DE202005018553U1 - Protective arrangement for an optical fiber comprises a hose additionally accommodating a conductor loop respectively with a specified electrical impedance and a measuring unit at its ends - Google Patents

Abstract

The protective arrangement for an optical fiber (40) comprises a protective hose (50) additionally accommodating a conductor loop (60) with a specified electrical impedance at one end and a measuring unit at the other end.

Description

The The invention relates to a protective device for an optical fiber, the the transmission of laser light is used.

at Working with laser light is uncontrolled from the given Beam radiation emerging radiation a significant health hazard in particular breaks lead from optical fibers for the uncontrolled discharge of laser light. Because of Occupational safety is therefore for High-performance optical cable (LLK) a safety system for breakage monitoring required. In case of damage to the LLK, the safety system should be triggered and Turn off the laser to prevent laser light from becoming uncontrolled in the workrooms spreads.

Safety systems according to the current state of the art, such as in the DE 30 41 649 A1 described, use a copper wire, which is introduced into the shell of the LLKs. If the LLK breaks, the copper wire is either severed mechanically or acts as a fuse, as the emerging laser radiation melts the copper wire. This interruption of the electrical contact (resistance measurement) then triggers the safety system.

From the DE 40 10 789 A1 a Lichtleitkabelsystem is known, which consists of a main optical fiber to be monitored with a transparent cladding and a cable sheath, wherein in addition to the monitored main optical fiber, a monitoring optical fiber is introduced into the cable sheath, which is parallel to the main optical fiber to be monitored and wherein the monitoring optical fiber This transceiver system detects failure of the monitoring optical fiber due to damage to the main optical fiber.

Furthermore, from the EP 0 320 108 A2 a fiber optic cable system is known in which a break or power loss is detected by monitoring the conductivity of conductive polymers (with metal particles) associated with the optical fiber.

The use of copper wire, as in the DE 40 10 789 A1 described, has the undesirable side effect that the light guide cable acts as an antenna. This results in an electromagnetic coupling of the outside world to the source of the laser radiation, the laser. In the laser, in particular in pulsed operation, electromagnetic oscillations and waves are generated, which are then emitted via the copper wire acting as an antenna into the room. Such an emission can hinder the work and is also not permitted under the current regulations for protection against electromagnetic radiation (electromagnetic compatibility). Therefore, costly protective measures, such as spacers or additional shields, must be taken, but the down-flexibility of the cable and increase its weight.

systems, which on the evaluation of a current flow through a conductor are based, in which falls below a minimum current, the light source often turns off only when the LLK breaks through or burned out. With the mentioned systems can only fatal damage be discovered at the LLK. So these safety systems can only then triggered when the LLK is complete destroyed is, if the LLK is broken or by radiation leakage from the main fiber blown.

The EP 0 113 104 in contrast, describes a fiber optic cable system in which the main optical fiber is closely adjacent to a plastic coated reference optical fiber. When the main optical fiber breaks, the adjacent plastic jacket of the reference optical fiber melts, so that the reference light guided in this fiber does not reach a detection unit. In this way, a fraction of the fiber is displayed.

outgoing from this prior art, it is an object of the present invention a device for monitoring of fiber optic cables available on the one hand, the resistance of the protective tube against destruction by escaping laser light increases and on the other hand the responsiveness of the monitoring of the Optical fiber significantly improved. Furthermore, on the one hand the Optical fiber in front of outside, e.g. mechanical influences protected On the other hand, people should be transported in front of the inside of the optical fiber Laser light protected become.

Is solved this task by the features of the independent claim.

• a) der Schutzschlauch einen zwei- oder mehrlagigen Aufbau besitzt, bestehend aus einer inneren Lage aus einem optisch transparenten, elektrisch isolierenden Material sowie mindestens einer darüberliegenden Lage aus einem nicht-transparenten Material, und
• b) eine Leiterschleife zusätzlich zur Lichtleitfaser durch den Schlauch geführt ist, bestehend aus zwei gegeneinander isolierten elektrischen Leitern, die an einem Ende des Schlauches über eine definierte elektrische Impedanz und am anderen Ende mit einer die Impedanz kontrollierenden Messeinheit verbunden sind,
• c) wobei die Isolation der beiden elektrischen Leiter so gewählt ist, dass sie durch die Wärmewirkung von austretendem Licht bei einem Bruch der Lichtleitfaser oder durch die Strahlung, entweder zur Beeinflussung der elektrischen Leiter oder zu einem direkten Kontakt der elektrischen Leiter führt oder mindestens einer der elektrischen Leiter durchtrennt wird, und
• d) eine Änderung des Widerstandes durch die Messeinheit detektierbar ist.

