WO2009122147A1

WO2009122147A1 - Wide angle optical, security device

Info

Publication number: WO2009122147A1
Application number: PCT/GB2009/000817
Authority: WO
Inventors: Milan Momcilo Popovich; Antoine Yvon Messiou
Original assignee: Milan Momcilo Popovich; Antoine Yvon Messiou
Priority date: 2008-04-04
Filing date: 2009-03-27
Publication date: 2009-10-08
Also published as: GB0806103D0

Abstract

A wide angle viewing apparatus for security applications is disclosed. The apparatus comprises an image inverter (8), a lens (31) and a screen (140). The image inverter and lens together form an erect image of an external scene at said screen. The screen (140) further comprises a Fresnel surface and a diffusing surface. The apparatus may further comprise at least one of a two way audio communication link (101, 103), an infrared sensor (117) and an ancillary illumination source (105, 107).

Description

WIDE ANGLE OPTICAL SECURITY DEVICE

REFERENCES TO RELATED APPLICATIONS

The present application claims the priority of United Kingdom patent application No. GB0806103.8 filed 4 April 2008.

The present application incorporates by reference in its entirety the PCT application: PCT/US2007/003646 by the present inventors entitled "WIDE ANGLE DISPLAY DEVICE" with international filing date: 26 November 2007.

BACKGROUND OF THE INVENTION

This invention relates to a viewing device, and more particularly to a wide angle viewing apparatus for security applications.

Door viewers for home security are well known. One common approach provides a peephole incorporating a miniature wide-angle lens. Peepholes suffer from the problem that the viewer's face must be pressed against a tiny hole.

United States Patent No. 4,082,434 discloses a wide-angle door viewer comprising a concave objective lens, an intermediate concave lens and a convex eyepiece lens. The eyepiece lens is positioned at a predetermined distance from the objective lens. The intermediate lens corrects the aberration of the erect virtual image formed by the objective lens. The eyepiece lens magnifies the image formed by the intermediate lens. A magnified final erect virtual image is formed on the eyepiece lens. The 4,082,434 apparatus suffers from the problem that the location of the virtual image makes it impractical to insert a diffusing screen to provide a real image. Therefore, the user's eye must be positioned close to the eyepiece lens. Further, the small effective diameter of the concave objective lens results in a dim image. Increasing the effective diameter of the concave objective lens to provide a brighter image will allow visual access from outside unless a shutter is incorporated into the viewer.

United States Patent No. 4,257,670 discloses an optical peephole device comprising three lens assemblies disposed serially along a common optical axis. The first assembly provides a doublet comprising a thick-edged meniscus and a double-concave lens. The second assembly comprises five identical plano-convex lenses equidistantly spaced from each other. The third assembly provides accommodation and comprises a plano-convex lens and an eyepiece. An erect virtual image formed by the meniscus is converted into an inverted real image by the plano-convex lens. The other plano-convex lenses correct aberrations and performs a second inversion on said inverted real image, such that the final erect real image is formed on the plano-convex lens. The disadvantage of the 4,257,670 apparatus is that although the image derived from the plano-convex lens is erect and real, the luminance of the final image suffers from the transmission losses incurred by the large number of lenses. As in the case of the 4,082,434 apparatus it is not possible to provide a real image and consequently the user's eye must be positioned close to the eyepiece. Furthermore, the device is not suitable for typical domestic door applications due to its large overall length.

United States Patent No. 4,892,399 by Ahn discloses a door viewer comprising two prisms of rectangular isosceles triangle shape in cross section whose hypotenuse surfaces abut horizontally, a front convex lens, an intermediate plano-convex lens and a plano-convex eyepiece lens. The front convex lens has a front concave surface and a rear convex surface to correct chromatic aberration. The convex surfaces of the intermediate and eyepiece lenses are positioned face to face with each other to correct barrel distortion. The door viewer casts an image onto a ground glass screen formed on or provided abutting the eyepiece lens.

