One can regard a three dimensional photoelastic model as a general system of linear retarders. These linear retarders can he individually separated out using a compensator. In the general case, a compensator can he used to cancel the combined effect of the linear retarders up to a certain required point along the light path. The discrete linear retarders from the point of entry of light to the model to the point along the light path under consideration can be replaced by a resultant optically equivalent model which is characterised by three discrete quantities called the characteristic parameters.1 If the compensator cancels the characteristic retardation completely and is orientated along the axis of the linear retarder of the optically equivalent model for any general light path, such a configuration of the compensator would correspond to the ideal case of compensation. However there could exist, for any particular three dimensional model, particular values of characteristic parameters, which, along with the value of retardation introduced by the compensator would result in a resultant optically equivalent model having the same primary and secondary characteristic directions2 as in the ideal case of compensation. This can lead to a misinterpretation of the results.In this paper three very simple methods have been described to ascertain the correct positioning of the compensator.
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