**4.1 Relation between reflection rate and RGB values measured by the color analyzer**

In this section, the physical meaning of the RGB values measured by the color analyzer is considered. As described in Section 2, the value measured by the color analyzer expresses the intensity of the reflected light from an object by spectroscopic RGB values of specific wavelengths in visible light. The correlation between the reflection rate and the RGB values was investigated using long-term samples irradiated by plasma, installed in the vacuum vessel of the LHD. The long-term irradiated samples are made of substrates of stainless steel and are placed on the first wall of the toroidal section at various locations from the inside to the outside of the torus. They are exposed to the plasma during one experimental cycle (see **Figure 6(b)** for the locations). The optical reflection spectra of these samples, which were removed from the vacuum vessel after the plasma experiment, were evaluated using spectroscopic ellipsometry. At the same time, the RGB values of the samples were measured with a color analyzer. **Figure 3** shows the relationship between the RGB values and the reflection rate. The reflection rates were obtained by averaging the values at the

#### **Figure 3.**

*The relationship between the RGB values measured by the colour analyzer and the reflectance measured by the spectroscopic ellipsometer [11].*

wavelengths of red (615 nm), green (540 nm) and blue (465 nm) from the optical reflection spectra. For reference, the unexposed samples are plotted on the same graph. The reflection rate and the RGB values show a linear relationship, indicating that the RGB values measured by the color analyzer express the reflection rate. The surface roughness of the sample may also affect the RGB values. However, the crosssection of the sample observed by Transmission Electron Microscopy (TEM) shows that the surface roughness is less than 100 nm, which is lower than the wavelength of visible light, so the effect of surface roughness is considered to be small.
