EP4378161A1

EP4378161A1 - Projection device and projection method

Info

Publication number: EP4378161A1
Application number: EP22757232.8A
Authority: EP
Inventors: Daniel Thomae
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 2021-07-30
Filing date: 2022-07-22
Publication date: 2024-06-05
Also published as: CN117730528A; KR20240043746A; DE102021119886A1; WO2023006605A1

Abstract

The invention relates to a projection device comprising an imaging module (2) which generates a multi-color image such that a first color sub-image with a first wavelength and a second color sub-image with a second wavelength are generated, a projection unit (3) which is supplied with the multi-color image and images said image into an exit pupil (6) such that the image can be perceived as a virtual image by a viewer when the eye (A) of the viewer is positioned in the exit pupil (6) and the viewer views the projection unit (3) at a specified viewing angle, wherein the projection unit (3) has a volume hologram which deflects the multi-color image into the exit pupil (6) for imaging purposes, and the volume hologram has a volume grid for each wavelength of the color sub-image, said volume grid having a respective deflection efficiency curve which is based on the viewing angle and which is maximal at the specified viewing angle such that a first efficiency ratio of the first deflection efficiency curve for the first wavelength to the deflection efficiency curve of the second wavelength is produced. The deflection efficiency curves for a specified angular range about the specified viewing angle are set such that the first efficiency ratio for the specified angular range is constant. The imaging module (2) is actuated such that when the multi-color image is generated, a first brightness ratio of the brightness of the first color sub-image to the brightness of the second color sub-image is inversely proportional to the first efficiency ratio such that the different deflection efficiency curves are compensated for and such that the viewer can perceive the multi-color image as a true-color virtual image at viewing angles from the specified angular range.

Description

Carl Zeiss Jena GmbH

Projection device and projection method

The present invention relates to a holographic projection device and a holographic projection method.

With such a holographic projection device, a volume hologram can be used to deflect the image to be imaged. Since the deflection efficiency of such volume holograms is different for different wavelengths depending on the viewing angle for a user, this can have the disadvantage that the projected image has an undesirable color cast towards the edge, which a viewer notices very quickly and is very annoying . Three wavelengths are generally used in a projection unit, one wavelength each being generally provided for blue, green and red. However, only two wavelengths can also be used.

Proceeding from this, it is therefore the object of the invention to provide a projection device in which this difficulty is eliminated as far as possible. Furthermore, a corresponding projection method is to be provided.

The invention is defined in the independent claims. Advantageous developments are specified in the dependent claims.

Since the image module in the projection device according to the invention is controlled in such a way that when the multicolored image is generated, a first brightness ratio of the brightness of the first partial color image to the brightness of the second partial color image is inversely proportional to the first efficiency ratio, the different deflection efficiency curves are thus compensated and the viewer can see the multicolored image for viewing angles from the predetermined angle range as a true-color virtual image. An opposite color cast is thus generated during image generation, which is present for the deflection due to the volume grating of the volume hologram. This is possible because the deflection efficiency curves for the predetermined angular range around the predetermined viewing angle are set in such a way that the first efficiency ratio is constant for the predetermined angular range. It can thus also be said that, according to the invention, the different deflection efficiency curves of the volume hologram, which are dependent on the viewing angle, are set for different wavelengths in such a way that the first efficiency ratio is constant for the predetermined angular range. At the same time, the brightness on the image module for the first and second color sub-image is set in such a way that the first brightness ratio is inversely proportional to the first efficiency ratio for the predetermined angular range. Thus, no undesired color line occurs for viewing angles within the predetermined angle range.

The volume gratings can be reflective or transmissive gratings. Likewise, the volume gratings in the case of a waveguide can also be edge-lit gratings.

Furthermore, the volume gratings can advantageously be formed in the same layer, which is also referred to as multiplexing. With this, the volume gratings can easily be produced in a simple manner.

Of course it is also possible to provide a separate layer for each volume grid. These layers are then preferably attached to one another.

The volume gratings can be, for example, exposed volume gratings. This is preferably understood to mean that the volume grating is exposed and optionally developed or bleached, so that a stable exposed volume grating is then present.

