WO2004031852A1

WO2004031852A1 - Contrast-increasing projection screen

Info

Publication number: WO2004031852A1
Application number: PCT/EP2003/010855
Authority: WO
Inventors: Christoph Rickers; Michael Vergöhl
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2002-09-30
Filing date: 2003-09-30
Publication date: 2004-04-15
Also published as: DE10245881A1; JP2006501504A; KR100982319B1; EP1546802A1; KR20050083706A; AU2003280347A1

Abstract

The invention relates to a projection screen for representing static or animated images with the aid of one or several monochromatic laser light sources. Said projection screen is provided with a coating that reflects in a spectrally selective manner and a structured lacquer layer made of a hardened lacquer or a structured film made from a holographic master as a layer reflecting in a spatially selective manner in order to increase spatial selectivity.

Description

Contrast-increasing screen

The present invention relates to a contrast-increasing screen for the display of static or moving images by means of projection through a narrow-band light source, such as one or more monochromatic light sources.

In order to be able to view projected images as unaffected by stray light as daylight or artificial room lighting, the reflectivity of the screen should be low for the entire wavelength range of visible light, except for the wavelengths that correspond to the radiation from the light source or light sources. For large-scale incident light projection, in particular of colored images using laser light sources or other more or less narrow-band light sources for several primary variances (such as red, green, blue [RGB] such as LCD projection or CRT projection), projection screens are therefore desirable which have a strongly wavelength-selective reflection behavior have: That is, the reflectivity of the screen for the wavelengths that correspond to the primary valences used for the projection should be as high as possible and as low as possible for the other wavelengths, for example from ambient light.

In the sense of the invention, reflection is understood to mean the total light intensity scattered or reflected back from the screen, based on the incident intensity.

In addition, the reflection of the screen should have a selectable spatial angle characteristic, the reflection taking place in a defined radiation angle range, so that little or no light is emitted into / from those spatial angle regions where there is no observer. Ideally, the reflection should take place in an angular range of +/- 40 °, measuring from the normal to the screen horizontally to the left or to the right. Due to the spatial angle characteristic, a specular reflection is avoided and instead a diffuse reflection is caused. In addition, the contrast is further increased compared to a Lambert radiator. In order to be able to perceive the static or moving images on a screen clearly and undisturbed by daylight or other ambient light or interference light, the screen should have spectrally and spatially selective reflectivity.

Various proposals have already been made to improve not only the spectral selectivity but also the spatial selectivity of the reflection from picture walls.

DE 197 47 597 describes a projection screen in which the spectral selectivity for monochromatic light such as laser light is brought about by a multilayer system which is composed of layers of dielectric materials which are alternately high and low refractive index. This multilayer system, which acts as an interference filter, increases the reflectivity for the wavelengths of the monochromatic laser light and reduces it outside the wavelength range of the projection light.

To adjust the angular characteristics, pigments are provided on the screen in a separate lacquer layer.

DE 199 01 970 describes a spectrally selectively reflecting screen for the projection with narrow-band, in particular monochromatic, light, such as is generated by lasers, for example, in which the spectral selectivity is brought about by a coating of cholesteric polymers. To set the spatial angle characteristic, the surface of the substrate to which the coating is applied is structured here.

However, it has been shown that when using pigments to adjust the angular characteristic, the desired diffuse reflectivity is obtained, but on the other hand, the particles cause high scatter. This high scattering means that light, which must first pass through this scattering layer before it can advance to the spectrally selective coating, is already spectrally unselectively emitted back in the scattering layer, so that as a result, the spectral selectivity of the screen is reduced or even destroyed.

If the angle characteristic is set by structuring the surface of the substrate, the spectrally selectively reflecting coating can in principle be applied to the structured surface itself, the structure being transferred to the coating. Alternatively, the spectrally selectively reflective coating can first be applied to the smooth substrate, and then the structured layer can be applied. In combination with interference filters e.g. The problem arises, for example, that in the case of a coating on a structured substrate, the spectral selectivity suffers, since the peak positions of the wavelengths in the spectrum change and the background reflects more, as discussed in more detail below in connection with FIG. 2. In the reverse order, that is to say in the event that the spectrally selectively reflecting coating is first applied to the smooth substrate, a scattering surface must be applied to the layer system. This is usually done by mechanical processing of the filter layer, whereby it is exposed to stresses and can be destroyed.

