DE102011056552A1 - Method and device for producing a holographic screen for electronic front projection - Google Patents

Abstract

The invention relates to a method and a device (48) for producing screen holograms for front projection with a covering of the screen (14, 96) by individual holographic image pixels (72), the screen pixels (72) being superimposed by superposition of at least one first illumination beam (20 ) and a second illumination beam (22) and different angular orientations of at least one of the illumination beams (22) with different emission directions are provided.

Description

The invention relates to a method for the production of holographic screens (e.g., screens) for electronic projection and to a device for producing a screen hologram.

Background of the invention

Electronic image projectors (beamer) for front projection of vivid color images are today an important supplement to flat screens because of the large image width that can be projected onto free space in rooms for a larger number of viewers. They have therefore become today an indispensable means of presenting information at meetings in business and in the classroom. Increasingly, electronic projectors are also used for movie playback in the cinema, for television and for playing DVDs for home theater. Other fields of application of this technique are projections of large-scale images in traffic and guidance simulators for driver or pilot training, in particular for motor vehicles, ships and aviation devices, such as aircraft or helicopters. Recently, portable image projectors - built into digital cameras and cell phones - are being offered for mobile playback of images and videos over short distances on the market.

Common feature of most image projectors, which are equipped with very different technology, is the projection of three to four superimposed images in the primary colors red, green and blue (RGB) and occasionally an additional color or white. The images to be superimposed are produced in separate color channels of the projector with liquid crystal modulators (LCDs), digitally controlled mirror modulators (digital mirror devices, digital light projectors, DLPs) and projected onto a screen at the same time as a slide projector with strong light sources.

Images of the mixed colors are created by overlaying the images of the primary colors. Discharge lamps (such as xenon and mercury lamps), light-emitting diodes (LEDs) or lasers (solid-state and semiconductor lasers) are optionally used as the light source. With lamps as a light source, the required primary colors are filtered out of their emission spectrum for projection by static color filters or rotating color wheels with limited bandwidth. The narrow emission lines of individual light-emitting diodes of different semiconductor systems can be used directly as primary colors. The same applies to some emission lines of lasers in the visible region of the spectrum, which are often used as sources of primary colors for image projection.

With lasers, as an alternative to a parallel projection, a serial image structure that uses the good bundling of laser beams can be carried out. Each RGB beam is modulated separately with the image information. All beams are brought to a common axis and passed over the screen with a double-axis scanner. Intersections of the rays with the screen then record the image serially during the scan, much like the electron beams of a television tube.

A serious disadvantage of projecting onto a screen, as compared to LCD and plasma monitors using the inherent lighting or direct backlighting of the screen, is their sensitivity to extraneous light, i. in particular room lighting or daylight of the environment. This is because the extraneous light from all directions as the projector light is backscattered from the conventional projection screens to the viewer. The superimposition of the two light components reduces the contrast and color saturation of the projected images. On the other hand, in the flat screens incoming extraneous light is absorbed from the outside in a blackened mask of the screen and thus does not get back to the viewer.

If clear walls or cinema screens are used as a screen for electronic projection in bright rooms, then the image disturbance by the ambient light can be overcome to an extent with an increase in the luminous flux of the projector. For this purpose, a very bright source is needed with the disadvantage of their higher power consumption and reduced life. When using lasers as a light source, the risk of eye damage is increased by more intense scattered laser light. The transmission quality of the images is always disturbed in this way. An optimal solution offers only a screen that brings the viewer the same quality of projection regardless of the strength of the extraneous light. To achieve this, the appearance of extraneous light on it should be completely avoided. At the same time, the light from the projector should not be attenuated on the way across the projection screen to the auditorium.

State of the screen technologies

Some manufacturers of electronic projectors have developed special screens that improve contrast and color saturation when projected in a bright environment compared to a white screen.

For example, the "HCS-80 ChromaVue-Contrast Home Theater Screen" offered by "Sony" integrates multiple dielectric reflective filter layers in the screen with a filter bandwidth adapted to the spectral width of the projector's red, green and blue primary colors. These color filters lie in the screen in front of an absorbing surface and therefore scatter predominantly incident light with the wavelengths of the projector back to the viewer. Since the reflectance of the color filter decreases with the angle of incidence, however, this measure is useless to suppress lateral extraneous light. The technical effort in the manufacture of these screens is considerable, and the achieved improvement in image quality is bought with a considerable weight and high price of the screen.

In a second projection screen "Supernova" from the company "dnp denmark as", the permitted angle of incidence and angle of radiation of the screen is narrowed with the aid of a transparent film with an embossed array of light-absorbing louvre foils, in order to suppress extraneous external light outside this angle. Here, however, no additional distinction of the colors of the forwarded light is made as in the first screen. Thus, only a slight improvement of the screen quality is achieved. A major disadvantage of this second screen is that the projection angle range and the viewing angle range must be very close to each other, which greatly limits its possible applications.

It is understood that screens of this type based on a variety of additional micro-optics of filters and absorption films in the production of very complex and costly and therefore not very well suited for cheap mass production. A desirable solution would be a screen, in addition to the function of the targeted distribution of the projector light in the auditorium, the two mentioned filter functions of the color and the angle of incidence in the distinction of useful and extraneous light united.

In the invention, this is to be achieved by means of holographic screens, which also provide a cost-effective mass production.

It is known that holographic images of real canvases, in particular with the help of phase holograms (volume holograms), can in principle be used as screens for front projection. They have the advantage, in particular over the previously described screens, that they can be inexpensively manufactured with the holographic replication methods after the production of a suitable master hologram. They can then be applied as a thin film on larger image display area. Holographic screens for front projection are e.g. produced as reflection phase holograms which have the particular advantage of e.g. to amplitude holograms and transmission phase holograms have a very high diffraction efficiency and an efficient suppression of the 0th order and the conjugate beam of a holographic recording.

Such screens are eg in the DE 197 00 162 B4 "Method of Making a Holographic Screen for Laser Projection". Here, first of all, the property of each hologram is used that it can only be reconstructed by illumination from a particular direction - which was determined when it was captured as the direction of the reference beam. This excellent direction is then chosen in the application of a hologram as the screen as the preferred direction of incidence of a projection of the projector. All light from other directions, is passed through the hologram, where it can then be absorbed separately. In addition, a phase hologram acts like a narrow spectral filter. This means that its image can only be reconstructed with light from the same spectral range around the wavelength that was used in its recording. This effective wavelength bandwidth is determined by the thickness and refractive index modulation of the hologram. If its central wavelength is set to be the same as the basic color of the projector and the bandwidth of the hologram is greater than or equal to the bandwidth of the basic color of the projector, then the entire projector light is selectively reflected by the screen hologram. But at the same time, light of other wavelengths, such as interfering extraneous light, is transmitted unhindered, where it then no longer bothers or can be absorbed in a black area.

In summary: in the prior art according to the DE 197 00 162 B4 At the same time, the reconstruction of the holographic screen requires the fulfillment of two conditions: first, the direction of projection must be the same as the direction of the reference beam; second, the color of the projector light must be identical to the wavelength of the holographic screen. Only then are images of the projector projected onto them visible. However, this also ensures that extraneous light from directions other than the optimal direction of the projector - even those with the specific color of the projector - passes unhindered the hologram and can no longer disturb the front projection. Similarly, extraneous light that falls from the direction of the projector but a different color has passed freely as the one specified for the hologram.

It is understood that in this solution for the suppression of extraneous light in a projection on a pre-determined when recording the hologram angle of incidence of the recording must be set during the projection. This is not a limitation for most applications, because projectors and screens are predominantly fixed installations in showrooms. There are two alternative applications of the thin screen hologram, either as an image display area in front of a black area that absorbs the passing extraneous light, or as an image area without an absorber wall. The first alternative would be e.g. for wall projections of advantage. The second, however, is for freestanding screens on glass panes, as well as windows and partitions, which then allow the viewer to keep an eye on the space behind the screen in addition to or as an alternative to the projection.

Since each color projector has at least three primary colors red, green and blue for the image reproduction of the entire color space, holographic screens for true color projection should also have at least three hologram images of these colors. At the same time, the spectral bandwidths of the holograms are adapted to the bandwidth of the projector. Either three hologram images exposed together or three separate hologram films stacked on top of each other are used in a hologram foil - each for a different base color. In an image projection, every single hologram only scatters its specific base color and lets the light of other colors through unhindered. Volume holograms, i. Hologram sheets having a multiple wavelength (5-20 μm) thickness were used. Compared to thin screens, they have the advantageous close tolerances of the projection angle of the projection wavelength and the spectral bandwidth.

