Machine-readable optical disc with reading-inhibit agent

0001This is a division of application Ser. No. 08/902,844, filed Jul. 30, 1997, now U.S. Pat. No. 6,011,772 which claims benefit to Provisional No. 60/026,390 filed Sep. 16, 1996.

BACKGROUND OF THE INVENTION

0002This invention relates to machine-readable optical discs of all types, including for example digital discs such as compact discs (CD's), digital video discs (DVD's), CDROM's, and the like.

0003Conventional optical discs have reached widespread acceptance as a low-cost, reliable storage medium for digital information including music, video, and data. One of the traditional advantages of optical discs as their long life.

0004However, in some applications, the long life of the conventional optical disc may represent a disadvantage. For example, if music, movies or software is to be made available for a limited time period, as in the rental, period for entertainment, the original optical disc must be returned at the end of the rental period.

0005A need presently exists for an improved machine-readable optical disc that eliminates the need for the return of an optical disc at the end of a rental period.

SUMMARY OF THE INVENTION

0006According to a first aspect of this invention, an optical disc comprising machine-readable, information-encoding features is provided with a barrier layer releasably coupled to the disc. This barrier layer is configured to prevent machine-reading of the disc. A reading-inhibit agent is included in the disc, and is activated by removal of the barrier layer. This reading-inhibit agent is operative, after it is activated, to alter the disc to inhibit reading of the disc. Both the barrier layer and the reading-inhibit agent can take many forms, as discussed by way of example below.

0007According to another aspect of this invention, an optical disc comprising machine-readable, information-encoding features is provided with a reading-inhibit agent that is activated by machine-reading the disc. This reading-inhibit agent is operative, after it is activated, to alter the disc to inhibit reading of the disc. In alternate embodiments, the reading-inhibit agent may be activated by optical radiation incident on the disc during machine-reading of the disc, or by rotation of the disc during machine-reading of the disc.

0008According to a third aspect of this invention, a method is provided for inhibiting reading of an optical disc. According to this method, an optical disc is provided comprising machine-readable, information-encoding features, and a reading-inhibit agent. The reading-inhibit agent is activated by optical radiation, and is operative, once activated, to alter the disc to inhibit reading. A reading device is provided to read the disc, and this reading device comprises a source of optical radiation. According to the method of this invention, the disc is read with the reading device, and the inhibit agent is concurrently activated with optical radiation from the source. The source of optical radiation that activates the reading-inhibit agent can either be the source of optical radiation that forms the reading beam, or a second source, separate from the reading beam source.

BRIEF DESCRIPTION OF THE DRAWINGS

0009<figref idref="DRAWINGS">FIGS. 1 through 3</figref> are partial cross-sectional views of three barrier layers suitable for use in embodiments of this invention.

0010<figref idref="DRAWINGS">FIGS. 4</figref>, <b>5</b>, <b>6</b>, <b>7</b> and <b>8</b> are partial cross-sectional views of optical discs that incorporate first, second, third; fourth, and fifth preferred embodiments of this invention, respectively.

0011<figref idref="DRAWINGS">FIGS. 9 and 10</figref> are plan views of optical discs that incorporate sixth and seventh preferred embodiments of this invention, respectively.

0012<figref idref="DRAWINGS">FIGS. 11 and 12</figref> are partial cross-sectional views of optical discs that incorporate eighth and ninth preferred embodiments of this invention, respectively.

0013<figref idref="DRAWINGS">FIG. 13</figref> is a plan view of an optical disc that incorporates a tenth preferred embodiment of this invention.

0014<figref idref="DRAWINGS">FIGS. 14 and 15</figref> are partial cross-sectional views of optical discs that incorporate embodiments of the invention employing galvanic cells.

0015<figref idref="DRAWINGS">FIG. 16</figref> is a partial cross-sectional view of a prior art compact disc.

0016<figref idref="DRAWINGS">FIG. 17</figref> is a partial cross-sectional view of a disc containing a reservoir.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

0017The present invention can be implemented in many different ways, and the following discussion will describe selected embodiments of the invention. These embodiments are intended as examples only, and not as an exhaustive list of all forms that the invention can take. Generally speaking, the embodiments discussed below can be classified into two groups. The first group uses a barrier layer to prevent premature activation of the reading-inhibit agent, while the second group does not use such a barrier layer.

0018In general, this invention can be used with the widest possible variety of optical discs comprising machine-readable, information-encoding features. <figref idref="DRAWINGS">FIG. 16</figref> shows a highly schematic cross section of an optical disc such as a prior art compact disc. <figref idref="DRAWINGS">FIG. 16</figref>, like all of the other figures, is not drawn to scale; selected features have been exaggerated in size for clarity of illustration. The disc of <figref idref="DRAWINGS">FIG. 16</figref> includes a substrate <b>10</b> which is formed with an array of information-encoding features such as pits <b>12</b>. The surface defining the information-encoding features <b>12</b> is covered with a reflective layer <b>14</b>, which may be, for example, formed of aluminum. The reflective layer <b>14</b> is in turn covered with a protective layer <b>16</b> which protects the reflective layer <b>14</b> from oxidation and physical damage. A reading beam aligned with the arrow <b>18</b> is incident on the surface of the substrate <b>10</b> opposite the information-encoding features <b>12</b>. This reading beam passes through the substrate <b>10</b>, is reflected by the reflective layer <b>14</b>, and then passes out through the substrate <b>10</b> for detection. Features <b>10</b>-<b>18</b> described above are completely conventional. As used herein, the term “information-encoding features” is intended broadly to encompass the widest possible range of such features, regardless of the particular encoding mechanism or reading beam interaction mechanism that is used.

