## 2 Materials and Method
### Light Sources and White Balance Function  
Because the photoelastic technique uses light, the response may vary depending on the optical characteristics of the light such as the wavelength band of the light source, color temperature, and color reproducibility. As seen from Equation (6), the wavelength of the incident light has a direct linear relationship with the principal stress difference appearing in the photoelastic material. In addition, depending on the setting of the digital camera when observing the photoelastic response, the acquired image data may be affected.  
In the experiment, we employed two types of light sources: a specially-designed RGB LED light source, featuring red, green, and blue LED panels arranged in a Bayer pattern, and a commercially available standard white light source. The maximum luminous intensity of the white light source was about 140 mW/m\({}^{2}\), the wavelength at the time of testing, P, was 449 nm, and the color temperature was 9259 K. However, the maximum luminous intensity of the RGB LED was about 15 mW/m\({}^{2}\), the wavelength at the time of testing, P, was 516 nm, and the color temperature was 9641 K. Although the peak wavelength of the RGB LED appeared in the green band, the measured luminous intensity in the blue (460 to 470 nm) and red (630 to 640 nm) wavelength bands was also manufactured to be about 15 mW/m\({}^{2}\). Figure 3 compares the spectral power distributions of two different light sources analyzed by the MK350S spectrometer manufactured by UPRtek (Miaoli County, Taiwan).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Types**} & \multirow{2}{*}{**Tensile Strength**} & \multirow{2}{*}{**Tensile Elongation**} & \multirow{2}{*}{**Shear Strength**} & \multirow{2}{*}{**Specific Gravity**} & \multicolumn{2}{c}{**Viscosity**} \\  & & & **(\%)** & & **(MPa)** & **(g/L)** & **(Pa.s)** \\ \hline Instant & & & & & & **Resin** & **Hardener** \\ glue & - & - & 15\(\sim\)25 & 1.10 & \multicolumn{2}{c}{Non-drip gel} \\ Epoxy A & 39 & 8 & 22.6 & 1.03 & 40\(\sim\)90 & 5.5–8 \\ Epoxy B & 35 & 9 & 30 & 1.00 & 67.5 & 7 \\ Epoxy C & 41 & 10 & 30 & 1.10 & 41\(\sim\)61 & 2\(\sim\)4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Material properties of the adhesives.  
Figure 3: Spectral power distribution: (**a**) white light; (**b**) RGB LED.  
The sensitivity of the digital camera was set to ISO 100; the aperture value was set to F5.6, and the shutter speed was set to 0.1 s. The color of light appears different depending on the temperature. Even for the same observation object, if the color temperature of the light source is low, the target object appears red, and in contrast, if the color temperature of the light source is high, the object appears blue.  
The white balance is a function that adjusts the balance of the three colors of the image according to the color temperature of the light source to be close to the actual color. The digital camera used in the experiment provides an automatic white balance function, and it is also possible to correct the image by manually setting the color temperature value. In this study, the automatic white balance function was compared with no color correction. After acquiring the images using an EOS 650D (Canon, Tokyo, Japan) camera and an EF-S 18-55 mm/3.5-5.6 IS II lens, the responses were analyzed. The resolution of the digital image was 350 DPI (dots per inch), which was set high enough to precisely observe the photoelastic response.