ILLUMINATION DEVICE FOR VEHICLES

Abstract

A hologram for an illumination device for vehicles and a corresponding illumination device are provided. The hologram has a plurality of holographic structures designed for a respectively associated wavelength, wherein the holographic structures have diffraction properties that are identical among one another.

Claims

1. A hologram for an illumination device for vehicles, comprising a plurality of superimposed holographic structures, wherein each of the plurality of holographic structures for a respectively associated reconstruction wavelength has identical diffraction directions for the reconstruction of an extensive holographic object, wherein the associated reconstruction wavelengths are different.

2. The hologram as claimed in claim 1, wherein each of the plurality of superimposed holographic structures for an image point of the holographic object locally has an associated group of Bragg planes, wherein the Bragg planes of different groups which are associated with the image point are locally parallel to one another and have distances corresponding to the respectively associated reconstruction wavelengths within each group.

3. The hologram as claimed in claim 1, wherein a thickness of an active layer of the hologram is greater than 50 μm.

4. The hologram as claimed in claim 3, wherein the thickness is greater than or equal to 140 μm.

5. The hologram as claimed in claim 1, wherein values of full width at half maximum for diffraction efficiencies associated with the wavelengths overlap at least for some of the wavelengths.

6. The hologram as claimed in claim 1, wherein at least three of the reconstruction wavelengths lie in a wavelength interval of 50 nm.

7. The hologram as claimed in claim 6, wherein the at least three of the reconstruction wavelengths lie in a wavelength interval of 25 nm.

8. The hologram as claimed in claim 1, wherein the plurality of superimposed holographic structures comprises more than three holographic structures.

9. The hologram as claimed in claim 8, wherein the plurality of superimposed holographic structures comprises more than five holographic structures.

10. An illumination device for motor vehicles, comprising: a light source arrangement for generating an illumination light beam, and a hologram comprising a plurality of superimposed holographic structures, wherein each of the plurality of holographic structures for a respectively associated reconstruction wavelength has identical diffraction directions for the reconstruction of an extensive holographic object, wherein the associated reconstruction wavelengths are different, and an optical arrangement for directing the illumination light beam onto the hologram.

11. The illumination device as claimed in claim 10, wherein the associated wavelengths are distributed over a spectral range which is greater than a full width at half maximum of a spectrum of the light source arrangement.

12. The illumination device as claimed claim 10, wherein each of the plurality of superimposed holographic structures for an image point of the holographic object locally has an associated group of Bragg planes, wherein the Bragg planes of different groups which are associated with the image point are locally parallel to one another and have distances corresponding to the respectively associated reconstruction wavelengths within each group.

13. The illumination device as claimed claim 11, wherein a thickness of an active layer of the hologram is greater than 50 μm.

14. The illumination device as claimed claim 13, wherein the thickness is greater than or equal to 140 μm.

15. The hologram as claimed in claim 10, wherein values of full width at half maximum for diffraction efficiencies associated with the wavelengths overlap at least for some of the wavelengths.

16. The illumination device as claimed claim 10, wherein at least three of the reconstruction wavelengths lie in a wavelength interval of 50 nm.

17. The illumination device as claimed claim 16, wherein the at least three of the reconstruction wavelengths lie in a wavelength interval of 25 nm.

18. The illumination device as claimed claim 10, wherein the plurality of superimposed holographic structures comprises more than three holographic structures.

19. The illumination device as claimed in claim 18, wherein the plurality of superimposed holographic structures comprises more than five holographic structures.

Description

[0053] The invention is explained in greater detail below on the basis of embodiments with reference to the accompanying drawings, In the figures:

[0054] FIG. 1 shows a hologram in accordance with the prior art,

[0055] FIGS. 2A to 2C show graphs for elucidating the diffraction efficiency of holograms of various thicknesses,

[0056] FIG. 3 shows a sectional view of an illumination device in accordance with one embodiment,

[0057] FIG. 4 shows a schematic illustration of a hologram in accordance with one embodiment, and

[0058] FIGS. 5A to 5C show graphs for elucidating the diffraction efficiency of holograms of various embodiments.

[0059] Various embodiments are explained below with reference to the accompanying drawings. It should be noted that these embodiments serve merely for elucidation and should not be construed as limiting. Moreover, elements of different embodiments can be combined with one another in order to form further embodiments. Variations, modifications and details which are described for components of one of the embodiments are also applicable to corresponding components of other embodiments.

[0060] FIG. 3 shows a sectional view of an illumination device in accordance with an embodiment.