Door viewers based on the principles of the Ahn device are capable of providing a small real image, typically 25-60 millimeters in size, that can be viewed from a small distance. A commercially available door viewer based on the Ahn invention, known as the Ultra Vista door viewer, is distributed via the internet website www.doorviewers.ca. The Ultra Vista door viewer provides a 132^° horizontal field of view and has an output image screen size of approximately 57 millimeters diameter. The image may be viewed from a range of approximately 2 meters and has the appearance of a miniature television display. The required door opening size is 56 millimeters for door thicknesses in the approximate range 20 to 45 millimeters. However, door viewers based on the Ahn invention suffer from the problem that the viewing screen size roughly determines the size of the door hole. It is therefore difficult to provide a large area screen using a viewer designed according to the principles of the Ahn invention.

There are several problems to be overcome in designing a door viewer with a small door aperture and a large area screen. To achieve a high image brightness the lens system requires a numerically low F- number, where F- number is defined as the focal length of the image projection lens divided by the effective aperture of the lens.

There are trade-offs to be made between the angle of surveillance, the range of screen viewing angles available to users, screen size and door size. Basic optical theory dictates that product of the entrance pupil area multiplied by the light collection solid angle corresponding to the field of surveillance should be roughly equal to the maximum screen viewing solid angle multiplied by the screen area. Providing a door viewer with a large viewing screen, a wide field of surveillance and a wide viewing angle will tend to increase the size of the entrance pupil. This in turn will increase the overall diameter of the lens and hence the size of door hole required.

In order to minimize the thickness of the door viewer the projection screen should have a large bend angle. In other words, the screen should be capable of directing light incident at a steep angle to the screen surface into an average direction substantially normal to the screen surface. It is difficult to maximize the photometric and screen thickness requirements simultaneously.

United States Patent No. 6,511,186 by Burstyn et al discloses a screen in which light rays having acute incidence angles of a screen are deflected into the viewing space by Total Internal Reflection (TIR) Fresnel lens elements or by diffractive elements. However, the apparatus disclosed by Burstyn is not suitable for numerically small F-number illumination due to the small dimensions of the Fresnel lens facets.

There is a need for a low cost door viewer that offers a large viewable area, ideally around 100 to 150 millimeters diagonal. The field of view should be 130 degrees horizontal. The installation requirements should be no more demanding in terms of door alterations and installer skill than existing technologies. The screen should be viewable from a range of around 2 meters and for a representative range of viewer heights. Desirably, the door hole size should be in the range 40-60 mm. The device should have minimal projection from the front or rear surfaces of the door. The device should provide means for eliminating stray light that may impair the quality of the output image.

Thus there exists a need for an improved door viewer that can provide a wide field of surveillance, a large area viewable image and a thin form factor requiring only a small door aperture.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved door viewer with a wide field of surveillance, a large area viewable image, and a thin form factor requiring only a small door aperture.

The objects of the invention are achieved in a first embodiment comprising a wide-angle lens system incorporating an image inverter, a multiple reflection lens system and a diffusing screen. The wide-angle lens system is optically coupled to the multiple reflection lens system and is disposed between the multiple reflection lens and the external scene. The multiple reflection lens system comprises at least a first transmitting surface operative to admit light from an external scene into the door viewer, a second transmitting surface for transmitting a first region of the field of surveillance towards a viewer; a third transmitting surface for transmitting a second region of the field of surveillance towards a viewer; a first reflecting surface; and a second reflecting surface. A first multiplicity of optical paths from said external scene to the viewer passes through the first transmitting surface, traversing at least one light refracting medium and passing through the second transmitting surface towards the viewer. A second multiplicity of optical paths from said external scene to said viewer passes through the first transmitting surface, undergoing a first reflection at the first reflecting surface and a second reflection at the second reflecting surface, and passing through the third transmitting surface towards the viewer, said paths traversing at least one light refracting medium. The first multiplicity of optical paths corresponds to incident light having an angle of incidence at the first transmitting surface less than or equal to a predefined value and said second multiplicity of optical paths corresponds to incident light having an angle of incidence at the first transmitting surface greater than said predefined value.

