Computer Optics

431-438

Authors: D.A. Kozlov, E.S. Kozlova, V.V Kotlyar

In this paper a research on conditions for resonance excitation in a homogeneous dielectric cylinder with square cross-section is conducted using a FDTD-method. It is shown that in a cylinder with square cross-section resonant modes similar to whispering gallery modes can be excited, which allows one to reduce the transverse dimensions of the focal spot. FDTD method demonstrates an acceptable accuracy of the resonant mode detection. A similar result is obtained for a cube on a dielectric substrate: it is possible to achieve the reduction of the transverse dimensions of the focal spot by changing the corresponding cube size.

451-457

Authors: V.V. Kotlyar, A.G. Nalimov, M.V. Kotlyar

We propose a new version of the metalens, namely, a polarization microlens (PML) that focuses the linearly polarized laser light in a subwavelength circular spot. The PML consists of a set of binary subwavelength gratings, which are laid out in a parquet manner within the rings of a Fresnel zone plate. A phase shift of π, which the zone plate should introduce into the light field in passing the zone boundary, is implemented in the PML under study using two diffraction gratings adjacent to the zone boundary, which form oppositely polarized transmitted beams. It is shown that the PML relief depth can be reduced using a high refractive index material. For amorphous silicon, the PML relief depth can vary from 50 nm to 120 nm. In this case, a circular focal spot having a diameter smaller than the diffraction limit is formed at distances to the PML ranging from 200 nm to 1300 nm. The minimum focal spot diameter is 0.372 of a wavelength.

467-474

Authors: K.V. Andreeva, M.A. Moiseev, S.V. Kravchenko, L.L. Doskolovich

A novel method for the design of total internal reflection based (TIR) optical elements generating complex two-dimensional narrow-angle light distributions is presented. The method consists of two parts: computation of a piecewise smooth solution and its subsequent approximation by a spline surface. Examples of TIR optical elements designed using the proposed approach are discussed. The simulation results demonstrate high performance of the proposed method: the luminous efficacy of the optical elements is 91.8% and the relative root-mean-square error is less than 8.6%.

482-488

Authors: V.P. Korolkov, A.S. Konchenko, V.V. Cherkashin, N.G. Mironnikov

A process of manufacturing conformal correctors for solid state YAG:Nd3+ lasers is discussed. It is proposed that a maskless lithography method should be used for fabricating a photoresist film with a desired thickness profile as an alternative to the proximity lithography based on half-tone masks. The use of a specular spectral scatterometry method for the testing of the conformal corrector shape at an early stage of photoresist profile formation is reported. A combination of these two methods makes the corrector manufacturing process significantly cheaper and faster.

501-507

Authors: S. A. Shoydin

For the case of 3D phase holograms, the impact of the form factor on the hologram diffraction efficiency is shown. The form factor concept is introduced by analogy with the classical definition, which is used for calculating the averaged interaction of complex-shaped objects. In the present context, the form factor concept is used for determining the average diffraction efficiency of the holograms formed with the help of beams distributed non-uniformly over the hologram field. It is shown that after calculating the corrections, they can be used in the same way as coefficients in the classical formulas of the diffraction efficiency. The impact of such a form factor only comes into play when there are two factor present at the same time, i. e. non-linearity of photoresponse and non-uniformity of the light beam distribution over the hologram field.

516-525

Authors: N.A. Lomov, L.M. Mestetskiy

A "width of a shape" concept considered in relation to variable-width objects is used as the integral shape descriptor for image recognition tasks. The approach to the definition of this concept is based on covering the figure of interest with certain-size disks. A function that defines the area of disk cover against the disk size is used as a shape descriptor. An analytical method for calculating the area of disk cover for polygonal figures is proposed. The possibility to approximate by polygonal figures binary raster images and objects with non-linear boundaries makes the approach universal. The proposed method is based on the medial representation of objects as a skeleton and a radial function. The method provides high accuracy and computational efficiency. The efficiency of the proposed shape descriptor is illustrated by the example of its application to the digital font recognition problem.

