Complete understanding of the role of intrinsic and extrinsic defects on the optical properties of functional semiconductors requires accurate hybrid experimental - theoretical analysis. In this respect, ab-initio calculation in the framework of density functional theory has been proven as a powerful and efficient theoretical tool. In the Kohn-Sham approach to density functional theory, while ground state propertiers are well described, studying excited state properties of materials using common exchange-correlation approximations has always been a challenging topic. Finding proper methods for excited state calculations that are both exact and fast is a major theoretical challenge. In this study, we employ the half-occupation technique as an effective method with a computatioinal cost comparable to that of common exchange-correlation functionals to explore Ce and Sm doped CaS crystals which are luminous materials with many applications (CaS is considered as an OSL material in this study). In the initial step, the accuracy of half-occupation technique was examined for calculating the band gap of some pure semiconductor materials within different functionals, and then the best functional was selected for calculations of the doped samples. Then the effects of Ce and Sm defects as well as vacancies on the electronic structure and band gap of CaS host semiconductor were assessed by considering several defect configurations. The obtained results are compared with the available experimental absorption and radioactive luminescence (RL) spectra to identify the more probable configuration of the pure and doped samples. It is argued that the obtained electronic structures are very helpful to understand the experimental observations of optical behaviour of the pure and doped CaS crystals.