using conventional ultrasonic testing because most incident ultrasound passes completely through these cracks. Nonlinear ultrasound inspection using sub-harmonic frequencies a promising method for detecting closed cracks. To implement this method, a sub-harmonic phased array (PA) is proposed to visualize the length of closed cracks in solids. A sub-harmonic PA generally consists of a single transmitter and an array receiver, which detects sub-harmonic waves generated from closed cracks. The PA images are obtained using the total focusing method (TFM), which (with a transmitter and receiving array) employs a full matrix in the observation region to achieve fine image resolution. In particular, the receiving signals are measured using a laser Doppler vibrometer (LDV) to collect PA images for both fundamental and sub-harmonic frequencies. Oblique incidence, which is used to boost sub-harmonic generation, inevitably produces various surface waves that contaminate the signals measured in the receiving transducer. Surface wave interference often degrades PA images severely, and it becomes difficult to read the closed crack's position from the images. Various methods to prevent or eliminate this interference are possible. In particular, enhancing images with signal processing could be a highly cost-effective method. Because periodic patterns distributed in a PA image are the most frequent interference induced by surface waves, spatial frequency filtering is applicable for removing these waves. Experiments clearly demonstrate that the spatial frequency filter improves PA images.

Optical detection of mechanically damaged areas on manufactured metal parts by spatial frequency filtering

An inspection of mechanically damaged areas on manufactured metal parts is necessary to produce high-quality products. A scanning probe on a sample is necessary for a conventional surface inspection system, which is time-consuming. We propose a novel high-speed detection method for defects on metal surfaces with rolling indentations. To obtain a large field of view in a measurement, we used a laser sheet that was expanded with a laser line generator and also used an expanded collimated beam, rather than a small laser spot as used in conventional techniques. Furthermore, we used an obliquely incident laser beam to suppress the effect of the rolling indentations surrounding defects, and also applied spatial frequency filtering to extract only defects. The spatial frequency filtering under oblique incidence is theoretically explained and defect extraction was investigated in experiments.

Eye movement control during scene viewing: Immediate degradation and enhancement effects of spatial frequency filtering

What controls how long the eyes remain fixated during scene perception? We investigated whether fixation durations are under the immediate control of the quality of the current scene image. Subjects freely viewed photographs of scenes in preparation for a later memory test while their eye movements were recorded. Using the saccade-contingent display change method, scenes were degraded (Experiment 1) or enhanced (Experiment 2) via blurring (low-pass filtering) during predefined saccades. Results showed that fixation durations immediately after a display change were influenced by the degree of blur, with a monotonic relationship between degree of blur and fixation duration. The results also demonstrated that fixation durations can be both increased and decreased by changes in the degree of blur. The results suggest that fixation durations in scene viewing are influenced by the ease of processing of the image currently in view. The results are consistent with models of saccade generation in scenes in which moment-to-moment difficulty in visual and cognitive processing modulates fixation durations.

新型航空镜间快门对光学传递函数的影响分析

The optical transfer function is the important parameter to evaluate the image quality of optical system. Based on the working property of one new type aviation lens shutter designed for the digital aerial photograph camera, the analysis of influence on the optical transfer function is presented. The lens shutter can change the optical pupil shape of the camera's optical system in the processing of control exposure, which means the point spread function is changed. Thus the optical transfer function is also changed. The relationship between the variety of aperture stop and the optical pupil function is built up by taking the optical system as the spatial frequency filter and adopting the Fourier transfer method and scalar diffraction theory as the theoretical foundation to analysis the influence of the shutter in the working period on the optical transfer function. Through the comparing analysis of the calculation and measurement results, they show that the maximum absolute error is 0.196 and the maximum relative repetitiveness. Furthermore, the design decreases the time ratio occupying whole error is -0.274 for the optical transfer function, which has better accuracy and exposure time for the beginning stage of shutter and the closing stage of shutter. It improves the comprehensive transfer function of system, which provides the theoretical reference for the optimization design of center shutter.

