Long-Term Effects of Cyclic Environmental Conditions on Paintings in Museum Exhibition by Laser Shearography

Abstract

To better evaluate current condition standards commonly used for the exhibition of canvas paintings, it is necessary to have a quantitative technique capable of measuring degradation components induced by changes in temperature and relative humidity, as well as the effects of ambient vibration and the thermomechanical effects of museum lighting. This paper presents advances in our development of a customized laser shearography system for temporal characterization of in-plane displacements of canvas paintings when subjected to changes in exhibition conditions. The shearography system performs concomitant measurements of gradients of displacement along two orthogonal shearing directions and is synchronized with a thermal IR camera to provide thermal maps of the area being analyzed. Recent innovations incorporated into the system include a real-time temporal phase unwrapping algorithm, and high-resolution Fast Fourier Transform (FFT) methods to calibrate applied shearing levels that allow a wide range of measuring resolutions. Examples will be presented that illustrate the system’s capabilities to detect cracks in the paint surface and measure and map associated strain vectors as a function of changes in condition parameters. Included are representative results of continuous 30 h recordings on American nineteenth century oil on canvas painting. Multi-domain data has been combined and correlated using the shearography and IR data from our system, temperature and humidity data from the museum’s climate control system, as well as activity log from museum’s security system.