GB2145535A

GB2145535A - Reflective optical elements

Info

Publication number: GB2145535A
Application number: GB08420159A
Authority: GB
Inventors: Alan Richard Dicey
Original assignee: Marconi Avionics Ltd
Current assignee: Allard Way Holdings Ltd
Priority date: 1983-08-26
Filing date: 1984-08-08
Publication date: 1985-03-27
Also published as: GB8420159D0; GB2145535B

Abstract

A reflective optical element comprising a holographically formed planar diffraction grating in which the fringe planes (5) intersect the major surfaces of the planar recording layer (3) in which the grating is formed. The element has the property that its plane of reflection is at a finite angle to its physical plane. Three methods of formation are disclosed and one particular application of the element is as the combiner of a head-up display unit. (see Figs. 9-11 and 7-not shown-respectively). <IMAGE>

Description

SPECIFICATION Reflective optical elements This invention relates to reflective optical elements.

It is an object of the present invention to provide a reflective optical element of planar form whose plane of reflection is at a finite angle with respect to the physical plane of the element.

According to the invention there is provided a reflective optical element comprising a planar diffraction grating holographically formed in a planar recording layer so that the planes containing the diffraction grating maxima and minima intersect the major surfaces of the recording layer.

The invention also provides a first method of manufacturing a reflective optical element according to the invention comprising irradiating a planar recording layer with two expanded collimated coherent light beams, the light beams being directed onto opposite sides of the layer at different angles to the layer so that interference of the beams within the layer forms a planar diffraction grating in thelayer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the layer.

The invention further provides a second method of manufacturing a reflective optical element according to the invention comprising irradiating a planar reflecting element with a single expanded collimated coherent light beam through a planar recording layer, the beam being directed at different acute angles with respect to the reflecting element and the recording layer so that interference within the layer of light incident on the recording layer before reflection at the reflecting element with light incident on the recording layer after reflection at the reflecting element forms a planar diffraction grating in the recording layer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the recording layer.

The invention also provides a third method of manufacturing a reflective optical element according to the invention comprising: producing a master reflective optical element according to the invention; and replicating said master by placing a recording layer in contact with the master, and irradiating said master through the recording layer with a single expanded collimated coherent light beam so that interference of light incident on the recording layer before reflection at said master with light incident on the recording layer after reflection at said master forms a planar diffraction grating in the layer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the layer.

The invention also provides a head-up display unit comprising a combiner through which an observer using the unit can view a scene and a projector arranged to project light representing a display onto the combiner for reflection to the observer, thereby to provide the observer with a view of the display superimposed on his view through the combiner, wherein said combiner comprises a reflective optical element according to the invention.

In one particular embodiment of such a display unit, said reflective optical element is constructed and positioned so that its physical plane is more nearly vertical than its reflecting plane.

One reflective optical element in accordance with the invention, one application of the element and several methods of making an element according to the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a diagram illustrating the element; Figure 2 is a ray diagram depicting the optical behaviour of the element of Figure 1; Figure 3 shows diagrammatically a known head-up display unit of the monocombiner type; Figure 4 is a representation of the instantaneous field of view obtained with the unit of Figure 3; Figure 5 shows diagrammatically a known head-up display unit of the dual combiner type; Figure 6 is a representation of the instantaneous field of view obtained with the unit of Figure 5.

Figure 7 illustrates how an element according to the invention can be used in place of the dual combiner of the display unit of Figure 5; Figure 8 is a representation of the instantaneous field of view obtained with a head-up display unit using an element according to the invention for its combiner; Figure 9 illustrates a first method of forming a reflective optical element according to the invention; and Figures 10 and 11 illustrate second and third methods of forming a reflective optical element according to the invention.

Referring to Figure 1, the element comprises two optically flat sheets 1 of light transparent material, such as glass, between which is sandwiched a plane gelatine hologram recording layer 3 in which has been formed a reflective hologram. The hologram is formed with the diffraction pattern tilted with respect to the plane of the gelatine layer, that is, with the planes containing the hologram diffraction pattern maxima and minima intersecting the major surfaces of the layer, as shown by the dashed lines 5 in Figure 1.

Such a hologram effectively comprises a vast number of narrow reflecting strips lying in the gelatine in planes parallel to one another at a finite angle to the plane of the layer.

