GB2186384A - Head-up display collimating element - Google Patents

Abstract

In an optical element (10) for use as a collimator for a head-up display system a convex lens (11) is in optical contact with a diffractive optical element (12) acting as a concave mirror for on-axis light at diffracting layer 14. Off-Axis spurious light (A, B) is diverged by the upper surface of the convex lens (11) and in a head-up display system susceptibility to image washout, by off-axis light is thereby reduced. Spurious light entering the element (10) is absorbed by an absorbent layer (15). The present invention reduces the problem of display washout and is particularly suited to systems wherein the axis is protected by other means, such as a blocking reflector in the object to image light path. <IMAGE>

Description

SPECIFICATION Improvements in or relating to optical elements This invention relates to optical elements and in part icularto collimating optical elements for use in headup display systems.

In a head-up display system (HUD) such as may be included in an aircraft cockpit, light from an object forming the required display is combined with an outside view from the cockpit The combination is achieved by directing lightfrom the display onto a combiner element placed in the observer's outside sight line, so that a superimposed image of the display may be observed. The image is collimated so that the superimposed image may be relaxing ly and accurately read whilst viewing a distant object. In some conventional HUDs the image is collimated by a concave mirror which also serves to direct light onto the combiner. Mirrors having conventional reflective surfaces and diffractive optical elements arranged to be equivalent to such mirrors are known as collimators.

Recently a HUD arrangement has been proposed in which a concave mirror collimator situated below the outside sight line directs light upwardly onto a combiner. In an aircraft installation the arrangement is such that the collimator is positioned in a near horizontal attitude extending towards the observer. In a military installation the collimator may extend rearwardlyto project over the ejection line, being retractable in the event of ejection. Such positioning and attitude gives rise to potential problems of dis play washout due to the light collecting properties of concave mirrors and damageto collimator surfaces if known collimating optical elements are employed in such an installation.

According to the present invention an optical elementforuse as a collimatorfor a head-up display system includes a convex lens in optical contact with a diffractive mirror arranged foroperation substanti- ally on axis.

To reduce the possibility of display washout due to spurious reflection the lower surface of the diffractive mirror advantageously has an anti-reflection coating or an energy absorbent layer bonded thereto, preferably with optical cement of matched refractive index. The non-concave surface reduces the problem of washout and provides advantageous mechanical properties for aircraft installation over the ejection line, where the collimator occupies cockpit space immediately in front of the aircraft pilot. For low susceptibility to washout by off-axis light the convex lens is arranged such that incident light is received at a convex surface.

In orderthatfeatures and advantages of the present invention may be fully appreciated an embodiment will now be described with reference to the accompanying diagrammatic drawings, of which: Figure 1 represents a collimator in accordance with the present invention, Figure2 shows a HUD for aircraft installation including the collimator of Figure 1, and Figure 3 represents an alternative embodiment of a collimator in accordance with the present invention.

In a collimator 10 (Figure 1) a plano-convex lens 11 is optically bonded to a diffractive optical element 12 arranged in accordance with known priciplesto act as a concave mirror at diffracting layer 14. A neutral density light energy absorbing layer 15 is bonded to diffractive element 12 with optical cement of mat ched refractive index.

The collimator 10 is included in a HUD for aircraft installation (referring to Figure 2) and situated below an outside view along a centre line 20. Light 21 from a display (not shown in the Figure) is deflected buy a reflector 22 on the collimator 10. The collimator 10 directs light upwardly to conventional combiner 23 for combination with the outside view along centre line 20 in accordance with known HUD principles, giving rise to an exit pupil 26. Collimator 10 is mounted substantially horizontally and projects over ejection line 24.

It will be appreciated that the convex upper surface of collimator 10 reduces, as compared with a conventional mirror collimator, the range of incidence angles overwhich spurious light, mainly sunlight, reflected from the collimator surface can be seen by the display observer 25. Convex mirror action atthe surface of lens 11 (referring again to Figure 1) diverges rays (such as rays A and B) reflected from the surface of the collimator 10 which results in a lower intensity for reflected rays viewed through exit pupil 26,thereby reducing the likelihood of display washout as compared with a HUD including a conventional collimator.It will further be appreciated that since diffractive element 12 is configured for op- eration substantially on axis, reflection from diffracting layer l4for light arriving at oblique angles of incidence (such as from direction A) is inefficient, further reducing susceptibility to washout. Itwill thus be realised that a collimator in accordance with the present invention takes advantage of the properties of diffractive optical elements used on axis or near axis.

