Helmet mounted display

Abstract: A a helmet mounted display including a visor (14) attached to the helmet (10) and having a partially reflecting semi-transparent portion (16) in the view of the wearer and an optical projector (18) for projecting an image onto the semi-transparent portion. The semi-transparent portion has a relatively higher reflectivity in a central area thereof and a relatively lower transparency in an annular area outside the central area. In one aspect of the invention, the heads-up display includes a projector having an image-forming surface whereon an image is formed and an optical system for focusing the image at separate sagittal and tangential foci between the image-forming surface and the semi-transparent portion.

Description: FIELD OF THE INVENTION This invention relates to the field of helmet displays. BACKGROUND OF THE INVENTION Helmet displays which allow for the simultaneous viewing of information and/or images generated by a computer or other source together with and/or superimposed on a direct viewing of a distant scene are well known. One type of such apparatus provides a small, flat, usually partially reflecting, mirror which projects an image from a CRT toward the eye of a viewer. Examples of such apparatus are described in U.S. Patent 4,364,636 to Ellis and U.S. Patent 4,722,601 to McFarlane. U.S. Patent 4,026,641 describes a system in which the CRT display is projected at an oblique angle onto a toric display reflector which reflects the image toward the eye of the observer. A second type of apparatus provides for the image to be projected off a semi-reflecting portion of the visor of the helmet. In general, if such reflection is made at an oblique angle off a spherical surface, the image, as viewed by the observer, is distorted. In order to at least partially relieve this problem, U.S. Patent 4,761,056 to Evans et al. provides a parabolic semi-reflecting portion on the helmet visor and a parabolic mirror for reflecting the image onto the semi-reflecting portion. Similarly, U.S. Patent 3,787,109 to Vizenor and U.S. Patent 3,923,370 to Mostrom are typical of systems in which the image is reflected off a semi-reflecting parabolic portion of a visor to the eye of the observer. U.S. Patent 4,878,046 to Smith interposes a beam splitter mirror in the line of view of the observer, so that the reflection from the semi-transparent surface, be it spherical or planar, is not distorted. U.S. Patent 4,081,209 describes a system in which the image of an LED display is reflected from a spherical semi-reflecting surface molded into the surface of the helmet visor. A prism, placed in the path of the projection from the LED display to the spherical and chroma aberrations caused by the off-axis reflection from the spherical surface and an additional cylindrical element is added to compensate for astigmatic effects. In this system the image is projected toward the eye from the source without any intervening focus. Such systems appear to be unable to compensate for a field of view of more than a few degrees, even with the addition of additional elements. Apparently in order to allow for compensation by this method, the semi-reflecting section is separate from, and at an angle with, the rest of the visor. None of the above mentioned apparatus deal with a further problem of such heads-up helmet mounted display systems, namely the effect of the edge of the semi-transparent surface, which appears to be present in all commercial systems. While the superimposition of data or other images on the directly viewed scene is well received, the effect of the edge of the semi-reflecting region is annoying to most users. For most of the above systems this effect is unavoidable since there is a physical edge or sharp change in curvaturc at the edge of the semi-transparent region. US Patent 3,302,515 describes a projection mirror where the reflectivity of the surface decreases from the edges to the centre so as to provide a uniformly lighted field. A tilted catadioptric helmet mounted display is disclosed in Optical Engineering vol.9, no. 8, August 1990, Bellingham,US at pages 849-854, the display comprising, a spherical reflective surface which is tilted with respect to the line of sight of the observer, an optical system for imaging a display to the location of an intermediate image. The optical system comprises a tilted spherical field lens which compensates for the astigmatism introduced by the tilt of the field lens. It is an object of the present invention to supply an improved, relatively wide angle, substantially non-distorting, heads-up projection system for a helmet utilising a spherical semi-reflecting portion of the visor of the helmet. It is also an object of the invention to provide an improved helmet mounted display using a semi-reflecting portion of a visor and having no discernible edge effect. Accordingly, the invention provides a helmet mounted display apparatus comprising an optical element to be mounted on an observer and having a surface comprising an at least partially reflecting portion to be arranged in the field of view of the observer, the observer's eye being located in a predetermined position; an optical projector for projecting an image for reflection from the at least partially reflecting portion including a display source whereon an image is formed; and an astigmatic optical system arranged for forming an intermediate image of the image displayed on the display source at separate sagittal and tangential surfaces between said astigmatic optical system and the at least partially reflecting portion, said at least partially reflecting portion having a substantially spherical shape, and being positioned, arranged with respect to the astigmatic optical system and the predetermined position of the eye of the observer, such that light originating from the intermediate image is reflected towards the position of the eye and such that the sagittal and tangential foci of the at least partially reflecting portion coincide, respectively, with