Thereafter, a protective device for an optical fiber is provided, consisting of a protective tube, at least one guided by the protective tube electrical conductor loop with a defined electrical impedance and a special insulation of the conductor loop. According to the invention, this protective device is characterized in that

• a) the protective tube has a two- or multi-layer structure, consisting of an inner layer of an optically transparent, electrically insulating material and at least one dar overlying layer of a non-transparent material, and
• b) a conductor loop in addition to the optical fiber is passed through the hose, consisting of two mutually insulated electrical conductors, which are connected at one end of the hose via a defined electrical impedance and at the other end with an impedance-controlling measuring unit,
• c) wherein the insulation of the two electrical conductors is chosen so that it leads by the heat effect of leaking light in a fraction of the optical fiber or by the radiation, either to influence the electrical conductors or to a direct contact of the electrical conductors or at least one of electrical conductor is severed, and
• d) a change of the resistance by the measuring unit is detectable.

• a. eine innerste Lage, die durch einen transparenten, ungefärbten Schlauch aus PTFE gebildet wird;
• b. darüber liegend eine nicht-transparente Lage, insbesondere eine metallische Lage, bevorzugt ein Geflecht, eine Umspinnung oder eine Bandierung aus blankem Kupfer,
• c. eine dem mechanischen Schutz dienende und den äußeren Abschluss des Schutzschlauches bildende Ummantelung.

In particular, an embodiment of the protective device for the transmission of laser powers below 500 W is preferred, wherein the protective tube has a multilayer structure (from inside to outside), comprising

• a. an innermost layer formed by a transparent, undyed PTFE tube;
• b. lying above a non-transparent layer, in particular a metallic layer, preferably a braid, a braiding or banding of bare copper,
• c. a mechanical protection serving and the outer end of the protective tube forming sheath.

• a. eine innerste Lage, die durch einen transparenten, ungefärbten Schlauch aus PTFE gebildet wird;
• b. darüber liegend eine erste nicht-transparente Lage, insbesondere eine metallische Lage, bevorzugt ein Geflecht, eine Umspinnung oder eine Bandierung aus blankem Kupfer,
• c. gefolgt von einer ersten, isolierenden Trennlage,
• d. darüber liegend eine weitere nicht-transparente Lage, insbesondere eine metallische Lage, bevorzugt ein Geflecht, eine Umspinnung oder eine Bandierung aus blankem Kupfer,
• e. gefolgt von einer weiteren, isolierenden Trennlage,
• f. die von einer Lage aus Zugentlastungselementen ummantelt ist,
• g. und eine dem mechanischen Schutz dienende und den äußeren Abschluss des Schutzschlauches bildende Ummantelung.

Furthermore, an embodiment of the protective device for the transmission of high laser powers over 500 W is preferred, wherein the protective tube has a multilayer structure (from inside to outside), comprising

• a. an innermost layer formed by a transparent, undyed PTFE tube;
• b. lying above a first non-transparent layer, in particular a metallic layer, preferably a braid, a braiding or banding of bare copper,
• c. followed by a first, insulating release liner,
• d. lying over another non-transparent layer, in particular a metallic layer, preferably a braid, a braiding or banding of bare copper,
• e. followed by another, insulating separating layer,
• f. which is encased by a layer of strain relief elements,
• G. and a sheath serving as a mechanical protector and forming the outer end of the protective tube.

Of the Structure of the protective tube according to the invention is not limited on the examples given here, but more layers in the aforementioned Inserted be, for manufacturing reasons, or to further increase the mechanical strength, or to fulfill other functions useful or necessary.

Further advantageous measures and embodiments are in the rest dependent claims described; The invention will be described with reference to the following figures described in more detail:

1 shows the schematic structure of the protective tube according to the invention in cross section.

2 shows the schematic structure of the protective tube according to the invention in a preferred embodiment for use with laser light lower power.

3 shows the schematic structure of the protective tube according to the invention in a preferred embodiment for use with very high power laser light in moving applications.

