Side-emitting light guide and method for its manufacture

Description

[0001] The invention relates generally to the technical field of light-emitting elements. Specifically, the invention relates to the use of light guides in lighting or display devices.

[0002] For lighting or display purposes, linear light sources may be desirable. A light emitter coupled with a side-emitting light guide can serve as such a light source. This arrangement is superior to an arrangement of many small emitters placed side by side, such as an LED string, in terms of both space-saving design and high robustness. With such a string, the high number of components means that individual LEDs can quickly fail, which is immediately noticeable and disruptive.

[0003] Another way to create linear light sources is with side-emitting optical fibers. These fibers have light emitters attached to one or both ends, which couple light into the fiber. Scattering elements are distributed within the fiber, scattering the light that passes through it. These light sources produce very homogeneous illumination. However, the total luminous flux is limited by the intensity provided by the light emitter.

[0004] Even if the optical fiber appears homogeneous to the eye, bending it can cause problems. When the fiber is bent, for example, during attachment to a substrate with clips, the scattering cladding, which is only loosely attached to the core, can detach or be stretched by the bend, becoming thinner at the outer edge and compressed in the center of the bend, potentially causing creases. This can create air gaps between the scattering cladding and the light-guiding core, which may persist even after realignment. These gaps then lead to locally varying light emission and thus inhomogeneous light intensity.

[0005] The invention is therefore based on the objective of providing a light source with a linear appearance that has a flexible light guide exhibiting homogeneous light emission with high luminance. This objective is achieved by the subject matter of the independent claims. Advantageous embodiments are specified in the dependent claims. A light source with a linear appearance is understood to be a light source that appears to the naked eye of an observer at a normal viewing distance as a luminous line without significant lateral extent. This line need not be straight, but can, in particular, also be curved.Accordingly, the invention provides a side-emitting optical fiber comprising at least one optical fiber configured as a side-emitting fiber, such that light guided in the fiber is scattered along its longitudinal direction, and a tube surrounding the fiber, wherein the tube is translucent and light-scattering, allowing light emitted by the fiber to pass through the tube while being scattered, and wherein the tube is surrounded by a transparent sheath. Both the tube and the sheath are made of plastic. The at least one optical fiber is loosely guided within the tube and is a side-emitting optical fiber.The hose has a wall thickness that is at least five times smaller than the outer diameter of the transparent sheathing, where the diameter of the transparent sheathing is at least 1 millimeter and the wall thickness of the hose is less than 200 micrometers.

[0006] Preferably, not just a single optical fiber, but a fiber bundle containing several optical fibers is guided within the tube. Dividing the fibers into multiple strands is advantageous for maintaining maximum flexibility of the optical fiber.

[0007] To ensure the flexibility of the optical fiber, at least one fiber, or the fiber bundle, is loosely guided within the tube 3. Accordingly, there is no connection between the tube and the adjacent fiber(s). This allows the fibers to move within the tube, particularly in the longitudinal direction, which facilitates bending and prevents stress on the fiber during bending.

[0008] Inside the tube, at least one fiber, preferably a plurality of light-conducting fibers in the form of a fiber bundle, is arranged, in which the fibers are each designed to emit light sideways, so that light coupled into the respective fibers is gradually scattered out along the longitudinal extent of the light guide, passes through the tube with further scattering, and is emitted to the outside through the sheath. The light guide thus has a structure of at least two layers, with the outer, transparent sheath and the tube forming an inner sheath.Because the tube is light-scattering, the angular distribution of the light emitted by the fibers is further homogenized, so that in a preferred embodiment, within an angular range of +45° to -45° in any spatial direction, the deviation of the emission intensity compared to 0° (perpendicular) is less than 55%, particularly preferably less than 30%, and most preferably less than 20%. This applies especially and also in curved sections of the optical fiber according to the invention.

[0009] The optical fiber can be manufactured using a method in which at least one light-conducting, side-emitting fiber is arranged in a light-scattering and translucent plastic tube, the tube containing the at least one light-conducting fiber is then surrounded by a transparent plastic sheath.

[0010] The optical fiber described here also avoids the problem of air gaps forming after bending. Even if a local air gap forms between the sheath and the tube, it no longer affects the light extraction and thus the local scattering efficiency, since the scattering and extraction of the light already takes place inside the tube in the individual light-conducting fibers.

[0011] The invention is described in more detail below, using the figures as an example. Brief description of the characters

[0012] Fig. 1 shows a light source with a side-emitting light guide. Fig. 2, Fig. 3 and Fig. 4 These are cross-sectional views through side-emitting fibers. Fig. 5 shows cross-sectional views through a light guide Fig. 6 is a variant with a single light-conducting fiber. Fig. 7 is a variant of the example of Fig. 5 with an effects section. Fig. 8 , Fig. 9 and Fig. 10The cross-sectional view shows variants of a light guide with a fastening element. Fig. 11 is an example of a light guide with additional channels in the longitudinal direction. Fig. 12 and Fig. 13 These are diagrams showing measured values ​​of the luminous flux of optical fibers before and after a bend. Fig. 14 shows the angle-dependent luminance for two optical fibers. Fig. 15 shows the luminance as a function of the longitudinal position along two light guides. Fig. 16 represents a light guide with a region surrounding the tube that has a lower refractive index. Fig. 17 shows the path of the light beam to a fiber optic cable. Fig. 18 shows simulations of light emission for a light guide with and without an air gap between the outer casing and the luminous inner part. Fig. 19 is a diagram of the apparent width of the luminous inner part as a function of the width of the air gap. Fig. 20shows a light source in which the light is coupled into the side-emitting optical fiber via another optical fiber. Detailed description of the invention

[0013] In Fig. 1 A light source 2 with a side-emitting optical fiber 1 according to this disclosure is schematically depicted. The light source 2 comprises a light emitter 4, which is optically coupled at one end 10 of the optical fiber 1 to at least one optical fiber 7, preferably to the optical fibers 7 of a fiber bundle 8, in order to couple light into one or more fibers 7. In general, without limiting it to the illustrated embodiment, semiconductor light emitters are preferred for the light source 2. These can include light-emitting diodes or semiconductor lasers. In this way, several light emitters of different colors can also be coupled to obtain illumination whose light color is adjustable. Unlike in Fig. 1Light emitters 4 can also be arranged at both ends 10, 11 of the light guide 1 in order to couple their light into the fibers 7.

