## 2 Materials and Methods
### Measurement Setup: Requirements and Procedures
#### 2.2.1 Experimental Difficulties  
A number of conditions may impair a successful optical characterization using DRS. The following list summarizes detrimental scenarios that should be avoided:  
* Different distances between light source and sample surfaces: excitation and/or detected diffuse reflectance intensity changes (\(I\propto 1/r^{2}\) with optical pathlength \(r\) for a point light source). Furthermore, focusing optics are frequently used to increase the illumination intensity. When the  
Figure 4: Metallic sample holder ring (A) containing the powder that is placed onto a base plate (B). A mirror (C) reflecting the excitation beam downwards and the emission beam sideways is located above the sample. axial position of the sample is altered, the excitation spot size may also vary, thereby modifying the incident intensity. This effect may be enhanced by possible focusing optics and/or apertures within the beam path. Likewise, since the detection arm collects diffusely reflected light from a defined solid angle, a change in the diffuse reflectance position results in an altered amount of light that is being relayed to the detector (cf. Figure 5. Note that even in the absence of an aperture, the detector will collect a different intensity depending on the sample position). As a result, even minimal deviations in the axial position can cause significant variation in the measured signal intensity yielding large errors in the observed diffuse reflectance, if the positions among the white standard and powder samples differs as will be shown at a later point (cf. Figure 11). * Thin layers: the sample is partly translucent. Light reaches the sample carrier and (depending on its optical properties) can be reflected or transmitted further, thereby significantly altering the backscattering properties. Depending on the material, we recommend well-pressed pellets with thicknesses of at least 3 mm to minimize such effects. * Irregularities of the sample-surface: bumps, gaps or slants. Remitted intensities change considerably under grazed incidence angles as quantified by [PERSON]'s Cosine law [1; 3]. With a severely disturbed matte surface, Lambertian behavior cannot be assumed anymore and diffuse reflectance is affected by geometrical instead of material specific effects. In this case, a re-preparation of the sample becomes necessary. * Unequal irradiation positions on sample surface: the smaller the irradiation area, the higher the influence of local sample inhomogeneities on the diffuse reflectance. This way, reproducibility is severely hampered and significant signal fluctuations between individual positions can be expected. Improvement is achieved by maximizing the irradiation area to the sample size, thereby averaging out local irregularities. Nevertheless, there must be a significant separation of illumination area and sample border in order to avoid light propagation through the sample onto the sample-holder wall, as only the sample must be illuminated. Exposure of any other component must be avoided. * Inappropriate monochromator bandwidths: for a synchronous scan, the excitation \(\Delta\lambda_{\mathrm{Ex}}\) and emission band \(\Delta\lambda_{\mathrm{Em}}\) need to share a spectrally high overlap. In particular, the transmission bandwidth of one monochromator must be a subset of the other such that a slight discrepancy in central wavelengths of the monochromators do not cause a sudden drop-off of the signal. If, for example, both monochromators transmit a bandwidth \(\Delta\lambda_{\mathrm{Ex}}=\Delta\lambda_{\mathrm{Em}}=0.2\) nm of a white light continuum (i.e., the spectral intensity is constant throughout the investigated range) but  
Figure 5: Detected luminance for two different sample positions. Black: ideal position, red: shifted. the central wavelength of the emission monochromator is detuned by 0.1 nm in comparison to the excitation monochromator, half of the signal is lost in this specific case.