An ellipsometer having a light source for generating a probe beam along a probe beam path. A polarizing beam splitter passes the probe beam along the probe beam path, at least in part, as the probe beam passes through the beam splitter in a first direction, and diverts the probe beam along a detection path, at least in part, as the probe beam passes through the beam splitter in a second direction that is substantially opposite of the first direction. A compensator variably retards at least portions of the probe beam along at least one axis of the compensator, thereby changing an orientation of the light passing through the compensator. Optics focus the probe beam on a spot on a substrate. A concave mirror receives the probe beam from the spot on the substrate as it travels along the probe beam path in the first direction, and sends the probe beam back along the probe beam path in the second direction. A detector receives the probe beam along the detection path. Preferably, all of the elements of the ellipsometer that are disposed along the probe beam path are fixed and do not rotate relative to the probe beam during measurement operations.