Photoelasticity principles

For optimum performance, CD measurements require a polarization modulated source. In principle, any of the polarization-selective optical devices discussed earlier could be mechanically moved to create the required modulation. However, this approach is problematic in that it is difficult to implement physically, the mechanical movement may introduce noise into the measurement situation, and there are limitations to the rate at which the polarization can be modulated. A preferable approach is to use an electronic device to effect the required phase retardation. Although a number of devices have been used for this purpose (e.g. magneto-optical, Kerr effect, etc.), modern CD instruments rely upon either the Pockels effect, or photoelastic modulation for this function. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

In the first two Sections, the basics of the Brewster angle ellipsometry and the operation principles of the photoelastic modulator (PEM) based ellipsometer will be overviewed. In the following, three different ellipsometric studies of the solid-liquid crystal interface wiU be presented. 

Experimental work [118-125], especially that using photoelastic techniques, has established the presence of residual stresses in joints but the results have often not been quantitative. For the simpler case of polymeric films coated onto metallic substrates, photoelastic techniques and a method based upon the bimetallic strip principle have often been employed [126-131]. Using the latter method, Danneberg [130] showed that for a wide range of epoxy-based coatings on an aluminium substrate thermal contraction was a major cause of internal stress and that the stresses generated were of the order of 0.08... 

Using a combination of linear and circular measurements, the direction and magnitude of the principle stress difference can be determined and the full 2D stress tensor can be reconstructed. However, in granular applications, the internal stress distribution of each particle is less important than the forces between particles. Section 2.4.3 explores techniques to extract forces directly from photoelastic images. 

Figure 13.47 Schematic view of the principle setup for RAS. Optical key components are the photoelastic modulator and two polarizers. Usually, a quartz modulator and the Rochon prisms are used. (From [137])...

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