Numerous murals containing abundant historical information have been preserved in the ancient architectural structures in our country. However, these murals have suffered varying degrees of damage after thousands of years, such as the formation of surface and subsurface cracks and voids. Optical inspection techniques, such as spectral analysis and imaging technology, are widely used in the restoration and preservation of cultural relics. However, these methods involve chemical composition analysis or primarily provide two-dimensional images, which cannot fulfill the demands of micro-defect detection. Digital holography enables three-dimensional surface profiling. Digital holography combined with excitation can be utilized for the detection of surface and subsurface defects in murals. Therefore, a portable deformation detection system based on digital holography is designed herein to meet the needs of in-situ defect detection in murals. The developed detector combined with acoustic sweep excitation is applied to the in-situ detection of defects in murals at the Palace Museum, Beijing. Combined with acoustic sweep excitation, this study confirms that digital holography can be used to determine the status of damage of the surface and subsurface of cultural relics through non-destructive methods. This research is conducive to the diagnosis of damage in cultural relics, as well as for analyzing defect formation and predicting defect growth, thereby providing a scientific basis for the restoration and protection of cultural relics.
Based on the principle of holographic interference on diffuse reflection surfaces, a portable deformation detection system was designed. First, an aluminum plate, the surface of which was coated with white particulate paint, was selected as the experimental sample. Holograms of the surface of the aluminum plate were captured after applying force excitation. After applying filtering techniques to the digital hologram and extraction algorithms to the deformation fringe phase, the feasibility of the technique and capability of the system for quantitative analysis were validated. With the combination of acoustic sweep excitation, experiments were conducted on mural samples and interior architectural wall samples to inspect the internal defects. Deformation fringes corresponding to surface and subsurface defects were obtained, confirming the effectiveness of the frequency sweeping excitation method using acoustic waves. The portable deformation detection system based on digital holography combined with acoustic excitation was used to analyze the murals at the Palace Museum in Beijing. In-situ micro-defect detection was performed on the western and southern walls of Ru Ting. Gaussian 1σ criterion and histogram segmentation methods were applied to eliminate the overall background phase from the deformation fringe phase to obtain a three-dimensional distribution of the defects in order to analyze the locations and contour features of the defects.
Firstly, theoretical analysis is used to prove that the principle of digital holographic interference on diffuse reflection surfaces is reasonable for extracting out-of-plane deformation data. Feasibility verification using aluminum plate samples reveals that the three-dimensional distribution of deformations could be quantified. The experimental system enables quantification of the out-of-plane deformation (Fig. 5). Combined with acoustic sweep excitation, sub-surface defects in mural samples and interior architectural wall samples are effectively detected. Different defects, such as voids and cracks, show distinct abnormal fringe patterns (Fig. 9, Fig. 13). In-situ micro-defect detection was performed on the murals on the southern and western walls of Ru Ting at the Palace Museum. By using phase extraction and spherical aberration phase elimination algorithms to determine the phase distribution of the deformations, the three-dimensional defect distribution could be determined. Visible cracks, paint peeling, and shallow-level defects such as micro-cracks, fractures, and voids are detected in the mural on the southern wall (Fig. 16). The main defects in the western wall mural are subsurface cracks and hollow spots. These results also prove that conventional sounds such as those made by tourists are still harmful to cultural mural relics (Fig. 17).
A deformation detection system is designed herein based on the principle of digital holographic imaging of a diffuse reflection surface, combined with the acoustic excitation method. The system is successfully applied to the detection of defects in real murals at the Palace Museum, providing information about the locations and contours of the defects. The system is capable of detecting real-time micro-defect-induced deformations on the diffuse reflection surface. The acoustic excitation method offers controllable parameters and simple operation, while being a non-contact approach. By processing holograms and applying background phase elimination algorithms, defect characteristics such as positions and contours can be extracted from the overall deformation phase, enabling accurate identification of defects. This study demonstrates that the portable holographic deformation detection system, in combination with acoustic sweep excitation, can effectively detect defects such as voids, cracks, and holes on mural artifacts. This system provides a scientific basis for diagnosing the health of mural artifacts, as well as for restoration and preservation. In future work, we will further investigate the effective range and safety threshold of acoustic wave excitation for detecting defects in cultural relics. We will also consider the spatial and ground conditions of structures like Ru Ting at the Palace Museum to design a high-precision scanning and stitching method for capturing holograms of the entire mural, focusing on detecting defects in the subsurface within a large area.