The exposure to generate the exposed volume grating can be carried out, for example, in such a way that a reference wave with a predetermined wavelength (e.g. 532 nm, 460 nm or 640 nm) is incident at a first angle of incidence (e.g. 0°) on a layer (which is a photosensitive volume holographic material comprises or is formed from it) into which the volume grating is to be exposed is directed and that a signal wave with the same wavelength is also directed onto the layer at a second angle of incidence (e.g. of 60°) which differs from the first angle of incidence , wherein the reference wave and the signal wave originate from the same laser, so that an interference field or interference volume with the desired number of interference maxima arises over the photosensitive volume holographic material of the layer and thus a desired refractive index modulation is formed. The change in refractive index produced is maximum at the interference maxima, so that the interference maxima determine the refractive index modulation. For example, photosensitive glasses, dichromated gelatins or photopolymers can be used as photosensitive volume holographic materials. These can e.g. B. applied to a PC film (polycarbonate film) and exposed there accordingly.

The refractive index modulation is understood here to mean, in particular, the amount of the maximum change or variation in the refractive index.

The volume hologram can be embedded in a transparent carrier. However, it is also possible for the volume hologram to be formed in the interface of the transparent carrier.

The transparent carrier can also be used as an image guide, which has a coupling region spaced apart from the volume hologram, via which the multicolored image is coupled into the image guide. In the image guide, the multicolored image can be guided through reflections up to the volume hologram. The volume hologram then decouples the guided light to the viewer.

The transparent carrier can be, for example, a windscreen or some other pane of a vehicle. However, it can also be a plane-parallel plate. Furthermore, it is possible for the transparent carrier to have curved boundary surfaces. The transparent carrier of the volume hologram can also be part of an optical system, which is arranged, for example, in the dashboard of a vehicle and directs the light from there to the viewer via the reflection on the windshield.

The transparent support can be made of glass or plastic.

In particular, the projection device can be designed in such a way that the generated virtual image can be perceived superimposed on the environment. For this purpose, the volume hologram is preferably also transmissive for the first and second wavelength.

The predetermined angular range can be an angular range of 2°, 3°, 4°, 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, 19° or 20°. The predetermined angular range can differ in the horizontal and vertical directions. For example, the horizontal angle can be 14° - 20° and the vertical angle 5° -7.5°. The predetermined viewing angle may be in the middle of the predetermined angle range. However, it can also lie outside the center of the predetermined angular range. A constant efficiency ratio for the predetermined angular range is understood here in particular to mean that the efficiency ratio for the predetermined angular range varies between the wavelengths (preferably in relation to the maximum value of the efficiency ratio in the predetermined angular range) by no more than 1%, 2%, 3%, 4th %, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16% or 17% changes.

It goes without saying that the features mentioned above and those still to be explained below 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 is explained in more detail below on the basis of exemplary embodiments with reference to the accompanying drawings, which also disclose features that are essential to the invention. These embodiments are provided for illustration only and are not to be construed as limiting. For example, a description of a

Embodiment with a variety of elements or components should not be interpreted in such a way that all of these elements or components are necessary for implementation. Rather, other embodiments may include 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 otherwise stated. Modifications and variations that are described for one of the embodiments can also be applicable to other embodiments. To avoid repetition, the same or corresponding elements in different figures are denoted by the same reference symbols and are not explained more than once. From the figures show:

1 shows a schematic view of a first embodiment of the holographic projection device 1 according to the invention; FIG. 2 shows an enlarged detailed illustration of the projection device 1 from FIG. 1 ;

3 shows a representation of the deflection efficiency of the volume gratings for three different ones

wavelengths; FIG. 4 shows a representation of the effective deflection efficiency, taking into account the brightness scaling of the imager for the wavelengths of FIG. 3; 5 shows a further embodiment of the holographic projection device 1 according to the invention;

6 shows a further embodiment of the holographic projection device 1 according to the invention;

7 shows a further embodiment of the holographic projection device 1 according to the invention, and FIG. 8 shows a partial sectional view of the projection device 1 from FIG.