[0061] The illumination device of FIG. 3 comprises a light source arrangement 30, which can have one or more light emitting diodes, for example, If the illumination device in FIG. 3 is a tail lamp, said light emitting diodes can be red light emitting diodes, in particular, i.e, emit in the red spectral range. For other types of illumination devices, light sources which emit in a different part of the spectrum (for example yellow for indicator lamps) or broadband white light (for example for front headlamps) can correspondingly be used. In addition, the illumination device in FIG. 3 has a reflection hologram 35, which, upon illumination by light from the light source arrangement 30, produces a luminous signature, i.e. diffracts or directionally scatters the light in order to reproduce a virtual object stored in the reflection hologram 35, from which virtual object the light then apparently emanates. Said virtual object can in particular lie outside the vehicle in which the illumination device from FIG. 3 is installed, in order thus to bring about the impression of an illumination device outside the vehicle (for example behind the vehicle in the case of a tail lamp).

[0062] In this case, the reflection hologram 35 is a reflection hologram according to the invention having holographic structures for a plurality of reconstruction wavelengths (also referred to simply as wavelengths hereinafter). The implementation of such a hologram according to the invention will be explained in greater detail later.

[0063] In the case of the embodiment in FIG. 3, beams 32 pass to a beam deflection region 31 of a light guiding body 34. The beams 32 are deflected in the beam deflection region 31 such that they illuminate the reflection hologram 35. The refractive index of the light guiding body 34 here is preferably close to the refractive index of the reflection hologram 35 in order to minimize reflection losses at internal interfaces.

[0064] Apart from the use of the hologram 35 according to the invention, the illumination device in FIG. 3 corresponds to an illumination device described in the German patent application 10 2017 124 296.1 cited in the introduction, and will not be described in any further detail. Said illumination device serves as one example for the use of the hologram 35 according to the invention. However, such a hologram according to the invention can also be used in other illumination devices for vehicles, for example other illumination devices described in the German patent application 10 2017 124 296.1, or else illumination devices described in DE 10 2016 117 969 Al in the name of the applicant, or else other illumination devices for vehicles in which a hologram is illuminated by a light source arrangement.

[0065] As explained in the German patent application 10 2017 124 296.1 in the name of the applicant, the beam 32 can also be guided in the light guiding body 34 by multiple reflection at the outer surfaces facing outward, in order thus to illuminate the entire reflection hologram 35, wherein a portion of the light incident on the reflection hologram is in each case diffracted or directionally scattered in order to generate the extensive virtual holographic object (also referred to simply as virtual object for short hereinafter). Locally a region 33 which generates a region 38—illustrated as a jigsaw piece—of the virtual object or else the entire virtual object will be considered for the following explanations. The virtual object has virtual image points 37, virtual beams 36 emanating for each image point. In this case, “virtual” means that for an observer the light beams apparently emanate from the reconstructed virtual object, but in reality are diffracted or directionally scattered toward an observer by the hologram 35 in such a way that the diffracted or scattered beams are superimposed so as to give rise to the impression of the virtual object. Even though a virtual object is used here as an example of a holographic object, the techniques illustrated here can also be applied to holograms that generate real holographic objects or objects lying in the plane of the hologram.

[0066] It should be noted that such holograms that store a virtual holographic object that is then reproduced should be differentiated from holograms that serve to provide an optical function such as a mirror function, for example. In the former case, a virtual, in particular three-dimensional, extensive object having defined dimensions, for example, arises in space, while in the second case the hologram recreates the effect of an optical element, for example of a mirror (plane mirror, parabolic mirror, etc.).

[0067] FIG. 4 shows a schematic cross-sectional view of part of a holographic layer 40, as an implementation example for the hologram 35 from FIG. 3, in particular the region 33 considered. Elements corresponding to the elements that have already been discussed with reference to FIG. 1 bear the same reference signs here and will not be explained again. In particular, in the embodiment in FIG. 4, too, an illumination beam 10 is incident on the holographic layer 40 at an angle, for example 70 degrees with respect to the normal 12, and diffracted object light beams 15 are generated in response to this illumination, said object light beams bringing about the impression of a virtual object for an observer.

[0068] In this case, the holographic layer 40 contains for each image point of the virtual object at least two groups of Bragg planes as holographic structures, wherein the groups are parallel to one another, but the distances between the Bragg planes are different between the groups. As a result, each group of Bragg planes is designed for a different wavelength, wherein as a result of the parallel course of the groups among one another the diffraction properties for the different wavelengths are identical in each case. A first group of Bragg planes 42 having a distance D1 and a second group of Bragg planes 43 having a distance D2 are illustrated in the example in FIG. 4. As already explained for FIG. 1, in FIG. 4, too, a plurality of further corresponding groups of Bragg planes running in other directions can be provided in order to obtain a corresponding emission characteristic of the object light beams corresponding to the object stored in the holographic layer 40.