The holographic front projection screen has the advantages of lighter weight, simpler structure and lower cost than the conventional angle and color discrimination screens available on the market and described above. Moreover, its production is done in the same way as holograms in their widespread applications in security and decoration techniques.

The recording technique of the above DE 197 00 162 B4 However, it has to struggle with the following fundamental technical difficulties. For the production of larger master holograms, very long exposure times are required with the laser power available today. It then requires a very high frequency stability of the laser used and high mechanical stability of the structures of the exposure device, which are difficult to meet with the current state of the laser and exposure technology. Furthermore, the production of a homogeneous scattering screen requires a uniform exposure over the entire cross section of the hologram. This is difficult to achieve because of the Gaussian intensity profile of a laser beam, ie its natural radial intensity decrease towards the edges. This then leads to a different exposure course in the three holograms of the primary colors and thus to undesired color disturbances in the image content.

To improve these inadequacies of the holographic screens, is in the DE 199 34 162 B4 First proposed is the exposure of the holographic screens to laser pulses of such short duration that the instabilities with their long time constant can no longer influence the exposure. Second, the holographic image of a front projection screen should not be stored as an entire image in a hologram with the aid of an expanded exposure beam, but rather by recording a plurality of individual images, ie individual holograms (pixel holograms). However, these can be imprinted with a focused beam with a fixed intensity and constant exposure time in a scanning process over a larger area. Pixel holograms can be stored so small and close together in the hologram that it would appear to the viewer as if it were a homogeneous projection surface. Using the same recording conditions as a homogeneous screen, each of these pixel holograms stores the advantageous characteristics of narrowing the effective projection angle, the projection wavelength, and the spectral bandwidth.

The in the DE 199 34 162 B4 proposed recording technique of a reflection hologram provides the scanning of such a hologram, which stands on the bottom with a natural screen in close optical contact (recording technique according to Denisyuk). For example, a pulsed laser is used. The scanner will be placed at the projector's later location as a beam source for the reconstruction of the holographic screen. In this in the DE 199 34 162 B4 proposed scanning method, the local scattered light distribution of each individual illuminated point of the screen (pixels) is recorded in the hologram by backscatter from the template and stored.

With this method, because of the fixed but adjustable intensity of the exposure laser at all scanned points across the cross section of the hologram, a homogeneous exposure with high color stability can be achieved. With a very short pulse duration of the exposure, the influence of laser instabilities and mechanical shocks on exposure efficiency and contrast of the recording can be handled. For the reproduction of the three or more primary colors sees the DE 199 34 162 B4 also several stacked holograms, as in a sandwich before.

The recording techniques of the above two publications DE 199 34 162 B4 and DE 197 00 162 B4 however, have the following shortcomings.

They are used to store fixed scattering properties of natural scatter templates in the hologram. One of these specified properties is the local angular distribution of the scattering intensity as a function of the angle of incidence of a light beam, which assumes its maximum for most scattering natural surfaces, if the radiation angle is equal to the angle of incidence (reflection angle). In a projection on the side of a large screen or with a large opening angle, this angular distribution of the scattering, as in 1 is shown, very unfavorable, since with her only a small part of the scattered light, in the angular range of the auditorium, passes. At the same time, the perceived luminance of the screen varies depending on the position of the viewer. A desirable scattering distribution of the screen would be as in 2 is shown, a scattering of the screen mainly in the direction of the viewer. However, this then requires a constant change of the radiation angle in their direction over the entire surface of the screen.

When using laser projectors, an additional serious problem arises, namely the formation of laser granulation, "speckles" of the image in the eye of the observer, which are very pronounced in natural canvases or other scattering surfaces as a template. Speckles are created by superimposing partial waves, which are scattered at the statistically distributed microroughness of the canvas surface and interfere with each other. These microroughness and speckles randomly distributed in the spatial frequency are again imaged and stored in the hologram during the recording of the screen and then lead to undiminished or increased speckle formation during image reproduction.

Both here described defects of the DE 197 00 162 B4 and the DE 199 34 162 B4 known methods are a consequence of the use of natural scattering materials of fine grains of the surface with statistical spatial distribution for the expansion of the projector light. Through them the radiation angle is clearly defined and a strong Specklebildung is inevitable.

To fix the first drawback, the fixed connection between the angle of incidence and the beam should be removed in an improved screen. Furthermore, it should be designed such that the projector light can be directed depending on its angle of incidence in variable radiation angles in the direction of the auditorium. In order to reduce the second disadvantage, the strong Specklebildung, it should be ensured that the wavefronts, which get from the screen into the eye of the observer, no irregular interference but at most to periodic intensity distribution in front of the screen and in the eye of the beholder who with relatively simple cost-effective known optical measures can be averaged out.

In order to achieve both, computer-calculated nanostructured scattering screens are being developed recently. In these, preferably binary diffractive surface structures are written by electron beam lithographic methods which allow an adapted change of the scattering angle. Such structures are eg of W. Freese et al. in the article: "Design of binary subwavelength multiphase level computer generated holograms" in Optics Letters, Vol. 35, pages 676-678 (2010) described. Here is written in a photoresist layer of the surface of a quartz glass plate, with very fine binary structures. These structures, of a size below the wavelength of light, then serve as master structures for their subsequent transfer into hologram materials, such as photopolymers and film materials. For this purpose, common methods of reexposure are used.

These calculated diffractive scattering elements, however, have the fundamental disadvantage that the scattering angle, as known from diffraction angles at gratings, depends on the wavelength and lead to different luminance and color distribution over each individual master. For each color, therefore, a separate master must be produced using this complicated process, and all masters and their copies must be matched to each other, so that individual color holograms with the same emission characteristics are created. Due to the small size of the master, which can be described electronically, an enlargement to the required level of a screen with a subsequent optical duplication method must be made subsequently.

The computer-calculated and electronically written hologram therefore needs several intermediate steps of the angle, color and Rescaling until the master etched in quartz glass, typically 6 inches in diameter, can be brought into the final format of the screen, eg 60 inches wide. In addition, the basic structure of the master must be converted into individual phase holograms for the different colors.

With this technique, holographic screens with a specific variable scattering characteristic can be produced over larger screen areas. The way to get there, however, is very long and expensive. There must also be no local single errors in the transmission, because they would immediately lead to a reduction in quality of the whole screen. But this is hardly avoidable in the many stages of this production.

By using diffractive structures of sub-wavelength size in each pixel of the hologram, only zero-order diffraction occurs, with simultaneous suppression of all higher orders. This creates a homogeneous wavefront of the radiated wave in the far field, which will then be speckelfrei with respect to each pixel. However, especially in a laser projection by the superposition of the waves of adjacent separate pixels with a separation of several wavelengths distance, similar to two-dimensional lattices, a periodic wave field with a varying intensity course develops. Because of the relatively large period spacing in this grid, the intensity profile is very finely structured and therefore difficult to remedy.

It is an object of the invention to disclose improved methods and apparatuses for producing a holographic screen for front projection compared to the prior art.

This object is solved by the subject matters of the independent claims. Advantageous embodiments are the subject of the dependent claims. According to a first aspect, the invention provides a method for producing screen holograms for front projection with coverage of the screen by individual holographic image pixels, wherein the screen pixels are provided by superposition of at least a first illumination beam and a second illumination beam and different angular orientations of at least one of the illumination beams with different emission directions ,

In an advantageous embodiment of the invention it is provided that the screen pixels are illuminated, preferably by optical lens arrangements, from both sides of a hologram film, wherein the hologram film is illuminated from the one side with the first illumination beam and from the other side with the second illumination beam.

In an advantageous embodiment of the invention, it is provided that the screen pixels are illuminated in each case in the same pixel volume of a hologram film by means of a pivoting device and a pusher and a common scanning movement of the two beams over the surface of the screen hologram.

In an advantageous embodiment of the invention it is provided that the screen pixels are produced in temporal succession by a common lateral and vertical grid-shaped scanning movement of the first and the second illumination beam relative to a hologram film.

In an advantageous embodiment of the invention, it is provided that the first illumination beam is a reference beam, which is always aligned with a axis alignment to a fixed point (P) in space, preferably as a plane wave, and / or that the second illumination beam is an object beam, which has, preferably with a part of a spherical wave, in a holgram film a common cutting volume with the reference beam, and whose axis and / or its aperture angle for producing the different pixels are set differently to produce the different angles of emission per pixel.