0000Embodiments that Utilize a Barrier Layer

0019The following embodiments of the invention utilize a barrier layer to prevent activation of the reading-inhibit agent until the barrier layer has been removed. <figref idref="DRAWINGS">FIGS. 1-3</figref> show three different types of barrier layers that can be used. In <figref idref="DRAWINGS">FIGS. 1-3</figref>, the reference symbol <b>20</b> is used to depict the optical disc, which includes information-encoding features <b>22</b> on the upper surface of the disc, in the orientation shown in the figures. In the embodiment of <figref idref="DRAWINGS">FIG. 1</figref>, a barrier layer <b>24</b> is releasably secured (as for example with a suitable adhesive) adjacent the surface of the optical disc <b>20</b> that carries the information-encoding features <b>22</b>. In the embodiment of <figref idref="DRAWINGS">FIG. 2</figref>, the barrier layer <b>26</b> is releasably secured to the surface of the disc <b>20</b> opposite the surface that carries the information-encoding features <b>22</b>. In the embodiment of <figref idref="DRAWINGS">FIG. 3</figref>, the barrier layer <b>28</b> is formed as a closed package which completely seals the optical disc <b>20</b> from contact with ambient oxygen and moisture. In this case, there is no need for the barrier layer <b>28</b> to be adhesively secured to the disc <b>20</b>. As used herein, a barrier layer which is releasably coupled to an optical disc may be coupled adhesively as shown in <figref idref="DRAWINGS">FIGS. 1 and 2</figref>, coupled by enveloping the disc as shown in <figref idref="DRAWINGS">FIG. 3</figref>, or coupled in any other way that reliably associates the barrier layer and the disc prior to removal of the barrier layer.

0020As pointed out below, the reading-inhibit agent can take many forms and can be applied at many different places on the optical disc <b>20</b>. Depending upon the reading-inhibit agent used and its location, the position and physical and chemical characteristics of the barrier layer <b>24</b>, <b>26</b>, <b>28</b> can be selected as appropriate.

0021It is not essential in all applications that the barrier layer cover an entire surface of the disc <b>20</b>. If the reading-inhibit agent is localized to a particular portion of the disc, the barrier layer may cover only an area adjacent to and aligned with that portion. Preferably, the barrier layer should prevent machine-reading of the optical disc until it is removed.

0000Reading-Inhibit Agents that Disrupt Readability of the Optical Disc by Controlled Degradation of the Reflective Layer

0022A first type of reading-inhibit agent disrupts the reflectivity of the reflective layer in optically read discs to such an extent that the encoded data is rendered unusable. By disrupting the readability of the disc at a known time after the initial use of the disc, or after removal of the barrier layer, the practical usage lifetime of the disc can be limited and controlled.

0023The reflective layer <b>14</b> that is conventionally used in optical discs is typically formed as a thin film of metallic aluminum. This aluminum film can be corroded by exposure to an oxidizing environment to such an extent that the film no longer has sufficient reflectivity to support optical reading of the disc. For example, water and oxygen from the atmosphere can form a suitable oxidizing environment for such an aluminum film. The rate and timing of the corrosion of the aluminum film can be controlled by several approaches, including control of the concentration of an oxidizing species, control of the solution pH, introduction of dissimilar metal couples, and introduction of chemical species to control solubility of aluminum. For example, in the case where atmospheric oxygen is the oxidant, a porous polymer film may be placed over the aluminum film to provide known permeability characteristics for moisture and oxygen from the atmosphere as it migrates to the aluminum film. In this case, corrosion can be substantially prevented by a barrier layer such as the barrier layer <b>24</b> of <figref idref="DRAWINGS">FIG. 1</figref> or the barrier layer <b>28</b> of <figref idref="DRAWINGS">FIG. 3</figref> until the barrier layer is removed prior to initial reading of the optical disc.

0024A key feature of optically read discs is the use of a reflective layer <b>14</b> as described above to reflect light from the interrogating light source, generally a laser operating with a principal wavelength in the visible portion of the spectrum, to the detector. The reflective layer <b>14</b> is most generally composed of metallic aluminum which is deposited on to the information-encoding features by sputtering a very thin film. This thin film is approximately 55 nanometers in thickness in conventional compact discs.

0025Conventional reflective layers are subject to corrosion reactions involving oxidation of the metallic aluminum and subsequent formation of aluminum compounds such as hydroxy salts which are not reflective: <br />Al→Al⁺³+3e⁻(oxidation), <br />Al⁺³3OH⁻→Al(OH)₃(compound formation).