Reflection from the bottom surface of the diffractive element 12 is suppressed by absorbing layer 15. To further reduce the possibility of washout the upper surface of lens 11 has an anti-reflection coating.

In the HUD system (referring again to Figure 2) the image of the display visible through exit pupil 26 is formed via an intermediate image 27 of the display formed in front of reflector 22, by relay optics 28. A feature of this arrangement isthatthe axis 29, ofthe collimator 10 is blocked to spurious light (for example sunlight from above) by reflector 22 and the properties of the collimator 1 to reduce washout by off-axis light is particularly advantageous. In accordance with the present invention the diffractive mirror of collimator 10 is arranged for substantially on-axis operation, and the extent to which off-axis light is accepted matched to the range of angles blocked by reflector 22.

In general the intermediate image 27 will not be plan r, but have a radius of curvature as defined by the properties ofthe relay optics 28. In a HUD system this curvature must be corrected at some point if the display is to be accurately presented. A particularly advantageous feature of a HUD system including an optical element in accordance with the present in vention isthatpartial correction is applied by convex lens 11. Full correction to produce correct collimation overthe full field of superimposed view may be applied if the radius of curvature of intermediate image 27 as defined by relay optics 28 is complementaryto convex lens 11.

Itwill be realised thatthe position of collimator 10 over ejection line 24 makes the collimator 10 vulner- able to damage by activity in the cockpit. In the present embodiment the upper surface of collimator 10 is the comparatively robust surface of convex lens 11,which is also less likely to collect debris than a conventional concave collimator.

So that features of the present invention may be further appreciated, an alternative embodimentwill now be described.

Acollimator30 (Figure 3) has a convex lens 31 in optical contact with a diffractive mirror 32 and having a plane upper surface 37. The substrate 33 which carries diffractive layer 34 is slightly concave and has an absorbing layer 39 bonded thereto. The collima tor30forms a collimated image of an object at35. In a display system the objectwill be an intermediate image 36 formed by relay optics (not shown) as previously described.

The present embodiment is particularly suitable for collimators where a shortfocal length is required and where the intermediate image 36 lies on a curved surface, as shown. For maximum rejection of extraneous lightthe upper surface of lens 31 may have a convex profile 38. A particular advantage of this embodiment isthat diffractive layer34 may be formed on a concave surface of substrate 33, which facilitates fabrication. An additional advantage is thatthe diffractive pattern of layer 34 may be formed by an exposure made from the same side as incident light in operation.

Claims (5)

1. An optical elementforuseasacollimatorfora head-up display system including a convex lens in optical contact with a diffractive mirrorarrangedfor operation substantially on axis.

2. An optical element as claimed in claim 1 and having a surface with an anti-reflection coating or energy absorbent layer bonded thereto.

3. An optical element as claimed in claim 1 or claim 2 and having a convex surface to receive inci- dent light.

4. An optical element as claimed in claim 1,2 or 3 and wherein the diffractive mirror is formed on a concave surface of a curved substrate.

5. A head-up display system as herein described with reference to the drawings.

5. An optical element substantially as herein des cribedwith reference to the accompanying drawings.

6. A head-up display system including an optical element as claimed in any preceding claim.

7. A head-up display system as claimed in claim 6 and including an axis blocking reflectorwhereinthe extentto which off-axis light is accepted by the diffractive mirror is matched to the range of angles blocked by the reflector.

8. A head-up display system as claimed in claim 6 or claim 7 and wherein correction to intermediate image curvature is applied by the convex lens in optical contact with the diffractive mirror.

Amendments to the claims have been filed, and have the following effect: (a) Claims 1-8 above have been deleted ortextually amended.

(b) New ortextually amended claims have been filed as follows:-

1. A head-up display system including a on-axis collimator which comprises a convex lens in optical contact with a diffractive mirror wherein the lens has a convex surface to receive incident light and wherein, in use, off-axis light is diverged by the convex surface of the lens.

2. A head-up display system as claimed in claim 1 wherein the collimator has a surface with an antireflection coating orenergyabsorbantlayer bonded thereto.

3. A head-up display system as claimed in any preceding claim and including an axis blocking reflector wherein the extent to which off-axis light is accepted by the diffractive mirror is matched to the range of angles blocked bythe reflector.

4. A head-up display system as claimed in any preceeding claim and wherein correction to intermediate image curvature is applied bythe convex lens in optical contact with the diffractive mirror.