WO1992016867A1

WO1992016867A1 - Helmet mountable display system and helmet provided with such a display system

Info

Publication number: WO1992016867A1
Application number: PCT/NL1992/000050
Authority: WO
Inventors: Bob Kruizinga; Inga Francisca Eduarda Moelans; Godelieve Ghislena Anna Augusta De Vos; Guido Lodewijk Maria Leo Brandt
Original assignee: Optische Industrie de Oude Delft NV
Current assignee: Optische Industrie de Oude Delft NV
Priority date: 1991-03-12
Filing date: 1992-03-12
Publication date: 1992-10-01
Anticipated expiration: 1993-09-12

Abstract

Display system designed for mounting on a helmet and comprising two image projection devices for mounting on either side of the helmet, which image projection devices each emit an image intended for an eye of the person wearing the helmet, and a visor provided with two optically transparent, reflecting optical elements, which optical elements receive the images emitted by the image projection devices and reflect them to the eye for which said image is intended, in which connection, to simplify the requirements to be imposed on the optical properties of the optical elements, the image projection devices are mounted in a plane which extends essentially horizontally through both eyes of the wearer of the helmet, and either the image projection device which emits the image reflected to the left eye is sited on the right-hand side of the helmet and the image projection device which emits the image reflected to the right eye is sited at the left-hand side of the helmet, or a flat optically transparent, reflecting element is mounted vertically between the eyes, which flat element reflects the image originating from the image projection device at the left-hand or the right-hand side, respectively, of the helmet to the optically transparent, reflecting optical element sited in front of the left or right eye, respectively. In order to be able to site the image projection devices further to the rear so that they fall completely outside the field of view of the wearer of the helmet, an optically transparent reflecting optical auxiliary element is provided which is mounted in the vicinity of the respective image projection device on or near the visor and which reflects the rays originating from the image projection device to the associated optically transparent, reflecting optical element or to the flat optically transparent, reflecting optical element, respectively.

Description

Title: Helmet mountable display system and helmet provided with such a display system,

The invention relates to a display system designed for mounting on a helmet, which display system comprises at least an image projection device for attachment to one side of the helmet, and a visor provided with at least one optically transparent, reflecting optical element, which reflecting optical element is designed to receive the image emitted by the image projection device and to render it visible for the wearer of the helmet.

Such a display system is known from the paper by R.J. Witherington entitled "Optical design of a holographic visor helmet mounted display" in SPIE, vol. 147, Computer Aided Optical Design (1978), pages 161-170, in particular Fig 1. Such a display system, in which an image projection device is fitted at both sides of the helmet at the level of the helmet wearer's chin and in which the optically trans¬ parent, reflecting optical element consists of two holographic optical elements which each receive the image intended for an eye of the helmet wearer, is disclosed by a paper by H.-D.V. Bδhm and R. Schranner entitled "Requirements of an HMS/D for a Night-Flying Helicopter" in SPIE, vol. 1290, Helmet Mounted Displays (1990), pages 93- 107, in particular Fig. 6.

Such display systems are used "-for the helmets of pilots of, in particular, military aeroplanes and helicopters, although civil applications are also expected in the future. The information to be projected may consist of weapon system and flight information and/or information originating from image sensors, such as images obtained from intensified residual light or from thermal infrared radiation. A pilot who is wearing such a helmet is able to fly with it in twilight and darkness and does not in principle need to avert his eyes from the surroundings in order to observe the necessary weapon system and flight information. In twilight, the information originating from the image sensors is projected onto the optically transparent, reflecting optical element, which is in practice a holographic optical element, on top of the surroundings actually observed.

Compared with the display systems already known for a fairly long time, in which an image combination device is sited in the helmet between the visor and the pilot's eyes, a display system in which the information intended for the pilot is projected directly from one or more image projection devices on the visor has the advantage that no optical elements are situated close to the pilot's eyes. In practice, it is not only found, specifically, to be unpleasant if optical elements are in fact close to the pilot's eyes, but that also implies a safety risk and makes the wearing of (sun)glasses by the pilot impossible. Examples of such display systems in which an image combination device is sited in the helmet between the visor and the pilot's eyes and in which use is made of holographic optical elements, whose properties have been extensively explained, are described in European Patent Applications 0 286 496 and 0 288 365.

