JPH03295040A - Optical information recording medium - Google Patents

Abstract

PURPOSE:To improve characteristics of the medium by successively forming a first transparent layer, first recording thin film layer, second transparent layer, second recording thin film layer, and third transparent layer on a substrate, so that phase transition of the first and second recording thin film layers is caused by irradiating with laser beam and can be optically detected. CONSTITUTION:On a transparent and flat substrate 1 such as glass, there are seccessively formed a first transparent layer 2 such as a transparent dielectric, first recording thin film layer 3, second transparent layer 4 such as a transparent dielectric, second recording thin film layer 5, and third transparent layer 6 such as a transparent dielectric. As for the recording thin film layers 3, 5, chalcogenide compds. such as SbTe, InTe, etc., are used which shows phase transition between amorphous and crystal states. As for the transparent layers 2, 4, 6, oxides such as SiO2, SiO, etc., are used. By this constitution, phase transition of the first and second recording thin film layers is caused by irradiating with laser beam and can be optically detected. Thereby, the medium shows little deterioration due to repetition of recording, and the medium has good optical characteristics.

Description

[Detailed description of the invention] Industrial field of application The present invention relates to an optical information recording medium that records and reproduces information at high speed and high density using light, heat, etc. When converged by It is possible to change a minute area, and it is also possible to read out changes in that minute area, but optical information recording media are those that utilize this for recording and reproducing information. The basic structure of an optical recording medium (referred to as "optical recording medium" or simply "medium") is that a recording thin film layer whose state changes in some way by laser spot light irradiation is provided on a base material with a flat surface. The following methods are used to record and reproduce signals. In other words, a plate-shaped medium is moved by a rotating means such as a motor or a translation means, and a laser beam is focused and irradiated onto the recording thin film surface of this medium.The recording thin film absorbs the laser beam and heats up.Laser If the light output is increased above a certain threshold, the state of the recording thin film changes and information is recorded.This threshold value is determined by factors such as the characteristics of the recording thin film itself, the thermal characteristics of the substrate, the relative speed of the medium to the light spot, etc. Even if the amount of recorded information depends on Therefore, it is desirable to have a material and structure that changes its state with a small laser power and exhibits a large optical change. Although compound thin films containing Te are known,
These so-called holes are recorded when the thin film is melted or evaporated by laser light irradiation to form small holes (＼ The reflected or transmitted light from this recording area and the surrounding area have different phases, so they are canceled out by interference. Reproduction is performed by detecting changes in the amount of reflected light or transmitted light that reaches the detection system after being reflected or diffracted.Also called phase change type, the crystal of the recording thin film material does not change shape. There are recording media that undergo optical changes due to structural changes.The materials include oxide thin films whose main components are amorphous chalcogenide thin trap tellurium and Te-T e 02 consisting of tellurium oxide (Tokukosho, 54). -3725 publication)
. A thin film containing t=Te-TeO2-Pd as a main component is also known (Japanese Unexamined Patent Publication No. 68296/1983). These are recorded by changing at least one of the extinction coefficient or refractive index of the thin film by laser light irradiation.
In this part, the amplitude of the transmitted light or reflected light changes, and as a result, the amount of transmitted light or reflected light that reaches the detection system changes, which is detected and the signal is reproduced.Phase change media do not involve changes in shape. Therefore, if a reversible state change is possible, recorded signals can be erased and rewritten.4 Ge-Te-
3b-s material (Japanese Patent Publication No. 47-26897), T
e-0-Ge-8b material (JP-A-59-185048
Although there are known materials such as Te-〇-Ge-3b-Au based material (JP-A-61-2594), Ge5b-Te-based material (JP-A-62-209742), etc. There are two states that can change reversibly, an amorphous state (or a glassy amorphous state) and a crystalline state, which stably exist. This is achieved by heating the thin film with laser light irradiation to raise its temperature, melt it, and then rapidly cool it to an amorphous state during the cooling process when the laser light irradiation ends. Crystallization is similarly achieved by heating the thin film with laser light irradiation to raise the temperature below the melting point to a temperature sufficient for crystallization, but even when the temperature is raised above the melting point, sufficient rapid cooling conditions cannot be obtained during cooling, so it is slowly cooled. Crystallization is also achieved when The ability to use the amorphous state and the crystalline state as information recording and erasing states, respectively ＼ On the other hand, it is optional whether to use each as the erasing state and the recording state.Length It is common to use the amorphous state as the recording state. Therefore, the explanation below will be based on such correspondence as a representative example.4 Generally speaking, optical discs are recorded and reproduced using laser light.Although optical characteristics are required, the wavelength of the laser light used It is necessary for the absorption to be large, and for the optical state difference (i.e., the amount of change) between the recorded state and the erased state (C unrecorded state) to be large. In order to prevent the recording thin film and the base material from being deformed or destroyed when erasing records, it is common to protect the recording thin film by sandwiching it between transparent layers made of inorganic dielectric material, etc., as shown in Figure 2. In addition to this protective function, a protective layer such as a dielectric material (hereinafter referred to as a transparent layer) also has the function of controlling the optical properties mentioned above and the function of controlling the heating and cooling conditions of the thin film. In the former case, the optical constant (refractive index) and thickness of the transparent layer, and in the latter case, the thermal constant (density of thermal conduction wind) and thickness are the parameters that influence the characteristics. Therefore, the transparent layer must be selected so that all of these properties are optimized according to the purpose.If sufficient optical properties cannot be obtained just by sandwiching the transparent layer, as shown in Figure 3, Although it has been proposed to provide a reflective layer made of metal or the like on the transparent layer to improve optical properties, it is also possible to improve the optical characteristics by providing a reflective layer made of metal or the like on the transparent layer. It is also optional to provide a protective material for overall mechanical protection of the thin film structure. This protective material can be formed, for example, by pasting a resin plate with an adhesive or by applying a resin dissolved in a solvent using a spin cord method and then drying it.The problem to be solved by the invention is phase-change optical recording. As described above, the medium can be made amorphous by heating the recording thin film material above its melting point to melt it and then rapidly cooling it.

