LABEL INDICATING AGEING OR DEGRADATION FOR OPTICAL DISC OR OPTICAL DISC ENCLOSURE

The invention relates to a label for an optical disc or an optical disc enclosure, and in particular to a method and device for providing a visual indication relating to the state of degradation of an optical disc.

Optical disc technology is extensively used nowadays as a data storage medium. One major disadvantage of using an optical disc as a data storage medium is the uncertainty over the lifetime of an optical disc.

There are three basic types of optical disc, these being CD, DVD and BD (Blu-Ray) discs, each type further having ROM, R, and RW formats. Each of these disc types uses a different data layer material (for example, moulded aluminum, silver mirror, organic dye, or phase-changing film, respectively). Assuming a disc is physically handled properly during use, it is the deterioration of these materials that is the primary cause for disc degradation and, ultimately, "end-of-life" for the disc. With CDs and DVDs, the user does not notice early degradation because the error detection and correction capability that is built into the system corrects a certain number of errors. The user notices a problem only when the error correction coding is unable to fully correct the errors.

One method for determining the end-of-life for a disc is based on the number of errors on a disc before the error correction occurs. The chance of disc failure increases with the number of errors, but it is impossible to define the number of errors in a disc that will absolutely cause a performance problem (minor or catastrophic) because it depends on the number of errors that remain after error correction, and their distribution within the data. When the number of errors (before error correction) on a disc increases to a certain level, the chance of disc failure, even if small, can be deemed unacceptable and thus signal the end-of- life of the disc.

The longevity of a ROM disc is determined by the extent to which the aluminum layer of the disc is exposed to oxygen. Oxygen, including pollutants, can migrate through the polycarbonate layer or the hard lacquer layer (CD label side and edge), carried in

by moisture. Oxygen or moisture can penetrate more easily through scratches, cracks, or delaminated areas in the label. Oxygen can also be trapped inside the disc during manufacturing.

If a disc is left in a very humid environment, moisture and oxygen will eventually reach the aluminum, causing it to lose its reflectivity. This is because the normally shiny aluminum, which resembles silver, becomes oxide-dull and much less reflective. The combination of high humidity and increased temperatures will accelerate the oxidation rate.

The life expectancy of a ROM disc therefore depends on the environmental conditions to which it is exposed over time. Generally, it is best to keep ROM discs in a dry, cool environment.

For "write once" discs (R-discs) that cannot be erased by CD or DVD drives, the mirror normally consists of silver. Silver is susceptible to corrosion if exposed to sulphur dioxide, which is an air pollutant that can penetrate the disc in the same way oxygen can with moisture. Similarly, silver also corrodes when exposed oxygen or moistures. R-discs use a dye-based layer (organic dye) for recording data. The organic dye used in the data layer of R-discs degrades naturally, but slowly over time. High temperatures and humidity will accelerate the process. Prolonged exposure to UV light can degrade the dye properties and eventually make the data unreadable. Heat build up within the disc, caused by sunlight or close proximity to heated light sources, will also accelerate dye degradation.

Manufacturers claim that CD-R and DVD-R discs have a shelf life of 5 to 10 years before recording, but no expiration dates are indicated on CD-R, DVD-R, or DVD+R packaging, nor are there published reports of tests to verify these claims.

With regard to rewriteable discs such as RW and RAM discs, these are generally not considered for long-term or archival use, and life expectancy tests are seldom performed for this type of medium. Rewritable discs use a phase-changing metal alloy film for recording data, and aluminum or silver for the reflective layer. The alloy film is not as stable as the dye used in R-discs because the material normally degrades at a faster rate. In RW discs one naturally also finds degradation of the reflecting silver or aluminum layers. The phase-changing film is affected primarily by heat, but also by ultraviolet

(UV) light in the aging process. The combination of high temperature and UV light may further accelerate the aging process.

The data on the phase-changing metal alloy film layer can be erased and rewritten a limited number of times (about 1,000 times for RW discs and about 100,000 times

for RAM discs). This rewriting does, however, affect disc life expectancy. In other words, RW or RAM discs archived after the first recording should have a longer life expectancy than those that have undergone several erase-recording cycles. Given the normal degradation rate alone, the life expectancy for RW and RAM discs will be less than that of R-discs. Add to that multiple rewrites, and the life expectancy can be even less.

