## 3 Diffuse Reflectance Spectroscopy: Analyzing Results
### Pitfalls and Artifacts
#### 3.3.2 Sample Positioning and Monochromator Settings  
Diffuse reflectance measurements are highly sensitive to slight modifications of the experimental conditions, especially in terms of geometry. Tilting or changing the axial position of the sample with respect to the white standard can have significant influence on the overall diffuse reflectance and is quantified in Figure 11 for TiO\({}_{2}\) against _CR30F_. For reference, the effect of too small monochromator bandwidths in the synchronous scan is shown as well.  
When the TiO\({}_{2}\) sample and the white standard are both identically tilted by 6\({}^{\circ}\), the diffuse reflectance spectrum essentially remains identical compared to the reference position. Varying the position or angle of only the sample (or only standard, respectively) does not alter the shape of the diffuse reflectance spectrum; however, it yields substantial deviations in the overall amplitude: For minor tilts (e.g., by an angle of 0.6\({}^{\circ}\)), a shift of the diffuse reflectance in the VIS range by 0.1 is observed whereas this offset grows to 0.2 upon axial repositioning (e.g., 0.3 mm). In this regard, a change of the diffuse reflectance upon tilting can likely be attributed to a local raise by a few 10 um.  
Figure 11: Comparison of the observed TiO\({}_{2}\) diffuse reflectance when the sample is displaced by 0.3 mm upwards (brown) or tilted by 0.6\({}^{\circ}\) (orange) compared to the white standard’s position. For comparison, an equal tilt of both sample and white standard by 6\({}^{\circ}\) (green) and at reference positions (black) are shown. The effect of both monochromators operating at identical (narrow) bandwidths is displayed in blue.  
Figure 10: Spectral output of the xenon arc lamp based on [94; 95] (left scale, blue dots) and quantum efficiencies of the photomultiplier tube (PMT) from [94; 96] (right scale, brown solid line) used in present studies.  
The drastic increase in diffuse reflectance for both misaligned positions is mainly caused by the different detected intensities due to a dissimilar optical path-length as was shown in Figure 5. Despite these serious differences in the VIS range, the diffuse reflectance becomes largely identical at the band gap for wavelengths \(\lambda<410\,\)nm. Hence, such errors may not appear evident at a first glance.  
With just marginally differing measurement conditions, distinctions between samples are challenging to interpret. Even general conclusions such as a comparison between the whiteness of two materials requires a careful evaluation of the setup to ensure an identical situation for all samples. Thus, careful alignment is of utmost importance to utilize the setup's sensitivity at its best.  
When both sample and white standard are affected in the same manner, e.g., by an equal tilt, diffuse reflectance becomes reproducible (cf. Figure 11). This is confirmed by a further measurement at the reference position with equal excitation and emission monochromator bandwidths of \(\Delta\lambda=0.2\,\)nm. Qualitatively, these two diffuse reflectance spectra exhibit similar amplitudes and shapes. However, owing to slight and irregular detuning between the two (narrow) monochromator slits, the diffuse reflectance is heavily affected and shows significant signal fluctuations throughout the visible spectral range.