Patent ID: 7612888

Claim:
A method for measuring an optical heterodyne interference, comprising the following steps of: generating a reference light and a measurement light having different frequencies, the reference light being one of a P-polarization light and a S-polarization light and the measurement light being the other of the P-polarization light and the S-polarization light; irradiating the measurement light on a target to be measured through a polarization light beam splitter and a first λ/4 wavelength plate which is positioned between the polarization light beam splitter and the target; irradiating the reference light upon a reference mirror through a second λ/4 wavelength plate which is positioned between the polarization light beam splitter and the reference mirror; obtaining an electric signal indicative of an interference light by a light receiving element arranged to detect the interference light of the measurement light reflected from the target and passed through the first λ/4 wavelength plate, the polarization light beam splitter, and a polarizing plate positioned between the polarization light beam splitter and the light receiving element, and of the reference light reflected by the reference mirror and passed through the second λ/4 wavelength plate, the polarization light beam splitter, and the polarizing plate; and adjusting an axis of the polarizing plate by a polarizing plate rotation mechanism so that the electric signal outputted by the light receiving element becomes a minimum when the reference light is shut off by a light shutoff mechanism positioned between the polarized light generator and the polarization light beam splitter so as to selectively shut off the reference light, thereby enabling measurement of a surface condition or a surface defect on the target to be measured.