The figure above is a schematic of the system installed at the National Air and Space Museum and the DOE headquarters in Washington, D.C., Light from the sulfur lamp is focused by a parabolic reflector so that it enters the light pipe within a small angular cone. Light travels down the pipe, reflecting off the prismatic film (A) that lines the outer acrylic tube. The prismatic film reflects the light through total internal reflection (C), an intrinsically efficient process. Some of the light striking the film (at A) is not reflected and "leaks out" of the pipe walls (B), giving the pipe a glowing appearance. A light ray that travels all the way down the pipe will strike the mirror at the end (D) and return back up the pipe. A special light-extracting surface (another type of reflecting film) is used to draw the light out of the pipe in a controlled manner to where it is most needed (E).