The invention concerns an optical measurement arrangement having an ellipsometer, in which an incident beam of polarized light is directed at an angle of incidence 0 onto a measurement location on the surface of a specimen. The control apparatus includes a first three-position switch and a second three-position switch, each of which has a non-pressed position, an optimum operated position, and an over-pressed position. The forward lands have a nominal aft-facing step, the aft lands have a nominal forward-facing step and the middle lands are nominally flush.