# Feasibility of LED-Assisted CMOS Camera: Contrast Estimation for Laser Tattoo Treatment
## 4 Discussion  
The purpose of the current study is to investigate the feasibility of using a multicolor LED-assisted CMOS camera to detect visual changes in tattoo after laser treatment in terms of image intensity and contrast. Regardless of the tattoo color, the proposed method was able to detect variations in the intensity from both the background and the tattoo area (Figures 3 and 4). A clear distinction was also found in the contrast changes between the pre and post treatment (black = 72% and red = 27%; Table 2), and corresponded well to the degree of tattoo reduction in the tissue (black = 89% and red = 27%; Figure 5). Therefore, we conclude that the proposed imaging method may provide an indirect assessment of laser tattoo treatment in terms of contrast variations. Unlike the black tattoo, the red tattoo yielded different contrast variations in the LED color-dependent manner, implicating that the multicolor LEDs can facilitate in a remission increase (i.e., scattering and specular reflection) from the tattoo area and eventually affect the contrast of the tattooed area after the treatment. Thus, the proposed LED-assisted CMOS camera can be a feasible tool to estimate the variations in the tattoo contrast after the laser treatment. According to previous studies [5; 11; 13; 17], black pigment is associated with constant and high light absorption in the visible spectra (ranging from 380 nm to 770 nm), leading to less light remission. In fact, the black tattoo demonstrated comparable contrast variations between the pre and post treatment under all the LED colors (Figure 3). Thus, significant light absorption (i.e., insignificant \begin{table}
\begin{tabular}{c c c c} \hline LED Color & Red & Green & Blue \\ \hline Black & 72.8 \(\pm\) 4.2 & 71.5 \(\pm\) 6.4 & 72.7 \(\pm\) 1.6 \\ Red & 0 & 28.7 \(\pm\)7.2 & 25.5 \(\pm\) 8.6 \\ \hline \end{tabular}
\end{table} 
Table 2: Relative contrast reduction (%) in tattoo after laser treatment. Figure 5: Variations in tattoo distribution before and after laser treatment: (**a**) cross-sectional images of tattooed tissue before (pre) and two weeks after (post) laser treatment for black (left column) and red (right column), and (**b**) degree of tattoo reduction after single laser treatment (bar = 500 \(\upmu\)m; N = 3; \({}^{*}p<0.001\)). remission) implies that any LED color may facilitate the CMOS camera in detecting the black tattoo position as well as the residual tattoo contrast after the laser treatment. Meanwhile, the optical responses of the red tattoo are contingent upon the incident wavelength over the visible spectra [5; 11]. Strong light reflection at 635 nm (up to 60%) could be responsible for the indistinctive intensity profiles as well as the negligible contrast variations from the treated red tattoo (Figure 4). However, both the green and blue LEDs presented evident intensity profiles as well as contrast decrease possibly due to the selective high light absorption of the red pigment at 450 nm and 532 nm. Hence, the LED color-dependent responses imply that multicolored tattoo can selectively be detected using various LED colors in association with high absorption characteristics. The partial removal of tattoo distribution in the skin after laser treatment substantiated the ostensible variations in contrast between the pre and post treatments (Figure 5). Despite the 1.5-fold higher total fluence (i.e., 1.5 J/cm\({}^{2}\) for black vs. 0.5 J/cm\({}^{2}\) for red), the black tattoo still yielded approximately three-fold higher contrast reduction as well as more tattoo removal than the red tattoo (contrast reduction = 71.8-72.8% for black vs. 23.2-28.7% for red; area reduction = 89% for black vs. 27% for red). Thus, a correlation between the detected contrast and the residual tattoo distribution still needs to be elucidated in a quantitative manner to evaluate the feasible performance of the LED-assisted camera for clinical applications.