The second reflecting surface surrounds the first transmitting surface. The first reflecting surface surrounds the second transmitting surface and the third transmitting surface surrounds both the first reflecting surface and the second transmitting surface. In a preferred operational configuration the second transmitting surface, the first reflecting surface and the third transmitting surface lie on a first single continuous surface and the first transmitting source and the second reflecting surface lie on a second single continuous surface. Said first and second single continuous surfaces enclose at least one refractive index medium.

At least one of the second or third transmitting surfaces of the multiple reflection lens system may have diffusing characteristics.

Each surface of the multiple reflection lens system may be characterized by one of a spherical, Fresnel, diffractive or aspheric optical surface form. Each surface of the multiple reflection lens system may have an anamorphic surface form. Each surface of the multiple reflection lens system may have a conical surface form.

At least one of the first and second reflecting surfaces of the multiple reflection lens system may function as a total internal reflection surface. At least one of the first and second reflecting surfaces of the multiple reflection lens system may have a reflective coating. The wide angle lens system and the multiple reflection lens system together form an image of the external scene on the diffusing screen.

The image inverter is an optical device operative to invert the input image in at least the vertical sense. The image inverter incorporates at least one air gap orientated to block the propagation of stray external light by means of total internal reflection

In the first embodiment of the invention the image inverter comprises a pair of identical upper and lower optical components. Each component further comprises an input surface that admits light from the external scene, a reflecting surface and an exit surface. The reflecting surfaces of said components are disposed back-to-back arranged back-to-back substantially overlapping and parallel to each other. Said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence. Each component comprises two optical elements disposed in sequence along the light path from the external scene. The first element has a first surface that provides the input surface of the image inverter and a second surface. The second element has a first surface of the same shape as the second surface of the first element and a second surface that provides the exit surface of the image inverter. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that will not result in a reflection at the planar reflecting surface. Incident ambient light that is not directed out of the inverter by the second surface of the first element passes through the input surface is reflected by said reflecting surface and finally passes through the exit surface. At least one of said first surface of said first element and said second surface of said second element may be curved. At least one of said first surface of said first element and said second surface of said second element may be tilted surfaces. At least one of said first surface of said first element and said second surface of said second element may conical surfaces.

The reflecting surface in each component of the inverter may be mirror coated or may alternatively be a total internal reflection surface. Desirably, said reflecting surfaces are aligned parallel to the horizontal viewing plane. In the case where the reflecting surfaces rely on total internal reflection, said surfaces would be separated by a small air gap. Where a mirror coating is used the reflecting surfaces may be in contact. Said input and exit surfaces may be planar. Alternatively, at least one of said input and exit surfaces may be curved.

In one embodiment of the invention at least one of the input or output surfaces of the image inverter is a Fresnel lens.

The diffusing screen is disposed between the multiple reflection lens system and the viewer. Said diffusing screen comprises a central portion disposed between the second transmitting surface of the multiple reflection lens system and the viewer and a surrounding portion disposed between the third transmitting surface and the viewer. The central portion of the diffusing screen is designed to bend rays emerging from the central portion of the multiple reflection lens into a predetermined viewing direction. The outer portion of the diffusing screen is designed to bend rays emerging from the outer portion of the multiple reflection lens into a predetermined viewing direction. All of the optical surfaces of the door viewer may have a common axis of symmetry.

In another embodiment of the invention similar to the first embodiment a further lens system is disposed between the second transmitting surface of the multiple reflection lens system and the central portion of the diffusing screen.

In another embodiment of the invention similar to the first embodiment the multiple reflection lens systems is divided into two air spaced portions such that the first and second multiplicity of ray paths each traverse at least one air space. The air space is enclosed by a pair of opposing optical surfaces. Said opposing surfaces may have any of the optical surface forms used in the first embodiment and may each comprise more than one type of optical surface form.

In another embodiment of the invention similar to the first embodiment the first multiplicity of optical paths corresponds to incident light having angles of incidence less than the critical angle at the first reflecting surface. The second multiplicity of optical paths corresponds to incident light having angles of incidence greater than or equal to the critical angle at the first reflecting surface.

