GB1590795A - Information-bearing disc and a method of production thereof - Google Patents

Description

(54) INFORMATION-BEARING DISC AND A METHOD OF PRODUCTION THEREOF (71) We, EASTMAN KODAK COMPANY, a Company organized under the Laws of the State of New Jersey, United States of America 343 State Street, Rochester, New York 14650, United States of America do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to discs having information encoded thereon, such as video discs, and to a method for mass producing duplicate discs from a master record.

A video disc is a recording of video information, which has a structure similar to a phonograph record. A preferred type of video disc has the video information recorded as a distribution of discrete optical apertures or discontinuities of varying sizes, arranged in a spiral path. Such optical apertures or discontinuities vary in shape from being round to being elliptical. A substantial portion of such optical apertures or discontinuities have a minimum dimension of less than one micron (10-6 metre). See the article by Broadbent in the Journal of the SMPTE, volume 83, July 1974, pages 554 to 559, which teaches a method of producing a master video disc, and a method of producing duplicate video discs from the master video disc.

An economical method of reproducing video discs is by photographically printing from a master disc, using methods such as taught by Jerome et al in the Journal of the SMPTE, volume 83, July 1974, pages 560 to 563. However, video discs having apertures or discontinuities with a minimum dimension less than a micron have not been successfully reproduced by photographic printing. Projection printing has not been successful because there is no known optical projection apparatus which can maintain sufficient resolution over an area the size of a video disc (about 30 cm in diameter). Contact printing has not been successful because contact between the master and the duplication material has not been intimate enough to produce satisfactory resolution over the whole area of the video disc.

In accordance with the present invention there is provided a method of photographically producing a duplicate disc from a master record having information encoded thereon in the form of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said method comprising the steps of 1) superimposing the master record on a duplication material, the duplication material comprising a photosensitive material having an absorption of actinic radiation which, under the influence of such actinic radiation, decays to a lower level, and 2) exposing said photosensitive material to actinic radiation through said optical apertures.

In accordance with the present invention there is also provided a disc comprising a layer containing information in the form of a distribution of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said layer comprising a developed photosensitive material which has been exposed to actinic radiation and which has an absorption of actinic radiation which under the influence of such actinic radiation has decayed to a lower level.

The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a master disc and a duplication material ready for contact printing; Figure 2 shows the result, without the present invention, of contact printing the master disc on to a duplication material; Figure 3a shows the behaviour of radiation upon passage through an aperture; Figure 3b is a transmission profile corresponding to the behaviour shown in Figure 3a; Figures 4, 5a and 5b and Sc illustrate qualitively the contact printing process of the present invention; Figures 5a', Sb' and Sc' show transmission profiles corresponding to the stages of the contact printing process shown in Figures Sa, Sb and Sc respectively; Figure 6 is a qualitative representation of the result of a contact printing process embodying the present invention; and Figure 7 is a cross-section through a portion of a master which includes an overcoat.

Figure 1 shows a portion of a master video disc 18 comprising a glass substrate 21 having a thin layer of metal 20, e.g. bismuth, coated thereon. A series of apertures in the thin metal layer 20 represents video information contained both in the size and the spacing of such apertures. Three of such apertures are shown in Figure 1 denoted by numerals 19a, 19b and 19c. A major portion of the apertures are less than 1 micron wide and less than 2 microns in length. Since the production of a master video disc 18, as taught by Broadbent, above, is a relatively expensive process, it is desirable to mass produce a large number of duplicate discs from the master video disc 18. One form of a duplication material 22 comprises a layer 23 of photosensitive material that forms a positive density image when exposed to an original, coated on a reflective layer 24, which may be of aluminium Both layers are supported by a flexible plastic substrate 25. It is desired to accurately form optical apertures in the photosensitive material 23 corresponding in size and spacing to the apertures on the master video disc 18. In order for the recorded video signal to be accurately reproduced, the size of each optical aperture in the duplicate disc must not vary from the size of its corresponding optical aperture in the master disc 18 by more than about 30 . Any distortion in aperture size or spacing leads to a distorted or noisy video signal upon playback of the duplicate disc which results from exposure and processing of duplication material 22.

