## 3 Materials and Methods
### Method for Evaluating Material Color Differences  
The image acquisition software was designed to acquire 80 images for a sample measurement, and frame averaging was used. This allowed the elimination of all noise sources except for the spatial non-uniformity of the digital camera response [36]. A video camera was used to capture the image of the material sample in the sRGB color space. Since the test material samples were a single color, the average of all pixel values was calculated and divided by 255 to move to the floating-point range [0, 1]. This resulted in non-linear scalar \(sR^{\prime},~{}sG^{\prime},~{}sB^{\prime}\) values that should be converted to CIE Lab* color space values using a color conversion algorithm. First, the \(sR^{\prime},~{}sG^{\prime},~{}sB^{\prime}\) values were transformed to linear \(sR,sG,sB\) values by [37]:  
\[P=\left\{ \begin{array}{cc}\frac{P^{\prime}}{12.92}&P^{\prime}\leq 0.04045\\ \left(\frac{P^{\prime}+0.055}{1.055}\right)^{2.4}&P^{\prime}>0.04045\end{array}  \right., \tag{2}\]  
where \(P=sR,sG,\) or \(sB\) are color space values. Conversion to the CIE \(XYZ\) color space with D65 white point was accomplished by using the color conversion matrix [37]:  
\[ \begin{bmatrix}X\\ Y\\ Z\end{bmatrix} = \begin{bmatrix}0.4124&0.3576&0.1805\\ 0.2126&0.7152&0.0722\\ 0.0193&0.1192&0.9505\end{bmatrix}\begin{bmatrix}sR\\ sG\\ sB\end{bmatrix} , \tag{3}\]  
Next, a nonlinear transformation of the \(XYZ\) values to the CIE Lab* color space was performed using the following formulas [38]:  
\[L^{*}=116~{}f\bigg{(}\frac{Y}{Y_{n}}\bigg{)}-16, \tag{4}\]  
\[a^{*}=500~{}\bigg{(}f\bigg{(}\frac{X}{X_{n}}\bigg{)}-f\bigg{(}\frac{Y}{Y_{n}} \bigg{)}\bigg{)}, \tag{5}\]  
\[b^{*}=200~{}\bigg{(}f\bigg{(}\frac{Y}{Y_{n}}\bigg{)}-f\bigg{(}\frac{Z}{Z_{n}} \bigg{)}\bigg{)}, \tag{6}\]  
let \(t=\frac{X}{X_{n}},~{}\frac{Y}{Y_{n}}~{}or~{}\frac{Z}{Z_{n}}\), then:  
\[f(t)= \begin{cases}\sqrt[3]{t}&t>\frac{216}{24389}\\ \frac{24389t+432}{3132}&otherwise\end{cases} , \tag{7}\]  
where \(X_{n}=0.950489,~{}Y_{n}=1.000000,~{}\text{and}~{}Z_{n}=1.088840\) for CIE standard illuminant D65. A color difference metric, \(\Delta E00\) (CIEDE2000) [39], was chosen because it is considered to be the most accurate and widely accepted method for measuring the difference between two colors. Developed by the International Commission on Illumination (CIE) in 2000, this metric is based on the CIE Lab* color space, which is a perceptually uniform color space that considers the non-uniformity of the color space and the correlated color temperature of the reference white. The formula corrects for the shortcomings of the older CIE94 formula. Given a pair of color values in CIELAB space \(L_{1}^{*},~{}a_{1}^{*},~{}b_{1}^{*}\) (before far-UVC exposition), \(L_{2}^{*},~{}a_{2}^{*},~{}b_{2}^{*}\) (after far-UVC exposition), and parametric weighting factors \(k_{L},k_{C},\) and \(k_{H},\) the \(\Delta E00\) can be calculated using the following formulas [40]:  
\[\Delta E00=\sqrt{\bigg{(}\frac{\Delta L^{\prime}}{k_{L}S_{L}}\bigg{)}^{2}+ \bigg{(}\frac{\Delta C^{\prime}}{k_{C}S_{C}}\bigg{)}^{2}+\bigg{(}\frac{\Delta H ^{\prime}}{k_{H}S_{H}}\bigg{)}^{2}+R_{T}\bigg{(}\frac{\Delta C^{\prime}}{k_{C} S_{C}}\bigg{)}\bigg{(}\frac{\Delta H^{\prime}}{k_{H}S_{H}}\bigg{)}}, \tag{8}\]\[\Delta L^{\prime}=L_{2}^{*}-L_{1}^{*}, \tag{9}\]  
\[\Delta C^{\prime}=C_{2}^{\prime}-C_{1}^{\prime}, \tag{10}\]  
\[\Delta H^{\prime}=2\sqrt{C_{1}^{\prime}C_{2}^{\prime}}\sin\biggl{(}\frac{\Delta H ^{\prime}}{2}\biggr{)}, \tag{11}\]  
\[C_{i}^{\prime}=\sqrt{\bigl{(}a_{i}^{\prime}\bigr{)}^{2}+\bigl{(}b_{i}^{*} \bigr{)}^{2}}\ \ \ i=1,\,2, \tag{12}\]  
\[a_{i}^{\prime}=(1+G)a_{i}^{*}\ \ \ i=1,\,2, \tag{13}\]  
\[G=0.5\biggl{(}1-\sqrt{\frac{\overline{C}_{ab}^{*}\,7}{\overline{C}_{ab}^{*} \,7+25^{\prime}}}\biggr{)}, \tag{14}\]  
\[C_{i,\,ab}^{*}=\sqrt{\bigl{(}a_{i}^{*}\bigr{)}^{2}+\bigl{(}b_{i}^{*}\bigr{)}^{ 2}}\ \ \ \ i=1,\,2, \tag{15}\]  
\[\overline{C}_{ab}^{*}=\frac{C_{1,ab}^{*}+C_{2,ab}^{*}}{2}, \tag{16}\]  
\[\Delta H^{\prime}=