Television – Video display – Projection device
Reexamination Certificate
1999-12-28
2003-06-17
Miller, John (Department: 2614)
Television
Video display
Projection device
C348S781000, C359S031000, C359S016000
Reexamination Certificate
active
06580469
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a projection device with a matrix located in an image plane for generating a video image, with optics for projection of this video image on a screen and with light source for illuminating the matrix.
b) Desecration of the Related Art
A technique of the type mentioned above is known from video projection devices, already commercially available at the present time, in which an LCD matrix is installed between a light source and optics like a transparency in a slide projector and is imaged by the optics on a screen.
The LCD matrix is controlled, for example, for displaying video images. In this way, a video picture can be imaged on a screen as a large image by means of the technique known from slide projection. This large-image technique is considered to be trend-setting because electronic picture tubes can not be used for very large images.
Aside from the above-mentioned transmitted light projection known for transparencies, incident light projection with an LCD such as is known from episcopes would also be suitable for the above purpose, wherein the LCD lies in the image plane of the episcope construction in this case.
When use of the incident light projection method is desirable, a mirror array or matrix can also be used instead of the LCD matrix. A matrix of this kind can be obtained as a circuit from Texas. Instruments, for example. In this case, a plurality of tilting mirrors, one for each picture point, arranged in a matrix are digitally controlled. In one digital state, a tilting mirror reflects the full light intensity; in the other state, the mirror receives and reflects the light at an angle at which it can no longer be thrown onto the screen. This means that the corresponding picture point is dark on the screen except for small proportions of scattered light.
The different degrees of brightness of the light for displaying the gray value or color value of a picture point are adjusted in that the mirrors are acted upon by a suitable pulse train so that only an intermediate value between full light intensity and dark is perceived by the eye of an observer for every picture point in time average.
The known episcope method and slide projection methods are disadvantageous in that a very high light output is required of the light source, whose heat output could, for example, destroy the LCD. For this reason, elaborate cooling is also required in projectors of this type which makes these devices heavy, inconvenient and expensive.
In addition, conventional episcope technique is not suitable for the tilting mirror technique because with the latter technique the best image is achieved when the light reaches the matrix only at a defined angle, i.e., precisely the adjustment angle of the tilting mirrors for the darkness value such that only a small surface of practically zero is opposed to the light source from the mirror for reflection. However, in the case of a concave-mirror structure such as is known from episcopes for illumination of an image, the defined angle cannot be adjusted. Therefore, as regards illumination which is advisably carried out at a defined angle in the case of tilting mirrors, a substantially greater light loss must be taken into account compared with the known episcope technique.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to reduce light loss in such projectors as far as possible and accordingly to provide a compact video device of the type mentioned above in which the problem of cooling is minimized in spite of a compact construction.
This object is met by providing an arrangement by which the video image is uniformly illuminated and, in particular, in that a device for mixing the light of the light source by means of multiple reflections is provided between the light source and the matrix, and the mixed light is directed to the matrix proceeding from the device.
The device, according to the invention, for mixing the light of the light source is used for generating the most uniform possible light density or luminance on the matrix. In conventional lamps there is always nonuniform luminance because of the filament or the volume always required for emission of light, so that a certain nonuniformity must always be taken into account in illumination. For this reason, the generated light spot on the transparency, on the image to be projected, or on the LCD matrix should always be expanded farther than the dimensions of the latter so as to enable a light field which is as homogeneous as possible. However, the light loss resulting from this is reduced according to the invention in that the light is repeatedly reflected back and forth in the device for mixing before it exits this device again, so that the origination of every light bundle in the emission volume of the light source is lost. The light field generated in this way then has a very uniform luminance after exiting the device for mixing. This device for mixing can be, e.g., a light-conducting fiber or a mirror system for reflecting back and forth.
The resulting advantage for mixing, particularly with respect to required output, can easily be illustrated by a numerical example. While conventional technique only assumes that an area twice as large as that necessary for the area of the image must be illuminated, the edge length can be reduced by up to one half in the uniform illumination according to the invention. This means that only a fourth of the output is required as a result of the device for mixing.
The extreme benefit gained by the device for mixing according to the invention derives above all in the improved adaptation of the size of the light spot to the area of the image. The drastic reduction in output achieved in this way follows from the quadratic dependence of the size of the light spot on the edge length to be illuminated.
However, it has also turned out unexpectedly that the video images generated by means of the device for mixing are of a substantially higher quality than is the case without the device. This results primarily from the fact that the more uniform illumination of the matrix carried out by means of the invention also permits a better reproduction of the image contents.
In a preferred further development of the invention, the matrix is formed of digitally controllable tilting mirrors. In comparison with the LCD matrixes mentioned above, the benefits of the device for mixing come into play in a particularly advantageous manner precisely in these matrixes by which a defined angle can also be adjusted because the light impinges essentially only from the direction of the device for mixing.
As was already stated, it is advisable in mirror matrixes that a defined angle can be maintained in a particularly simple manner by means of a corresponding geometric construction of the projection device and a suitable geometric arrangement of the device for mixing with respect to the matrix.
The light loss can then be further reduced, according to an advantageous further development of the invention, when coupling-in optics are provided between the light source and the device for mixing. The coupling-in optics ensure that the greatest possible amount of light reaches the device for mixing from the light source. Therefore, it also prevents an avoidable light loss.
In the same way, another further development of the invention advantageously results in a reduction in the required amount of light in that coupling-out optics are provided between the device for mixing and the matrix, wherein the matrix can be illuminated by the coupling-out optics. In particular, the angle for the illumination of the matrix with the digitally controllable tilting mirrors can be adjusted with the coupling-out optics by means of a corresponding geometric configuration in such a way that the highest possible contrast for high-quality images can be achieved.
It is further provided according to a preferred further development of the invention that a field lens is arranged in front of the matrix. By means
Meissner Hans-Juergen
Piehler Eberhard
Pudenz Juergen
Rieche Gerd
Schmidt Dietrich
Carl Zeiss Jena GmbH
Miller John
Reed Smith LLP
Yenke Brian
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