In an advantageous embodiment of the invention, it is provided that the axis of the object beam is directed with a rotary and / or displacement device to the common cutting volume and / or that the beam axis and the opening angle of the object beam are adjusted to a desired angular range of a viewing space of the screen hologram illuminate.

In an advantageous embodiment of the invention, it is provided that the first and the second illumination beam are generated by a common laser.

In an advantageous embodiment of the invention it is provided that the first and the second illumination beam are guided to their associated relative to a holgrammfilm movable lens arrangements by optical fibers, preferably by monomode glass fibers.

In an advantageous embodiment of the invention, it is provided that the second illumination beam is optionally used directly for the exposure of a single image pixel or split into several modes in order to illuminate a plurality of image pixels simultaneously.

According to a further aspect, the invention provides a device for producing screen holograms for a projection by forming the screen holograms from individual holographic image pixels, comprising:
a light source for a holographic recording of interference of a first illumination beam and a second illumination beam, and
a scanning device for guiding the first illumination beam and the second illumination beam movable relative to a hologram film,
wherein the scanning device is configured and arranged such that the axis of the first illumination beam is aligned with a fixed point (P) in space relative to the hologram film at each pixel exposure, and the axis and / or an aperture angle of a focusing second illumination beam relative to the hologram film are directed variably in an angular range of an auditorium.

In an advantageous embodiment of the invention, a pivoting device for different adjustment of the angular orientation of the second illumination beam per pixel is provided.

In an advantageous embodiment of the invention, it is provided that the light source has a pulsed laser whose pulse length is designed to expose individual holographic image pixels.

In an advantageous embodiment of the invention, a Strahlleiteinrichtung for guiding the first illumination beam and the second illumination beam is provided and formed such that the Lichtwegdifferenz of the first and second illumination beam from a common laser as a light source to the Holgrammfilm over the entire scanning range away shorter than the Coherence length of the laser is.

In an advantageous embodiment of the invention, it is provided that a splitting device, preferably a transmission grating, is provided for splitting the second illuminating beam into a plurality of plural modes for the simultaneous exposure of a plurality of pixels.

A particularly preferred use of a screen hologram, which can be produced by a method and / or a device according to one of the preceding claims, for front projection, is characterized in that, in a parallel image projection, the beam path of a projector is not perceptibly moved by the spectator's eye to form a speckle pattern to screen the screen pixel in space, or in a serial image projection of the diameter of the screen pixels and the projection beam against the diameter of a still be resolved by the eye of a viewer on the screen resolving element is selected so that a determination of a Specklemusters by the movement of the projection beam for serial image projection ,

In an advantageous embodiment of the invention, it is provided that a first optical fiber, in particular monomode glass fiber, is provided for guiding the first illumination beam from the light source to a movable relative to the Holgrammfilm write head of the first illumination beam and that a second optical fiber, in particular monomode glass fiber for directing the second illumination beam from the light source to a write head of the second illumination beam movable relative to the hologram film.

A preferred embodiment of the invention provides a method of producing screen projection holograms for electronic projection with coverage of the screen by individual holographic screen pixels, wherein the screen pixels have different emission directions, by superposition of two illumination beams of a reference and an object beam, by optical lensing from both sides of the hologram film are each time in the same pixel volume of the hologram film, illuminated by means of rotating and sliding device and common scanning movement over the entire surface of the screen hologram.

It is preferred that the pixels are produced in temporal succession by common lateral and vertical scanning scan movement of the two lens arrays along the film.

It is preferable that the reference beam always illuminate a plane wave with an axis orientation to a fixed point P in space.

It is preferable that the object beam represents a part of a spherical wave having a common intersection volume with the reference beam in the hologram film.

It is preferred that the axis of the object beam is directed with a turning and shifting device to the common cutting volume.

It is preferable that the beam axis and the aperture angle of the object beam are adjusted to illuminate a sufficient angular range of a viewer space of the screen hologram

It is preferred that the object beam and reference beam originate from a common laser and are guided through monomode optical fiber up to the movable lens arrays of the object beam and reference beam on the hologram.

It is preferred that the screen pixels formed in the individual exposures and scan movement are so close together that no unexposed gaps in the entire screen hologram arise.

A further preferred embodiment of the invention provides an apparatus for producing on-screen holograms consisting of individual holographic electronic projection screen pixels with a narrow-band laser source for holographic recording of the interference of object and reference beams, the apparatus comprising a scanning device for guiding the two beams across the screen is designed so that the axis of the reference beam are each time aligned to a fixed point in space and the axis of the focused object beam and its opening angle variably in the angular range of the auditorium.

It is preferred that the device has a laser source which has a sufficiently short exposure time to produce individual holographic screen pixels during scan movement.

It is preferred that the optical path difference of the object beam and the reference beam from a common laser source to the hologram be shorter than the coherence length of the laser source despite the scanning movement with an adjustment of the optical fiber length

It is preferable that, optionally, the fiber mode exiting from the glass fiber of the object beam is simultaneously used for exposing a plurality of image pixels directly to the exposure of a single image pixel or after the passage of the mode through a transmission grating into multiple orders.

It is preferred that the size of the screen pixels is set so that they can not be resolved by the viewer's eye in front of the screen and at the same time their speckle pattern in the room is averaged out with an imperceptible by the eye movement of the beam path of the projector in a parallel image projection to the screen ,

It is preferred that the diameter of the screen pixels and the projection beam of a serial image projection with respect to the diameter of the resolution element of the eye on the screen is selected so that a sufficient averaging of the perceived speckle pattern takes place solely by the movement of the projection beam.

In the following the invention will be explained in more detail with reference to the accompanying drawings. Showing:

1a . 1b : Angular distribution of projector light originating at point P on a conventional screen and holographic screens made as a direct image of a conventional screen relative to the positions of viewers in front of the wall.

2a . 2 B : Desired improved angular distribution of the light emitted by a screen compared to that in the 1a and 1b to the state of the art illustrated situations.

3a . 3b . 3c : Cross section of reference beam (plane wave) and object beam (focused wave) in the hologram with focus point in front of the hologram. Shown are cross sections at the upper edge of the hologram in 3a , in the middle in 3b and lower edge of the hologram in 3c ,

4a . 4b . 4c Same appearance as at 3a . 3b and 3c but with a focal point of the object beam behind the hologram.

5a . 5b ; 5c : Reconstruction of hologram images after 3a . 3b and 3c using a plane wave as a section of the projector beam from the projector source in point P.

6a . 6b . 6c : Reconstruction of hologram images after 4a . 4b and 4c with a plane wave from projector source in point P.

7a . 7b : A first example of a construction of a hologram exposure apparatus.

8a . 8b A second example of the construction of a screen hologram exposure apparatus.

9 : a schematic representation of an example of a beam path of the reference and object beam to the hologram.

10 : a first example of a close juxtaposition of the hologram pixels in the Screen hologram along a line and between adjacent lines in a circular beam.

11 A second example of a close juxtaposition of the hologram pixels in the screen hologram along a line and between adjacent lines in a hexagonal beam.

12 A series of separate focus points of the closely spaced hologram pixels in a plane outside the hologram material. The axis of the image pixels are also indicated.

13 : an exemplary division of the object beam into a plurality of exposure beams of the different orders in a transmission grid hologram between the glass fiber and the exposure optics of the object beam.

14 : some examples of the exposure pattern of the object beam after beam splitting in the transmission grating in 13 ,

15 : an example of stacking the holograms of the three colors red (R), green (G) and blue (blue) on a transparent support plate (T) with an additional absorber layer (A).

16 : an example of stacking the holograms of the RGB colors on a support plate (T) without absorber layer.

17 : an example of a parallel image projection on a holographic screen with representation of the screen structure within the circle of the image resolution of the viewer's eye.

18 : an example of a serial laser image projection on a holographic screen showing the screen structure within the circle of the image resolution of the observer's eye and the intersection of the laser beam with the surface of the hologram.

19 : A schematic representation of an example of an implementation of the variation Δs of a lateral displacement of an incident beam from the projector onto a screen pixel in an angle change Δφ or a transverse path change ΔS behind the screen hologram in the auditorium.

Embodiments of the invention will now be described with reference to the illustration in the drawings.

1a shows a side view of an angular distribution of incident light projection according to the prior art, as shown in the DE 197 00 162 B4 and the DE 199 34 162 B4 is described. 1b shows this angular distribution seen from above. Shown is that of projector light 10 originating at point P on a conventional canvas 12 and on holographic screens 14 as a direct reflection of a conventional canvas 12 produced relative to the positions of viewers 15 in front of the wall.

