EP1776613A1

EP1776613A1 - Image wall having a reduced speckle effect

Info

Publication number: EP1776613A1
Application number: EP05781451A
Authority: EP
Inventors: Jan GÄBLER; Christoph Rickers; Frank Neumann
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2004-08-10
Filing date: 2005-08-08
Publication date: 2007-04-25
Also published as: DE102004038969A1; WO2006018170A1

Abstract

The invention relates to an image wall for projection using narrow-band light sources, especially laser projection. Said image wall comprises a polycrystalline layer that is provided with a surface structure which allows speckle effects to be reduced. Also disclosed are image walls representing a molding of a polycrystalline layer, said molding reproducing the surface structure of the polycrystalline layer.

Description

Screen with reduced speckle effect

The present invention relates to a screen with reduced speckle effect for projection methods with narrow-band light sources, in particular for laser light.

Among the visualization methods, for example with cathode ray tubes (CRT), liquid crystal display (LCD), plasma screen and projection devices, the projection methods are of particular importance. They allow a large-scale display of images with high resolution.

Projectors with narrow-band light sources are used for the projection methods. One example is lasers, which can be used to produce monochromatic light.

Similar to color television, laser light sources of the three primary valences red, green and blue are used for this purpose.

Laser projection systems, such as scanning laser projection methods, offer the possibility to project sharp images on almost arbitrarily shaped bodies due to unlimited depth of field. Compared to conventional methods, they have outstanding bright-dark contrast values of 1: 50,000 to 1: 100,000 compared to 1: 5,000 in conventional devices. In addition, a much larger color space makes it possible to realize more brilliant images. Due to the large color space and the high contrasts in darkened rooms, 3D laser effects can be used to achieve subjective 3D effects.

A general problem with the projection of images is the poor contrast. This is caused by the fact that not only the projection light but also the ambient light is reflected on the screen. By using spectrally narrow-band light sources, the contrast can be increased considerably by using image walls which have a spectrally selective reflective coating. Due to the spectrally selectively reflecting coating, essentially only light of the projector is reflected and the ambient light is absorbed or transmitted. Such spectrally selective image walls are for example Game in German Patent DE 199 01 970 C2 and the German Patent Application DE 19747 597 A1.

For the image formation, scattering surfaces are required, that is to say wallboards with a rough surface structure. Due to the rough Oberflächen¬ structure, the light is reflected in different spatial directions and can be seen from different observation points.

If laser light is reflected on scattering surfaces, so-called speckle effects can occur. The cause of speckle effects is a reflection of the light beam at two points of the scattering surface whose lateral distance is on the one hand smaller than the resolution of the eye of a viewer, but on the other hand the points are at different distances away from the observer. This case can occur, for example, in scattering surfaces with an irregular surface, in which two closely adjacent points at which the light beam is scattered have a different height. The Wellen¬ trains reflected at these points have due to the different distance from the viewer on low transit time differences. However, the wave trains are imaged on the retina only at one point and because of the differences in transit time there are interference effects. Subjectively, the effect is to be seen as a statistical distress or a graininess in the image and should therefore be avoided.

In order to avoid the speckle effect, numerous solutions have been proposed, which in principle can be subdivided into the following categories:

Approaches to avoiding interference as well

Approaches based on an averaging of the speckle effect, also called "Ver¬ lubrication".

To implement these approaches, measures can be taken on the projector, the screen or both.

DE 101 18 662 A1 describes a screen in which the speckle effect is reduced by avoiding the interference. To resolve the interferences Renz an image wall material is proposed, which causes a volume scattering of the light. Here, the illuminated object is coated with a volume-scattering coating, for example polytetrafluoroethylene. Nach¬ part of this method is the need to adjust the coherence length of the laser light used to the thickness of the volume-scattering coating, so that there is a limitation on the usable projectors. Also, it is generally difficult to combine a volume scattering coating with the desired spectrally selective reflection.

A method for smearing the speckle effect is known from US 5,272,473 be¬. There it is proposed to arrange a sound source on a screen. The acoustic waves generated by the sound source pass through the screen and stimulate them to vibrate. Due to the oscillating Bild¬ wall different speckle patterns are generated depending on the vibration state of the reflected Laserlichtstrah- len, which averaged during the integration time of the detector (eye), that smeared, so that reduces the speckle contrast. Depending on the sound frequency and screen dimension, however, bellies and nodes can occur in which there is no smearing and thus the speckle effect fully reappears.

