## 1 Introduction  
Various scientific techniques are required to determine the manufacturing techniques and conservation conditions of cultural heritage. In particular, for multilateral recording and imaging from the external surface to the inside of an artifact, nondestructive testing techniques employing electromagnetic waves (e.g., X-rays, ultraviolet rays, infrared rays, terahertz waves, and microwaves) and visual inspection processes are essential. Nondestructive testing techniques are widely employed in structural integrity assessments in mechanical and civil engineering as well as the analysis of cultural heritage (e.g., earthquake, ceramics, murals, and stone pagodas). Various studies have reported the discovery of new feature information that could not be confirmed using only the human eye [1, 2]. Most previous studies primarily focused on two-dimensional image analysis using a single technology [3, 4, 5, 6, 7], and even if three-dimensional (3D) shape information was acquired,the results were frequently dependent on external surface information [8; 9]. This is apparently caused by the limitations of applied technologies and the complexity of data processing techniques. Recently, various scanners and computer image processing algorithms have been developed [10; 11; 12], and 3D digital technologies are becoming increasingly ubiquitous, enabling the stereoscopic recording and data fusion of cultural heritage [13; 14; 15; 16; 17]. Three-dimensional digital technologies are mainly divided into optical scanning and X-ray computed tomography (CT) techniques depending on the type of the physical energy source used and the corresponding light reflection or transmission response. Optical scanning acquires information about the external geometry of an artifact using reflection data by employing a laser or structured light sources (with white or blue light), and X-ray CT determines the internal geometry of an artifact as tomographic information through the transmission properties of X-rays [18; 19; 20]. Both scanning techniques have considerably different purposes; thus, it is important to select an appropriate technique based on the required precision of the external geometry and internal structure. Both technologies have excellent applicability and high resolution. Thus, they have been extensively employed in conservation research for a long time, for instance, for the documentation of archeological sites and artifacts, shape analysis, condition diagnosis, digital restoration, and reproduction. In particular, X-ray CT is actively employed for applicable artifacts because it provides vital information for the interpretation of cultural heritage production techniques and the institution of conservation plans [21]. Thus, optical scanning and X-ray CT are excellent technologies that have led to important discoveries and contributions to the conservation of cultural heritage. However, the purposes and results of the two technologies differ distinctly. These variances lead to valuable interpretations when the technologies are individually utilized; however, much greater synergy is attained when complementary data are combined. Specifically, digital recording data can be enhanced if the external surface information obtained via optical scanning and the internal shape constructed via X-ray CT are combined into a single 3D model. For this purpose, multifusion technologies that synthesize different types of digital images have been utilized in the medical [22; 23] and industrial fields (e.g., mechanical and civil engineering) [24]. In addition, fusion image results with high accuracy have been reported [25; 26; 27; 28]. A study on 3D imaging modalities that fuse internal and external shapes has also been conducted in the domain of cultural heritage [29]. This article presents the overall workflow for aligning 3D optical scanning and X-ray CT mesh models and their interactive visualization. However, it is difficult to apprehend the deviation in the registration accuracy when using the two technologies.