In solar thermal research, there is a lot of attention has been paid on optimal design of solar reactors and cavity receivers in order to overcome inherent technical challenges. While many of these studies focus on the geometry and flow optimization of the reactor/receiver, few have focused on control of light entry into the cavity via auxiliary embedded mechanisms. Because the natural fluctuation of solar radiation affecting the thermal behavior of these reactor/receivers during daytime, it is important to address this challenge. In order to cope with transient nature of solar energy, several modular devices have been conceived. They are designed based upon concepts featuring control of set of blades or interchanging apertures to adjust light entry. These techniques yield promising performance on maintaining the desired reactor temperature and solar to fuel efficiencies. Towards that effort, an extensive fundamental background in designing, manufacturing, and testing of such mechanisms has been accumulated by our research group. This paper provides an overview of three successful mechanical apertures developed by our group to regulate flux entry into solar thermal receivers. The overview covers methodological aspects commonly encountered during the design process. A thorough comparison of performance records of these mechanisms are given per their actuation methods. Effect of each aperture mechanism and their individual change of cross-sectional shape is being evaluated through extensive optical simulation. The simulation results give insights on their respective optical performance by quantifying their radiative energy gain and losses. The paper also presents a numerical method coupling optical model to a thermodynamic analysis yielding accurate estimation of reactor temperature per experimental validation. Finally, numerical simulation showed that closed loop control of aperture size can efficiently regulate the temperature throughout a sunny day. Results of this research highlight the advantage of adopting variable apertures in solar cavity receivers.