## 1 Introduction  
Diffuse reflectance spectroscopy (DRS) of (nanoscaled) powders is a widely applied, powerful tool in material sciences and industry [1; 2; 3]. The scattered intensity of an incident light beam is scanned as a function of the wavelength over the sample and the data set is analyzed with respect to its macroscopic optical characteristics, such as the overall reflectivity. Moreover, DRS potentially enables the determination of the absorption characteristics of the particles themselves, even for particles on the sub-micron length scale. The absolute values as well as the dispersive features of the absorption coefficient are significant for a variety of materials with large commercial potential, such as pigments or phosphors. Absorption properties determine the macroscopic optical features of dielectric powders to a great extent, but are also responsible for the losses of internally emitted light. The success in the determination of the absorption features goes along with a thorough knowledge on the relation between the diffusely reflected (remitted) signal intensity and the optical properties of the dielectric materials as well as the powder characteristics including particle size distribution and filling factor. The particular type of light-interaction with powders has been addressed worldwide in a series of theoretical studies over the last decades (cf., e.g., [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14]). Alike, an innumerable amount of experimental methods and procedures for powder sample preparation have been developed and can be found as an established state-of-the-art tool in nearly any physico-chemical laboratory today. However, the complexity of the underlying problem of light-matter interaction within a sample characterized by a pronounced disorder on the nanoscale is connected with a number of challenges along the entire procedure covering sample preparation, measurement, analysis and identification of artifacts up to the final determination of the absorption coefficient over a large spectral range. As an example, the experimental detection of diffusely scattered light in a large solid angle using integrating spheres already is far from trivial and sometimes not even feasible, e.g., if the internal coating is not virtually perfectly wavelength-independent, a substitution correction is missing, fluorescence or phosphorescence occurs, or a gliding-off of a (brittle powder) sample contaminates the sphere [15; 16; 17; 18; 19]. Accordingly, the application of DRS often lags behind its capabilities and is primarily used for the determination of macroscopic optical parameters. Moreover, there exists no in-depth end-to-end instruction in literature so far. This paper addresses the challenging task of the optical characterization of pure, dielectric (nano-)powders with the aim to provide an end-to-end instruction from appropriate sample preparation up to the determination of absorption spectra. Particular emphasis is placed on the presentation of an innovative preparation procedure for producing powder pellets that allows the ability to obtain samples showing an ideal Lambertian scattering behavior while also being highly reproducible. As a result, a reliable experimental access to diffuse reflectance spectroscopy and ensuing determination of the absorption coefficient \(\alpha\) using thick, non-translucent samples is provided. The original approach makes use of a minimized experimental procedure in order to curtail the number of free parameters and thus also to circumvent potential error sources. Powder pellets lacking extraneous host materials are used while integration spheres are skipped entirely in order to provide an alternative route for material characterization. These experimental simplifications enable the possibility for Monte Carlo simulations of light propagation along multiple scattering, diffraction and absorption events within the powder pellet.