In the embodiment shown in Fig. 1, the holographic projection device 1 according to the invention comprises an image module 2 for generating a multicolored image and a projection unit 3, which here comprises a holographic beam splitter 5 integrated into a windshield 4 of a vehicle, on which the multicolored image (representative the beam path of a light beam L is drawn in) in the direction of an exit pupil 6 of the projection unit 3 is deflected in such a way that a user who positions his eye A in the exit pupil 6 can perceive the multicolored image as a virtual image when he is looking along a predetermined direction 7 looks at the projection unit 3 (or here at the holographic beam splitter 5).

The image module 2 can include an image generator 8 and a control unit 9 with a processor 10, the control unit 9 controlling the image generator 8 to generate the multicolored image. The imaging device 8 can be an LCD module, an OLED module or an LCoS module or a tilting mirror matrix. Furthermore, the imager can

Have ground glass, which is not shown here. The system can also have a light source that is not directly assigned to the imager, such as a laser, for illuminating the imager, which is not shown. The multicolored image is generated by means of the image generator 8 in that, for example, three partial color images are generated with different wavelengths. This can be, for example, a blue color sub-image with a wavelength of 460 nm, a green color sub-image with a wavelength of 532 nm and a red color sub-image with a wavelength of 640 nm. The partial color images can be generated simultaneously or alternately so quickly one after the other that a user can only perceive the superimposition as a multicolored image. As can be seen in particular in the enlarged partial view of FIG. 2, the holographic beam splitter 5 has a photopolymer layer 11 in which a volume holographic grating is written for each of the three wavelengths. The three grids are thus superimposed in the same volume (namely in the photopolymer layer 11), so that what is known as multiplexing occurs. Each of the three volume holographic gratings is designed in such a way that it is reflective for one of the three wavelengths mentioned (for example with a bandwidth of ±3% of the central wavelength) and transmits the remaining wavelengths. The reflection is to be understood as diffraction at the lattice structure of the volume hologram. In this case, the reflectivity of the individual volume holographic gratings, which corresponds to the diffraction efficiency, as will be described in detail below, is set such that there is an effective reflectivity of approximately 45%. This is mainly due to the fact that, for the purpose described, reflectivities of 100% are not permitted in the windshield 4 of the vehicle for safety reasons. For other applications where such security issues are not important, the volume holographic gratings may well be designed to have a reflectivity greater than 45%.

The holographic beam splitter 5 is designed for the predetermined viewing direction 7 with a predetermined viewing angle cd of 62.5° (relative to the normal 12, at the point where the normal 12 intersects the windshield 4). However, deviating viewing directions 13 and 14 can also occur, for which the individual volume holographic gratings have different reflectivities, since for each of the volume holographic gratings there is a reflection efficiency profile that is dependent on the viewing angle and is different for the individual volume holographic gratings. This would lead, for example, to the perceptible virtual image having an increasing red cast as the angular deviation from the predetermined viewing angle increases. With large exit pupils 6, as shown in FIGS. 1 and 2, these different viewing angles are already available to the user for different positions in the virtual image. The predetermined viewing angle is only fulfilled when viewing the center of the image. Viewing direction 13 or 14 can already exist at the edge of the image, so that the individually perceived virtual image would already have a red cast away from the center of the image.

The viewing directions 13 and 14 thus define an angular range around the predetermined viewing direction 7 for which there should be at least one true-color projection of the virtual image into the exit pupil 6 . This can be, for example, a range of ±2° based on the predetermined viewing angle cd. The individual volume holographic gratings are designed in such a way that they have the diffraction efficiencies shown in FIG. 3 as a function of the viewing angle a, the viewing angle in degrees being plotted along the abscissa and the diffraction efficiency and thus the reflectivity in percent being plotted along the ordinate. The curve K1 shows the diffraction efficiency of the grating for 460 nm, the curve K2 shows the diffraction efficiency for the wavelength 532 nm and the curve K3 shows the diffraction efficiency for the wavelength 640 nm. Each of the curves K1-K3 has its maximum at the predetermined viewing angle cd of 62.5° and then falls in the diffraction efficiency as the viewing angle becomes larger or smaller, so that the diffraction efficiency curves shown in FIG. 3 are present. The slope is now chosen so that for each viewing angle the ratio of the diffraction efficiencies is equal to the ratio of the diffraction efficiencies at the predetermined viewing angle cd. In the example described here as an example, the diffraction efficiency for the predetermined viewing angle cd is 66.4% for the wavelength 460 nm, 45.7% for the wavelength 523 nm and 18.8% for the wavelength 640 nm. The diffraction efficiency adjustment is achieved by the volume holographic grating for the three wavelengths in a common material layer with the refractive index of 1, 5 and a layer thickness of 10 pm with a refractive index modulation of 0.015 for 460 nm, 0.0125 for 532 nm and 0.0085 for 640 nm can be written. The diffraction efficiency curves of the curves K1, K2 and K3 are therefore in the ratio 2.4:1:0.7.