There was therefore a need for a possibility of being able to adjust the spatial angle characteristic of spectrally selectively reflecting picture walls without the problems mentioned above occurring. In addition, it should be possible to carry out the adjustment in a simple manner and, overall, enable the spatially selective reflectivity of picture walls to be improved.

The object according to the invention is achieved by a contrast-increasing screen which contains a substrate and a spectrally selectively reflecting coating, the screen having at least one spatially reflecting layer, the spatially reflecting layer being selected from a layer which is formed from a curable lacquer, and a holographically structured film. The invention further relates to the use of curable lacquers and holographically structured foils for the formation of such spatially reflecting layers.

Any known spectrally selectively reflecting coating can be used for the screen according to the invention, as described, for example, in the aforementioned German patent application DE 197 47 597 A1, in the international application WO 98/36320 and in the German patent DE 199 01 970 C2 which are expressly referred to in this context and which are fully incorporated into the present application.

According to the invention, the spatially selective reflection is brought about by a layer with a structured surface, which consists of a curable lacquer, or is a holographically structured film in which the surface and / or its volume has been structured and / or modified using known holographic methods is.

Holographically structured foils are known per se and are e.g. already used by POC in combination with a spectrally unselective filter as a contrast-increasing screen (LORS). However, the contrast increase achieved here is insufficient because it only results from the exclusive remission of light from a defined solid angle range.

If the lacquer is used, the structuring results in the course of the hardening of such lacquer layers by polymerization and crosslinking of the starting materials used, which leads to shrinkage processes which lead to the microfolding of the surface. This micro-folding results in the desired structuring on the surface of the lacquer layer.

Suitable methods and materials for structured lacquer layers that can be used according to the invention are described in principle in German patent application DE 198 42 510 A1. In general, there is a manufacturing process here of decorative and functional surfaces on rigid or flexible substrates, but without referring to specific applications. The methods and materials described there can in principle also be used for the production of the structured lacquer layer used according to the invention.

^"The lacquers that can be used for the present invention are preferably electron-beam or UV-curing coloring and lacquer layers. In principle, these coloring or lacquer layers are obtained by starting mixtures of polymerizable and crosslinkable mono- and oligomers with or without The photoinitiator is applied in a conventional manner to a suitable base and hardened by means of suitable radiation, the extent of the microfolding and thus the appearance of the structuring vary depending on the monomer / oligomer system used, layer thickness, UV wavelength, Art of the substrate and coating technology, so that the structuring can be specifically adjusted as required by simply varying the parameters mentioned.

Suitable paints or paint systems are described in detail in DE 198 42 510 A1.

Examples of suitable starting materials are acrylates, epoxies, vinyl ethers, unsubstituted and substituted styrenes and mixtures thereof. These can be in suitable solvents. The acrylates preferably have a functionality of 2 or more.

These materials are usually cured by irradiation with monochromatic UV light with a wavelength suitable for the respective system. Monochromatic UV light of a wavelength is preferably used for the curing, which is still able to directly form polymer radicals for the polymerization and crosslinking, is in the UV absorption range of the coating component and enables curing with an acceptable photon dose. In principle, all wavelengths that are able to effect curing in the irradiated zone of the lacquer layer can be used to generate the micro-folding are used, provided they correspond to the absorption spectra of the paint components.

Examples of commercially available emitters which are suitable for the present invention are an excimer UV laser which emits monochromatic UV light at 172 nm and 222 nm, as is available, for example, from Heraeus Noblelight. An argon excimer laser with a wavelength of 126 nm is also suitable.

Since air absorbs short-wave UV radiation with ozone formation, the radiation for curing and folding the lacquer layer should preferably take place in an inert atmosphere.

It has been shown that as the wavelength decreases, the depth of penetration of the photons into the lacquer layer is smaller and a finer structuring is obtained. Thus, a fine, invisible microfolding is generally formed with wavelengths of less than 200 nm and larger structures that are visible with longer wavelengths of 200 nm and more. It has also been shown that with longer-wave radiation (222 nm) the peak-valley ratio of surface roughness and ripple increases more with increasing thickness of the lacquer layer than with short-wave radiation (172 nm).

