Optical: systems and elements – Projection screen – With reflector or additional screen
Reexamination Certificate
1999-07-14
2001-08-28
Mahoney, Christopher E. (Department: 2851)
Optical: systems and elements
Projection screen
With reflector or additional screen
Reexamination Certificate
active
06282023
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a front projection screen having a front side and a rear side and, viewed from the front side, successively comprising a louvre screen and a reflecting or back-scattering screen surface.
A projection screen of the type described in the opening paragraph is known from, for example, the English-language abstract of Japanese patent application JP-A 62-147444.
The projection screen described in this abstract comprises a louvre screen whose blinds are horizontally oriented so that a part of the ambient light incident from above will be absorbed. The ambient light is in fact incident on the screen from all directions. In this way, at least a part of the ambient light is prevented from reaching the viewers' space. However, the signal light comes from a well-defined direction. Provided that the projector is situated at a suitable location, the signal light will be passed unhindered by the louvre screen. The image contrast is thus enhanced by the measure described in said abstract. Furthermore, the known image projection screen has a scattering layer which is provided on the screen surface. Since the layer is present between the screen surface and the louvre screen, the viewing angle of the screen is determined by the louvre screen.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a projection screen having an even better contrast ratio, also in the presence of ambient light, in combination with a sufficiently large viewing angle.
To this end, a first foil is present at the light-incident side of the louvre screen, which foil has a transparent or scattering effect, dependent on the angle of incidence.
The present invention is based on the recognition that a distinction is to be made between the illumination which is wanted for the screen and the illumination which is unwanted for the screen. Projector light comes from a well-defined direction, namely the direction of the projector, whereas ambient light is incident on the screen from all directions.
The foil used has a discriminating effect on the angle of incidence of the light. Within a given angle range around the normal on the foil, the foil has a light-scattering effect, whereas it is transparent at angles outside this range and does not influence the through-going light.
Such a foil is known per se and is available, for example, under the name of Lumisty of the firm of Sumitomo Chemical Company and is described in the article: “A Novel Polymer Film that Controls Light Transmission” by M. Honda et al. in Progress in Pacific Polymer Science 3, 1994, pp. 159-169.
The foil is available for different angle ranges. A very suitable foil for the application mentioned above is a foil which, when provided on the screen, has an angle range of between, for example −15° and +15° in the horizontal direction, within which range the foil has a scattering effect, whereas it is transparent at angles outside this range. The angle range within which the scattering effect occurs may also be asymmetrical around the normal on the foil.
Moreover, the foil is situated at the light-incident side of the screen so that the ultimate viewing angle for the viewer will correspond to the angle range in which the A foil has a scattering effect.
When ambient light is incident on the foil, the light incident within a given angle with respect to the normal, will be scattered, whereas the rest will be passed without being scattered. At the louvre screen, the ambient light which was scattered by the foil will be passed, whereas the rest, in other words, the light passed unhindered by the foil, will be absorbed. In fact, the louvre screen is also angle-selective. Light at an angle of incidence within a given angle range will be passed, whereas light at an angle of incidence outside this angle range will be obstructed. Subsequently, back-scattering or reflection will take place on the screen surface, dependent on the implementation of this surface.
The screen surface may be implemented, for example, as a reflecting surface. In that case, the ambient light which was scattered by the foil and passed by the louvre screen in the direction of the screen surface will be reflected on the screen surface within the same angles. Consequently, this light will reach the viewers' space via the louvre screen and the foil. The part of the ambient light which is reflected by the screen into the viewers' space thus only relates to the ambient light having an angle of incidence within the angle range in which the foil has a scattering effect.
If the surface has a back-scattering effect, a part of the ambient light will be absorbed by the louvre screen upon return of the light to the exit surface of the screen, because the angle distribution is extended due to the back-scattering.
The angles of incidence of the signal light are within the angle range in which the foil has a scattering effect so that, in the case of a reflecting surface, substantially all the signal light will reach the viewers' space. If the surface is implemented as a back-scattering surface, the signal light which, upon incidence on the screen, has angles of incidence within a given angle range will be scattered through a much larger angle range. Consequently, a part of the signal light will be obstructed by the louvre screen and thus be prevented from reaching the viewers' space. However, since also more ambient light will be obstructed in the case of a back-scattering surface, the contrast ratio of the projected image is substantially not influenced detrimentally.
A preferred embodiment of the front projection screen according to the invention is characterized in that a second foil is present between the louvre screen and the screen surface, which second foil has a transparent or scattering effect, dependent on the angle of incidence.
Due to the presence of the second foil, there is extra scattering in the angle range in which scattering already occurs. This has the advantage of a better image homogeneity within the angle range, which is important for the signal light.
A further embodiment of the front projection screen according to the invention is characterized in that a polarizer is present at the light-incident side of the first foil, said polarizer having a transmission direction which corresponds to the direction of polarization of signal light to be projected.
When the projector supplies polarized light and when the transmission direction of the polarizer corresponds to the direction of polarization of the signal light, the signal light will be passed. Substantially half the ambient light will be obstructed. When using a polarizer only, approximately half of the light incident on the screen reaches the viewers' space again. When using a foil and a louvre screen as described above, the ambient light incident on the screen within a given angle of incidence will reach the viewers' space, independent of the direction of polarization. Now, it is proposed to add a polarizer to the combination of one foil or two foils with a louvre screen. As a result, only the ambient light which is incident on the screen within a given angle of incidence and also has the transmission direction of polarization of the screen will reach the viewers' space. The result is an improved contrast ratio.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 2652852 (1953-01-01), Van Orden
patent: 3248165 (1966-04-01), Marks et al.
patent: 5193015 (1993-03-01), Shanks
patent: 5210641 (1993-05-01), Lewis
patent: 6040941 (2000-03-01), Miwa et al.
patent: 6064521 (2000-05-01), Burke
patent: 6130735 (2000-10-01), Hatanaka et al.
patent: 6166793 (2000-12-01), Hayashi et al.
M. Honda et al., “A Novel Polymer Film that Controls Light Transmission”, Progress in Pacific Polymer Science 3, 1994, pp. 159-169.
Fumitaka Yajima, Seiko Epson Corp., “Reflection Type Screen”, Jul. 1987, Abstract.
Edmund Scientific Tech Library,
Mahoney Christopher E.
U.S. Philips Corporation
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