One of the stated properties in the prior art is the local angular distribution of the scattering intensity as a function of the angle of incidence of a light beam, which assumes its maximum for most scattering natural surfaces when the radiation angle is equal to the angle of incidence (reflection angle). In a projection on the side of a large screen or with a large opening angle, this angular distribution of the scattering, as in 1 is shown, very unfavorable, since with her only a small part of the scattered light, in the angular range of the auditorium, passes. At the same time, the perceived luminance of the screen varies depending on the position of the viewer 15 ,

A desirable scattering distribution of the screen would be as in 2 is shown, a scattering of the screen mainly in the direction of the spectators 15 , However, this then requires a constant change of the radiation angle in their direction over the entire surface of the screen. 2a shows a side view comparable to 1a to the desired improved angular distribution of the screen 12 . 14 radiated light compared to the in the 1a State of the art situation. 2 B shows a top view comparable to 1b to the desired improved angular distribution of the screen 12 . 14 emitted light compared to in 1b illustrated situation according to the prior art.

The following describes a method for producing a holographic screen and an apparatus for producing a holographic screen for the electronic projection, with which the distinction of the ambient light and the projection light 10 with respect to their direction of incidence and color via screen holograms, also with the aid of holography for discriminating angles of incidence and reconstruction wavelengths in pixel phase holograms. Next, on the one hand, an adaptation of the Winkelabstrahlcharakteristik over the entire cross section of the holographic screen to the location of the viewer in front of the screen allows and on the other hand, when using lasers as a projector light source, the emergence of Bildspeckles be significantly reduced to allow their averaging with simple means.

For this purpose, according to one embodiment, a method for the production of Projection screen holograms, in particular a method for the production of screen projection holograms for electronic projection with a coverage of the screen by individual holographic screen pixels proposed in which the screen pixels have different emission directions, by superposition of two illumination beams, a reference and an object beam, by optical lens assembly from both sides of the hologram film, each time in the same pixel volume of the hologram film, are illuminated by means of rotating and sliding device and common scanning movement over the entire surface of the screen hologram.

Further, according to an embodiment, an apparatus for producing on-screen holograms consisting of individual holographic screen pixels for electronic projection with a narrow-band laser source for the holographic recording of the interference of object and reference beam is proposed which comprises a scanning device for guiding the two beams across the screen is designed so that the axis of the reference beam are aligned each time to a fixed point in space and the axis of the focused object beam and its opening angle variable in the angular range of the auditorium.

In the method according to a preferred embodiment of the invention, a screen for projection is produced by the exposure of a large plurality of single shots, the screen pixels, as phase holograms, each pixel being chosen so small that it is not recognized as a single element by the eye of an observer can. This means, for example, that assuming angular resolution of the eye of 1 arc minute, i. about 0.3 millirad, which corresponds to a resolution element of 0.3 mm diameter at a viewing distance of 1 m, the image pixels should be significantly smaller, e.g. 0.1 mm. At a typical reading distance on the display of 33 cm then less than 0.1 mm or e.g. 0.03 mm. These pixel sizes are still very large in comparison with the wavelength of the light of about 0.0005 mm, and the desired filtering functions with respect to the wavelength and incident direction of incident light described above can be realized here in the same way as in a large hologram.

The composition of all the screen pixels imprinted as miniaturized phase holograms then gives the whole screen as a master hologram. The pixels are imprinted over the entire screen hologram with a continuous wave emitting or pulsed laser using a fast scan feed.

The preferred embodiments of the invention suggest that the holographic screens should not be fabricated as a holographic image of a diffusing surface, as is the case in the prior art, rather than as an imprint of a computer-computed and electron-beam lithographic process written diffractive structures.

Instead, the preferred embodiments suggest that each image pixel be accommodated in the hologram by the superposition of at least two previously adjusted wavefronts of an object beam and a reference beam that can be formed with refractive optics. Holographic structures that result from this superimposition of planar and spherical waves are directly comparable with the so-called classical Fresnel zone or phase plates, which can perform the same functions as lenses and concave mirrors of classical optics.

The following recording technique is proposed in detail: Individual image pixels are combined with the direct superposition of planar wave and curved wave in e.g. imprinted two crossed beams that meet from the opposite direction to the hologram always in the same place in the hologram. To expose a larger hologram area, both beams are guided along a scan track and orthogonal thereto along a plurality of parallel scan tracks. The two-sided exposure of object and reference beam corresponds to the usual recording technique of reflection holograms. Here, the plane wave beam (first illumination beam) corresponds to the reference beam, and the concave-curved wave front centered beam near the hologram (second illumination beam) represents the object beam. Displacement of the focal point of the object beam outside the plane of the hologram is proposed. which has the advantage that there is a better overlap for all angles of incidence of the two exposure beams in the hologram material. To form high contrast interference, the same linear polarization of the two beams is advantageous. Since the angles of incidence of the beams are partly large, it is advantageous for a good efficiency of the exposure to choose the plane of incidence of the beams on the hologram as a common polarization direction.

As suitable materials for phase holograms, for example, silver halides, photopolymers and dichromated gelatin can be used. The exposure sensitivity of the various materials to saturation is in the range of 0.1-1000 mJ / cm ² depending on the material and color of the exposure. The silver halide material is related to conventional black and white photo films and has the advantage of that it can be used for both continuous wave and pulsed exposure with a sensitivity of typically 3 mJ / cm ² to saturation of the material. The photopolymer is characterized by a simple development process with UV light and baking rather than a chemical process and its good environmental stability. Its sensitivity to saturation is about 50 mJ / cm ² . Since their exposure process proceeds much more slowly with a time constant of about 1 ms compared to the time constant of silver halides in the picosecond range, photopolymers are mainly suitable for continuous-wave illumination. On the other hand, dichromate gelatins are relatively insensitive and hygroscopic and should be sealed between glass plates, making them less suitable for screen hologram applications.

Preferred embodiments therefore suggest using silver halide for pulsed exposure as a base material for the production of master holograms and photopolymer for producing copies by the usual exposure, e.g. using contact copies from the master. However, with a correspondingly slow exposure, photopolymer would be suitable for both master holograms and their copies.

The 3a . 3b . 3c show a cross section of reference beam 20 (plane wave) and object beam 22 (focused wave) in the hologram 26 with focus point 24 in front of the hologram 26 , Shown are cross sections at the upper edge of the hologram 26 in 3a , in the middle in 3b and bottom edge of the hologram 26 in 3c , at 27 the concave mirror function of the object beam is indicated. The axis 28 of the reference beam 20 is always directed to the origin of the projector light in point P; the axis 30 of the object beam 22 is directed towards the auditorium. Common pivot point of the two axes 28 . 30 is their crossing point S in the hologram 26 ,

The 4a . 4b . 4c show the same representation as at 3a . 3b and 3c but with a focus point 24 of the object beam 22 behind the hologram 26 ,

In particular, the show 3 and 4 a cross-sectional image of the hologram 26 with the proposed writing heads 36 . 34 or the lens systems 31 . 32 of the reference beam 20 or of the object beam 22 in the 3a . 3b and 3c and in the 4a . 4b and 4c each in positions of exposure at the top, in the middle and at the bottom of the hologram 26 , The reference beam 20 (plane wave) should always be directed to a common point P in the room, the location of the later beam exit of the projector. The object beam 22 (spherical wave) becomes simultaneously, as in the 3 and 4 is shown from the opposite side of the hologram 26 with the help of a lens system 32 towards the hologram 26 focused. This happens along an axis 30 entering the auditorium of the hologram 26 directed at the playback.

In the overlapping area of both beams 20 . 22 between the curved (spherical) and the plane wavefronts in the hologram 26 it comes to interference. This interference pattern of the spatial modulation of the refractive index of the volume hologram becomes in the hologram 26 stored. The wavefront of the object beam 22 corresponds to a spherical section of a spherical wave, ie a spherical cap whose center is outside the hologram 26 towards the print head 34 of the object beam 22 can lie ( 3 ), or alternatively towards the write head 36 of the reference beam 20 ( 4 ). Another possible alternative not shown here would be the superimposition of the plane wave of the reference beam 20 with the focus 24 of the object beam 22 in the hologram material itself.

The axis 30 of the object beam 22 is during the scanning of the two rays 20 . 22 and their superposition along the hologram 26 varies continuously with a goniometer about a pivot point S in the hologram material in two spatial directions as in FIG 3 and 4 is shown after recording the desired course of the beam angle, as in 2 is shown to get towards the auditorium. The reference beam 20 becomes a small section of the hologram on the way towards the fixed point of space P. 26 , typically of 100 μm diameter, at any time with the aid of a second goniometer in two spatial directions around S as a fulcrum illuminate. At the same time, the object beam traverses 22 the same section in which then the desired interference figure is imprinted.