535-542

Authors: A.Y. Denisova, V.V. Sergeye

In the article we offer a novel approach to calculating multichannel image histograms using hierarchical data structures. The proposed methods result in a histogram-tree which has many advantages over existing data structures traditionally used for multidimensional histograms. The suggested data structure requires a significantly smaller memory space for storage in comparison with a histogram that is presented as a frequency table for all possible brightness values (histogramhypercube). The histogram-tree provides a dramatic decrease in speed in comparison with the list of unique brightness values and their frequencies (histogram-list). Moreover, the histogram-tree allows one to approximate the probabilities of brightness values which are not presented in the analyzed image in contrast with the histogram-list. The possibility of histogram approximation is very important for practical applications. Such approximation for histogram-tree is constructed by means of tree reduction. Nodes with frequencies lower than a given threshold are removed from the histogram-tree. The reduced histogramtree benefits from the decrease of the required memory space while maintaining the accuracy of the original probability density distribution representation. We propose two algorithms for constructing a histogram-tree. The “depth” algorithm operates with image pixels sequentially. For each pixel it constructs appropriate branches and nodes until the maximum depth of the tree is achieved. This algorithm has a recursive realization and is more time efficient than the other one. The “across” algorithm performs a multiple scan of the image, each time filling just one tree level. This algorithm allows tne to operate with images with a larger number of channels than the “depth” algorithm because it is more memory efficient in the runtime when approximating a histogram. Both algorithms build the histogram-tree significantly faster when compared with the construction of the histogram-list. The theoretical estimates and experimental results described in the article demonstrate that computing histogram of multichannel images using histogram-tree has many advantages over the histogram-hypercube and histogram-list.

552-559

Authors: S.A. Borminsky, A.V. Solntseva, B.V. Skvortsov

The article describes a method for optoelectronic control of liquid volume in a tank. Measurements can be performed remotely without tank calibration. The method is based on the fact that there is a near-wall region where the liquid surface has a higher reflectivity. This region occurs due to surface tension or minor ripples. With this method, first, an image of liquid surface is taken with a photo-camera. Then, a near-wall liquid-air boundary is identified in the image before calculating the current area of the liquid surface. When the tank is filled for the first time, the device records the liquid area for every level. A calibration table is automatically created without applying additional tools. To avoid the error caused by inaccurate photo-camera positioning, a special mathematical model is devised. It uses reference marks made on tank walls horizontally. An algorithm for recognition of liquid boundaries in the image is developed. Measurements of the liquid volume conducted using a laboratory prototype have shown the reduced error to be within 0.5%, provided that the digital photo-camera is not rigidly fixed.

572-582

Authors: D.V. Mikhaylov, A.P. Kozlov, G.M. Emelyanov

In this paper we look at two interrelated problems of extracting knowledge units from a set of subject-oriented texts (the so-called corpus) and selecting texts to the corpus by analyzing the relevance to the initial phrase. The main practical goal here is finding the most rational variant to express the knowledge fragment in a given natural language for further reflection in the thesaurus and ontology of a subject area. The problems are of importance when constructing systems for processing, analysis, estimation and understanding of information. In this paper the text relevance to the initial phrase in terms of the described fragment of actual knowledge (including forms of its expression in a given natural language) is defined by the total numerical estimate of the coupling strength of words from the initial phrase jointly occurring in phrases of the text under analysis. The paper considers known variants of such estimation procedures and their application for the search of distinct components which reflect the initial phrase in the texts selected to the topical text corpus. These components correspond to words and their combinations. In comparison with the search of such components on a syntactically marked text corpus, the method for text selection offered in this paper enables a 15-times reduction (on average) in the output of phrases which are irrelevant to the initial one in terms of either the described knowledge fragment or its expression forms in a given natural language.

588-593

Authors: S.A. Degtyarev

In this work, using a finite element method implemented we simulate in the Comsol software focusing a TE-polarized Gaussian beam by 3D dielectric conical microaxicon tapers with smaller-than-30-degrees vertex angles. The simulation results have shown that, as is the case with the 2D axicons, with minor changes in a vertex angle, the 3D axicon can alternatively show either focusing or defocusing properties. However, the angle intervals in which the 2D and 3D axicons show focusing or defocusing properties are different.