DE102021110734A1 - Optical system for floating holograms - Google Patents

Abstract

An optical system (110) comprises an imaging holographic optical element (130) which generates a floating hologram (150). An upstream light-forming holographic optical element (120) effects spectral filtering of the light (90).

Description

TECHNICAL AREA

Various examples relate to a system that includes multiple holographic optical elements to create a floating hologram.

BACKGROUND

Techniques are known for creating a floating hologram using an imaging holographic optical element (HOE). Such a levitated hologram is created in a volume placed outside of the imaging HOE. This means that the hologram is offset from the imaging HOE. This creates an optical "floating effect", the hologram stands freely in space.

It was found that in the case of holograms with great depth or a large distance from the imaging HOE, particularly high demands must be made on the quality of the illumination of the imaging HOE.

SHORT SUMMARY

Accordingly, it is an object of the invention to provide an optical system which can produce a high-quality floating hologram.

This object is solved by the features of the independent patent claims. The features of the dependent claims define embodiments.

According to various examples, a system of multiple HOEs is used to create a high quality hologram. In particular, an imaging HOE that is set up by a suitable exposure in such a way that it generates a hologram with a desired motif when subsequently illuminated can be used.

Furthermore, a light-shaping HOE can be used; the light that illuminates the imaging HOE can be shaped by the light-shaping HOE.

An optical system thus includes an imaging HOE. The imaging HOE is set up to create a floating hologram based on light. This floating hologram is placed in a volume outside of the imaging HOE. Furthermore, the optical system includes a light source. The light source is set up to emit the light along a beam path to the imaging HOE. The optical system also includes a light-shaping HOE. This is arranged in the beam path between the light source and the imaging HOE and is set up to apply one or more light-shaping functionalities to the light.

In principle, a wide variety of light-shaping functionalities can be provided, for example individually or also cumulatively. Exemplary light-shaping functionalities are, for example: spectral filtering, ie selection of a smaller wavelength range of the incident light; Filtering in angular space, that is, for example, selection of a smaller angular spectrum with which the light propagates along the beam path; as well as arrangement of the light in spatial space, for example to deflect the light towards the imaging HOE and/or to illuminate the imaging HOE homogeneously.

A method of manufacturing an optical system includes providing an imaging HOE. This is set up to create a floating hologram based on light. The levitated hologram is placed in a volume outside of the imaging HOE. The method also includes providing a light source. This is set up to emit the light along a beam path to the imaging HOE. The method also includes providing a light shaping HOE. This is arranged in the beam path between the light source and the imaging HOE and set up to apply one or more light-shaping functionalities to the light.

The features set out above and features described below can be used not only in the corresponding combinations explicitly set out, but also in further combinations or in isolation without departing from the protective scope of the present invention.

character list

• 1 12 is a schematic view of an optical system including a light-shaping HOE and an imaging HOE in series along a ray path of light, according to various examples.
• 2 illustrates an example implementation of the optical system from FIG 1 according to various examples.
• 3 12 illustrates spectral filtering that can be provided by the light-shaping HOE according to various examples.
• 4 illustrates an example implementation of the optical system from FIG 1 according to various examples.
• 5 illustrates an example implementation of the optical system from FIG 1 according to various examples.
• 6A 12 illustrates an exemplary integration of an optical system according to various examples into an interior screen of a motor vehicle.
• 6B FIG. 12 is a perspective view of the optical system implementation of FIG 2 .
• 7 Figure 12 is a flow diagram of an example method.

DETAILED DESCRIPTION OF EMBODIMENTS

The properties, features and advantages of this invention described above, and the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the exemplary embodiments, which are explained in more detail in connection with the drawings.

The present invention is explained in more detail below on the basis of preferred embodiments with reference to the drawings. In the figures, the same reference symbols designate the same or similar elements. The figures are schematic representations of various embodiments of the invention. Elements depicted in the figures are not necessarily drawn to scale. Rather, the various elements shown in the figures are presented in such a way that their function and general purpose can be understood by those skilled in the art.

Techniques that make it possible to create a floating hologram are described below. The hologram can display an image, such as a button or a sign.

For this purpose, an optical system is used, which includes several HOE.

The hologram, which is generated by means of a corresponding optical system, can have a particularly large floating height and/or a particularly large depth effect. For example, a distance between a volume in which the hologram is displayed under appropriate illumination of an imaging HOE and the imaging HOE could be no less than 60% of the lateral dimensions (perpendicular to the distance) of a refractive index modulated region of the imaging HOE.

