In-line measurement of phase-shift non linearity

One of the principal limitations of phase-shift interferometry is the shift non linearity. We propose two methods for measuring this non linearity, on e applicable to the registration of a limited number of interferograms (3 o r 4) and the other applicable to many interferograms (typically 16). We see two main advantages in these methods: first, they require no extra instrum entation, and, secondly, shift non linearity is measured in situ, that is p articularly important in the case of mechanical phase shift. Experiments co nfirm the validity of the corrections, allowing in our case reduction of sy stematic errors by 30 to 45%.