The front ring can be made of acrylic, stainless steel, brass, steel, etc. The viewing window is held in place by the front ring. Some rings are a permanent crimp design, while others can be removed providing dial/pointer access for recalibration or repair. The ring which retains the window is bayonet, threaded, snap-on or with screws, depending on the application and case type. Window materials are typically clear acrylic, polycarbonate, flat glass or safety glass.