[0014] Generally, the fiber bundle 8 runs inside a tube 3. This tube is in turn surrounded by a sheath 5. Both the tube 3 and the sheath 5 are made of plastic. This gives the optical fiber 1 high flexibility, allowing it to be easily routed and fixed with a bend 12. In the example shown, the optical fiber 1 has been provided with a bend 12 that deflects by 90°.

[0015] A side-emitting optical fiber typically differs from data transmission optical fibers in its shorter intended length, since the luminance decreases with increasing length for a given brightness of the coupled light emitter. In a further development of the invention, without being limited to specific embodiments, it is provided that the optical fiber 1 has a length in the range of 0.5 meters to 100 meters, preferably up to 50 meters. Longer lengths are particularly suitable in conjunction with lasers as light emitters. For decorative lighting and other lighting applications, shorter lengths up to 105 meters, preferably 2.5 to 5 meters, are particularly suitable. In the medical field, among other applications,As a component, for single or multiple use, in facilities for diagnostic and therapeutic treatments, lengths of less than 0.5 meters, preferably less than 0.1 meters, particularly preferably 0.01 to 0.05 meters are suitable, with a preferred diameter of the light guide 1 of approximately 1 mm and thinner.

[0016] Fig. 2Figure 1 shows a cross-sectional view of an embodiment of a light-conducting, side-emitting fiber 7. A fiber bundle 8 for the optical fiber can then be produced by combining several such fibers 7. Without limiting itself to the example shown, the fiber 7 generally comprises a fiber core 72 surrounded by a fiber cladding 70, wherein the fiber core 72 has a higher refractive index than the fiber cladding 70, so that light in the fiber core 72 can be guided in the usual manner under total internal reflection at the interface 73 with the fiber cladding 70. A light-scattering core 74 runs longitudinally along the fiber core 72. This core partially scatters the light guided in the fiber core 72 so that it can exit the fiber 7. In this way, light is continuously scattered out along the fiber 7.

[0017] Fig. 3 Figure 7 shows an example of another embodiment of a light-conducting fiber. Just like the fiber according to Fig. 3 The fiber 7 has a fiber core 72 surrounded by a fiber cladding 70 with a lower refractive index. This embodiment is based on the presence of light-scattering elements 75 at the interface 73 between the fiber core 72 and the fiber cladding 70. These elements change the direction of the light that would otherwise be reflected at the interface 73 by scattering, so that some of the light is scattered out. Generally, the light-scattering elements can also be present in the fiber cladding 70. In particular, the light-scattering elements located near or at the interface then cause the light to scatter out. Fig. 4Another embodiment is in which light-scattering elements 75 are distributed in the fiber core 72. In general, according to one embodiment, the light-conducting fibers 7 have a fiber core 72 and a fiber cladding 70, which has a lower refractive index than the fiber core 72, wherein at least one light-scattering element 74, 75 is arranged in the fiber 7 such that this light guided in the fiber core 72 is gradually scattered out in the longitudinal direction of the fiber 7. In the case of the Fig. 2 A single light-scattering element in the form of a core 74 extending longitudinally along the fiber 7 is provided. However, several cores 74 can also be present. All illustrated embodiments can also be combined with one another. Thus, the embodiment according to Fig. 2The fibers also include additional light-scattering elements 75 at the interface 73 or in the fiber core 72. The at least one fiber 7 can be made of glass or plastic. The light-guiding fibers 7 are side-emitting optical fibers. This embodiment is advantageous because the high transparency of the glass can be utilized over longer distances, and a flexible optical fiber 1 can still be created due to the use of a fiber bundle with several thin fibers.

[0018] Fig. 5Figure 1 shows a cross-section of a side-emitting optical fiber 1. The optical fiber 1 comprises a fiber bundle 8 with several light-conducting, side-emitting fibers 7 that scatter light along the longitudinal direction from the fibers 7 and thus also from the optical fiber 1, creating a thin, elongated, or linear illumination. The fiber bundle 8 is surrounded by a tube 3. The tube is light-scattering and translucent. This allows light emitted from the fibers 7 of the fiber bundle 8 to pass through the tube 3, where the light is generally at least partially scattered in the wall of the tube 3. The tube 3 is further surrounded by a transparent sheath 5. Both the tube 3 and the sheath 5 are made of plastic. This creates a flexible optical fiber 1 that is particularly bendable.

[0019] The tube 3 is generally preferably made of thin walls. This keeps the diameter of the assembly consisting of fiber bundle 8 and tube 3 as small as possible. This is advantageous for achieving high luminance. For this reason, the wall thickness of the tube and the diameter of the optical fiber 1, or the sheath 5, should differ as significantly as possible. A large ratio of outer diameter to wall thickness of the tube 3 also reduces the bending radii of the tube and the fiber bundle 8 when bent, thus preventing breakage of the fibers 7 in the fiber bundle 8 if the optical fiber 1 is bent too sharply. It is therefore provided that the tube 3 has a wall thickness that is at least five times smaller than the outer diameter of the transparent sheath 5.Furthermore, it is provided that the diameter of the transparent sheathing is at least 1 millimeter, preferably at least 2 millimeters, in order to limit the bending radii in the fiber bundle 8.

[0020] It is generally also stipulated that the wall thickness of the tube 3 is at most 200 micrometers, and in particular at most 100 micrometers. This ensures high flexibility of the inner part of the optical fiber with the tube 3 and the fiber bundle 8 contained therein. At the same time, this results in high luminance of the optical fiber and a homogeneous angular distribution of the laterally emitted light, particularly in combination with the use of scattering particles whose diameter is more than 100 times smaller than the wall thickness of the tube 3, most preferably more than 150 times smaller, and most preferably more than 200 times smaller. Since the scattering particles generally have a size distribution, the diameter of the scattering particles refers to the average value (also called D50).

