# A Self-Color-Changing Film with Periodic Nanostructure for Anti-Counterfeit Application
## 4 Conclusions and Future Work  
This paper proposes a design of an anti-counterfeiting security film based on a frequency-selective surface (FSS). Our film can display different colors on each side of the film and the colors are swapped when the film is reversed, due to the film's FSS behavior.  
In this design, aluminum nanoparticles are arranged in an array on a two-dimensional plane that is embedded in PET. This arrangement allows us to adjust the wavelengths based on the separation distance, as well as the dimension of the cubes. The performance of the film is evaluated by an FEA method.  
The parametric studies show that the nanocube's width (W) and length (L) are the critical parameters in the film design. By adjusting the size of the nanocube appropriately, it is possible to obtain the transmittance characteristic that changes the color of the light when passing through or reflecting off the film. The condition requires that the size of the nanocube must be smaller than the wavelength of the incident light. This technology has a low cost when it is mass produced. It has a simple structure and the color-changing phenomenon can be observed clearly by the naked eye. Additionally, to bring it to industrial use, the authors plan to create the best-sized prototypes as presented in this paper by roll-to-roll nanoimprint lithography or other technology [38, 41].  
The average efficiency of light transmittance in the 550 nm to 700 nm region exceeds 50% and the average reflectance is less than 10%. The average light transmittance efficiency in the 400 nm to 550 nm region is less than 10% and the average reflectance is more than 50%, enabling our film to perform well as an LPF.  
We also present a method for analyzing the colors of nanofilm by converting the transmittance to color. This technique yields colors that are close to the actual colors obtained experimentally. The CIE chromaticity diagram confirms the correctness of our analyzed colors with the colors that are seen by the human eye. The result shows less than 13% of color error and reaffirms the accuracy and efficiency of our method.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample No.**} & \multicolumn{2}{c}{**CIE (x,y) Values**} & \multicolumn{2}{c}{**CIE (x,y) Values**} & \multirow{2}{*}{**\% Error**} \\  & \multicolumn{2}{c}{**(Proposed)**} & & [38] & \\ \cline{2-6}  & **x** & **y** & **x** & **y** & **x** & **y** \\ \hline
1 & 0.3089 & 0.2856 & 0.3208 & 0.3000 & 3.7 & 4.8 \\
2 & 0.3755 & 0.4071 & 0.4083 & 0.4083 & 8.0 & 0.3 \\
3 & 0.4397 & 0.3856 & 0.4083 & 0.3417 & 7.7 & 12.8 \\
4 & 0.3789 & 0.4153 & 0.3670 & 0.4000 & 3.2 & 3.8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Color error analysis.  
**Author Contributions:** Conceptualization, A.S. and T.T.; methodology, A.S. and T.T.; validation, A.S.; formal analysis, A.S.; investigation, A.S.; writing--original draft preparation, A.S. and T.T.; writing--review and editing, A.S. and T.T.; visualization, A.S. and T.T.; supervision, T.T.; project administration, T.T.; data curation, T.T.; funding acquisition, T.T. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors wish to express their gratitude toward IoT Lab, Department of Electronic and Telecommunication Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi, Bangkok, Thailand, for their support.  
The authors declare no conflict of interest.