ILLUMINATION MODULE FOR ILLUMINATING A SURFACE AND IMAGE GENERATOR UNIT HAVING SUCH AN ILLUMINATION MODULE

An illumination module for illuminating a surface has a beam source emitting illumination radiation, an extensive deflection hologram arranged at a distance from the surface to be illuminated, and a collimator optical unit at which the illumination radiation is directed, which collimates the illumination radiation and which emits the latter as collimated radiation incident on the deflection hologram, wherein the deflection hologram is designed such that it deflects the incident collimated radiation in the direction toward the surface to be illuminated and at the same time acts as a diffuser.

PRIORITY

This application claims the priority of German patent application DE 10 2021 111 673.2 filed May 5, 2021, which is hereby incorporated herein by reference in its entirety.

FIELD

The present invention relates to an illumination module for illuminating a surface, and to an image generator unit having such an illumination module.

BACKGROUND

By way of example, such an illumination module and such an image generator unit can be used for a head-up display (HUD). Such HUDs may contain volume-holographic optical units, which are deflecting grating structures that comprise a significant wavelength dependence (dispersion). As a result, the observation angle of the HUD changes with the wavelength, resulting in an HUD blur in the case of broadband illumination. Thus, an image generator unit for such an HUD should comprise spectral lines that are as narrowband as possible.

SUMMARY

An object of the invention to provide an illumination module for illuminating a surface, by means of which the difficulties described at the outset can be rectified as completely as possible.

By way of the illumination module, it is possible in particular to use a very narrowband illumination radiation for illuminating the surface. On account of the diffuser property of the deflection hologram, a uniform illumination of the surface can advantageously be achieved. Moreover, there advantageously is a reduction in speckle if the illumination radiation is coherent radiation since a mixing of coherence regions not capable of interfering is obtained on account of the diffuser property.

By way of the illumination module, it is hence possible to obtain very narrowband illumination of the surface to be illuminated.

In particular, the beam source may comprise a laser. Thus the beam source is able to output illumination radiation at one wavelength. The beam source may also comprise a plurality of lasers, with the result that the illumination radiation is formed by a plurality of wavelengths (each of which is a very narrowband). For example, this may relate to wavelengths from the red, green, and blue wavelength range.

The deflection hologram may be in the form of a volume hologram in particular. Further, the deflection hologram can be a reflective hologram. Alternatively, it is possible that the deflection hologram is a transmissive hologram.

On account of the diffuser property of the deflection hologram, the deflection hologram also comprises a scattering property which leads to an individual incident beam being deflected within a scattering cone, whereby the desired diffuser effect is achieved.

A substrate body which is transparent to the illumination radiation may be provided, the deflection hologram being formed on the lower side of said substrate body and the upper side of said substrate body being spaced apart from the lower side and being the surface to be illuminated.

Further, the substrate body can comprise a side face through which the collimated radiation enters the substrate body and then strikes the deflection hologram.

An input coupling hologram which is struck by the collimated radiation can be formed on the side face, the input coupling hologram deflecting the incident collimated radiation toward the deflection hologram.

In particular, the input coupling hologram may bring about a deflection in a first plane and the deflection hologram may bring about a deflection in a second plane, with the two planes intersecting. They can preferably intersect at an angle of 90°.

The collimated radiation preferably strikes the input coupling hologram at an angle of incidence of greater than 60°, greater than 65°, greater than 70°, or greater than 75°. Preferably, the angle of incidence is less than 90°, 89°, 88°, 87°, 86°, 85°, 84°, 83°, 82°, 81°, 80°, 79°, 78°, 77°, 76° or 75°.

The input coupling hologram can be a volume hologram. Further, the input coupling hologram can be designed as a transmissive hologram or as a reflective hologram.

In general, what still needs to be mentioned in the context of the development in which the beam source emits illumination radiation at a plurality of wavelengths (e.g., an RGB application) is that the input coupling hologram, if designed as a volume hologram, and the deflection hologram, if designed as a volume hologram, may each be designed such that the volume hologram for the plurality of wavelengths is designed as a layer system (i.e., a respective hologram per wavelength) or as a multiplex hologram (structures for all wavelengths in a hologram).

