Holographic projection screen and method of production

Details Holographic projection screen and method of production Holographic projection screen and method of production

1996-02-15

2001-09-11

Spyrou, Cassandra (Department: 2872)

Optical: systems and elements

Holographic system or element

Having multiple object beam or diffuse object illumination

C359S030000

Reexamination Certificate

active

06288805

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a holographic projection screen and its method of production.
A projection system consists of an emitting image of small size, which is magnified by suitable optics and projected onto a screen. The emitting image is generally an active or non-active transmitting element illuminated by a collimated light source.
When the projection is so-called front projection, the observer is placed on the same side as the projector in relation to the screen, which then reflects the image towards the observer.
When the projector illuminates the rear of the screen in relation to the observer, it is a back-projecting screen.
Given the low brightness of the images coming from the projector, it is preferable for the screen to be directional and to have gain so that a large part of the light emitted by the screen reaches the observer's eye.
Moreover, in the case of front-projection screens, it may be advantageous for reasons of convenience to be able to position the projector, not in the center in relation to the projection screen, but to the sides or above or below. This type of projector operation is usually obtained by off-centering the emitting image in relation to the optical axis of the projection system.
However, in order for the light scattered by the screen to reach the observer, it would be necessary in this case for the screen with gain to possess a luminance indicatrix off-axis in relation to the direction of the reflected mid-ray. Thus, as is shown in
FIG. 1
, an incident beam F
1
(projection beam) is reflected off the screen SC, in a conventional manner, along a direction F
2
. In order for it to reach the observer, it is necessary for it to be directed along a lobe direction F
3
.
Such a screen, being on principle outside the laws of geometrical reflection, cannot be produced in a conventional structure. However, it is known how to produce, in conventional optics, screens which, in transmission, bring about these scattering functions with “off-axis” gain. These are complex structures of the Fresnel-lens type. As a consequence, standard front-projection systems use screens either with very low gain (movie theater), or of the Lambertian type.
Moreover, the low contrast of the projected image, once the ambient illumination increases, should be noted as a major drawback of front-projection devices. This is connected with the very nature of the screen, of the white-surface type, which rescatters a large part of the incident light irrespective of its source.
Devices of the “back-projection” type, the scattering screen of which possesses a “black matrix” which enables the contrast of the image to be substantially increased for the same ambient illumination, are thus generally preferred.
In order to illustrate the state of the art, the following document describing various projection devices is cited:
M. KAWASHIMA et al, “Display and projection devices for HDTV”, IEEE Transactions on Consumer Electronics, Vol. 34, No. 1, February 1988, 100-110.
SUMMARY OF THE INVENTION
The subject of the invention is a screen produced by a holographic method more specifically suited to front projection and making it possible to provide the various desired characteristics illustrated in FIG.
2
.
The quantity of light which reaches the observer's eye depends:
on the direction of the mid-ray coming from the image to be projected, reflected by the screen towards the observer;
on the lobe of light scattering around the mid-ray.
The main characteristics desired are:
re-direction, at each point of the screen, of the mid-ray coming from the projection direction into the objective of the observer (horizontal direction, for example): off-axis operation. The orders of magnitude of the angles of incidence on the screen are specified in FIG.
2
. They can be varied from one end of the screen to the other;
scattering of the light around the mid-ray;
screen gain: anisotropic scattering lobe, wider in the horizontal than in the vertical, typically plus or minus 15° in the vertical for plus or minus 30° to plus or minus 45° in the horizontal, depending on the applications;
production of these functions for the three primary colors, red, green and blue, without color shift;
minimum scattering of the ambient light towards the observer in order to obtain better contrast.
The invention therefore relates to a holographic projection screen which includes at least one layer made of photosensitive material in which at least one index grating may be recorded by interference of an optical recording wave with an optical object wave, the thickness of the layer made of photosensitive material being greater than or equal to n
0
&Lgr;
2
/2&pgr;&lgr;
n
0
being the average index of the photosensitive material;
&Lgr; being the average spacing of the index grating;
&lgr; being the recording wavelength.
The invention also relates to a method for producing a holographic screen, characterized in that:
at least one layer of a photosensitive material is produced, enabling at least one layered index grating to be recorded by interference of an object wave with a recording wave, the thickness of this layer being greater than or equal to n
0
&Lgr;
2
/2&pgr;&lgr;
n
0
being the average index of the photosensitive material;
&Lgr; being the average spacing of the index grating;
&lgr; being the recording wavelength;
the layered index grating is recorded by illuminating the layer with the aid of an object wave making a first angle of incidence with the plane of the layer and of a recording wave making a second angle of incidence with the plane of the layer.


REFERENCES:
patent: 3851948 (1974-12-01), Gange et al.
patent: 5046793 (1991-09-01), Hockley et al.
patent: 5187597 (1993-02-01), Kato et al.
patent: 5856048 (1999-01-01), Tahara et al.
T.A. Shankoff, Phase Holograms in Dichromated Gelatin, Applied Optics, vol. 7, pp. 2101-2105, Oct. 1968.*
P. Hariharan, Criteria for thin holograms and volume holograms, Optical Holography (Cambridge Studies in Modern Optics: 2), pp. 58-59, Cambridge University Press 1984, ISBN 0 521 31163.

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