Oblique incidence technique

An additional oblique incidence technique for producing surface waves is illustrated in Figure 18j. A surface wave velocity measurement can tell us something about the material properties of a test specimen. A clear understanding of dispersion associated with non-homogenieties with depth can be useful. 

A. Pilarski, J. L. Rose, J. Dale, K. Balasubramaniam, and D. Lecuru, An ultrasonic oblique incidence technique for adhesive bond quality evaluation. Proceedings of the 4th European NDT Conference, September 13-17, 1987, London, UK, Vol. 4, pp. 2237-2244 Pergamon Press, New York (1987). 

Pilarski, A. and Rose, J.L., A transverse-wave ultrasonic oblique-incidence technique for interfacial weakness detection in adhesive bonds. J. Appl. Phys., 63, 300-307 (1988). 

In the following summary of contrast enhancement techniques, it is assumed that specimens are being observed in transmission, that they are not self-luminous, and that the light source is not imaged onto the specimen by the microscope condenser. All these assumptions describe typical conditions for LCP microscopy. Figures 5 and 6 show ray diagrams for a normally incident and obliquely incident beam of parallel rays, respectively. In both cases, the objective back focal plane contains the Fraunhofer diffraction pattern of the specimen. 

For oblique incidence, the reflection is stronger, and moreover refraction occurs, further enhancing contrast. Consequently, oil droplets in water (m = 1.09) can readily be observed in a simple light microscope. Figure 9.5 gives the minimum size needed for particles to be visible with various microscopic techniques. Flowever, measures to enhance contrast are often needed if m is close to unity. 

Analytical expressions similar to those for spherical particles have been derived for infinite-length cylinders in perpendicular incidence as well as in oblique incidence, for elliptic cylinders, and for spheroids (see Refs. 168 and 169). With increasing complexity of the shape of the particle, even with as little change as from sphere to spheroid, the analytical solution to the problem becomes formidable. Then, the use of numerical solution techniques may be preferable to analytical techniques. 

ANL s ultrasonic viscometer is a nonintrusive in-line device that measures both fluid density and viscosity. The design of the viscometer is based on a technique that measures acoustic and shear impedance. The technique was first applied by Moore and McSkimin (1970) to measure dynamic shear properties of solvents and polystyrene solutions. The reflections of incident ultrasonic shear (1-10 MHz) and longitudinal waves (1 MHz), launched toward the surfaces of two transducer wedges that are in contact with the fluid, are measured. The reflection coefficients, along with the speed of sound in the fluid, are used to calculate fluid density and viscosity. Oblique incidence was commonly used because of better sensitivity, but mode-converted waves often occur in wedges that do not exhibit perfect crystal structure and lack well-polished surfaces. For practical applications, we use the normal-incidence arrangement. 

The theory can be generalized to the case of oblique incidence/ Martinoty and Candau found that the viscosity coefficients determined by the ultrasonic technique compare fairly well with those derived from capillary flow. 

This is a widely used optical technique for measuring film thicknesses. The technique uses polarized light at oblique incidence and yields information on the optical constants of... 

Ellipsometry was in fact the earliest optical technique to be applied to the study of electrode processes. It involves the determination of the change in polarisation state of an obliquely incident light beam upon specular reflection at a surface. In order to fully define a monochromatic light beam it is necessary, in addition to knowing the frequency, amplitude, and direction of propagation, to include information about the electric and magnetic vectors which describe the polarisation state. Since these vectors are orthogonal and related in amplitude it is, in fact, only necessary to consider one of them, and it is the electric vector that is usually chosen. If this vector lies in a plane then the beam is said to be plane polarized, and further if this plane lies parallel to the plane of incidence at a surface the beam is said to be p-polarised, whereas a beam polarised in a plane perpendicular to the plane of incidence is referred to as s-polarised. For any beam it is possible to resolve the electric vector into its s and p components, and when these components are of the same frequency but different phase and amplitude the beam is said to be elliptically polarised. This name arises from the... 

In [141], a simple probe-pump laser technique was proposed to investigate the change of Xb in CLCs under the influence of intense laser radiation. An argon laser beam (A = 514 nm), incident normal to the liquid crystal cell, was used as a pump beam, and an oblique incident laser beam from a laser diode X = 670 nm) as a probe beam. A nonabsorbing isothiocyanate mixture with 5 = 510 nm was investigated. An increase of the pitch at very low laser intensities (100 700 mW) was observed which depends on the polarization of the beam. 

Oblique incidence ultrasonic test techniques like those illustrated in Fig. 7 are also quite useful. In fact, it has been found in earlier studies that the normal beam shear wave incidence technique is often more sensitive to subtle interfacial weakness problems than a normal beam longitudinal incident wave. It is difficult, however, to produce shear wave incidence in a normal beam mode because of the somewhat permanent adherence to the substrate that is required. As a result, it was... 

A review of various aspects of the normal beam technique, either pulse-echo or through-transmission is presented next along with sections on oblique incidence, guided waves and acoustic microscopy. 

Other ultrasonic techniques which show promise are oblique incidence waves (Pilarski et al, 1990) and Lamb waves (Bork and Challis 1995). Rose and co-workers (Puthillath and Rose 2010 Rose et al. 1996 Rose and Ditri 1992 Rose et al. 1995) have used guided ultrasonic waves to detect defects in adhesively bonded joints. 

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