In order to achieve true-color projection, the control unit 9 controls the image generator 8 in such a way that the brightnesses for the blue, green and red color sub-image are exactly inversely proportional to the relationship described for the curves K1-K3. The brightness ratio for the blue, green and red color partial image is therefore 0.4:1:1.4, so that after deflection at the holographic gratings in the photopolymer layer 11 there is effective reflectivity (i.e. with brightness correction), as in Fig. 4 is shown schematically. In Fig. 4, the viewing angle in degrees is plotted along the ordinate and the diffraction efficiency, taking into account brightness correction, in percent along the abscissa. The three curves K1', K2' and K3' are more or less superimposed for the desired viewing angle range, so that the virtual image is projected with true colors. The user only sees a certain decrease in brightness as the distance from the center increases, but this is much less disturbing than the color cast effect described.

As can also be seen from FIG. 4, the maximum reflectivity for the predetermined viewing angle cd is approximately 46%, so that the transmission is at least 54%. It is of course possible for the projection device 1 according to the invention to have further optical elements, for example for minimizing aberrations. So mirrors and lenses can be used. As shown schematically in FIG. 5, an optical system 15 made up of a plurality of optically active surfaces can be arranged between the image generator 8 and the holographic beam splitter 5, which is shown here schematically as a lens. These optics 15 are necessary for the correction of optical aberrations, such as dynamic distortion, which inevitably occur in the system shown in FIG. 1 when imaging the image generator 8 via the volume hologram 5 as the only effective surface. The volume hologram 5 can also be placed in the optical system 15 so that the conventional Fresnel reflex is used on the windshield 4 to reflect the image into the driver's field of vision. If the volume hologram is placed in the optical system 15, the diffraction efficiency of the volume hologram can be greater than 46%.

6 also shows a modification in which the light from the image generator 8 is coupled into the windshield 4 via a coupling element 16 (e.g. deflecting mirror) and is guided there by at least one reflection to the photopolymer layer 11, on which the coupling described is carried out.

Instead of the windshield 4, any other transparent body can also be used for the projection device 1 according to the invention. This transparent body can be designed as a plane-parallel plate. However, it is also possible for at least one boundary surface (for example the front and/or rear side) to be curved.

The photopolymer layer 11 can be embedded in the transparent body as shown with the windshield in Figs. 1, 2, 5 and 6. However, it is also possible that the photopolymer layer is formed on the front or back of the transparent body. Furthermore, a cover layer can also be provided on the photopolymer layer 11 .

The projection device 1 according to the invention can also be designed to be placed on the user's head and for this purpose have a holding device 32 which can be placed on the user's head and which can be designed, for example, in the manner of a conventional eyeglass frame. In this case, the projection device 1 can have a first and a second spectacle lens 33 , 34 which are fastened to the holding device 32 . The Holding Device

32 with the lenses 33, 34 can be designed, for example, as sports glasses, sunglasses and/or glasses to correct ametropia, with the user having the first lens

33 the virtual image can be reflected in his field of vision. The image module 2 can be arranged in the area of the right-hand side of the spectacles of the holding device 32, as shown schematically in FIG.

As can best be seen from the enlarged, schematic partial sectional view in FIG. 8 , the first spectacle lens 33 has a rear side 37 and a front side 38 . The back 37 and the front 38 are curved here. However, it is also possible that they are flat. The curvature can be a spherical curvature or an aspheric curvature. If the virtual image is to be visible in superimposition with the environment, an effective deflection efficiency in the range of, for example, 50% can again be present. If the surroundings should not be visible, the deflection efficiency can be chosen to be larger.

