Optics: image projectors – Composite projected image – Multicolor picture
Reexamination Certificate
1999-11-30
2001-10-30
Mahoney, Christopher E. (Department: 2851)
Optics: image projectors
Composite projected image
Multicolor picture
C353S031000, C353S094000, C348S750000, C348S757000
Reexamination Certificate
active
06309072
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a video projection system for projecting more than one picture in which a light source for generating at least one light bundle, means for intensity modulation and means for spatial light bundle modulation, wherein these means are controlled by a video signal, and at least one projection surface for displaying a determined quantity of individual pictures are arranged in the direction of light of a beam path.
b) Description of the Related Art
There are known projectors with light sources which can derive from two fundamental principles:
1. Projectors based on the conventional imaging principle, e.g., LCD projectors and DLP projectors. These projectors will be referred to hereinafter as “picture-imaging projectors”;
2. Projectors working with light bundles which are deflected in two dimensions. These projectors are also referred to, for example, as “laser display” or “laser projectors”.
The above-mentioned projectors can be designed exclusively for showing monochrome pictures or also for showing black-and-white pictures. In these cases, it is sufficient to convert the electronic picture information into an intensity modulation or brightness modulation of the light. The picture impression depends first on the adjustment of picture brightness with reference to the ambient light conditions. The maximum picture brightness is limited by the light output which is modulated by 100%. The picture impression is further determined by the level of contrast. In adjusting the contrast, the modulation amplitude is fixed; that is, the gray values of the video signal are correlated with correspondingly modulated light outputs.
If a plurality of projectors of the same type which are arranged adjacent to one another show identical pictures, it is only necessary to adjust the parameters of brightness, contrast and spectral intensity distribution of the projectors relative to one another. With four projectors for four pictures, this means that there could be 12 possible adjustments.
Further, it must be ensured that the means for picture modulation and associated controlling electronics in the projectors operate with entirely identical characteristics. Only in this way will it be achieved in practice that a plurality of projectors receiving the same video information also show the same pictures.
The degrees of freedom for adjusting the projectors are increased almost three-fold when generating color images. In the case of a color picture projector, this can be explained by the fact that three different colored monochrome systems are put together in practice: a system for the primary color red, a system for the primary color green, and a system for the primary color blue. Accordingly, there are 9 possibilities for adjusting the color effect, contrast and brightness of a picture. Therefore, with four projectors, there are already 36 degrees of freedom for adjustment! The following picture properties relevant for the human faculty of sight must be extensively identical in a plurality of projectors: contrast (light-dark ratio), gradation (channel gradation), convergence (color overlap), picture sharpness, picture composition (pixel structure), brightness (luminous density), color intensity (saturation) and color shade (hue). In particular, color differences (differences in color stimulus values) between projectors which project pictures directly adjacent to one another are determined in an extremely sensitive manner by a human observer. The objective of matching the projection characteristics of a plurality of projectors to the greatest possible extent can be achieved in practice only at considerable expense with respect to technology, cost, personnel and time.
In order to project large pictures, a plurality of individual pictures are usually combined to form a total picture. For this purpose, each individual picture is generated by a complete projector.
Arrangements comprising a plurality of LCD projectors, for example, are known. According to EP 0 731 603 A2, each of these projectors contains a white light source from which the three primary colors, red, green and blue, are filtered out. Constructions containing three light sources, each of which provides a primary color, are also known. These light sources are temperature radiators. Solutions making use of temperature radiators have substantial deficiencies. The observer of the projected picture expects to be presented with a picture of uniform brightness and color depiction. However, this cannot be realized when each projector generating an individual picture has its own light source. It cannot be expected that the brightness of the independent light sources of each projector and their color-reproducing characteristics will really be identical and that they will actually remain identical permanently.
A primary problem consists in that the projection lamps age differently and unforeseeable changes in the spectral intensity distribution occur through this process. Tests have shown that the color temperature of individual projection lamps, even of the same type and age, show substantial differences in radiating characteristics. As a result, the individual projectors will reproduce different colors for the individual picture in question. In the course of aging, the color temperatures of the individual projection lamps change very differently.
Another source for differences in the spectral intensity distribution are differences and tolerances in the supply of power to the individual light sources or cathode ray tubes of the projectors. All of these negative influences lead to a considerable waste of time and material on the part of the operator of an installation of this kind to produce and maintain a balance of brightness and color effect in the individual projectors. In practice, the picture composed of the individual images must satisfy at least modest requirements with respect to quality. The process of matching the individual projectors is very time-consuming and complicated. For example, if brightness is increased by changing the supply voltage or the supply current to a projection lamp, the color temperature also changes. This influence can only be compensated by a new color matching of the projector. When a projection lamp fails and is replaced with a new one, this process becomes still more uneconomical as the new projection lamp will differ substantially with respect to light output and color temperature from those that have already been in use for a long period of time. Accordingly, it is often necessary to replace all of the projection lamps in the system at the same time when one projection lamp fails. Again, this increases costs.
Further, It is known from EP 0 589 179 that the projection lamps (temperature radiators) in picture-imaging projectors can be replaced with laser light sources. Laser light sources have the advantage over temperature radiators that they can emit light in defined wavelengths, i.e., different laser light sources of one type emit light of exactly the same wavelength. This is a great advantage, for one, in that the spectral intensity distribution is identical in laser light sources of the same construction.
However, difficulties arise even in this case with a monochrome projection system, and increased technical expenditure is required to operate a plurality of laser light sources for the individual pictures in such a way that the total picture gives a uniform impression of brightness.
A picture-imaging laser projection system generating color pictures also needs three laser light sources in the primary colors red, green and blue for each projector. One example of this is a DLP system according to EP 0 589 179 A1 in which three laser light sources generate laser light bundles of different wavelengths. The light bundles are spatially combined and expanded to the size of a DMD array by means of optics. A projection objective images the DMD array on the projection surface.
In a multiple arrangement of these projectors, the ratios of th
Cruz Magda
LDT Gmb&H Co. Laser-Display-Technologie KG
Mahoney Christopher E.
Reed Smith LLP
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