Therefore, each layer that makes up the medium, especially the recording thin film layer and the transparent layer, becomes high temperature when irradiated with laser light for recording and erasing, and does not deteriorate due to thermal cycles.As a result, the medium characteristics deteriorate, that is, the amount of change decreases.Repeatability inferior to
Or an increase in noise causes may occur. The causes of this deterioration involve multiple mechanisms and have not been completely elucidated, but the main possible causes are listed below.

One is a phenomenon in which when the recording thin film is melted by heating up due to laser light irradiation, compositional segregation and phase separation occur due to temperature distribution, resulting in loss of reversible phase change characteristics. There are also physical destruction phenomena such as deformation and pinhole formation due to thermal expansion and increased vapor pressure of the recording thin film itself. Furthermore, thermal deterioration of the dielectric material constituting the transparent layer, such as deformation due to changes in the crystal structure and changes in optical constants, and physical destruction such as deformation that follows the deformation of the recording thin film layer. As a countermeasure to suppress such a phenomenon, it goes without saying that selecting the composition of the constituent materials, but also measures in terms of the structure of the medium can be considered. In order to suppress segregation and phase separation of the recording thin film material, it is effective to reduce the thickness of the recording thin film. If the recording film thickness is sufficiently small compared to the area melted by laser irradiation, there will be little segregation or phase separation.This is because the interface of the transparent layer has the effect of suppressing the movement of substances, and if the film thickness is small, the melted volume of the recording thin film material This is thought to be due to the fact that the area of the interface is larger compared to , so the effect is larger.

In addition, if the film thickness is small, the cooling rate of the molten part increases and the temperature gradient in the film thickness direction becomes small, which may suppress segregation and phase separation. In the case of a structure in which a recording thin film layer is sandwiched between transparent layers as shown in Figure 2, one problem is that the amount of laser light absorbed decreases when the recording thin film layer is made smaller, resulting in poor sensitivity. There is also the disadvantage that the amount of optical change cannot be obtained and the reproduced signal becomes smaller.