The element thus now has the property that its plane of reflection is angled to the physical plane of the gelatine layer. Hence, as illustrated in Figure 2, incident and reflected light rays I and R lie at equal angles 8 with respect to the normal HN to the plane of reflection, i.e. the 'holographic' normal, rather than to the normal SN to the physical plane of the element 7.

One particular application of a reflecting element according to the invention will now be described with reference to Figures 3 to 7.

Referring to Figure 3, a conventional headup display unit arrangement comprises a projector comprising a cathode ray tube 11 and an optical system 1 3 which projects a collimated image of a display produced on the screen of the cathode ray tube 11 onto a semi-reflective glass plate combiner 1 5 for reflection to an observer. The observer is thus provided with a view of the display superimposed on his view through the combiner 1 5 of the scene outside the aircraft, or other vehicle in which the head-up display unit is mounted.

The instantaneous field of view of the display provided by such an arrangement is limited to a "porthole" 1 7 which is the image of the exit Pupil (usually defined by the final collimating element of the optical system) of the optical system 1 3. The shape of the instantaneous field of view (IFOV) as seen by the observer is typically of the form, shown in Figure 4, the "double-bubble" shape being due to the fact that the "portholes" seen by each eye of the observer overlap.

An improvement in the vertical field of view is highly desirable, particularly for combat aircraft, and has been achieved in some designs by the use of a combiner of the dual combiner type as shown in Figure 5. In this arrangement a second glass plate combiner 1 9 positioned in parallel spaced relation with the conventional single combiner 1 5 on the side of the combiner 1 5 further from the observer reflects a second image of the exit pupil, forming a second overlapping "porthole" 21 for each eye, and the instantaneous field of view extends to the shape shown in Figure 6.

A dual combiner arrangement does however have certain intrinsic problems. Firstly, the observer views some parts of the outside scene through one combiner and other parts through two combiners. Graded coatings on the combiners can give near constant bright ness and chromatic uniformity of the outside scene view from the design eye position, but from other head positions the observer may observe a colour band across the combiner which could be interpreted as an artificial horizon. Similarly, display brightness unifor mity is degraded from certain eye positions.

Secondly, the combiners must be aligned very precisely if double imaging is to be avoided where their fields of view overlap. Thirdly, an extension of the combiner 1 5 nearer the observer to cover the field of view of the other combiner 1 9 is required and this mades it harder to fit inside the aircraft ejection line and canopy constraints.

These problems can be largely overcome by replacing the two glass plates of a dual combiner with a single holographic reflective optical element in accordance with the present invention. The projector of the unit is arranged to produce a narrow waveband e.g.

generally green, light display, and the holographic combiner reflective optical element is designed in accordance with known principles to reflect only light in that narrow waveband.

As shown in Figure 7 the holographic combiner element 23 is positioned with its physical plane more steeply inclined than the planes of the dual combiner glass plates 1 5 and 1 9 so as to lie approximately in the plane of the combiner 1 5 at its lower end and approximately in the plane of the combiner 1 9 at its upper end. However, it is constructed so that its plane of reflection is parallel to those of the combiners 1 5 and 19, i.e. so as to reflect incident light at the same angle as the combiners 1 5 and 1 9. The holographic combiner thus reflects an approximately elliptical image of the exit pupil to each eye, giving an instantaneous field of view, as depicted in Figure 8.

A single holographic combiner according to the invention is smaller and lighter than an equivalent dual glass plate combiner system, can be constructed to give improved ejection line and canopy clearance capabilities, cannot give double images, presents a uniform display brightness with no discontinuities and gives uniform attenuation of the outside scene view.

In general, using a reflective optical element according to the invention for the combiner of a head-up display unit allows the physical plane of the combiner to be angled independently of the required plane of reflection giving the optical designer more freedom to fit larger field of view optics into a confined space such as the cockpit of an aircraft.

Several methods of manufacturing a reflective optical element in accordance with the invention will now be described.

