Patent Description:
In line with current styling trends, lighting devices for motor vehicles are becoming smaller and smaller, especially narrower, with reduced height, and at the same time they are becoming more and more widespread, for example to have a continuous, non-divided light signature at the front and/or rear of the vehicle. In order to create a fine, thin light line and also to simplify the manufacturability of the entire system, thin and in particular flexible light guides can be used into which light is coupled at the front end and which exits laterally distributed over the length of the light guide.

An lighting device of the type mentioned above is known from <CIT>. The lighting device comprises a plurality of light emitting diodes (LED), from which light is emitted when the lighting device is in operation. The lighting device also includes a reflector that reflects the light onto a light coupler. The light coupler is made of transparent material so that the light inside the light coupler is guided from an entry surface to an exit surface. The entry surface has a larger diameter than the exit surface. Furthermore the entry surface of the light coupler is convex. Additionally, the lateral surface of the light coupler is concavely curved. At the exit surface of the light coupler there is the frontal entry surface of a light guide, from whose lateral surface the light can exit distributed over the length of the light guide. The chosen design of the light coupler ensures that the light at least partially impinges on the comparatively small entry surface of the light guide and is coupled into it.

A disadvantage is the comparatively complex design of the light coupler. Furthermore the light coupler cannot guarantee an effective coupling of the light into the light guide with a small cross section of the entry surface of the light guide.

From the <CIT> devices are known, which utilize holographic 4D plenoptic capture and display technologies to generate a light field function to provide glasses-less vision correction for observers with imperfect vision, and to project an image according to the generated light field function. Furthermore, methods are disclosed for calibrating a four-dimensional light field for a user with an uncorrected visual acuity.

From the <CIT> an optical system is known having a light source, a uniforming device and an optical fiber type lightguide, wherein the uniforming device comprises a first optical transmission portion connected to the lightguide and a second optical transmission portion connected to the first optical transmission portion including a plurality of optical fibers stretching across the inside thereof which are arranged in such a manner that a light-receiving end of each of the optical fibers is placed at a randomly assigned position on a light-receiving end of the second optical transmission portion and on the other hand a light-emitting end of each of the optical fibers is placed at a randomly assigned position on a light-emitting end of the second optical transmission portion, thereby uniforming the distribution of the quantity of light emitted therefrom.

A tutorial is known form <NPL>. The tutorial gives an overview of the transverse Anderson localization of light in one and two transverse dimensions. A pedagogical approach is followed throughout the presentation, where many aspects of localization are illustrated by means of a few simple models. The tutorial starts with some basic aspects of random matrix theory and light propagation through and reflection from a random stack of dielectric slabs. Transverse Anderson localization of light in one- and two-dimensional coupled waveguide arrays is subsequently established and discussed. Recent experimental observations of localization and image transport in disordered optical fibers are discussed. More advanced topics, such as hyper-transport in longitudinally varying disordered waveguides, the impact of nonlinearity, and propagation of partially coherent and quantum light, are also examined.

The problem underlying the present invention is the creation of an lighting device of the type mentioned at above, which enables an effective coupling of the light into the light guide even with a small cross-section of the entry surface of the light guide.

According to the invention, this is achieved by a lighting device of the aforementioned kind with the characteristic features of claim <NUM>. The subclaims concern preferred embodiments of the invention.

According to claim <NUM> it is intended that the light guiding within the light coupler is based on the Anderson Localization, preferably the Transversal Anderson Localization. The Transversal Anderson Localization ensures that light can propagate in the light coupler essentially exclusively in the direction in which the entry surface and the exit surface are opposite each other. This ensures that the light entering the entry surface exits the exit surface almost completely and can be coupled very effectively into the light guide if the entry surface of the light guide is suitably arranged.

It may be provided that the light coupler comprises at least two light-guiding materials having refractive indices different from each other, wherein for example one of the materials may be air. Two light-guiding materials with different refractive indices can lead to the occurrence of the Transverse Anderson Localization. The effect is greater if the difference in refractive indices is as large as possible. The transparent, light-guiding materials can be especially plastic or glass.

It is possible for the light coupler to comprise a plurality of fibers, preferably the individual fibers having a cross-section smaller than <NUM>. The aim is to achieve a cross-section of the individual fibers that is smaller than the wavelength of the visible light. The wavelength of visible light lies in the range from <NUM> to <NUM>. Due to a fiber cross-section smaller than <NUM> a light guiding based on the Transversal Anderson Localization is efficiently possible.

