Optical: systems and elements – Projection screen – Embedded particles
Reexamination Certificate
2002-04-17
2004-12-07
Mahoney, Christopher E (Department: 2851)
Optical: systems and elements
Projection screen
Embedded particles
C359S455000, C359S456000
Reexamination Certificate
active
06829086
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the projection field, and more specifically concerns screens used both for front and for rear projection. Front projection is the projection of an image onto one side of a screen which by convention will be called hereinafter the front of the screen, for displaying images on the front of a screen. In the current state of technology, this type of projection is done in a darkened room, the conventional example being projection onto glass bead cinema screens.
Rear projection is projection of the picture onto one side of a screen which will be called by convention hereinafter the back of the screen for displaying images on the other side of the screen, which by convention will be called the front of the screen. Such screens are notably used for large scale projection or so-called picture walls; these screens, when they have sufficient contrast, are used in a normally lit theatre. As the projector, conventional analog projectors such as those of the 3-tube type can be used or one can also use, as is done in the equipment currently marketed by the applicant, digital devices such as the digital Micro-Mirror devices sold by Texas Instruments known as the DMD. Rear projection screens can also be used in other applications, for example as a screen for filtering a collimated or slightly divergent light, i.e. with an angle of divergence less than or of the order of 20°. Such screens can be used in signs for highways, or as directional filters on cathode ray tubes.
The ideal properties of a rear projection screen are as follows:
good luminescence or transmitivitty, in other words an ability to transmit light forwardly of the screen so that projected pictures are effectively displayed to the public, and that they are not at all or only slightly reflected back to the projector or absorbed by the screen;
high light absorption in the front to back direction so that ambient light is not reflected towards the public at the same time as the light projected from the back;
good resolution, in other words the ability to distinguish two projected points that are close to each other; controlled directivity, in other words the possibility of controlling the solid angle within which the rays passing through the screen are delivered; from this point of view, one can generally define a gain for the screen by comparing its characteristics with those of a diffusing reflector screen formed from a magnesium oxide layer on a backing support.
The ideal properties of a projection screen are substantially the same:
an ability to reflect light projected onto the front of a screen towards the public so that projected pictures are effectively reflected towards the public and are not or only slightly absorbed by the screen;
good resolution, in other words the ability to distinguish two projected points that are close to each other;
good resolution, in other words the ability to distinguish two projected points that are close to each other; controlled directivity, in other words the possibility of controlling the solid angle within which the rays passing through the screen are delivered; from this point of view, one can generally define a gain for the screen by comparing its characteristics with those of a diffusing reflector screen formed from a magnesium oxide layer on a backing support.
In the current state-of-the-art, projection screens are only used in a darkened hall, and the behaviour of the screen as regards ambient light is not a property that is considered.
Conventionally, the nominal contrast of a rear projection screen is defined as the ratio L0/(l×R) between the light L0 delivered by the screen and the product of the light l incident on the screen and the reflection R of the screen. This definition applies both to projection as well as to rear projection. In the case of rear projection, a reflection that is too high of light in a backward sense decreases the contrast of a projected image and can prevent the screen being used other than in a darkened room; as the rear projector is a box, rear projection tolerates minimal ambient lighting unlike front projection. This obviously creates a problem for applications such as control rooms or outdoor applications such as for example projection in sports stadiums.
Various rear projection screens have been proposed. The oldest and simplest solution is to use a frosted glass screen. A screen formed by a plate of frosted glass with a pitted surface constituting a Lambert frosting, with isotropic light scattering: the transmissivity of such a screen is consequently 50% and its gain is 1. Its front-to-back reflection is of the order of 10% which makes the use of frosted glass difficult under ambient light conditions. The Stewart Film Screen Corporation offers improved frosted glass screens having an oval-type forward gain and non-isotropic forward light scattering. Transmissivity is still around 50% but in the direction of use of the screen, gain is better than unity. Briefly, these frosted glass screens have high resolution but low contrast, which is typically of the order of 10. The company HIP is proposing screens formed from a thin film diffuser on a transparent substrates on which black dots embedded in the diffusing thin-film are deposited; these black dots reduce light reflection and increase screen contrast; nevertheless they also lower transmissivity and lead to information loss. Transmissivity is of the order of 50% and contrast typically between 50 and 100.
It is also known, for television applications, to provide a screen with a lenticular structure. Such screens have a wavy structure in a horizontal direction, unvarying by translation in the vertical direction. The waviness allows spreading of light horizontally thereby increasing viewing angle in this direction. The inclusion of diffusing cores such as diffusing bubbles inside the material has also been proposed to ensure controlled scattering in the vertical direction as well as in the horizontal direction: viewing angle in the vertical direction remains reduced and is in any case linked to bubble concentration; the use of such bubbles decreases screen resolution. Maximum resolution is fairly low in view of the minimum size of the wavy patterns which is of the order of 0.3 mm. With a wavy pattern size of the order of 0.8 to 1 mm, such screens are generally used for video. For high resolution graphic applications, such screens pose local or whole screen moiré patterning problems.
Such a screen is disclosed in European patent application 0,241,986; in this document, a black matrix is deposited between the undulations in order to improve contrast; this black matrix has the disadvantage of absorbing a part of the information. Transmissivity of such screens is of the order of 55% and contrast around 100. Dai Nippon Printing and Philipps offer such screens. At the SIG 99 (Symposium of international display) held at San José, Calif. from May 16 to 20, 1999, Dai Nippon Printing presented a new lenticular screen having a layer absorbing ambient light applied directly to the outer cylindrical surface of the undulations; claimed improvement over the previous product is as follows:
tinted screen
New screen
Optical
54%
57%
transmission
Return R of
11%
6%.
ambient light
Screen contrast and brightness remain pretty average.
J. L. Tedesco et al, Holographic Diffusers for LCD 9-32, mentions, for rear projection applications, the use of a screen formed from a Fresnel lens, a conventional diffuser and a lens matrix. The Fresnel lens forms an image of the lens aperture in a mid portion of the image space. The diffuser provides a limited diffusion of the image in the vertical direction and the lens matrix ensures image spreading in the horizontal sense. At the SIG 99, Sarnoff Corporation presented a new improved black matrix lens-structure screen without however stating how the black matrix had been improved; contrast appears good at but brightness remains fairly average in view of a transmission which at the best is 60%.
Briefly such lens-structure sc
Kenyon & Kenyon
Mahoney Christopher E
Synelec Telecom Multimedia
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