The collimated radiation strikes the deflection hologram at an angle of incidence of greater than 60°, greater than 65°, greater than 70°, or greater than 75°. In particular, the angle of incidence is less than 90°, 89°, 88°, 87°, 86°, 85°, 84°, 83°, 82°, 81°, 80°, 79°, 78°, 77°, 76° or 75°.

An antireflection layer can be formed on the input coupling hologram and/or a half wave plate layer can be formed between the input coupling hologram and the side face. By way of example, the input coupling hologram and the half wave plate layer can each be formed as a film, with the result that they can be provided together as a film stack.

The illumination radiation emitted by the beam source can be guided to the collimator optical unit via a free beam section, an optical fiber or a combination of optical fiber and free beam section.

The surface to be illuminated can also be an exposed hologram which shows the desired 3-D effect upon illumination. In particular, the illumination radiation can be coherent radiation.

An illumination module according to certain embodiments is used in the image generator unit. A flat light modulator is arranged in the surface to be illuminated or in a surface conjugate thereto, said light modulator, for image creation purposes, modulating the collimated radiation, which is incident thereon and which was deflected by the deflection hologram, in order to create an image. By way of example, the flat light modulator can be a liquid crystal display or a tilting mirror matrix. If it is a liquid crystal display, it is frequently formed on a transparent substrate body. In this case, a further half wave plate film could optionally be introduced between the light modulator and substrate in order to match the polarization direction of the illumination to the preferred direction of the LCD (depends on how the polarizer of the LCD is arranged). The side of the substrate body (which may also be referred to as lower side) opposite the liquid crystal display can then be provided with the deflection hologram. This renders a very compact design possible. Further, the substrate body may then comprise the side face on which the input coupling hologram may subsequently be formed.

The illumination module and/or the image generator unit may comprise a control unit which serves to control the beam source and/or the flat light modulator.

Hence, the illumination module may serve as background illumination or as an edge-lit diffuser in the image generator unit.

The illumination module or the image generator unit may be part of an HUD.

It goes without saying that the features mentioned above and the features yet to be explained hereinafter can be used not only in the specified combinations but also in other combinations or on their own without departing from the scope of the present invention.

The invention will be explained in even greater detail below on the basis of exemplary embodiments with reference to the accompanying drawings, which likewise disclose features essential to the invention. These exemplary embodiments are provided for illustration only and should not be construed as limiting. For example, a description of an exemplary embodiment having a multiplicity of elements or components should not be construed as meaning that all of these elements or components are necessary for implementation. Rather, other exemplary embodiments may also contain alternative elements and components, fewer elements or components, or additional elements or components. Elements or components of different exemplary embodiments can be combined with one another, unless stated otherwise. Modifications and variations that are described for one of the exemplary embodiments can also be applicable to other exemplary embodiments. In order to avoid repetition, elements that are the same or correspond to one another in different figures are denoted by the same reference signs and are not explained repeatedly.

DETAILED DESCRIPTION

In the exemplary embodiment shown inFIG.1, the illumination module1for illuminating a surface is used to illuminate a liquid crystal display (LCD)2formed on an upper side3of a substrate block4. Thus, together the illumination module1and the liquid crystal display2form an image generator unit B, by means of which images can be created in a manner known per se. To this end, further provision can be made of a control unit5for controlling the liquid crystal display2.

In this case, the illumination module1further comprises a laser6, the laser radiation of which is guided via a fiber7to the fiber output8of the fiber7and is output by the fiber output8. The emitted laser radiation9is then incident on a collimator optical unit10of the illumination module1. The collimator optical unit10creates collimated radiation11with a diameter of 30 mm. However, to simplify the illustration, only a central ray of the collimated radiation11is depicted inFIG.1. The collimated radiation11is deflected by a first and a second deflection mirror12,13(at points P1and P2) (FIG.1andFIG.2) and directed at an input coupling hologram15formed on a side face14of the substrate block4. In this case, the angle of incidence of the collimated radiation11on the input coupling hologram15is approx. 75° to 80° and is chosen such that the collimated radiation11covers the entire input coupling hologram15in the y-direction as a result. In this case, the extent of the input coupling hologram15in the y-direction is approx. 70 mm. As may be gathered fromFIG.1, for example, the input coupling hologram15comprises a flat and rectangular shape with a shorter side of approx. 30 mm such that the entire surface of the input coupling hologram15is illuminated by the collimated radiation11on account of the deflection of the collimated radiation11by means of the two deflection mirrors12and13.