0026The oxidation of the aluminum metal is balanced by a reduction reaction such as the following: <br />O₂+2H₂O+2e⁻→4OH⁻(in neutral or alkaline solutions). <br />2H⁺2e⁻→H₂(in acidic solutions). <br /> The corrosion reaction typically involves an electrolyte film on the surface of the aluminum to form an ionic path between the oxidation and reduction sites on the aluminum surface. In the example of atmospheric oxygen, a film or layer of water on the surface is one suitable electrolyte. The rate of corrosion will be influenced by the availability of the oxidizing species (e.g. oxygen or hydronium, H+), the addition of soluble salts to influence the conductivity of the electrolyte, the addition of chlorides to alter the stability of the normally protective aluminum oxide film, pH buffers to influence the stability of the normally protective aluminum oxide layer or to influence the reduction reaction, or the addition of complexing agents to dissolve protective aluminum oxides or to keep aluminum corrosion products in solution. Such salts and other complexing agents may be deliberately added in a layer of material placed next to the aluminum layer. Addition of a hygroscopic material and salts to this layer can also aid in collecting atmospheric moisture for subsequent release as liquid water solution at the corrosion reaction site. The hygroscopic material or salts effectively lower the dew point of the aluminum surface, the relative humidity at which a liquid film forms on the metal surface.

0027Cupric and ferric chloride are specific examples of oxidizers that may be incorporated into an electrolyte layer next to the aluminum layer to accelerate corrosion of the aluminum. These materials offer several advantages. If the metal cation is reduced to the metallic state in the oxidation reaction, the metal (e.g. copper or iron) deposited on the aluminum surface forms local cathodes that can accelerate corrosion of aluminum in adjacent areas. If the metal cation is not completely reduced to the metallic state, the cuprous or ferrous species may react with oxygen to restore the oxidizing power of the solution.

0028<figref idref="DRAWINGS">FIG. 4</figref> shows one preferred embodiment of this invention which includes a substrate <b>10</b> and a reflective layer <b>14</b> as described above. In this case, an electrolyte layer <b>30</b> is applied adjacent to the reflective layer <b>14</b>. The electrolyte layer <b>30</b> contains substances which aid the corrosion reactions, such as hygroscopic salts, pH buffers, complexing agents for aluminum, and the like. The electrolyte layer <b>30</b> is in turn covered with an outer layer <b>32</b> of a material which is permeable to environmental moisture and oxygen. The permeable layer <b>32</b> is in turn initially covered by a barrier layer <b>24</b> as described above. The barrier layer <b>24</b> prevents oxygen and water from reaching the permeable layer <b>32</b> during storage and transport. When a user wishes to read information from the optical disc of <figref idref="DRAWINGS">FIG. 4</figref>, the user removes the barrier layer <b>24</b>. Oxygen and water vapor from the atmosphere then diffuse through the permeable layer <b>32</b> at a controlled rate. The water vapor can be, for example, collected by hygroscopic materials in the electrolyte layer <b>30</b>, and subsequently made available to aid in the aluminum corrosion reactions discussed above.

0029Based on typical corrosion rates for aluminum, and an assumed reflective layer thickness of 55 nanometers, the reflective layer may be degraded adequately to prevent machine-reading of the optical disc in, for example, 1 to 100 hours after removal of the barrier layer <b>24</b>, depending upon the availability of moisture, and the parameters of the electrolyte layer <b>30</b> and the permeable layer <b>32</b>.

0030Table 1 illustrates the relationship between the corrosion rate i_corr, the rate of aluminum film removal L, and the time t_{(55 nm)}to corrode 55 nanometers of aluminum. In Table 1, L is estimated using Farady's law.

0031<tables id="TABLE-US-00001" num="00001"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="105pt" align="center" /><colspec colname="2" colwidth="28pt" align="center" /><colspec colname="3" colwidth="84pt" align="center" /><thead><row><entry namest="1" nameend="3" rowsep="1">TABLE 1</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row><row><entry>i_corr</entry><entry>L</entry><entry>t_{(55 nm)}</entry></row><row><entry>(uA/cm2)</entry><entry>(nm/hr)</entry><entry>(Hours)</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="1" colwidth="105pt" align="char" char="." /><colspec colname="2" colwidth="28pt" align="char" char="." /><colspec colname="3" colwidth="84pt" align="char" char="." /><tbody valign="top"><row><entry>0.1</entry><entry>0.1</entry><entry>442.3</entry></row><row><entry>1</entry><entry>1.2</entry><entry>44.2</entry></row><row><entry>10</entry><entry>12.4</entry><entry>4.4</entry></row><row><entry>100</entry><entry>124.4</entry><entry>0.4</entry></row><row><entry namest="1" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0032If desired, metallic films or pieces of a more noble metal (for example a metal such as copper or silver, or carbon) can be placed in electrical contact with an aluminum reflecting layer <b>14</b> and with an electrolyte layer <b>30</b> containing oxygen as described above or other suitable oxidizing species. In this case the galvanic couple due to the presence of the more noble element will result in more rapid and directed corrosion of the aluminum reflecting layer <b>14</b> than would otherwise occur in the absence of that second, more noble element.

0033Additionally, if desired the reflective layer <b>14</b> can be sputter-coated in such a manner that the reflective layer <b>14</b> itself includes more noble elements such as copper in the reflective film itself. The aluminum alloy film will have a higher corrosion rate than a purer aluminum film due to the formation of localized cathodes at the sites of the more noble elements.