With a corresponding adjustment of the numerical aperture of the lens of the write head 32 of the object beam 22 can different opening angles of the object beam 22 and the diameter of his focus 24 and then also the radiated beam in the hologram reproduction can be adjusted.

It is understood that the object beam 22 does not necessarily have to represent a rotationally symmetric wave, which is focused with a conventional lens, but it can also be formed with a lens having different focal lengths in the two axes. This leads to the reconstruction of each pixel being an asymmetrical beam with different opening angles arises in the x and y directions. This would be advantageous, for example, if an asymmetrical illumination of an auditorium is desired.

It is, as already basically in the prior art according to the DE 199 34 162 B4 It is proposed that the pixels of all primary colors of the projection be imprinted in a single hologram in succession or simultaneously with the corresponding exposure lasers. Alternatively, it is proposed to record for each individual color completely identical holograms with the same method and structure as master holograms, so-called H1 holograms, which can then be produced with the usual duplication methods as copies, so-called H2 holograms, for the applications. It is understood that the optical components used such as glass fiber and optics for the design of the reference beam 20 and the object beam 22 then should be achromatic.

Since screens for spectral ranges outside the visible range e.g. UV and IR range for use in simulators for UV cameras and IR cameras (night and thermal imaging cameras) are also required in the market, further preferred embodiments also provide that with the hologram materials and lasers suitable for holographic screens with the same method and devices as in the visible range can be produced.

The 5a . 5b and 5c show reconstruction of hologram images 3a . 3b and 3c using a plane wave as a section of the projector beam from the projector source at point P. The 6a . 6b and 6c show reconstruction of hologram images 4a . 4b and 4c with a plane wave from projector source in point P.

The reconstruction of the hologram 26 As with other reflection holograms, this is done with the aid of the conjugate beam of the reference beam, ie with beams 38 which have their origin here in the spatially fixed source point of the projector P as in 5 in the case of exposure after 3 and in 6 the case of exposure in 4 is shown. For individual pixels, these are individual approximately parallel rays 38 from the expanded projection beam 40 , which contain the individual basic colors of the projection. Here arises like the 5 and 6 show for each position of an incident beam then an expanded beam 42 with an aperture angle of the angle of the focused object beam 22 equivalent.

Because the axis 30 of the object beam 22 continuously while recording from one position of the scan to another over the entire area of the hologram 26 is changed, this also applies to the reproduction of the ray bundles from these positions of the hologram 26 be radiated. For example, if a parallel beam 38 from the location of the projector P to the hologram 26 falls as schematically in 5 and 6 for the three positions at the upper edge, in the middle and at the lower edge of the hologram in each case a), b) and c), different beams are formed 42 from the hologram 26 However, they all face spectators with different viewing angles from the hologram 26 are directed.

A preferred embodiment provides for the technical realization, therefore, that the two write heads 34 . 36 for reference beam 20 and object beam 22 around two crossed orthogonal axes in pivoting directions 44 . 46 rotatable, ie in a goniometer 47 . 50 are mounted with the common pivot point S in the middle of the hologram material, as in 7a is indicated. The writing heads 34 . 36 both are around the two (x, y) coordinate axes on the hologram 26 during the scan with corresponding angle drives evenly rotated to the respective axis orientation, the reference beam 20 to the projector location P and the object beam 22 towards the viewer 15 maintain and at the same time the same place in the hologram 26 illuminate.

The 7a and 7b show the construction of a first embodiment of an exposure device 48 for screen holograms. Two exposure heads 34 . 36 with reference beam 20 from behind and object beam 22 from the front are always with goniometers 47 . 50 to the common place S of the hologram 26 directed while both writing heads 34 . 36 with the help of a line scan 52 in the x-direction and with the help of interlaced lines 54 in y-direction together along the whole surface 56 of the hologram 26 to be moved. At the same time, the orientation of the reference beam 20 to the origin of the projector P and the orientation of the object beam 22 continuously adjusted to the auditorium.

The 8a and 8b show the structure of a second embodiment of an exposure device 48 for screen holograms with lighting heads or writing heads 34 . 36 in gimbal turning device 58 Alternatively to the first embodiment of 7a and 7b with goniometers 47 . 50 , In the second embodiment, a correction means 59 for additional correction of the height and side position of the illumination head of the object beam 22 for the common exposure in point S indicated.

9 shows the beam path of the reference beam 20 and object beam 22 to the hologram 26 over two monomode fibers 60 . 62 with an optical path length difference from the separation point of the beams O to their cross point S in the hologram, which is smaller than the coherence length of the laser 66 should be. In the separation point O, the two beams 20 . 22 through a beam splitter 64 from a laser beam 68 from a laser 66 generated.

Overall, in the 7 to 9 preferred embodiments of exemplary exposure devices 48 for the exposure of the whole hologram 26 shown. 7a shows an exposure device 49 for the reference beam 20 and object beam 22 as two around their two axes (x, y) each in a goniometer 47 . 50 in the swing directions 44 . 46 swiveling writing heads 34 . 36 on both sides of the hologram 26 that work together with a biaxial pusher 31 over the whole hologram surface 56 , which lies in the plane (x, y), are displaceable. Here is hinted how both writing heads 36 . 34 are both pivotable about their two axes, and can be moved together in a grid pattern in the two coordinate axes along the (x, y) plane. 7b shows the construction of the exposure device 48 from 7a in supervision. Here is shown how the whole hologram 26 line-like with the two writing beams in the (x, y) plane can be traversed.

The control of the entire sequence of the exposure, ie the scan in two directions, the angular position of the goniometer 47 . 50 the writing heads 36 . 34 and the irradiation parameters of the exposure laser 66 happens via a microprocessor (μP) which is controlled by a corresponding control program of a computer (PC), as shown in 7a is indicated.

As an alternative, the writing heads could 36 . 34 instead of goniometers 50 in 7 in a gimbal - cardanic turning device 58 - as in 8a and 8b is shown, are rotated. Here, however, would be an additional position corrector in two axes on one of the arms, for example on the arm, the lighting head - write head 34 - the object beam 22 carries, beneficial, as in the 8a - Correction device 59 - is hinted at a simultaneous exposure of the same pixel volume in the hologram 26 with the two rays 20 . 22 to achieve.

It is understood that it is also possible to use the hologram 26 even in two axes under two fixed writing heads 32 . 34 , which then continue to perform only the necessary angular movements to move. Also, a uniaxial movement of the hologram, for example, when unwinding the hologram film from a drum feasible, while the two write heads 36 . 34 then in their orthogonal direction along the hologram 26 be scanned.

9 shows an embodiment of a preferably provided beam guidance of a laser 66 up to the writing heads 36 . 34 for the reference beam 22 and the object beam 22 , Preferably, the laser beam becomes 68 over a monomodige flexible glass fiber 60 . 62 , like in the 9 is shown, each of the write heads 36 . 34 fed. This ensures that the beam guidance and the optical path length to the point S in the hologram 26 , despite the fast angular movements of the writing heads 36 . 34 around the two orthogonal axes x and y and simultaneous scanning movement along the axes, remains stable. The use of monomode glass fibers 60 . 62 ensures a sufficiently high beam quality for the production of the desired interference in the hologram 26 regardless of their transmission length.

The two rays 20 . 22 Reference and object beam should be from the same laser 66 , which preferably has only one longitudinal resonator mode with high frequency stability, so that their interference ability is ensured at all times. The splitting of the laser beam 68 in these two rays 20 . 22 is for example with the help of a splitter prism 70 as a beam splitter 64 provided in the point O. This divider prism 70 may be either a polarization splitter prism or a semipermeable prism. The beam splitting is also possible with a glass fiber beam splitter (not shown). To interference in the hologram 26 It is advantageous that the difference of the light path from the point O to the exposure volume S at the hologram 26 over the two separate glass fibers 60 . 62 to the two heads 36 . 34 shorter than the coherence length of the laser 66 , Preferably, this condition also applies to any position of the heads 36 . 34 during the exposure of the whole hologram 26 ,

Furthermore, it is advantageous if the linear polarization of the two writing beams at the exposure location S for the formation of interferences is the same at all times, which is the case with known measures, such as, for example, polarization-maintaining glass fibers 60 . 62 or with the installation of polarizers with the same orientation in the two write heads 36 . 34 for the reference beam 20 and the object beam 22 is easy to carry out. To achieve a desired maximum possible contrast of interferences between reference beam 20 and object beam 22 is the same beam intensity in the overlay volume S of both beams 20 . 22 in the hologram 26 advantageous. With the arrangements of the writing heads 36 . 34 in the 3a . 3b and 3c and in the 4a . 4b and 4c as well as in the beam guidance in 9 is ensured that this is always the case. However, possible differences in intensities with known continuous beam attenuators in one of the beam paths of the write heads 36 . 34 be easily balanced.

