It is well known that compact discs have become exceedingly popular in terms not only of the entertainment industry for audiovisual amusement, but also in the computer industry where they are used to carry programs and other data in audio and audiovisual modes.

With this popularity and the size and value of the market comes the eternal risk of plagiarism, ie piracy, which with modem technology becomes ever simpler, with a concomitant financial loss to the rightful owners of the original material.

Counterfeiting is a major industry in itself and many and varied efforts have been devoted to making more difficult the copying of data from their carriers and to provide the carriers with a unique identifying signature.

Compact discs carry the important data on essentially one side, the other side usually carrying branding, decorative matter and general information regarding the contents of the disc. It has been proposed to create on a face of the compact disc a hologram of a suitable image which will be unique to the disc in question and to its originator, thereby providing a security badge to identify the disc as a genuine product. Products not carrying the relevant image will thus be counterfeit.

One signal advantage of a hologram is the difficulty of copying it and accordingly its use as a security badge is important and will be well known in other fields such as credit and bank cards.

As indicated above, various attempts have been made to enable a compact disc to carry a hologram, and indeed there are commercial examples of discs which depict a hologram on their non-data carrying surface. Such holograms are generally of a decorative character and do not readily constitute a security badge.

Other proposals involve the application of a hologram to the actual data-carrying surface of the disc. Unfortunately, the manner of application has proved to involve a multi-stage production operation and in some instances has resulted in the integrity of the data being compromised by the superimposition of the hologram onto the data carrying surface with consequential impairment to the acquisition of data by the laser which reads the disc.

Optical discs have data stored in the form of a spiral track comprising'pit'areas of reduced effective reflectivity in a surface of'land'areas which reflect substantially all of the light from the laser in the playing device. The reduced reflectivity of the pits is achieved in one technique by arranging for their depth to coincide with one-quarter of the wavelength of the interrogating laser in the playing device. Thus although the bottom of the pit may be perfectly smooth and effectively a plane mirror, light reflected from it will be out-of-phase when compared with light reflected from land areas.

One previous method proposed for decorating the surface of CD's with a hologram has avoided placement of the hologram in the data replay areas of the disc in order to prevent data corruption. In such a method the hologram area is spatially separated from the data-containing area to avoid cross-talk between the hologram fringes and the data structure. In this way the laser beam in the playing device does not read the areas containing a hologram. This restriction of the available hologram area is inconvenient, and thus forces limitations on the use of holography as a security device or as a promotional novelty.

Another earlier proposal involves a method where the hologram fringe structure is contained in a lacquer coated on the reverse, ie non-data, side of the disc after pressing the data into the first side. Such a method is inconvenient from the point of view of both expense and the time taken in producing discs.

Classically, a holograph of the'Benton'or'Rainbow Hologram'image type is recorded as a plane sinusoidal wave structure in the surface of a suitable substrate. The holographic fringes are predominantly linear and relatively simple to define. The fringes recorded in plane grating-type'holograms'and dot-matrix images are even more orderly and mathematically predictable in their structure. In the case of complex colour transmission holograms, however, these sinusoidal wave structures are complicated by the fact that they take the form of a compound addition of the amplitude of the individual colour component waves.

This complexity of the structure is in fact advantageous in avoiding the creation of surfaces which may tend to be confused by the data reader system as having digital qualities.

Holographic image design is of critical importance in this respect such that highlight zones in the holographic image are carefully controlled to avoid their coincidence with the surface of the resist, whereby areas of high level dissolution of resist can inadvertently be created.

In embossed holography, such phase recordings as described here can be manifested in the form of a thickness variation in an embossable lacquer coated onto a carrier layer which is relatively dimensionally stable. The surface of a compact disc (or DVD) is an ideal carrier in terms of such dimensional stability.

The CD data track, conversely, is essentially a digital (0/1) stream of information. Cross-talk or interference between the signals can be avoided by virtue of the incompatibility of the sinusoidal holograph with the digital requirements of a reader system. In fact, the pit structure of the data track

provides a diffraction effect which is potentially destructive towards the optical effectiveness of the hologram, but which, by virtue of its effective fringe spacing can be arranged to be non-deleterious towards the holographic decoration of the disc, since the diffraction may appear at a quite different viewing angle to the holographic image. More difficult, and more important, is to avoid the deletion of the fidelity of the data track by spurious modulations caused by the holographic structure when encountered by the laser beam of the reading device.

One object of the present invention is to differentiate between the type of structure capable of carrying a digital optical signal, and the type of structure which can effectively carry a holographic interference recording.

A further object of the present invention is to decorate the whole or a part of the active surface of optical discs, namely compact discs or digital versatile discs, with one or more holographic images.

A still further object of the present invention is to provide a method of producing in a single step such an optical disc which carries both data and at least one holographic image on its active surface.

One aspect of the invention thus provides such a method of production including the step of limiting the total thickness of the holographic structure to a fraction of the depth of the data'pits'as compared to the'land'areas of the data recording structure.

