1. Measurement methods and instruments used

1.1 Reminder of the principle of phase scanning interferometry

The Fizeau interferometer is used to measure flat, spherical or slightly aspherical surfaces. Fringe analysis can be performed statically (known as workshop interferometry), but the phase-shifting interferometry method is the most widely used. By recording intensity images for several known phase shift values, either by moving a reference surface (also known as a reference mirror or standard mirror) or the mirror to be measured, or by varying the wavelength, appropriate algorithms are used to reconstruct the surface in comparison with the reference surface