Spatial filter

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

Metasurfaces with Asymmetric Optical Transfer Functions for Optical Signal Processing

Abstract: Metasurface thin films created from arrays of structured optical elements have been shown to perform spatial filtering of optical signals. To extend their usefulness it is important that the symmetry of their response with changes to the in-plane wave vector k_{p}→-k_{p} can be tailored or even dynamically tuned. In this Letter we use a general theory of metasurfaces constructed from nondiffracting arrays of coupled metal particles to derive the optical transfer function and identify the physical properties essential for asymmetry. We validate our theory experimentally showing how the asymmetric response of a two-dimensional (planar) metasurface can be optically tuned. Our results set the direction for future developments of metasurfaces for optical signal processing.

Apparatus and method of image enhancement through spatial filtering

Abstract: An image enhancement apparatus and method are disclosed. The apparatus consists of a spatial frequency filter (12) where zero order mask diffraction information is reduced in an alternative pupil plane of the objective lens (9), specifically just beyond the mask plane. By introducing an angular specific transmission filter (12) into this Fraunhofer diffraction field of the mask (8), user accessibility is introduced, allowing for a practical approach to frequency filtering. This frequency filtering is accomplished using a specifically designed interference filter (14, 15) coated over a transparent substrate (16). Alternatively, filtering can also be accomplished in a complementary region near the wafer image plane or in both near-mask and near-wafer planes.

Adaptive mean filtering for noise reduction in CT polymer gel dosimetry

Abstract: X-ray computed tomography (CT) as a method of extracting 3D dose information from irradiated polymer gel dosimeters is showing potential as a practical means to implement gel dosimetry in a radiation therapy clinic. However, the response of CT contrast to dose is weak and noise reduction is critical in order to achieve adequate dose resolutions with this method. Phantom design and CT imaging technique have both been shown to decrease image noise. In addition, image postprocessing using noise reduction filtering techniques have been proposed. This work evaluates in detail the use of the adaptive mean filter for reducing noise in CT gel dosimetry. Filter performance is systematically tested using both synthetic patterns mimicking a range of clinical dose distribution features as well as actual clinical dose distributions. Both low and high signal-to-noise ratio (SNR) situations are examined. For all cases, the effects of filter kernel size and the number of iterations are investigated. Results indicate that adaptive mean filtering is a highly effective tool for noise reduction CT gel dosimetry. The optimum filtering strategy depends on characteristics of the dose distributions and image noise level. For low noise images (SNR approximately 20), the filtered results are excellent and use of adaptive mean filtering is recommended as a standard processing tool. For high noise images (SNR approximately 5) adaptive mean filtering can also produce excellent results, but filtering must be approached with more caution as spatial and dose distortions of the original dose distribution can occur.

The effects of small displacements of spatial filters.

Abstract: In a coherent optical system any displacement of a spatial filter from its optimum position degrades the performance of the system. In this paper the performance of a matched filtering system is related to the degree of displacement for both uniform and nonuniform noise spectral densities. Static displacements of the filter normal or parallel to the optical axis are treated, as well as random motion of the filter normal to the optical axis. Displacements normal to the optical axis cause the performance to decrease more rapidly than those parallel to the axis. For both kinds, the positioning tolerances are more critical when the noise spectral density is nonuniform. A dual frequency-plane optical precessor is described; this processor helps overcome the tolerance problem when the noise is nonuniform because the fixed part of the matched filter can be placed in one frequency plane and the changeable part in another.

Frequency multiplexed optical spatial filter

Abstract: A frequency multiplexed optical spatial filter comprises a thin film of a photochemical hole burning material. The filter stores a plurality of images in the photochemical hole burning medium. The images are formed at different optical frequencies and coexist in the same volume of the storage medium. Individual images are accessed by changing the frequency of the illuminating radiation.