A normalization procedure for an ellipsometric system having a rotating optical element such as a polarizer or compensator is disclosed. In operation, a first DC component is extracted from the measured output signals obtained during the first 180 degrees of rotation of the optical element and a second DC component is extracted from the output signals obtained during the second 180 degrees of rotation of the optical element. The first DC component is used to normalize the output signals obtained during the first 180 degrees of rotation of the optical element and the second DC component is used to normalize the output signals obtained during the second 180 degrees of rotation of the optical element.