A method of maintaining the spatial relationship between the focal point of a first focussing lens and the end of an optical fiber which directs electromagnetic radiation to a detector in an ellipsometer, comprising the steps of: a) providing a sample and a source of electromagnetic radiation and an optical fiber with an accessible end; b) orienting the sample such that a beam of electromagnetic radiation produced by said source is caused to obliquely impinge thereupon, reflect therefrom, and via the first focusing lens enter the accessible end of said optical fiber; c) causing a portion of the beam of electromagnetic radiation reflected from the sample to enter a multi-element alignment detector via a second focusing lens, and positioning said multi-element alignment detector so that signals are developed by each of its elements (Q1, Q2, Q3, Q4); wherein the said sample is an alignment sample; and, the method comprises the steps of: d) removing said alignment sample and placing a test sample in its place; e) adjusting the orientation of the test sample so that the multi-element alignment detector provides substantially the same output as was achieved in step c; with the result being that the portion of the electromagnetic beam not caused to enter said multi-element alignment detector, is caused to enter the accessible end of the optical fiber via said first focusing lens; wherein the known signals developed by each of the elements of said multi-element detector are utilized to effect change in the vertical position of the sample, the vertical position being along the perpendicular from the sample surface, by using the fact that if the vertical position of the sample is not appropriate to direct the reflected beam onto the multi-element alignment detector elements, no signal will be developed thereby. a) providing a sample and a source of electromagnetic radiation and an optical fiber with an accessible end; b) orienting the sample such that a beam of electromagnetic radiation produced by said source is caused to obliquely impinge thereupon, reflect therefrom, and via the first focusing lens enter the accessible end of said optical fiber; c) causing a portion of the beam of electromagnetic radiation reflected from the sample to enter a multi-element alignment detector via a second focusing lens, and positioning said multi-element alignment detector so that signals are developed by each of its elements (Q1, Q2, Q3, Q4); wherein the said sample is an alignment sample; and, the method comprises the steps of: d) removing said alignment sample and placing a test sample in its place; e) adjusting the orientation of the test sample so that the multi-element alignment detector provides substantially the same output as was achieved in step c; with the result being that the portion of the electromagnetic beam not caused to enter said multi-element alignment detector, is caused to enter the accessible end of the optical fiber via said first focusing lens; wherein the known signals developed by each of the elements of said multi-element detector are utilized to effect change in the vertical position of the sample, the vertical position being along the perpendicular from the sample surface, by using the fact that if the vertical position of the sample is not appropriate to direct the reflected beam onto the multi-element alignment detector elements, no signal will be developed thereby.