Disc manufacturers sometimes specify the expected lifetime of an optical disc. However, the problem is that the lifetime of a disc is very difficult to predict and depends on many external factors such as the handling of the disc, and environmental conditions. Also, at present there is no standardized way of predicting the lifetime of a disc. Manufacturers test a disc by using accelerated aging methodologies with controlled extreme temperature and humidity influences over a relatively short period of time. In the same way, optical discs are tested against UV light exposure. However, it is not always clear how a manufacturer interprets its measurements for determining the end-of-life of a disc. Few, if any, life expectancy reports for these discs have been published by independent laboratories. Expectations vary from 5 to 100 years for optical discs. This unknown lifetime reduces user confidence in the use of optical discs as a storage medium.

JP 60-239927 discloses a known system that determines the degradation of a disc during playback by monitoring the signal level of reflected light, thus enabling a warning to be provided to a user that the data should be transferred to a new disc. However, such a system suffers from the disadvantage that the warning system relies on the apparatus actually reading data from the disc, which can lead to a loss of data if the disc suddenly stops working.

JP 03-119533 discloses another known system in which an indicator is placed around the periphery of a disc, and in which a dye fades with respect to time. However, such a system suffers from the disadvantage of not providing a true indication of the state of degradation of the optical disc.

In addition to the above mentioned disadvantages, if an optical disc is not used for a long period of time, for example because the disc is merely being used as a back-up disc, the end-of-life of an optical disc may have passed while the optical disc has been in storage, for example in an optical disc enclosure 1 in a personal disc library 3, as shown in Figure 1.

The aim of the present invention is to provide a label for an optical disc or an optical disc enclosure for providing an indication to a user of the state of degradation of an optical disc, without having the disadvantages mentioned above.

According to the invention, there is provided a label for providing an indication of the state of ageing or degradation of the optical disc to a user. The label comprises a transparent cover layer and an indicator layer for providing a visual indication of the state of ageing or degradation of the optical disc. The transparent cover layer and the indicator layer are configured to act in combination, such that the exposure of the indicator layer via the transparent cover layer to environmental parameters, causes the indicator layer to change visual appearance in relation to the ageing or degradation of the corresponding optical disc.

The invention has the advantage of enabling the sate of degradation of a disc to be conveyed to a user, without having the disadvantages associated with the prior art.

According to another embodiment of the present invention, there is provided an enclosure for an optical disc, the enclosure comprising a label as defined in the claims. The invention has the advantage of enabling a user to determine the state of degradation of an optical disc without having to remove the optical disc from its enclosure.

According to another embodiment of the present invention, there is provided an optical disc comprising a label as defined in the claims.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the following drawings in which:

Fig. 1 shows how optical discs are typically stored in a library; Fig. 2 illustrates how optical discs are stored according to the present invention;

Fig. 3 shows a label according to a first embodiment of the present invention; Fig. 4 shows a label according to a second embodiment of the present invention; Fig. 5 shows how a label for an enclosure of an optical disc changes visual appearance in accordance with the present invention;

Fig. 6 shows an example of an indicator used in a label of the present invention.

Figure 2 shows an example of an optical disc library 1 for storing optical disc enclosures 3. Each optical disc enclosure 3 comprises a label 5 for providing a visual indication about the state of degradation or ageing of a corresponding optical disc. By "corresponding" optical disc it is meant the optical disc that was supplied with the enclosure 3, or the optical disc that is being stored in the enclosure 3.

The labels 5 can be placed on a convenient location on the disc enclosure 3. For example, the labels 5 can be placed on the side of a DVD enclosure. When new, the labels 5 will be transparent. Upon ageing, however, the labels 5 will change color, ultimately indicating that data should be backed-up onto another disc. As will be seen from Figure 2, the different colored labels on the various enclosures 3 indicate that the corresponding optical discs have aged differently, and hence have different levels of degradation. Each label 5 is a thin film, which can be glued onto any surface.

