Optical: systems and elements – Projection screen – Unitary sheet comprising plural refracting areas
Patent
1998-12-09
2000-08-01
Metjahic, Safet
Optical: systems and elements
Projection screen
Unitary sheet comprising plural refracting areas
G03B 2160
Patent
active
060975393
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a rear projection screen of the kind that is illuminated from behind and that has at the front side light dispersing mediums for reception of light beams from a image source with a view to showing a image with mainly homogeneous luminosity over a wide viewing angle. The light dispersing elements consist of a number of tightly positioned, substantially parallel and in the application position of the screen vertically running lens elements, that have two triangular lenses, an intermediate cylinder lens and a lens adjacent to the triangular lenses.
Such projection screens are applied in various devices for producing a visible image to the viewer, e.g. projection of radar images, aeroplane simulators, television, traffic control lights, microfilm readers, video games, video monitors with projected image and for projection of movies through rear projection. In such devices a light source, placed behind the screen, projects light forward along a projection axis against the screen in preparation for generating, at the level of the screen, an image which is spread to all viewers in front of the screen.
When a large number of viewers are present they will normally spread horizontally, and thus it is desirable to have a broad dispersion of the light horizontally over a wide angle. This is especially the case with television sets with a rear projected screen, where there a large number of viewers sitting in front of the screen at the horizontal level over a relatively wide angle relative to the screen.
One of the problems you have to cope with, in connection with rear projection systems, is that most of the light is projected along the projection axis, which means that the intensity of the image increases, the closer the viewer sits to the projection axis. Colour video devices with rear projection screens normally apply three cathode ray tubes, that is to say one tube for each of the primary colours, i.e. red, green, and blue, which tubes project the image to the screen through their own projection lens. In a conventional horizontal arrangement of the cathode ray tubes the green tube is normally positioned centrally on the projection axis, while the red and the blue cathode ray tube are placed with their optical axes at an angle of 5 to 10 degrees with the projection axis of the green tube. Unless the screen compensates for these displaced positionings, a phenomenon called colour shifting will occur. This phenomenon expresses itself through the fact that, if the luminosities of the three colours is normalized at the center of the viewer group, the luminosity relationship varies with the angular position in the horizontal plane all over the viewing angle. This implies that a viewer's perception of the image depends on his place in the horizontal plane in front of the screen.
Add to this that, when rear projected projection screens are used in and exposed to ambient light, the contrast of the projected image is affected by the light reflection on the front side of the screen. Thus, it is desirable to reduce the reflection of ambient light from the front side of the screen. For reduction of the light reflection various masking technologies have been suggested, where a black, non-reflective sheet has been inserted between the lenses, or all the front of the screen without black stripes has been mattered.
Various rear projected screens have been suggested for the purpose of increasing the viewing angle in the horizontal plane. U.S. Pat. Nos. 4,418,986 and 4,509,822 describe such a system, where a screen is applied which has a rear sheet like a Fresnel lens, which is able to collimate the beams from the image source in parallel beams, and a front sheet, which is constructed with a dispersion lens with vertical continuous ribs/vertices for spreading the light over a specified horizontal viewing angle. According to the well-known technique, the front side of the screen is substantially divided into two lens types, a cylindrical lens for dispersal of the light for a narrow forward directed visual fiel
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Mahoney Christopher E.
Metjahic Safet
Scan Vision Screen APS
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