Advantageously this fraction is chosen to be of the order of the ordinary production limits of smoothness of the stamping master employed to produce the disc.

Hologram structures of acceptable commercial image brightness can be made to conform to these limits using various methods developed for the purpose.

According to a further aspect of the invention one method includes the steps of exposing a hologram latent image at a low level of energy into a photoresist layer which is subsequently exposed to high energy exposure of the data-track (1/0) information.

Conveniently modified solvent processing is then applied to the layer to obtain a suitable surface profile where the data-pits are cleared to the glass substrate whilst the holographic data are confined to the limited modulation of the land surface.

In a still further aspect of the invention a method involves sequentially coating the photo-resist in two layers, one of which is dyed in such a way as in use during playback to inhibit the transmission of the holographic laser, but to admit the light of the data laser by virtue of its differing wavelength.

Accordingly the invention additionally provides a method for the playback of optical discs using lasers of differing wavelengths to produce the two image components for data and hologram.

In a refinement of the method of using lasers of differing wavelengths, photoresist layers of differing spectral sensitivity are used to separate the sensitivity of the two layers to the two laser wavelengths.

By way of example, a krypton laser operating at 413 nm. will provide light with exceptional actinic effect on one resist type, whereas the holographic exposure made at 458 nm. with an argon laser may be more active towards a spectrally sensitized resist only.

An alternative method includes the step of re-coating the solvent-processed imaged data resist with a second thin coating to receive the holographic image.

To prevent the second coating from gathering in the pre-cut'pits', an aerosol method of coating may be used with the disc inverted relative to the ordinary coating regime.

Dilution of photoresist solution with abnormal quantities of solvent for use in high speed spin-coating equipment enables exceptionally thin layers to be successfully produced down to an almost infinitesimal limit of a few nanometres of thickness.

Use of abnormally dilute, cold, or brief processing techniques can be used to treat these thin layers in an appropriate way during their development to obtain the most efficient hologram.

Shipley 1800 photoresist is an example of a resist capable of producing such structures, and other products with more suitable gamma characteristics may in the future provide tolerance to ease the criticality of the processing of the master recording.

Photoresist coatings may be typically coated on circular glass substrate discs.

The use of a red iron oxide surface coating or other absorbent barrier layer to this glass can be used to eliminate unsatisfactory effects of internal reflection of the laser light within the substrate glass to improve the fidelity of both the hologram and the data recordings.

These coated discs are sequentially imaged with data track and then moved in light-safe containers to a holographic imaging table system for imaging with holographic fringes or vice-versa, dependent upon which of the alternative dual layer imaging techniques above is to be used. Transmission holograms of the 2D/3D and Stereogram types are examples of the type of imaging which can be transferred into the resist from such a table system. Digitised dot-matrix systems

for holographic imaging can also be used, and direct imaging of plane gratings through masks may be an alternative decoration technique.

The holographic information may also be placed onto the unexposed photo resist by means of micro embossing from a separate pre-recorded medium such as a metal shim. This metal shim will carry the holographic pattern which on contact with the unexposed photo resist will be impregnated into the resist under a predetermined pressure, leaving behind the impression of a holographic image without causing any detriment to the unexposed photo resist layer. The resist is then ready for digital encoding without interference from the holographic information.

In order to reduce the crosstalk tendency of such well-defined fringes, however, we have devised a masking system which can be used to protect the data track from contamination.

Unlike the contact mask methods which have been published in the past, we have obtained critical registration of the mask sufficient to protect the track zone by coating in one example fine grain silver halide (< < 20nm. grains) high-contrast emulsion directly onto the surface of the photoresist prior to exposing the data track. The resist is thus exposed simultaneously with the silver halide emulsion during the data imaging step. Subsequently, other negative masking images, such as logo shaped window apertures, or windows to protect the critical outer extremity of the data spiral track, which is sometimes rendered otherwise noisy by other process parameters or imperfections, can be exposed with red light into the sensitized silver halide emulsion. If the layer is developed and fixed and dried the black silver can selectively and accurately protect the underlying photoresist areas from exposure to the holographic image or images which will cover other parts of the disc.

Formulations for a suitable fine-grain silver halide gelatin emulsion have been published by Bjelkhagen in his book'Silver Halide Recording Materials for Holography and their Processing'Springer Series in Optical Science. After imaging the hologram information, the silver halide mask can be washed entirely away from the photoresist before it is returned in darkness to be machine processed in a routine way.

A principal advantage of the present invention is that its methods rely on being able to generate optically readable discs in a single production step whereby the disc carries both data and a holographic image on the active surface in the absence of any compromise in the integrity of the data thereby enabling both the data and the image to co-exist. The high cost of double handling techniques currently proposed in the art is thus avoided, the only additional cost of manufacture residing in the preparation of the purpose-modified metal master stamper for the discs, thus representing a significant saving in production time and expense.

It will be understood that while the present invention as described above delimits a number of methods of achieving the objectives set forth, the list of such methods is not exhaustive but the present invention embraces other procedures which provide the same end result.