Science / Medicine : Erasable CD Developed

A new type of compact disc that can be repeatedly erased and written on has been developed by researchers at Optical Data Inc. of Beaverton, Ore. Compact disks can store data at a density 10 times greater than that of the magnetic media used in personal and other computers, and are now widely used for audio and video recordings because of their high storage density and fidelity of reproduction. But they are not widely used in computers because they cannot readily be written on.

Compact discs store data in digital form as smooth surfaces and pits in a thin metal film coated on a plastic disc. The pits and surfaces represent the “0� and “1� of the binary code used in computers. They are read with a small laser, which can distinguish between the two surfaces.

The new discs are formed from two layers of plastic with different properties and dyed different colors, according to chemist Nancy E. Iwamoto. Heat from a laser stores data by causing small bumps to form on the surface that can be read by scanning them with the same laser at a lower power. A second laser causes both layers of plastic to return to their original shapes, erasing the bump.