Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2000-01-13
2001-10-09
Henry, Jon (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S004000, C349S201000, C348S343000, C348S832000, C353S070000, C353S080000
Reexamination Certificate
active
06301027
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to projection systems and more particularly to a projection system having 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 a color image formed by projecting three monochromatic image components of the color 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
. The display panel would typically be based on reflective LDC or MEMs display technology. The display panel generates the image components that are 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 designed 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 component corresponding to that particular peak wavelength is displayed on the display panel
22
. The image component 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 image components that correspond to the tristimulus color lights allows the displayed image on the projection screen to appear to be in color, due to fusion of the three image components within the eye integration time. 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 image components 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 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.
A concern with conventional projection systems, such as the projection system
10
, is that a large housing is required to incorporate the components of a complete projection system into an integrated device. This is partly due to the fact that the image generator and the projection screen need to be separated by a significant distance, increasing the minimum depth of the housing.
Therefore, what is need is an efficient projection system having a compact optical configuration that can display color images of high quality.
SUMMARY OF THE INVENTION
A projection monitor and a method of displaying an output image on a viewing screen of the monitor utilize a number of reconfigurable holographic optical elements (HOEs), allowing the components of the monitor to be physically arranged in a compact configuration. 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 and/or generating virtual (holographic) images. Preferably, the compact configuration of the monitor's components minimizes the required depth of the monitor, making the monitor ideally suited for desktop use.
Each reconfigurable HOE of the monitor includes a hologram that is sandwiched between two electrode layers. The hologram is a holographic photopolymeric 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 monitor includes an image generator, a mirror, a holographic reflector and a viewing screen that are collectively contained within a housing. The projection monitor further includes two conventional optical devices whose main function is to assist in the image transfer but which are not of key significance to the understanding of the invention. Preferably, the depth of the housing is approximately half the height of the viewing screen. The image generator of the monitor is configured to sequentially generate and project three monochromatic image components for each color image to be displayed on the viewing screen. The composite color image is formed on the viewing screen by sequentially displaying the three projected monochromatic images that correspond to the tristimulus colors, i.e., red, blue and green. With a sufficiently fast display rate, the sequentially displayed monochromatic images form a single image that will appear to be in color, due to fusion of the three image components within the eye integration time.
The mirror and the holographic reflector operate to direct the projected image components from the image generator to the viewing screen. The mirror is positioned in the line of sight of the image generator and is positioned to reflect the image components to the holographic reflector. The holographic reflector is positioned to receive the image components from the mirror and to reflect the image components to the viewing screen. The optical devices of the monitor are situated in the path of the propagating image components to correct off-axis aberrations that are caused by the physical arrangement of
DigiLens Inc.
Henry Jon
Law Offices of Terry McHugh
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