Rear projector

Details Rear projector Rear projector

1998-08-26

2001-05-15

Eisenzopf, Reinhard (Department: 2614)

Television

Video display

Projection device

C348S744000, C348S756000, C348S751000, C348S782000, C348S804000, C348S787000, C353S074000, C353S077000, C353S078000, C353S079000, C359S453000, C359S456000, C359S460000, C359S196100

Reexamination Certificate

active

06233024

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a rear projector with a housing on whose front side is arranged a screen for showing a video picture, with a brightness-modulated and color-modulated R-G-B light source for emitting a light bundle, with a deflecting device for scanning this light bundle, wherein the light bundle is deflected by the deflecting device over a total angle &agr; with respect to vertical scanning, and with a deflecting mirror which is located in the housing and arranged at an angle &dgr;, wherein the light bundle exiting from a virtual vertex of the total deflection angle &agr; is deflected on the screen by the deflecting mirror.
b) Description of the Related Art
Rear projectors of this kind are known from the German DE 43 24 849 C2. The deflecting mirror known from this reference is intended to reduce the construction depth. The reasoning behind this is as follows: Due to the laws governing geometrical optics, a determined optical path is predetermined by the image size. Particularly in a rear projector according to DE 43 29 849 C2, image generation relies on angular deflections of light bundles. Therefore, a magnification of the image is always tied to an increase in the dimensions of the device. This means that the construction depth that can be achieved is always limited for a given image size.
In order to provide small housing depths, that is, the flattest rear projectors possible, the optical beam path is folded via the deflecting mirror. While only one deflecting mirror is used for this purpose in accordance with the reference cited above, devices are also known from DE 31 52 020 A1 and U.S. Pat. No. 4,003,080 in which three deflecting mirrors are used.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to optimize known rear projectors with respect to construction depth.
This object is met in that the virtual or real vertex located in front of the deflecting mirror lies at a location where the smallest angle &bgr; of the light bundle to the surface of the screen during deflection is less than 20° and the angle &dgr; is given by &dgr;≦45°−&agr;/4+&bgr; wherein angle &dgr; is determined for curved mirror surfaces between the projection surface and the chord between the extreme points of incidence of the light bundle on the deflecting mirror during vertical scanning over the angle &agr;.
Based on this teaching, the construction depth of a rear projector can be optimized at any deflection angle &agr; contingent upon the device. In particular, it has turned out that the deflecting mirror which lies closest to the screen with reference to the light propagation makes the greatest contribution for such optimization. If only one deflecting mirror is used, the light bundle reaches the latter directly after it has exited from the deflecting device. In this case, the above-mentioned vertex is a real point and is located at the point of deflection in the deflecting device or in the exit pupil of optics arranged subsequent thereto.
If a plurality of deflecting mirrors are used, the vertex to be considered in this case in front of the last deflecting mirror with respect to the light path is a mirror image and is therefore virtual. Regardless of whether the vertex is virtual or real, the same principle is applicable.
In order to provide a compact construction, the deflecting mirror should be as close as possible to the projection screen. The limits to be adhered to for optimization are substantially dependent on the size of the screen and on the dimensions of the rest of the housing. Surprisingly, however, it has turned out that the desired behavior can be described, independent from the construction shape in other respects, by the smallest possible angle &bgr; of the light bundle relative to the vertical line. Accordingly, it is maintained that this angle should be less than 20°. It could have been assumed that the angle should be as small as possible, that is, virtually 0°. However, an angle other than 0° but especially less than 5°, 10° or 20°, in a particularly compact rear projector also allows for sufficient space for operating controls and loudspeakers at the front. Given a selection of suitable conditions where other projection conditions remain similar, an enlargement of the angle &bgr; leads to smaller angle &dgr; of the deflecting mirror relative to the screen which plays a substantial part in determining the achievable reduction in the depth of the housing. With respect to &dgr;, it has turned out to be especially advantageous when this angle is selected according to the inequality indicated above.
The teaching is applicable for plane mirrors as well as for curved mirror surfaces. However, while the meaning of angle &dgr; is uniquely defined for plane mirrors, a corresponding condition must be found for curved mirror surfaces.
When it is considered that the angle &dgr; substantially defines the depth for the housing of the rear projector based on the position of the edges of the deflecting mirror, it can be seen that the angle &dgr; in the case of curved mirror surfaces has a similar meaning for plane mirrors when it is determined as the angle between the projection surface and the chord between the projection surface and the extreme points of incidence of the light bundle during vertical scanning over angle &agr;. In this case, one of the extreme points of incidence is given at deflection angle 0° and the other is given at the total deflection angle &agr;.
A more compact video device can be provided, according to an advantageous further development of the invention, in that the vertex is virtual and is generated by additional deflecting mirrors in the rear projector, that is, when the optical path is folded by additional mirrors.
It can be seen from the above-mentioned inequality for angle &dgr; that the choice of this angle is relatively open. Further, any curved mirror surfaces can be used for deflecting mirrors. At very small angle &dgr; or large curvatures, however, disadvantages can be expected for video projection in that geometric imaging errors occur such as those known in the case of oblique projection of slides. In particular, trapezoidal images are generated in this case.
These geometry errors should be avoided as far as possible. In view of this requirement, the selection of the angle &dgr; would ultimately be restricted further. On the other hand, it would be desirable to keep the angle &dgr; very small in the interest of the smallest possible construction depth without having to allow for any kind of restriction of &dgr;.
In order to be able to keep the construction depth as small as possible, therefore, according to an advantageous further development of the invention, a computing device is provided for recalculating the video picture prior to projection with respect to distortion due to oblique projection or curvature of the deflecting mirror and a control device is provided which controls the deflection and/or intensity modulation of the light bundle, so that there appears on the screen in the image direction a virtually undistorted video picture which diverges from an equal distribution of lines by less than 30%, particularly less than 10%, with respect to line spacing.
Accordingly, any mirror shapes and very small angles of &dgr; can be used in that the image for image generation is distorted in an opposite sense with respect to the geometric errors resulting from the construction, so that after the geometrically distorting deflection a virtually undistorted image is formed again on the projection screen. The limit for permissible image errors is essentially subjective. In this regard, tests have shown that video pictures in which line spacing deviates from an equal distribution in the range of 30%, especially less than 10%, are still perceived as acceptable by viewers.
Image distortions of this kind for compensation of image errors are known. In general, this compensation can be achieved by calculating the image displayed in a distorted manner by defle

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