Photomask

In electronics, a photomask is a special stencil which is used to create a printed circuit board. When light is shone through the photomask, certain parts where the light hits the unused circuit board are strengthened, much like what toothpaste does to the hard surfaces your teeth. When a circuit board is put through an acid bath, the parts where the light shone through the photomask stay in place, while the parts that were covered by the shadow of the photomask are eaten away by the acid.