Furthermore, in a structure using a reflective layer as shown in Figure 3, when the thickness of the recording thin film layer is reduced, it is possible to maintain optical characteristics to a considerable extent by selecting the thicknesses of the two transparent layers. However, there is a problem that the margin for the thickness of the transparent layer is small and high-precision film thickness control is required, which increases manufacturing costs.Also, since a reflective layer is required, the manufacturing process increases, resulting in high costs and reliability. There are many problems in laminating different materials from the viewpoint of this.Furthermore, in the structure having this reflective layer, there is also the problem that recording and reproduction cannot be performed with a laser beam from the reflective layer side.The present invention solves the above problems. The purpose of the present invention is to provide an optical information recording medium.4 Means for Solving the Problems In order to solve the above problems, a recording thin film layer that causes an optically detectable change when irradiated with a laser beam is provided on a substrate. The provided optical information recording medium includes a first transparent layer, a first recording thin film layer, a second transparent layer, a second recording thin film layer, and a third transparent layer, each of which is sequentially provided on a base material. If the second recording thin film layer is made of a material that undergoes an optically detectable phase change due to laser beam irradiation, the above-mentioned structure can be used, even if the thickness of the recording thin film layer is sufficiently small. Optical properties can be maintained by selecting the thickness of the transparent layer.

Next, detailed effects will be explained using specific examples.

As shown in FIG. 1, the structure of the recording medium of the embodiment includes a transparent layer 2 such as a transparent dielectric material on a base material 1, a recording thin film layer 3, a second transparent layer 4 such as a transparent dielectric material, and a second transparent layer 4 such as a transparent dielectric material. A recording thin film layer 5 and a transparent layer 6 such as a third transparent dielectric material are sequentially coated.Furthermore, a transparent protective material 7 is provided thereon. In addition to this, although not shown in the figure, it is also possible to use a configuration in which no protective material is applied.1. % In this case, if we consider air (refractive index 1.0) instead of the protected forest 7, it is optically equivalent and the same effect can be obtained. t
The optical properties can be controlled by appropriately selecting l, t3 and t5.

In this case, it is desirable to use a material with a different refractive index from the base material 1 for the transparent layer 2, and if the refractive index of the transparent layer 2 is equal to that of the base material l, optically This is equivalent to the case where the recording thin film layer 3 is provided directly on the substrate, and the transparent layer 2 no longer contributes to controlling the optical properties.Also, a transparent and smooth flat plate made of glass, resin, etc. may be used as the substrate 1. There may be groove-like irregularities on the surface for tracking guides.As the protective material 7, a material prepared by dissolving resin in a solvent and applying and drying it, or a material made by bonding a resin plate with adhesive, etc. can be used, but the recording thin film can be used. The recording thin film material used for layers 3 and 5 is a material that undergoes a phase change between amorphous and crystalline, such as 5bTeK.
InTe & GeTeSn & 5bSeX, Te5
eSbK 5nTeSeiInSeK TeGe5
nOK TeGe5nAuK TeGe5nSbi
A chalcogen compound such as TeGeSb is used. Also, AgZnX InSb undergoes a phase transition between crystals.
Other metals and compounds can also be used.

Transparent layers 2, 4, and 6 are 5102, S10, TlO2
, MgO1Ge○2 etc. 5iaN4, BN, A
Nitriding such as IN￥IJ, ZnS, Zn5e, ZnTe
, sulfides such as PbS, or a mixture thereof can be used.

As a method for forming these thin film layers, a vacuum evaporation method using a multi-source evaporation source, a sputtering method using a mosaic composite target, or the like can be used.