0034<figref idref="DRAWINGS">FIG. 14</figref> is a schematic view of an optical disc <b>80</b> which includes an aluminum layer <b>82</b> and a copper layer <b>84</b>, separated by an electrolyte layer <b>86</b>. The metal layers <b>82</b>, <b>84</b> may be configured for example as a conventional two-sided DVD to encode information, and the copper layer <b>84</b> provides sufficient reflectivity for conventional reading. The metal layers <b>82</b>, <b>84</b> are connected electrically in any convenient manner, for example by a metal foil <b>88</b> or a conductive adhesive (e.g. an epoxy filled with carbon, silver or copper particles). The three layers <b>82</b>, <b>84</b>, <b>86</b> and the foil <b>88</b> form a galvanic cell, in which the aluminum layer <b>82</b> is the anode that corrodes relative to the more noble metal. The electrolyte layer <b>86</b> provides ionic continuity between the layers <b>82</b>, <b>84</b>, while the foil <b>88</b> provides electronic contact.

0035<figref idref="DRAWINGS">FIG. 15</figref> shows an optical disc <b>80</b>′ that is similar to the disc <b>80</b> of FIG. <b>14</b>. Primed reference numerals are used in <figref idref="DRAWINGS">FIG. 15</figref> for elements corresponding to elements <b>82</b>-<b>88</b> of FIG. <b>14</b>. In <figref idref="DRAWINGS">FIG. 15</figref> the area of the copper layer <b>84</b>′ is greater than the area of the aluminum layer <b>82</b>′ to increase the aluminum corrosion rate. Also, openings <b>90</b>′ are provided through the copper layer <b>84</b>′ and the adjacent polycarbonate layer <b>92</b>′ to further increase the aluminum corrosion rate. Preferably, the openings <b>90</b>′ are located in an area of the disc <b>80</b>′ not containing stored information, such as the central portion of the disc <b>80</b>′.

0036As shown in <figref idref="DRAWINGS">FIG. 5</figref>, it is not essential in all embodiments that atmospheric oxygen and water be used as the oxidizing species. For example, as shown in <figref idref="DRAWINGS">FIG. 5</figref>, microcapsules <b>34</b> can be provided between the barrier layer <b>24</b> and the permeable layer <b>36</b>. These microcapsules can contain any suitable oxidizing species and electrolyte. In this example removal of the barrier layer <b>24</b> ruptures at least some of the microcapsules <b>34</b>, thereby releasing electrolyte and oxidant into the permeable layer <b>36</b>. The electrolyte and oxidant migrate through the permeable layer <b>36</b> and come into contact with the reflective layer <b>14</b> in order to initiate a controlled corrosion process. This embodiment is less sensitive to the availability of atmospheric moisture than the embodiment of FIG. <b>4</b>.

0037From the foregoing it should be apparent that the reading-inhibit agent can take many forms, including electrolytes, oxidizing species, various elements more noble than the reflective metal, and permeable films that control the rate at which atmospheric oxygen and water reach the reflective layer. In various embodiments the inhibit agent can take the form of films, or it can be contained in various ways, including by use of microcapsules.

0038The following paragraphs detail test results related to the use of hygroscopic salts, placed on an aluminum surface, to pick up water from the atmosphere and form an electrolyte film. The hygroscopic salts may be sufficiently corrosive by themselves, or alternately they may be used in conjunction with other salts and complexing agents to provide the desired aluminum removal rate. The salts are preferably applied in the anhydrous form to the surface, and are then protected by a barrier to exclude moisture from the salts. Activation of the corrosion process occurs when the barrier is removed.

0039The corrosion approach is based on the principle that a dry salt will come to equilibrium with its environment. In the process of coming to equilibrium, the salt can either dissolve, to form an electrolyte solution, or become drier. Table 1a lists the humidity above saturated solutions of several salts in a closed environment. If the salt is placed in air with higher humidity than the table value, it will pick up water. If the humidity is lower than the table value, the solution will lose water. The salts used in this application include magnesium chloride and quaternary ammonium amine chlorides.

0040<tables id="TABLE-US-00002" num="00002"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 1a</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>Humidity Above Saturated Solutions of Various Salts</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="28pt" align="left" /><colspec colname="1" colwidth="77pt" align="left" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="91pt" align="center" /><tbody valign="top"><row><entry /><entry>Solid Phase</entry><entry>t° C.</entry><entry>% Humidity</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="28pt" align="left" /><colspec colname="1" colwidth="77pt" align="left" /><colspec colname="2" colwidth="21pt" align="center" /><colspec colname="3" colwidth="91pt" align="char" char="." /><tbody valign="top"><row><entry /><entry>H₃PO₄•1/2H₂O</entry><entry>24</entry><entry>9</entry></row><row><entry /><entry>LiCl•H₂O</entry><entry>20</entry><entry>15</entry></row><row><entry /><entry>KC₂H₃O₂</entry><entry>20</entry><entry>20</entry></row><row><entry /><entry>Pb(NO₃)₂</entry><entry>20</entry><entry>98</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0041Lithium chloride and potassium acetate were tested as the candidate salts. To these, either potassium hydroide (KOH) or trisodium phosphate (TSP) were added to increase the aggressiveness of the electrolyte. Placement of dilute solutions of either KOH or TSP on the disc surface quickly dissolved the aluminum film. With these aggressive salts, complexing agents, such as citrate, were not needed to remove any passive films on the aluminum.