The glass fibers used 60 . 62 are preferably monomode fibers with a typical core diameter in the visible range of 2 ω _f = 5 μm and a numerical aperture NA _f = 0.11. The whole radiation angle is α _f = 12.6 °. If a total beam angle of the pixels of the hologram of α _o = 45 °, ie with a numerical aperture of the lens of NA _o = 0.38, is desired for the object beam, this results for the focus diameter of the object beam 22 of 2 ω _o = 2 ω _f NA _f / NA _o = 1.45 μm.

As a lighting laser 66 For the master hologram, a pulsed laser is preferable to a continuous wave laser. With the use of a pulsed laser with a very short pulse duration, ie typically in the range of a few nano- or picoseconds, the interference capability of the two write beams despite any movement of the write heads 32 . 34 , the fiberglass 60 . 62 and mechanical instabilities of the entire structure at any time secured, especially if the optical path length difference from the laser 66 to the hologram 26 along the two fibers 60 . 62 shorter than the coherence length of the laser 66 , Pulsed lasers are, for example, Q-switched lasers with a pulse duration of nanoseconds and pulse repetition frequency of eg 50 kHz and coherence length of several tens of centimeters or mode-locked lasers with a pulse duration in the picosecond range with a pulse repetition frequency of eg 100 MHz and a typical coherence length of a few millimeters. The individual RGB colors can be produced by frequency doubling or frequency splitting in optical parametric oscillators with subsequent sum frequency or difference frequency formation. Such a way is eg in the DE 199 34 162 B4 , "Method and apparatus for the production of screen holograms, as well as screen hologram" already proposed, as mentioned above. It is understood that other lasers that provide the required RGB colors with sufficient power and frequency purity can also be used.

The exposure track in the hologram 26 will preferably consist of a plurality of parallel lines similar to the tracks of lines in television tubes. So that as complete as possible exposure of the hologram 26 takes place, adjacent pixels are the individual pulses of the laser 66 be generated as close as possible to each other. In addition, the pixels of adjacent lines should also be as close together as possible. This superimposition of adjacent pixels in the lines and from one line to the next is shown in FIG 10 shown schematically.

10 shows a close juxtaposition of the hologram pixels 72 in the screen hologram 26 along a line 74 and between adjacent lines in a circular beam.

It is desirable to achieve complete saturation of the exposure in each individual pulse in the hologram material. Then all the information about the current orientation of the reference beam will be 20 and the object beam 22 and the opening angle of the object beam 20 in every pixel 72 for their location clearly in hologram 26 frozen.

To cover the entire hologram surface with pixels as seamlessly as possible 72 to achieve saturation of their exposure, it is proposed that not only circular beams with a Gaussian distribution of their radial intensity distribution are used for the exposure, but also beams which are generated by means of known refractive beam shaping elements in the write heads 36 . 34 in beams with a constant intensity ("top hat" intensity distribution) to be converted. With the known beam-shaping elements and any other beam cross-sections than a circular, for example, as a rectangle or other polygon produced and strung together during the scan, as in 11 is pictured.

11 shows a close juxtaposition of the hologram pixels 72 in the screen hologram 26 along a line 74 and between adjacent lines 74 at a hexagonal beam.

12 shows a series of the separated focus points 24 the tightly packed hologram pixels 72 in a plane outside the hologram material. The axis 30 the image pixel 72 are also indicated.

Each pixel exposure is an image of a spherical wave, that of a focus 24 goes out, in front of or behind the hologram 26 lies. These foci 24 are at a certain distance from the row 74 the pixel 72 of the hologram 26 , as in 12 is indicated, and are clearly separated because of their smaller size. In the reconstruction with the conjugate reference beam, this spherical wave is again made exactly as if it had its origin in this focus 24 depending on the exposure arrangement in front of or behind the hologram 26 as shown in Figures 5a-5c and 6a-6c for the last case. It needs to understand the propagation of the waves from the individual pixels 72 and their overlay in the room behind the screen will be noticed that the Source of each pixel wave the location of the focus 24 of the object beam 22 is and that the overlapping area of both beams 20 . 22 includes an interference structure, which is a three-dimensional image of the entire beam path of the object beam 22 from this focus 24 to the spectator 15 in the hologram 26 is stored. As previously estimated for an opening angle of the object beam of 45 °, the diameter of the focus would then be 1.45 μm and the pixel diameter would be 100 μm with a focal distance from the hologram of 120 μm.

For a typical number of 10,000 lines for a 1m x 1m hologram with a target pixel diameter of 0.1mm, then 10,000 pixels would have to be along the line 74 to be written. With a line feed rate of 5 m / sec and a pulse frequency of the laser 66 of 50 kHz would be the duration of the exposure of a line 74 then 1 m in length, then 0.2 sec, and with a delay to the start of the next line inscription of 0.8 sec, the exposure time of the entire hologram takes 10,000 seconds or 2.8 hours. With a pixel diameter of 10 microns, the focal distance to the screen would shorten to 12 microns, the number of pixels 72 per line 74 and the number of lines 74 increase to 100,000 and at the same writing speed the total exposure takes 78 hours.

All parameters - the pixel size, the line scan speed and the line density - should depend on the exposure sensitivity of the hologram material, the beam power of the laser 66 and its pulse repetition frequency can be adjusted. Assuming a pulse laser with a pulse frequency of 50 kHz and again of a pixel diameter of 100 microns and a mean beam power of 1 W, then the pulse energy is 2 × 10 ^-5 J. The exposure intensity per pixel 72 with an area of 10 ^-4 cm ² is then 2 × 10 ^-1 J / cm ² = 200 mJ / cm ² .

However, this value exceeds the saturation value of a silver halide hologram material (3 mJ / cm ² ) by a factor of 67. Either the average power of the laser 66 this factor can be reduced to 15 mW or the number of exposures can be reduced with a single pulse with appropriate beam splitting and thus the total exposure time of the hologram.

It should be understood that these numerical examples, which relate to some typical readily realizable parameters of scanners, illumination optics, hologram materials and lasers, serve only to demonstrate the basic practicability of the invention and that these quantities vary widely from case to case can be.

13 shows an exemplary division of the object beam 22 into several exposure beams of different orders in a transmission grating hologram between fiber 62 and exposure optics of the object beam 22 ,

In order to achieve a better distribution of the individual exposures over the entire hologram surface, it is proposed according to a particularly preferred embodiment not only to carry out a single exposure at each time, but possibly to perform a series of exposures simultaneously. To accomplish this, it is proposed in front of the focusing optics of the object beam 22 in the parallel beam path, as in 13 is shown, a line grid 76 or a cross grid 78 to integrate, with a splitting of the beam 22 is performed in several diffraction orders, as in 14 will be shown.

14 shows some examples of the exposure patterns of the object beam 22 after beam splitting in the transmission grating in 13 ,

Becomes a line grid 76 used, then the focusing optics generates in addition to the imaged line 80 the 0th order focus on the beam axis a series of lines 82 the higher orders and at a cross grid 78 a number of points 84 as a square network around the central point 86 the 0th order as the center. In this case, the Bragg grating formula Nλ = 2 g sin φ between the angle φ of the diffraction orders N = 0, 1, 2, 3 .... and g of the lattice constant at the wavelength of the beam λ, wherein at the cross lattice 78 the two lattice constants g _x and g _y must be inserted in the orthogonal directions x and y with the formation of N × M pixels. Here, therefore, the available laser pulse energy can be distributed simultaneously to a row N × M of pixels, which then have an N × M larger area coverage than a pixel 72 can be reached at any time of the scan.

This beam splitting only affects the object beam 22 , The associated reference beam 20 For example, as an expanded planar beam, all N × M pixels may be included simultaneously or with a similar division method in N × M single beams for superposition with the corresponding split object beams 22 be brought to the overlay.

15 shows an example of a stack of holograms of the three colors red (R), green (G) and blue (blue) on a transparent support plate T with an additional absorber layer A.

16 shows an example of stacking the holograms of the RGB colors on a support plate T without absorber layer.