4 shows the schematic structure of the fiber optic cable monitoring for evaluation with a single circuit for the combined plug-in, break and laser light monitoring between the contacts 31 and 32 ,

5 shows the schematic structure of the optical fiber cable monitoring for evaluation with separate circuits for the plug and break monitoring between the contacts 31 and 33 and for the laser light monitoring between the contacts 32 and 33 ,

According to the invention therefore a protective device for an optical fiber is provided as in the 1 is shown in more detail:
The protective device for an optical fiber 40 , which is sometimes referred to as a "light guide", consists of a protective tube 50 , wherein the protective tube is constructed at least two layers, consisting of an inner layer 51 of an optically transparent, electrically insulating material and at least one overlying layer of a non-transparent material 52 . 53 . 54 , And at least two in addition to the light guide through the hose out, mutually insulated electrical conductors 61 and 62 attached to one end of the hose via an electrical impedance 23 and connected at the other end to an impedance-controlling measuring unit, the isola tion 63 and 64 the two electrical conductors 61 and 62 is chosen so that it is caused by the thermal effect of leaking light at a fraction of the light guide or by the radiation, either to influence the electrical conductors or direct contact of the electrical conductors or at least one of the electrical conductors is severed and a change in impedance can be detected by the measuring unit.

For the optical fiber 40 It is a commonly used optical fiber, which consists of an optical fiber core 41 which consists of an optical fiber jacket 42 coated, in turn, with a coating 43 (cladding) is provided.

The protective hose 50 consists according to the invention of several layers. At least one of the outer layers 52 . 53 . 54 According to the invention, it is non-transparent or optically dense, so that laser light emerging from the fiber can in no case escape to the outside.

Since this material inevitably heats up quickly due to the laser light, there is a risk that it may be destroyed in the sequence. This is the case in particular when high-energy laser radiation is guided in the optical fiber. To avoid this is an inner situation 51 inserted, which is transparent to the laser light and therefore does not heat itself directly. The distance between a possible breakage of the fiber to the outer jacket is thereby additionally advantageously increased, so that the laser light can be distributed and the outer shell is not so easily destroyed.

The transparent inner layer 51 is preferably a tube made of a non-colored fluoropolymer, eg PTFE, since this material is very temperature resistant. At lower loads (if the cable is not used for highest, but only for medium laser power) can also be another, cheaper transparent plastic can be used, for example polyamide. In order to minimize the risk of destruction of the nontransparent outer sheath when the laser light exits, the non-transparent material of the overlying layer may be designed to be optically reflective inward, so that it is not heated by the laser light, or it may be around the transparent tube 51 around an additional location 52 be attached from an optically reflective, highly thermally conductive material, preferably a dense braid of bare copper wires or a metal spiral hose. The use of bare copper, which forms the non-transparent layer lying over the transparent inner layer, or forms part of it, eg in the form of a braiding, a braiding or banding, is particularly preferred since this material has a very high degree of reflection for optical radiation in the infrared range, a high thermal conductivity and a relatively high melting point combined.

The actual fiber breakage sensor not only reacts to the melting of a lying close to the fiber electrical conductor 62 , but also to the reduction of electrical resistance to a second electrical conductor 61 , which is also close to the fiber. Preferably, both are conductors 61 and 62 combined into a thin cable, this is particularly preferred then a miniature coaxial cable 60 , The diameter of this cable 60 should be less than 1 mm, more preferably a diameter between 0.8 and 0.5 mm. The electrical conductors 61 . 62 of the cable can be made of copper strand, but the use of steel strands or wires is also possible.

The electrical insulation 63 . 64 of the cable 60 consists of one of the insulating materials commonly used for electrical strands, for example of PVC (polyvinyl chloride), PU (polyurethane), PA (polyamide), PTFE (polytetrafluoroethylene, eg Teflon) or FEP (perfluoroethylene propylene). In contrast to the inner transparent layer of the protective tube is the insulation material of the cable 60 colored, preferably black, for example by adding carbon, whereby the laser light is absorbed particularly well, so the cable heats up very quickly and is thereby destroyed, causing the two conductors 61 . 62 get into electrical contact, but at least the contact resistance is significantly reduced. As a result, laser light emerging from the fiber can already be detected long before the electrical conductor melts. Thus, the ability to detect an error is significantly improved. In particular, this system is also able to detect an error already when due to a defect of the optical fiber 40 initially only a small amount of laser light escapes from the fiber. The particularly high degree of safety is thus achieved by combining on the one hand the particularly sensitive laser light detection resulting from the inventive arrangement of optical fiber, electrical conductors and their isolation to each other, and on the other hand, the high protective effect of the protective tube by the multilayer structure according to the invention keeps the laser light emerging from the fiber inside and resists destruction for a particularly long time.

