Patent ID: 8860949

Claim:
An interferometer which measures a displacement of an object to be measured by observing a fluctuation in intensity of interfering light generated by dividing light emitted from a light source into two light beams and overlaying the two light beams including a light beam from the object, the interferometer comprising: a light-receiving unit including a light-receiving area including a plurality of partial areas and configured to detect a light intensity distribution in one beam of the interfering light by the plurality of partial areas and configured to detect light intensity in each part of the one beam of the interfering light by each of the plurality of partial areas and output each signal about the light intensity detected by each partial area, the light intensity distribution being a distribution in the light-receiving area; and a processing unit configured to calculate a value of an index indicating a phase shift in the one beam of the interfering light using a difference between the signals output from the plurality of partial areas, wherein the value of the index indicating the phase shift in the one beam of the interfering light changes with an adjustment of the interferometer.