Patent ID: 8379222

Claim:
A Fizeau interferometer comprising: a laser light source emitting a laser light; an imaging device detecting interferograms; a reference spherical surface configured to reflect part of the laser light emitted from a laser light source as a reference light, configured to allow the other part of the laser light to pass through as a measurement light and configured to incident the measurement light to a spherical surface to be measured; and a measuring apparatus configured to measure a form of the spherical surface using interferogram of the reference light and the measurement light reflected by the spherical surface, wherein: the measuring apparatus includes; an intensity obtaining section configured to determine a center position of the reference spherical surface where a focal point of the reference spherical surface is aligned with a focal point of the spherical surface to be measured, and obtain intensity maps of interferograms at n positions (n is 3, 5, or 7) by moving a position of the reference spherical surface along an optical axis of the laser light at equal intervals from a first position to a second position, a distance from the first position to the center position is equal to a distance from the center position to the second position; and a form calculating section configured to set a coefficient of the intensity map of the interferogram at an i th position (i is an integer and 1≦i≦n−1) equal to a coefficient of the intensity map of the interferogram at an (n−i+1) th position in a phase analysis and calculate the form of the spherical surface using the intensity maps of the interferograms obtained by the intensity obtaining section.