Optical: systems and elements – Projection screen – Unitary sheet comprising plural refracting areas
Reexamination Certificate
2000-03-30
2001-10-16
Mahoney, Christopher E. (Department: 2851)
Optical: systems and elements
Projection screen
Unitary sheet comprising plural refracting areas
C359S460000
Reexamination Certificate
active
06304378
ABSTRACT:
The invention relates to a projection screen of the type that is illuminated from behind and which has at its front side light dispersing means for receiving beams from an image source with a view to showing an image with substantially homogeneous luminosity over a wide viewing angle. The light dispersing means consist of a number of closely positioned, substantially parallel and in the application position of the screen vertically extending lens elements that have two inclining lens crests, and an intermediate lens.
Such projection screens are used in various apparatuses for generating an image which is visible to the viewers, eg projection of radar images, aeroplane simulators, television, traffic control lights, microfilm readers, video games, video monitors with projected image and for the projection of movies through rear projections. In such apparatuses a light source, placed behind the screen, projects light forwards along a projection axis towards 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 wide dispersion of the light horizontally over a wide angle. This is the case in particular with television sets with a rear-projected screen wherein several viewers are sitting in front of the screen at the horizontal level over a relatively wide angle relative to the screen.
One of the problems encountered 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 is to the projection axis. Colour video devices with rear projection screens normally use three cathode ray tubes, viz one tube for each of the primary colours, ie 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 usually positioned centrally on the projection axis, while the red and the blue cathode ray tubes are arranged with their optical axes at an angle of from five to ten 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 in that, in case the luminosities of the three colours are normalised at the centre 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.
Furthermore, when rear projected 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. Various masking technologies have been suggested for the reduction of light reflection, wherein a black, non-reflective sheet has been inserted between the lenses, or the entire front of the screen without black stripes has been dulled.
Various rear-projected screens have previously been suggested for the purpose of increasing the viewing angle in the horizontal plane. U.S. Pat. Nos 4,418,986, 4,469,402, 5,428,476, and 4,509,822 disclose such a system where a screen is used 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 configured with a dispersion lens with vertical, continuous ribs/tops for distributing the light over a specified horizontal viewing angle. According to the well-known technique, the front side of the screen is in essence divided into two lens types; a cylindrical lens for dispersal of the light for a narrow, forwardly directed field of vision, and a total-reflecting lens for further increasing the viewing angle.
Among experts it is well known that it is technically difficult to accomplish a homogeneous light dispersion when using the well-known lens constructions since the well-known systems are constructed with one or two lenses —one lens that transmits the light within a narrow horizontal viewing area, eg +/−25°, and a lens with inner total-reflection that spreads the light from +/−25-30° up to +/−60°, respectively. It applies to both lens types that it is technically difficult to accomplish sufficient overlapping between the two lens types, and especially when the rear projection screen is used for video projection where usually three projection cathode ray tubes are used—one cathode tube for each fundamental colour, and where the optical axes of these usually have a mutual angle of 7-12°.
If eg the green image is projected at right angles on the rear side of the screen, the blue and red images shall be projected against the rear side of the screen with an inclination of eg the mentioned 7° relative to the normal of the screen. However, this may have the effect that a person watching the screen diagonally from the front will see an image which is dominated by either blue or red—depending on whether the relevant person is closest to the optical axis of the projector emitting the blue image or to the projector emitting the red image. In the following, this colour distortion will be designated ‘colour-shading’.
The rear projection screen according to the present invention is characterized in that two lens crests are used whose axes of symmetry incline toward each other or away from each other, respectively, and whose axes of symmetry further form an angle relative to the normal to the plane of the screen, and wherein a further lens has been inserted between two said lenses, the axis of symmetry of which is normal to the plane of the screen. The use of a plurality of lenses that are approximately identical, but having different inclinations, provides a more homogenous luminosity.
Thus, the light coming from behind will be reflected one hundred percent when it hits the inner side of two symmetrical lens crests since the precondition for the total reflection is present. A light beam that hits the inner side of the left lens part in this way will be radiated through the lens top. In this way, through the differences in lens inclinations in combination with the inserted, transmitted side lenses at the foot of the inclining lens crests, it is accomplished that the light is dispersed across a wider fan despite the lenses being approximately identical. It also follows that the colour-shading explained above will be neutralised as a consequence of the total-reflected light radiated through the reflecting lenses being spread in a arc of an angle which is approximately 10-30 percent wider than the angle which the optical axes of the light source form in combination.
A viewer viewing the image diagonally from the front will thus perceive three merged images since the lens crests and the transmitted lenses will deflect the light in such a way that beams from the three light sources will be perceived as being approximately parallel. At the same time the advantage will be accomplished which is associated with lenses with one hundred percent reflecting sides; viz the image can be viewed under a wide side-viewing angle. In order to ensure that also the intermediate lenses, where the forwardly directed light is radiated through the tops are blended in such a way that the colour-shading problem is solved, the lenses are made with different curvatures. This means that the relation between the volume of light that is radiated through the asymmetrical lens tops may vary within wide ranges in accordance with the severity of the colour shading problem in the particular appliance.
REFERENCES:
patent: 3578841 (1971-05-01), Elmer
patent: 4418986 (1983-12-01), Yata et al.
patent: 4468092 (1984-08-01), Inoue et al.
patent: 4469402 (1984-09-01), Yata
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Mahoney Christopher E.
Scan Vision Screen APS
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