In the process of determining the complete state of stress at a general point in a three-dimensional photoelastic model (in a nondestructive or a semidestructive manner), the concept of an optically equivalent model plays an important role. An optically equivalent model is defined by three parameters, which are termed characteristic parameters. This paper first surveys the direct and indirect methods used to determine the characteristic parameters and then presents a new indirect method for determining these parameters. An algorithm has been developed to determine the characteristic parameters unambiguously by the new method. The new method is verified experimentally and results are compared with theoretical characteristic parameters determined using Jones calculus.