It may be provided that the light coupler comprises a plurality of first fibers having a first refractive index and a plurality of second fibers having a second refractive index different from the first refractive index. The alternating fibers correspond to the two transparent, light-guiding materials with different refractive indices.

It is possible for the first and the second fibers to be arranged randomly next to one another in transverse directions, said transverse directions being perpendicular to the direction of propagation of the light propagating from the at least one entry surface to the exit surface. As a result, the at least two optical materials with different refractive indices are arranged stochastically or randomly along two transverse dimensions of the light coupler and run homogeneously along the third dimension, the third dimension corresponding to the direction of propagation of the light propagating from the at least one entry surface to the exit surface. The refractive index is thus constant in one dimension along the respective fiber and randomized over all fibers along the two remaining dimensions. This favors the occurrence of the Transversal Anderson localization, so that the light propagates essentially exclusively in the third dimension or in the direction in which the entry surface and the exit surface are opposite each other.

It may be provided that the light coupler is made by compressing, heating and drawing the plurality of first and second fibers or a plurality of fibers with air inclusions in random arrangement, so that by fusing the different fibers or the fibers with air inclusions a mixed light-guiding material with at least two different refractive indices is produced. By heating and drawing, the cross-section of the fibers can be reduced to dimensions smaller than <NUM>. Furthermore, this results in a firm bond between the individual fibers. If only one type of fibers with air inclusions is used, the compressing, heating and drawing of the air inclusions creates elongated air ducts extending between the entry surface and the exit surface. The fibers and the air ducts are arranged stochastically or randomly. In both cases this light-guiding material shows in a special way the desired material properties for a light guide based on the Transversal Anderson Localization. The material has a statistically varying transverse refractive index and a longitudinally constant refractive index in the direction between the entry surface and the exit surface.

It is possible that the light coupler has a decreasing cross-section starting from its entry surface, wherein the shape of the light coupler corresponds to a truncated cone or a truncated pyramid and the smaller diameter of the truncated cone or the truncated pyramid faces the entry surface of the light guide. It may be provided that the extension of the exit surface of the light coupler is smaller than or equal to the extension of the entry surface of the light guide, in particular wherein the diameter of the exit surface of the light coupler is smaller than or equal to the diameter of the entry surface of the light guide. By such a design the light can be directed and bundled to the smaller light entry surface of the light guide like in a funnel. This results in an effective coupling of the light impinging on the light coupler into the optical fiber.

It is possible that the exit surface of the light coupler abuts the entry surface of the light guide, preferably being bonded to the entry surface of the light guide. This enables a very effective coupling of the light into the light guide. It may be advantageous that the light guide has a recess into which the light coupler projects, or in that the light coupler has a recess into which the light guide projects.

It may be provided that no optical component is arranged between the light source and the light coupler, in particular that the entry surface of the light coupler is arranged directly opposite the light emission surface of the at least one light source. This reduces the losses before coupling.

It is possible for the lighting device to comprise several light sources each with a light emitting surface, which are arranged next to one another, in particular in one or two directions, the lighting device is configured so that, during operation of the lighting device, light emanating from the light emitting surfaces of the light sources enters the entry surface of the light coupler, in particular at least substantially completely.

It may be provided that the extent of the light emitting surface of the at least one light source or the extent of the surface area formed by the light emitting surfaces of the light sources arranged next to one another is smaller than or equal to the extent of the entry surface of the light coupler. This ensures that essentially all light emitted by the at least one light source or by the light sources enters the light coupler.

It is possible that the light guide is an extruded plastic light guide or that the light guide is a glass light guide with an entry surface diameter of less than <NUM> or that the light guide is a glass light guide bundle with a common entry surface having a diameter of less than <NUM>. Such light guides may be flexible and used to produce a fine, thin line of light, for example at the front and/or rear of the vehicle.

It may be provided that the at least one light source is a light emitting diode (LED). Despite the design of the at least one light source as a light emitting diode, the light generated by the light source can effectively enter the possibly very small light entry surface of the light guide by using the light coupler. Without a light coupler this was previously only possible with laser diodes.

The invention is explained in more detail below on the basis of the attached schematic drawings and shows.

In the figures, identical and functionally identical parts are provided with the same reference signs.

The first version of a lighting device according to the invention shown in <FIG> comprises a light source <NUM>, which is designed as a light emitting diode arranged on a printed circuit board <NUM>. The light emitting diode or the light source <NUM> has a light emitting surface <NUM> (see <FIG>) from which light <NUM> is emitted when the lighting device is in operation (see <FIG>).