According to a preferred embodiment, urethane diacrylate is chosen as the lacquer component, preferably together with a flexible reactive thinner. With such flexible systems, fine matt gloss structures can be obtained. A radiator as mentioned above can be used for curing.

For the production of the structured lacquer layer, the starting components can be present in pure form, diluted with organic solvents or as an aqueous dispersion. The starting components can be mixtures of radiation-polymerizable mono- / oligomers and liquid or dissolved, non-radiation-polymerizable polymers. For processing it has have proven to be advantageous if the viscosity of the starting components at the time of exposure is less than 10,000 mPas.

Basically, higher-viscosity starting materials can also be processed, although supporting measures are generally required. In this way, higher-viscosity acrylates can be structured with thermal support.

Systems with the composition described above have a shrinkage which is advantageous for microfolding and have the required UV reactivity.

The thickness of the lacquer layer is chosen in a range that is customary for picture walls. The lacquer layer used according to the invention preferably has a thickness which is in a range from 5 to 15 μm.

However, it goes without saying that, in principle, lacquer layers can also be used for the present invention, in which the hardening takes place by mechanisms other than by irradiation with electron beams or UV light, as long as there is a corresponding structuring of the surface, for example by radiation of different wavelengths or Heat etc.

In the case of holographically structured foils, the function results from a surface relief that is reproduced by a holographically fabricated master. Of particular advantage here is the full control over the structuring of the light-scattering surface that is possible with this type of production. The completely random, non-periodic structures that can be obtained can be understood as randomly arranged microlenses. The function of the foils is not dependent on the wavelength and is equally given with white, monochromatic, as well as with coherent and incoherent light. The random structure of the holographically structured foils to be used according to the invention destroy undesired moiré effects and color diffraction. The incident light can be precisely controlled in very defined areas. The remi- Tiert light can thus be limited to a solid angle range, whereby the light yield can be optimized.

With the holographically structured film, different solid angle ranges can be selected vertically and horizontally. This is particularly advantageous for screens, since areas where there are no viewers, such as the ceiling area, can be hidden and, moreover, the rooms in which screens are used often have a greater width than height.

To explain the invention in more detail, exemplary embodiments are described below with reference to the figures.

These show in:

FIG. 1 shows two embodiments of the screen according to the invention with different arrangements of the spatially selectively reflecting layer,

FIG. 2 shows a comparison of the optical performance of an image wall according to the invention with a planar substrate with an image wall with a conventionally structured substrate,

FIG. 3 shows an illustration of the angle dependence of the reflection of an image wall according to the invention with a varnish layer of different fineness.

FIG. 4 shows a schematic illustration of the development of a spectrally selectively reflecting coating using a genetic algorithm, and

FIG. 5 shows a diagram of the spectral profile of an image wall obtained by a method according to FIG. 4.

As shown in the two embodiments a and b in FIG. 1, the spatially selectively reflecting layer 1 used according to the invention can be arranged in any way, it being possible for it to be located above (FIG. 1a) or below (FIG. 1b) the spectrally selective coating 2. In the illustration shown, it is located directly above or below the spectrally selective coating 2, the individual layers being arranged on a substrate 3.

While the structured lacquer layer / film, which acts as a spatially selectively reflecting layer 1, ensures the scattering of the light projected onto the screen, illustrated here by the arrows 4, 5, 6 and 7, the spectral selectivity is achieved by the coating 2. According to a further embodiment of the present invention, the holographically structured film can be used as a substrate. In this case, the spectrally selectively reflecting coating 2 is applied to the smooth back of the film.

Based on Figure 1a, this means that the substrate 3 shown there is omitted. If, in addition, a multilayer system is used as the spectrally selectively reflecting coating 2, such as an RGB filter, the structure must be adapted accordingly by producing the layer sequence during coating inversely to the coating sequence on a separate substrate, such as in FIG. 1a shown, carried out, and the changed incidence medium is taken into account that is now no longer air but the holographically structured film.

