## 2 Materials and Methods
### Measurement Setup: Requirements and Procedures
#### 2.2.3 Signal-To-Noise Ratio  
The quality of the finally determined diffuse reflectance spectrum is very much dependent on the signal-to-noise ratio (SNR) of the experimentally determined intensity signals. Straightforwardly, the SNR can be improved by increased integration times related with signal detection. This presumes a stable setup for the entirety of the measurement procedure as well as a reference detector to sample the stability of the incident intensity of the probing light. Furthermore, the noise of the reference detector has to be taken into account. Alternatively, multiple fast scans with subsequent mathematical averaging over the individual data sets may be preferred. In terms of reproducibility, a fixed routine is imperative, explicitly including a startup procedure allowing for thermal stabilization of excitation lamp and detectors. A common, subsequently applied three-fold scan would then consist of:  
1. Measure white standard. 2. Measure sample. 3. Measure white standard. \begin{table}
\begin{tabular}{l l l} \hline
**ws** & & **Limits** \\ \hline \multirow{4}{*}{MgO} & \(\bullet\) & Not particularly stable [72] \\  & \(\bullet\) & Hygroscopic [73] \\  & \(\bullet\) & Weak UV diffuse reflectance [18; 72] \\ \hline \multirow{2}{*}{TiO\({}_{2}\)} & \(\bullet\) & Weak UV diffuse reflectance [64] \\  & \(\bullet\) & May fluoresce under UV excitation [74] \\ \hline \multirow{4}{*}{BaSO\({}_{4}\)} & \(\bullet\) & Reflectance deteriorates over time [15] \\  & \(\bullet\) & Fragile material that can be degraded by water and humidity [75] (p. 52) \\  & \(\bullet\) & Weak UV diffuse reflectance for \(\lambda<400\,\mathrm{nm}\)[18; 72] \\  & \(\bullet\) & May fluoresce when irradiated around \(360\,\mathrm{nm}\)[72] \\  & \(\bullet\) & Prone to water adsorption and strongly absorbs around \(1.4\), \(1.9\) and \(2.7\,\mathrm{\SIUnitSymbolMicro m}\)[65; 72] \\ \hline \multirow{4}{*}{Sintered PTFE} & \(\bullet\) & Not as Lambertian as fine powders [64] \\  & \(\bullet\) & Comparably low refractive index requires thicker samples (\(\geq\)6–8 mm to reach its peak reflectance) [67; 76] and may exhibit the _translucent blurring effect_[72] \\  & \(\bullet\) & Can develop static charge, that way attracting dust and dirt [72] \\  & \(\bullet\) & Soft material [72] \\  & \(\bullet\) & Demanding preparation of sintered PTFE-plate (cutting from bulk material and rasping into particular shape/size) \\  & \(\bullet\) & Very expensive [64; 75] \\ \hline \multirow{4}{*}{\(\alpha\)-Al\({}_{2}\)O\({}_{3}\)} & \(\bullet\) & Can have a slight absorption peak at approx. \(700\,\mathrm{nm}\) depending on powder and/or its production \\  & \(\bullet\) & Very high Mohs hardness (\(\approx\)9 [77]) can cause abrasion of containers during grinding, thus contaminating the white standard \\ \hline \end{tabular}
\end{table} 
Table 2: Materials commonly applied for white standards (ws) in diffuse reflectance measurements with their respective optical and material properties important for diffuse reflectance spectroscopy (DRS). This procedure has the distinct advantage to account for slower temporal fluctuations of the incident sampling light as the wavelength-dependent diffuse reflectance of the sample (s) is scaled to the average of the two diffuse reflectance measurements of the white standard (ws):  
\[R_{\infty}(\lambda)=\frac{I_{\text{s}}(\lambda)}{\frac{1}{2}\left(I_{\text{ws} }^{(1)}(\lambda)+I_{\text{ws}}^{(2)}(\lambda)\right)} \tag{1}\]  
with the spectrally measured intensities \(I(\lambda)\). Alternatively, if intensity drifts occur on shorter timescales, the measurement time per sample can be reduced by drastically decreasing integration times per wavelength and successively switching back and forth between sample and white standard:  
1. Measure white standard. 2. Measure sample. 3. Measure white standard. 4. Measure sample. 5. Measure white standard. 6....