[0021] The ratio of the diameter of the sheath 5 to the diameter of the tube 3 is preferably at least 1.5 / 1 in order to achieve a high luminance for a given diameter of the light guide 1 and to avoid excessively tight bending radii of the fiber bundle 8. Unlike in Fig. 5As shown, the sheathing 5 does not necessarily have to have a circular cross-section. The same applies to the tube 3. In the example shown, the tube 3 already conforms to the shape of the fiber bundle 8, and its cross-section is therefore not exactly circular. In such a case, the mean diameter of the sheathing 5 and the tube 8 can be assumed to be in the ratio of 1.5 to 1 given above. However, it is also possible to consider the cross-sectional areas. It is generally preferred that the ratio of the cross-sectional area enclosed by the outer contour of the sheathing 5 to the cross-sectional area enclosed by the outer contour of the tube 3 is at least 2.25 / 1. With a thin wall thickness of the tube 3, the cross-sectional area enclosed by the outer contour of the tube corresponds approximately to the cross-sectional area of ​​the fiber bundle 8.

[0022] According to a preferred embodiment, the fiber bundle 8 has a diameter in the range of 0.5 mm to 5 mm, preferably from 1 mm to 3 mm. Accordingly, for non-circular geometries, the cross-sectional area of ​​the fiber bundle is 0.19 mm² to 19.7 mm², preferably 0.78 mm² to 7.1 mm².

[0023] It has proven particularly suitable with regard to both the optical and mechanical properties of the optical fiber 1 if the tubing 3 is designed as a plastic tube shrunk around the fiber bundle 8. Shrinking allows for a good adaptation of the tubing 3's shape to the contour of the fiber bundle 8 without excessively compressing the bundle. Furthermore, such shrink tubing is available with a wall thickness of 30 µm or less, which further increases the optical fiber's luminance. If necessary, the plastic tubing can also be shrunk only partially or in sections around the fiber bundle. Generally, it is advantageous if the fibers 7 remain loosely bundled within the tubing 3, or are guided loosely within the tubing 3 without any material or form-fitting connection. This prevents tensile stresses on individual fibers 7 and the associated fiber breakage when the optical fiber 1 is bent.

[0024] For the light-conducting fibers 7, diameters in the range of 10 to 250 µm, preferably 20 µm to 100 µm, preferably 30 µm to 70 µm are suitable in order to achieve good flexibility of the fiber bundle 8.

[0025] Polyester is one suitable material for the tube 3. To impart light-scattering properties to the tube 3, light-scattering particles can be included as an additive in the plastic. According to one embodiment, the plastic contains oxide particles. Particularly suitable are high-refractive-index oxides, such as titanium oxide. In one embodiment, a tube 3 is thus provided in the form of a polyester tube containing oxide particles, especially titanium oxide particles. The scattering particles embedded in the tube 3 preferably have a diameter of less than 1 µm for more than 50% of the tube to increase the emission homogeneity.

[0026] A polymer blend has proven particularly suitable as a plastic for the coating 5. A polymer blend combines the properties of both high transparency and high flexibility. An aliphatic polyurethane can preferably be used as one component, or one of the polymers, of the polymer blend. This is particularly elastic and can be combined with other plastics to form a highly transparent polymer blend. In particular, such an aliphatic polyurethane can also be a thermoplastic elastomer. Alternatively or additionally, another thermoplastic elastomer can also be a component of the polymer blend. According to a further embodiment, at least one of the polymers of the polymer blend is a thermoplastic elastomer.

[0027] In another embodiment, one of the polymers in the polymer blend is a polycarbonate. Polycarbonates are advantageous for the transparency of the coating. The same applies to PMMA. According to yet another alternative or additional embodiment, one of the polymers in the polymer blend is therefore a polymethyl methacrylate.

[0028] A particularly suitable combination is a polymer blend, which generally contains polymethyl methacrylate and thermoplastic polyurethane. These two components can, in particular, form the polymer blend on their own, or, if other components are present, they can be the two components with the two largest proportions in the blend mixture.

[0029] According to a further embodiment, at least one of the plastics of hose 3 and sheath 5 contains polymer particles. These particles can further improve the mechanical properties. For example, polymer particles are known as additives for increasing impact strength. A very transparent, yet impact-resistant and abrasion-resistant, and therefore scratch-resistant, plastic can be obtained according to an advantageous embodiment if at least one of the plastics of sheath 5 and hose 3 contains polymer particles with cross-linked PMMA. This embodiment is particularly suitable in combination with a polymer blend containing PMMA. According to a further embodiment of the invention, a polymer blend is provided which contains PMMA as one component and additionally polymer particles with cross-linked PMMA.

[0030] With the optical fiber structure 1 described here, it is also possible to use additional additives, either as an alternative or supplement to impact modifiers, which would otherwise be detrimental to the optical properties. In particular, it is intended to include at least one additive in the sheath 5, which could be a flame retardant or a UV stabilizer. The optical properties of the optical fiber 1 are hardly affected by these additives in the sheath 5, since the transmission in the light-conducting fibers is not disrupted and the light travels only a short distance through the sheath 5 after passing through the tube 3.

[0031] Fig. 6 shows a variant of the example of Fig. 5Instead of a fiber bundle 8, this variant has a single side-emitting light-conducting fiber 7. As shown, the fiber 7 can be thicker compared to fibers in a fiber bundle to couple in as much light as possible. On the other hand, the overall cross-section of the optical fiber 1 can also be smaller than in an embodiment with a fiber bundle. As shown, the single fiber 7 is generally guided loosely in the tube 3, or not connected to the tube 3 at all, to facilitate bending of the optical fiber 1 and to avoid irreversible changes after a bending process. In the example shown, the fiber corresponds to the structure according to [reference to relevant figure]. Fig. 2 .

[0032] In one embodiment of the light guide 1, it is provided that the light guide 1 comprises an elongated optical effect section 19, which is connected to the sheath 5 and, in particular, embedded in the sheath 5 and extends along the longitudinal direction of the light guide 1, and whose properties influence the light emitted by the tube 3 with regard to color, brightness, or direction of emission. An example of this is shown in the variant of Fig. 7In this example, an effect section 19, in the form of a diffusely reflecting strip, is arranged in the casing. This modifies the spatial emission of the light such that an angular region around the light guide 1 is shaded, while the brightness in the remaining angular region is increased. The color of the emitted light can also be influenced by a suitable coloring or pigmentation of the effect section. For example, a light guide 1 can be provided whose tube 3 with fiber bundle 8 appears as a bright line in a first color against the background of light diffusely reflected at the effect section in a different color.