the separate sagittal and tangential surfaces of the intermediate image, the tangential and sagittal foci corresponding to the line of sight defined by the predetermined position of the eye and the position of the at least partially reflecting portion, characterised in that the astigmatic optical system comprises a toroidal mirror. The optical element may be a visor attached to a helmet adapted to be worn by the observer. The astigmatic optical system may comprise a lens of prismatic effect. The toroidal mirror may be operative to focus the image at said separate sagittal and tangential surfaces. Preferably the projector includes a focusing element situated at an area containing the sagittal focus. More preferably, one face of the element is placed at the sagittal focus and a second face is placed at the tangential focus. Alternatively, the optical projector includes a focusing element placed at the tangential focus and/or a focusing element placed at the sagittal focus. The apparatus may include at least one folding mirror in the optical path of the image to allow for a more compact projection system. Preferably said helmet mounted display does not include any intervening elements between an observing eye of the observer and the at least partially reflecting portion. The at least partially reflecting portion may comprise a partially reflecting semi-transparent portion in the view of the observer and the semi-transparent portion may have a relatively higher reflectivity in a central area thereof and a relatively lower reflectivity in an annular area outside the central area. The reflection from the at least partly reflective portion may be specular. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and advantages of the invention will become more apparent in view of the detailed description of a preferred embodiment of the invention as illustrated by the accompanying drawings in which: Figs. 1 and 2 are respective front and side views of a helmet incorporating a helmet mounted display in accordance with a preferred embodiment of the invention; Fig. 3 is an unfolded drawing of the optical path of a display system of the preferred embodiment of Figs. 1 and 2; Figs. 4 and 5 are schematic diagrams illustrating one method for producing a semi-reflecting surface according to a preferred embodiment of the invention; and Figs. 6 and 7 are schematic diagrams illustrating a alternative preferred method for producing a semi-reflecting surface according to a preferred embodiment of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figs. 1 and 2 show respective front and side views of a helmet 10 in accordance with a preferred embodiment of the invention. Helmet 10 comprises a body portion 12 of standard construction and a mostly transparent visor 14 rotationally attached to the sides of body portion 12 such that it can be rotated upwards and out of view of the wearers. Visor 14 has, preferably, a substantially spherical shape and includes a partially-reflecting semi-transparent spherical portion 16, which will be described in more detail below, placed in the line of sight of the observer. Mounted in a housing 19 above the visor is a projection system 18 for projecting information onto the semi-transparent portion 16 to form a helmet mounted display enabling the wearer to view the projection from system 18 superimposed on the outside scene. Projection system 18 includes a display 20 having a substantially flat display surface 22. Display 20 can be for example a CRT display, a LED display, a plasma display, an electroluminescent display, a liquid crystal display or any other suitable display. Light from an image which is generated at display surface 22 is reflected from a toroidal folding mirror 24, which may be either a front surface mirror or a reflection coating placed on the back surface of a suitable lens, via a lens 26 to a folding mirror 28. The image, after reflection from mirror 28, passes through a preferably prismatic lens 30 to semi-transparent portion 16 whence it is reflected toward the eye of the wearer of the helmet. The path of the central ray of the image is shown on Figs. 1 and 2 by dotted line 32. As is well known, images which are reflected off-axis from a spherical surface have both astigmatic and spherical aberrations. An unfolded drawing of the optical path of the projection system is shown in Fig. 3. Lens 30 comprises a central, preferably prismatic portion 30A and two spherical focusing portions 30B and 30C. It should be noted that, in a preferred embodiment of the invention, one of the spherical focusing surfaces is placed at or near the tangential focus of surface 16 and the other spherical focusing surface is placed at or near the sagittal focus of surface 16. This placement allows for separate adjustment of the focal lengths of the two reflection axes. Lens 30 acts as field optics for the system and partially corrects for the astigmatism caused by off-axis reflection from surface 16. Lens 26 is designed to correct for some of the spherical and chroma aberrations caused by the oblique reflection from spherical surface 16. Finally, toroidal mirror 24 corrects for the astigmatic effect of the off-axis reflection from surface 16, i.e., it focuses the light from surface 22 at the tangential and sagittal focuses of surface 16. As is more clearly seen in Fig. 3, prismatic lens 30 and lens 26 are placed at an angle to the optical axis of the system in order to aid in correcting for coma and spherical aberrations. In a preferred embodiment of the invention, the semi-transparent portion is not uniformly reflecting. Preferably, the central area of the semi-transparent portion has the desired uniform reflection and the reflectivity is decreased from the central area to the edge of the semi-reflective portion so that the edge is not noticeable to the wearer. The present inventors have found that a decreasing portion having a angle (as viewed by the wearer) of 50 mrad is sufficient to cause the edge effect to disappear where portion 16 has a reflectivity