0042Further, tests were conducted by placing the salts onto the unprotected aluminum layer of CDs. Some of the CDs were then left exposed to room air while others were placed in desiccators with relative humidities of 20% and 8.5%. The relative humidities in the desiccators were controlled by solutions of sulfuric acid; the specific gravity of the sulfuric acid solution was selected to provide the desired relative humidity. During these experiments, ambient relative humidities ranged from 20 to 30 percent. Four salts were used: potassium acetate (KAc), lithium chloride (LiCl), KOH, and TSP and were mixed as shown in Table 1b: The concentration of salt in the solution on the disc surface depended on the amount of water that was absorbed.

0043<tables id="TABLE-US-00003" num="00003"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="1"><colspec colname="1" colwidth="217pt" align="center" /><thead><row><entry namest="1" nameend="1" rowsep="1">TABLE 1b</entry></row></thead><tbody valign="top"><row><entry namest="1" nameend="1" align="center" rowsep="1" /></row><row><entry>Salts Mixtures</entry></row></tbody></tgroup><tgroup align="left" colsep="0" rowsep="0" cols="4"><colspec colname="offset" colwidth="14pt" align="left" /><colspec colname="1" colwidth="63pt" align="left" /><colspec colname="2" colwidth="56pt" align="center" /><colspec colname="3" colwidth="84pt" align="center" /><tbody valign="top"><row><entry /><entry>SALT</entry><entry>TSP</entry><entry>KOH</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row><row><entry /><entry>KAc (4 grams)</entry><entry>1.31 g or .13 g </entry><entry>0.58 g or 0.06 g</entry></row><row><entry /><entry>LiCl (4 grams)</entry><entry>1.31 g or 0.13 g</entry><entry>0.58 g or 0.06 g</entry></row><row><entry /><entry namest="offset" nameend="3" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0044When LiCl was placed on the disc's aluminum surface under ambient conditions, droplets of water formed on the salt mass within 30 minutes; with KAc it took 3 hours. The water droplets formed with LiCl were clearly visible to the unaided eye; the droplets formed with KAc could be observed with the use of a magnifying glass. After these samples were allowed to stand overnight, the aluminum with LiCl showed partial corrosion, while the aluminum with KAc was intact.

0045The tests also showed that KOH alone was highly hygroscopic and corroded the discs under all conditions. Within the limitations of existing equipment under the driest conditions KOH corroded the aluminum surface in all tests. The water retained in the KOH was sufficient to corrode the aluminum surface, even when a glove bag was used to apply the KOH, and a dry desiccator was used to store the sample.

0046At 20% RH, the LiCl (alone and in mixtures) continued to form water droplets on the disc surface and to attack the aluminum. In the 8.5% RH desiccator, visible water droplets did not form, in agreement with the table values.

0047TSP did not attack the aluminum when placed on the surface by itself, even under ambient conditions. TSP was not sufficiently hygroscopic to form an aggressive electrolyte film. However, when used in conjunction with LiCl at 20% RH, enough water was picked up to form an aggressive solution, which attacked the aluminum. A mixture of LiCl and TSP did not attack the aluminum in the 8.5% RH desiccator (no breakthrough after four days).

0048These tests demonstrated that the corrosion process can be activated by ambient moisture down to at least 20% relative humidity, and probably down to 15% based on published values for LiCl. Other salts or drier KOH may allow one to go to even lower humidities.

0000Reading-Inhibit Agents that Operate by Absorbing Optical Radiation of the Reading Beam

0049The digital video disk (DVD) format uses a 650 nm laser to read information from the disk. If this reading beam is absorbed to a significant degree, the return signal from the disk is attenuated. By including a light-absorbing material in the disk, it is possible to attenuate the reading signal enough to prevent the disk from being read. Preferably, the light-absorbing material is strongly absorbing at the wavelength of the reading beam. Many compounds absorb at 650 nm, and they usually appear blue or green in color.

0050In order to allow the disc to be read on its first use, the light-absorbing material is initially nonabsorbent at the wavelength of the reading beam. Over time, for example four to 24 hours, this light-absorbing material becomes absorbing at the wavelength of the reading beam in response to some environmental stimulus. One approach is to use a compound for the light-absorbing material that is initially colorless, but which oxidizes to a new compound which is colored upon exposure to oxygen in the atmosphere, or some other oxidant. Many compounds are known which exhibit this behavior. Four compounds which may be particularly appropriate are given in Table 2 (in their oxidized form).

0051<tables id="TABLE-US-00004" num="00004"><table frame="none" colsep="0" rowsep="0"><tgroup align="left" colsep="0" rowsep="0" cols="3"><colspec colname="offset" colwidth="35pt" align="left" /><colspec colname="1" colwidth="56pt" align="left" /><colspec colname="2" colwidth="126pt" align="center" /><thead><row><entry /><entry namest="offset" nameend="2" rowsep="1">TABLE 2</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row><row><entry /><entry>Compound</entry><entry>Color Index Number</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></thead><tbody valign="top"><row><entry /><entry>Indigo Carmine</entry><entry>73015</entry></row><row><entry /><entry>Methylene Blue</entry><entry>52015</entry></row><row><entry /><entry>Thionin</entry><entry>52000</entry></row><row><entry /><entry>Gallocyanine</entry><entry>51030</entry></row><row><entry /><entry namest="offset" nameend="2" align="center" rowsep="1" /></row></tbody></tgroup></table></tables>

0052The colorless precursor to the light-absorbing material is incorporated in the optical disc somewhere along the path taken by the laser light of the reading beam. For instance, the colorless precursor can be compounded within the material (typically polycarbonate) that makes up the substrate <b>10</b>, or the colorless precursor can be included in a coating on a surface of the substrate <b>10</b>.

