  Nowadays, photovoltaic (PV) technology has reached a high level of maturity
in terms of module efficiency and cost competitiveness in comparison with other
energy technologies. As PV has achieved high levels of deployment, the
development of devices that can help to reduce PV operation and maintenance
costs has become a priority. Soiling can be cause of significant losses in
certain PV plants and its detection has become essential to ensure a correct
mitigation. For this reason, accurate and low-cost monitoring devices are
needed. While soiling stations have been traditionally employed to measure the
impact of soiling, their high cost and maintenance have led to the development
of innovative low-cost optical sensors, such as the device presented in this
work and named "DUSST" (Detector Unit for Soiling Spectral Transmittance). The
thermal characterization of the components of DUSST and the methodology used to
predict soiling transmittance losses are presented in this study. The results
show that the losses can be predicted with an error lower than 1.4%. The method
has been verified with an experimental campaign with naturally soiled coupons
exposed outdoors in Ja\'en, Spain.