[0033] In general, without being limited to the example shown, according to another embodiment, a functional element extending longitudinally along the optical fiber 1 is guided within the tube together with the at least one light-conducting fiber 7. Such a functional element can, in particular, be an electrical conductor 27 or a reinforcing element 28. A metal or plastic wire or a rope is suitable as the reinforcing element 28. An electrical conductor 27 can also simultaneously serve as a stabilizing reinforcing element 28. Furthermore, it is also conceivable to arrange such a functional element outside the tube 3 within the sheathing 5.

[0034] The design of the light guide 1 with a plastic sheath 5 enables its manufacture using a continuous forming process, for example, by extrusion. This process allows even more complex geometries to be realized very easily. In one embodiment, the sheath 5 is provided with a fastening element 14 extending axially along the light guide 1 in the form of a spring or rib 24. Fig. 8 Figure 1 shows an example in which two such ribs 24 are formed in the sheathing 5. In contrast to the illustration, the fastening element can also be a separate element anchored in the sheathing 5. Figure 2 shows an example of this embodiment. Fig. 9The rib 24 can, for example, be a prefabricated element made of a different plastic than the material of the sheath 5, or it can be a metal element. The rib 24, in the form of a separate element, can be anchored in the sheath 5 during extrusion, just like the tube 3. To mount the light guide 1, the light guide with the rib 24 can then be inserted into a corresponding groove of a component to be fitted with the light guide 1.

[0035] Fig. 10This is an example of a further embodiment. In this embodiment, a fastening element 14 extending axially along the optical fiber 1 is provided in the form of a groove 25. The groove 25 can be inserted directly into the sheath 5, as shown, or formed from the shape of the sheath 5. In the examples shown so far, the tube 3 with the fiber bundle 8 was arranged more or less centrally in the cross-section of the sheath. The example of Fig. 10 This also shows that an eccentric arrangement is possible.

[0036] According to yet another embodiment, the casing 5 can have at least one axially extending channel 15, in particular in the form of a cavity 16. An example of such an embodiment is shown. Fig. 11In this example, there are three channels 15, each forming cavities 16. Unlike the illustration, the channels 15 can also be filled, for example with cables, wires, or a filler material. Fig. 11 This is also an example of how the cross-section of the tube 3 containing the fiber bundle 8 does not have to be circular. Rather, many variations in cross-sectional shape can be achieved with this arrangement. In this example, the tube has the cross-sectional shape of a sector of a ring, as do the two laterally adjacent channels 15.

[0037] In general, the optical fibers 1 described here are characterized by high resistance to changes in their optical properties after bending. This is because the light emission characteristics are essentially determined by the tube 3 and the fibers 7 contained within it. In particular, according to one embodiment, the additional light losses caused by bending the optical fiber 1 are less than 0.1 times the total light intensity per turn of the optical fiber 1 when the bending radius is 21 mm. Furthermore, the additional light losses caused by bending the optical fiber 1 can be less than 0.3 times the total light intensity per turn of the optical fiber 1 when the bending radius is 12 mm.These properties apply particularly to optical fibers 1 with an outer diameter, or a diameter of the cladding, of 3 mm or less, especially to optical fibers with a diameter in the range of 1 to 3 millimeters. The additional light losses, beyond those already caused by the side-emitting optical fiber, are otherwise recognizable as sections that appear brighter compared to adjacent areas. Accordingly, the light emission then appears uneven along the optical fiber, with one or more brighter areas. Furthermore, this additionally emitted light is missing further along the optical fiber, so that it is then darker overall. Fig. 12 and Fig. 13 This is shown by the measurements of the luminous flux before and after winding the optical fiber with one or more turns. Fig. 12shows the flow in its original state and after wrapping with one and two turns with a bending radius of 21 millimeters.

[0038] Fig. 13 Figure 1 shows a similar diagram with measured values ​​after winding with one to three turns and a bending radius of 12 millimeters. Curve (a) in both diagrams shows the measured values ​​for an optical fiber 1 as described herein. For comparison, curves (b) and (c) show measured values ​​for conventional side-emitting polymer optical fibers. The measured values ​​are normalized to the value without bending. As can be seen, the emitted intensity of the optical fiber described here is hardly affected by the strong bending. Even after winding with three turns and a bending radius of 12 mm, the flux is over 70% of the original value ( Fig. 13In contrast, state-of-the-art optical fibers (curves (b) and (c)) show reductions in flux of almost half after two turns with a bending radius of 21 mm ( Fig. 12 ) and with three turns and a bending radius of 12 mm by almost 2 / 3 ( Fig. 13 ).

[0039] The optical fiber described here also achieves a particularly high homogeneity of the emitted light. This results in particular from the fact that the angular distribution of the light emitted by the individual light-conducting fibers 7 is further homogenized by scattering in the wall of the tube 3. In one embodiment, the optical fiber 1 has an angular distribution of the emitted light in which the asymmetry A = (I 45° - I 135° ) / (I 45° + I 135° ) between the light intensity I 45° of the light emitted at an angle of 45° to the direction of light transmission and the light intensity I 135° of the light emitted at an angle of 135° to the direction of light transmission is: A < 0.1, preferably A < 0.05. This asymmetry expresses the difference between the emission of light with a directional component in the forward direction and the emission of light with a directional component in the reverse direction (i.e., opposite to the direction of light transmission).The low asymmetry in the radiation is determined by . Fig. 14 clarifies. In Fig. 14 Two curves (a) and (b) are shown, displaying measured values ​​of the angle-dependent luminance for two optical fibers. Curve (a) shows the measured values ​​for an optical fiber 1 according to this disclosure with a diameter of 2 mm. Curve (b) shows, for comparison, an optical fiber with a fiber bundle, but without a translucent, light-scattering tube 3. It is evident that the emission from an optical fiber according to this disclosure is almost perfectly homogeneous, i.e., angle-independent. In contrast, curve (b) shows a significant forward scattering. The emission with a directional component in the direction of the optical fiber is considerably stronger compared to the emission against the direction of the optical fiber.

