# Study of Crack Growth of Transparent Materials Subjected to Laser Irradiation by Digital Holography
## 5 Conclusions  
DH has the advantages of having high resolution, being non-destructive, and operating in real-time in the imaging study of crack growth; therefore, DH is used to study the effect of laser irradiation on crack growth at the highest resolution. With this study, a method for detecting crack growth using a single-pulse laser with DH is proposed. The effect of different wavelengths of laser on crack growth is also investigated for Dammar varnish material, which is commonly used as an oil painting sealant.  
It is concluded that near-infrared and infrared light irradiation is more effective than near ultraviolet and visible light irradiation for Dammar varnish materials. In addition, further experiments were conducted to study the phase changes of different parts of the crack under laser radiation. Based on this experimental study by DH, it is found that if there are tiny cracks on the surface material of the oil painting, the cracks easily expand and deteriorate under infrared radiation, thus causing damage to the oil painting; therefore, the light environment of infrared irradiation should be avoided as much as possible for preserving oil paintings. Similarly, in laser cleaning of older or more fragile oil paintings, the infrared wavelength laser needs to be used with caution to avoid irreversible damage to the artwork.  
The proposed digital holography system can be made compact and portable and is thus feasible for cultural heritage preservation. The unique advantages of digital holography have been proved to be applicable to the study of minor damage in the field of cultural heritage; therefore, future work will include the study of tiny defects of non-transparent objects using digital holographic systems.  
\begin{table}
\begin{tabular}{l c c c c} \hline
**Irradiation Wavelength (nm)** & **740 + 750** & **840 + 850** & **940 + 950** & **1040 + 1050** \\ \hline Experimental group X = 457 & 0.072 & 0.087 & 0.090 & 0.101 \\ Control group X = 457 & 0.001 & 0.001 & 0.005 & 0.004 \\ Experimental group X = 1625 & 0.136 & 0.138 & 0.134 & 0.176 \\ Control group X = 1625 & 0.016 & 0.016 & 0.023 & 0.018 \\ Experimental group X = 2525 & 0.168 & 0.166 & 0.163 & 0.198 \\ Control group X = 2525 & 0.008 & 0.013 & 0.006 & 0.014 \\ \hline \end{tabular}
\end{table}
Table 1: Crack growth distance between experimental and control groups at different irradiation wavelengths (unit: \(\mu\)m).  
**Author Contributions:** Conceptualization, V.T. and W.Z.; methodology, V.T. and W.Z.; investigation, Z.C.; data curation, Y.L. and Y.C.; writing--original draft preparation, W.Z. and Y.L.; writing--editing, Y.L.; writing--review, V.T., Y.Y. and H.Z. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Natural Science Foundation of China (No. 61975112 and No. 52075314), the Major State Research Development Program of China (2020 YFE024600), and the Shanghai Key Laboratory of Intelligent Manufacturing and Robotics.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Conflicts of Interest:** The authors declare no conflict of interest.