Figure 2 shows an attempt, without the present invention, to contact print a master video disc 18 on to duplication material 22. The condition of contact between the master disc 18 and duplication material 22 in the vicinity of the apertures 19a, 19b and l9c illustrates a common contact printing problem. At apertures 1 9a and 1 9b,loss of intimate contact due to a foreign object 17, such a speck of dust or dirt, is illustrated. Foreign object 17 might also be a localized imperfection in the layer of photosensitive material 23 that serves to increase the effective thickness of the layer 23 in the vicinity of the apertures. Foreign object 17 could also be a localized surface defect on the master video disc 18. At aperture 19c, the ideal, but often not realized, condition of intimate contact between the layer of photosensitive material 23 and the thin layer of metal 20 is shown.

When illuminated by radiant energy, each of the apertures causes diffraction which causes the radiation passing therethrough to diverge. Aerial images (images in space which are not focused on a surface) of the apertures formed by the diffracted radiation in the near field (i.e., in the image space directly beneath the apertures) contain increasing amounts of unwanted structure (fringing) and geometrical distortion as the distance from the apertures to such images becomes greater than a few wavelengths of the exposing radiation. In practical situations, the exposing radiation has appreciable spectral range so that the interference related image structure (fringing) tends to even out, but the degradation of the image due to geometrical distortion remains. Thus, exposure of the layer of photosensitive material 23 by radiation passing through the apertures 19a and 19b (where intimate contact has been lost) results in degraded (distorted) images 1 9a ' and 1 9b ' in the photosensitive layer 23. When it is recognized that 30 A (the general tolerance limit for aperture size distortion) is only about .0075 of the wavelength of actinic light, it is readily apparent that images 19a' and 19b' are unacceptably distorted. The imagewise exposure made through aperture 19c (resulting in image 1 9c ') is of higher fidelity (has less distortion), due to the intimate contact between the thin layer of metal 20 and the layer of photosensitive material 23. Such contact gives the near field diffracted image less distance in which to degrade.

While it would appear that the solution to the problem would be to eliminate foreign objects, such as foreign object 17, and to eliminate surface imperfections on the layer of photosensitive material 23 and master video disc 18, a deeper understanding of the problem leads to a different and preferable solution.

Figure 3a shows an aperture 10 (similar to the apertures discussed above) having a transmission profile as shown in Figure 3b. As a plane wave of radiant energy 12 passes through the aperture 10, a near field diffraction image is produced. The spatial irradiance distribution of such image depends upon the distance from the aperture 10. The lengths of the arrows 14, in Fig. 3a, quantitatively represent the relative flux and direction of diffracted radiation at the aperture 10. It is seen that while maximum radiation occurs in the forward direction, considerable radiation is directed laterally. Upon contact printing, both images 19a' and 19b' are distorted by the spreading of the diffracted exposing radiation, i.e., diffracted radiation impinging on image planes that occur at increasingly greater depths throughout the volume of the imaging layer gives rise to the increasingly distorted aperture images 19a' and 19b', discussed above.

The present invention recognizes that if one could confine the diffracted radiation in such a way that the angular distribution of the near field diffracted radiation that reaches any particular image plane is made sufficiently narrow, the occurrence of unwanted image distortion would be minimized, and the contact printing process (described in connection with Figure 2) would be significantly less sensitive to losses in intimate contact between the master video disc 18 and the duplication material 22 )or alternatively thicker layers of photosensitive material 23 could be used, if intimate contact is maintained).

Figure 4 qualitatively illustrates the workings of the present invention showing the contact printing of a single aperture 26. The master video disc 18 and the duplication material 28 appear similar to those discussed above. The duplication material 28, however, comprises a material such as photosensitive layer 27 having certain optical properties which, in effect, control the diffracted, laterally diverging exposing radiation discussed above.

