Spatial filter

About: Spatial filter is a research topic. Over the lifetime, 6170 publications have been published within this topic receiving 100451 citations.

Hybrid optical processor for pattern recognition and classification using a generalized set of pattern functions.

Abstract: A pattern recognition and classification system has been studied which computes the inner products of an input pattern with a generalized set of pattern functions. The system utilizes a filter whose impulse response consists of an amplitude superposition of a set of generalized pattern functions in phase-coded form and has a space–bandwidth product the same as that of a matched filter for one pattern function. Each pattern function in the general set may correspond to a different variation (e.g., scale or rotation) of the object to be detected. Because the amplitude superposition of the phase-coded pattern functions takes place in the digital computer, and the filter is created as a computer-generated hologram, the biasing problem of conventional, multiple exposure (intensity superposition) holograms is significantly reduced. This makes it possible to encode many more pattern functions than was previously possible using multiple exposure techniques. Furthermore, the use of computer-generated holograms eliminates the need to generate a transparency for each pattern function and complex phase code. To facilitate real-time operation a hybrid system was constructed consisting of a liquid crystal light valve for incoherent-to-coherent image conversion, a TV camera and image digitizer for image analysis, and a laser scanner to produce the computer-generated holograms. Both the TV camera/digitizer and the laser scanner systems were interfaced to a digital computer for automatic operation. Experimental results using the hybrid system are presented for pattern recognition of rotated and scaled objects and pattern classification. In the second half of the paper we provide an analysis on the SNR of the processor employing the coded-phase technique, on the diffraction efficiencies of computer-generated filters (amplitude superposition) vs conventional filters (intensity superposition), and on the number of pattern functions required for a certain recognition task.

Influence of tip modulation on image formation in scanning near-field optical microscopy

Abstract: Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection.

The use of film nonlinearities in optical spatial filtering.

Abstract: Optical data-processing systems that use spatial filters for detecting two-dimensional signals are usually arranged so that the data are placed in a space domain and the signals to be detected are placed in a frequency domain as holographically recorded Fourier transforms. We propose to interchange the roles of the data and the signal by introducing the data into the system as a Fourier transform hologram and by placing the signal in the space plane. Furthermore, the nonlinear behavior of the film used to record the Fourier transform hologram can be used to good advantage. These modifications combine to give a system that (1) acts as an adaptive filtering system because the noise suppression part of the matched filter is based on a specific noise sample rather than on averages of many noise samples, (2) is less sensitive to the position of the element that must be changed to perform a search for the scale or orientation of the signal, and (3) may reduce the aberrations that are sometimes caused by spatial carrier-frequency functions. Experimental results support the theoretical conclusions.

Direction of Arrival Estimation in the Spherical Harmonic Domain Using Subspace Pseudointensity Vectors

Abstract: Direction of arrival DOA estimation is a fundamental problem in acoustic signal processing. It is used in a diverse range of applications, including spatial filtering, speech dereverberation, source separation and diarization. Intensity vector-based DOA estimation is attractive, especially for spherical sensor arrays, because it is computationally efficient. Two such methods are presented that operate on a spherical harmonic decomposition of a sound field observed using a spherical microphone array. The first uses pseudointensity vectors PIVs and works well in acoustic environments where only one sound source is active at any time. The second uses subspace pseudointensity vectors SSPIVs and is targeted at environments where multiple simultaneous soures and significant levels of reverberation make the problem more challenging. Analytical models are used to quantify the effects of an interfering source, diffuse noise, and sensor noise on PIVs and SSPIVs. The accuracy of DOA estimation using PIVs and SSPIVs is compared against the state of the art in simulations including realistic reverberation and noise for single and multiple, stationary and moving sources. Finally, robust performance of the proposed methods is demonstrated by using speech recordings in a real acoustic environment.