## 1 Introduction  
Photoelastic measurement is a method used to measure the level of stress and strain within a material [1]. This technique leverages the property of birefringence, using transparent substances to visually examine stress and strain. Given its advantage of offering comprehensive visual accessibility, the photoelastic method has gained increased recognition. Initially, it was employed for stress analysis in infrastructures such as dams and bridges. [PERSON] [2] compared theoretical results with the photoelastic phenomenon for plane stress and plane strain problems and applied it to experiments on the Morris Dam. [PERSON] et al. [3] analyzed the bending of beams using photoelasticity under various loading conditions. [PERSON] and [PERSON] [4] determined the elastic stresses acting inside the soil mass of a cofferdam using the photoelastic technique. While experiments using photoelasticity were primarily focused on the analysis of structures, over time, its application has been expanded to granular materials, including geotechnical engineering lab experiments to study soil behavior and aid in the development of mechanical models. In 1957, [PERSON] [5] and [PERSON] [6] demonstrated that photoelasticity could be used to visualize the transmission of force in a packed particle assembly. Later, [PERSON] and [PERSON] [7] simulated granular material as a two-dimensional assembly of disks and utilized photoelastic techniques to determine the magnitude and direction of contact forces between the disks. [PERSON] [8] and [PERSON] [9] proposed applications using photoelasticity to address large displacement problems such as shear box, cone penetration, and reinforcement pullout. The subsequent research actively utilizes photoelasticity to understand the formation and impact of the force-chain network in granular materials [10; 11; 12], as well as explore the effects of the shearing rate [13] and packing density on slip behavior [14], and jamming in granular materials [15]. Recently, studies have been carried out on diverse subjects, such as the fabrication of particle models of various shapes using 3D printing [16], the impact of stress relaxation from temporary halts in pile penetration processes [17], and the visualization of the soil arching effect [18]. Nonetheless, most of these studies utilize transparent glass or polymer-based substances. While the transparent photoelastic approach offers high-quality fringe visibility and contrast, it also imposes a certain restriction as the material chosen for the model grains must be transparent. This leads to an issue where the mechanical properties of these model grains, made from transparent material, differ from those of mineral grains which occupy most soil particle types. In this regard, the reflective photoelastic coating technique emerges as a practical alternative for constructing the model grains. [PERSON] [19] was the pioneer in studying the concept of photoelastic coating. Following that, [PERSON] and others [20] delved into the reinforcement issues related to photoelastic coating, proposing correction factors to interpret the fringe patterns for practical engineering purposes. [PERSON] [21] explored various facets of the reflective technique, including the optical setup of a reflection polariscope, the relationship between stress and strain-optics for a photoelastic coating, and the correction factors required for applying the reflection photoelastic method. Importantly, [PERSON] discovered that the type of adhesive used to affix the photoelastic material to the sample greatly affected the photoelastic response. The light source's wavelength band and the measurement camera's setup were also identified as significant factors. More recently, thanks to the advancement of digital photoelasticity, researchers have been actively working on observing and analyzing RGB color information using polychromatic light, replacing monochromatic light, in order to compute the fringe order [22; 23; 24; 25; 26; 27].