Solar energy market is still growing in Morocco and in other countries around the world. The development and deployment of any solar project require a prefeasibility study before the establishment of the plant. The most important parameters influencing the site selection or the project feasibility are solar radiation and the climatic conditions. Soiling is a limiting efficiency factor that drastically affect the optical and the electrical performances of the solar plants. The objective of the current study is to evaluate and compare the soiling impact, from an optical point of view, on both PV and CSP technologies by investigating the reflectance and transmittance drop of glass and mirror samples exposed at Green Energy Park research facility (Morocco) during the dry period of the year. The results show that the effect of soiling is very important in the dry period of the year, it can reach up to 35% of soiling loss after only one week for the mirror samples. For the glass samples, the drop can reach up to 12%. Besides, it has been found that the CSP mirrors are three times more affected by soiling than PV glass samples.