WO2001002912A1

WO2001002912A1 - System for generating an image

Info

Publication number: WO2001002912A1
Application number: PCT/US2000/010356
Authority: WO
Inventors: Milan M. Popovich
Original assignee: Digilens Inc.
Priority date: 1999-07-01
Filing date: 2000-04-18
Publication date: 2001-01-11
Also published as: AU4249100A

Abstract

A system for generating an image including an illumination system comprising a light source and a plurality of lasers (32, 34, 36) to produce a plurality of laser beams. The system further includes a signal generator operable to modulate the plurality of beams with image information and a multiplexing device (50) operable to combine the plurality of laser beams into a single laser beam (54). A scanner (60) is positioned to receive and scan the single laser beam to produce an intermediate image. The system also includes an optical diffuser (18) positioned such that the intermediate image is formed thereon and operable to project a resultant image having a viewable angular range greater than a viewable angular range of the intermediate image.

Description

SYSTEM FOR GENERATING AN IMAGE

BACKGROUND OF THE INVENTION

The present invention relates generally to image generating devices, and more particularly, to compact display systems, such as those used in head mounted displays.

Head mounted displays have received considerable attention as a technique for displaying high magnification, large field of view, and high definition virtual images.

The head mounted display generally includes a support member for mounting the display on a head of a user and various optical and display components. The components are arranged to magnify an image displayed on a compact image display panel and to display the magnified image ahead of the user through the optical system. The user typically does not directly observe an image displayed on a monitor or screen, but instead observes a magnified virtual image converted from the image displayed on the display panel. The head mounted display thus provides a compact arrangement for displaying to the user a larger image than displayed on a small microdisplay panel.

Recently, microdisplay devices have been developed that produce a viewable image typically of size 12 mm diagonal, but which can be as small as 4.8 mm x 3.6 mm. This is advantageous in terms of reducing manufacturing cost, however, problems have been encountered in attempting to magnify these small images for comfortable viewing by a user. Typically, such displays have an emission angle of approximately 10 to 20 degrees. For example, if the microdisplay device is based on liquid crystal technology, there is only a narrow angle over which a high contrast image is viewable. An optical system used to magnify the image displayed on the microdisplay must provide a sufficiently large exit pupil. The exit pupil needs to be large enough to allow for rotations and side to side movement of the eye and slippage of the display. Exit pupil size usually has to be traded off against field of view. However, due to the Lagranage invariant of classical optical theory, the size of the exit pupil and field of view must be constrained as follows:

Exit Pupil Dimension x Field of View = Display Dimension x Emission Angle.

Since both the display dimension and the emission angle are small, the viewing geometry is typically limited to an exit pupil of 6 mm diameter and a field of view of 40° with a display diagonal of 12 mm and an emission angle of 20°. If larger fields of view are required, the exit pupil may be too small to be usable for comfortable viewing of the image.

Furthermore, it is often difficult to provide a sufficiently bright image with a microdisplay. In order to produce a multicolor image with a microdisplay that produces only monochrome images, a sequence of images are typically displayed and illuminated sequentially with red, green, and blue lights produced by a light source (e.g., array of light emitting diodes (LEDs)). The projected emission area of the light source on the surface of the microdisplay must generally conform to the size of the microdisplay. The practical problems of coupling conventional light sources to a very small display area can give rise to significant light losses, resulting in a displayed image that is insufficiently bright. There is, therefore, a need for an image generating system, which provides a relatively large field of view and exit pupil and a sufficiently bright image that can be comfortably viewed by a user with a compact wearable display.

SUMMARY OF THE INVENTION

A system for generating an image is disclosed. The system includes an illumination system comprising a plurality of lasers operable to produce a plurality of laser beams. The system further includes a signal generator operable to modulate the plurality of laser beams with image information and a multiplexing device operable to combine the plurality of laser beams into a single laser beam. A scanner is positioned to receive and scan the single laser beam to produce an intermediate image. The system also includes an optical diffuser positioned such that the intermediate image is formed thereon and operable to project a resultant image having a viewable angular range greater than a viewable angular range of the intermediate image.

Th optical diffuser may comprise three holographic optical elements, each having a hologram recorded therein which is optimized to diffract red, green, or blue light.

The image generating system may also include a reflective optical device positioned to transmit the resultant image from the optical diffuser to a user for viewing. The reflective device may also include holographic devices. The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is schematic of an image generating system of the present invention.

Fig. IB is a schematic of a portion of the system of Fig. 1 A showing additional detail.

Fig. 2 is a schematic of three holographic optical elements each optimized to diffract red, green, or blue light.

Fig. 3 is a schematic illustrating optics for projecting a resultant image formed by a diffuser of the image generating system of Fig. 1.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily apparent to 5 those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention.

Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

Referring now to the drawings, and first to Fig. 1, an image generating system, generally indicated at 10, is shown. The system 10 may be used in a head mounted display, handheld display or other compact display devices. The system 10 includes a display system, generally indicated at 14, an optical projection system 16 positioned to receive an image displayed by the display system 14, and an optical diffuser 18. The projection system 16 is configured to form a real, focused intermediate image I having a generally uniform illumination distribution on the diffuser 18. The intermediate image I does not have to be in perfect focus. The intermediate image I may be slightly defocused and distorted such that its aberrations compensate for those of the optical system used to magnify the intermediate image into the final image, as is well known by those skilled in the art of optical system design. The diffuser 18 is configured to allow a resultant image to be projected for viewing by a user. The optical system 16 and diffuser 18 are utilized to overcome constraints imposed by the relatively small

angular range (emission angle α) over which the display system 14 is viewable (as

indicated by arrows 20). The image source is characterized by the width of the beam multiplied by the angle over which it is scanned (i.e., angle a). According to the Lagrange theorem, this corresponds (in viewing space) to the product of the field of view and exit pupil. The theorem further dictates that each of the two products are equal to the size of the intermediate image, h, multiplied by the emission angle θ at the intermediate image plane (Fig. IB). If final viewing optics 90 are used to create an image at infinity with a field of view ψavA an exit pupil of dimension D, then

according to the Lagrange theorem, the following relationship applies:

d x = h x θ = D x ψ

The small emission angle α imposes a significant constraint on the size of the exit pupil

of the viewing system, as well as the field of view. The diffuser 18 increases the angular bandwidth of ray bundles originating at the intermediate image I plane, thereby

increasing the angular range (emission angle θ) over which the image is viewable (as

indicated by arrows 22). This effectively increases the Lagrange invariant of the overall system 10. The size and polar diagram of the resultant image after diffusion by the diffuser 18 are such as to satisfy the Lagrange invariant for the required field of view of the exit pupil for the overall system 10. For example, the optical system 10 may form the resultant image with a one to one magnification of the image displayed by the display system 14. In this case the product of the field of view and exit pupil can be increased by arranging the components so that the diffuser 18 has a diffusion characteristic such that the resultant image has a larger emission angle than that of the light from the input image display. The resultant image may be used as a source image for further optics (shown in Fig. 3 and described below) which project the image to the user's eye or eyes. The display system 14 includes an illumination system comprising red, green, and blue laser sources 32, 34, 36 coupled to a video signal generator 40. Various laser sources may be used, including those which are well known in the art such as an argon laser, diode laser, helium-neon laser, YAG laser, krypton laser, dye laser or other laser sources. Some of the larger lasers may need to be introduced to the display via a fiber optic link. In fully integrated wearable or head mounted displays, miniature diode lasers are preferred. Light emitting diodes, which have reasonably narrow bandwidths, may also be used. Combinations of lasers and light emitting diodes (e.g., red and green lasers and a blue light emitting diode) may also be used. The signal generator modulates the laser beams with image information. The red, green, and blue signals may be derived from a camera or a DVD player, for example. Additional electronic circuitry may be required to condition the video electrical signal to match the input requirements of the modulation device. The modulation may be achieved by controlling the input electrical signal applied to each laser source 32, 34, 36 or by modulating the laser beams 42, 44, 46 using, for example, an acousto-optic modulator

(not shown). The acousto-optic modulator operation is based on the diffraction of light by a column of sound in a suitable interaction medium. An optical multiplexing system 50 combines the individual red, green, and blue laser beams 42, 44, 46 into a single beam 54. The multiplexing system 50 comprises mirrors and prisms arranged to combine the three beams 42, 44, 46 into a single beam 54 containing the red, green, and blue laser beams. The multiplexing system 50 preferably provides good quality optical surfaces and accurate alignment to ensure optimum registration of the red, green, and blue laser beams. The beam 54 is deflected in the x and y directions by a beam scanning device 60 under the control of a controller 62. The scanning device 60 is preferably an electro-mechanical device comprising rotating or vibrating mirrors, for example. The scanning device 60 may also include acousto-optic devices. The beam scanning device 60 forms an image on diffuser projection system 16. The scanner 60 is preferably compact and light weight with low power consumption. Separate scanners are provided for X and Y directions. A vibrating mirror may be used for the horizontal scan direction and a galvanometrically oscillated mirror may be used for the vertical scanning direction, for example. The vibrating mirror may be based on micro-electromechanical (MEMs) technology, for example.