Specifically, the photosensitive layer 27 initially has a high absorption to the wavelength (or wavelengths) of radiation used for exposure in the contact printing process; but as exposure takes place, the absorption decays to a lower level in proportion to the amount of radiation absorbed. These optical properties are known per se as Aufrollen, or the Aufroll effect (see U.S. Patent No. 2114468). In effect, therefore, radiation diffracted at large angles by aperture 26 is confined via absorption to the upper surface portion of the photosensitive layer and does not significantly contribute to degradation of the imagewise exposure of planar regions in the photosensitive layer that are at greater depths in the photosensitive layer.

Thus, for example, in Figure 4, which depicts the contact printing process at some intermediate state, such that full exposure of the photosensitive layer 27 has not yet been accomplished-but such that a volume 30 of the photosensitive layer 27 lying beneath aperture 26 has been exposed and rendered transparent to further exposing radiation-point "0" lying directly beneath aperture 26 on plane "a" within photosensitive layer 27 can be exposed while point "p" which lies outside the geometrical shadow of aperture 26 on plane "a" cannot be exposed at this time. Exposure will continue until the bottom plane of the photosensitive layer 27 that is coincident with the top surface of reflective layer 24 becomes transparent over an area that is centred beneath aperture 26 and has the same size and shape as aperture 26. Moreover, since a given planar area of photosensitive medium cannot be exposed until the region of the medium lying above it is exposed to render it transparent, radiation reflected from the reflective layer beneath the photosensitive material cannot affect the image-wise exposure unless severe overexposure is carried so far that radiation diffracted at relatively large angles by the aperture 26 can reach the layer beneath the photosensitive medium.

It has been found through experimentation that use of a photosensitive layer 27 having the described optical properties leads to a contact printing process significantly less affected by losses in contact between the master video disc 18 and the duplication material 28. Also, thicker photosensitive layers that ultimately have higher transmission contrast between exposed and non-exposed regions can be used.

Figures 5a, 5b and Sc represent the contact printing process in time sequence and serve to offer further explanation of the present invention. Initially, the master video disc 18 is placed in contact with the duplication material 28. Upon exposure, a first group of photons transmitted to the surface of the photosensitive layer 27 "sees" a transmission profile as shown in Figure 5a'. As discussed in connection with Figures 3a and 3b, such an aperture causes diffraction having an angular intensity distribution shown quantitatively by arrows 14 in Figure 3a. Due to the above described optical properties of the photosensitive layer 27, however, a portion 26' (defined by the broken line) of the photosensitive layer 27 directly under and centred on the aperture 26 becomes transparent, the exact shape of the transparent portion of the photosensitive layer 27 being determined by the angular intensity distribution of radiation (see Fig. 4). A second group of photons transmitted to an unexposed plane in the volume (beneath the surface) of the imaging layer 27 does not therefore, see the transmission profile of Figure 5a', but rather see a transmission profile determined both by the aperture 26 and by the transparent portion 26' near the top surface of the photosensitive layer 27 (see Figure 5b). The transmission profile of this effective aperture is shown in Figure 5b' and, importantly, contains no sharp edges or corners. It is well known in optics that radiation is diffracted at large angles by sharp edges. The transmission profile of Figure 5b' can be expected, therefore, to cause diffraction that is confined to a narrower solid angle than diffraction caused by the transmission profile of Figure 5a'. The net effect is that of directing more radiation in the forward direction and less radiation in the lateral directions. Similar to the first group of photons, the second group of photons causes a portion 26" (defined by the broken line in Fig. 5b) of the photosensitive layer 27, to become transparent. As shown in Figure 5c, a third group of photons exposes a third plane (farther beneath the surface), with an effective aperture comprising the aperture 26 and the transparent portion 26" of the photosensitive layer 27. Again, due to the shape of the resultant transmission profile (Figure Sc') there is diminished diffraction in the lateral direction.