An advantage of the screen according to the invention is that spatial selectivity is obtained by means of a thin, flexible, structured film or foil, the scattering properties of which can be adjusted very finely in the course of production.

A further advantage, which occurs in particular in the case of an arrangement according to FIG. 1a, in which the structured lacquer layer / film is applied to the spectrally selective coating 2, lies in the fact that the structuring of the surface as provided according to the invention means that the spectrally selective initially applied Coating 2 is not exposed to any mechanical influences that could damage or even destroy it.

If the holographically structured film is used, the structure per se was created before the coating - this avoids the possible destruction of the filter by subsequent embossing and also has the advantage of coating on a smooth surface (back of the film or the substrate) to be able to.

In principle, any material can be used as the substrate 3, as is known per se for the production of such picture walls. Examples of material alien are glass or plastic. The substrate 3 can be transparent or non-transparent. In the case of a transparent substrate 3, the screen can be used, for example, for a projection on transparent glass or plastic surfaces, such as on window panes, as a head-up display or the like. In the case of a non-transparent substrate 3, the substrate can be highly absorbent, for example by being colored black, so that it shows minimal remission for all wavelengths of visible light. The substrate can be made of a flexible or rigid material. An example of a flexible substrate is a plastic film. If the holographically structured film is used for the spatially selectively reflecting layer 1, it can also be used directly as a substrate. In this case, the holographically structured film should be chosen to be transparent.

If the spatially reflective layer 1 is applied to the spectrally selective coating 2 as shown in FIG. 1a, a clear lacquer or a transparent film is selected.

If, on the other hand, the spatially reflective layer 1 is applied between the substrate 3 and the spectrally selective coating 2, an arbitrarily transparent or non-transparent material that is more or less transparent to light can be used. In this case, for example, the lacquer layer can be colored with suitable dyes and / or pigments. For example, instead of the non-transparent substrate mentioned above, a correspondingly absorbent lacquer layer can be applied to a substrate which is transparent per se.

When using a structured lacquer layer, it was also observed that the addition of pigments is beneficial for the formation of the microfolding. To promote microfolding, corresponding materials which act as nuclei for the microfolding can therefore be present in the lacquer layer.

As already stated above, any spectrally selectively reflecting coating known per se can be used for the screen according to the invention. The spectrally selective coating 2 can thus be formed from one or more cholesteric polymer layers, as described in DE 199 01 970 C2. Spectral selectivity is achieved on the basis of the properties of cholesteric polymers which, as a single layer, have the ability to circularly polarized light of a certain handedness (ie either right or left circular) and thus 50% of the unpolarized light in a certain wavelength band To reflect ΔΛ. For optimal reflectivity, the screen should have at least 2 mutually enantiomeric cholesteric polymer layers with corresponding selectivity for this wavelength for each selected wavelength. In this case, since cholesteric enantiomers reflect oppositely circularly polarized light, both the right-hand and the left-hand circularly rotating portion of the unpolarized light in the relevant wavelength band is reflected.

A screen that is particularly suitable for RGB radiation should therefore have at least six layers of cholesteric polymers, two adjacent layers being enantiomeric to each other and reflecting the blue, red or green light, so that a total reflection of approximately 100% for all RGB Wavelengths can be achieved.

The spectrally selective coating 2 can be constructed from a multi-layer system of at least two dielectric materials with different refractive index. The at least two layer materials are applied alternately on the substrate, so that a low-index layer and a high-index layer are arranged alternately on the substrate. The respective layer thicknesses of the high or low refractive index layers of a system can be the same or different. For example, one or more periods of a high-index layer with a first layer thickness and a low-index layer with a second layer thickness can be provided. In this case one speaks of a periodic arrangement.

The thicknesses of the high-index and low-index layers can also vary; in this case one speaks of a non-periodic arrangement. Examples of suitable dielectric materials for the above coating 2 of layers with low and high refractive index materials are the oxides or nitrides of silicon, aluminum, titanium, bismuth, zircon, cerium, hafnium, niobium, scandium, magnesium, tin, zinc, yttrium and indium. Preferred examples for low refractive index materials are SiO ₂ and MgS ₂ and in particular Al ₂ O ₃ .