[0040] The arrangement according to this disclosure also leads to a significantly higher luminance compared to previous side-emitting polymer optical fibers. This is demonstrated by the example of... Fig. 15 In the diagram shown here, the luminance was measured as a function of the longitudinal position along the optical fiber 1. Because the light is gradually scattered out of the optical fiber 1, the luminance decreases with increasing distance from the end of the optical fiber containing the light emitter. Since the fibers are bundled together with the tube to form a tight bundle with a small diameter, a higher luminance and thus an apparent higher brightness (curve (a)) results compared to an optical fiber in which the fibers are guided in a simple transparent tube (curve (b)).

[0041] One factor influencing luminance is refraction at the outer surface of the cladding 5. In the case of a light guide 1 with a circular cross-section, the outer surface of the cladding acts like a cylindrical lens. This magnifies the actively luminous part of the light guide 1, namely the tube 3 with the fiber bundle 8 contained within it. This optical magnification, however, reduces the luminance accordingly. Even though this effect does not directly affect the total emitted luminous flux, the actively luminous part appears larger, but dimmer. This effect is disadvantageous when the light guide is used as a design element that creates optical accents in the form of a luminous line. At low luminance, the light guide may, for example, no longer be visually noticeable in a bright environment, thus losing its function as an optical accent.It would therefore be desirable to design a light guide 1 with an inner, side-emitting, in particular strand-shaped or elongated part and a sheathing surrounding this part in such a way that an optical magnification effect caused by refraction at the outer surface is at least reduced at the outer surface of the sheathing 5.This problem is generally solved by an elongated side-emitting optical fiber 1, which has an inner elongated part, which can also be called a core, and a sheath 5 surrounding the inner part, wherein light can be guided in the inner part by coupling at one of the ends 10, 11 of the optical fiber 1, which is gradually directed laterally out of the inner part through the sheath 5 to the outside by scattering or refraction, so that the inner part, viewed through the sheath 5, appears as an elongated, in particular line- or thread-shaped, luminous element, wherein a layer surrounding the inner part is present between the inner part and the sheath 5, which has a refractive index that is lower than the refractive index of the sheath 5.

[0042] In a further development of the embodiments described above, the inner part is formed by the tube 3 with the fiber bundle 8 contained therein, since the tube 3 is that part of the optical fiber 1 that appears as a line or thread-like light source. An example of such an embodiment, namely an optical fiber 1 with a layer 9 surrounding the tube 3 having a lower refractive index, is shown. Fig. 16As mentioned, the tube 3 and the fiber bundle 8 form the inner part 6 enclosed by the layer 9. In the simplest case, which is particularly effective with regard to the optical effect, there is a gap or space between the sheath 5 and the inner part 6, so that the layer 9 is formed by a fluid layer, in particular an air layer or, more generally, a gas layer. In a further development of the previous embodiments, there is a fluid-filled, in particular gas-filled, and most preferably air-filled gap between the tube 3 and the sheath 9.

[0043] In another embodiment, the inner part 6 can also be formed by a single side-emitting fiber. Generally, the inner part 6 represents the part that appears as the luminous element. This luminous element is generally defined by the scattering elements that appear luminous and represent the points of origin of the emitted light for the observer.

[0044] Assuming a fixed connection between a white, luminous core, or inner part, and a transparent sheath, the light emanating from the inner part is uniformly distributed across all angles through the transparent sheath 5 and directed outwards at its surface 50 according to the law of refraction. It should be noted that an observer at a distance many times greater than the cable diameter (which is usually the case in practice) can only perceive with their eye the rays emitted from the sheath that are essentially parallel to each other. All rays emitted from the surface of the sheath at other angles are not projected onto the retina and are therefore not perceived. The sketch of the Fig. 17This illustrates the underlying path of the rays. Using the law of refraction, the limiting case can be determined, identifying the location of the outermost parallel rays still perceptible to the eye, which then constitute the perceived diameter of the light. According to the law of refraction, the angles α, β of this light ray 20 are given by: sinα sinβ = n Ummantelung n Luft

[0045] With sinα = b R and sinβ = r R This results in the following for the perceptible diameter D vis: D vis = 2 ∗ r ∗ n Mantel

[0046] This diameter corresponds to twice the distance b to Fig. 17With a refractive index of approximately ncladding = 1.5 for the transparent cladding and a diameter of 2·r of dcore = 2.1 mm for the white luminous core, or inner part 6, the resulting perceived luminous diameter Dvis is 3.2 mm. This corresponds to the outer diameter of the transparent cladding 5. Therefore, although a smaller luminous core is used compared to the outer diameter, this does not lead to the desired increase in luminance due to a smaller perceived area.

[0047] Only with smaller diameters of the inner part, for example 0.3 mm, is a luminous diameter of in this case 0.45 mm perceived, which is much smaller than the outer diameter of the light guide 1 of 3.2 mm and thus leads to the desired increase in luminance.

[0048] However, if a layer 9 with a lower refractive index is present, in particular an air gap between the luminous core or inner part 6 and the transparent outer layer 5, additional refraction of the light occurs at the inner surface of the transparent outer layer, namely at the interface between the outer layer 5 and layer 9, resulting in new conditions for the perceived luminous diameter. This can no longer be calculated simply analytically, but can be determined numerically or graphically via simulation. It turns out that, surprisingly, a perceived luminous diameter can even be achieved that is slightly smaller than the diameter of the white, luminous core, or inner part 6. The effect of the layer 9 on increasing the luminance arises primarily from the shape of the interface between layer 9 and the outer layer.In general, the interface 18 forms a refractive surface that is substantially, or at least partially or sectionarily, concave. According to a preferred embodiment, the interface 18 between the sheath 5 and the layer 9 is therefore designed to form a concave refractive surface, so that it acts as a refractive surface of a diverging lens or a defocusing optical element in the plane perpendicular to the longitudinal direction of the optical fiber 1.