Alternatively, JP 2000-081602 A describes a screen with Flüssigkristall¬ materials, wherein the liquid crystals are caused by applying a high-frequency low-voltage signal in vibration. The vibrating liquid crystals, in turn, cause rapidly varying speckle patterns and thus averaging (blurring) of the contrast. Also, this type of image walls can only be poorly combined with the desired contrast-enhancing spectrally selective reflective coatings and is also limited in terms of realizable dimensions.

DE 100 26 973 A1 describes a screen for the laser projection technique with improved throwing power, so that good images can be achieved. For this purpose, a screen is selected, which consists of a glass ceramic or an ent¬ mixed glass. By special temperature-time treatment, the molded body produced from the melt is partially crystallized, so that the - A -

holding screen receives next to an amorphous glass content of a polycrystalline portion.

The polycrystalline portion in the screen causes an increase in the scattering of the incident light. However, the increase in the amount of grit should not be so high that a good view through the glasses is prevented. According to DE 100 26 973 A1, the crystal formation takes place as well as the scattering within the layer material. There is no suggestion to provide the surface of the screen with the structure of a polycrystalline layer, that is to say to provide an irregular surface on which the incident light is then reflected.

On the contrary, it is proposed to reduce the surface to reflect to avoid reflection with extraneous light waves. Reflection, however, occurs only on smooth surfaces, because irregular surfaces reflect the light not uniformly as in a mirror, but in many directions.

An object of the present invention is to provide a screen, in particular for laser projection, with a reduced speckle effect, which does not have the aforementioned disadvantages of the prior art. In particular, it is an object of the present invention to provide a screen with reduced speckle effect, which can be readily combined with contrast-enhancing coatings and is not subject to any restrictions with regard to the narrow-band light source used, such as the laser source.

This object is achieved by a screen whose surface has the structure of a polycrystalline layer, with smooth crystal facets, wherein the crystal facets form a structured surface topography by different spatial orientation.

The invention thus encompasses image walls with polycrystalline layers as well as image walls with a surface structure which corresponds to the surface structure of polycrystalline layers. These are, for example, impressions (replicas) of polycrystalline surfaces. In the invention, the crystal facets each act as micromirrors. The concept of the mirror is already showing that it is a reflection on the surface and not inside the screen. Also therein lies a difference to DE 100 26 973 A1.

For the purposes of the invention, the term "screen" includes projection surfaces of all kinds.

The present invention is based on the basic idea of avoiding reflection of the incident light at closely adjacent points which are imaged on a point of the retina, and which due to their spatial arrangement lead to path differences of the reflected light, so that it is on the retina to interference effects and thus to the generation of the speckle effect.

According to the invention, therefore, a property of polycrystalline layers is exploited for the image wall to form smooth crystal facets. Due to the smooth surface of the facets, light incident on a facet is reflected evenly, with no path differences.

A second requirement for image walls is that the light is remitted into different solid angles, so that the image is visible to the viewer from different points of view. This requirement is taken into account according to the invention in that the facets of the crystal tips form a structured surface topography in that they are oriented in different spatial directions and have different angles of inclination to the vertical with respect to the screen.

For the purposes of the present invention, "alignment in different spatial directions" and "different angles of inclination" means that the surface topography is such that the cross section or cross section of the incident light beam comprises an environment of different structure, so that the light beam is reflected in different directions becomes. The arrangement and / or size of the facets is selected according to the invention so that within the cross-section of the incident light beam, a reflection mission takes place essentially in all solid angles. Cross section in the sense of the invention is the area per pixel, which is swept by the incident light beam.

Thus, the facet surfaces can be larger than the beam cross section of the incident light. In this case, their spatial arrangement within the effective cross-section should allow a remission into the desired solid angles. Preferably, the beam cross section of the incident light should be at least as large or larger than the facet surface.

For example, conventional laser light has a beam cross-section, which generally covers several facets of a polycrystalline layer, so that it is remitted in various directions, as is necessary for the realization of an image wall. However, due to the smoothness of the facets, it is also avoided at the same time that points of the surface below the resolution of the eye which could produce a path difference are returned in the same direction.

According to the invention, the speckle effect is thus reduced by:

1. the smoothness of the crystal facets of the polycrystalline layer, so that the ein¬ radiating light is reflected without optical delay and thus no interference and no speckle can occur, and

2. By the size and / or arrangement of the facets within the effective cross section of the incident light beam, the light in different

Directions is reflected and thus viewable from different points of view.