The imaging HOE can be implemented as a volume HOE, i.e. having a variation of the refractive index in 3-D. A corresponding refractive index modulated area has a 3-D extent. This variation in refractive index breaks the light with a diffraction pattern, thereby forming the hologram. The bulk HOE is distinguished from a surface HOE, where modulation of the surface of a substrate gives rise to the diffraction pattern. For example, the surface could be wavy.

The imaging HOE can be implemented as a transmission HOE or as a reflection HOE. In a transmission HOE, the refractive index modulated region is illuminated from one side and the hologram is created in a volume facing the opposite side. With reflection HOE the refractive index modulated region is illuminated from one side and the hologram is created in a volume facing the same side.

For example, it would be possible for the optical path of the light to impinge on the imaging HOE in edge-lit geometry. This means that the imaging HOE has a substrate on which the refractive index modulated region is applied. The beam path is coupled into the substrate on the narrow side, then passes through the substrate - e.g. glass or polymethyl methacrylate - before it hits the refractive index-modulated area. Typically, the substrate has a layer thickness that is significantly greater than the layer thickness of the refractive index-modulated area. The so-called reconstruction angle describes the angle at which the light hits the refractive index-modulated area. This can be arranged along a surface of the imaging HOE. Light that is not diffracted by the refractive index modulated region to create the hologram can then undergo total reflection at the surface of the imaging HOE and be reflected back into the substrate; as a result, the reproduction of the hologram is not disturbed by "background light". An absorbing material can absorb such back-reflected light (beam dump). The diffracted light hits the surface at smaller angles and is allowed to escape to create the hologram.

The optical system can include a light source. This is set up to emit the light along a beam path to the imaging HOE.

The light source preferably emits light in the visible spectrum, in particular between 380 nm and 780 nm.

In the various examples described herein, one or more light emitting diodes may be used as the light source. Light-emitting diodes are particularly simple, durable and inexpensive and have sufficient optical properties, in particular with regard to the coherence of the light emitted, with regard to a large number of lighting functions, in particular holographic lighting functions. Light-emitting diodes are particularly efficient.

For example, a light emitting diode could have a light emitter (active area that emits photons) that has dimensions between 0.5×0.5 mm ² and 1×1 mm ² . It can be particularly advantageous to use small emitter areas for the applications mentioned.

In principle, it is helpful if the reconstruction wave - i.e. the wave front of the light during illumination - corresponds as closely as possible to the reference wave when recording the hologram - i.e. with the wave front of the light during exposure. The exposure takes place with lasers, which in principle represent a point light source. Accordingly, it is advantageous if the LEDs used for the reconstruction have the smallest possible emitter areas and thus do better justice to the assumption of a point light source.

Various examples are based on the finding that a further improvement in the illumination of the imaging HOE can be achieved by using a light-shaping HOE, which is arranged in the beam path between the light source and the imaging HOE.

The light-shaping HOE can implement various light-shaping functionalities. Overall, the illumination of the imaging HOE can be improved as a result.

Some such light-shaping functionalities that can be provided by the light-shaping HOE are described below in connection with Table 1. Short description Exemplary details I spectral filtering The light-shaping HOE can be set up to perform spectral filtering of the light. This means that light of certain wavelengths is passed from the light-shaping HOE in the direction of the imaging HOE - through appropriate diffraction - while light of other wavelengths is not passed in the direction of the imaging HOE. Details with the spectral filtering are later in connection with 3 explained. Spectral filtering of this type makes it possible to achieve particularly monochromatic illumination or illumination with a comparatively narrow wavelength spectrum. As a result, the hologram can be produced with particularly good quality. II Angular Spectrum Filtering The light-shaping HOE can also be configured to filter the angular spectrum of the light. The angular spectrum is characterized by the shape of the wave front of the propagating light along the beam path. A plane wave would, for example, cause the imaging HOE to be illuminated from only one angle, or the light to propagate along the beam path without divergence. In one example, a reduced divergence of the light along the beam path can be generated by filtering the angular spectrum. So the light can be collimated. By reducing the divergence, almost flat wave fronts of the light can be generated. After the filtering, the angular spectrum could be, for example, less than 2°, optionally less than 1°, further optionally less than 0.5°. More generally, the angular spectrum can be matched by the filtering to the angular spectrum of reference light used in the exposure of the light-shaping HOE. A hologram of particularly good quality can be generated by such filtering in angular space. III Light shaping in local space The light-shaping HOE can also be set up to direct the optical path of the light towards the imaging HOE. As a result, compact designs of the optical system can be achieved. The light shaping HOE can be configured to homogeneously illuminate the imaging HOE. Such a homogeneous illumination of the imaging HOE can mean, in particular, that the entire area of the imaging HOE is illuminated with predominantly the same light intensity. A deviation in intensity along the imaging HOE is preferably less than 20%, more preferably less than 10% and particularly preferably less than 5%. In particular, a ratio of minimum intensity (or minimum illumination or irradiance) to maximum intensity (or maximum illumination or irradiance) can be <0.8.