A drawback of the display system described in the abovementioned paper is that very high requirements are imposed on the optical properties of the holographic optical element and of the image projection device because the angle of incidence of the beam of rays originating from the image projection device and the angle of reflection desired for reflection by the holographic optical element to the pilot's eye form, with the normal to the surface of the holographic optical element, an angle which is such that the reflection condition according to which the angle of incidence is equal to the angle of reflection is nowhere near fulfilled. Although it is nevertheless possible in a holographic optical element to reflect the beam of rays in the desired direction despite the fact that the reflection condition is not fulfilled, this requires a holographic optical element that, quite apart from the optical power of the substrate on which the holographic optical element is mounted, i.e. the curvature of the surface of the visor, has a high optical power. Together with the boundary condition that the holographic optical element must have a high diffraction efficiency, this has the consequence that very severe aberrations are produced in the image observed, for which aberrations it appears to be virtually impossible to compensate in practice, and that the holographic optical element exhibits very considerable colour errors.

So that the wearer of the helmet is provided with an optimum image, it is necessary, in embodiments of display systems of the present type, to site the image projection devices far forward with respect to the face of the wearer of the helmet in view of the optical properties of the image projection devices and of the holographic optical elements. However, the wearer of the helmet finds it disturbing in many cases that he is able to see the image projection devices at the edge of his field of view and that they to some extent impede his field of view in the lateral direction. The siting of the image projection device far forward also has the drawback that the centre of gravity of the helmet is situated far forward, and this is extremely tiring, in particular when the helmet is worn for a fairly long time at a stretch and does not permit a high g loading in military aeroplanes because the head always tends to bend forwards.

The object of the invention is to offer solutions to these problems and, for this purpose, it provides a display device of the abovementioned type in which the at least one image projection device is mounted in a plane which extends essentially horizontally through both eyes of the wearer of the helmet and in which measures are either taken which result in the light rays originating from the image projection device, which is sited at one side of the helmet, traversing a path which is directed at the optically transparent, reflecting optical element placed in front of the eye situated at the opposite side of the helmet, or measures are taken which result in the light rays originating from the image projection device, which is sited at one side of the helmet, traversing a path which comprises an optically transparent, reflecting optical auxiliary element mounted in the vicinity of the respective image projection device on or near the visor, or both the abovementioned measures are taken.

As a result of said measures, it is found that, by departing from the existing idea that a projection device which is mounted near a side of the helmet must emit its image to the eye at the corresponding side of the head, it is possible to reduce the requirements to be imposed on the optical power of the holographic optical elements appreciably, as a result of which considerably lower aberrations are found to be possible. In certain cases, it is even no longer necessary to make use of an expensive holographic optical element, but a much cheaper semi- transparent mirror element is adequate. The .invention also relates to a helmet provided with a display system of the abovementioned type. It is also found that, as a result of said measures, it is possible to site the image projection devices considerably further to the rear with respect to the face of the wearer of the helmet without concessions having to be made at the same time to the quality of the images observed by the wearer.

A further embodiment of the invention is characterised in that a flat, optically transparent, reflecting optical element is sited in the path of the light rays originating from the image projection device at one side of the helmet and directed at the optically transparent, reflecting optical element placed in front of the eye situated near the opposite side of the helmet, which flat, optically transparent, reflecting optical element extends in a vertical plane, which vertical plane extends straight forward.

As a result of siting, a flat, optically transparent, reflecting optical element between the eyes, the holographic optical element is able to reflect the image in a direction situated at the other side of the normal to the direction of incidence. As a result of this, the holographic optical element will exhibit many fewer colour errors, and in some cases none at all.

In certain cases, it is even no longer necessary to make use of an expensive holographic optical element, but a much cheaper semi-transparent mirror element is adequate. The invention also relates to a helmet provided with a display system of the type described above.

The invention will be explained below in greater detail on the basis of an exemplary embodiment with reference to the drawing, wherein:

Figure 1 shows a diagrammatic side view of a helment provided with a known display device;

Figure 2 shows a diagrammatic front view of a helmet provided with a known display device;

Figure 3 shows a diagrammatic plan view of the helmet according to Figures 1 and 2, in which the path of a beam of rays is indicated;

Figure 4 shows a diagrammatic plan view of a helmet provided with an embodiment of a display device according to the invention, in which the path of a beam of rays is also indicated; Figure 5 shows a diagrammatic front view of a helmet provided with a display device according to Figure 4;

Figure 6 shows a diagrammatic plan view of another embodiment of a display device according to the invention; Figure 7 shows a diagrammatic plan view of yet another embodiment of a display device according to the invention;

Figure 8 shows a diagrammatic front view of a further embodiment of a display device according to the invention; Figure 9: shows a diagrammatic plan view of a first embodiment of the embodiment according to Figure 8; and

Figure 10 shows a diagrammatic plan view of a second embodiment of the embodiment according to Figure 8.