[0049] Taking the foregoing considerations into account, in a further development of the invention it is generally preferred that the inner part 6 has a diameter of at least 0.75 millimeters. Below this diameter the effect is also present, but less noticeable due to the already small diameter.

[0050] Fig. 18Figures (a) and (b) show two simulations of the beam path for an optical fiber 1 with an air gap (Figure (a)) and without an air gap (Figure (b)). The air gap is omitted in Figure (a) for simplicity.

[0051] It is clearly evident that the apparent diameter D1 of the luminous inner part 6 is significantly smaller for an observer in the embodiment with an air gap than the diameter D2 in the embodiment without an air gap (partial image (b)). It is even possible that the apparent diameter is smaller than the actual diameter. Thus, the air gap, or more generally the layer 9 with the lower refractive index, effectively increases the luminance.

[0052] The thickness of layer 9, preferably the air gap, is of little overall relevance. This is shown in the diagram of the Fig. 19 In this diagram, half the apparent width is b, as it appears in Fig. 17The apparent width b is plotted as a function of the air gap width. Half the apparent width b corresponds to half the apparent diameter of the inner part 6, or the apparent radius. As can be seen from the diagram, the effect of the air gap width on the apparent width of the inner part 6 is minimal. The layer 9 can therefore be made very thin. In general, without limiting it to the example shown, one embodiment provides that the ratio of the diameter of the inner part 6, D6, to the diameter D9 of the layer 9 is given by: 1 − D 6 D 9 ≤ 0 , 3

[0053] As mentioned, the optical fibers 1 do not need to have a circular cross-section. The above relationship (3) can be formulated accordingly for the cross-sectional areas of the boundary curves of the inner part 6 and the layer 9. In this case, according to a further embodiment, the following applies: 1 − A 6 A 9 ≤ 0 , 09 . In this context, A 6 denotes the area enclosed by the outer boundary curve of the cross-section of the inner part 6 and A 9 denotes the area enclosed by the outer boundary curve of the cross-section of the layer 9.

[0054] It is usually sufficient to prevent or eliminate the connection between the tube 3 or the surface of the inner part 6 and the sheath 5. A gas-filled gap, in particular an air gap of sufficient width, is then maintained by the surface roughness. This also applies when the optical fiber is bent with virtually any bending radius. Alternatively, a coating containing particles can be present on the tube 3 or, more generally, on the inner part 6, with the particles separating the sheath 5 and the surface of the inner part and creating a gap. For a gas-filled gap, a dry gaseous medium with a dew point of no more than -20 °C is preferably used to prevent condensation in the gap. Suitable gases include, for example, dry air or an inert gas filling.

[0055] The light guide 1, as described here, is particularly suitable for contour lighting or, more generally, for decorative lighting, and thus also as a design element for the interiors of buildings and vehicles. As such a design element, the light guide can be used for lighting in the interiors of buildings and their furnishings, e.g., in furniture, or on or near equipment and machinery, as well as in the exterior of buildings, such as on facades, and for interior or exterior lighting of vehicles. Vehicles can include wheeled and rail vehicles, ships, and aircraft.

[0056] Particularly in vehicles such as automobiles, aircraft, ships, and / or trains, the fiber optic cable can be used as part of the interior trim. In the field of interior design, the fiber optic cable can be used as part of furniture, especially vehicle seats, living room furniture, and / or kitchens. Further applications include... Use as part of a headlight (40), in particular a vehicle headlight, for the illumination of runways for aircraft, for use together with other light guides and / or other side-emitting step-index fibers to form a planar structure which in turn can form a lighting body, for backlighting displays, for use as ambient or demarcation lighting in vehicles, ships, aircraft, buildings, roads, wayfinding signs, road signs, textiles, for use as safety lighting with self-illuminating phosphorescent additives.

[0057] Another application is its use in or on medical devices, for example as accent, contour, or safety lighting. It is also intended to use the light guide 1 or the light source 2 as a device, or at least as a component of a device, for a medical therapy procedure, in particular for photodynamic therapy (PDT) for tumor therapy, for endovenous laser therapy (EVLT) for the treatment of varicose veins, for laser-induced interstitial thermotherapy (LITT), or for applications in the fields of ophthalmology, dentistry, and dermatology. In the latter, it is also intended for supporting wound healing.With regard to use in medical technology, it is advantageous if the plastic(s) used, including polymer blends, preferably comprise biocompatible plastic materials that are listed, for example, according to standards EN ISO 10993-1:2018 or EN ISO 10993-5:2009 or USP Class VI. Furthermore, the material used should be selected to be sterilizable, particularly by ethylene oxide (EO), as this sterilization method is of particular interest for disposable or single-use applications in the medical technology sector, as described in ISO 11135:2014.Particular attention must be paid to ensuring that the material is chlorine-free, as otherwise chlorine-containing compounds may be formed during the EO process, which can be toxic and can only be incompletely removed after the sterilization process.

[0058] An embodiment of a light source 2, which is suitable among other things and especially for medical applications, is shown. Fig. 20In this embodiment, the light from the light emitter 4 is coupled into a further optical fiber 21, which is optically connected to one end 10 of the side-emitting optical fiber 1. This embodiment is particularly advantageous if the further optical fiber 21 is also flexible. Preferably, a short section is used for the side-emitting optical fiber. In a further development of this embodiment, optical fibers with a length in the range of 5 mm to 50 mm are used. The optical fiber 1 serves as a diffuser for the light supplied to the further optical fiber 21. For medical applications, the diffuser can be positioned on or inside the body of a patient being treated, and the light can be emitted there via the optical fiber 1. The further optical fiber 21 can be fused or bonded to the optical fiber 1 for optical coupling.For high light intensities, it is generally advisable to use a laser 40 as a light emitter 4.

[0059] Without limiting itself to the specific example shown, a light source 2 is provided in which the side-emitting optical fiber 1 is connected to a further flexible optical fiber 21, and in which a light emitter 4, preferably a laser 40, is coupled to the further flexible optical fiber 21, such that the light from the light emitter 4 is coupled into the side-emitting optical fiber 1 via the further optical fiber 21 and emitted along the side-emitting optical fiber 1. This light source 2 can be used in particular for the medical applications mentioned above.

