The new method for identification of optical marks containing transformed phase masks (TPMs) is considered. A TPM placed in an optical correlator produces several sharp correlation peaks on its output. We choose the distances between peaks as basic geometrical features of a TPM. The increase of quantity of basic features leads to the increase of security level of documents and valuable papers. However, the amount of such features is limited and is depending from many factors. We picked the probability of false identification of the nearest feature as the criterion of separation of features on independent classes. The optical/digital device for identification of credit cards based on a joint transform correlator architecture was used for the study of TPMs. To estimate the law of distribution of basic features inherent to the same class, we carried out experiments with the series of TPMs. We have shown, that the additional features allow to increase the number of basic features, separated on different classes. These experiments also have allowed to estimate the error of features measurements.