Method and apparatus for measuring shape

A shape measuring apparatus, comprising:

• a light source that irradiates a surface to be inspected and a reference spherical surface with light emitted from the light source so as to generate an interference fringe pattern produced by interference between light to be inspected reflected on the surface to be inspected and reference light reflected on the reference spherical surface;
• a wavelength measuring device that measures the wavelength of the light emitted from the light source;
• a lens having a reference spherical surface;
• a position control mechanism configured to hold an object to be inspected having a surface to be inspected and configured to move the object to be inspected relative to the reference spherical surface;
• an imaging unit that receives light reflected from the reference spherical surface and light reflected from the surface to be inspected;
• and
• a control unit,
• wherein by driving the position control mechanism, the surface to be inspected is step-scanned in multiple steps along an scan axis passing through the center of curvature of the reference spherical surface,
• wherein in each step of the step scanning, the phase corresponding to a shape measurement region in the interference fringe pattern obtained using the imaging unit is measured, and the wavelength of the light is measured using the wavelength measuring device;
• wherein after defining the distance between any point on the surface to be inspected and the center of curvature of the reference spherical surface as a function of wavenumber including the measured phase and wavelength as parameters, the wavenumber in each step is calculated from the relationship of the function between adjacent steps, and the moving distance between each step is calculated;
• and
• wherein a measurement value of the distance between any point on the surface to be inspected and the center of curvature of the reference spherical surface is calculated from the calculated wavenumber and the function, a design value of the distance between any point on the surface to be inspected and the center of curvature of the reference spherical surface is calculated using the calculated moving distance, and a shape error of the surface to be inspected is calculated from the difference between the measurement value and the design value.