The protective tube can be designed differently depending on the desired degree of protection. A particularly simple embodiment is described below and in the 2 shown:
The inner situation 51 is formed by the transparent inner shell, which is preferably made of undyed or not colored fluoropolymer such as PTFE (Teflon) or FEP in the form of a tube consists. The wall thickness should be about 1 mm. Above is a non-transparent metallic layer 52 , Preferably formed as a dense braid of bare copper, also possible is a wound banding of bare copper band, or a braiding of copper wires. The outer end as mechanical protection forms a flexible plastic jacket 54 that in the non-transparent position 52 is extruded. Here, the same materials can be used as for coats of electrical cables, such as polyurethane (PU) or other thermoplastics. Such a protective hose is particularly suitable for the transmission of laser power below 500 W for fixed installation.

Furthermore, a further embodiment of the protective tube is preferred, which is particularly suitable for the transmission of highest laser powers in permanently flexible applications, for example on a robot. Such an embodiment is in the 3 shown:
The inner situation 51 is formed by the transparent inner jacket, which preferably consists of undyed PTFE (Teflon), the wall thickness should be min least 1 mm. Above is a first non-transparent metallic layer 52 , preferably formed as a dense braid or braiding of bare copper wires. Above is a thin separation layer serving as insulation 71 , which consists for example of wound Teflon tape or an extruded thermoplastic (eg PU). Above is a second, further non-transparent metallic layer 52 , preferably formed as a dense braid or braiding of bare copper wires. Above this is another separation layer serving as insulation 71 , Above it is a network of ropes, strands or wires, which are stranded with the lowest possible number of strokes. This layer serves for strain relief and is provided by individual strain relief elements 55 (Ropes, strands or wires) formed, which, for example - but not exclusively and exclusively - are ropes made of aramid fibers (Kevlar) or steel wires. Under mesh is understood in this context, a more or less dense network. The outer end forms a flexible plastic jacket 54 on the strain relief layer 55 is extruded. Here, the same materials as for coats of electrical cables can be used, such as polyurethane or other thermoplastics.

Of the Structure of the protective tube is not limited to the examples given, rather, you can inserted further layers be, for manufacturing reasons, or to further increase the mechanical strength, or to fulfill other functions useful or necessary. For example, e.g. the integration of isolated conceivable electrical wires with which transmit signals for control tasks can be. Crucial here is that required for the protective effect components according to the invention are present and are not impaired in their functioning.

The fiber optic cable has the following typical structure ( 4 and 5 ): on the input side of the fiber optic cable is a Lichtleitkabelstecker 11 which fits to a corresponding socket (not shown) of the laser source. The laser source delivers the laser beam 14 which is in the optical fiber 40 is coupled. There are electrical contacts on the plug 26 . 27 , which are short-circuited when plugging in the plug by the mating contacts located in the socket, or are forcibly opened when pulling the plug. Also on the input plug 11 a cable is attached, with which the conductors of the electrical safety loop are led out of the fiber optic cable, and that with an electrical plug's rule 20 is completed. At this connector are the contacts 31 . 32 respectively. 31 . 32 and 33 tapped, where the circuit for monitoring the conductor loop or the electrical impedance is connected. The optical fiber 40 will be together with the ladders 61 . 62 or the cable 60 inside the protective tube 50 guided. On the output side of the fiber optic cable is again a plug 12 , in which in the same way as at the input plug 11 electrical contacts 24 . 25 are short-circuited when plugging the plug through the located in the socket (eg on a laser processing optics) mating contacts, or be forcibly opened when pulling the plug. Inside the output connector 12 is the electrical impedance 23 , In both plugs 11 . 12 There are also temperature switches 21 . 22 , At the output connector 12 leaves the laser beam 15 the optical fiber 40 ,

Depending on the evaluation circuit used, the individual components of the monitoring can be interconnected differently. The simplest case is in 4 shown in which all the components of the monitoring system, ie plug monitoring 24 . 25 . 26 . 27 , Temperature switch 21 . 22 , combined fracture protection with laser light detection 61 . 62 , connected in series. In order for a cross-circuit to be detected as a fault, an electrical impedance must be present at the other end of the fiber-optic cable 23 additionally be connected in series, eg a defined electrical resistance. The control circuit must then match the resistance of the conductor loop to the contacts 31 and 32 and switch off the laser as soon as the resistance deviates significantly from the setpoint value, ie a switch-off must take place if the resistance becomes too small or too high. Circuits with such a behavior are under the term window comparator known.

