The authentication of identities within a realm in which some identities are authenticated using direct authentication, and some identities are authenticated using federated authentication. In order to accomplish an uncomplicated mechanism for detecting artifacts in clinical applications, an impedance signal is measured through a first electrode set attached to the skin surface in a measurement area of a patient’s body, the impedance signal being indicative of the impedance of the signal path formed between individual electrodes of the set. A focus mechanism is either directly or indirectly coupled to both image detector assemblies.