Interferometric technique

Figure 3.12. Experimental configuration and velocity profiles demonstrating the use of VISAR interferometric techniques in pressure-shear instrumentation to determine in-plane shear motion as well as longitudinal (P-wave) motion (Chhabildas and Swegle, 1980).

This Fourier transform process was well known to Michelson and his peers but the computational difficulty of making the transformation prevented the application of this powerful interferometric technique to spectroscopy. An important advance was made with the discovery of the fast Fourier transform algorithm by Cooley and Tukey 29) which revived the field of spectroscopy using interferometers by allowing the calculation of the Fourier transform to be carried out rapidly. The fast Fourier transform (FFT) has been discussed in several places 30,31). The essence of the technique is the reduction in the number of computer multiplications and additions. The normal computer evaluation requires n(n — 1) additions and multiplications whereas the FFT method only requires (n logj n) additions and multiplications. If we have a 4096-point array to Fourier transform, it would require (4096) (4095) or 16.7 million multiplications. The FFT allows us to reduce this to... 

CC)Spark Cinematography. See Stationary Film with Multiple Spark Camera DD)Spark Photography. When it is required to have a source of light of brief duration (below 1 microsec) an elec spark in air (or in inert gas) is used. Sparks may be used in "Shadow , Schlieren and "Interferometric techniques... 

Because of the incredible precision of interferometric techniques, this measured velocity is altogether one percent of the earth s circumference velocity derived from the orbital motion. Very-long-baseline interferometry (VLBI)— which is an exhaustively improved Pogany experiment—can detect Ago 10-9 in the earth s rotation. 

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

Since, however, each model involves some assumptions, the calculation of h2 always renders certain inaccuracy. The most important problem in the three-layer model concerns the position of the plane that divides the hydrophobic and hydrophilic parts of the adsorbed surfactant molecule. In some cases it seems reasonable to have this plane passing through the middle of the hydrophilic head of the molecule, in others the head does not enter into the aqueous core. That is why it is worth comparing film thicknesses determined by the interferometric technique using the three-layer model, to those estimated by other methods. An attempt for such a comparison is presented in [63]. Discussed are phospholipid foam films the thickness of which was determined by two optical techniques the microinterferometric and FT-IR (see Section 2.2.5). The comparison could be proceeded with the results from the X-ray Reflectivity technique that deals not only with the foam film itself but also with the lamellar structures in the solution bulk, the latter being much better studied. Undoubtedly, this would contribute to a more detailed understanding of the foam film structure. 

A completely different behaviour exhibit NBF from NaDoS solutions. They do not change their thickness with pa and Cei alterations. However, their properties depend on the composition of the initial surfactant solution (see 0(Cei) and ta(Cei) dependences in Section 3.4.1.1). The thickness of NBF determined from h(Cei) dependence is approximately equal to the doubled thickness of the adsorption layer as assumed by Perrin [318]. This is confirmed by NBF obtained from other surfactants. It should be bom in mind that the interferometric technique employed to measure film thickness gives directly the optical difference in the path of the beams reflected by the two film surfaces. When the thickness is calculated from optical measurements a refractive coefficient, being a function of film structure, should be chosen (see Sections 2.1.3 and 3.4.1). 

We have found that the differential interferometric technique to be particularly useful in this regard (32). 

Improvements in the single side-band performance of a mixer-based receiver can be made by filtering the unwanted side band before it is down-converted in the mixer. Such a scheme, which is described in detail by Goldsmith (1982) is based on interferrometric techniques. We will not discuss single side-band filtering any further, except to note that it is a particularly apposite demonstration of the use of optical techniques to process the radiation in the spectrometer. We will discuss the use of interferometric techniques in Section IX as a means to realize a reflection mode spectrometer. These few examples indicate the flexibility of application of optical techniques to problems of instrument design in the FIR. 

Evidently more definitive experiments are required to explain the MEDI process. These will involve the use of correlation techniques, such as the covariance approach outlined here. Additional information will be provided by the use of area detectors to define the ion trajectories [57, 58]. In the future, experiments will involve the coherent control of the dissociation dynamics by using interferometric techniques. 

Methods for the analysis of phosgene in the presence of HCl have involved interferometric techniques [1235,1967]. In one example, the refractive index of the gas mixture was measured by use of an interferometer, in order to calculate the composition of the mixture [1967]. In another example, a rapid method for the continuous analysis of phosgene (in the off-gases from the production of alkyl isocyanates) involved an interferometric technique in which the number of interference iines were correlated as a function of phosgene concentration [1235]. 

There have been some initial studies of thermal-wave detection using the techniques described above. Ash and his colleagues have performed some imaging experiments with the laser interferometric technique, (8-11) while Amer and his colleagues have used both the laser interferometric and a laser deflection (surface deformation) technique for spectroscopic studies on amorphous silicon. (12-13) These various investigations were all performed at low to moderate modulation frequencies (<100 kHz) only. 

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