As shown inFIG.1andFIG.3, the substrate block4comprises a lower side16, on which a deflection hologram17is formed. The deflection hologram17has a flat embodiment and is spaced apart from the liquid crystal display2(in the z-direction on account of the extent of the substrate block4). The deflection hologram17is preferably arranged parallel to the liquid crystal display2.

In this case, the deflection hologram17is designed such that (at the point P4for the central ray of the collimated radiation11) it deflects the incident collimated radiation11(which is incident on the deflection hologram17on account of the deflection—at the point P3for the central ray of the collimated radiation11—by means of the input coupling hologram15) such that it propagates substantially in the z-direction and consequently runs through the substrate block4, which is transparent to the laser radiation9,11, to the upper side3and thus illuminates the liquid crystal display2from behind.

The deflection of the collimated radiation11by means of the input coupling hologram15is implemented such that the deflected collimated radiation11illuminates the entire deflection hologram17in the x-direction. An angle of incidence ranging from 75 to 80° is again chosen to this end.

However, the deflection hologram17does not only carry out the above-described deflection in the direction of the upper side3, but additionally also comprises the function of a diffuser. As indicated inFIGS.1to3, each beam of the collimated radiation11which strikes the deflection hologram17is additionally scattered such that a scattering cone18is created. Hence, individual rays of the collimated radiation11are mixed on the upper side3and are incident on the deflection hologram17at different points of incidence. As a result, the coherence length is also reduced, as depicted schematically inFIG.4. To simplify the illustration, the assumption is made inFIG.4that the deflection hologram17is not a reflective deflection hologram17like in the above-described embodiment, but rather is a transmissive deflection hologram17. As is clearly evident from this illustration, the created scattering cones18lead to a mixing of the points of incidence on the deflection hologram17up to the upper side3, with the result that a more uniform illumination of the upper side3is possible. Moreover, this advantageously also reduces the coherence length, as indicated by the hatched region19inFIG.4. This advantageously leads to a speckle reduction.

The embodiment described inFIGS.1to3is distinguished by a very high degree of compactness. Hence, it is possible to provide an illumination module1and an image generator unit B, which each have a very compact embodiment. The laser radiation9of the laser6need not be guided to the collimator optical unit10via a fiber. Naturally, a free beam system is also possible. A combination of a fiber7and a free beam system or free beam section is also possible.

What can be gathered from the schematic detailed view of the substrate block4with the deflection hologram17and the layer arrangement for the input coupling hologram15depicted in the exploded representation inFIG.5is that, in addition to the input coupling hologram15, a half wave plate film20may also be provided between the input coupling hologram15and the side face14and an antireflection coating21may be provided on the input coupling hologram15. In the case of polarized radiation, the half wave plate film20leads to the polarization direction being rotated through 90°.

Only one deflection mirror12is required in the exemplary embodiment shown inFIG.6, which shows a plan view in the same manner as inFIG.2.

In the previous description, the assumption was made that the laser6emits only laser radiation9at one wavelength. In this way, a monochromatic image can be created by means of the liquid crystal display2. Naturally, it is also possible for the laser6to be designed such that it for example emits red, green, and blue laser radiation, with the result that this can be used to create a color image.

The laser radiation can be very narrowband on account of the use of the laser6. Despite this narrowband laser radiation, it is possible to obtain a very homogeneous and uniform illumination on the surface3, and hence on the liquid crystal display2. At the same time, unwanted speckle can be reduced or avoided.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it will be apparent to those of ordinary skill in the art that the invention is not to be limited to the disclosed embodiments. It will be readily apparent to those of ordinary skill in the art that many modifications and equivalent arrangements can be made thereof without departing from the spirit and scope of the present disclosure, such scope to be accorded the broadest interpretation of the appended claims so as to encompass all equivalent structures and products. Moreover, features or aspects of various example embodiments may be mixed and matched (even if such combination is not explicitly described herein) without departing from the scope of the invention.