Abstract: An ellipsometer or reflectometer includes a light source creating a mono or polychromatic probe beam. The probe beam is focused by one or more lenses to create an illumination spot on the surface of the subject under test. A second lens (or lenses) images the illumination spot (or a portion of the illumination spot) to a detector. The detector captures (or otherwise processes) the received image. A processor analyzes the data collected by the detector. An aperture is positioned along the beam bath between the light source and the detector. The aperture has an elliptical shape to give the image received by the detector a circular shape. The circular shape maximizes the amount of energy that may be received using a square test area.