The choice of impedance 23 can be adapted to the conditions of use of the LLK. If the LLK is preferably operated with low to medium laser power, for example with powers below 500 W, then the light escapes in the event of a defect of the fiber, in any case 63 . 64 melted; the severing of the conductors 61 and or, 62 is less likely then. The resistance of the conductor loop ( 60 ) will sink sooner. A particularly fast response of the monitoring circuit is achieved in this case, when the impedance has a relatively high resistance value, for example in the range of 1 MΩ, and the monitoring circuit checks the deviation of the impedance in a relatively narrow window, eg in case of overshoot or undershoot of the resistance value by 30% to 50%. Then an error is detected even with only a weak conductivity of the insulation. On the other hand, the measurement currents required for this resistance range are very low, which increases the risk of signal interference and thus could cause false tripping. To prevent the risk of electrical interference, it is more preferable to have the impedance 23 to be provided with a mean resistance value, for example in the range of 1 kΩ to 10 kΩ. This impedance range is also preferred when using the LLK for very high laser powers (eg over 500 W), since in this case, if the fiber breaks, the conductors 61 and or 62 melt through very quickly and therefore more likely to increase the resistance of the conductor loop 60 occurs. The window of the monitoring circuit is in this case to adapt to this resistance range.

Will in this way both the impedance 23 as well as the window of the monitoring circuit adapted to the intended use of the LLK, so to speak coded, then it is achieved at the same time that only suitable LLK can be used at the respective laser source. This makes it possible, for example, to adapt the thickness of the protective tube and thus the protective effect to the intended use of the LLK and to make a LLK cheaper for lower laser powers, as an erroneous use of a laser source with high performance by encoding the impedance 23 and the corresponding adjustment of the window of the monitoring circuit is excluded.

Another possibility of interconnection is in 5 shown. Here are between the contacts 31 and 33 the plug-in monitoring 24 . 25 . 26 and 27 , the temperature switch 21 and 22 and the breakage protection 62 connected in series. An impedance in this circuit is not necessary because a cross-circuit can not occur because the return 66 the conductor loop constructively separated from the breakage protection 62 is housed. As a result, the resulting conductor loop is between the contacts 31 and 33 very low impedance and only needs to be controlled to increase the resistance or interruption. As a result, for example, a so-called emergency stop switchgear can be connected directly here. The impedance 23 This version is only required for laser light detection by the conductors 61 and 62 (as a cable 60 summarized) is formed. The control of the impedance takes place as in the example of 4 eg with a window comparator between the contacts 31 and 32 instead of.

11

Lichtleitkabelstecker, input side

12

Lichtleitkabelstecker, output side

14

Laser beam, coupling side

15

Laser beam, outcoupling

20

multipolar electrical connector

21

temperature switch (Opener)

22

temperature switch (Opener)

23

defined electrical impedance (resistance or capacitance)

24

Contact for plug-in monitoring

25

Contact for plug-in monitoring

26

Contact for plug-in monitoring

27

Contact for plug-in monitoring

31

connection for circuit to control the electrical impedance

32

connection for circuit to control the electrical impedance

33

connection for circuit to control the electrical impedance

40

optical fiber

41

Optical fiber core

42

Optical fiber cladding

43

An optical fiber coating (Cladding)

50

flexible Conduit

51

transparent Hose (inner layer of the protective hose)

52

non-transparent Position of the protective tube (e.g., Cu braid or metal

helix)

53

location the protective hose with several electrical conductors

54

outer location of the protective tube

55

Strength members (e.g., steel wires or aramid fibers)

60

conductor loop (two-pole cable (e.g., coaxial cable))