Mechanisms for similarity matching in disparity measurement

Early neural mechanisms for the measurement of binocular disparity appear to operate in a manner consistent with cross-correlation-like processes. Consequently, cross-correlation, or cross-correlation-like procedures have been used in a range of models of disparity measurement. Using such procedures as the basis for disparity measurement creates a preference for correspondence solutions that maximize the similarity between local left and right eye image regions. Here, we examine how observers’ perception of depth in an ambiguous stereogram is affected by manipulations of luminance and orientation-based image similarity. Results show a strong effect of coarse-scale luminance similarity manipulations, but a relatively weak effect of finer-scale manipulations of orientation similarity. This is in contrast to the measurements of depth obtained from a standard cross-correlation model. This model shows strong effects of orientation similarity manipulations and weaker effects of luminance similarity. In order to account for these discrepancies, the standard cross-correlation approach may be modified to include an initial spatial frequency filtering stage. The performance of this adjusted model most closely matches human psychophysical data when spatial frequency filtering favors coarser scales. This is consistent with the operation of disparity measurement processes where spatial frequency and disparity tuning are correlated, or where disparity measurement operates in a coarse-to-fine manner.

Coloring of radiation scattered by polymer-dispersed liquid crystals

We analyze the effects of coloring of a beam traversing a light-scattering medium. Spectral investigation of the effects of coloring has been carried out using a solution of liquid crystal in a polymer matrix (polymer-dispersed liquid crystals – PDLC). It is shown that the result of coloring of the beam at the output of the medium depends on the magnitudes of the phase delays of the singly forward scattered partial signals. We consider the influence of interference coloring effect on the transmission scattering and spatial-frequency filtering of the radiation which has passed through the PDLC.

Tunable acoustic waveguides in periodic arrays made of rigid square-rod scatterers: theory and experimental realization

The tunable and the engineering possibilities of waveguides in periodic arrays made of rigid square-rod scatterers are theoretically and experimentally reported in this work. Due to the square shape of the scatterers, the control of their orientation with respect to the direction of the incident wave can be used for moulding the propagating acoustic waves inside the periodic structure. On the one hand, the plane wave expansion with supercell approximation is used to obtain the band structure of the periodic system. On the other hand, the scattering of waves in finite periodic arrays is analysed using the finite elements method. Experimentally, a prototype made of rigid square-rod scatterers is used to validate the theoretical predictions. A spatial-frequency filter and some applications in waveguiding for audible sound are discussed in this work. Good agreement between theory and experiments and the high tunability of the system are demonstrated.

Advanced optics experiments using nonuniform aperture functions

A method to create instructive, nonuniform aperture functions using spatial frequency filtering is described. The diffraction from a single slit in the Fresnel limit and the interference from a double slit in the Fraunhofer limit are spatially filtered to create electric field distributions across an aperture to produce apodization, inverse apodization or super-resolution, and apertures with phase shifts across their widths. The diffraction effects from these aperture functions are measured and calculated. The excellent agreement between the experimental results and the calculated results makes the experiment ideal for use in an advanced undergraduate or graduate optics laboratory to illustrate experimentally several effects in Fourier optics.

The Talbot effect: recent advances in classical optics, nonlinear optics, and quantum optics

The Talbot effect, also referred to as self-imaging or lensless imaging, is of the phenomena manifested by a periodic repetition of planar field distributions in certain types of wave fields. This phenomenon is finding applications not only in optics, but also in a variety of research fields, such as acoustics, electron microscopy, plasmonics, x ray, and Bose–Einstein condensates. In optics, self-imaging is being explored particularly in image processing, in the production of spatial-frequency filters, and in optical metrology. In this article, we give an overview of recent advances on the effect from classical optics to nonlinear optics and quantum optics. Throughout this review article there is an effort to clearly present the physical aspects of the self-imaging phenomenon. Mathematical formulations are reduced to the indispensable ones. Readers who prefer strict mathematical treatments should resort to the extensive list of references. Despite the rapid progress on the subject, new ideas and applications of Talbot self-imaging are still expected in the future.

