Materials with specific optical properties are needed in numerous technical fields. For example specific layers are applied onto synthetic materials in order to generate mirror properties (DE 39 27 087 C2), or multilayer systems are applied onto window panes in order to provide thermal protection (DE 197 45 881 A1).
Multilayer filter systems, which have specific reflection and/or transmission properties (WO 95/17690), are also employed in the case of liquid crystal displays. These filter systems comprise for example in each pixel region filters for red, green and blue, with each filter having a common dielectric broadband mirror. The difference between the filters consists in the tuning thickness between the mirrors for each filter. Use of the common dielectric broadband mirror simplifies the process of production of mosaic arrays in the RGB filter.
Furthermore a color filter grouping for a surface image sensor is known, which comprises several color filters on the sensor, which selectively filter out different colors (U.S. Pat. No. 4,979,803). Each color filter comprises at the bottom a semitransparent and partially reflecting silver layer. On this silver layer is disposed a transparent and absorption-free dielectric layer having a thickness which is selected such that it corresponds to the wavelengths of the light to impinge onto the image sensor.
A further known optical filter is comprised of two mirror layers which form an optical resonance space with a dielectric layer being disposed between these mirror layers (U.S. Pat. No. 5,726,805).
Another filter with variable wavelengths of modifying etalon type comprises at least two reflecting surfaces whose spacing from each other is not constant but rather increases or decreases monotonically from the spacing in a selected direction in a light-sensitive plane of the etalon (EP 0 442 738 A2).
Interference filter groupings on the basis of a thin metal filter and comprising at least one dielectric layer between two metal layers are disclosed in U.S. Pat. No. 6,031,653. The two metal layers are of a thickness which is less than the penetration depth of the radiation.
Furthermore is known an optical filter in which several optical layers are disposed one on the other, which are alternately comprised of a high-refraction material, TiO2, and a low-refraction material, SiO2 (U.S. Pat. No. 5,731,898).
None of the filters listed above reflects spectrally selectively in the primary valences of red, green and blue, as a consequence of which the filters are not suitable for a laser projection screen.
Projection screens must have special optical properties in order to ensure brilliant image representation. This applies especially to laser projection, in which a deflectable laser beam, into which the primary colors red, green and blue are coupled, scans the screen (cf DE 196 40 404 A1). The projection screens must have reflection maxima in the red, green, blue wavelengths of approximately 629 nm, 532 nm and 447 nm, respectively. In addition, a broadband background light must be absorbed, for example in a subjacent black layer.
A further requirement made of the properties of laser projection screens comprises that the reflection must not involve a mirror-like reflection but rather a diffuse reflection in a preselectable solid angle range in order for the viewing of the projected images to be possible from different locations. Reflections in a spatial region in which no observer is present must be avoided in order to minimize intensity losses. The screen must thus not be a Lambert radiator in which the luminance is constant in all directions of a half space. According to DIN standard 19045, Bereich Betrachtungsbedingungen {Area: Viewing Conditions}, the reflection should be up to ±40° in the horizontal direction, measured from the projection axis, and ±10° in the vertical direction, measured from the visual axis of a central viewer. In addition, the shift which can be observed of the reflection characteristic with changed reflection angle—the so-called color flop—should also be largely suppressed.
Lastly, projection screens should meet the requirement that they also ensure good recognizability of the projected images in non-darkened rooms.
The invention therefore addresses the problem of providing a material for projecting screens which is suitable for laser projection. This problem is solved according to the present invention.
The invention consequently relates to a material with spectrally selective reflection in the primary valences of red, green and blue. This material is preferably applied onto laser projection screens in order to make projection possible even in daylight. In a first solution, onto a substrate, for example glass or synthetic material, a first layer of aluminum is applied, which is followed by a second layer of SiO2, on which, in turn, a layer of NiCr is disposed. The latter is in contact with air. A second solution comprises five layers, of which the first is comprised of TiO2, the second of SiO2, the third of TiO2 the fourth of SiO2 and the fifth of TiO2. The first layer is here connected with the substrate, while the fifth layer is in contact with air. The first solution is much simpler than the second solution.
In an embodiment of the invention, a material is provided having spectrally selective reflection in the ordinary valences of red, green and blue, having a first layer highly reflecting over the entire range of visible light: a second layer, essentially non-absorbing over the entire range of visible light, which is disposed on the first layer and whose optical thickness is between approximately 600 nm and 900 nm, with the optical thickness being defined as 4 .n .d where n is the index of refraction and d the geometric thickness: a third leyer, partially transmissive over the entire range of visible light, which is disposed on the second layer, with the third layer having a geometric thickness of approximately 20 to 80 rim.
In an alternative embodiment, the material has a first layer of a material with a high index of refraction which is approximately 400 to 600 nm thick; a second layer of a material with a low index of refraction which is approximately 127 nm thick and which is disposed on said first layer: a third layer of a material with a high index of refraction which is approximately 400 to 600 um thick and which is disposed on said second layer: a fourth layer of a material with a low index of refraction which is approximately 72 nm thick and which is disposed on said third layer: a fifth layer of a material with a high index of refraction which is approximately 400 to 600 nm thick and which is disposed on the fourth leyer. The material with a high index of refraction in at least one of said first layer, the third layer and the fifth layer is selected from TiO2. Ta2O5, TiOxNyand the material with a low index of refraction in said second and fourth layers are each SiO2,
The advantage attained with the invention comprises in particular that utilizing only three layers, one metallic layer and two dielectric layers, the above stated requirements are met in a laser image screen. It is here especially advantageous that the metallic layer can be produced cost-effectively, for example by vapor deposition in film coating installations. In comparison to sputter processes the metal layer can be applied at approximately the 10- to 100-fold rate. The dielectric layer can alternatively be applied with the cost-effective vapor deposition technology or the sputter technology. The requirements are alternatively also fulfilled with the aid of a five-layer system of dielectric materials with a high index of refraction. However, the production of the five-layer system is significantly more costly than the production of the three-layer system. Both layer systems can be readily produced as a large-area system by means of a DC or a medium frequency sputter process. Consequently the expensive production of the fully dielectric layer systems commonly used up to now becomes superfluous. The reproducibility of the layer thickness and the indices of refraction is better than 1%.
It is understood that projection operation under daylight conditions entails a certain desaturation of the colors. Daylight contains a component in the RGB range. If the laser is switched off and only daylight falls onto the coated screen, the screen appears white since the reflected daylight component in the RGB range yields the color white through an additive color mixture. This white is superimposed onto the laser colors such that the color saturation decreases slightly in comparison to pure spectral colors. The decrease of the color saturation, however, is less than is the case with conventional white screens, since a large component of the daylight spectrum, which is outside of the RGB maxima, is absorbed.
Embodiment examples of the invention are depicted in the drawings and will be described in further detail below.