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A set consisting of linear optical retarders, three-dimensional photoelastic models, some crystals, twisted fibres, etc. is optically equivalent to an elliptic retarder, and can be described by its three parameters. New experimental methods for determining the optical parameters of an elliptic retarder are presented with a brief review of the existing methods. The Stokes vector representation of polarized light is used in developing these methods. It is shown that it is possible to find the integral order of the retardation (connected with the elliptic retarder) only in certain cases where this retardation can be gradually increased from zero. Some experimental results are presented as supporting evidence for these methods. The relative accuracies of these methods are discussed. A combination of different methods is recommended for obtaining maximum experimental accuracy.

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A simple and accurate method of measurement of absolute retardation using holography is described. In this, the retardation added in the object beam between the exposure causes a shift in the fringe pattern in the reconstruction. This is achieved by rotating the direction of polarization of the beam between the exposures. The method makes use of a single view, and it has some specific advantages over the present methods in vogue. Some experimental results are presented as supporting evidence. The method is useful for the measurement of general displacements, strains, and material properties.