GB1602373A - Holographic imaging - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO HOLOGRAPHIC IMAGING (71) I, THE SECRETARY OF STATE FOR DEFENCE, Whitehall, London SWIA 2HB, do hereby declare the invention,for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to holographic imaging, and in particular to holographic imaging head up display systems.

Recent developments in holography have led to the use of holograms as optical elements in head up display systems. Holographic optical elements have advantages over conventional optical elements in having, for example, low mass and much reduced sensitivity to surface finish. Holograms also offer potentially lower cost than comparable conventional optical elements.

A hologram suitable for use as a lens can be constructed by recording in a suitable medium the interference pattern between light from a monochromatic point source at an object location and a reference beam from a point at a desired image location. For an image at infinity the reference beam is parallel. Light from the object point source and the reference beam must be coherent. In use, an object emitting or illuminated by monochromatic light of the wavelength used in construction of the hologram is placed at the object location and an image of the object is formed by the hologram at the image location, which can be infinity. If, in use, the image is viewed from the same side of the hologram as the object location the hologram is said to be of the reflection type.

Alternatively if. in use, the image is viewed from the opposite side of the hologram to the object location then the hologram is said to be of the transmission type.

According to the present invention a holographic imaging head-up display system includes a plurality of monochromatic objects, each emitting or illuminated by substantially monochromatic light of a different wavelength, a multiply exposed hologram arranged for deviation of light from each object to form a collimated image of each object, which images appear overlaid within a single image field when viewed from a suitable position, the hologram being positioned such that the overlaid images may be viewed superimposed on an external view through the hologram.

A multiply exposed hologram may be constructed by exposing a suitable medium to an interference pattern between coherent monochromatic light from point sources at each object locations and from a collimated reference source. Each exposure may be carried out sequentially to record the interference fringe pattern between coherent monochromatic light from a point source at a different object location and from a collimated reference source of the desired wavelength.

Alternatively a multiply exposed hologram may be formed by using monochromatic light of any wavelength. If a hologram is used to form an image of an object emitting or illuminated by monochromatic light of a wavelength different from the wavelength used in the formation of the hologram the result is a different focal length and a different angle of deviation, in accordance with the formula A = 2d sin 0; where A is the wavelength, S the angle of deviation and d the fringe spacing. Hence it can be seen that a hologram of a desired focal length and angle of deviation for light of a particular wavelength may be formed with monochromatic light of another wavelength by an exposure recording in a suitable medium the interference fringe pattern between two suitably positioned coherent sources.

The present invention is equally applicable to head up display systems including holograms of both the transmission and reflection types.

In a preferred arrangement of a holographic imaging head up display system in accordance with the present invention Cathode Ray Tubes (CRTs) are included to produce displays which act as objects for the hologram. Filters may be included, one associated with each CRT, to remove light of unwanted wavelengths emitted by CRT phosphors so that substantially monochromatic light from each display reaches the hologram. Alternatively a chromatic dispersion corrector, for example a diffraction grating or a second hologram, associated with each CRT may be included so that images of each object produced by light over a band of wavelengths emitted by the CRT appear coincident. Alternatively a single CRT having a split screen with areas of different phosphors may be used.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a head-up display system, Figure 2 is a side elevation, and Figure 3 shows the view from the observation position of a head-up display system.

A CRT l0aofFig. 1 producesasubstantially monochromatic display, light from one point of which is represented by the beam 1 la. A diffraction grating 1 2a is arranged to diffract light from the CRT through a filter 1 7a which has a suitable pass band so that monochromatic light from the display is incident upon a transmission hologram 18. The hologram produces a collimated image of the display which can be viewed from a suitable observation position, such as 14, superimposed on a view through the hologram. Overlaid images are also produced of displays on two other CRTs 10b and 1 0c which have associated diffraction gratings 1 2b and 1 2c and filters 1 7b and 17c. The bath of light from one point on each display is represented by beams 11 b and 11 c respectively.

The formation of a collimated image of the CRT 10b at the observation position 14 superimposed on a view through the hologram 18 is shown in elevation in Fig. 2.

The CRTs, diffraction gratings and filters each have an associated shield such as that shown at 15 (not shown in Fig. 1) to prevent light from sources other than the display being incident upon the hologram along the family of paths 11 and upon the CRT faces.

Fig. 3 shows the view from the observation position. A second shield 16 is positioned to prevent light of the wavelengths used for imaging being incident upon the hologram substantially from above the line of sight.

It will be appreciated that there are many alternative arrangements of the shields. In particular, the second shield 16, could be frequency selective blocking only light of those frequencies used for imaging, or it could be of slatted construction arranged to be clear along the line of sight and to block light from above.

It will be realised that conventional optical elements, such as for example, a field flattening lens, may advantageously be included to improve image quality, particularly where a large field of view is desirable.

It will be realised that there are many alternative phosphors which may be used in CRTs forming objects for holographic imaging head up display system in accordance with the present invention. Preferably a phosphor which emits light of wavelengths within a narrow band of wavelengths is used.

such as for example gadolinium oxysulphide (green) and europium doped yttrium vanadate (red).

