A method of characterizing a sample utilizing a beam of electromagnetic radiation, comprising the steps of: a) providing an alignment sample, a source of electromagnetic radiation, a multi-element alignment detector and a data detector; b) orienting the alignment sample such that at least a portion of a beam of electromagnetic radiation produced by said source is caused to obliquely impinge thereupon, reflect therefrom and enter the data detector at a base angle-of-incidence and plane-of-incidence; c) causing a portion of the beam of electromagnetic radiation reflected from the alignment sample to enter said multi-element alignment detector, and monitoring signals from each element of said multi-element alignment detector; d) removing said alignment sample and placing a test sample in its place; e) obtaining data from said test sample by causing electromagnetic radiation from said source to reflect from said test sample and enter said data detector while monitoring the output of each element (Q1, Q2, Q3, Q4) in said multi-element alignment detector; characterized in that the method further comprises the step of f) characterizing said test sample by analyzing the data obtained from the data detector in step e), in combination with signals from elements of the multi-element alignment detector in step e) which identify differences from signals determined therefrom in step c), said difference signals serving to provide information to compensate offsets in intended and actually realized angle-of-incidence and plane-of-incidence of the test sample as compared to the base angle-of-incidence and plane-of-incidence of the alignment sample achieved in step b). a) providing an alignment sample, a source of electromagnetic radiation, a multi-element alignment detector and a data detector; b) orienting the alignment sample such that at least a portion of a beam of electromagnetic radiation produced by said source is caused to obliquely impinge thereupon, reflect therefrom and enter the data detector at a base angle-of-incidence and plane-of-incidence; c) causing a portion of the beam of electromagnetic radiation reflected from the alignment sample to enter said multi-element alignment detector, and monitoring signals from each element of said multi-element alignment detector; d) removing said alignment sample and placing a test sample in its place; e) obtaining data from said test sample by causing electromagnetic radiation from said source to reflect from said test sample and enter said data detector while monitoring the output of each element (Q1, Q2, Q3, Q4) in said multi-element alignment detector; characterized in that the method further comprises the step of f) characterizing said test sample by analyzing the data obtained from the data detector in step e), in combination with signals from elements of the multi-element alignment detector in step e) which identify differences from signals determined therefrom in step c), said difference signals serving to provide information to compensate offsets in intended and actually realized angle-of-incidence and plane-of-incidence of the test sample as compared to the base angle-of-incidence and plane-of-incidence of the alignment sample achieved in step b).