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.