Preferred embodiments provide for applications in the visible range an identical production of eg three master holograms (H1) in each of the primary colors red, green and blue. These three or more holograms are then transferred by the usual replication method of holography by exposure to any number of copies (H2). Each of these RGB H2 holograms, which typically have a thickness of 10-30 μm, may be laminated to a backing plate T or a flexible backing film by known methods. They form together with a possible absorber layer A or without these the end products of the opaque or transparent holographic screens, whose cross-sectional images in the 15 and 16 are shown. The 15 shows a cross section of screens which include an absorber A on the opposite side of the support plate T and the 16 of screens without the absorber layer A, to allow for a look-through and then be free for the corresponding applications in space.

The holographic screens described herein according to embodiments of the invention may be used for projectors 90 can be used either with a parallel or serial screen layout. The first are projectors 90 As was mentioned at the beginning with imaging chips such as LCD and DMD modulators, the images of which are projected on the screen with the aid of light-intensive lamps in the filtered-out colors RGB or individual light-emitting diodes or lasers of the individual RGB colors, as in 17 is pictured. The holgraphischen screens according to the embodiments, but also for laser projectors 92 with a serial image construction with the help of biaxial scanners 94 used as in 18 will be shown.

17 shows an embodiment of a parallel image projection on a holographic screen 96 with representation of the screen structure within the circle of the image resolution of the viewer's eye. Reference A1 denotes the area of the image resolution of the eye on the screen. Reference B1 denotes pixels of the projector 90 on the screen. Reference C1 denotes hologram pixels on the screen. Reference D1 denotes the focal point 24 of the hologram pixel and also the source of the radiation from the pixel of the holographic screen 96 ,

18 shows an embodiment for a serial laser image projection on a holographic screen 96 showing the screen structure within the circle of the image resolution of the viewer's eye and the intersection of the laser beam 98 with the surface of the hologram 26 , In this case, the reference character A2 denotes the range of the image resolution of the eye on the screen. Reference B2 denotes the circle of intersection of the projector beam with the screen. Reference C2 denotes hologram pixels on the screen. Reference character D2 denotes the focal point 24 the hologram pixel, also the source of the radiation from the pixel of the holographic screen 96 ,

It is understood that the RGB wavelengths of the projectors 90 . 92 for optimal image reproduction with the wavelength of the holographic screen 96 integrated RGB holograms should match.

It will now be the reproduction of a homogeneous in their irradiance projection of a primary color of a light source on the holographic screen 96 to be viewed as. Above all, the interest here from the eye of an observer 15 of the screen 96 perceived modulation of the intensity distribution, which is referred to as subjective speckles. For parallel projection, narrow-band lamps and LEDs with a bandwidth of about 20 nm and extremely narrow-band lasers with a bandwidth of 0.01 nm are suitable as light sources. For serial projection, on the other hand, extremely narrowband lasers with continuous wave or pulsed emission are best suited.

In the parallel projection, as in 17 is shown, the area of the resolution of the eye on the screen, which is referred to as the circle A1, comprises the individual image pixels B1, the individual hologram pixels C1 and individual laser sources or foci of the hologram pixels D1. This means that for each frame of the projection the eye simultaneously sees the light waves of a two-dimensional grid of grid points D1 within the circle A1. This two-dimensional grid creates the classic lattice interference on the viewer's retina. First, at the incidence of a plane light wave of the projector on each of the grid points diffraction occurs at a circular aperture having a half angle φ _{1 of} the first zero point with the diameter at the wavelength λ with sin φ ₁ = 1.22 λ / 2ω ₀ . At λ = 0.5 × 10 ^-6 m and 2ω ₀ = 1.45 μm as in the above numerical example, φ ₁ = 22.5 ° ie equal to the previously defined half beam angle of the pixel 2φ ₁ = α _o = 45 °. The second zero is then at φ ₂ = 50 ° and can be neglected, ie it is essentially the 0th-order diffraction.

Since the diffracted waves of several adjacent grid points are superimposed, a second superimposed intensity modulation with a shorter period with the zeros ψ = λ / 2g, where g the lattice constant, ie the pixel spacing in the hologram, and ψ denote the diffraction angle. For g = 100 μm, for example, ψ = 0.25 × 10 ^-2 rad ^ 0.14 ° is obtained as the period spacing of the intensity variations, which corresponds to 10 times the value of the smallest resolution of the eye. This pattern is then repeated at the frame rate of the projector, for example 50 Hz.

However, this intensity pattern can be averaged out if it is changed at least at the same frequency as the frame rate, eg if it is during a lighting period of the projector 90 is moved by the factor M over the resolution limit of the eye along two or two orthogonal directions, eg in (x, y) in the screen plane. Suitable averaging methods are described in the EP 1260850 B1 "Methods and apparatus for eliminating stationary image interference in image projections with temporally or spatially coherent light, as well as system for image projection" described. If the degree of modulation of this intensity variation I as contrast denotes C = (I _{max -I min} ) / (I _max + I _min ) between a minimum and maximum value, this can be reduced by the root of the number M of the resolution elements which are reduced by the movement C'= C / √M.

Every ray of light from the projector 90 the on the hologram 26 falls, is due to the focusing effect of each pixel 72 greatly influenced in its spread before it reaches the eye of the beholder. As a result, there is only a very small induced variation in the propagation of the waves of the projector 90 necessary in order to achieve a sufficient evaluation of the interference in the eye of the beholder, as in 19 is shown schematically.

19 shows a schematic representation of the implementation of the variation Δs a lateral displacement of an incident beam from the projector 90 to a screen pixel in an angle change Δφ and a transverse path change ΔS behind the screen hologram in the auditorium. The focusing effect of a screen pixel 72 is indicated by a dashed lens.

A lateral variation of the incident beam on the screen of Δs = 1 μm causes, for example with the focal length of a hologram pixel of f = 120 μm, an angle change of φ = Δs / f = 1/120 = 0.8 × 10 ^-2 rad = 0.5 °. This is eight times larger than the 3 mm diameter pupil of 3 mm from the screen, which is φ = 1/1000 = 10 ^-3 rad. A 50-fold movement of the intensity pattern above this limiting resolution would thus require a primary movement of the projection beam by 50 μm. If this movement is performed in both the x and y axes, the reduction in contrast factor C'/ C = 1 / √ 50 ² = 1/50 would be achieved. However, this movement on the projection beam requires very little movement of the beam path in the projector, since the image magnification factor of the projector optics is typically 1: 100, ie between the image size on the image size on the screen. Thus, a lateral movement of the beam path in the projector itself of only 0.5 microns would be required to achieve this averaging, which is technically easy to implement.

In a serial projection, the in 18 is shown, the range of eye resolution A2 includes the laser scan spot B2. This illuminates the hologram pixels C2 at the same time with their associated foci D2 at the same time. Here, however, B2 moves very fast within A2, in which case a first evaluation of the pattern takes place over the number of pixel holograms within the resolution range, which is determined by the ratio of the projector beam diameter and the diameter of the resolution range. A further evaluation is achieved via the line frequency by the partial superimposition of the adjacent lines, which is 50 kHz, for example, at a refresh rate of 50 Hz.

Unlike the parallel image projection where the averaging of the laser speckles causes an additional movement in the beam path of the projector 90 makes necessary, in the serial projection an additional averaging is usually not necessary because of the projection beam 98 even in constant motion, both along a single line 74 , as in 18 is indicated as well as successively from line to line. This is a series of pixels within the resolution range of the eye, the speckle modulation of the intensity curve is averaged out in time without additional movement in the projector beam path in the eye. However, the efficiency of this averaging depends on the ratio of the cross section of the scan beam on the screen to the cross section of the resolution range of the eye and the cross section of the holographic screen pixels, as well as the degree of superposition of the adjacent lines. Here also plays a role for the averaging, whether the projection laser 100 CW laser is a continuous wave laser or a pulsed laser, since with pulsed lasers discrete image pixels of the laser and continuous wave lasers produce continuous traces of image pixels.

The speckle formation of the screen proposed here 96 is significantly changed according to the previous explanations to the speckles of a conventional screen or its direct holographic image, ie by his Grid structure limited to only one specific spatial frequency. This periodic diffraction pattern can, in a parallel image projection because of the focusing effect of the image pixels by very small two-dimensional fluctuations in the beam path of the projector 90 be very efficiently averaged. Here are the required variations in the beam path of the projector 90 to the screen so low that they can not affect the image resolution on the screen. In a serial image projection provides the movement of the projection beam 98 within the resolution range of the eye itself, with an appropriate design of the cross-section of the image pixel and the projection beam, for sufficient speckle suppression without further action on the projector would be required.