In principle, all materials which can be deposited as polycrystalline layers can be used as material for the polycrystalline layer of the screen according to the invention.

Examples of materials for polycrystalline layers are diamond, silicon, indium, indium arsenide, gallium arsenide, cadmium selenide, perylene-tetracarboxylic dianhydride, zinc oxide, aluminum oxide, gallium gadolinium garnet (Ga ₃ Gd ₅ O ₁₂ , GGG), yttrium gadolinium garnet (Y ₃ Gd ₅ O ₁₂ , YGG) and yttrium aluminum garnet (Y ₃ Al ₅ O ₁₂ , YAG).

The polycrystalline layers of the invention can be obtained by means of commonly known conventional deposition methods, such as chemical vapor deposition and physical vapor deposition.

A preferred example of the chemical vapor deposition is the akti¬ fourth chemical vapor deposition, the activation can be done for example with plasma or with hot wires. Suitable methods for the activated chemical vapor deposition are known per se. Suitable processes are described, inter alia, in DE 196 29 456 C1, DE 198 50 346 and DE 195 30 161 C2, to which reference is made in full here.

Further, in this context, Wild, C; Koidl, P., Müller-Sebert, W .; Walcher, H .; Kohl, H .; Herres, N .; Locher, R .: "Chemical vapor deposition and characterization of smooth {100} -faceted diamond films" in: Diamond and Related Materials 2 (1993), pp. 158-168 and Wild, C; Herres, N .; Koidl, P .: "Texture formation in polycrystalline diamond films" in: Journal of Applied Physics 68 (1990) 3, August 1, p. 973-978.

The activated chemical vapor deposition is particularly suitable for the production of polycrystalline diamond layers.

Suitable examples of the physical vapor deposition are sputtering and vapor deposition.

The surface topography can be adjusted by simple variation of the process parameters. Examples are the gas phase composition or temperature. Further measures may be the variation of the substrate temperature by cooling or heating tables.

Variation of the substrate temperature, for example, supports the formation of oriented polycrystalline diamond films. The characterization of the facet geometry such as smoothness, size and solid angle can be done with contactless optical methods. Thus, a confocal microscope or other methods using, for example, a scanning electron microscope (SEM) or an atomic force microscope (AFM) can be used for the measurement.

The deposition of polycrystalline layers usually takes place on a substrate. For the screen according to the invention, the layer including Sub¬ strat can be used. Examples of suitable substrates are silicon, ceramics, as used inter alia for diamond layers, hard metals, niobium, titanium, steel or graphite.

According to one embodiment, the layer may be detached from the substrate and only the layer used for the screen.

The screen according to the invention with reduced speckle effect can in principle be used for all projection methods in which speckle effects can occur.

In particular, it is intended for use in processes with narrow-band light sources. These are usually laser light sources today. It is understood, however, that the present invention is not limited to laser light sources, but also includes light sources having comparable properties.

Hereinafter, the present invention will be explained in detail with reference to the attached figures and preferred embodiments.

Show it:

Figure 1 a), b) and c) schematically shows the growth of a polycrystalline layer after van der Drift;

FIG. 2 an oblique view of a polycrystalline diamond layer in an oblique view;

Figure 3 a), b) and c) polycrystalline diamond layers with different

crystallite;

Figure 4a) and b) oriented polycrystalline diamond layers; and

Figure 5a) and b) a replica of polyacrylate in different magnifications.

The growth of polycrystalline layers as used according to the invention is shown schematically in FIGS. 1a) to c). The desired polycrystalline layers are formed by - as shown in Figure 1a) - ein¬ individual separate crystallites, which are applied for example by a previous seeding on the desired substrate, initially grow unhindered in all directions of the free half-space. The directions are indicated by arrows. An enlargement can be found in the circle.

The growth in all free directions continues until the crystallites collide laterally. Growth can then take place only from the substrate surface, as indicated in Figure 1b) by arrows. As a result, a polycrystalline "rod-shaped" structure is obtained, as shown in FIG. 1 c).

At the top of the layer, the crystallites can grow freely and assume the crystal shape characteristic of the respective material. Thus, FIG. 2 shows an SEM image of a polycrystalline diamond layer in an oblique view, which reveals the pyramidal or tetrahedral shape typical of diamond layers.