0053Preferably, the rate at which atmospheric oxygen reaches the colorless precursor is controlled in order to render the optical disc unreadable at a selected time after the barrier layer is removed. The rate at which oxygen reaches the colorless precursor should be selected such that the optical disc can be read at least once before sufficient color is generated to make the optical disc unreadable. The rate at which oxygen reaches the colorless precursor should be high enough to ensure that the optical disc becomes unreadable within the desired time period (for example four to 24 hours). Various methods can be used to control the rate at which oxygen reaches the colorless precursor. If the light-absorbing compound is contained within the body of the substrate <b>10</b>, the amount of the absorbing compound can be adjusted as appropriate for the application; higher loadings will result in quicker obscuration. The rate at which the absorbing compound becomes absorbing to the reading beam can be lowered by lowering the concentration of the absorbing compound in the substrate, or by applying an outer coating to the substrate which acts as a semipermeable oxygen barrier.

0054Alternately, the absorbing compound can be placed as shown in <figref idref="DRAWINGS">FIG. 6</figref> in a layer <b>38</b> on a surface of the substrate <b>10</b>. The rate of the oxidation reaction can be controlled in this case by choosing a matrix such as a suitable polymer for the absorbing compound layer having the appropriate barrier properties. Alternately, an additional coating layer can be employed over the absorbing layer, and this additional coating can act as a semipermeable oxygen barrier which allows oxygen to reach the absorbing layer at the desired rate.

0055As shown in <figref idref="DRAWINGS">FIG. 6</figref>, a barrier layer <b>26</b> is used to protect the absorbing layer <b>38</b> from atmospheric oxygen during storage and transport. The barrier layer can also take the form of an air-tight package, as shown in FIG. <b>3</b>.

0000Reading-Inhibit Agents that Operate by Altering Physical Dimensions of the Optical Disc

0056Certain embodiments of the invention use a reading-inhibit agent which alters its physical dimension when activated. A superabsorbing polymer is one such material, for example a polymer or copolymer containing a carboxylic or alcohol moiety. For example, a water-absorbent resin may be formed from a cross-linked polymer or a copolymer of acrylic acid, methacrylic acid, methylacrylate-vinylacetate, starch-ethyl acrylate, starch-acrylonitrile, carboxymethyl cellulose, ethylene oxide, vinyl alcohol, acrylamide, and the like.

0057Such materials can be used in several ways to make an optical disc unreadable, for example as the material absorbs ambient moisture. The absorption of such moisture creates a volume change in the material, which can be used to cause a combination of any of the following effects to prevent reading: delamination, a change in the refractive index, or a change in spinning characteristics.

0058For example, as shown in <figref idref="DRAWINGS">FIG. 7</figref>, a superabsorber layer <b>42</b> can be placed between two digital video disc substrates <b>40</b>. The entire digital video disc is then protected with an encapsulating barrier layer <b>28</b> similar to that shown above in FIG. <b>3</b>. When the barrier layer <b>28</b> is removed, ambient moisture is allowed gradually to reach the superabsorber layer <b>42</b>. As the superabsorber layer absorbs moisture, it will increase in volume, thereby causing the digital video disc to delaminate and preventing further reading of the disc.

0059In the example of <figref idref="DRAWINGS">FIG. 8</figref>, a superabsorber layer <b>44</b> is placed on the readable surface of a digital video disk <b>40</b>, and this superabsorber layer is protected by a barrier layer <b>26</b>. When the barrier layer <b>26</b> is removed, the superabsorber layer <b>44</b> will absorb ambient moisture and increase in volume. This volume increase causes a significant change in the refractive index of the material, which renders the digital video disc unreadable.

0060As shown in <figref idref="DRAWINGS">FIG. 9</figref>, a superabsorber layer <b>48</b> may be placed either partially or completely around a spindle hole <b>46</b> of the digital video disk <b>40</b>. This superabsorber layer <b>48</b> is protected by a barrier layer (not shown in <figref idref="DRAWINGS">FIG. 9</figref>) prior to use. When the barrier layer is removed, ambient moisture will gradually cause the superabsorber layer <b>48</b> to expand. If the superabsorber layer <b>48</b> is placed as shown in <figref idref="DRAWINGS">FIG. 9</figref>, this can cause the spindle hole <b>46</b> to assume an eccentric position, thereby rendering the optical disc unreadable. Alternately, if the superabsorber layer <b>48</b> extends substantially around the spindle hole <b>46</b>, the superabsorber layer <b>48</b> may expand to the point where the spindle bole <b>46</b> is too small to fit on the spindle of the reading device.

