Lanthanide-doped nanoparticles exhibit exceptional optical properties that enable their use in a variety of applications, including spectroscopy and laser cooling. Cryogenic spectroscopy of these nanoparticles allows for the investigation of their behavior under extreme conditions and the assessment of their suitability for optical refrigeration. In this work, we present the measurement of emission spectra of monocrystalline lanthanide-doped nanoparticles, with the aim of optimizing their spectral characterization in cryogenic environments. Specifically, we analyzed samples doped with Yb and with ErYb. Moreover, the emission spectra obtained at various known temperatures serve as a necessary tool for performing spectroscopic thermometry on these particles and for evaluating their potential for laser-based cooling. The experimental setup, based on a cryostat, was specifically developed and implemented for these measurements, and its construction and commissioning were also a key part of this study.