Example 1 To gel manila with the composition of Ge2Sb2Te6 which is a phase change material as a recording thin film Antimony and tellurium 3
Use the original compound. Formation methods include Ge, Sb, and Te.
Although the recording thin film is formed in an amorphous state even when using the electron beam evaporation method using two evaporation sources, the optical constants of the amorphous state obtained by depositing only Ge2SbaTes with the above composition on a glass plate are as follows: Complex refractive index at a wavelength of 830 nm n+k i is 4. 8+1. 3i was heat-treated at 300°C for 5 minutes in an inert atmosphere to make it into a crystalline state.5. 8+3. As an example of the present invention, as shown in FIG. ZnS (with a refractive index of 2° 20) was deposited to a thickness of t1 by electron beam evaporation, and then Ge2Sb2Te6 was formed to a thickness of t2 as the recording thin film layer 3 by the method described in Example 1, and ZnS was further deposited as a transparent layer 4. Thickness t3
Similarly, the above-mentioned Ge2Sb2'I'es is deposited to a thickness of t4 as a recording thin film layer 5, and ZnS is deposited to a thickness of t5 as a transparent layer 6. The reflectance (amplitude reflectance) before and after heat treatment, that is, in the amorphous state and in the crystalline state, is Rw and Rd, respectively, and the difference △R (=Rw - Rd) is calculated as follows: The absorption coefficients Aw2 and Ad2 of the recording thin film layer 2 and the absorption coefficients Aw4 and Ad4 of the recording thin film layer 4 in the state are expressed as the film thickness t1 of each layer,
Reflectance and absorption calculated by changing t2, t3, t4, and t5 are calculated using the matrix method from the complex refractive index and film thickness of each layer. (See Chapter 3) Also, assuming that the base material 1 and adhesive protective material 7 have infinite film thickness (ignoring the effect of the base material-air field adhesive protective material-air interface), the reflectance R is Table 1 shows the conditions and calculation results for the ratio of light incident on the base material that exits into the base material. Table 1 shows a typical condition where the thickness of the two recording thin film layers is equal (t
2=t4), the transparent layer thickness t1 has the largest reflectance change △R among the thicknesses of each recording thin film layer.
Table 1 shows the calculation results for the combinations of , t3, and t4.As shown in Table 1, the absorption of the two recording thin film layers differs when the film thicknesses t2 and t4 are 15 nm or more, but they are almost equal when the film thicknesses are 10 nm or less. It is also known that the absorption of each recording thin film layer is the result of multiple reflections of the entire multilayer structure.If the thickness of the recording thin film layer, especially the recording thin film layer on the side where the laser light enters, is large, the absorption of the recording thin film layer will reach other alignment thin film layers. The absolute value of the amount of laser light becomes small, making it impossible to equalize the absorption of the two recording thin film layers.

