Full-field measurement of principal mechanical strains via the stereo-digital images correlation technique (stereo-DIC) during thermoforming of thin thermoplastic sheets was recently reported as a potential method to control the wall-thicknesses under the assumption of material’s incompressibility. The accuracy of mechanical strains during forming thermoplastic sheets are not only dependent of reconstruction and correlation errors of stereo-DIC, but also of the considered reference state of boundary conditions following the temperature-regulation of thermoplastic sheets. This paper introduces an experimental method based on high speed imaging techniques such as IR-thermography and stereo-DIC techniques to detect damped out-of-plane undulations which are associated with the initiation of dynamic inflation operations of thin sheets. The aim is to rely on the characteristic durations of these short-time events to objectively separate between mechanical strains which are affected by the uncontrollable change of boundary conditions from the global pressure-induced strains. An application case is presented to demonstrate that damped out-of-plane undulations are beneficial to avoid ill-posed stereo-DIC problems particularly when the boundary conditions are sensitive to regulation operations of temperature.