Reflected-light interferometry

When two fluid interfaces have a high radius of curvature, such as in the pseudoemulsion film, the distance between the interference patterns is too small to be measured by common reflected light interferometry. In this case, differential interferometry can be used for imaging the interface profile [40-45]. (Another technique for studying curved films is the controlled drop tensiometer, as was shown in section 2.)... 

Thickness is one of the main parameters of a foam film. The most widely employed technique for its determination is interferometry. It is based on the comparison between the intensities of the light falling on the thin film and that reflected from it. This technique permits to evaluate the thickness of equilibrium as well as thinning films. It has been used by Perrin [48] and Wells [49] with soap films. The intensity of the reflected light was measured with an interferometer comparing the two parts of the visual field (in a microscope). Thus the film thickness was determined with an accuracy of about 0.5 nm. 

Differential interferometry in reflected light allows measurement of the shape of the upper reflecting surface. This method was used by Nikolov et al. - " - "" to determine the contact angle, film and line tension of foam films formed at the top of small bubbles floating at the surface of ionic and nonionic surfactant solutions. An alternative method is the holographic interferometry applied by Picard et to study the properties of bilayer lipid membranes in solution. Film... 

Most commonly, film thickness measurements are obtained using reflected-light thin-film interferometry. 

Deflection detection schemes have been developed that use self-sensing piezoresistive cantilevers [104] or optical interferometry [105, 106]. But in nearly all practical systems cantilever deflection is measured with an optical lever system [107], as shown in Fig. 3.22. A small spot of laser light is reflected from the back of the cantilever and the reflected light is directed to an adjustable mirror. This in turn reflects the light onto a position-sensitive detector, a four-quadrant photo diode, shown schematically in Fig. 3.23. 

Interferometry is based on the fact that light reflected from the front and back interfaces of a film travels different distances, producing interference effects. The method has been applied to Langmuir-Blodgett films (Section XV-7) and to soap films (Section XrV-8) [147-149]. 

The advent of lasers allowed optical interferometry to become a useful and accurate technique to determine surface motion in shocked materials. The two most commonly used interferometric systems are the VISAR (Barker and Hollenbach, 1972) and the Fabry-Perot velocity interferometer (Johnson and Burgess, 1968 Durand et al., 1977). Both systems produce interference fringe shifts which are proportional to the Doppler shift of the laser light reflected from the moving specimen surface. Both can accommodate a speci-... 

Optical interferometry can be used to measure surface features without contact. Light reflected from the surface of interest interferes with light from an optically flat reference surface. Deviations in the fnnge pattern produced by the interference are related to differences in surface height. The interferometer can be moved to quantify the deviations. Lateral resolution is determined by the resolution of the magnification optics. If an imaging array is used, three-dimensional (3D) information can be provided. 

The two limitations of optical interferometry, the one-quarter wavelength of light limit and the low resolution, have been addressed by using a combination of a fixedthickness spacer layer and spectral analysis of the reflected beam. The first of these overcomes the minimum film thickness that can normally be measured and the second addresses the limited resolution of conventional chromatic interferometry. 

Interferometry exploits the superposition of electromagnetic waves to measure some physical property that probes the original state of the waves. Interferometers typically have light beams that are split by beam splitters (BS) (at least one per interferometer), reflected off mirrors, and measured by either one or two detectors. The path length difference and/or the phase difference are measured. 

Laser interferometry employs the principle of optical interference to recover the sought acoustic information from the light reflected from, or scattered by, a surface under ultrasonic vibration. Its non-contact nature makes laser probing a preferred alternative to contact methods in studying surface waves, their diffraction and damping by intrinsically rough surfaces. 

OCT uses interferometry and a reflection-based edge-detection algorithm to define the thickness of tlie circumpapiUary RNFL Measures the retardation of light reflected from the birefringent RNFL fibers and provides an estimated RNFL thickness... 

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