This is an optical device bundling lots of glass optical fibers. It transfers the light and image with high efficiency and low distortion. They are used in CRT's, camera tubes, CCD coupling, and image intensifiers. They replace optical lenses in a number of applications and there is no focal point to consider. These are great to experiment with as you can see the image of newsprint follow the curve of the face plate. If you have light reflected from the flat side, you'll see a bulls eye that varies in diameter as you change plate's orientation. These are polished, in great condition, and have serial numbers on the edge. Flat on one side and concave on the other. 35mm diameter by 6mm thick.