The growth rate of the crystallites is generally higher, the lower the inclination of the direction of growth of a crystallite to the perpendicular to the substrate surface, d. H. the more parallel the course of the growth direction is to the perpendicular to the surface. Crystallites with a higher inclination to the vertical, that is, crystallites with more tilt, grow slower. Due to the slower growth, they are overgrown by the crystallites, which have a lower inclination to the vertical and thus faster growth. The growth of the more highly tilted crystallites comes to a standstill and the faster growing crystallites prevail. This situation is shown in FIG. 1 c), in which schematically overgrown crystallites are discernible.

By overgrowing slower growing crystallites, the size of the crystal tips of the intersecting crystallites increases with the layer thickness. In this way, therefore, the crystallite size can be set arbitrarily. It can be less than 100 nm but also several 100 microns and more.

Thus, FIG. 3 a) shows a polycrystalline diamond layer with a small crystallite size of approximately 1 μm in plan view, FIG. 3 b) shows a polycrystalline diamond layer of average crystallite size with approximately 6 μm in plan view, with the line shown at the bottom right each 5 microns. A cross-section through a polycrystalline diamond layer with very large crystallite sizes of about 150 μm, on the other hand, is shown in FIG. 3 c), the darkly shaded line at the top left in the figure representing 100 μm.

As already mentioned above, the shape of the crystal tips can be adjusted by varying the coating parameters. Thus, in Figures 1 to 3 statistically oriented layers are shown. Likewise, oriented polycrystalline layers can be used. The layers can have uniform topographies. These may be formed, for example, from pyramidal or plateau-shaped tips, as shown in Figures 4 a) and b). FIGS. 4 a) and b) show exemplary oriented polycrystalline diamond layers which have a pyramidal topography (FIG. 4 a) or a topography with plateaus (FIG.

According to a further aspect, the present invention comprises image walls whose surface topography corresponds to the surface topography of the above-mentioned polycrystalline layers according to the invention. These can be obtained by molding, also called replication, of the polycrystalline layers according to the invention. The polycrystalline layer acts as a master, from which an impression is made.

The resulting replica can be used even for the screen, which then represents a negative image of the master.

In accordance with another embodiment, replication may be further replicated. The replicate obtained here then shows the original positive surface structure of the polycrystalline layer used as the master.

Like the polycrystalline layer, the replicate may be used as such or together with a substrate for the screen.

Generally, for replication, the structure of the master surface is transferred to a polymeric material. Suitable techniques are a) pressing thermoplastic polymer such as polypropylene and polyamide onto the master surface, b) transferring the master structure to a cold polymer by means of embossing or rolling, c) filling the master surface with a curable monomer oligomer formulation which hardens or crosslinked while in contact with the master, and d) injection molding, wherein, for example, the master is introduced into a suitable mold, and the polymer is optionally injected under pressure. For the replication, a material is applied to the layer that takes over the topography in a suitable replication quality. Examples are metals, polymers or other materials such as glass.

Thus, metals can be deposited by means of galvanic processes.

As materials for replication, polymers such as thermoplastic or thermosetting polymers can be used. The curing can take place by means of conventional methods, for example by irradiation, such as UV light, irradiation with electrons (EB) or another suitable source, or by using a liquid hardener component.

Preferably, the replication is carried out according to method c). Process c) is a three-stage process in which the master is filled with a polymer formulation of sufficiently low viscosity, the polymer is cured and the impression is removed from the master.

Examples of particularly suitable polymers are acrylates and epoxides, in particular acrylates of low viscosity, which can be cured by means of UV / EB.

FIGS. 5a and b (an illustration of the Leibnitz Institute for Surface Modification eV (IOM)) show an impression of polyacrylate obtained according to method c), wherein FIG. 5b shows an enlargement of FIG. 5a, and FIG. 5a the line shown at the bottom right corresponds to 10 μm and in FIG. 5b to 3 μm. These figures clearly demonstrate the extraordinary imaging accuracy that can be obtained with the replication method described above. In particular, with the latter method c) replicas of microstructures can be obtained in high quality, which have a geometric Ge accuracy within the order of magnitude of 10 nm.

For replication, the polycrystalline layer used as the master can be subjected to suitable pretreatment processes. Thus, the non-stick properties can be improved with a non-stick coating or by electrochemical treatment. Also, the polycrystalline layers can be deposited with a doping electrically conductive, so that electrochemical at the polycrystalline layer Processes can be performed. If, for example, diamond is used as the layer material, doping with boron can be carried out in order to make the layer electrically conductive.