The projection system 16 receives the image displayed by the display system 14 and forms the intermediate image I on the diffuser 18. Projection optics of the projection system 16 focus and magnify the display images so that the images can be properly diffused by the diffuser 18. The projection system 16 may be formed from conventional refractive optical elements, holographic diffractive elements (as described below for the diffuser 18), or a combination thereof. Cylindrical, prismatic, or off-axis aspheric optical components may also be included to correct for geometric aberrations in off-axis optical configurations such as those required to implement wearable displays, as is well known by those skilled in the art. The optical system 16 may also include reflective optical elements (not shown) to fold the optical path to further reduce the size of the image generating system 10.

The diffuser 18 preferably comprises a stack of holographic optical elements 80, 82, 84 (Figs. 1 and 2). The holographic optical elements 80, 82, 84 each include a hologram, which may be a Bragg (thick or volume) hologram or Raman-Nath (thin) hologram. The Bragg holograms provide high diffraction efficiencies for incident beams with wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition and within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition. The hologram is used to control transmitted light beams based on the principles of diffraction. It is to be understood that the holographic optical elements 80, 82, 84 may also be reflective rather than transmissive as shown in Fig. 1 and described above. In the case of a reflective holographic device, the arrangement of the image generating system 10 is modified to utilize reflective properties of the hologram rather than the transmissive properties described herein.

The light that passes through the hologram is diffracted by interference fringes recorded in the hologram to form an image. Depending on the geometry of the fringes recorded, the hologram is able to perform various optical functions which are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations. The hologram may be configured to perform operations such as deflection, focusing, or color filtering of the light, for example. The interference fringes may be created by applying beams of light or may be artificially created by using highly accurate laser writing devices or other replication techniques, as is well known by those skilled in the art. The hologram may be recorded in conventional holographic materials such as photopolymer material supplied by DuPont Holographic Materials, of Wilmington, Delaware.

Each holographic optical element 80, 82, 84 is holographically configured such that only a particular monochromatic light is diffracted by the hologram. The optical element 80 has a hologram which is optimized to diffract red light, the optical element 82 has a hologram which is optimized to diffract green light, and the optical element 84 has a hologram which is optimized to diffract blue light. The elements 80, 82, 84 have a diffusing function recorded therein. The diffusing function may be achieved by introducing diffusion elements (e.g., computer generated holograms) into one or more of the beams during recording of the hologram. Preferably, separate holographic diffusion elements are used for red, green, and blue wavelengths to prevent color distortion. The holograms for the different wavelength bands may also be recorded in a single holographic element, however, this may result in cross-talk between the different colors. The holographic optical elements may be passive since the red, green, and blue images are projected simultaneously rather than sequentially, as described in U.S.

Patent Application Serial No. 09/484,494, which is incorporated herein by reference.

Since the intermediate image I formed on the diffuser 18 is multicolored, there is no need for the holographic diffraction elements of the diffuser to be switchable.

After diffusion by the diffuser 18, light from the resultant image is transmitted to the final viewing optics, generally indicated at 90 (Fig. 3). The optics may include a reflective holographic diffusion device comprising a stack of holographic optical elements 92, 94, 96 which are operable to diffract red, green, and blue wavelengths, respectively. The holograms recorded in the elements 92, 94, 96 are operable to magnify and collimate the resultant image and transmit the image for viewing by a user (as indicated by arrows 126). The diffusion device 90 may also be transmissive.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. An image generating system comprising:

an illumination system comprising a plurality of lasers operable to produce a plurality of laser beams;

a signal generator operable to modulate said plurality of laser beams with image information;

a multiplexing device operable to combine said plurality of laser beams into a single laser beam;

a scanner positioned to receive and scan said single laser beam to produce an intermediate image; and

an optical diffuser positioned such that the intermediate image is formed thereon and operable to project a resultant image wherein a product of a viewable angular range and exit pupil of said resultant image is controlled by a product of size and emission angle of said intermediate image to optimize viewable angular range and exit pupil.

2. The image generating system of claim 1 further comprising a reflective optical device positioned to transmit the resultant image from the optical diffuser to a user for viewing.

3. The image generating system of claim 2 wherein the reflective optical device comprises three holographic optical elements each having a hologram recorded therein which is optimized to diffract red, green, or blue light.

4. The image generating system of claim 1 wherein the optical diffuser comprises a plurality of holographic devices.

5. The image generating system of claim 4 wherein said plurality of holographic devices comprises three holographic optical elements each having a hologram recorded therein which is optimized to diffract red, green, or blue light.

6. The image generating system of claim 1 further comprising a controller operable to deflect said single laser beam in at least two directions.

7. The image generating system of claim 1 wherein the scanner is an acousto- optic device.

8. The image generating system of claim 1 further comprising a transmissive optical device positioned to transmit the resultant image from the optical diffuser to a user for viewing.