TAB. 1: Various light-shaping functionalities that can be provided by the light-shaping HOE. A homogeneous angle and wavelength spectrum of the illumination of the imaging HOE can be achieved by means of such light-shaping functionalities, so that a hologram can be reconstructed which has a large distance to the refractive index-modulated region of the imaging HOE and has a large depth of focus.

In principle, different implementations for the light-shaping HOE are conceivable. For example, it would be possible for the light-shaping HOE to deflect the beam path into reflection geometry. That is, a reflection HOE can be used.

A reflection HOE is wavelength-selective, which means that only light from a narrow wavelength spectrum is diffracted efficiently for a specific exit angle. As a result, the spectral filtering according to Table 1: Example I can be achieved. For example, a full width at half maximum of the wavelength spectrum of the light after spectral filtering could be achieved, which is not larger than 10 nm, in particular not larger than 5 nm. This can achieve a better reconstruction of the image in the form of the hologram, because smearing and ghosting - the could otherwise occur with broadband illumination of the imaging HOE - can be avoided.

1 illustrates aspects related to an optical system 110. 1 FIG. 12 is a schematic representation of the optical system 110 set up to generate a hologram 150. FIG.

The optical system 110 includes a light source 111. The light source 111 can be implemented by one or more light emitting diodes. The light source 111 is set up to emit light 90 along a beam path 81 . The light 90 is used to create the hologram 150 .

Various optical components 171 , 120 , 130 are arranged along the beam path 81 .

For example, it would be possible for a refractive or mirror-optical optical element 171, 172 to be arranged in the beam path 81 between the light source 81 adjacent to the light source 111. This refractive or mirror-optical optical element is set up to collect the light 90 . A greater light yield can be achieved in this way.

For example, the optical element 171, 172 could be implemented by a concave mirror or a lens.

The light 90 propagates further along the ray path 81 toward a light-shaping HOE 120. Various light-shaping functionalities that may be provided by the light-shaping HOE 120 have been discussed above in the context of TAB. 1 described.

The light 90 - after being shaped by the light-shaping HOE 120 - then propagates further along the optical path 81 towards an imaging HOE 130. The imaging HOE 130 is configured to generate the levitated hologram 150 based on the light 90.

Various structural implementations of the optical system 110 are conceivable. Some implementations are described below, for example in connection with 2 .

2 illustrates aspects related to the optical system 110. Particularly illustrated 2 an exemplary structural implementation of the optical system 110. In the example of FIG 2 the optical system 110 does not include a refractive or mirror-optical optical element which would be arranged in the beam path 81 between the light source 111 and the light-shaping HOE 120 .

The light source 111 emits the light 90 with a significant divergence, ie with a comparatively wide angular spectrum. 2 shows an example of rays of the light 90 along the beam path 81 (“ray tracing”).

The light 90 impinges on the light shaping HOE 120 . The light-shaping HOE 120 includes a substrate 122 and a refractive index-modulated region 121. The light-shaping HOE 120 redirects the light 90 along the optical path in reflection geometry.

Because reflection holograms, such as reflection hologram 120, are wavelength selective - that is, they diffract light for a specific angle only efficiently for a specific range of wavelengths - spectral filtering results. Spectral filtering is also in 3 shown. 3 illustrates the efficiency of diffraction into a given solid angle as a function of wavelength. Illustrated is the wavelength spectrum 601 of the incident light (shown with the dashed line) and the wavelength spectrum 602 (solid line) of the diffracted light. The full width at half maximum 612 of the spectrum of the diffracted light is not more than 30%, optionally not more than 40%, further optional not larger than 50% of the full width at half maximum 611 of the emission spectrum of the light source, ie the spectrum of the incident light. In particular, the full width at half maximum 612 of the diffracted light is not larger than 10 nm, optionally larger than 5 nm.

Now again referring to 2 : due to the spectral filtering, the light 90 incident on the imaging HOE 130 has a narrower bandwidth than the light 90 emitted by the light source 111 .

In 2 Also shown is the angle of reflection 125 at which the light-shaping HOE 120 reflects the light along the optical path 81 . In addition, the angle of incidence 126 of the light 90 on the imaging HOE 120 is also shown. These angles 125, 126 correspond to the angles at which reference light strikes the imaging HOE 120 from two different laser sources when the imaging HOE 120 is exposed.