The lighting device further comprises a light coupler <NUM>, which has an entry surface <NUM> and an exit surface <NUM>. During operation of the lighting device, the light emitted from the light emission surface <NUM> enters the entry surface <NUM> and is guided inside the light coupler <NUM> to the exit surface <NUM>. The light coupler <NUM> has a decreasing cross section starting from its entry surface <NUM>, whereby the shape of the light coupler <NUM> corresponds to a truncated cone or a truncated pyramid. The larger diameter of the truncated cone or the truncated pyramid faces the light emitting surface <NUM> of the light source <NUM>.

The extension of the light emission surface <NUM> of the light source <NUM> is smaller than the extension of the entry surface <NUM> of the light coupler <NUM> (see <FIG>). Furthermore, the entry surface <NUM> of the light coupler <NUM> is located directly in front of the light emission surface <NUM> and projects beyond the light emission surface <NUM> in the directions parallel to the plane of the printed circuit board <NUM>. This ensures that essentially all light emanating from the light source <NUM> enters the light coupler <NUM>.

The light coupler <NUM> can consist of two transparent, light-conducting materials with different refractive indices, which can be designed in particular as first and second fibers. The fibers can be randomly arranged next to each other in transverse directions, the transverse directions being perpendicular to the direction in which the entry face <NUM> and the exit face <NUM> are opposite each other.

The refractive index is thus constant in one dimension along each fiber and randomized across all fibers along the two remaining dimensions, resulting in an effect known as Transversal Anderson Localization. Accordingly, the light inside the light coupler <NUM> propagates essentially exclusively in the third dimension or in the direction in which the entry surface and the exit surface are opposite each other.

The lighting device further comprises a light guide <NUM>, which has an entry surface <NUM> facing the exit surface <NUM> of the light coupler <NUM>. The light guide <NUM> may have non-imaged, suitable structures which cause the light to emerge laterally from the outer surface of the light guide <NUM>, distributed over the length of the light guide <NUM>.

The light guide <NUM> can be very thin and flexible. The light guide <NUM> can be an extruded plastic light guide, for example. Alternatively, the light guide <NUM> can be a thin glass light guide with an entry surface diameter of less than <NUM>, for instance a diameter of <NUM>. Alternatively, the light guide <NUM> can be a thin glass light guide bundle with a common entry surface having a diameter of less than <NUM>, for instance a diameter of <NUM>. Such light guides can be flexible and can be used to create a fine, thin line of light, for example at the front and/or rear of the vehicle.

The extension of the exit surface <NUM> of the light coupler <NUM> is smaller than the extension of the entry surface <NUM> of the light guide <NUM>. It may be provided that the exit surface <NUM> of the light coupler <NUM> lies against the entry surface <NUM> of the light guide <NUM>, preferably glued to it.

In the design according to <FIG> and <FIG>, the light guide has a recess <NUM> into which the end of the light coupler <NUM> provided with the exit surface <NUM> projects. Alternatively, the light coupler <NUM> may have a recess not shown in the illustration into which the light guide <NUM> can protrude.

The design of the lighting device according to <FIG> comprises four light sources <NUM> designed as light emitting diodes, each with a light emitting surface <NUM>. The light emitting diodes are arranged next to each other in two directions on the circuit board <NUM> (see <FIG>).

Here, the extension of the surface area formed by the adjacent light emitting surfaces <NUM> of the light sources <NUM> is smaller than the extension of the entry surface <NUM> of the light coupler <NUM> (see <FIG>). This ensures that essentially all light emitted by the light sources <NUM> enters the light coupler <NUM>.

Claim 1:
Lighting device for a motor vehicle, comprising
- at least one light source (<NUM>) with a light emitting surface (<NUM>),
- a light coupler (<NUM>) having an entry surface (<NUM>) and an exit surface (<NUM>), the entry surface (<NUM>) having a greater extent than the exit surface (<NUM>), wherein the light coupler (<NUM>) comprises at least one light-guiding material such as plastic or glass, and
- a light guide (<NUM>) with an entry surface (<NUM>), wherein the lighting device is configured so that during operation of the lighting device light (<NUM>) emanating from the light emitting surface (<NUM>) of the at least one light source (<NUM>) enters the entry surface (<NUM>) of the light coupler (<NUM>), is guided within the light coupler (<NUM>) from the entry surface (<NUM>) to the exit surface (<NUM>), emerges from the exit surface (<NUM>) of the light coupler (<NUM>) and enters the entry surface (<NUM>) of the light guide (<NUM>),
characterized in that the light guiding within the light coupler (<NUM>) is based on the Anderson Localization, preferably the Transversal Anderson Localization.