Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
1999-07-09
2001-01-16
Henry, Jon (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S004000, C359S022000, C359S033000, C359S576000, C349S005000, C349S201000, C353S049000
Reexamination Certificate
active
06175431
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to projection systems and more particularly to a projection system that utilizes one or more holographic optical elements.
DESCRIPTION OF THE RELATED ART
Projection systems operate to display an image or a sequence of images by projecting the image(s) onto a screen. These systems tend to be designed for multiple viewers and usually require the projection screen to have a large viewing area in order to achieve large fields of view at comfortable viewing distances. The projected images vary in size, depending on the particular projection system. For example, projection systems that are used in theaters are able to display projected images that are extremely large, while projection systems that are used in offices for presentations are typically limited to much smaller projected images.
With reference to
FIG. 1
, an exemplary prior art projection system
10
is shown. The projection system
10
includes a projector
12
and a projection screen
14
. The projector and the screen operate to display an input image generated by projecting the input image onto the screen. The projector
12
is comprised of an image generator
16
and projection optics
18
. The image generator
16
contains a light source
20
that provides white illumination light to an input image display panel
22
. As an example, the input image display panel
22
may be a reflective liquid crystal display (LCD) panel which is illuminated by the light from the light source. The display panel generates the input image that is to be projected onto the screen.
The image generator
16
also includes a color filter
24
that is positioned between the light source
20
and the display panel
22
. The color filter
24
operates to pass only a selected portion of the illumination light, with the selected portion having a particular peak wavelength. The color filter is controllable to sequentially transmit tristimulus color lights, i.e., red, blue and green lights. Typically, the color filter is a rotating disc having three transparent regions. These regions contain pigments to filter the illumination light based on wavelength. The rotation of the disc allows alternating colors to be sequentially transmitted through the disc. When light of a particular peak wavelength impinges upon the display panel
22
, an input image corresponding to that particular peak wavelength is displayed on the display panel
22
. The input image is then projected toward the projection screen
14
through the projection optics
18
. This process is repeated for the other two colors. The sequential projection of the input images that correspond to the tristimulus color lights allows the displayed image on the projection screen to appear to be in color. The size of the displayed image on the projection screen is primarily determined by the projection optics
18
and the distance between the projection optics and the projection screen.
The projection optics
18
includes one or more zooming lenses (not shown) to magnify the projected input image from the image generator
16
. These zooming lenses are selectively repositioned within the projection optics to provide specific magnification powers. The projection optics may also include one or more focusing lenses (not shown) to focus the projected image on the projection screen
14
. The projection screen may be configured for “reflective viewing,” i.e., viewing from the side of the projection screen facing the projector
12
, or for “transmissive viewing,” i.e., viewing from the side of the projection screen opposite to the projector.
U.S. Pat. No. 5,737,040 to Ichikawa et al. describes a projection system that utilizes a passive hologram array to function as a color filter. The Ichikawa et al. projection system includes a transmissive LCD which receives illumination light from a light source as backlight and which projects an input image in a direction away from the light source. The passive hologram array of the Ichikawa et al. projection system is a single panel that contains a number of micro-holograms. Each micro-hologram operates to disperse incident illumination light based on wavelength, such that each color that is filtered from the illumination light enters a predetermined liquid crystal cell of the LCD. The input image displayed on the LCD is then magnified and projected onto a projection screen. The passive hologram array increases the intensities of the color lights that are transmitted to the LCD from the light source, which consequently increases the brightness of the projected image displayed on the projection screen.
While the prior art projection systems operate well for their intended purposes, what is needed is an efficient projection system having a compact optical configuration and having sophisticated display functions that are enabled by holographic optical elements.
SUMMARY OF THE INVENTION
A projection system and a method of displaying a projected input image on a projection screen of the system utilize one or more reconfigurable holographic optical elements (HOEs) to manipulate propagating light in the system. The reconfigurable HOEs may be designed to perform simple optical functions that are commonly associated with traditional optical devices, such as those performed by lenses, prisms and mirrors. However, the reconfigurable HOEs are also designed to perform sophisticated optical manipulations, such as varying the light intensity with respect to a specific direction.
Each reconfigurable HOE includes a hologram that is sandwiched between two electrode layers. The hologram is a holographic photo-polymeric film that has been combined with liquid crystal. The presence of the liquid crystal allows the hologram to exhibit optical characteristics that are dependent on an applied electrical field. Preferably, the hologram is a Bragg-type hologram, having a high diffraction efficiency. The electrode layers may be made of Indium Tin Oxide (ITO), which typically has a transmission efficiency of greater than 80%.
The reconfigurable HOE has at least two optical operating states, a diffractive state and a passive state. The diffractive properties of the reconfigurable HOE primarily depend on the recorded holographic fringes in the photopolymeric film. In the diffractive state, the reconfigurable HOE diffracts propagating light in a predefined manner. In the passive state, the reconfigurable HOE does not optically alter the propagating light. Initially, the hologram of the reconfigurable HOE is in the diffractive state, such that received light is diffracted in the predefined manner. However, when an electrical field is generated in the hologram by applying voltage to the electrode layers of the reconfigurable HOE, the operating state of the hologram switches from the diffractive state to the passive state.
In a first embodiment of the invention, the projection system includes a color filter that contains a single reconfigurable HOE stack and includes projection optics having a number of reconfigurable HOE stacks. The term “reconfigurable HOE stack” is herein defined as a set of at least three reconfigurable HOEs that are designed to collectively operate on the propagating tristimulus color lights. Each reconfigurable HOE of the stack in the color filter is holographically configured to diffract a particular tristimulus color light of a polychromatic illumination light when in the diffractive state. In operation, the reconfigurable HOEs of the color filter are sequentially placed in the diffractive state to sequentially diffract the tristimulus color lights to a conventional display panel of the system. The display panel generates an input image that is projected through the projection optics for display on a projection screen of the system.
The reconfigurable HOE stacks of the projection optics are holographically configured to optically manipulate the projected image so that the displayed image on the projection screen is enlarged by a selectable magnification power. Each reconfigurable HOE stack is configured to magnify the projected
Popovich Milan M.
Storey John J.
Waldern Jonathan D.
DigiLens Inc.
Henry Jon
Law Offices of Terry McHugh
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