This reflection angle 125 may correspond to the Brewster angle of the substrate 122 material. This means that the light 90 directed by the imaging HOE 130 is linearly polarized. In this way, undesired interactions due to different polarizations of the light 90 during the exposure of the light-shaping HOE 120 can be avoided.

The angle of incidence 126 - in the example shown 2 the angle of incidence 126 is 0°, that is perpendicular to incidence on the light-shaping HOE 120; in principle, however, other values would also be possible - is selected in such a way that Fresnel reflections of the light 90 are oriented away from the imaging HOE 130 . As a result, the quality of the illumination of the imaging HOE 130 can be additionally increased.

In 2 a so-called reconstruction angle 135 is also shown. The reconstruction angle 135 denotes the direction along which the light 90 along the optical path 81 impinges on the refractive index modulated region 131 of the imaging HOE 130 . This reconstruction angle 135 is defined by the reflection angle 125, the relative location of the light-shaping HOE 120 to the imaging HOE 130, and the refraction at the air-to-substrate 132 interface.

Then, based on the light 90, the hologram 150 is generated in a volume 159 that is located at a distance 155 from the refractive index modulated region 131 of the imaging HOE 130. FIG. A floating hologram 150 is thus generated.

In various examples, it is possible for one or more further beam shapes of the components to be arranged along the beam path 81 between the light source 111 and the light-shaping HOE 120 . For example, a lens 171 - cf 4 - or a mirror 172 - compare 5 - be used. As a result, the light yield can be increased, ie a larger amount of the light 90 emitted by the light source 111 can be used to illuminate the imaging HOE 130 .

Such a refractive or mirror-optical optical element 171, 172, which is arranged in the beam path 81 between the light source 111 and the light-shaping HOE 120, can collect/shape the light in a horizontal and/or vertical direction. "Vertical" denotes a direction in the plane of the drawing; "horizontal" means a direction perpendicular to it (cf. 6B) . Accordingly, rotationally symmetrical, cylindrical or anamorphic optics can be used.

6A 12 illustrates aspects related to an integration of the optical system 110 with an interior screen 201 of a motor vehicle. It shows that the imaging HOE 130 is arranged in a recess in the interior panel 201 flush with the interior panel 201, and the hologram 150 - in the example shown an on/off button - offset to the surface of the interior panel 201 in a volume in the interior of the Motor vehicle is shown.

• Die Schwebehöhe des Hologramms 150 - das heißt der Abstand 155, vgl. 2 - kann größer als 20 mm, zum Beispiel 30 mm sein.

Exemplary geometric variables for such an integration are listed below:

• The flying height of the hologram 150 - i.e. the distance 155, cf. 2 - can be larger than 20 mm, for example 30 mm.

The reconstruction angle 135 (compare 2 ) can typically be in a range from 60° to 80°, for example at 70°.

The substrate 132 of the imaging HOE 130 can be made of glass, for example, and have a thickness 134 (in 2 shown) of 20 mm. This thickness 134 can be at a larger reconstruction angle 135 or a smaller lateral dimension 135 (also in 2 shown) of the refractive index-modulated area 131 can also be selected to be smaller.

The distance between the light-shaping HOE 120 and the coupling surface of the substrate 132 of the imaging HOE 130 is chosen such that the beam of light 90 from the light source 111 to the light-shaping HOE 120 is not cut by the substrate 132 of the imaging HOE 130 (bottom right corner of the substrate 132 in 2 ).

Furthermore, it can be desirable to choose the distance from the light source 111 to the light-shaping HOE 120 as large as possible, so that the light source 111 has the best possible properties of a point light source. At the same time, a greater distance between the light source 111 can also illuminate a larger area of the imaging HOE 130, for example perpendicular to the plane of the drawing in FIG 2 or along and/or perpendicular to the lateral dimension 135 (the corresponding depth direction is in 6B visible). On the other hand, the distance selected should not be too large in order to form as much light 90 as possible from the light source 111 in the vertical direction with the light-shaping HOE 120 (that is, corresponds to the height of the light-shaping HOE 120). For example, a range of 50 mm to 100 mm has been identified as helpful as a distance, for example 70 mm in particular.

Depending on the size of the lateral dimensions 135 of the imaging HOE 130, the parameters of the reconstruction angle 135 of the imaging HOE 130 and the distance between the light source 111 and the light-shaping HOE 120 can be used to set the best possible lighting situation.

7 FIG. 12 is a flow diagram of an exemplary method of manufacturing an optical system. For example, using the method of 7 the optical system 110 can be manufactured according to one of the examples discussed above. Optional blocks are in 7 shown with dashed lines.

