Vertical louvres or fins may be used on east or west facing windows to stop sunlight from entering the room.
While the fin between the window and the sun blocks direct sun, the one on the other side of the window will reflect diffuse light into the room.
Fins can be fixed, for instance as elements of the facade, or moveable. If they are moveable, then the amount of blocking can be adjusted. More light can be admitted in winter, while most of the sun is kept out of the building during the hot summer months.