Referring to Figure 9, in the most general method of hologram manufacture, a laser beam from a source 25 is split into two parts which are then steered, expanded and collimated by a beam splitter 27, mirrors 29, lenses 31 and collimators 33 to provide two intersecting, coherent beams directed onto a hologram recording layer 35 from opposite sides of the layer. This technique can be used to produce a reflecting optical element accord ing to the invention by arranging for the angles of incidence of the beams on the layer 35 to differ so that interference of the beams within the recording layer 35 forms a tilted planar diffraction grating in the layer 35 in which the planes containing the diffraction grating maxima and minima are at a finite angle to the plane of the layer i.e. they intersect the major surfaces of the layer.

in another method of hologram manufacture a single collimated beam is reflected by a mirror to form the second, intersecting beam.

Referring to Figure 10, in this case a recording layer 37 is placed adjacent a mirror 39, and an expanded collimated beam of coherent light produced by a source 41, lens 43, pinhole 45 and a collimator 47 is directed onto the exposed side of the recording layer 37 at an acute angle. A diffraction grating is then produced by interference between light directly incident on the recording layer 37 before reflection at the mirror 39 with light incident on the recording layer 37 after reflection at the mirror 39. This technique can only produce diffraction patterns whose maxima are concentric spheres, or, in the limiting case, parallel planes, and thus is only used to produce reflection holograms.

To produce the tilted diffraction grating required in an optical element according to the invention using this second method, the plane of the recording layer 37 must be at an angle with respect to the mirror 39, as shown in Figure 11 so that the directly incident and reflected beams in the layer 37 are at different angles to the normal to the plane of the layer 37.

In a third method of producing an optical element according to the invention, a master reflective optical element according to the invention, e.g. produced by either of the two above mentioned methods is used. The master is placed in contact with the recording layer and a single expanded collimated coherent beam is used to expose the recording layer. Thus, the third method is precisely as illustrated in Figure 10 except that the mirror 39 is replaced by a reflective optical element in accordance with the invention.

The required tilted diffraction grating is then produced in the layer by interference between directly incident and reflected beams as described above, these two beams necessarily being at angles to the normal to the physical plane of the recording layer which differ by an amount determined by the master.

Claims

1. A reflective optical element comprising a pianar diffraction grating holographically formed in a planar recording layer so that the planes containing the diffraction grating maxima and minima intersect the major surfaces of the recording layer.

2. An element according to Claim 1 wherein said layer is sandwiched between a pair of optically flat sheets of light transparent material.

3. An element according to Claim 1 or Claim 2 wherein said recording layer consists of gelatine.

4. A reflective optical element substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

5. A method of manufacturing a reflective optical element according to Claim 1 comprising irradiating a planar recording layer with two expanded collimated coherent light beams, the light beams being directed onto opposite sides of the layer at different angles to the layer so that interference of the beams within the layer forms a planar diffraction grating in the layer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the layer.

6. A method of manufacturing a reflective optical element according to Claim 1 comprising irradiating a planar reflecting element with a single expanded collimated coherent light beam through a planar recording layer, the beam being directed at different acute angles with respect to the reflecting element and the recording layer so that interference within the layer of light incident on the recording layer before reflection at the reflecting element with light incident on the recording layer after reflection at the reflecting element forms a planar diffraction grating in the recording layer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the recording layer.

7. A method of manufacturing a reflective optical element according to Claim 1 comprising: producing a master reflective optical element according to Claim 1; and replicating said master by placing a recording layer in contact with the master, and irradiating said master through the recording layer with a single expanded collimated coherent light beam so that interference of light incident on the recording layer before reflection at said master with light incident on the recording layer after reflection at said master forms a planar diffraction grating in the layer in which the planes containing the diffraction grating maxima and minima intersect the major surfaces of the layer.

8. A method of manufacturing a reflective optical element according to Claim 1 substantially as hereinbefore described with reference to Figure 9, Figure 10, or Figure 11 of the accompanying drawings.

9. A head-up display unit comprising a combiner through which an observer using the unit can view a scene and a projector arranged to project light representing a display onto the combiner for reflection to the observer, thereby to provide the observer with a view of the display superimposed on his view through the combiner, wherein said com biner comprises a reflective optical element according to Claim 1.

10. A head-up display unit according to Claim 9 wherein said reflective optical element is constructed and positioned so that its physical plane is more nearly vertical than its reflecting plane.

11. A head-up display unit substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.