[0069] In other words, the same object for the same direction of the illumination light beam 10 is written into the holographic layer repeatedly for different wavelengths. In this way, the plurality of wavelengths are diffracted by the holographic layer 40 in an identical way. In this case, the plurality of groups of Bragg planes shown in FIG. 4 are present at each point of the hologram and for each image point to be represented, such that overall the hologram has a plurality of superimposed structures for the reconstruction of a virtual object with identical diffraction properties, wherein each of the plurality of superimposed structures is designed for a different wavelength.

[0070] In this case, the holographic layer 40 is preferably relatively thick, for example thicker than 50 μm or thicker than 140 μm, in order to achieve correspondingly sharp diffraction characteristics, as already explained with reference to FIG. 2.

[0071] In this case, the wavelengths associated with the different groups can be relatively close together, but can also be further away from one another. They preferably lie in a range which corresponds approximately to the width of a spectrum of a light source used or goes somewhat beyond that. This will now be explained on the basis of various embodiments with reference to FIG. 5.

[0072] FIG. 5 shows, in a manner similar to FIG. 2, the diffraction efficiency q in percent for a case in which the object was written into the holographic layer for five different wavelengths. The number of five wavelengths should be understood here merely as an example. In other words, in such a case, there are five groups of Bragg planes for each direction of Bragg planes (that is to say for a respective image point), wherein the groups are parallel among one another, but have different distances between the Bragg planes between the groups according to the different wavelengths.

[0073] In addition, FIGS. 5A to 5C schematically show the spectrum 20 of a light emitting diode, as already explained with reference to FIGS. 2A to 2C.

[0074] In all cases in FIGS. 5A to 5C, the hologram thickness was 250 μm, resulting in correspondingly sharp, or narrow, peaks of the diffraction efficiency

[0075] In the embodiment in FIG. 5A, the individual peaks are spaced relatively far apart from one another and distributed equidistantly between approximately 610 nm and 655 nm. The “outer” peaks therefore lie at places where the spectrum 20 of the light emitting diode has only a low intensity.

[0076] As long as the spectrum of the light emitting diode is as represented by the curve 20, principally the central peak, the peak to the left of the central peak and the peak to the right of the central peak make the main contribution to the diffracted light. Since the peaks are very sharp, hardly any widening of fine structures of the resulting virtual object occurs. On the other hand, by virtue of the further groups of Bragg planes, the use of the spectrum of the light emitting diode is thus improved overall compared with the case in FIG. 2C.

[0077] In addition, a drift, e.g. thermal drift, of the light source arrangement can be compensated for by the arrangement in FIG. 5A. Thermal drift or other fluctuations can cause the spectrum 20 to shift toward higher or lower wavelengths. In this case, one of the two outer peaks in FIG. 5A then makes a greater contribution (the one in whose direction the spectrum shifts), while other peaks make a smaller contribution. A decrease in the intensity of the resulting luminous signature on account of thermal drift can be prevented or at least reduced in this way.

[0078] For this purpose, generally the wavelengths of the different groups can be distributed over a greater range, in particular greater than the full width at half maximum of the spectrum of the light source used.

[0079] In the embodiment in FIG. 5B, the peaks are close together and even overlap. In this way, a central portion of the spectrum of the light emitting diode 20, i.e. that portion with the greatest intensity, can be utilized well.

[0080] In the example in FIG. 5C, the peaks are distributed approximately over the full width at half maximum of the spectrum 20. Here a central range of the spectrum is still utilized well, and a drift of the spectrum is also at least approximately compensated for to a certain extent.

[0081] It should be noted that combinations of the possibilities illustrated in FIGS. 5A to 5C are also possible. By way of example, a plurality of overlapping peaks can be arranged in a central range of the spectrum of the respective light source arrangement, in a manner corresponding to Bragg planes having plane distances that differ from one another only slightly, and further peaks can be arranged further away from a center of the spectrum.

[0082] In this way, in various embodiments, it is possible to achieve an improved utilization of the spectrum of a light source arrangement such as a light emitting diode or light emitting diode arrangement with at the same time high angle selectivity and thus little widening of fine structures of a virtual object to be represented.

[0083] The above embodiments serve merely for elucidation and should not be interpreted as restrictive.