Figure 3 shows a label according to a first embodiment of the present invention. The label 5 comprises an indicator layer 101 covered by a transparent cover layer 103. An adhesive layer 105 is provided for attaching the label 5 to a disc enclosure. A barrier layer 107 is provided for preventing the adhesive layer 105 from penetrating the indicator layer 101. According to the first embodiment, the label 5 is covered by a seal 109, which has to be removed either by the manufacturer (for example when an optical disc is inserted into the enclosure), or by the user once the optical disc has been purchased (for example for a R or RW disc).

It will be appreciated by a person skilled in the art that various reactive dyes and photo-bleaching agents can be used as the indicator layer 101.

For example, the indicator layer 101 could comprise a dye containing leuco methylene blue, which is a well-known indicator agent in redox-based titrations. The coloring mechanism is then based on the oxidation of leuco methylene blue (transparent) into methylene blue (blue). In other words, using this dye as the indicator layer 101 will result in the reactive dye going from transparent to blue when oxidized.

It will be appreciated that the transparent cover layer 103 and the indicator layer 101 forming the label 5 must be tuned, or configured in combination, in such a way that the ageing of the indicator layer 101 is a true measure of the ageing of the corresponding disc. In other words, the indicator layer 101 has properties which, when exposed to similar environmental parameters, changes visual appearance in relation to the expected end-of-life of the corresponding optical disc.

Figure 4 shows a second embodiment of the invention. As with Figure 3, the label 5 comprises an indicator layer 101 covered by a transparent cover 103. An adhesive layer 105 is provided for attaching the label 5 to a disc enclosure. A barrier layer 107 is provided for preventing the adhesive layer 105 from penetrating into the indicator layer 101. Attached to the adhesive layer 105 is a protective layer 111, which a user can remove prior to attaching the label 5 to the optical disc enclosure, using the adhesive layer 105.

According to this embodiment, the label 5 is added as a separate item inside the optical disc enclosure, and is covered by a protective package (not shown). For example, the protective package should be non-transparent, and preferably contain a protective gas, (such as N ₂ ). The user can then decide when and where to place the label 5. In other words, when the user wants to use the indicator label, the user opens the bag, peels off the bottom protective layer 111 and sticks the indicator label to the required surface. The labels 5 can also be purchased separately in large quantities by the consumer.

Preferably, the indicator layer 101 can be configured such that it can be tuned. For example, the reactive dye can also contain Sn(II) 2-ethyl hexonoate, which makes it possible to tune the coloring mechanism. In this manner, the color transformation can be controlled by another oxidation reaction, relating to the transition of Sn(II) 2-ethyl hexanoate into Sn(IV) 2-ethyl hexanoate. This latter conversion is not an equlibrium reaction, as opposed to the methylene blue redox couple. The oxidation potential of Sn(II) 2-ethyl hexanoate is much lower than that of leuco methylene blue. Consequently, in the presence of an oxidant (oxygen from the ambient, H ₂ O, etc.) all the Sn(II) salt will first oxidized and form Sn(IV).

When all Sn(II) is consumed, the oxidation and coloration of leuco methylene blue will start. The concentration of Sn(II) 2-ethyl hexanoate in the dye layer will determine the incubation time before the disc starts to turn blue. The time can be controlled by the amount of Sn(II) 2-ethyl hexanoate in the reactive dye. Hence, adding or substrating the Sn(II) 2-ethyl hexanoate content in the dye layer will increase or reduce, respectively, the incubation time before the label 5 starts to turns blue.

Another way of controlling the incubation time before the label 5 starts to turn blue is to cover the dye with oxygen diffusion barriers such as silicon nitride (Si ₃ N ₄ ) or polymers, for example. There are a number of possible materials that can be used as diffusion barriers. The diffusion of oxygen through the diffusion barrier is governed by the equation:

C(x,t) = C _s erfc(-==) (1)

2Dt

Therefore, the addition of a oxygen diffusion barrier increases the incubation time for coloring of the disc. As can be seen from equation (1), the incubation time can be extended considerably in this way.