Thus, the effective aperture formed in the photosensitive layer 27 and seen by successive groups of photons as they expose successive planes in the photosensitive layer 27, evolves in time and, as a reuslt of such evolution, each successive group of photons sees an effective aperture having a transmission profile lacking sharp edges and corners. The elimination of sharp edges and corners, as discussed, results in less diffraction in the lateral directions. This, of course, further confines the lateral extent of the volume of the photosensitive layer which is rendered transparent to a region immediately below the aperture 26 of the master disc 18.

The end result is an image, in the photosensitive layer 27, of the aperture 26 having significantly less distortion than that which would be obtained were the photosensitive layer 27 not to possess the optical properties required by the present invention.

Figure 6 shows the identical arrangement of Figure 2 with the sole exception that the layer of photosensitive material 23 has been replaced by a photosensitive layer 27 having optical properties in accordance with the present invention. The results of the contact printing operation are qualitatively represented by the broken lines 30. It is seen that even where intimate contact is lost (around apertures 1 9a and 19b) the resulting image in the photosensitive layer 27 does not have the distorted shape shown in Figure 2. While less than perfect contact still exists, the effects of such imperfect contact have been minimized and aperture size distortion kept to an acceptable level, thereby permitting duplicate video discs to be mass produced from a master video disc by contact printing.

As a practical matter, since the duplicate video disc is for consumer use, a photosensitive material is needed at a reasonable cost having optical properties in accordance with the present invention. One such material having these optical properties is a diazo film. The diazo film contains a diazonium salt which is highly absorptive of actinic radiation in the 400 nm range (blue light) and a coupler whose dye absorbs at longer wavelengths. Each diazonium salt molecule, however, is photolyzed by light which it absorbs to give material which does not absorb in this range. Once the particular diazonium salt molecule has absorbed a photon and decomposed, it is, in effect, rendered transparent to other photons. Those portions of the diazo film which are exposed with radiation of a relatively high level therefore become transparent to the exposing radiation at a relatively rapid rate. On the other hand, portions of the diazo film which receive low exposure levels become transparent at a proportionately slower rate. The diazo film appears yellow to the eye (blue light is absorbed) before exposure to actinic radiation. Those portions of the diazo film exposed to actinic radiation (blue light) become transparent to such radiation, thereby producing the angular confinement of diffracted light and the evolving aperture effect (described above) of the present invention. In the case of a diazo film, processing by exposure to ammonia (or other alkaline material) produces a sharp increase in the visual contrast between the unexposed yellow areas and the exposed transparent areas by causing unexposed diazonium salt molecules and coupler molecules to react and form a relatively dense visual dye. For example, the visual dye may be one which absorbs strongly at a wavelength of 633 nanometres, which is produced by a He-Ne laser.

One two-component diazo material which has worked well has the following formulation: Cellulose acetate butyrate 70.0g 5-Sulphosalicylic Acid 1.4g 3-(o-Tolylcarbamoyl)-2-naphthol 5.88g 4-Diethylaminobenzenediazonium 4.62g tetrafluoroborate Acetone 780.2g The above diazo material was exposed through a master video disc to a carbon arc lamp, and developed by heating at 80"C in the presence of damp ammonia. The resulting duplicate video disc had substantially the same signal-to-noise ratio as the master video disc.

Other two-component diazo materials may be used, as long as the diazonium salt is sensitive to and absorptive of radiation which is being used in the contact printing process, and the coupler forms an azo dye which absorbs radiation used in retrieving information from the duplicate video disc.

Of course, a one-component diazo material may be used if wet or semi wet processing is available.

Also, heat processable diazo materials may be used.

Further examples of one and two component diazo materials and heat processable diazo materials are set forth in Kosar, Light Sensitive Systems. New York: John Wiley, 1965, on pages 194 to 320.

Other photographic systems which possess the desired optical properties include dye bleach and photochromic systems. Both offer high initial absorption in the wavelength region at which they are sensitive. In dye bleach systems, photogenerated excited species are bleached by added materials, e.g. 1-allyl-2-thiourea, to give products transparent in the initially absorbing wavelength region. Photochromic systems photoisomerize upon exposure to products absorbing in a different spectral region from the starting material.

