Complex conjugate hologram display

Aberration in an image reflected from a warped surface such as an automobile windshield is corrected by providing an image to the windshield which is predistorted to compensate for the aberration. The predistorted image is created and stored in a hologram by a recording technique wherein a first transmission hologram which records the virtual image of an object is illuminated with a conjugate beam. The beam which is transmitted by the first transmission hologram is reflected off the windshield onto a second hologram recording plate, which is also illuminated by a reference beam passed through the windshield. The desired predistorted image is thereby stored in the second hologram recording plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The subject invention relates to displays and more particularly to a 
hologram for removing aberrations generated by a mirror in an image viewed 
in a head-up automotive display. 
2. Description of Related Art 
Prior art holographic head-up displays have been associated with aircraft 
applications and have been relatively complex and expensive. See generally 
U.S. Pat. Nos. 4,447,128 and 3,885,095 assigned of record to Hughes 
Aircraft Company. In proposed head-up automotive displays, the windshield 
serves as a combiner mirror which generates aberrations in the viewed 
image. In conventional head-up displays, the source is matched to the 
aberrations of the combiner by adding complex relay optics between the 
source and the combiner to warp the intermediate image presented to the 
combiner. These relay optics typically have 7-10 elements, which are 
tilted, decentered and sometimes aspheric. They cost thousands of dollars 
and are bulky and heavy. Such an approach is clearly not suitable for a 
low cost automotive display that requires minimal space in the instrument 
panel. 
Holographic procedures for forming holograms in general are well-known and 
can be found in known reference works, such as "Optical Holography" by 
Collier et al., Academic Press, New York, N.Y. (1971). However, to date 
such procedures have not been applied to automotive displays or to the 
problems of aberrations in such displays. 
SUMMARY OF THE INVENTION 
According to the invention, an object is generated as a 3-D warped 
holographic image. The warping or predistortion of the image is designed 
to compensate for the aberrations imparted by the windshield. The 3-D 
hologram, including predistortion, is made from the conjugate of the 
original image. The technique adds no appreciable weight or cost to the 
system since the 3-D hologram consists of only a thin flat film. The 
technique is particularly suited to situations where the display consists 
of a finite number of fixed images--as opposed to a fully programmable 
display. 
The invention thus results in an improved, head-up display suitable for use 
in automobile environments and in which windshield aberrations are 
accommodated in a low-cost compact manner.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description is provided to enable any person skilled in the 
optical and automotive fields to make and use the invention, and sets 
forth the best modes contemplated by the inventor of carrying out his 
invention. Various modifications, however, will remain readily apparent to 
those skilled in the art since the generic principles of the present 
invention have been defined herein specifically to provide a relatively 
economical and easily manufactured complex conjugate hologram display. 
FIG. 1 shows the preferred embodiment in which an image 11 is generated by 
a hologram 13 which is illuminated by a source 19. The image 11 is 
reflected by a combiner glass 15 to the viewer at 17. The combiner glass 
15 may be, for example, an automobile windshield. The shape of the 
combiner/windshield 15 is chosen for aerodynamic, rather than optical 
characteristics, and therefore, causes large aberrations in the image 11. 
The sequence of steps used to make the hologram 13 best illustrates how 
these aberrations are cancelled by the hologram 13. 
FIG. 2 shows the final display in which the viewer 17 sees an image shown 
as a star 21 which appears to be at the distance L from the viewer 17. As 
shown, the image also appears beyond the windshield 15. In the case of an 
automobile, the viewer 17 may thus view the object without directing his 
attention away from the road. In order to prevent aberrations in the 
viewed image, a hologram H1 is made of the object 21 at a distance L from 
the observer 17. 
The hologram H1 records the virtual image of the object 21. The first step 
in the process of making the hologram 13 is illustrated in FIG. 3. The 
hologram H1 is made by illuminating the object 21 and the holographic 
recording material or plate H1' with illumination produced by a laser 23 
which is split by a beam splitter 26. The split beams 28, 29 are then 
passed to lenses 25, 27 which direct the illumination onto the star object 
21 and the hologram recording plate H1'. 
In Step 2 (FIG. 4), the hologram H1 is reconstructed with the conjugate 
beam 31, which would reconstruct the real image of the object 21 back at 
its original position. By interposing the warped combiner windshield 15, 
the rays 33 from the hologram H1 are reflected downward. A reference beam 
35 is also directed through the windshield 15 onto a second hologram plate 
H2'. Together, the reference beam 35 and downward reflected beam 33 create 
a hologram H2. The hologram H2 thus records the object rays as aberrated 
by reflection from the combiner/windshield 15. FIG. 4 also illustrates the 
warped real image 18 of the object 21 created below the second hologram 
recording plate H2'. 
In Step 3 (FIG. 5), the image as seen by the observer 17 is reconstructed 
by illuminating the hologram H2 with its original reference beam provided 
by the illumination source 19. The hologram H2 generates the aberrated 
wavefront which, when reflected by the warped combiner 15, presents the 
viewer 17 with the undistorted image of the original object 21 at distance 
L. FIG. 5 also illustrates the warped virtual image 22 of the object 21 
created below the hologram H2. 
The hologram H2 can be a reflection hologram (FIG. 6A) in which case the 
illumination of the source 19 is reflected off the hologram H2 onto the 
windshield/combiner 15. Alternatively, the hologram H2 can be a 
transmission hologram (FIG. 6B) in which case the illumination of the 
source 19 is transmitted through the hologram H2 onto the 
windshield/combiner 15. If the windshield shapes and mountings vary 
appreciably, one may compensate for aberrations by generating a custom 
hologram for each windshield. 
The hologram members according to the preferred embodiment can be 
relatively inexpensive in mass production with substantially little weight 
addition to the vehicle. They further eliminate the need for expensive 
optics to achieve holographic head-up displays where a holographic image 
is reflected onto or transmitted to a windshield or similar combiner 
means. 
Various modifications to the above described invention may be readily 
apparent to those skilled in the optical and automotive fields in view of 
the above described generic concepts. Therefore, the scope of the present 
invention should be interpreted solely from the following claims.