Claims

patent claims

1. Projection device with an image module (2), which generates a multicolored image in that a first

partial color image with a first wavelength and a second partial color image with a second wavelength are generated, a projection unit (3) to which the multicolored image is supplied and which projects it into an exit pupil (69) in such a way that an observer can perceive it as a virtual image, when his eye (A) is positioned in the exit pupil (6) and he looks at the projection unit (3) at a predetermined viewing angle (co), wherein the projection unit (3) has a volume hologram that contains the multicolored image for imaging in the exit pupil (6) deflects, wherein the volume hologram has a volume grating for each wavelength of the partial color images, each of which has a deflection efficiency profile that is dependent on the viewing angle and that is maximum for the predetermined viewing angle (cd), so that a first efficiency ratio of the first deflection efficiency profile for the first wavelength to Deflection efficiency curve of the second wavelength was present gt, wherein the deflection efficiency curves for a predetermined angular range around the predetermined viewing angle (cd) are set such that the first

Efficiency ratio is constant for the predetermined angular range, the image module (2) being controlled in such a way that when the multicolored image is generated, a first brightness ratio of the brightness of the first color sub-image to the brightness of the second color sub-image is inversely proportional to the first efficiency ratio, so that the different deflection efficiency curves are compensated and the viewer can perceive the multicolored image for viewing angles from the predetermined angle range as a true-color virtual image.

2. Projection device according to claim 1, wherein all volume gratings are formed in the same layer (11).

3. Projection device according to claim 1 or 2, wherein the volume gratings are designed as reflective volume gratings.

4. Projection device according to one of the above claims, wherein the volume hologram is embedded in a transparent carrier.

Projection device according to one of the above claims, wherein the projection unit has an image guide in which the multicolored image is coupled and guided by reflection to the volume hologram, which deflects the multicolored image and thereby decouples it from the image guide.

Projection device according to one of the above claims, wherein the image module also generates a third color sub-image with a third wavelength, wherein due to the deflection efficiency curve of the volume grating for the third wavelength there is a second efficiency ratio of the first deflection efficiency curve for the first wavelength to the deflection efficiency curve of the third wavelength and the deflection efficiency curves for the predetermined angular range around the predetermined viewing angle (cd) are set in such a way that the second efficiency ratio is constant for the predetermined angular range, the image module (2) being controlled in such a way that when the multicolored image is generated, a second brightness ratio of the brightness of the first partial color image to the brightness of the third color field is inversely proportional to the second efficiency ratio.

A projection device according to claim 6, wherein the first wavelength is in the blue wavelength range, the second wavelength is in the green wavelength range and the third wavelength is in the red wavelength range.

Projection process in which a multicolored image is generated in that a first partial color image is generated with a first wavelength and a second partial color image is generated with a second wavelength, the multiple colored image is supplied to a projection unit (3) which projects it into an exit pupil (69) so depicts that a viewer can perceive it as a virtual image when his eye (A) is positioned in the exit pupil (6) and he looks at the projection unit (3) at a predetermined viewing angle (cd), the projection unit (3) a has a volume hologram that deflects the multicolored image into the exit pupil (6) for imaging, wherein the volume hologram has a volume grating for each wavelength of the partial color images, which in each case has a deflection efficiency profile that is dependent on the viewing angle and is maximum for the predetermined viewing angle (cd), see above that a first efficiency ratio of the first deflection efficiency profile f for the first wavelength to the deflection efficiency profile of the second wavelength is present, wherein the deflection efficiency curves for a predetermined angular range around the predetermined viewing angle (co) are set such that the first efficiency ratio for the predetermined angular range is constant, with a first brightness ratio of the brightness of the first color partial image to the brightness of the second color partial image being reversed when the multicolored image is generated is proportional to the first efficiency ratio, so that the different deflection efficiency curves are compensated for and the viewer can perceive the multicolored image for viewing angles from the predetermined angular range as a true-color virtual image.