[0060] The basic function of the side-emitting light guide 1 is achieved by light scattering in or on the tube 3. In addition to its scattering properties, the light guide 1 can also generally have filtering properties or alter the spectrum of the emitted light compared to the spectrum of the coupled light. In this way, a variety of lighting effects can be achieved. According to one embodiment, the tube 3 contains pigments or other colored scattering particles. These scattering particles absorb portions of the light depending on the wavelength, so that the reflected or scattered light also differs spectrally from the light guided in the core and acquires a color. According to another alternative or additional embodiment, at least one of the plastics of the tube 3 and the sheath 5 is colored. The dye generally does not increase scattering, so the plastic remains transparent.The dye according to this embodiment is therefore not a pigment. The partial spectral absorption of the dye alters the spectral distribution of the light passing through the plastic. Optionally, the dye is provided by the polymer itself or a polymer component, in contrast to the alternative of dye molecules dissolved in the polymer matrix.

[0061] Another way to spectrally influence light is by converting it to other wavelengths, i.e., by converting light of one wavelength into light of at least one other wavelength using a suitable conversion material. This is advantageous because it minimizes the loss of intensity when changing the spectral characteristics of the emitted light. Such conversion is achieved, in particular, by incorporating a photoluminescent material into the optical fiber 1. The photoluminescence can be either fluorescence or phosphorescence. The photoluminescent material can be contained in one or more of the components of the optical fiber 1 that come into contact with the light, especially in the core 3 and the outer sheath 5. For example, the tube 3 can contain photoluminescent particles 30.For example, blue light can be injected and scattered at tube 3, partially converted into yellow light, and then mixed to emit white light. Alternatively, a photoluminescent material can be incorporated into the casing to convert at least some of the intensity of the light exiting tube 3. This illustrates the different optical effects that can be achieved. With a colored casing 5, the perceived color of the brightly glowing tube 3 is altered. With a photoluminescent dye in the casing 5, a halo of light is created around the brightly glowing tube 3, and this halo has a different color than the light directly emitted by the tube 3.

[0062] By using phosphorescent additives, a glow-in-the-dark effect can be created in the light guide, even when the light source is switched off. This allows for the creation of emergency lighting, for example, for use in hospitals, vehicles, or aircraft. Furthermore, the phosphorescent particles in the light guide are also charged by external light, so such light guides can also be used as passive emergency lighting or passive accent lighting.

[0063] Due to their insensitivity to bending, the optical fibers according to the invention are particularly advantageous for use in photobioreactors or any other type of photobiological process. The optical fibers can be attached to the outside of (transparent) reactors or, regardless of the transparency of the reactor wall, to the inside of such reactors, since the optical fibers are impermeable to liquids and resistant to chemicals.

[0064] All of the aforementioned variants can also be integrated or applied to sections of the optical fiber, while other sections lack these functionalities. This allows for a wide variety of effects. Furthermore, sections can be configured to have no laterally emitting light, enabling low-loss transmission of light to more distant locations.

[0065] Generally, radiation sources for the optical fiber can be used in a wavelength range of 150 nm to 15 µm, depending on the fiber materials used and the desired application. Any type of laser source, LEDs, arc lamps, incandescent lamps, or any other suitable radiation source is conceivable. Reference symbol list

[0066] 1 optical fibers 2 light source 3 Hose 4 Light emitter 5 sheathing 6 inner part of 1 7 fiber 8 Fiber bundles 9 layer 10, 11 End of 1 12 bend in 1 13 gap 14 Fastener 15 channel 16 cavity 18 Interface between 5 and 9 19 optical effects section 20 beam of light 21 additional optical fiber 24 rib 25 Nut 27 electrical conductor 28 Reinforcement element 40 Laser 50 Outer surface of 5 70 Fiber sheath 72 Fiber core 73 Interface between 70, 72 74 Soul 75 Light-scattering element

Claims

1. A side-emitting light guide (1), comprising - at least one light-conducting fibre (7) in the form of a side-emitting fibre, such that light guided in the fibre (7) is scattered outward in a distributed manner along the longitudinal extension thereof, and - a tube (3) surrounding the fibre (7), wherein - the tube (3) is configured so as to be light-scattering and translucent, such that light emitted from the fibre (7) can pass through the tube (3) while being scattered, wherein - said tube (3) is surrounded by a transparent sheathing (5), the tube (3) and the sheathing (5) being made of plastic, wherein the at least one fibre (7) is loosely guided within the tube (3) and the at least one light-conducting fibre (7) is a side-emitting glass fibre; and wherein the tube has a wall thickness which is smaller by at least a factor of five than the outer diameter of the transparent sheathing (5), wherein the diameter of the transparent sheathing is at least 1 millimetre and the wall thickness of the tube is less than 200 micrometres.

2. The side-emitting light guide (1) according to the preceding claim, comprising at least one of the following features: - a fibre bundle (8) comprising a plurality of light-conducting fibres (7) is guided within the tube (3), wherein the light-conducting fibres (7) are side-emitting glass fibres, preferably the fibre bundle (8) is loosely guided within the tube (3); - the diameter of the transparent sheathing is at least 2 millimetres.

3. The side-emitting light guide (1) according to any one of the preceding claims, characterized by at least one of the following features: - the ratio of the diameter of the sheathing (5) to the diameter of the tube (3) is at least 1.5:1; - the ratio of cross-sectional areas of the cross-sectional area enclosed by the outer contour of the sheathing (5) to the area enclosed by the outer contour of the tube (3) is at least 2.25:1; - a fibre bundle (8) comprising a plurality of light-conducting fibres (7) is guided within the tube (3) and the fibre bundle (8) has a diameter in the range from 0.5 mm to 5 mm, preferably in the range from 1 mm to 3 mm; - the at least one light-conducting fibre (7) has a diameter in the range from 10 to 250 µm, preferably from 20 µm to 100 µm, preferably from 30 µm to 70 µm; - a fibre bundle (8) comprising a plurality of light-conducting fibres (7) is guided within the tube (3) and the cross-sectional area of the fibre bundle (8) is in a range from 0.19 mm2 to 19.7 mm2, preferably in a range from 0.78 mm2 to 7.1 mm2.