0061<figref idref="DRAWINGS">FIG. 10</figref> shows another embodiment in which the superabsorber layer <b>50</b> is mounted near the outer rim of the digital video disk <b>40</b>. As before, the superabsorber layer <b>50</b> is initially protected by a barrier layer (not shown in FIG. <b>10</b>). Once the barrier layer is removed, the superabsorber layer <b>50</b> absorbs atmospheric moisture, thereby rendering the disc sufficiently out of balance to prevent reliable reading.

0062In all of the examples discussed above, the rate at which the super-absorber layer absorbs moisture can be modified by placing a semipermeable barrier over the exposed surface of the superabsorber layer. This barrier can regulate the diffusion of ambient moisture to the superabsorber layer, which in this way controls the time period during which the optical disc is readable after the barrier layer has been removed.

0000Reading-Inhibit Agents that Operate by Scattering the Reading Beam

0063As discussed above, a laser beam is typically used as a reading beam for optical discs. If the reading beam is scattered or otherwise attenuated to a significant degree, the disc cannot be accurately read. For example, as shown in <figref idref="DRAWINGS">FIG. 11</figref>, a digital video disc <b>40</b> can be provided with a layer <b>52</b> that includes a material such as a solvent that will alter the optical characteristics of the adjacent portion of the digital video disc <b>40</b>. For example, a polycarbonate exposed to solvent is known to craze, i.e. to form a diffuse, opaque film or layer, which scatters the reading beam. Suitable solvents include organic liquids or vapors such as acetone, xylene and the like. Depending upon the concentration of the solvent and the exposure time, various rates of loss of transparency can be obtained. Other coatings in addition to polycarbonates can exhibit the same effective behavior by slight dissolution in an organic solvent followed by deposition on the surface of the disc as the solvent evaporates or is lost. The redeposition process may also include a recrystalization of a glassy coating layer. This redeposition results in a less transparent and therefore less readable surface on the disc. The layer <b>52</b> of <figref idref="DRAWINGS">FIG. 10</figref> can include microencapsulated solvent beads which will rupture on removal of the adjacent barrier layer <b>26</b>.

0000Embodiments that Include Reading-Inhibit Agents without Barrier Layers

0064As pointed out above, it is not essential in all embodiments that a barrier layer be included. Rather, in some embodiments it is the act of reading the disc that activates the reading-inhibit agent. For example, optical radiation associated with disc reading, or rotation associated with disc reading can activate the reading-inhibit agent.

0065As shown in <figref idref="DRAWINGS">FIG. 12</figref>, one such embodiment includes an optical disc <b>54</b> which includes a reading-inhibit agent <b>56</b> adjacent one surface. In this case the reading-inhibit agent <b>56</b> is a photoactive material that, when activated by suitable optical radiation, is suitably changed in optical or physical characteristics so as to inhibit further reading of the disc. The photoactive material can alternately be dispersed in the bulk of the disk and can for example change from clear to opaque at the wavelength of the reading beam upon exposure to suitable optical radiation. As shown in <figref idref="DRAWINGS">FIG. 12</figref>, the disc <b>54</b> is installed in a reading device <b>58</b>. The reading device <b>58</b> includes a first optical source such as a laser <b>60</b> that directs the reading beam <b>62</b> against the disc <b>54</b>. Returning radiation from the disc <b>54</b> is sensed by a detector <b>64</b>, in the conventional manner. In this embodiment, the reading device <b>58</b> further includes a second optical source <b>66</b>. The second optical source <b>66</b> destroys or degrades the optical transmission or reflection required to read the disc. The second source <b>66</b> may be a conventional source such as a high pressure arc, an incandescent bulb, a fluorescent lamp, or a laser. As the disc <b>54</b> is read, radiation from the second source <b>62</b> interacts with the reading-inhibit agent <b>56</b> to inhibit further reading of that portion of the disc <b>54</b>. The second source <b>62</b> is arranged such that the second source <b>62</b> does not illuminate any portion of the disc <b>54</b> until after that portion of the disc <b>54</b> has been read by the reading beam <b>62</b>.

0066In alternate embodiments the reading beam <b>62</b> itself may initiate optical changes in the read inhibiting agent <b>56</b>, thereby dispensing with the need for the second source <b>62</b>.

0067Alternately, when the second source <b>62</b> is used, the need for a separate read inhibit agent <b>56</b> may be eliminated. In this case, the second source <b>66</b> may for example be a passively q-switched microchip laser focused on the surface of the disc. The effect of this laser is to create scattering centers by ablating the read surface of the disc. The scattering centers reduce the optical transmission of the disc to the reading beam <b>62</b>.

0068In either case, the second source <b>66</b> should be interlocked in a way that prevents consumer tampering, and should track in a way so as not to interfere with the initial reading of the disc. When the second source <b>62</b> is of sufficient power to provide the ablating action described above, access to the information on the disc will be denied almost immediately after it is read.

0069<figref idref="DRAWINGS">FIG. 13</figref> shows another embodiment having a reading-inhibit agent which is activated by the act of reading the disk. In this case an optical disc <b>70</b> includes a reservoir <b>72</b> that contains a reading-inhibit agent, such as a suitable solvent The reservoir <b>72</b> includes an opening <b>74</b>. When the disc is first rotated in order to be read, solvent passes out of the reservoir <b>72</b> via the opening <b>74</b>, and in this way a small quantity of solvent is released to the disc. The solvent can degrade the optical characteristics of the disc, as discussed above, to prevent reading of the disc a predetermined time after the solvent has left the reservoir. As one example, the reservoir <b>72</b> may be formed in a region bounded by two concentric annular ridges, similar to the stacking rings conventionally used in current optical discs.