Both embodiments of the screen according to the invention, the polycrystalline layer and the impression (replication) of the polycrystalline layer can be additionally provided, as required, with further layer systems known for image walls. For example, they can be equipped with optically effective layer systems. An example of optically effective layer systems are layer systems which act as filters. For example, layer systems may be used that preferentially transmit the wavelengths of the deployed projector, such as the three wavelengths of a laser projector used, while fully absorbing the other wavelengths. As mentioned above, the contrast can be increased with such layers.

It is also possible to use reflection layers with which transparent layers or transparently molded image walls can be made reflective.

Claims

claims

1. Screen with reduced speckle effect for a projection method, wherein the surface of the screen has the structure of a polycrystalline layer, with smooth crystal facets, wherein the facets form a structured surface by unterschiedli¬ ge spatial orientation.

The screen according to claim 1, wherein the screen is a polycrystalline layer.

3. Screen according to claim 1, wherein the screen is a positive or negative impression of a polykristalli¬ NEN layer according to claim 2.

4. Screen according to one of the preceding claims, wherein the polycrystalline layer or the impression auf¬ brought on a substrate or cantilever is formed.

A display panel according to any one of the preceding claims, wherein a polycrystalline layer selected from a randomly oriented layer and an oriented layer is used.

6. Screen according to one of the preceding claims, wherein the polycrystalline layer has been obtained from a Schichtmate¬ material selected from diamond, silicon, indium, indium arsenide, Galliumarse- nid, cadmium selenide, perylene-tetracarboxylic dianhydride, zinc oxide, Alumini¬ oxide , Gallium gadolinium garnet, yttrium gadolinium garnet, and yttrium aluminum garnet.

The screen according to claim 6, wherein the layer material is a doped material.

The screen according to claim 3, wherein the material for the impression is a material which takes over the topography of the polycrystalline layer.

A display panel according to claim 8, wherein the material is selected from a metal, glass or polymer.

The display panel of any of claims 8 or 9, wherein the material is a polymer selected from thermoplastic and thermosetting polymers.

11. A display panel according to any one of claims 8 to 10, wherein the material is a polymer selected from a curing monomer / oligomer formulation cured with ultraviolet or electron beam.

12. The screen according to claim 11, wherein the polymer is selected from acrylates and epoxides.

The screen according to any one of the preceding claims, wherein the polycrystalline layer is a diamond layer.

14. Screen according to one of the preceding claims, wherein the screen additionally contains one or more further layer systems selected from optically active layer systems and reflection layers.

15. A method for producing a screen according to any one of claims 1 to 14, wherein the deposition of the polycrystalline layer by means of chemical vapor deposition, activated chemical vapor deposition, physical vapor deposition or galvanic process takes place.

16. The method of claim 15, wherein the activation in the activated chemical vapor deposition takes place with plasma or with hot wires.

17. A method for producing a screen according to claim 15 or 16, wherein as a layer material selected from diamond, silicon, indium, indium arsenide, gallium arsenide, cadmium selenide, perylene-tetracarboxylic dianhydride, zinc oxide, aluminum oxide, gallium-gadolinium garnet,

Yttrium gadolinium garnet, and yttrium aluminum garnet is used.

18. A method for producing a screen according to claim 17, wherein a doped material is used as the layer material.

19. The method according to claim 17 or 18, wherein diamond is used as layer material.

20. A method for producing a screen according to any one of claims 3 to 14, wherein as material for the replication, a material is used, the

Topography of the polycrystalline layer takes over.

21. The method of claim 20, wherein the material is selected from a metal, a glass or a polymer.

22. A method for producing a screen according to claim 21, wherein a polycrystalline layer is replicated by 1) filled with a low viscous curing polymer formulation, the cured 2) Polymerformulie¬ tion and 3) the resulting replica is released from the template.

23. The method according to claim 22, wherein the curable polymer formulation used is an acrylate or epoxide which is cured by ultraviolet irradiation or electron beam irradiation.

24. The method of claim 23, wherein as the curable polymer, an acrylate is used.

25. The method according to any one of claims 15 to 24, wherein the screen is combined in addition with one or more layer systems combined under optically effective layer systems and reflection layers.

26. Use of a screen according to one of claims 1 to 14 for a laser projection method.