4. The side-emitting light guide (1) according to any one of the preceding claims, wherein the light-conducting fibres (7) or the at least one light-conducting fibre (7) comprise(s) a fibre core (72) which is surrounded by a fibre cladding (70), with the fibre core (72) having a higher refractive index than the fibre cladding (70), characterised in that the one or more light-conducting fibres (7) comprise at least one of the following features: - at least one light-scattering inner core (74) extends inside the fibre core (72) along the longitudinal extension of the light-conducting fibre (7); - light-scattering elements (75) are provided at the interface (73) between the fibre core (72) and the fibre cladding (70); - at least one scattering zone including scattering centres is provided at the interface (73) between the fibre core (72) and the fibre cladding (70), at least partially or in sections thereof; - light-scattering elements are distributed throughout the fibre core (72) and / or fibre cladding (70).

5. The side-emitting light guide (1) according to any one of the preceding claims, wherein a fibre bundle (8) comprising a plurality of light-conducting fibres (7) is guided within the tube (3), characterised in that the tube (3) is in the form of a plastic shrink tube (30) shrunk at least partially or at least in sections around the fibre bundle (8).

6. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that the plastic of the sheathing (5) comprises a polymer blend, preferably having at least one of the following features: - at least one of the polymers of the polymer blend is an aliphatic polyurethane; - at least one of the polymers of the polymer blend forms a thermoplastic elastomer; - one of the polymers of the polymer blend is a polymethyl methacrylate; - one of the polymers of the polymer blend is a polycarbonate, wherein, preferably, the polymer blend comprises polymethyl methacrylate and thermoplastic polyurethane in each case.

7. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that the plastic of the sheathing (5) comprises at least one additive, in particular a UV stabilizer, an impact modifier or a flame retardant.

8. The side-emitting light guide (1) according to any one of the preceding claims, characterized by at least one of the following features: - the light losses caused by a bending of the light guide (1) with a bending radius of the light guide (1) of 21 mm are less than 0.1 times the total light intensity guided within the light guide per winding of the light guide (1); - the light losses caused by the bending of the light guide (1) with a bending radius of the light guide (1) of 12 mm are less than 0.3 times the total light intensity guided within the light guide per winding of the light guide (1).

9. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that the light guide (1) exhibits an angular distribution of the emitted light, in which the following applies for the asymmetry A = (I45° - I135°) / (I45° + I135°) of the light intensity I45° of the light emitted at an angle of 45° relative to the light guiding direction and the light intensity I135° of the light emitted at an angle of 135° relative to the light guiding direction: A < 0.1, preferably A < 0.05.

10. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that the sheathing (5) has a fastening element (14) in the form of a rib (24) or a groove (25), which extends axially along the light guide (1).

11. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that the sheathing (5) has at least one channel (15), in particular in the form of a cavity (16), which extends along the axial direction.

12. The side-emitting light guide (1) according to any one of the preceding claims, characterized by at least one of the features: - the light guide (1) comprises an elongated optical effect section (19) connected to the sheathing (5), in particular embedded in the sheathing (5), which extends along the longitudinal extension of the light guide (1), the nature of which influences the light emitted from the tube (3) in terms of colour, brightness or emission direction; - the light guide (1) is configured to change the spectrum of the emitted light compared to the spectrum of the injected light; - the tube (3) contains pigments or coloured scattering particles; - at least one of the plastics of the core (6) and the sheathing (5) is coloured; - the light guide (1) contains a photoluminescent material.

13. The side-emitting light guide (1) according to any one of the preceding claims, characterised in that a fluid-filled, in particular gas-filled, gap is provided between the tube (3) and the sheathing (9).

14. The side-emitting light guide (1) according to any one of the preceding claims, which has an inner elongated part (6) and a sheathing (5) surrounding the inner part, wherein light is conductible in the inner part (6) by being injected at one of the ends (10, 11) of the light guide (1), said light being gradually directed laterally outward from the inner part (6) through the sheathing (5) due to scattering or refraction, so that the inner part (6), when viewed through the sheathing (5), appears as an elongated, in particular line-shaped or thread-like luminous element, wherein a layer (9) surrounding the inner part is provided between the inner part (6) and the sheathing (5), said layer having a refractive index that is lower than the refractive index of the sheathing (5), wherein preferably the layer (9) comprises a gas layer, wherein particularly preferably the following is true in each case: - for the ratio of the diameter D6 of the inner part (6) to the diameter D9 of the layer (9) the following applies: 1 - D6 / D9 ≤ 0.3; or - the following applies: 1 - A6 / A9 ≤ 0.09, wherein A6 is the area enclosed by the outer boundary curve of the cross-section of the inner part (6), and A9 is the area enclosed by the outer boundary curve of the cross-section of the layer (9); and wherein, most preferably, the interface (18) between the sheathing (5) and the layer (9) forms a concave refracting surface.

15. A method for producing a side-emitting light guide (1) according to any one of the preceding claims, wherein at least one light-conducting, side-emitting fibre (7) is arranged inside a light-scattering and translucent tube (3) made of plastic, and wherein the tube (3) with the at least one light-conducting fibre accommodated therein is surrounded by a sheathing (5) made of plastic.

16. A light source (2) comprising a light guide (1) according to any one of claims 1 to 14, wherein a fibre bundle (8) comprising a plurality of light-conducting fibres (7) is guided within the tube (3), and at least one light emitter (4) which is optically coupled to the fibres (7) of the fibre bundle (8) at one end (10, 11) thereof in order to inject light into the light-conducting fibres (7), wherein preferably, in the light source (2), the side-emitting light guide (1) is connected to a further flexible light guide (21), and a light emitter (4), preferably a laser (40), is coupled to the further flexible light guide (21), such that the light of the light emitter (4) is injected into the side-emitting light guide (1) via the further light guide (21) and is emitted along the side-emitting light guide (1).

17. Use of a light guide (1) according to any one of claims 1 to 14 or of a light source (2) according to claim 16 as contour lighting or accent lighting, or in a medical technology device.