Fig.11 report a typical set of data and their fitting within the interval determined as described above vs. normalized thickness; tin side is at a normalized thickness equal to 1. Raw data derive form the measurement of the fringe pattern frequency; the subset of data coming from the region of the image which satisfies the image quality criteria are highlighted in green color in Fig.11. Polynomial interpolation will use only this subset of filtered data. A large number of raw data are discarded when applying the image quality criteria. Again data fitting is done by 4th order symmetrical polynomial curve and a zero value is imposed to the integral of the thickness stress distribution. Surface stress can be determined after extrapolation using the polynomial model. The surface stress values obtained by the polynomial curve (about 81 MPa) are compatible to those obtained with the automated scattered light method described in paragraph 3 and with the GASP system, which were shown previously.