Optical data storage compact disc and method for manufacturing the same

DETAILED DESCRIPTION OF THE DRAWING AND PREFERRED EMBODIMENTS Referring to the drawings, there is shown an optical data storage disc in the form of a compact disc (or CD) which carries a digital audio recording.
The disc comprises a substrate 10 of polycarbonate typically 1.
2 mm thick having one side formed with a pattern of indentations or pits 11 defining the digital recording.
This side of the disc is metallised, as indicated at 12, and further coated with a layer 14 of thermoplastics material, the surface of which is embossed, as indicated at 15, with a relief pattern defining a holographic image.
The embossed surface 15 is metallised, as indicated at 16, and further covered by a coating of protective lacquer 17, onto which printing may be applied (e.
g.
using transparent or translucent inks) as indicated at 18.
The holographic relief pattern may extend over any desired area of the disc.
We have found it possible to provide and emboss the layer 14 over the indented side of the disc, without degrading the quality of indentations and without affecting the process of reading the digital information in the usual manner, i.
e.
using a laser beam directed at the opposite or underside surface of the disc, the holographic image being viewed from the upper side of the disc.
The upper side may be printed with human-readable information, but instead any required information may be carried in the hologram.
In one method of manufacture, the disc 10 is formed by injection moulding and then vacuum metallised (often by sputtering) and the metallised disc is then spun-coated with the layer 14 of embossable lacquer, which is then cured or dried.
The layer 14 is then embossed using heat and pressure, applied by one of several available methods.
For example, the layer 14 may be impressed by a flat-bed platen normally heated for example by a conventional electric element behind the embossing shim or by passing an electric current through the shim.
The shim profile may differ from a standard flat profile, to take into account variations across the disc and reduce the chances of air entrapment