Oversampling smoothness: an effective algorithm for phase retrieval of noisy diffraction intensities.

Coherent diffraction imaging (CDI) is high-resolution lensless microscopy that has been applied to image a wide range of specimens using synchrotron radiation, X-ray free-electron lasers, high harmonic generation, soft X-ray lasers and electrons. Despite recent rapid advances, it remains a challenge to reconstruct fine features in weakly scattering objects such as biological specimens from noisy data. Here an effective iterative algorithm, termed oversampling smoothness (OSS), for phase retrieval of noisy diffraction intensities is presented. OSS exploits the correlation information among the pixels or voxels in the region outside of a support in real space. By properly applying spatial frequency filters to the pixels or voxels outside the support at different stages of the iterative process (i.e. a smoothness constraint), OSS finds a balance between the hybrid input-output (HIO) and error reduction (ER) algorithms to search for a global minimum in solution space, while reducing the oscillations in the reconstruction. Both numerical simulations with Poisson noise and experimental data from a biological cell indicate that OSS consistently outperforms the HIO, ER-HIO and noise robust (NR)-HIO algorithms at all noise levels in terms of accuracy and consistency of the reconstructions. It is expected that OSS will find application in the rapidly growing CDI field, as well as other disciplines where phase retrieval from noisy Fourier magnitudes is needed. The MATLAB (The MathWorks Inc., Natick, MA, USA) source code of the OSS algorithm is freely available from http://www.physics.ucla.edu/research/imaging.

数字全息显微像平面相位畸变的自动补偿研究

The automatic phase aberration compensation of off-axis digital holographic microscopy is studied theoreticaly and experimentaly.Firstly,the average gradient auto-focus function is used to achieve the optimal reconstruction distance of the digital hologram recorded by CCD.After numerical apodization and spatial frequency filtering,the numerical reconstruction is performed by the product of the hologram with modified digital reference wave.Finally,with the automatic phase aberration compensation of image plane,the accurate phase distribution is achieved.This method uses only one single digital hologram and can retrieve automatically and rapidly the real phase distribution of the object.

Pigeons use low rather than high spatial frequency information to make visual category discriminations.

Pigeons were trained to discriminate photographs of cat faces from dog faces. They were then presented with test stimuli involving high- and low-pass spatial frequency filtering. Discrimination was maintained with both types of filtered stimuli, though it was increasingly impaired the more information was filtered out, and high-pass filtering impaired discrimination more than low-pass filtering. The pigeons were then exposed to hybrid stimuli in which high-pass filtered dog faces were combined with low-pass filtered cat faces, and vice versa. Response to hybrid stimuli was determined more by the low spatial frequency content than by the high-frequency content, whereas humans viewing the same stimuli at corresponding viewing distance respond more strongly to the high-frequency content. These results are unexpected given that, compared with humans, pigeons' behavior tends to be controlled by the local details of visual stimuli rather than their global appearance.

Eliminating distortions of nonperiodic structures in images

Methods for reconstructing distorted images of microstructures are proposed. The spatial-frequency filtering of a distorted image, as a result of which the distribution of the Fourier transform phases of this image is adjusted according to the distribution of zeros of the visualization system’s frequency characteristic, and the amplitude distribution is transformed so that the Fourier spectrum’s integrated frequency characteristic (IFC) is modified into a function that decays according to the power law.

Spatial-frequency Fourier polarimetry of human blood plasma in the diagnostics of pathological changes

A method of polarization mapping of the optical-anisotropic polycrystalline networks of the blood plasma albumin and globulin proteins with adjusted spatial-frequency filtering of the coordinate distributions of the azimuth and ellipticity of the polarization of laser radiation in the Fourier plane is proposed and substantiated. A set of criteria of diagnosing prostate cancer based on the statistical correlation and fractal analysis of the spatial-frequency filtered polarization distributions generated by dendritic networks of albumin and globulin spherulitic networks has been detected and substantiated.