A further example of a suitable phosphor is yttrium oxide which exhibits a dominant emission wavelength of 627 nm. A secondary peak exists at approximately 617 nm which may be suppressed by a filter as hereinbefore described. With such an arrangement collimated images having a resolution to 1 nrad may be achieved without chromatic dispersion correction.

WHAT I CLAIM IS: 1. A holographic imaging head up display system including a plurality of monochromatic objects, each emitting or illuminated by substantially monochromatic light of a different wavelength, a multiply exposed hologram arranged for deviation of light from each object to form a collimated image of each object which images appear overlaid within a single image field when viewed from a suitable position, the hologram being positioned such that the overlaid images may be viewed superimposed on an external view through the hologram.

2. A holographic imaging head up display system as claimed in claim 1 wherein the objects include one or more Cathode Ray Tubes (CRTs).

3. A holographic imaging head up display system as claimed in claim 2 wherein a filter is associated with each CRT.

4. A holographic imaging head up display system as claimed in claim 2 wherein associated with each CRT is a chromatic despersion corrector so that images of each object produced by light of wavelengths within a band of wavelengths emitted by the CRT appear coincident.

5. A holographic imaging head up display system as claimed in any one of claims 2, 3 or 4 and wherein a single CRT has a split screen, 6. A holographic imaging head up display system substantially as herein described with reference to the drawings.

7. A vehicle having a holographic imaging head up display system as hereinbefore claimed in any one of claims 1 to 6.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. hologram. Alternatively a chromatic dispersion corrector, for example a diffraction grating or a second hologram, associated with each CRT may be included so that images of each object produced by light over a band of wavelengths emitted by the CRT appear coincident. Alternatively a single CRT having a split screen with areas of different phosphors may be used. An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of a head-up display system, Figure 2 is a side elevation, and Figure 3 shows the view from the observation position of a head-up display system. A CRT l0aofFig. 1 producesasubstantially monochromatic display, light from one point of which is represented by the beam 1 la. A diffraction grating 1 2a is arranged to diffract light from the CRT through a filter 1 7a which has a suitable pass band so that monochromatic light from the display is incident upon a transmission hologram 18. The hologram produces a collimated image of the display which can be viewed from a suitable observation position, such as 14, superimposed on a view through the hologram. Overlaid images are also produced of displays on two other CRTs 10b and 1 0c which have associated diffraction gratings 1 2b and 1 2c and filters 1 7b and 17c. The bath of light from one point on each display is represented by beams 11 b and 11 c respectively. The formation of a collimated image of the CRT 10b at the observation position 14 superimposed on a view through the hologram 18 is shown in elevation in Fig. 2. The CRTs, diffraction gratings and filters each have an associated shield such as that shown at 15 (not shown in Fig. 1) to prevent light from sources other than the display being incident upon the hologram along the family of paths 11 and upon the CRT faces. Fig. 3 shows the view from the observation position. A second shield 16 is positioned to prevent light of the wavelengths used for imaging being incident upon the hologram substantially from above the line of sight. It will be appreciated that there are many alternative arrangements of the shields. In particular, the second shield 16, could be frequency selective blocking only light of those frequencies used for imaging, or it could be of slatted construction arranged to be clear along the line of sight and to block light from above. It will be realised that conventional optical elements, such as for example, a field flattening lens, may advantageously be included to improve image quality, particularly where a large field of view is desirable. It will be realised that there are many alternative phosphors which may be used in CRTs forming objects for holographic imaging head up display system in accordance with the present invention. Preferably a phosphor which emits light of wavelengths within a narrow band of wavelengths is used. such as for example gadolinium oxysulphide (green) and europium doped yttrium vanadate (red). A further example of a suitable phosphor is yttrium oxide which exhibits a dominant emission wavelength of 627 nm. A secondary peak exists at approximately 617 nm which may be suppressed by a filter as hereinbefore described. With such an arrangement collimated images having a resolution to 1 nrad may be achieved without chromatic dispersion correction. WHAT I CLAIM IS:

1. A holographic imaging head up display system including a plurality of monochromatic objects, each emitting or illuminated by substantially monochromatic light of a different wavelength, a multiply exposed hologram arranged for deviation of light from each object to form a collimated image of each object which images appear overlaid within a single image field when viewed from a suitable position, the hologram being positioned such that the overlaid images may be viewed superimposed on an external view through the hologram.

2. A holographic imaging head up display system as claimed in claim 1 wherein the objects include one or more Cathode Ray Tubes (CRTs).

3. A holographic imaging head up display system as claimed in claim 2 wherein a filter is associated with each CRT.

4. A holographic imaging head up display system as claimed in claim 2 wherein associated with each CRT is a chromatic despersion corrector so that images of each object produced by light of wavelengths within a band of wavelengths emitted by the CRT appear coincident.

5. A holographic imaging head up display system as claimed in any one of claims 2, 3 or 4 and wherein a single CRT has a split screen,

6. A holographic imaging head up display system substantially as herein described with reference to the drawings.

7. A vehicle having a holographic imaging head up display system as hereinbefore claimed in any one of claims 1 to 6.