Transparent displays allow observers to see the objects behind the displays as well as information on the displays. For this reason, transparent displays have received attention as next-generation displays. Visibility of the background objects when they are seen through the on-screen contents is very important for designers of on-screen contents as well as observers of transparent displays. In this paper, factors affecting the visibility of the seethrough objects are examined first. Each of selected factors is formulated. Finally, visibility of the see-through objects is defined as a function of the selected factors. Validity of the proposed model for visibility of the see-through objects is verified by comparing them with results from human visual experiments.