Figures 1 and 2 show a pilot wearing a helmet 1 which is provided with a visor 2. These igures essentially correspond to Figure 6 of the abovementioned paper by Bδhm and Schranner. Mounted at the left-hand side (as viewed by the wearer of the helmet) of the helmet is an image projection device 3, known per se, which is capable of projecting an image onto a holographic optical element 4 mounted on the inside of the visor 2, which image is intended for the pilot's left eye 7. In the same way, an image projection device 5 which is capable of projecting an image onto a holographic optical element 6 also mounted on the inside of the visor 2, which image is intended far the pilot's right eye 7', is mounted at the right-hand side of the helmet. The specific structure of such an image projection device and of such a holographic optical element do not need to be discussed within the scope of the present application since the structure and the operation thereof, and also the diverse possible variants, are well-known to the person skilled in the present art and because the specific configuration thereof is of no importance for the essence of the present invention.

Figure 3 shows diagrammatically in plan view the path of a beam of rays 8 emitted by the image projection device 5 and intended for the holographic optical element 6.

A beam of rays emitted by the image projection device 3 and intended for the holographic optical element 4 travels in a corresponding way and therefore requires no further explanation. In Figure 3, the normal to the surface of the holographic optical element is indicated by N. From Figure

3 it is evident that, for example, the central ray of the beam 8 in the figure has an angle of incidence with the normal to the holographic optical element which is equal to and that the angle of reflection by the holographic optical element to the eye 7' is equal to j5, α and β being unequal and, in addition, being situated on the same side of the normal, whereas, according to the reflection condition. the following should apply: α = β , a and β being situated on either side of the normal. Although it is nevertheless possible, due to the specific properties of a holographic optical element, to reflect the beam of rays 8 in the direction of the eye 7' despite the fact that the reflection condition is not fulfilled, this requires a holographic optical element which, quite apart from the optical power of the substrate on which it is mounted, has a very high optical power. Together with the boundary condition that the holographic optical element should have a high diffraction efficiency, this has the consequence that very severe aberrations are produced in the image observed, for which aberrations virtually no compensation appears to be possible in practice.

Figures 4 and 5 show diagrammatically the same view as in Figures 3 and 2, respectively, but a solution according to the invention is indicated therein. The solution shown is that the image projection devices 3 and 5 are mounted on either side of the head of the wearer of the helmet in a plane which extends essen¬ tially horizontally through the two eyes of said wearer,, so that little or no deflection of the beam of rays emitted by the image projection device in the vertical direction is necessary, and that the signals which originate from sensors, image pick-ups etc. and which are intended for the pilot's right eye 7' are fed to the image projection device 3 which is situated on the left-hand side of the pilot's head. The image projection device 3 emits a beam of rays 8' in the direction of the holographic optical element 6, which reflects said beam to the eye 7' .

As is evident from Figure 4, the central ray of the beam 8' in the figure now has an angle of incidence with the normal to the holographic optical element which is equal to a' , while the angle of reflection by the holographic optical element 6 to the eye 7' is equal to β* . As is evident from Figure 4, the condition α'-= β ^% can now be fulfilled, so that the requirements which are imposed on the optical power of the holographic optical element 6 are appreciably less than in the case of Figure 3, so that the additional requirements of a high diffraction efficiency without unacceptable aberrations which can no longer be compensated for being produced can be fulfilled more easily. Now that the reflection condition can be fulfilled, it is also possible to make use of a much cheaper semi-transparent mirror element instead of an expensive holographic element. A further advantage of this solution is that the visor on which the holographic element is mounted does not need to have a specific optical power.

If an image projection device is also mounted on the other side of the helmet, which is not necessary, a beam of rays emitted by the image projection device 5 and intended for the holographic optical element 4 travels in a corresponding way to the beam emitted by the image pro¬ jection device 3 and therefore offers the same advantages, so that this does not therefore require any further explanation.