The incubation time before the coloring of the label commences is related to the oxidation of leuco methylene blue (transparent) into methylene blue (blue).

Preferably, to mimic the degradation in the corresponding disc even better, the indicator layer 101 may be sandwiched between a polycarbonate substrate and cover sheets, similar to a corresponding disc.

Another way of controlling the incubation time before the label starts to turn blue is to tune the oxygen permeability of the transparent cover layer 103, (or of the polycarbonate substrate or the cover-sheet in the example described in the preceding paragraph). The oxygen permeability of the transparent cover layer 103 or the polycarbonate substrate determines the oxygen diffusion through it.

The labels 5 can be produced as larger sheets, which are then cut into small pieces (typically 1 cm ² ). Alternatively, the polycarbonate embodiment may be used for the narrow side of the disc casing, so that the label is automatically displayed when the discs are stored in the format shown in Figure 2. The proposed indicator means mentioned above, leuco methylene blue, is a useful candidate as it traces oxidation by oxygen or water. Oxidation is a serious risk for the performance of the corresponding disc, as it will deteriorate the mirror layer, which has been found to be one of the main lifetime problems in climate tests. This is not the only cause for degradation, however. The label should also be able to monitor temperature and UV- exposure induced reduction of disc performance. UV-sensitive organic compounds that change color upon exposure will be known to a person skilled in the art. Temperature indicators are also well-known, but for this application one would prefer a monitor that measures some accumulated value (integration of heat exposure over time). An organic molecule that starts to decompose at, say, 50 ⁰ C into colored fragments may be used for this purpose.

As will be appreciated from the embodiments described above, the label 5 provides a visual indication to a user about the state of degradation of an optical disc.

According to another aspect of the present invention, as an alternative to placing the label on the enclosure of the optical disc, the label may also be placed on the optical disc itself. This has the advantage of being able to provide an indication of the state of degradation of the disc by simply attaching a label, rather than having to alter the construction of the disc itself. It is noted that each of the features mentioned in relation to the application of the label to an optical disc enclosure also apply to the application of the label to an optical disc per se.

When placed over a portion of the optical disc that covers an area of the data layer, it will be appreciated that the label is configured such that it does not interfere with the operation of the optical disc.

Figure 5 shows how the color of a label 5 changes over time, thereby providing an indication of the state of degradation of the optical disc. At time T=o the indicator layer 101 is clear, thereby indicating that the corresponding disc is in an excellent state. At time T=Ti the color has changed slightly, thereby indicating that the corresponding disc is becoming slightly degraded. At time T=T ₂ the color has changed significantly, thereby indicating that the data stored on the corresponding disc should be transferred as soon as possible to a new disc. In this way, the indicator layer 101 is configured such that it provides an indication of the extent to which the optical disc is degrading.

Figure 6 shows the reversible reduction-oxidation process of leuco methylene blue (transparent) into methylene blue (blue).

The invention has the advantage of enabling the degradation of a disc to be monitored by simply looking at the optical disc, or the optical disc enclosure in its stored position.

The invention also has the further advantage that a large number of separate labels can be made in large quantities at the same time, thereby reducing the cost of a single label.

It is noted that the term "changing color" includes amongst other things the changing from a "clear" state into a particular color, changing from one color into another color, or changing from a shade of one color into another shade of the same color. Although the invention described in the various embodiments is described in relation to the color of the indicator means 101 changing in response to environmental conditions, it will be appreciated that some other form of visual characteristic could also be made to change. For example, the indicator means 101 could be configured to provide another visual effect, such as a "blistering look" or a "cracked look" as the indicator changes

over time, thereby providing an indication to a user that the data should be transferred to a new disc.

It will be appreciated that the indicator layer 101 can be placed in any position within the stack of layers in the label 5. In addition, it will be noted that the removable protective layer 111 in the embodiment of Figure 4 can be omitted if the label 5 is applied directly to an enclosure during manufacture. Likewise, the label 5 of Figure 3 could have a protective layer similar to the layer 111 in Figure 4, so that a label with a removable seal 109 can be applied by a user.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim.