Portable electronic speckle interferometry device for the damages measurements in veneered wood artworks

Abstract

Deterioration of wood artwork is often connected to mechanical material degradation that starts on microscopic scales. Insight into decay mechanisms can be obtained by monitoring surface deformation and displacement fields. This paper presents the application of Electronic Speckle Pattern Interferometry (ESPI) to detect damages of wood samples invisible into the surface. Two different damages were created on the model simulating cabinet wood panels: delaminating and worm galleries, all defects are invisible by naked eye. All optical arrangements of speckle interferometry are divided in two types of fundamentally different speckle patterns: photographic speckle patterns, which contain information only about the light wave amplitude, and holographic speckle patterns, which contain both phase and amplitude information for comparison potentialities of two approaches we present the results obtained by thermography and ESPI investigations of the wood panel under test. The fiber-optic ESPI set-up based on a He–Ne CW laser has been developed and used for studying the possibility to reveal the invisible damages and determine their locations, sizes and shapes. For this end the two digital holograms of the test object, corresponding to the non-heated and heated states of object, are captured at two video frames of the CCD camera, and then processed in a PC. The resulted fringe pattern has the information about the damages. The purpose of the work was determining the possibilities of ESPI method in revealing of different kinds of invisible damages. Our results indicate that developed variant ESPI system is well adapted to reveal the under surface damages in veneered wood samples. The set-up may be used in out-of-laboratory conditions and without severe anti-vibration preoccupations. The comparison of results obtained by developed ESPI system and thermo camera shows the higher sensitivity the ESPI system. The thermo camera does not indicate the presence of under surface damages like delaminating and worm galleries in contrast with ESPI system which is well adapted to extract information about these defects. Applying ESPI set-up we have determined the presence of different kind of damages located under surface: big delaminating and little worm galleries. The developed ESPI set-up is capable of predicting the position, shape and size of revealed damages. The results presented in this paper show that the ESPI technique is a promising tool for testing the wood artworks.