An apparatus for measuring a sample surface having a rotationally symmetric aspheric surface comprising:
- a white light source to output white light;
- an interferometer arranged to receive the output light from the light source and to output collimated light;
- a diffractive optical element having an annular diffraction grating composed of concentric annular areas, the diffractive optical element arranged to i) receive the collimated light from the interferometer, and ii) via a reference surface thereof, divide the collimated light incident thereon from the interferometer into a reference light and a measuring light,
- the reference surface of the diffractive optical element arranged to reflect back the reference light to the interferometer, and pass the measuring light toward the sample surface,
- the diffractive optical element being disposed between the interferometer and the sample surface along an optical path of the measuring light, the diffractive optical element deflecting the measuring light, by an angle determined by a wavelength of the measuring light and outputting the measuring light to the sample surface, the diffractive optical element deflecting a sample light that is the measuring light reflected back from the sample surface and outputting the sample light to the interferometer,
- the diffractive optical element arranged to deflect the received sample light to be incident on the reference surface, wherein the reference surface combines the sample light and the reference light into an interference light, the interference light being outputted to the interferometer;
- a wavelength analyzer arranged to analyze a wavelength of the interference light received from the interferometer, the wavelength analyzer comprising a reflecting-type diffraction grating plate, the wavelength analyzer outputting the interference light only when an inclination angle of the reflecting-type diffraction grating plate relative to an axis of the interference light inputted thereto and the wavelength of the interference light satisfy a predetermined condition;
- an imaging element arranged for receiving the interference light from the wavelength analyzer and forming an interference fringe image from the received interference light from the wavelength analyzer;
- an optical distance calculator arranged to calculate an optical distance between the reference surface and the sample surface along the optical path of the measuring light;
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- an analyzing control unit connected to i) receive optical distances from the optical distance calculator, and ii) receive interference fringe images from the imaging element and iii) analyze a shape of the sample surface based on each of the interference fringe images taken on a wavelength-by-wavelength basis of the interference light and the optical distance at the time of taking each of the interference fringe images.