In the devices according to Figures 3 and 4, the wearer of the helmet sometimes finds it disturbing that he is able to see the image projection devices 3 and 5 from the corners of his eyes and that these impede his field of view in a lateral direction. According to a further exemplary embodiment of the invention, which is shown in Figures 6 and 7, to overcome this drawback two additional optically transparent, reflecting optical auxiliary elements 9 and 10 mounted on or near the visor are provided, and these may again be holographic optical elements or semi-transparent mirror elements. The object of the element 9 is to receive the beam of rays originating from the image projection device 3, which is now sited further to the rear on the helmet, and to reflect it to the element 6 or the element 4, respectively. The element 10 receives a beam of rays, originating from the image projection device 5, in the same way and reflects it to the element 4 or the element 6, respectively. As is evident from Figure 6, it is still approximately true for the element 6 that α''= ,9*', so that the same advantages are retained as in the embodiment according to Figure 2, while the image projection devices can nevertheless be kept outside the field of view of the wearer of the helmet by siting them further to the rear. Moreover, this has the additional advantage that the weight distribution of the equipment mounted on the helmet becomes more favourable, and this is found in practice to be very important. The auxiliary elements 9, 10 can be mounted on the surface of the visor in the same way as the elements 4 and 6, but it is also possible to mount the auxiliary elements 9 and 10 in the immediate vicinity of the visor, and this offers the possibility of giving the auxiliary elements a surface shape which is other than that of the surface of the visor. It is further pointed out that, although separate optically transparent, reflecting optical elements 4, 9 and 6, 10 are shown in both the embodiment according to Figure 4 and in that according to Figures 6 and 7 for each image projection device, it is also possible in principle to use a single continuous holographic optical element or semi-transparent mirror element, of which, in the embodiment according to Figure 4, the right-hand section, seen in plan view, receives the beam originating from the image projection device 3 and intended for the eye 7* and reflects it, and the left-hand section receives the beam originating from the image projection device 5 and intended for the eye 7. In addition, in the embodiment according to Figures 6 and 7, the section furthest to the left fulfils the function of the element 9 and the section furthest to the right fulfils that of the element 10. In the case of the projection of images obtained by image intensification or infrared detection of scenes which are situated at a fairly large distance from the wearer of the helmet, it is also possible to feed the image information intended for the right and the left eye to the right-hand projection device and the left-hand projection device, respectively, which then project their images, according to the invention, into the left eye and the right eye, respectively. This is possible because the parallax in the image no longer plays any part for fairly large distances.

Figures 8 and 9 diagrammatically show a front and plan view of a further embodiment of the invention.

The essence of the embodiment shown is that the image projection devices 3 and 5 are mounted on either side of the head of the wearer of the helmet in a plane which extends essentially horizontally through the two eyes of said wearer, so that little or no deflection of the beam of rays emitted by the image projection device is necessary in the vertical direction, and that the signals which originate from sensors, image pick-ups etc. and which are intended for the pilot's right eye 7' are fed to the image projection device 5, which is situated on the right-hand side of the pilot's head. The image projection device 5 emits a beam of rays 8 in the direction of a flat, vertically sited, reflecting element 11, such as an optically transparent holographic element which is situated between the eyes. Element 11 reflects the beam of rays in the direction of the holographic optical element 6, which reflects said beam to the eye 7*. As is evident from Figure 9, the ray of the beam 8 which is now central in the igure has an angle of incidence with the normal to the holographic optical element which is equal to • , while the angle of reflection by the holographic optical element 6 to the eye 7• is equal to β' . As is evident from Figure 9, the condition α'= S' can now be fulfilled, so that the colour errors of the holographic optical element 6 are appreciably smaller than in the case of Figure 3.

Now that the reflection condition can be fulfilled, it is even possible to make use of a much cheaper semi- transparent mirror element instead of an expensive holographic element.

If an image projection device 3 is also mounted on - li ¬ the other side of the helmet, which is not necessary, a beam of rays emitted by the image projection device 3 travels to the rear side of the element 11, which also acts reflectively, and is then reflected to the holographic optical element 4, which reflects the beam of rays to the left eye 7.

In the embodiment according to Figure 9, the wearer of the helmet may in some cases find it disturbing that he is able to see the image projection devices 3 and 5 out of the corners of his eyes and that they to some extent impede his field of view in the lateral direction. In order to overcome this drawback, according to a further exemplary embodiment of the invention which is shown in Figure 10., two additional optically transparent, reflecting optical auxiliary elements 9 and 10, which may again be holographic optical elements or semi-transparent mirror elements, are provided which are mounted on or near the visor. The object of the element 9 is to receive the beam of rays originating from the image projection device 3, which is now sited further to the rear on the helmet, and to reflect them to the element 11, which reflects the beam of rays to the element 4. In the same way, the element 10 receives the beam of rays originating from the image projection device 5 and reflects it to the other side of element 11, which reflects the beam of rays to the element 6.