61

ladder 1 of the bipolar cable

62

ladder 2 of the two-pole cable

63

isolation

64

isolation

66

electrical ladder

71

separating layer (e.g., thermoplastic or banding)

Claims (17)

Protective device for an optical fiber, consisting of a protective tube, at least one guided by the protective tube electrical conductor loop with a defined electrical impedance and a special insulation of the conductor loop, wherein a. the protective hose ( 50 ) has a two- or multi-layered structure, consisting of an inner layer ( 51 ) of an optically transparent, electrically insulating material and at least one overlying layer ( 52 ) of a non-transparent material, and b. a conductor loop ( 60 ) in addition to the optical fiber ( 40 ) through the hose ( 50 ), consisting of two mutually insulated electrical conductors ( 61 . 62 ) at one end of the hose over a defined electrical impedance ( 23 ) and at the other end connected to an impedance controlling measuring unit, c. the isolation ( 63 . 64 ) of the two electrical conductors ( 61 . 62 ) is selected so that it is caused by the thermal effect of leaking light in a fraction of the optical fiber or by the radiation, either to influence the electrical conductors or to a direct contact of the electrical conductors or at least one of the electrical conductors is severed, and d. a change of the resistance is detectable by the measuring unit. Protective device according to claim 1, characterized in that the optical fiber ( 40 ) is suitable for high-energy laser radiation. Protective device according to claim 1 or 2, wherein the inner transparent layer ( 51 ) of the protective tube ( 50 ) is a tube of a non-colored fluoropolymer. Protective device according to one of claims 1 to 3, wherein between the inner transparent layer ( 51 ) and an overlying non-transparent situation ( 52 ) of the protective tube ( 50 ) is an internally reflecting intermediate layer. Protective device according to claim 4, wherein it is in the intermediate layer is a metallic layer. Protective device according to claim 5, wherein the metallic Interlayer is a copper braid. Protective device according to claim 5 or 6, wherein the metallic intermediate layer is a flexible metal spiral hose. Protective device according to one or more of claims 1 to 7, wherein the two adjacent to the optical fiber ( 40 ) through the hose ( 50 ) guided electrical conductor ( 61 . 62 ) to a cable ( 60 ) are summarized. Protective device according to claim 8, wherein the through guided the hose Cable is a coaxial cable. Protective device according to claim 9, wherein a diameter of the cable ( 60 ) between 0.5 and 0.8 mm is particularly preferred. Protective device according to one or more of the preceding claims 1 to 10, wherein the protective tube ( 50 ) has a multilayer structure (from inside to outside) comprising a. an innermost layer ( 51 ), which is formed by a transparent, undyed tube of PTFE; b. lying above a non-transparent position ( 52 ), in particular a metallic layer, preferably a braid, a braiding or banding of bare copper, c. a mechanical protection serving and the outer end of the protective tube ( 50 ) forming casing ( 54 ). Protective device according to one or more of the preceding claims 1 to 10, wherein the protective tube ( 50 ) has a multilayer structure (from inside to outside) comprising a. an innermost layer ( 51 ), which is formed by a transparent, undyed tube of PTFE; b. above it a first non-transparent position ( 52 ), in particular a metallic layer, preferably a braid, a braiding or banding of bare copper, c. followed by a first, insulating separating layer ( 71 ), d. another non-transparent position ( 52 ), in particular a metallic layer, preferably a braid, a braiding or banding of bare copper, e. followed by another, insulating separating layer ( 71 ), f. that of a layer of strain relief elements ( 55 ) is encased, g. and a mechanical protection serving and the outer end of the protective tube ( 50 ) forming casing ( 54 ). Protective device according to claim 12, wherein the strain relief elements ( 55 ) Ropes are made of aramid fibers. Protective device according to claim 12, wherein the strain relief elements ( 55 ) Are steel wires. Protective device according to at least one of the preceding claims, wherein the insulation ( 63 . 64 ) which together with the optical fiber ( 40 ) through the protective hose ( 50 ) guided electrical conductor ( 61 . 62 ), which form the conductors used to monitor the impedance, consists of an insulating material such as PVC, PU, PA, PTFE or FEP. Protective device according to claim 15, wherein the insulating material dyed black by adding carbon. Protective device according to one or more of the preceding claims, wherein non-transparent layers ( 52 ), insulating release liners ( 71 ) and the mechanical protective layers ( 54 . 55 ) are arranged alternately.