The optical method of reflected caustics is used for the determination of axial force in polymeric members and small components. The mechanical properties of the polymeric member are not required. The method has the advantage of simplicity. It is proposed that the member itself be used to determine the load in a two-force machine and structural elements. The test models are statically loaded in tension. A classification of errors with respect to their sources and influence on the results are discussed. The mechanical and optical parameters are investigated to optimize the design of the experiment. Two-loading apparatus are used to verify the results obtained using the proposed method.