As is evident from the figure, it is still approxi¬ mately true that o' ' = β' • , so that the same advantages are retained in the embodiment according to Figure 9, while the image projection devices can nevertheless be kept outside the field of view of the wearer of the helmet by placing them further to the rear, which has, moreover, the additional advantage that the weight distribution of the equipment mounted on the helmet becomes more favourable, which is found to be very important in practice. The auxiliary elements 9 and 10 can be mounted on the surface of the visor in the same way as the elements 4 and 6, but it is also possible to mount the auxiliary elements 9 and 10 in the immediate vicinity of the visor, and this offers the possibility of giving the auxiliary elements a surface shape which is other than that of the surface of the visor. It is further pointed out that, although separate optically transparent, reflecting optical elements are shown in both the embodiment according to Figure 9 and in that according to Figure 10, it is in principle also possible to use a single continuous holographic optical element or semi- transparent mirror element of which the right-hand section, seen in plan view in the exemplary embodiment according to Figure 9, receives the beam originating from the image projection device 5 and intended for the eye 7' and reflects it, and the left-hand section receives the beam originating from the image projection device 3 and intended for the eye 7. In addition, in the embodiment according to Figure 10, the section furthest to the left fulfils the function of the element 10 and the section furthest to the right fulfils that of the element 9.

Claims

1. Display system designed for mounting on a helmet, which display system comprises at least an image projection device for attachment to one side of the helmet and a visor provided with at least one optically transparent, reflecting optical element, which optically transparent, reflecting optical element is designed to receive the image emitted by the image projection device and to render it visible for the wearer of the helmet, characterised in that the at least one image projection device is mounted in a plane which extends essentially horizontally through the two eyes of the wearer of the helmet and in which measures are either taken which result in the light rays originating from the image projection device, which is sited at one side of the helmet, traversing a path which is directed at the optically transparent, reflecting optical element placed in front of the eye situated near the opposite side of the helmet, or measures are taken which result in the light rays originating from the image projection device, which is sited at one side of the helmet, traversing a path which comprises an optically transparent, reflecting optical auxiliary element mounted in the vicinity of the respective image projection device on or near the visor, or both the abovementioned measures are taken.

2. Display system according to Claim 1, characterised in that an image projection device is provided at both sides of the helmet and in that two optically transparent, reflecting optical elements are provided, each image projection device emitting a beam of rays which is directed at, and is reflected by, the optically transparent, reflecting optical element which renders the image visible to the eye of the wearer of the helmet situated near the opposite side of the helmet.

3. Display system according to Claim 1 or 2, charac¬ terised in that the optically transparent, reflecting optical element is a semi-transparent mirror element.

4. Display system according to Claim 1 or 2, characterised in that the optically transparent, reflecting optical element is a holographic element.

5. Display system according to at least one of Claims 1-4, characterised in that an optically transparent, reflecting optical auxiliary element which is mounted in the vicinity of the respective image projection device on or near the visor and which reflects the rays originating from the image projection device to the associated optically transparent, reflecting optical element is provided in the beam path of a specific image projection device to the associated optically transparent, reflecting optical element.

6. Display system according to at least one of Claims 1-5, characterised in that the optically transparent, reflecting optical elements and, optionally, optically transparent, reflecting optical auxiliary elements form one optically transparent, reflecting surface.

7. Display system according to Claim 1, characterised in that a flat, optically transparent, reflecting optical element is sited in the path of the light rays originating from the image projection device at one side of the helmet and directed at the optically transparent element placed in front of the eye situated near the opposite side of the helmet, which flat, optically transparent, reflecting optical element extends in a vertical plane, which vertical plane extends straight forward.

8. Display system according to Claim 7, characterised in that an image projection device is provided at both sides of the helmet and in that two reflecting optical elements are provided, each image projection device emitting a beam of rays which is directed at, and reflected by, the flat optically transparent, reflecting optical element, which reflects whichever of the beams to the reflecting optical element, which reflects the image and makes it visible to the eye of the wearer of the helmet situated at the same side edge of the helmet.

9. Display system according to Claim 7 or 8, characterised in that the optically transparent reflecting optical element is a semi-transparent mirror element.

10. Display system according to Claim 7 or 8, characterised in that the optically transparent, reflecting optical element is a holographic element.

11. Display system according to at least one of Claims 7 - 10, characterised in that an optically transparent, reflecting optical auxiliary element which is mounted in the vicinity of the respective image projection device on or near the visor and which reflects the rays originating from the image projection device to the flat optically transparent, reflecting optical element which reflects the rays to the associated optically transparent, reflecting optical element is provided in the beam path of a specific image projection device to the associated optically transparent, reflecting optical element.

12. Display device according to at least one of Claims 7 - 11, characterised in that the optically transparent, reflecting optical elements and optional optically transparent, reflecting optical auxiliary elements form one optically transparent, reflecting surface.

13. Helmet provided with a display system according to at least one of Claims 1 - 12.