GB2161615A

GB2161615A - A holographic display system

Info

Publication number: GB2161615A
Application number: GB08515947A
Authority: GB
Inventors: Nile F Hartman
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1984-07-09
Filing date: 1985-06-24
Publication date: 1986-01-15
Also published as: DE3523032A1; GB8515947D0; GB2161615B; CA1245085A; DE3523032C2; JPS6135416A; US4613200A

Description

1 GB 2 161 615A 1

SPECIFICATION

Holographic display system The present invention relates to a holographic display system.

Systems commonly used in aircraft to display images of instruments in the field of view of a pilot are termed "head-up displays" (HUDs). HUDs generally employ conventional optical elements or holographic elements to project an image of an instrument into the pilot's field of view as the pilot looks through the forward window of the aircraft.

In U.S. Patent 3,737,212 a first system of image projection is indicated in a HUD, wherein 10 the image supplied by a cathode raty tube (CRT) is projected onto a "combining glass" in an aircraft cockpit. The image produced by the CRT is projected by optical elements including a mirror and a lens between the CRT and the combining glass. A second system deqcribed in that patent includes a combining glass, in the form of a spherical diffraction lens, which provides an image to the pilot as a consequence of a CRT image source having a concave parabolic face.

While the second described system sets forth the desirability of providing a holographically recorded diffraction lens, it also emphasizes the necessity of using CRT's with rare earth doped phosphors that emit high energies of the light in narrow wavelength bands, in order to reduce inherent aberrations.

In U.S. Patent 2,418,111 a holographic HUD system is also disclosed. In that system, a 20 complex optical system is used in which includes relay lenses containing tilted and decentered optical elements employed to compensate for the aberration present in the holographic optical element used as a combiner.

There prior art patents illustrate conventional approaches to achieving a visibly perceivable

HUD image against a bright background. However, the use of exotic phosphors to achieve 25 bright narrow-band display illumination or expensive optics to provide image correction for broader-band sources have prevented widespread use of this otherwise desirable display concept.

According to the invention there is provided a holographic display system comprising; a source of radiation generating a visible image; a first reflective hologram positioned to reflectively diffract said image radiation; a second reflective hologram positioned to reflectively reconstruct and direct said image radiation diffractively reflected from said first hologram towards a viewer, whereby said image appears to said viewer as a virtual image of said source of radiation.

Further according to the invention there is provided a holographic display system comprising; 35 an illuminated -source of indicia; a first reflective holographic recording positioned to diffract illumination from said source; a second reflective holographic recording positioned generally parallel to said first recording for receiving said diffracted illumination and providing a reconstructed virtual image of.said indicia.

The present invention is intended to solve the problems of the prior art HUDs and provide low 40 cost, high image quality HUD system. In this invention, a pair of reflective holographic optical elements (HOE) are utilized to achieve holographic dispersion correction and allow for a broadband light source to produce a highly contrasted image to the viewer.

Because of its simplicity and reliability qualities, the present invention is inherently suited for use in an automotive environment and the preferred embodiment is so shown. However, it is forseen that the present invention could be employed to many environments where HUD systems are desired. Such environments could include aircraft, trucks, video games, simulators or other displays where it is desired to superimpose visual indicia in the field of view of an individual.

In the automotive environment, the present invention employs one of the HOEs installed in 50 the windshield within the field of view of an intended driver. A second reflective HOE is installed under the dashboard of the vehicle. The system generates an image of display instrumentation that is superimposed in the driver's forward field of view through the windshield. The display source, in this instance, is a vacuum fluorescent display (VFD) that has a fairly broadband spectrum (on the order to approximately 460-580nm). The two reflective HOEs are constructed 55 utilizing a fairly central wavelength for the object and reference beams with respect to the display source spectrum. Those HOEs, when properly mounted, in combination with the display source, provide a virtual image of the VFD indicia, free from the chromatic dispersion that typically characterizes holographic display techniques. Because the optical elements of the system are holographically recorded, a lensing function can be incorporated into the elements 60 during recordation. As a result, a virtual image, free of chromatic aberrations, is observable at a specified position.

The invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 illustrates the preferred embodiment of the present invention; 2 GB 2 161 615A 2 Figure 2 is a ray trace diagram illustrating dispersion correction by the present invention; Figure 3 illustrates the construction technique used to form one of the reflective holographic optical elements shown in Fig. 1; and Figure 4 illustrates the construction technique used to form the other of the reflective 5 holographic optical elements shown in Fig. 1.

The heads-up display system (HUD) of the present invention is shown in Fig. 1 as utilizing two parallel holographic optical elements HOE:E1 and HOEE2 to project a virtual image 10, of an indicia display source 10. The virtual image 10' is superimposed in the driver's field of view as the driver looks at the forward scene through the windshield 40. In the preferred embodiment, the HOE#2 is part of the laminate or is attached to the windshield 40 of a vehicle.10 The second element is located under the dash 30, out of the direct viewing path of the driver.

The indicia display source 101 in the preferred embodiment, is a conventional broadband vacuum fluorescent display (VFD) of the type commonly employed in some Ford Motor Company vehicles to digitally indicate the vehicle speed. That display source is merely illustrative of many available broadband sources that may be employed. The angles and, as well as distances D and OR,, are selected as a result of the slope of the windshield 40, the predicted location of the driver's eyes, the desired location of the Virtual image plane 10' and the space constraints under the dashboard 30.

Fig. 2 presents a ray tracing model to illustrate the chromatic dispersion correction mechanism of the system. Reflective HOE#1 represents the element positioned under the 20 dashboard 30 of the automobile parallel to reflective HOE#2 that is coplanar with the windshield 40. A ray originating from a point P on the VFD 10 emitting face and impinging on HOE#1 is polychromatic and is represented as containing wavelengths A, (blue), A, (green) and X, (red). The relationships between the wavelengths are such that AB, ""AG, <XR Both HOE#1 and HOE#2 behave as Bragg type diffraction gratings causing the incident 25 beams to be diffracted and dispersed at angles related to the wavelength of the incident light.