Even in the case of other materials used in the above-mentioned phase change recording thin film, it is necessary that the film thickness be approximately 15 to 20 nm or less in view of its optical constants. If this absorption differs, the magnitude of the recorded state of the two layers differs during recording, making it impossible to obtain a desired reproduction signal. Therefore, it is desirable that the two be approximately equal.Furthermore, the calculation results show that even if the thickness of each transparent layer changes by about λ/32 in optical length, in this case about 12 nm, there is no major change in the characteristics and there is a large margin in film thickness. I don't understand. For example, when the recording thin film layer thickness t2=t4=5 nm, the transparent layer thickness tl is 16
5-189nm, t3 is 165-189nrrK
Even when t5 is in the range of 1l30-165n, the reflectance change ΔR is 17% or more, and when the recording thin film layer thickness t2=t4=10 nm, the transparent layer thickness tl is 141-165 nm, and t3 is 16
It was found that the reflectance change ΔR could be 22% or more even in the range of 130 to 153 nm.Within these film thickness ranges, the film thickness t of transparent layer 2 and transparent layer 6 was
Even if a combination in which 1 and t5 are equal is possible, in that case, the recording thin film layers 3 and 5 are made of the same material and have the same thickness, t2=
t4, the materials of the transparent layers 2 and 4.6 are appropriate, and the refractive index of the base material and the protective material are equal.8 Therefore, the overall film thickness structure is symmetrical in the film thickness direction. Even when viewed from the protective material side, the optical properties are the same.In this case, as mentioned above, the thickness of the recording thin film layer and the transparent layer has an optical length of approximately λ/32, so the thickness of each layer should be approximately within that range. In addition, since the optimum value of the transparent layer thickness does not change greatly when the recording thin film layer thickness is 5 nm and 10% m, the margin for the recording thin film layer thickness is also large in this film thickness range. , t4 should be approximately equal.Furthermore, if the optical thickness of the base material and the protective material are equal, the characteristics will be the same whether recording or reproducing is performed from the base material side or from the protective material side. Based on the above results, it can be seen that by appropriately selecting the thickness of each layer, a configuration with a large change in reflectance can be obtained even when the recording thin film layer is sufficiently thin (less than 20% m).Based on this calculation, the following method is used. A width of 0. Thickness 1. Formed a groove track with a depth of 6 μm and a depth of 65% m. Using a PC resin disk of 2 mm in diameter and 200 mm in diameter, a ZnS thin film of 177% m was deposited using the above method while rotating it in a vacuum, and then a recording thin film of Ge2Sb2Tes was similarly deposited in an amorphous state with a film thickness of 5 nm. A thin film was deposited to a thickness of 177% m, and Ge2Sb2Tea was similarly formed as a recording thin film to a thickness of 5 nm in an amorphous state.
An nS thin film was deposited to a thickness of 153% m.A multilayer thin film with the same structure was also formed on a glass substrate of 18 x 18 mm and a thickness of 0.2 mm.Furthermore, a film was formed on a resin disk, and the same PC resin disk was cured with UV rays. A sample formed on a glass substrate with a protective material pasted with a transparent adhesive to form an optical recording medium was heated at 300°C for 5 minutes in an argon atmosphere to crystallize the entire surface. When I measured the reflectance from the side, it was 8 gourds before heat treatment.
After heat treatment, this medium with a reflectance change of 18% at 26% was rotated, and a semiconductor laser beam with a wavelength of 830 nm was focused at a linear velocity of 10 m/sec using a lens system with a numerical aperture of 0.5 to remove the substrate. The recording thin film was focused from the side using a known focus control means, and the groove track was irradiated while being tracked using a known tracking control means.9 When the surface of the recording thin film was continuously irradiated with an output of 8 mW, there was no recording of the irradiated area. The thin film crystallizes into a circle.Furthermore, the recording thin film is partially made amorphous by irradiating light modulated with a single frequency of 5 MHz and a duty of -50% using an output of 16, and recording is performed in row 1.
．． X, where 1 mW continuous output was irradiated and the reflected light was detected by a photodetector and played back, the playback signal amplitude was observedh CN ratio 53 dB (band resolution 30
kHz2) was obtained, and the laser light output was modulated between two levels of 8 mW and 16 mW to generate a single frequency 5M
When the same track was alternately irradiated with two wavelengths, Hz and 2MH2 (both with a duty of 50%), and the signals were overwritten, the CN ratio of the reproduced signal was 50 even after 104 recordings.
dB was obtained and that good recording was being performed. Using a PC resin disk with a thickness of 1° 2 mm and a diameter of 200 mm on which a groove track of 6 μm and a depth of 65% m was formed, a ZnS thin film of 153 nm was deposited in the same manner as in Example 1 while rotating it in a vacuum.
Further, as a recording thin film, Ge2Sb2Tea was deposited in an amorphous state to a thickness of 10% as in Example 1, and a ZnS thin film was further deposited to a thickness of 17?m. Further, as a recording thin film, Ge2Sb2Tea was similarly formed in an amorphous state with a thickness of 10% m, and a ZnS thin film was further deposited to a thickness of 141% m.
The film can also be formed on a glass substrate of 18 mm x 0.2 mm thick.Furthermore, when the film is formed on a resin disk, the same PC resin disk is pasted with an ultraviolet curable adhesive and a protective material that adheres is provided to form an optical recording medium. A sample formed on a glass substrate was heated at 300°C for 5 minutes in an argon atmosphere to crystallize the entire surface, and the reflectance from the substrate side was measured before and after crystallization. Before heat treatment, it was 18%.
After heat treatment, a reflectance change of 24% was obtained at 42%.9 This medium was rotated and a semiconductor laser beam with a wavelength of 830 nm was focused at a linear velocity of 10 m/see using a lens system with a numerical aperture of 0.5. Irradiated from the base material side as in 1.
When the surface of the recording thin film was continuously irradiated with an output of 7 mW, the recording thin film in the irradiated area was crystallized.Furthermore, the recording thin film was similarly irradiated with light modulated at a single frequency of 5 MHz and a duty of -50% at an output of 15. Line 1. Record partially amorphous. The place where the continuous output of k1mW was irradiated and the reflected light was detected with a photodetector and played back.
The reproduced signal amplitude was observed, and a CN ratio of 56 dB was obtained.Furthermore, the laser light output was modulated between two levels of 7 mW and 15 mW, and the single frequency was alternated between two wavelengths of 5 MHz and 2 MHz (both with a duty of 50%). When irradiating on the same track and overwriting the signals, 105
Even after recording twice, a CN ratio of 51 dB was obtained for the reproduced signal, confirming that good recording was being performed. A groove track with a thickness of 6 μm and a depth of 65 nm was formed. Using a PC resin disk of 2 mm in diameter and 200 mm in diameter, a ZnS thin film of 153 nm was deposited in the same manner as in Examples 1 and 2 while rotating it in a vacuum, and GeaSbaTes was further deposited as a recording thin film in Example 1.
, 2, a ZnS thin film was deposited in an amorphous state with a film thickness of 165 nm, and Ge2s bpTes was similarly deposited as a recording thin film.
A thin film of ZnS was formed in an amorphous state with a thickness of m, and a ZnS thin film was further deposited to a thickness of 153 nm.A multilayer thin film with the same structure was also formed on a glass substrate of 18 x 18 mm and a thickness of 0.2 mm.Further, the film was formed on a resin disk. An optical recording medium was formed by attaching a PC resin disk with an ultraviolet curable adhesive and a protective material with a total thickness of 1.2 mm.The sample formed on a glass substrate was heated at 300℃ for 5 minutes. The entire surface was crystallized by heating in an argon atmosphere, and the reflectance from the base material side was measured before and after crystallization. Before heat treatment, it was 16%.
After heat treatment, a reflectance change of 23% was obtained at 39%.The medium was rotated and a semiconductor laser beam with a wavelength of 830 nm was focused at a linear velocity of 10 m/sec using a lens system with a numerical aperture of 0.5. As in Examples 1 and 2, irradiation was performed from the substrate side.9 When the recording thin film surface was continuously irradiated with an output of 6.5 mW, the recording thin film in the irradiated area was crystallized, and t was similarly irradiated with an output of 15. Frequency: 5MHz Duty: 50
Recording is performed by irradiating light modulated by % to make the recording thin film partially amorphous. Amplitude observed h CN ratio 55d
When recording and reproducing were performed from the protective material side in the same manner, the same characteristics were obtained. Effects of the Invention According to the present invention, since the recording thin film layer is thin, it does not deteriorate even after repeated recording. It is possible to obtain a rewritable phase-change optical recording medium that is difficult to use and has good optical characteristics.