Preferred examples of high-index materials are tantalum oxide, titanium oxide and niobium oxide and Si ₃ N ₄ .

Examples of preferred combinations of high-index and low-index materials are silicon dioxide as the low-index material and titanium dioxide in the rutile phase or in the anatase phase as the high-index material, and the combinations SiO ₂ / Si ₃ N ₄ and in particular Al ₂ O ₃ / Si ₃ N ₄ . Another example of a suitable material is the mixing system

Coatings of this type made of a multi-layer system consisting of a sequence of alternately high and low refractive index dielectric materials act as an interference filter with which the wavelengths of the projection light are selectively reflected.

As an alternative to realizing the interference filter by means of a multi-layer system consisting of high and low refractive index layers, a filter system with a refractive index that is continuously and periodically modulated over the filter thickness can be used as a spectrally selective coating 2 - a so-called rugate filter.

To produce such a filter with a refractive index modulated over the layer thickness, the above-mentioned mixing system Si 1 can be used, for example. _x . _y O _x N _y are used, the refractive index being set here by varying the nitride concentration.

Other mixing systems consisting of a low refractive index and a high refractive index component such as mixing systems selected from SiO ₂ / TiO ₂ , SiO ₂ / Ta ₂ O ₅ and SiO ₂ / Nb ₂ 0 ₅ can be used. The refractive index is set here via the mass ratio of the components.

For the production of such spectrally selective coatings 2 on the basis of mixing systems, for example, cosputter processes or CVD processes, as are generally known, can be used.

Methods for producing the multilayered layer systems from dielectric materials are known per se and are described, for example, in DE 197 47 597 A1 and WO 98/36320. Examples of suitable coating processes are vacuum coating processes such as magnetron sputtering and electron beam evaporation.

It was found that by reducing the difference in the refractive indices, the angle dependency of such a layer system of low and high refractive index layers can be reduced. For example, by replacing the low-index material with a material with a higher refractive index, the angle dependency can be reduced. The angle dependency, and thus the shift of the peaks of the primary valences in the spectrum, is less for a layer system Al ₂ O ₃ / Si ₃ N ₄ than for a system SiO ₂ / Si ₃ N ₄ . Here, the low-index material SiO _{2 was} replaced by Al ₂ O ₃ , which has a higher refractive index.

In Figure 2, the optical performance of spectrally selective coatings on a planar substrate and on a structured substrate are compared. A dielectric multi-layer system was used as the spectrally selectively reflecting coating, as described above. The comparison makes it clear that the spectrally measured reflection of an image wall according to the invention according to FIG. 1b, in which the spectrally selectively reflecting coating 2 is deposited on a planar substrate 3, is significantly higher than the spectrally measured reflection of an image wall, in which the coating 2 on a structured substrate is applied. This result illustrates the advantage of a coating on planar substrates with subsequent structuring or, as is possible with the holographically structured film, structuring on the substrate side facing away from coating 2, as is provided according to the invention.

With the method according to the invention, the scattering behavior of a given screen can be adjusted by simply modifying the structuring of the lacquer layer or the holographically structured film which form the spatially selectively reflecting layer 1, so that the angular characteristic of the screen can be varied as required.

FIG. 3 shows the angle dependence of the reflection from picture walls according to the invention with a lacquer layer as a spatially reflecting layer 1, which differ in the angle characteristic. The differences in the angular characteristics result from the use of varnishes with different finishes.

According to the invention, not only can higher reflections be achieved since planar substrates can be used, but also the angular characteristic of a screen can be set individually, depending on the requirements, by simply modifying the structuring of the lacquer layer or the film.

As already explained in connection with FIG. 1, the spectrally selectively reflecting coating 2 can in principle be located above or below the substrate 3. Preferably, however, the geometric distance between the selectively reflecting coating 2 and the scattering surface, that is to say the structure of the lacquer layer or the film, should be as possible be small, which can be achieved, for example, by minimizing the thickness of the lacquer layer or film.

If the scattering surface and the spectrally selectively reflecting coating 2 are too far apart, the sharpness of the image suffers and "double or ghost images" can develop.

