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Articles of interest (July 2001)


Elastic laser spans the spectrum

A laser that changes colour when it stretches has been developed by a team of scientists from Germany and the US. Heino Finkelmann of Albert-Ludwigs University in Freiburg, Germany, and co-workers based the device on a liquid crystal with a spiral structure that behaves like a spring. It emits green light in its unstressed state, but this shifts to red when the spring is stretched (H Finkelmann et al 2001 Advanced Materials at press). The tunable laser could be invaluable in the communications industry to send light of different wavelengths through fibre optic cables.


Polarized light scattering by microroughness and small defects in dielectric layers by Thomas A. Germer

JOSA A, Volume 18, Issue 6, 1279-1288 June 2001

ABSTRACT

The polarization of light scattered by the surface of a material contains information that can be used to identify the sources of that scatter. Theories for light scattering from interfacial roughness of a dielectric layer and from defects in that dielectric layer are reviewed. Methods for calculating the Mueller matrix or the Stokes vector for scatter from multiple sources and for decomposing a Stokes vector into contributions from two nondepolarizing scattering sources are derived. The theories are evaluated for a specific sample and geometry. Results show that some incident polarizations are more effective than others at discriminating among scattering sources, with s -polarized light being least effective. The polarization of light scattered from interfacial roughness depends upon the relative roughness of the two interfaces and the degree of correlation between the two interfaces. The scattering from defects in the film depends on the depth of the defect and differs from that from any one of the cases of interfacial roughness. The scattering from defects randomly distributed in the film and for small dielectric permittivity variations in the film is also calculated. Experimental results are presented for a 52-nm SiO 2 film thermally grown on microrough silicon. 2001 Optical Society of America [Optical Society of America ]


Variational approach to moire pattern synthesis by Guy Lebanon, Alfred M. Bruckstein

JOSA A, Volume 18, Issue 6, 1371-1382 June 2001

ABSTRACT

Moire phenomena occur when two or more images are nonlinearly combined to create a new superposition image. Moire patterns are patterns that do not exist in any of the original images but appear in the superposition image, for example as the result of a multiplicative superposition rule. The topic of moire pattern synthesis deals with creating images that when superimposed will reveal certain desired moire patterns. Conditions that ensure that a desired moire pattern will be present in the superposition of two images are known; however, they do not specify these images uniquely. The freedom in choosing the superimposed images can be exploited to produce various degrees of visibility and ensure desired properties. Performance criteria for the images that measure when one superposition is better than another are introduced. These criteria are based on the visibility of the moire patterns to the human visual system and on the digitization that takes place when the images are presented on discrete displays. We propose to resolve the freedom in moire synthesis by choosing the images that optimize the chosen criteria. 2001 Optical Society of America [Optical Society of America ]


Angular Spectrum of Light Transmitted Through Turbid Media: Theory and Experiment
Alexander A. Kokhanovsky, Reiner Weichert, Michael Heuer, Wolfgang Witt

Applied Optics-OT, Volume 40, Issue 16, 2595-2600 June 2001

ABSTRACT

Measurements of the angular spectrum of light transmitted through turbid slabs with monodispersions of polystyrene spheres have been performed. The results obtained are compared with theoretical calculations, based on the small-angle approximation of the radiative transfer theory. The experimental data and the theoretical results coincide with a high accuracy, which allows us to develop the laser diffraction spectroscopy of optically thick light-scattering layers. [Optical Society of America ]


Noninvasive Characterization of Continuous-Profile Blazed Diffraction Gratings
Anu Huttunen, Ilkka Kallioniemi, Jyrki Saarinen

Applied Optics-OT, Volume 40, Issue 16, 2618-2625 June 2001

ABSTRACT

Smooth, continuous-profile diffractive optical elements and other regular microstructures demand advanced characterization methods. Here the problem of reconstructing the profiles with optical scatterometry is addressed for the first time to our knowledge. We represent the profile of a blazed grating with an analytic Chebyshev series. The coefficients of the series are inferred from the far-field diffraction pattern, when the grating profile is illuminated with coherent light. An advanced neural-network model is used as a nonlinear statistical estimator. The reconstruction error is found to be less than 4% as normalized to the depth of the profile, even with realistic measurement errors included in the simulations. [Optical Society of America ]


Multiple-Reflection Effects in Photoelastic Stress Analysis by Achintya K. Bhowmik

Applied Optics-OT, Volume 40, Issue 16, 2687-2691 June 2001

ABSTRACT

The interpretation of fringes observed in photoelastic stress measurements made with coherent well-collimated optical radiation such as a laser beam and slab specimens with parallel surfaces is affected by multiple internal reflections of light within the sample, which are usually negligible when incoherent light is used. An analysis of the multiple-reflection effects in photoelastic measurements involving the plane polariscope configuration is presented. The results show that the isochromatic fringes are modified by the interference of multiply reflected waves. The multipass differential phase accumulations that display oscillatory magnitudes as functions of the model thickness and the optical wavelength result in a shifted and altered intensity profile across the isochromatic fringes. It is shown that for large values of reflectivity, as in the case of samples with reflective coating or partial mirrors, the bright fringes split into multiple peaks. [Optical Society of America ]