As stated above, a master video disc for use in video disc duplication typically employs a thin bismuth coating on a support, the coating containing a distribution of tiny holes which have been machined by a laser. Such a coating is susceptible to damage when pressed in intimate contact with a duplication material during the contact printing process. Trapped dust or dirt may scratch the layer; also, the mere rubbing of the duplication material on the master video disc can cause the bismuth layer to peel off its substrate. It is apparent, therefore, that conventional master video discs are not well suited for high volume contact printing applications.

By virtue of the present invention, a master video disc, having video information in a bismuth coating or otherwise, may be provided with a protective overcoat, thereby preventing the intimate contact which the prior art has found so necessary. Referring to Figure 7, a master video disc 29 comprises a substrate 31 having an image bearing layer 32 coated thereon. The size and spacing of apertures, such as apertures 33a, 33b and 33c, define the video information content of the image bearing layer 32. The image bearing layer 32 may assume various forms, one of which is the laser-machined bismuth layer discussed above.

Coated over the image bearing layer 32 is a surface layer 34. The surface layer 34 is relatively thin, less than a micron in thickness, yet comprised of a durable material able to withstand the harsh environment encountered in the contact printing process. The thickness of the surface layer 34 depends upon the particular application. Some applications require a thickness less than half a micron while others permit a thickness of 5 microns. A duplication material, like the duplication material 28 of Figures 4 and 5, may be placed in direct contact with the master video disc 29, without causing any harm to image bearing layer 32.

In addition to being durable, the surface layer 34 of the master video disc 29 should preferably adhere tightly to the image bearing layer 32 in order to prevent separation of the two layers during use. Depending upon the materials chosen for the surface layer 34, a sub-layer (not shown) may be used between the surface layer 34 and the image bearing layer 32 to promote adhesion.

If the surface layer 34 is also compliant, dirt and dust (or other foreign objects) pressed between the master video disc and the replicate video disc during contact printing will become embedded in the surface layer 34. While such a property may appear to be a disadvantage, such is not necessarily the case. For example, if the surface layer 34 can be stripped from the master video disc 29, and a fresh surface layer coated thereon in its place, the compliant surface layer 34 serves to "clean up" the contact printing operation, at least insofar as keeping such foreign objects from the replicate video discs.

It may further be desirable to use a material for the surface layer 34 which is conductive. As a result of its conductivity, such as surface layer 34 would not exhibit the electrostatic attraction to dust seen in some non-conductive materials.

WHAT WE CLAIM IS: 1. A method of photographically producing a duplicate disc from a master record having information encoded thereon in the form of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said method comprising the steps of 1) superimposing the master record on a duplication material, the duplication material comprising a photosensitive material having an absorption of actinic radiation which, under the influence of such actinic radiation, decays to a lower level, and 2) exposing said photosensitive material to actinic radiation through said optical apertures.

2. A method according to claim 1 wherein said photosensitive material is a diazo material comprising a diazonium salt and a dye coupler.

3. A method according to claim 2 comprising the further step of developing said photosensitive material by exposure to an alkaline material.

4. A method according to claim 1, 2 or 3 wherein the information is video information.

5. A method according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

6. A disc comprising a layer containing information in the form of a distribution of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said layer comprising a developed photosensitive

