Diffraction efficiency of low-resolution Fresnel encoded lenses

A mathematical model to describe the behavior of low-resolution Fresnel lenses encoded in any low-resolution device (e.g., a spatial light modulator) is developed. From this model the diffraction efficiency is calculated in terms of all the parameters that characterize these lenses.     

Diffraction theory of optimized low-resolution Fresnel encoded lenses

A mathematical model describing the behavior of low-resolution Fresnel encoded lenses (LRFEL's) encoded in any low-resolution device (e.g., a spatial light modulator) has recently been developed. From this model, an LRFEL with a short focal length was optimized by our imposing the maximum intensity of light onto the optical axis. With this model, analytical expressions for the light-amplitude distribution, the diffraction efficiency, and the frequency response of the optimized LRFEL's are derived.     

Charge sharing in common-grid pixelated CdZnTe detectors

The charge sharing effect in pixelated CdZnTe (CZT) detectors with a common anode steering grid has been studied. The impact on energy resolution of weighting potential cross-talk and ballistic deficit due to cathode signal shaping has been investigated. A detailed system modeling package considering charge induction, electronic noise, pulse shaping, and ASIC triggering procedures has been developed to study the characteristics of common-grid CZT detectors coupled to the VAS_UM/TAT4 ASIC. Besides an actual common-grid CZT detector coupled to VAS_UM/TAT4 ASIC, a prototype digital read-out system has been developed to better understand the nature of the charge sharing effect.     

Method for fabricating pixelated, multicolor polarizing films

Pixelated, multicolor polarizing filters-of potential use in full-color displays-were produced by what we believe to be a novel method, i.e., masked evaporation of silver and gold onto glass substrates partially covered with separated sub-micrometer-wide strips of oriented poly(tetrafluoroethylene) (PTFE), prepared by friction deposition. The evaporated metal films preferentially nucleated at the glass surface and, consequently, formed parallel arrays in between the PTFE strips. The structures thus produced feature a strong angle-dependent absorption of polarized visible light, allowing for optical switching between red and blue and between green and yellow.     

Effect of extended defects in planar and pixelated CdZnTe detectors

We evaluated a spectroscopy-grade 15×15×7 mm³ CdZnTe (CZT) crystal with a high μτ-product, >10⁻² cm²/V, but impaired by microscopic extended defects, such as walls of dislocations, low-angle and sub-grain boundaries, and Te inclusions. First, we evaluated a planar detector fabricated from this crystal using a Micro-scale X-ray Detector Mapping (MXDM) technique. Then, we fabricated from the same crystal a pixel detector to study local non-uniformities of the electric field. The measured X-ray response maps confirmed the presence of non-uniformities in the charge transport, and they showed that the global- and local-distortions of the internal E-field correlated to the extended defects and space-charge buildup on the side surfaces.     

Continuous parallax in discrete pixelated integral three-dimensional displays

An evaluation of the retention of continuous parallax in pixelated integral three-dimensional image displays is presented. The integral image capture process is first considered, to provide a starting point for the investigation. The complementary display system is then examined in detail. The viewing geometry of the display system is analyzed to provide a foundation for the work to follow, and an experimental investigation and simulations of the characteristics of emitted ray bundles are presented. Next, an analytical model of decoding lenslet array operation is derived, leading to an understanding of the process responsible for production of continuous parallax in replay. It is found that if the lateral resolution of the lenslet is matched to that of the display, continuous parallax is retained in the replayed image, where the finite aberration-limited resolution of the lenslet acts to produce a low-pass reconstruction filter. A condition is derived for optimal continuous parallax in replay, based on a relationship between pixel width and lenslet rms spot size.     

One-step multichannel pattern recognition based on the pixelated structure of a spatial light modulator

We present an architecture in which a multichannel correlator can perform simultaneous optical pattern recognition. Processing in parallel is made possible by use of the different diffraction orders produced by the pixelated structure of the liquid-crystal spatial light modulator employed to display the input scene. We codify additional quadratic phases in the filters to separate the correlation information corresponding to each channel. We demonstrate that the system can recognize different targets simultaneously. Good agreement between experimental and numerically simulated results is obtained.     

Suppression of the zero-order diffracted beam from a pixelated spatial light modulator by phase compression

Phase compression is used to suppress the on-axis zero-order diffracted (ZOD) beam from a pixelated phase-only spatial light modulator (SLM) by a simple modification to the computer generated hologram (CGH) loaded onto the SLM. After CGH design, the phase of each SLM element is identically compressed by multiplying by a constant scale factor and rotated on the complex unit-circle to produce a cancellation beam that destructively interferes with the ZOD beam. Experiments achieved a factor of 3 reduction of the ZOD beam using two different liquid-crystal SLMs. Numerical simulation analyzed the reconstructed image quality and diffraction efficiency versus degree of phase compression and showed that phase compression resulted in little image degradation or power loss.     