Comment: Photoelastic Fringe Multiplication - see D. Post papers



Microscope-based static light-scattering instrument
M. T. Valentine, A. K. Popp, D. A. Weitz, P. D. Kaplan

Optics Letters, Volume 26, Issue 12, 890-892 June 2001

ABSTRACT

We describe a new design for a microscope-based static light-scattering instrument that provides simultaneous high-resolution images and static light-scattering data. By correlating real space images with scattering patterns, we can interpret measurements from heterogeneous samples, which we illustrate by using biological tissue. [Optical Society of America ]


Zooming in on atoms

Physical Review Letters 86, 5510-5513 (11 June 2001)

John Spence and Christoph Koch describe a new holographic method that can provide atomic-resolution images of crystal and other nanoscale structures.

 



Visual Inspection on the Internet

 

Visual inspection is one of the most common methods for materials examination. Information on visual examination can be obtained by browsing the Internet, and in this connection, the following websites may be of interest to the non-destructive testing community.

The New Zealand NDT Association offers information on a wide range of NDT methods and presents a good introduction to visual inspection.

The NCSA Engineering with Visual Analysis homepage describes the use of computer visualization to assist in the identification and characterization of flaws not always detectable by the conventional NDT methods.

One application of visual analysis is the stimulation and visualization of acoustic microscope measurements used to detect delamination in composite materials. Information on the NCSA's current visual analysis applications and facilities can be obtained through their website In addition, multimedia modules for material and engineering scientists can be obtained free of charge using anonymous ftp( see INSIGHT Vol.37, N 8, pp620-623)

Results from the use of direct volume visualization techniques for automatic detection of explosives are presented at the Invasion Technologies Inc. homepage.

The Tops NASA server provides technical information sheets on specific research projects, such as stroboscopic light sheet flow visualization technique for condition monitoring of helicopter rotors, and laser light sheet flow visualization technique.

A request for further information, publication or technical references is also available on-line for each technical information sheet.

Another NASA-related source of information on visual inspection

This site offers a full description of a machine vision system for automated visual surface flaw detection on orbiting space platforms. Detailed information on a an inspection robot remotely operated by an operator for both visual and non-visual inspections also presented.

An introduction to computer-integrated visual inspection for automatic examination of circuit boards.

Similarly, information on automated geometric visual inspection of 3D objects can be obtained here

The homepage of Visual Inspection Technologies Inc. describes products and services offered in the field of remote visual inspection It is well designed and advertises videoimagescopes, fiberscopes and boroscopes.

The UXR homepage provides technical information on a wide range of boroscopes and fiberscopes, The time taken to download images of the equipment is the only drawback of the site.

Remote visual inspection is relied upon for the detection ,analysis and documentation of defects normally inaccessible to the human eye. Olympus Systems presents equipment for remote visual inspection in complex machinery and inside pipes

An application of visual inspection to the paper industry gives technical data of a visual inspection winder to control quality of pre-limited material is described.

The Web site of the Center for Applied Optical Sciences describes various research areas from holography to optical metrology.

A well illustrated introduction on visual non-destructive testing on works of art proposed by the Applied Optics Group of the University of L'Aquila

One can learn of the use of holographic contouring techniques to locate and size defects on a fresco 1667 or a carving of the XIIIth century. A supercomputer analysis of a holographic inferograms for displacement using the Fourier transform method is presented by the D.L Gould . You can see his work using AVS visualization software.

The server of the Basic Industrial Research Laboratory of the Northwestern University in Illinois offers technical reports which cover a wide variety of subjects from the application of infrared and visual imaging for the inspection of protective coating to optical global bridge inspection techniques .

Technical information in a D-sight surface inspection system to visually enhance signs of impact damage and interlayer corrosion on aircraft parts

The Optical Metrology Laboratory at the Center for Applied Optics has extensive facilities for analysis of complex surfaces, measurement of ultra-smooth surfaces and precision coatings. Optical scattering and other standard tools, such as stylus and non-contact profilometers, are available to provide analysis in several formats.

Model and vibration analysis of bonded and composite structures can be carried out using holographic infermetry as described at the homepage of the Polaris Research Group .

It is also worth consulting the Cordis site to be aware of current NDT research projects carried out through the Brite-Euram scheme, including topics such as pulsed digital holography and shearography.

Finally, information on a ' futuristic' project for holographic fullbody security scanning can be enjoyably read here.