Further, it is possible to obtain an optical recording medium that is easy to manufacture and inexpensive, with a large margin in the film thickness of each layer.

Furthermore, since no metal reflective layer is used, it is possible to obtain an optical recording medium with high reliability, a simple manufacturing process, and a low cost.

Furthermore, by applying the same optical properties to the recording thin film layer from both the substrate side and the protective material side, it is possible to obtain a rewritable phase-change optical recording medium that can be recorded and reproduced from either side.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional diagram showing the configuration of one embodiment of the present invention; Figures 2 and 3 are schematic cross-sectional diagrams showing the configuration of a conventional example for comparison.・Base material 2.4.6...Transparent layer 3, 5...Recording thin film layer 7...Reflective layer 8...Protective material

Claims (5)

[Claims] (1) An optical information recording medium in which a recording thin film layer that causes an optically detectable change upon irradiation with a laser beam is provided on a base material, wherein a first transparent layer and a first recording layer are provided on the base material. A thin film layer, a second transparent layer, a second recording thin film layer and a third transparent layer are respectively provided in sequence, and the first and second recording thin film layers can be optically detected by a phase change caused by laser light irradiation. An optical information recording medium characterized by being made of a material that undergoes change. (2) The thickness of the first and second recording thin film layers is 2
The optical information recording medium according to claim 1, wherein the optical information recording medium has a particle diameter of 0 nm or less. (3) The optical information recording medium according to claim 1, wherein the first recording thin film layer and the second recording thin film layer have approximately the same absorption of laser light. (4) An optical information recording medium comprising a recording thin film layer that causes an optically detectable change upon irradiation with a laser beam on a base material, wherein a first transparent layer and a first recording layer are provided on the base material. A thin film layer, a second transparent layer, a second recording thin film layer, a third transparent layer and a protective material are provided in sequence, and the first and second recording thin film layers are made of the same material, and the first and second recording thin film layers are made of the same material, and , the second and third transparent layers are made of the same material, the first recording thin film layer thickness and the second recording thin film layer thickness are approximately equal, and the optical thickness of the first transparent layer is approximately the same as the optical thickness of the first transparent layer. An optical information recording medium characterized in that the third transparent layer has substantially the same optical thickness. (5) An optical information recording medium comprising a recording thin film layer that causes an optically detectable change when irradiated with a laser beam on a base material, the recording thin film layer having a transparent protection on the side opposite to the base material. An optical information recording medium characterized in that the optical thickness of the protective material is approximately equal to the optical thickness of the base material.