Implementation of a spatially multiplexed pixelated three-dimensional display by use of a holographic optical element array

A pixelated holographic stereogram is proposed and experimentally studied for the emulation of a spatially multiplexed composite three-dimensional (3-D) pixel display. With this approach, pixelated holograms are utilized to compose spatially multiplexed images. Each composite pixel in the holographic optical element array has a diffraction pattern that scatters light into predefined spatial directions. Under reconstruction, each pixel generates different intensities along a range of viewing angles. When the composite holographic pixel array is assembled, it has the capability to deliver 3-D effects. The technique, together with a novel recording scheme that is designed to synthesize a computerized 3-D display system based on this concept, is described in some detail.     

Chromatic aberrations of radial gradient-index lenses. II. Selfoc lenses

Simple equations used for analyzing chromatic aberrations of Selfoc lenses were derived in terms of Buchdahl chromatic coordinates and Buchdahl dispersion constants. The equations that employ gradient-index chromatic constants Ψ(1) and Ψ(2) are used for selecting suitable ion-exchange pairs to design an achromatic Selfoc lens.     

Assessing interactions in the brain with exact low-resolution electromagnetic tomography

Scalp electric potentials (electroencephalogram; EEG) are contingent to the impressed current density unleashed by cortical pyramidal neurons undergoing post-synaptic processes. EEG neuroimaging consists of estimating the cortical current density from scalp recordings. We report a solution to this inverse problem that attains exact localization: exact low-resolution brain electromagnetic tomography (eLORETA). This non-invasive method yields high time-resolution intracranial signals that can be used for assessing functional dynamic connectivity in the brain, quantified by coherence and phase synchronization. However, these measures are non-physiologically high because of volume conduction and low spatial resolution. We present a new method to solve this problem by decomposing them into instantaneous and lagged components, with the lagged part having almost pure physiological origin.     

Harmonic diffractive lenses

The harmonic diffractive lens is a diffractive imaging lens for which the optical path-length transition between adjacent facets is an integer multiple m of the design wavelength λ(o). The total lens thickness in air is mλ(o)/(n - 1), which is m times thicker than the so-called modulo 2π diffractive lens. Lenses constructed in this way have hybrid properties of both refractive and diffractive lenses. Such a lens will have a diffraction-limited, common focus for a number of discrete wavelengths across the visible spectrum. A 34.75-diopter, 6-mm-diameter lens is diamond turned in aluminum and replicated in optical materials. The sag of the lens is 23 μm. Modulation transfer function measurements in both monochromatic and white light verify the performance of the lens. The lens approaches the diffraction limit for 10 discrete wavelengths across the visible spectrum.     

Multiple-foci Luneberg lenses

A family of variable-index spherically symmetric lenses is presented that provide the light focusing at two (or three) points, subject to an appropriate division of the aperture.     

Integrating GWTM and BAT algorithm for face recognition in low-resolution images

In biometrics, face recognition is one of the important identification methods with various applications such as, video surveillance, defence, human/computer interactions and many more. The current face recognition systems perform well using the frontal images with high resolution. In contrast, the utilisation of low-resolution (LR) images degrades the performance of face recognition systems. Hence, this paper integrates the Gabor filter?+?wavelet?+?texture (GWTM) operator and the BAT algorithm to increase the performance, while deploying the LR images. The proposed algorithm integrates the uniqueness of Gabor features, the robustness of local features and the wavelet features to handle the inter-person and intra-person variations. This paper utilises the spherical SVM classifier to enhance the recognition performance. Finally, the proposed GWTM operator is compared with other existing algorithms such as, GOM, LBP and LGP based on the parameters of accuracy, FAR and FRR. The proposed GWTM operator attains the highest accuracy of 95% and a minimum FAR of 5%. The results prove that the proposed GWTM yields a performance improvement of 5, 3, 4 and 15% over the GOM, LBP, LGP and GWTM, respectively, in the absence of the BAT algorithm.     

Variable power phase lenses

A method is given to construct a phase lens capable of creating an optical aberration of variable power that is described by a single Zernike polynomial function whose meridional index is 1 or greater. The phase lens is created from two identical phase elements, each creating a single Zernike aberration, that can be rotated with respect to each other, thereby increasing the aberration effect from zero to twice the value of either. This is possible because these aberrations are vectorlike. Results are given from the testing of an example that was manufactured and designed to produce coma (Zernike term Z(3,1)).     

High-performance fluidic adaptive lenses

High-performance fluidic lenses with an adjustable focal length spanning a very wide range (30 mm to infinite) are demonstrated. We show that the focal length, F-number, and numerical aperture can be dynamically controlled by changing the shape of the fluidic adaptive lens without moving the lens position mechanically. The shortest focal length demonstrated is less than 30 mm for a 20-mm lens aperture. The fluidic adaptive lens has a nearly perfect spherical profile and shows a resolution better than 40 line pairs/mm in a plano-convex structure and 57 line pairs/mm in a biconvex structure.