In block 3005, an imaging HOE is first provided. For example, the imaging HOE 130 may be implemented according to the examples described above.

For example, block 3005 could include exposing the imaging HOE 130 to reference light from multiple interfering laser light sources. In this way, the refractive index-modulated region can be formed on a corresponding substrate. This defines the reconstruction angle 135 .

In principle, techniques for exposing an imaging HOE are known to the person skilled in the art, so that no further details need to be given here.

In block 3010, providing a light-shaping HOE occurs. For example, the light-shaping HOE 120 can be provided according to the examples described above.

Block 3010 may include exposing the light-shaping HOE 120 to reference light from multiple interfering laser light sources. In particular, the reflection angle of the light-shaping HOE can be defined in this way. The angle of reflection corresponds to the angle of illumination from one of the interfering laser light sources, and this angle can be set equal to the Brewster's angle of the light-shaping HOE.

To the in TAB. To achieve the light-shaping functionalities discussed in FIG. 1, the light-shaping HOE can be designed in particular in reflection geometry; In principle, however, an implementation as a transmission HOE would also be possible. A corresponding grating that diffracts and reflects incident light can perform spectral and angular space filtering, as in TAB. 1 discussed, provide. In addition, the suitable size and arrangement of the light-shaping HOE in relation to the imaging HOE from block 3005 can ensure that the refractive index-modulated region of the imaging HOE, especially in edge-lit geometry, is illuminated homogeneously.

In block 3015, a light source may be provided. In particular, this can be arranged at a suitable distance from the light-shaping HOE.

Then, in block 3020, the optical system obtained in this way could optionally be integrated into a panel, for example an interior panel of a motor vehicle.

Of course, the features of the embodiments and aspects of the invention described above can be combined with one another. In particular, the features can be used not only in the combinations described, but also in other combinations or taken on their own, without departing from the field of the invention.

Claims (11)

Optical system (110) comprising: - an imaging holographic optical element, HOE (130), which is set up to generate a floating hologram (150) based on light (90), which is arranged in a volume (159) outside the imaging HOE (130), - a light source (111) which is set up to emit the light (90) along a beam path (81) towards the imaging HOE (130), and - A light-shaping HOE (120) which is arranged in the beam path (81) between the light source (111) and the imaging HOE (130) and which is set up to perform spectral filtering of the light (90). Optical system (110) according to claim 1 , wherein the light-shaping HOE (120) is further set up to reduce an angular spectrum with which the light (90) propagates along the beam path (81). Optical system (110) according to claim 1 or 2 , wherein the light-shaping HOE (120) is further set up to deflect the light (90) along the beam path (81) towards the imaging HOE (130). Optical system (110) according to claim 3 , wherein the light-shaping HOE (120) deflects the light (90) along the beam path (81) in reflection geometry. Optical system (110) according to claim 4 , wherein a reflection angle (125) with which the light-shaping HOE (120) reflects the light (90) along the optical path (81) corresponds to the Brewster angle of a material of the substrate (122) of the light-shaping HOE (120). Optical system (110) according to one of claims 3 until 5 , wherein an angle of incidence (126) of the light (90) along the beam path (81) onto the light-shaping HOE (120) is selected such that Fresnel reflections of the light (90) are oriented away from the imaging HOE (130). Optical system (110) according to one of the preceding claims, wherein the optical path (81) impinges on the imaging HOE (130) in edge-lit geometry. Optical system (110) according to any one of the preceding claims, wherein the light source (111) comprises a light-emitting diode with an emission spectrum (601), wherein the spectral filtering allows a portion of the light (90) in the range up to 50% of a width (611) of the emission spectrum (601) to pass. An optical system (110) according to any one of the preceding claims, further comprising: - A refractive or mirror-optical optical element (171, 172), which is arranged in the beam path (81) between the light source (111) and the light-shaping HOE (120), and which is set up to transmit the light emitted by the light source (111). (90) onto the light shaping HOE (120). Optical system (110) according to one of the preceding claims, wherein a distance (155) between the volume (159) and the imaging HOE (130) is not less than 60% of a lateral dimension (135) of a refractive index modulated region (131) of the imaging HOE. A method for manufacturing an optical system (100), the method comprising: - providing an imaging holographic optical element, HOE (130), which is set up to generate a levitated hologram (150) based on light (90) which is in is arranged in a volume (159) outside of the imaging HOE (130), - providing a light source (111) which is set up to direct the light along a beam path (81) to send out to the imaging HOE, and - providing a light-shaping HOE (120), which is arranged in the beam path between the light source and the imaging HOE and which is set up to perform spectral filtering of the light.

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