This paper presents projector-based illumination techniques for creating correct occlusion effects for optical see-through setups. We project view-dependent occlusion shadows onto the real surfaces that are located behind virtual objects. This results in a perfect occlusion of real objects by virtual ones. We have implemented and tested our approach in the context of the Virtual Showcase display. We describe hardware extension for projecting light into the showcase and present our rendering techniques for displaying occlusion shadows for single and multi-user environments as well as for single and multi-light-projector configurations. We also report on the limitations of our system for multi-user situations and describe our experiences with a first experimental prototype.