of 10%. For higher reflectivities larger decreasing portions are required-for example, 100 mrad at 30% reflectivities. In general an angle of 50-300 mrad is preferred, with about 130 mrad being especially preferred. Useful improvement is often achieved with angles as low as 50 mrad. Figs. 4 and 5 illustrate one method of producing a semi-reflecting coating having the desired edge. Fig. 4 shows a vacuum system including a vacuum bell 38 containing visor 14. Within the vacuum bell is a heated boat 40 containing the coating material for the semi-transparent portion 16. Spaced from and between boat 40 and visor 14 is a mask 42 having an opening of the generally the same shape as the semi-transparent portion. The size of the opening and the spacing of mask relative to boat 40 and visor 14, as well as the size of the boat will be determined by simple geometric considerations as illustrated in Fig. 5 taking into consideration the size of central area and the desired angle of the decreasing portion. As shown in Fig. 5, portions of visor 14 adjacent to the center of the hole in mask 42 will be subject to evaporation from the entire boat 40, while outlying portions will "see" less of the boat, and will have a thinner layer deposited on them. Lines 44 indicate the portion of boat 40 (i.e., all of the boat) viewed by a point 46. Lines 48 indicate that off-center point 50 still sees the entire boat while lines 52 show that point 54 sees only a portion of the boat and is therefore coated by a thinner film of material. Thus, if a reflecting material is used to coat a portion of the surface of visor 14 with a thin film so that the central coated portion is semi-transparent, then near the edges the amount of coating will be reduced, reducing the reflectivity at the edges. Fig. 6 shows an vacuum coating system similar to that of Fig. 4, except that the stationary mask is replaced by a rotating mask 56 closely spaced from visor 14. The aperture in mask 56, shown more clearly in Fig. 7 is not circularly symmetric and rotates about point 58. In this situation, the central portion, marked "a" on the curve at the bottom of Fig. 7 will be uniformly coated, while portions outside the central portion, marked "b" on the curve will have less coating the farther they are from the central portion, due to the decreased amount of time they view boat 40. Although the present invention has been shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that various changes, additions and deletions in form and detail thereof can be made therein without departing from the spirit and scope of the invention, which is limited only by the claims.

Claims: A helmet mounted display apparatus comprising: an optical element to be mounted on an observer and having a surface comprising an at least partially reflecting portion (16) to be arranged in the field of view of the observer, the observer's eye being located in a predetermined position; and an optical projector (18) for projecting an image for reflection from the at least partially reflecting portion including: a display source (22) whereon an image is formed; and an astigmatic optical system (24, 26, 30) arranged for forming an intermediate image of the image displayed on the display source (22) at separate sagittal and tangential surfaces between said astigmatic optical system and the at least partially reflecting portion (16), said at least partially reflecting portion (16) having a substantially spherical shape, and being positioned, arranged with respect to the astigmatic optical system (24, 26, 30) and the predetermined position of the eye of the observer, such that light originating from the intermediate image is reflected towards the position of the eye and such that the sagittal and tangential foci of the at least partially reflecting portion (16) coincide, respectively, with the separate sagittal and tangential surfaces of the intermediate image, the tangential and sagittal foci corresponding to the line of sight defined by the predetermined position of the eye and the position of the at least partially reflecting portion (16), characterised in that the astigmatic optical system (24, 26, 30) comprises a toroidal mirror (24). Apparatus as claimed in claim 1, wherein the optical element is a visor (14) attached to a helmet (10) adapted to be worn by the observer. Apparatus as claimed in claim 1 or 2, wherein the astigmatic optical system (24, 26, 30) comprises a lens (30A) of prismatic effect. Apparatus as claimed in any one preceding claim, wherein said toroidal mirror (24) is operative to focus the image at said separate sagittal and tangential surfaces. Apparatus as claimed in any one preceding claim, including a focusing element (30B) situated at an area containing the sagittal focus. Apparatus as claimed in any one preceding claim, including a focusing element (30C) situated at an area containing the tangential focus. Apparatus as claimed in any one preceding claim and including at least one folding mirror (24) in the optical path of the image to allow for a more compact projection system (18). Apparatus as claimed in any one preceding claim, wherein said helmet mounted display does not include any intervening elements between an observing eye of the observer and the at least partially reflecting portion (16). Apparatus as claimed in any one preceding claim, wherein the at least partially reflecting portion (16) comprises a partially reflecting semi-transparent portion in the view of the observer and wherein the semi-transparent portion has a relatively higher reflectivity in a central area thereof and a relatively lower reflectivity in an annular area outside the central area. Apparatus as claimed in any one preceding claim, wherein the reflection from the at least partially reflecting portion (16) is specular.

Inventors: GILBOA PINHAS, GOLD ALEXANDER, YAELI JOSEPH, GILBOA, PINHAS, GOLD, ALEXANDER, YAELI, JOSEPH
Applicants: ELBIT SYSTEMS LTD, ELBIT SYSTEMS LTD.
Application year: 1992
Main classification: G02B27
IPC codes: G02B5, G02B27