Optics: image projectors – Polarizer or interference filter
Reexamination Certificate
2000-10-02
2002-08-06
Adams, Russell (Department: 2851)
Optics: image projectors
Polarizer or interference filter
C353S079000, C353S084000, C359S443000, C359S449000
Reexamination Certificate
active
06428169
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to an image display system with a projector for emitting projection light for generating an image on a light-backscattering projection surface.
b) Description of the Related Art
Image display systems have been common for a long time and are used, for example, in slide projection applications in the home or for showing films in cinemas.
In the last thirty years, image display systems have been developed in which LCD projectors, DLP projectors or laser projectors are used for writing an image with a light bundle.
Known image display systems use light-backscattering projection surfaces to present images, wherein the diffuse reflectance of commercially available projection surfaces in the visible wavelength range is largely independent from wavelength. Such projection surfaces are provided with a frame and a stand or suspension and are also referred to as image screens or projection screens. Projection screens are standardized, for example, according to DIN 19045.
The projection surface diffusely reflects (backscatters) the light originating from the projector back to the observer who visually perceives an image.
When ambient light acts upon the projection surface, it is also scattered from the projection surface toward the observer. As a result, even lowintensity ambient light drastically worsens the contrast (light/dark ratio) and color saturation (colorfulness) of the projected image. The ambient light interfering with the image display is usually sunlight or artificially generated light corresponding at least in some proportion to sunlight. In extreme cases, the projected image is practically entirely obliterated, for example, when sunlight strikes the projection surface directly when projecting images.
In order to overcome this problem, images are preferably projected at night or in very dark rooms as is known, for instance, in cinemas or planetaria. Also, presentations in conference halls or schools require rooms offering at least the possibility of darkening. The darkening of rooms requires that the building be outfitted with special lighting equipment and light openings which must be closed when necessary so as not to admit any light. However, the disadvantage in darkened rooms is that many activities can no longer be performed without sufficient illumination, e.g., writing. For this reason, it has always been necessary heretofore, e.g., in the case of presentations, to find a compromise between the necessary ambient light and the quality of the image display.
Recently, the light output of projectors has been considerably augmented, particularly by means of more efficient light sources. This has entailed a considerable expenditure in material and energy, although only a very limited improvement in contrast and color saturation has been achieved. Up to the present, daylight projection is possible only to a very limited extent even with very lightintensive projectors, but is increasingly in demand for purposes of advertisement and presentation without any satisfactory solution being available so far.
In order to reduce the influence of ambient light on the quality of the projected image, WO 98/36320 describes a projection screen comprising a plurality of layers. The uppermost layer or sequence of layers, considered in the projecting direction, reflects only the projection light. All other light components are absorbed by a lower layer. A projection surface of this type has a substantially dark effect in ambient light and is therefore known as a “black wall”. A visibly improvement in contrast would be expected with a projection screen of this type when using laser light for projection. However, this improvement is not yet so significant that the demand for projection in a conventionally illuminated environment has been met.
U.S. Pat. No. 2,590,906 or U.S. Pat. No. 5,568,316 describe interference filters which, as narrow-band reflection filters, are adapted exactly to the wavelengths of the laser light source, e.g., they can be used in connection with a projection screen known from WO 98/36320.
On this basis, it is the object of the invention to provide an image display system with a projection screen with which an image can be displayed whose contrast and color saturation is substantially retained even in comparatively intense ambient light. For this purpose, the projection screen should also be suitable for large-image display and should enable large projection angles and large observation angles. The invention has the aim of achieving a substantial improvement in the colorfulness (color saturation) of colors and in the contrast of a projected image perceived by the observer even when illuminated by artificial light and/or sunlight.
According to the invention, this is achieved by an image display system with a projector for emitting projection light for generating an image on a light-backscattering projection surface, wherein the wavelengths of the projection light lie in one or more discrete wavelength regions in the visible spectrum and the projection surface is covered on the projector side by an optical filter which passes only light with wavelengths corresponding to those of the projection light.
It is completely surprising for the person skilled in the art that the projection characteristics of contrast and color saturation of a projected image can be maintained virtually unaffected by the influence of the ambient light by covering the light-backscattering projection surface on the projector side with an optical filter layer of this kind.
In contrast, the basis of the known solution according to WO 98/36329 is that, initially, the projection light is essentially reflected in an upper layer, while the ambient light is transmitted through this upper layer into a lower layer and is absorbed in the latter.
However, in the invention the ambient light is prevented from passing through the top layer. Only the projection light reaches the light-backscattering projection surface. This surface lies below (considered in the direction of incident light) the blocking layer formed by the optical filter. Accordingly, practically no light ambient light, or only very little ambient light, reaches the surface on which the image display is realized.
It must be taken into account that the formation of a projected image is attributed not only to an interaction of the projection light with the surface of the projection surface, but that the projection light also, as a rule, can penetrate into the surface layer of the projection surface, where it is subjected to a volume scattering. The light that is backscattered as a result of the volume scattering reemerges at the surface of the projection surface as backscattered light.
The invention follows a completely novel path in that the ambient light is maintained at the same distance, as far as possible, from the projection surface which backscatters the light toward the observer.
A further advantage of the image display system according to the invention is that ambient light components which should penetrate the optical filter due to the fact that its blocking effect is only finite, impinges again on the optical filter after being backscattered at the projection surface, so that its filtering effect for the ambient light is multiplied. Therefore, in the invention, the contrast and color saturation of the image projection is degraded only to an insignificant degree by the ambient light.
A significant advantage of the invention is that the existing, standardized and commercially available projection screens can also be used as a basic component in the projection surface according to the invention.
In contrast to the “black wall” known from the prior art, the invention can select from a large number of existing “white screens” with their specific characteristics.
In the invention, the spectrum of the projection light can be determined according to the image to be displayed. For example, the image may be a monochromatic image or a polychromatic image. The components of
Deter Christhard
Wunderlich Joerg
Adams Russell
Cruz Magda
Reed Smith LLP
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