The behaviour of each diffracted light beam can be described by the familiar Bragg equation X 30 2 nd sin 0., = X; OB, = arcsin ( -) 2 nd where d = grating spacing, n = refractive index of medium, A = wavelength of an incident beam, and 01r = the half-angle between the incident and diffracted ray, Bragg angle Reflective HOE#1 and HOE#2 are chosen to be geometrically identical in construction and the diffracted rays from HOE#1 are used in reconstructing the virtual image of the point source in HOE#2. The inherent wavelength dispersion of the diffracted X,, AG and X, rays from HOE#1 at 20,, 20G and 20R respectively, causes those rays to impinge on HOE#2 at different angles of incidence (exaggerated in Fig. 2). The incident rays on HOE#2 are, in turn, diffracted by 45 HOE#2, according to the Bragg equation. The diffracted rays from HOE#2 are thereby dispersion corrected and appear to originate from a virtual image point P' on the opposite side of the HOE#2 from which it is viewed. Thus, the image is recombined as a virtual image of point P appearing at P, free of chromatic dispersion.

The generation of an image, free from dispersion, requires the use of two reflective holograms 50 with nearly identical grating characteristics. If the grating characteristics differ significantly, that is if 0,,, varies by more than a few degrees (5) for the two holograms, some dispersion will be evident to the viewer as smearing of the image. In addition, the two holograms must be properly aligned with respect to each other to ensure the generation of an image of optimum quality and maximum image brightness.

Figs. 3 and 4 illustrate the geometries utilized in the fabrication of the two reflective holographic elements for the present invention. The two elements are constructed as non-image bearing holograms utilizing a monochromatic point source object beam and a collimated monochromatic reference beam of a wavelength that is selected to be centered in the broad- band of wavelengths generated by the VFD 10.

The focal length of the HOEs manufactured in the geometrically identical configuration are defined by the distance of the monochromatic point source forming the diverging object beam from the plane of the recording medium plate. In this case, the focal properties of HOE#1 in Fig. 3 can be described by the simple thin lens equation:

3 GB 2 161 615A 3 1 1 1 0C, RC, fCJ where Oc, = separation of diverging beam and recording plate, object beam Rc, = separation of the photo plate and point source forming reference beam, and fc, = focal length of HOE In the configuration shown in Figs. 4 and 5 the reference beam is collimated and Rc, = w.

Thus, the equation reduces to the form 1 1 OC, f,' and fC 1 OC, Similarly, the focal length of HOE#2, constructed as illustrated in Fig. 4, is fC2 = 0C2 The focal length of each resulting reflective HOE is defined as the object beam path length, in 30 these recording configurations.

The image position illustrated in Fig. 1 relative to the windshield is defined by the following equation:

OR2 0C2 0C, o', OR, - OC, where + D OC2 = separation of point source of the diverging beam (object beam) and recording medium used to fabricate reflective HOE#2 4 5 0,1 = separation of point source of the diverging beam (object beam) and recording medium used to fabricate reflective HOE#1 0', = separation of display panel 10 and reflective HOE#1 D = separation of HOEs, and OR2 = location of HUD image relative to the reflective HOE#2 In the equation, the subscripts 1 and 2 refer to the appropriate HOE while the subscripts R and C refer to reconstruction and construction conditions, respectively.

The magnification of the displayed image is defined by the ratio of the image distance from the windshield hologram, HOE#2, to the path length of the diverging object beam used to 55 construct the windshield hologram. The magnification can be determined using the following equation.

m= 0,,0,, + D(O,, - 0,1) OC1OR, + (OR, - Oc,)(D - 0 C2) Since the magnification of the HUD image is controlled by the proper choice of the construction and reconstruction parameters, the system allows the HUD image to be selectively positioned 65 4 GB 2 161 61 5A 4 over a large range of distances relative to the windshield and provides for a variable image magnification.

Claims

1. A holographic display system comprising:

a source of radiation generating a visible image; a first reflective hologram positioned to reflectively diffract said image radiation; a second reflective hologram positioned to reflectively reconstruct and direct said image radiation diffractively reflected from said first hologram towards a viewer, whereby said image 10 appears to said viewer as a virtual image of said source of radiation.

2. A display system as in Claim 1, wherein said first and second reflective holograms are geometrically identical in construction.

3. A system as in Claim 2, wherein said holograms are positioned in generally parallel nianes.

4. A system as in Claim 3, wherein said holograms are non-image bearing elements. 15

5. A system as in Claim 3, wherein said second hologram is mounted on the windshield of a vehicle and said source and first hologram are mounted below said windshield out of the direct vision of said viewer.

6. A holographic display system comprising:

an illuminated source of indicia; a first reflective holographic recording positioned to diffract illumination frOm said source; a second reflective holographic recording positioned generally parallel to said first recording for receiving said diffracted illumination and providing a reconstructed virtual image of said indicia.

7. A display system as in Claim 6, wherein said first and second reflective holographic recordings are identically recorded to contain identical diffraction patterns.

8. A display system as in Claim 7, wherein said first and second reflective holographic recordings are identically recorded with non-image bearing diffraction patterns.

9. A system as in Claim 8, employed in an automotive vehicle, wherein said source and said first reflecting holographic recording are located beneath the dashboard of said vehicle, said 30 second reflective holographic recording is co-planar with the windsield of said vehicle and said virtual image appears to a driver of said vehicle as being external to said windshield.

10. A holographic display system substantially as hereinbefore described with reference to the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1986. 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.