The method described here and the device described here are characterized, inter alia, by the fact that primarily no image structure is embossed into a hologram, but that a varying optical function, i. a holographic optical element is imprinted that processes incident light differently depending on the angle of incidence and color and where each incident beam is redirected from one projector to another. The hologram thus carries no image structure and changes the incident light on the hologram only its direction of radiation and expansion.

For this purpose, no intensity modulation or image modulation is impressed on the object beam, but only the angle of incidence of the object beam 22 from pixel 72 to pixels 72 changed.

Preferably, the light is basically over at least two glass fibers 60 . 62 to every pixel 72 as a reference beam 20 or object beam 22 guided. This allows the free mobility of both write heads 36 . 34 about the different locations (x, y) with, for example, a scanning movement and at the same time a constantly varying angle setting of the writing heads 36 . 34 eg in a goniometer 47 . 50 ,

A particularly advantageous property of the screen produced by the method described here 96 is its fundamental speckle-poverty in laser illumination and the unique possibility of the periodic residual speckles due to the focusing function of each individual pixel 72 very efficiently with some conventional speckle suppression techniques completely eliminate.

LIST OF REFERENCE NUMBERS

10

 projector light

12

 canvas

14

 holographic canvas

15

 spectator

20

 Reference beam (first beam)

22

 Object beam (second beam)

24

 focus point

26

 hologram

27

 Concave mirror function

28

 Axis (reference beam)

30

 Axis (object beam)

31

 Lens system (reference beam)

32

 Lens system (object beam)

34

 Write head (object beam)

36

 Write head (reference beam)

38

 Ray (to reconstruct the hologram)

40

 projection beam

42

 ray beam

44

 pan direction

46

 pan direction

47

 Goniometer (reference beam)

48

 exposure device

49

 exposure means

50

 Goniometer (reference beam)

51

 Pusher

52

 line scan

54

 skips

56

 Surface of the hologram

58

 Cardanic turning device

59

 corrector

60

 Glass fiber (reference beam)

62

 Fiberglass (object beam)

64

 beamsplitter

66

 laser

68

 laser beam

70

 splitter prism

72

 pixel

74

 row

76

 line grid

78

 cross grating

80

 Line 0th order

82

 Lines of higher order

84

 Points

86

 central point 0th order

90

 Projector for parallel image construction

92

 Projector for serial image construction

94

 scanner

96

 holographic screen

98

 Laser beam (projector)

100

 Laser (projector)

A

 absorber layer

A1

 Image resolution area of the eye on the screen

B1

 Pixel of the projector on the screen

C1

 Hologram pixel on the screen

D1

 Focus point of the hologram pixel

A2

 Image resolution area of the eye on the screen

B2

 Pixel of the projector on the screen

C2

 Hologram pixel on the screen

D2

 Focus point of the hologram pixel

uP

 microprocessor

PC

 computer

O

 Separation point of the rays

P

 origin

S

 intersection

T

 support plate

R

 red

G

 green

B

 blue

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 19700162 B4 [0014, 0015, 0019, 0023, 0026, 0085]
• DE 19934162 B4 [0020, 0021, 0021, 0022, 0023, 0026, 0085, 0105, 0122]
• EP 1260850 B1 [0149]

Cited non-patent literature

• W. Freese et al. in the article: "Design of binary subwavelength multiphase level computer generated holograms" in Optics Letters, Vol. 35, pages 676-678 (2010) [0028]

Claims (15)

Method for producing screen holograms for front projection with a covering of the screen ( 14 . 96 ) by individual holographic image pixels ( 72 ), where the screen pixels ( 72 ) by superposition of at least one first illumination beam ( 20 ) and a second illumination beam ( 22 ) and different angular orientations of at least one of the illumination beams ( 22 ) are provided with different Austrahlrichtungen. Method according to Claim 1, characterized in that the screen pixels ( 72 ), preferably by optical lens arrangements ( 30 . 31 ), illuminated from both sides of a hologram film, the hologram film from the one side with the first illumination beam ( 20 ) and from the other side with the second illumination beam ( 22 ) is illuminated. Method according to one of the preceding claims, characterized in that the screen pixels ( 72 ) each time in an equal pixel volume of a hologram film by means of a pivoting device ( 47 . 50 . 58 ) and a sliding device ( 51 ) and a common scanning movement of the two beams ( 20 . 22 ) over the area ( 56 ) of the screen hologram. Method according to one of the preceding claims, characterized in that the screen pixels ( 72 ) in temporal succession by a common lateral and vertical grid-shaped scanning movement of the first and the second illumination beam (US Pat. 20 . 22 ) are produced relative to a hologram film. Method according to one of the preceding claims, characterized in that the first illumination beam is a reference beam ( 20 ), which is aligned, preferably as a plane wave, with an axis alignment always to a fixed point (P) in space and / or that the second illumination beam is an object beam ( 22 ), which, preferably with a part of a spherical wave, in a hologram film has a common cutting volume (S) with the reference beam ( 20 ), and whose axis ( 30 ) and / or its opening angle for producing the different pixels ( 72 ) can be adjusted differently in order to determine the different angles of emission per pixel ( 72 ). Method according to claim 5, characterized in that the axis ( 30 ) of the object beam ( 22 ) with a swivel ( 50 . 58 ) and / or sliding device ( 51 ) is directed to the common cutting volume (S) and / or that the beam axis ( 30 ) and the opening angle of the object beam ( 22 ) to illuminate a desired angular range of an auditorium of the screen hologram. Method according to one of the preceding claims, characterized in that the first and the second illumination beam ( 20 . 22 ) by a common laser ( 66 ) be generated. Method according to one of the preceding claims, characterized in that the first and the second illumination beam ( 20 . 22 ) to their associated relative to a holgrammfilm movable lens arrays ( 31 . 32 ) by optical fibers, preferably by monomode glass fibers ( 60 . 62 ). Method according to one of the preceding claims, characterized in that the second illumination beam ( 22 ) directly to the exposure of a single image pixel ( 72 ) is used or split into multiple modes to produce multiple image pixels ( 72 ) at the same time. Contraption ( 48 ) for the production of screen holograms for a front projection by forming the screen holograms from individual holographic image pixels ( 72 ), with: a light source ( 66 ) for a holographic recording of an interference of a first illumination beam ( 20 ) and a second illumination beam ( 22 ), a scanning device ( 51 . 52 . 54 ) for guiding the first illumination beam ( 20 ) and the second illumination beam ( 22 ) movable relative to a hologram film, the scanning device ( 51 . 52 . 54 ) is designed and arranged such that the axis ( 28 ) of the first illumination beam ( 20 ) is aligned relative to the hologram film at each pixel exposure to a fixed point (P) in space and that the axis ( 30 ) and / or an opening angle of a focusing second illumination beam ( 22 ) are directed variably relative to the hologram film in an angular range of an auditorium. Apparatus according to claim 10, characterized in that a pivoting device ( 50 . 58 ) for different adjustment of the angular orientation of the second illumination beam ( 22 ) per pixel, Apparatus according to claim 10 or 11, characterized in that the light source is a pulsed laser ( 66 ) whose pulse length for the exposure of individual holographic image pixels ( 72 ) is trained. Device according to one of claims 10 to 12, characterized in that a beam guiding device ( 60 . 62 ) for guiding the first illumination beam ( 20 ) and the second illumination beam ( 22 ) and is designed such that the light path difference of the first ( 20 ) and the second illumination beam ( 22 ) from a common laser ( 66 ) as the light source up to the hologram film over the entire scan area is shorter than the coherence length of the laser ( 66 ). Device according to one of claims 10 to 13, characterized in that a splitting device, preferably with a transmission grating ( 76 . 78 ), for splitting the second illumination beam ( 20 ) into a plurality of plural modes for simultaneously exposing a plurality of pixels ( 72 ) is provided. Use of a screen hologram, which can be produced by a method and / or a device according to one of the preceding claims, for front projection, wherein in the case of a parallel image projection, the beam path of a projector ( 90 ) from the eye of the spectator ( 15 ) is imperceptibly moved to a speckle pattern of the screen pixels ( 72 ) in space, or in a serial image projection the diameter of the screen pixels and the projection beam ( 98 ) is selected in relation to the diameter of a resolution element that can still be resolved by the eye of a viewer on the screen in such a way that an evaluation of a speckle pattern by the movement of the projection beam ( 98 ) for serial image projection.

Images