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. Further examples of one and two component diazo materials and heat processable diazo materials are set forth in Kosar, Light Sensitive Systems. New York: John Wiley, 1965, on pages 194 to 320. Other photographic systems which possess the desired optical properties include dye bleach and photochromic systems. Both offer high initial absorption in the wavelength region at which they are sensitive. In dye bleach systems, photogenerated excited species are bleached by added materials, e.g. 1-allyl-2-thiourea, to give products transparent in the initially absorbing wavelength region. Photochromic systems photoisomerize upon exposure to products absorbing in a different spectral region from the starting material. As stated above, a master video disc for use in video disc duplication typically employs a thin bismuth coating on a support, the coating containing a distribution of tiny holes which have been machined by a laser. Such a coating is susceptible to damage when pressed in intimate contact with a duplication material during the contact printing process. Trapped dust or dirt may scratch the layer; also, the mere rubbing of the duplication material on the master video disc can cause the bismuth layer to peel off its substrate. It is apparent, therefore, that conventional master video discs are not well suited for high volume contact printing applications. By virtue of the present invention, a master video disc, having video information in a bismuth coating or otherwise, may be provided with a protective overcoat, thereby preventing the intimate contact which the prior art has found so necessary. Referring to Figure 7, a master video disc 29 comprises a substrate 31 having an image bearing layer 32 coated thereon. The size and spacing of apertures, such as apertures 33a, 33b and 33c, define the video information content of the image bearing layer 32. The image bearing layer 32 may assume various forms, one of which is the laser-machined bismuth layer discussed above. Coated over the image bearing layer 32 is a surface layer 34. The surface layer 34 is relatively thin, less than a micron in thickness, yet comprised of a durable material able to withstand the harsh environment encountered in the contact printing process. The thickness of the surface layer 34 depends upon the particular application. Some applications require a thickness less than half a micron while others permit a thickness of 5 microns. A duplication material, like the duplication material 28 of Figures 4 and 5, may be placed in direct contact with the master video disc 29, without causing any harm to image bearing layer 32. In addition to being durable, the surface layer 34 of the master video disc 29 should preferably adhere tightly to the image bearing layer 32 in order to prevent separation of the two layers during use. Depending upon the materials chosen for the surface layer 34, a sub-layer (not shown) may be used between the surface layer 34 and the image bearing layer 32 to promote adhesion. If the surface layer 34 is also compliant, dirt and dust (or other foreign objects) pressed between the master video disc and the replicate video disc during contact printing will become embedded in the surface layer 34. While such a property may appear to be a disadvantage, such is not necessarily the case. For example, if the surface layer 34 can be stripped from the master video disc 29, and a fresh surface layer coated thereon in its place, the compliant surface layer 34 serves to "clean up" the contact printing operation, at least insofar as keeping such foreign objects from the replicate video discs. It may further be desirable to use a material for the surface layer 34 which is conductive. As a result of its conductivity, such as surface layer 34 would not exhibit the electrostatic attraction to dust seen in some non-conductive materials. WHAT WE CLAIM IS:

1. A method of photographically producing a duplicate disc from a master record having information encoded thereon in the form of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said method comprising the steps of 1) superimposing the master record on a duplication material, the duplication material comprising a photosensitive material having an absorption of actinic radiation which, under the influence of such actinic radiation, decays to a lower level, and 2) exposing said photosensitive material to actinic radiation through said optical apertures.

2. A method according to claim 1 wherein said photosensitive material is a diazo material comprising a diazonium salt and a dye coupler.

3. A method according to claim 2 comprising the further step of developing said photosensitive material by exposure to an alkaline material.

4. A method according to claim 1, 2 or 3 wherein the information is video information.

5. A method according to claim 1 substantially as hereinbefore described with reference to the accompanying drawings.

6. A disc comprising a layer containing information in the form of a distribution of discrete optical apertures of varying sizes, a substantial portion of said apertures having a minimum dimension less than a micron, said layer comprising a developed photosensitive

material which has been exposed to actinic radiation and which has an absorption of actinic radiation which under the influence of such actinic radiation has decayed to a lower level.

7. A disc according to claim 6 wherein before development said photosensitive material comprised a diazonium salt and a dye coupler.

8. A disc according to claim 7 wherein said photosensitive material has been rendered light insensitive by processing with an alkaline material.

9. A disc according to claim 6, 7 or 8 comprising a reflective layer positioned adjacent said photosensitive material.

10. A disc according to claim 9 wherein said reflective layer is aluminium.

11. A disc according to any one of claims 6 to 10 which is flexible.

12. A disc according to any one of claims 6 to 11 wherein the information is video information.

13. A disc according to claim 6 substantially as hereinbefore described with reference to, and as shown in, Figures 1 and 4 to 6 of the accompanying drawings.