In an embodiment preferred according to the invention, as shown for example in FIG. 1a, the structured lacquer layer 1 is applied to the spectrally selectively reflecting coating 2. In this case, the optical properties of the lacquer can be taken into account when designing the coating 2 in order to obtain an optimization of the optical properties of the screen. For example, by adjusting the chemical composition of the lacquer, the refractive index of the lacquer can be adapted to the properties of the coating and a so-called index adaptation can be obtained.

According to a further embodiment, an anti-reflective coating 8 for anti-reflective treatment can additionally be provided on the spatially reflective layer 1 for an embodiment as shown in FIG. 1a, for example, in which the spectrally selectively reflective coating 2 is arranged below the spatially reflective layer 1. By providing an anti-reflective coating 8, any intrinsic reflections that may occur on the surface of layer 1, which can be approximately 4%, can be reduced to less than 1%.

In principle, customary anti-reflective coatings can be used for this. An example of a suitable anti-reflective coating is a layer system consisting of TiO ₂ (11 nm) -SiO ₂ (40 nm) -TiO ₂ (110 nm) -SiO ₂ (85 nm). Anti-reflective coatings can be deposited on the surface of the screen, for example, by a vacuum coating process (vapor deposition or sputtering) or by a wet chemical coating process (sol-gel process). Magnetron sputtering or electron beam evaporation can be mentioned as suitable methods. According to a further preferred embodiment, the spatially selectively reflecting image wall according to the invention contains a spectrally selectively reflecting coating 2, which is optimized with regard to the fact that, in addition to the highest possible contrast, it offers the best compromise between a small layer thickness, a small number of layer materials to be used and a small number Has individual layers.

Conventional layer systems for coatings as have been described above are produced by selecting the materials for the individual layers, the respective layer thickness and the number of layer thicknesses while specifying a defined discrete target spectrum. The layer systems obtained in this way show a contrast that is suitable for practical use, but are not optimized with regard to further desirable properties for a screen in order to achieve the best possible image impression.

However, it is desirable to have layer systems which not only have improved contrast, but also have this improved contrast with the smallest possible thickness of the overall layer system and of the individual layers with the smallest possible number of layers. Furthermore, the screen should enable suppression or at least suppression as far as possible of the so-called color flop effect. A color flop is understood to mean a change in the viewer's color impression, which is caused by a change in the viewing angle. The reason for this is the angle-dependent reflection transmission behavior of interference filters.

Depending on requirements, further limit conditions can be specified for the screen. One possibility for improving the color neutrality is that the screen wall has the same peak heights for the primary valences. Color neutrality means that e.g. B. projected white actually appears as white and not e.g. B. has a red cast. Conversely, it was found according to the invention that a corresponding color neutrality can be achieved by comparing the intensities of the primary valences of the respective projector with the relevant screen. The white balance is carried out here Adjust the color neutrality directly on the projector itself and not on the screen. In this case, significantly higher contrast values can be achieved with the same layer thicknesses than with a screen with the same peak heights for the primary valences. The reason for this is the possibility of reducing the green reflection, for which the human eye is very sensitive.

The spectral sensitivity of the human eye can also be taken into account in order to produce an optimal image impression on the viewer.

The layer system should have an optimal profile with regard to the properties mentioned above, in particular with a predetermined number of layers.

In order to obtain a spatially selectively reflecting screen with the above-mentioned properties, a spectrally selectively reflecting coating is preferably used for the screen according to the invention, which coating is obtained by using an evaluation method based on the colorimetry instead of the conventional optimization for a predetermined reflection spectrum based. For this purpose, the colorimetric evaluation is combined with an optimization algorithm that does not require a predetermined initial design as input for the optimization, which essentially already reproduces the target spectrum. A so-called genetic algorithm has proven to be particularly suitable for this purpose, which is known per se and is described, for example, in Heistermann J., "Genetic Algorithms - Theory and Practice of Evolutionary Optimization", BG Teubner, 1994, or at Goldberg, DE, "Genetic Algorithms in Search, Optimization, and Machine Learning", Addison-Wesley, Reading, 1989. When combined with colorimetric evaluation methods, this algorithm, which is robust and easy to implement, can be used for the optimization of optical coatings ,

