Speckle interferometric techniques, the most attractive in NDTE, rely upon the "cyclic correlation" of speckle pattern with itself. The cyclic correlation is induced by the superposition of two different speckle fields (or a speckle field and a smooth reference beam). The correlation fringes, which are obtained by subtracting the intensities of two speckle patterns imaged on a photosensing device, may represent different components of displacement (or deformation) depending on the optical setup adopted.
The "aperiodic decorrelation", which unavoidably affects the speckle pattern in any speckle interferometric technique, is regarded as a disturbance which may lower, and even destroy, the visibility of fringes. if the microstructure of the surface does not significantly alter, the aperiodic decorrelation is solely due the same macrodeformations under measurement.

Starting from the classical theory of fully developed speckle, a numerical simulations has been carried out to investigate the statistical properties of a speckle pattern integrated over the finite area picture elements of the sensing device. The approach may be employed to analyze any speckle method and hence optimize the performance of a speckle interferometer.

In the paper the analysis has been carried out to design a non interferometric technique based on the decorrelation of a speckle pattern, caused by the surface warping, imaged through an annular aperture. Evidence of the applicability of this speckle pattern decorrelation technique to non destructive testing of aeronautical components is provided by the experimental results reported in the paper. REFERENCES

1. M. L. Luchi, A. Poggialini, "Statistical Analysis of Intrinsic Noise in Speckle Correlation Fringes", basic Metrology and Applications, Papers dedicated to Professor Anthos Bray for his 70th anniversary, Levrotto & Bella, Torino, Italy, pp. 161-166, 1994.
2. F. M. Furgiuele, L. Pagnotta and A. Poggialini, "Damage Evaluation of Fatigued Composite Laminates by Speckle Interferometry", Proceedings of the 2nd International Seminar, University of Sheffield (U. K. ), pp. 14-25, 1994.
3. L. Pagnotta and A. Poggialini, "Controlli non Distruttivi mediante Technic di Decorrelazione Speckle", Proc. of the 8th National Conference of the AIPnD, Torino, Italy, pp. 903-916, 1995.