0000Additional Embodiment

0070<figref idref="DRAWINGS">FIG. 17</figref> shows a cross-sectional view that illustrates one form of a disc <b>100</b> containing a reservoir <b>102</b> as discussed immediately above. One or more capillary-tube-sized passages <b>104</b> are radially oriented to allow a suitable reading-inhibit agent (such as a solvent or a corrosive agent as discussed above) to flow from the reservoir <b>102</b> radially outwardly to the region of the disc that stores information via information-encoding features. The reservoir <b>102</b> and the passage <b>104</b> are closed by a silicone membrane <b>108</b> that defines an array of vents <b>110</b>, <b>112</b>. In this example, the vents <b>110</b>, <b>112</b> are formed as pin pricks. The silicone membrane <b>108</b> is covered by a polycarbonate sheet <b>114</b> that defines vents on <b>116</b>, <b>118</b> aligned with the vents <b>110</b>, <b>112</b>, respectively.

0071A releasable, peel-off label <b>120</b> is removably secured by a suitable adhesive to the polycarbonate layer <b>114</b>. This peel-off label <b>122</b> includes a tab <b>122</b> to facilitate removal and a protrusion <b>124</b>. The protrusion <b>124</b> passes through an opening in the polycarbonate layer <b>114</b> and presses the silicone membrane <b>108</b> into the passage <b>104</b> to create a mechanical valve that stops the flow of reading-inhibit agent radially outwardly from the reservoir <b>102</b>. Optionally, the passage <b>104</b> may also include a valve element <b>106</b> of a material that is dissolved by the reading-inhibit agent. For example, a valve element <b>106</b> of aluminum can be used in cases where the reading-inhibit agent is corrosive to aluminum Preferably, the reservoir <b>102</b> includes a wick <b>103</b> made of cotton or microfiber to retain fluid in the reservoir <b>102</b>. The passage <b>104</b> may have a cross-sectional size of 0.02 inch.

0072Preferably, the peel-off label <b>120</b> is sized such that the label must be removed in order to allow the disc <b>100</b> to be read. Once the label <b>120</b> has been removed, the vents <b>110</b>, <b>112</b> are opened, and the protrusion <b>124</b> is removed. This allows the silicone membrane <b>108</b> to relax upwardly, thereby opening the passage <b>104</b>. When the disc <b>100</b> is rotated during a reading operation centrifugal force causes the reading-inhibit agent in the reservoir <b>102</b> to flow radially outwardly via the passage <b>104</b> onto the information-encoding portion of the disc <b>100</b>.

0073In some embodiments the reading-inhibit agent may be selected so as not to interfere with normal reading of the disc <b>100</b> until a selected time after the reading-inhibit agent has contacted the information carrying portion of the disc. As an alternative, when the optional valve element <b>106</b> is used, the valve element <b>106</b> prevents the reading-inhibit agent from reaching the information carrying portion of the disc <b>100</b> until the valve element <b>106</b> is dissolved by the reading-inhibit agent. In this way, the plug <b>106</b> provides a timed release of the reading-inhibit agent onto the information carrying portion of the disc.

0074Tests have shown that two-pass transmission of the disc typically must fall to about 45 percent of the original value before a significant number of reading errors occur, and to approximately 30 percent of the original value before the disc becomes unplayable.

0000Conclusion

0075The optical discs described above have a short effective life, limited either by the number of times the disc is played (e.g. one, two or more times), or by the passage of time after the disc is dispensed (e.g. a selected number of hours after the disc is sold or rented, after the consumer opens a package, or after the disc is inserted into a disc player). The effective life of the disc may be limited in response to reading of the disc, opening of the disc, or rotation of the disc. Various methods for limiting the effective life of the disc have been described, including physical, chemical, and electrochemical methods. Physical methods include the diffusion of air or a component of air such as oxygen, resulting in physical and/or chemical effects; the use of optical activation to cause a physical change in the disc; or the use of physical forces or the removal of forces associated with rotation of the disc or removal of a label to cause a physical change in the disc. Chemical methods include a layer of the disc interacting with a chemical applied when the package is opened or by the vendor at the time of sale. Electrical or electrochemical methods include the use of an electrochemically active system to accelerate corrosion.

0076It should be apparent from the foregoing detailed description that the present invention can be implemented in a wide variety of forms. Barrier layers can take the form of sheets or patches on a surface of the disc, or of encapsulating packaging. In some cases barrier layers are not required. Reading-inhibit agents can take many forms, including materials which change optical or physical characteristics of the reflecting layer, or various other components of the optical disc. Reading-inhibit agents can be employed as microencapsulated materials, materials formed in layers over selected regions of a disc, or materials incorporated into other components of a disc. Reading-inhibit agents may extend over the entire information-encoding surface of the optical disc, or alternately may be limited to selected portions, for example portions that encode indexing or other introductory information.

0077It should therefore clearly be understood that the foregoing detailed description is intended by way of illustration, not limitation. It is only the following claims, including all equivalents, that are intended to define the scope of this invention.