Television – Video display – Projection device
Reexamination Certificate
1999-07-12
2003-10-28
Hsia, Sherrie (Department: 2614)
Television
Video display
Projection device
C348S756000, C348S779000, C348S040000, C348S041000, C345S006000, C345S419000, C359S455000
Reexamination Certificate
active
06639631
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates generally to the field of projection television receivers, and in particular to projection television receivers having screens providing improved visibility at wide horizontal viewing angles. A holographic screen is provided with a high gain, resulting in a substantially higher brightness when viewed at an angle normal to the screen than when viewed at an angle displaced from normal in a horizontal plane. This characteristic generally is considered undesirable in a projection television; however by employing a high gain holographic screen, the brightness is acceptable for viewing at a wide viewing angle, and can exceed the brightness of a conventional screen out to ±50° from normal.
2. Background Information
Design of a projection television system involves many choices of design criteria that affect color shift and various other brightness characteristics.
Projection television screens need at least three image projectors to form respective images of different colors, for example, red, blue and green. A projection screen receives images from the three projectors on a first side and displays the images on a second side with controlled light dispersion of all the displayed images. One of the projectors, usually green- and usually in the center of an array of projectors, has a first optical path in a substantially orthogonal orientation with the screen. At least two of the projectors, usually red and blue and usually positioned on opposite sides of the central green projector in the array, have respective optical paths converging toward the first optical path in a non orthogonal orientation relative to the screen, defining angles of incidence. As a result of this positioning scheme for the projectors, the image appearing on the screen is color shifted when viewed from different angles and the image is brighter at the center of the screen than at the edges of the screen or exhibits non-uniform brightness. It would be advantageous to reduce color shift and to improve brightness uniformity in projection screen television systems.
Color shift is defined as the change in the red/blue or green/blue ratio of a white image formed at the center of a projection screen by projected images from red, green and blue projection tubes, when viewed at different angles in the horizontal plane, by observations made at the peak brightness vertical viewing angle.
The color shift problem is caused by the need for at least three image projectors for respective images of different colors, for example, red, blue and green. A projection screen receives images from the at least three projectors on a first side and displays the images on a second side with controlled light dispersion of all the displayed images. One of the projectors, usually green and usually in the center of an array of projectors, has a first optical path in a substantially orthogonal orientation with the screen. At least two of the projectors, usually red and blue and usually positioned on opposite sides of the central green projector in the array, have respective optical paths converging toward the first optical path in a non orthogonal orientation defining angles of incidence. Color shift results from the non orthogonal relationship of the red and blue projectors, relative to the screen and to the green projector. As a result of the color shift, color tones may differ at every position on the screen. The condition in which the color tone difference is large is often referred to as poor white uniformity. The smaller the color shift, the better the white uniformity.
Color shift is denoted by a scale of numbers, in which lower numbers indicate less color shift and better white uniformity. In accordance with a common procedure, values for the red, green and blue luminance are measured at the screen center from a variety of horizontal viewing angles, typically from at least about −40° to +40°, to as much as about −60° to +60°, in 5° or 10° increments. The positive and negative angles represent horizontal viewing angles to the right and left of screen center, respectively. These measurements are taken at the peak vertical viewing angle. The red, green and blue data is normalized to unity at 0°. One or both of the following equations (I) and (II) are evaluated at each angle:
C
(
Θ
)
=
20
·
log
10
(
red
(
Θ
)
blue
(
Θ
)
)
;
(
I
)
C
(
Θ
)
=
20
·
log
10
(
green
(
Θ
)
blue
(
Θ
)
)
(
II
)
where &thgr; is any angle within a range horizontal viewing angles, C(&thgr;) is the color shift at angle &thgr;, red(&thgr;) is the red luminance level at angle &thgr;, blue(&thgr;) is the blue luminance level at angle &thgr; and green(&thgr;) is the green luminance level at angle &thgr;. The maximum of these values is the color shift of the screen.
In general, color shift-should be no larger than 5, nominally, on any commercially acceptable screen design. Other engineering and design constraints may sometimes require that the color shift be somewhat higher than 5, although such color shift performance is not desirable and usually results in a perceptibly inferior picture with poor white uniformity.
Screens for projection television receivers are generally manufactured by an extrusion process utilizing one or more patterned rollers to shape the surface of a thermoplastic sheet material. The configuration is generally an array of lenticular elements, also referred to as lenticules and lenslets. The lenticular elements may be formed on one or both sides of the same sheet material or on one side only of different sheets which can then be permanently combined as a laminated unit or otherwise mounted adjacent to one another so as to function as a laminated unit. In many designs, one of the surfaces of the screen is configured as a Fresnel lens to provide light diffusion. Prior art efforts to reduce color shift and improve white uniformity have focused exclusively on two aspects of the screen. One aspect is the shape and disposition of the lenticular elements. The other aspect is the extent to which the screen material, or portions thereof, are doped with light diffusing particles to control light diffusion. These efforts are exemplified by the following patent documents.
In U.S. Pat. No. 4,432,010 and U.S. Pat. No. 4,536,056, a projection screen includes a light-transmitting lenticular sheet having an input surface and an exit surface. The input surface is characterized by horizontally diffusing lenticular profiles having a ratio of a lenticulated depth Xv to a close-axis-curvature radius R
1
(Xv/R
1
) which is within the range of 0.5 to 1.8. The profiles are elongated along the optical axis and form aspherical input lenticular lenses.
The use of a screen with a double sided lenticular lens is common. Such a screen has cylindrical entrance lenticular elements on an entrance surface of the screen, cylindrical lenticular elements formed on an exit surface of the screen and a light absorbing layer formed at the light non convergent part of the exit surface. The entrance and the exit lenticular elements each have the shape of a circle, ellipse or hyperbola represented by the following equation (III):
Z
(
x
)
=
Cx
2
1
+
[
1
-
(
K
+
1
)
C
2
x
2
]
1
2
(
III
)
wherein C is a main curvature and K is a conic constant.
Alternatively, the lenslets have a curve to which a term with a higher order than 2nd order has been added.
In screens making use of such a double sided lenticular lens, it has been proposed to specify the position relationship between the entrance lens and exit lens, or the lenticular elements forming the lenses. It has been taught, for example in U.S. Pat. No. 4,443,814, to position the entrance lens and exit lens in such a way that the lens surface of one lens is present at the focal point of the other lens. It has also been taught, for example in JP 58-59436, that the eccentricity of the entrance lens be substantially equal to a reciprocal of the refractive
Hall, Jr. Estill Thone
O'Donnell Eugene Murphy
Pfile Wendy Rene
Fried Harvey D.
Hsia Sherrie
Laks Joseph J.
Thomson Licensing S.A.
Tripoli Joseph S.
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