The mode of operation of the genetic algorithm for optimizing spectrally selectively reflecting coatings for picture walls is explained in more detail below with reference to the flow chart in FIG. The functioning of the genetic algorithm is based on the evolution and recombination strategies of nature. The basic idea is that out of a number of individuals that together form a generation, only those individuals are selected to generate a new generation who have the best characteristics in terms of their environment. In the present case, an individual is understood to mean a layer system with its layer thicknesses and material properties. The parameters of the layer system such as layer thicknesses and materials are called genes. The algorithm first generates a population of individuals by randomly assigning materials and layer thicknesses (referred to as "static population" in FIG. 4). These individuals are then evaluated and sorted by quality. A loop consisting of the steps recombination, mutation, Evaluation and selection (selection of the better individuals, ie strata), whereby bad strata are largely discarded.Repeatedly performing the loop improves the quality of the population on average and that of the best individual absolutely until an optimum is reached The advantage of the genetic algorithm compared to standard hill-climbing algorithms lies in its parallelism and the fact that currently worse individuals are involved, who can discover new search areas in the fitness function through further recombination in later generations. This is essential for a global search H.

There are a variety of genetic algorithms that can be used in principle for the above procedure.

For the production of the picture walls used according to the invention, the evaluation and thus optimization takes place on the basis of colorimetric considerations. Since the evaluation is based on the color metric, a specification of a discrete target spectrum can be dispensed with. The contrast results from this

^ Blue ⁺ RGreen ⁺ R Red k = (D

3 Y

where Y is the standard color value and thus a measure of the brightness of the reflection spectrum, k indicates the contrast that the screen achieves for primary valences of the corresponding wavelengths with respect to ambient light.

To carry out the algorithm, a modified formula (1) can be used for evaluating the contrast, the major change being that the sum is replaced by a product:

k = -i _r (π R,) + - (2)

Y ι = B, G, R Rσ

and σ _{R is} the standard deviation of reflectivity and C is an empirical factor. N is usually set to 3 for three primary valences. Using this exponent, a weighting between the reflected brightness and the resulting contrast is also possible by selecting other values.

To minimize or eliminate the color flops effect, the layer system must be designed so that the change in the reflected intensities caused by changing the viewing angle is the same for all wavelengths of the projection light. As a basis for the evaluation by the algorithm, the spectra of an individual (layer system) are calculated for different viewing angles and the change in the reflected intensities for the wavelengths of the primary valences is compared, using standard deviations of these values.

If necessary, the color neutrality of the image can also be adjusted by means of white balance, the intensity of that reflected by the screen

Light one of the wavelengths of the primary valences (red, green, blue) with the Intensity of the monochromatic light of the corresponding wavelength as it is emitted by the projector is adjusted.

On the basis of the procedure described above, the genetic algorithm is used to evaluate a layer system which does not require a fixed specification for a discrete reflection spectrum and which increases the contrast, at the same time suppressing the color flop.

The combination of a genetic algorithm described and an evaluation based on the colorimetry results in spectrally selectively reflecting coatings which contain a significantly improved contrast with an optimally low overall layer thickness and number of individual layers.

For example, coatings with a contrast of at least 2.5 and a thickness of the entire layer of less than 4.5 μm can be achieved with this.

FIG. 5 schematically shows the spectral profile of a spectrally selectively reflecting coating which has been obtained using the genetic algorithm described above.

It is a coating of a layer system made of low and high refractive index dielectric materials. The coating consists of twelve individual layers, with SiO ₂ as the low-index material (n = 1.46) and Si ₃ N ₄ as the high-index material (n = 2.05), which have been deposited on a glass substrate

The layer structure starting from the glass substrate is as follows:

Glass

Si ₃ N ₄ 239 nm, SiO ₂ 210 nm,

Si ₃ N ₄ 324 nm; SiO ₂ 319 nm, Si ₃ N ₄ 435 nm, SiO ₂ 197 nm, Si ₃ N ₄ 22 nm, SiO ₂ 241 nm, Si ₃ N ₄ 72 nm, SiO ₂ 372 nm, Si ₃ N ₄ 249 nm, SiO ₂ 35 nm. This coating shows a contrast improvement k of 3.55. A detailed description of the development of spectrally selective coatings using a genetic algorithm, in particular of layer systems made of low and high refractive index dielectric materials, can be found in Ch. Rickers, M. Vergöhl, C.-P. Klages in: "Design and manufacture of spectrally selective reflecting coatings for use with laser display projection screens", Applied Optics, Volume 41, No. 16, June 2002, to which reference is made in full for the purposes of the present invention.

It remains to be noted that for the increase in the contrast, it is less a particularly high reflection in the range of the laser wavelengths that is decisive, but rather the lowest possible reflection in the spectral range outside of it. In the genetic algorithm, this effect can be taken into account by influencing the weighting with which the background is suppressed. This is done by varying the exponent of Y in equation (2) accordingly.

It was also found that reducing the peak width of the wavelengths of the laser light in the spectrum is only of limited use for increasing the contrast, since this means that a sharp decrease in the reflected intensity when viewed at a different angle has to be accepted Increasing the angle dependency.

Claims

Expectations

1. Contrast-increasing screen, which contains a substrate (3) and a spectrally selectively reflecting coating (2), characterized in that the screen has at least one spatially reflecting layer (1), the spatially reflecting layer (1) being selected from one Layer formed from a hardenable lacquer and a holographically structured film.

2. Contrast-increasing screen according to claim 1, characterized in that the spatially reflective layer (1) is a hardened layer made of at least one material selected from acrylates, epoxides, vinyl ethers, unsubstituted or substituted styrenes.

3. Contrast-increasing screen according to claim 2, characterized in that the spatially reflective layer (1) is a hardened layer based on acrylates.

4. Contrast-increasing screen according to one of the preceding claims, characterized in that the spatially reflecting layer (1) consists of a UV-curable material.

5. Contrast-increasing screen according to one of the preceding claims, characterized in that the spectrally selectively reflecting coating (2) is a layer system of two or more layers, at least one combination of a low and high refractive index material being used as the layer material.

6. Contrast-increasing screen according to one of claims 1 to 4, characterized in that the spectrally selective coating (2) is a layer system of at least two layers made of a cholesteric polymer.

7. Contrast-increasing screen according to one of the preceding claims 1 to 4, characterized in that the spectrally selective coating (2) is a filter system with continuously and periodically modulated over the filter thickness

Is refractive index. (JL _L

8. Contrast-increasing screen according to one of the preceding claims, characterized in that the spectrally selectively reflecting coating (2) is a combination of cholesteric polymer layers and dielectric layers.

9. Contrast-increasing projection screen according to one of the preceding claims, characterized in that the spectrally selectively reflecting coating (2) has a contrast k of at least 2.5 and a total layer thickness of less than 4.5 μm.

10. Contrast-increasing screen according to one of the preceding claims, characterized in that the spectrally selectively reflecting coating (2) is a layer system selected from SiO ₂ / Si ₃ N ₄ and Al ₂ O ₃ / Si ₃ N.

11. Contrast-increasing screen according to one of claims 1 to 10, characterized in that the material for the substrate (3) is selected from a transparent and non-transparent material.

12. Contrast-increasing screen according to claim 11, characterized in that the material for the substrate (3) is selected from a flexible and rigid material.

13. Contrast-increasing screen according to one of the preceding claims, characterized in that the substrate (3) is planar.

14. Contrast-increasing screen according to one of the preceding claims, characterized in that the holographically structured film acts as a substrate.

15. Use of a hardenable lacquer layer as a spatially selectively reflecting layer (1) for a screen.

16. Use according to claim 15, characterized in that the curable paint view consists of an electron beam or UV curable paint.

17. Use according to one of claims 15 to 16, characterized in that the lacquer layer consists of a hardenable lacquer based on a material selected from acrylates, epoxides, vinyl ethers, unsubstituted and substituted styrenes.

18. Use according to claim 17, characterized in that the curable lacquer layer consists of an acrylate-based material.

19. Use according to one of claims 16 to 18, characterized in that the lacquer layer consists of a UV-curable lacquer.

20. Use of a holographically structured film as a spatially reflecting layer (1) for an image wall with a spectrally selectively reflecting coating (2).

21. Use of a holographically structured film as a substrate for an image wall with a spectrally selectively reflecting coating (2).