The objects of the invention are achieved in a further embodiment in which the multiple reflection lens of the first embodiment is divided into first and second optical elements. In said alternative embodiment of the invention the image inverter comprises a pair of identical upper and lower components. Said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence. Each component further comprises an input surface, a reflecting surface and an exit surface. In said alternative embodiment of the invention the first element of the MLR has a first surface that admits light from the wide-angle lens and a second surface. The second element has a first surface of the same shape as the second surface of the first element and a second surface. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that did not result in a reflection at the planar reflecting surface. At least one of said first surface of said first element and said second surface of said second element may be curved. At least one of said first surface of said first element and said second surface of said second element may be tilted surfaces. At least one of said first surface of said first element and said second surface of said second element may conical surfaces. Incident ambient light that is not directed out of the multiple reflection lens by the second surface of the first element is imaged according to the principles of the first embodiment of the invention.

The objects of the invention are achieved in one embodiment comprising a wide-angle lens and a diffusing screen. Said embodiment eliminates the multiple reflection lens. The wide- angle lens incorporates an image inverter. The wide-angle lens may further comprise at least one separated optical element disposed between the inverter and the screen. The wide-angle lens may further comprise at least one separate optical element disposed between the external scene and the inverter. The wide-angle lens forms an image of the external scene on the screen. The image inverter comprises a pair of identical upper and lower optical components. Said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence. Each component further comprises an input surface that admits light from the external scene, a reflecting surface and an exit surface. The reflecting surfaces of said components are disposed back-to-back substantially overlapping and parallel to each other. Each said optical component is divided into first and second optical elements disposed in sequence along the light path from the external scene. The first element has a first surface that provides the input surface of the image inverter and a second surface. The second element has a first surface of the same shape as the second surface of the first element and a second surface that provides the exit surface of the image inverter. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that will not result in a reflection at the planar reflecting surface. Incident ambient light that is not directed out of the inverter by the second surface of the first element passes through the input surface is reflected by said reflecting surface and finally passes through the exit surface. At least one of said first surface of said first element and said second surface of said second element may be curved. At least one of said first surface of said first element and said second surface of said second element may be tilted surfaces. At least one of said first surface of said first element and said second surface of said second element may conical surfaces.

In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen. In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen incorporating a Fresnel surface and a diffusing surface

In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen incorporating at least one Fresnel surface.

In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen incorporating at least one diffusing surface.

In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen incorporating at least one curved surface.

In one group of embodiments of the invention there is provided viewing apparatus comprising an image inverter a single element lens and a screen incorporating at least one curved refracting surface.

In one embodiment of the invention at least one ancillary light source may be attached externally near the input surface of the wide angle lens. The illumination source may be controlled by a controller disposed in proximity to the screen providing an on/off switch and/or means for controlling the level of illumination. Alternatively, the illumination source may be remotely controlled by means of a remote controller. In one embodiment of the invention externally mounted light sensor may be used to match illumination from said ancillary light source to the external light level.

In one embodiment of the invention the illumination from said ancillary light source may be initiated automatically by detecting activity and presence in proximity to the door using a passive infrared sensor.

In one embodiment of the invention an interface for providing two way audio communications between the subject and a person viewing the screen may be provided. The apparatus incorporates an audio transmitter/receiver operative to transmit and receive audio signals mounted in proximity to the input surface of the wide angle lens. The apparatus further incorporates an audio transmitter/receiver operative to transmit and receive audio signals mounted in proximity to the screen. The audio transmitter/receivers may be switched on/off using a control device mounted near to the viewer screen or using a hand-held remote control device.

In one embodiment of the invention the subject may be illuminated by scavenging room light and traversing at least part of the input light path in reverse. In an alternative embodiment of the invention directed at scavenging room light the illuminated light may be captured via ports distributed around the periphery of the viewer screen.

In one embodiment of the screen may incorporate a tilt facility to improve visibility of the screen for persons outside the design height range. In one embodiment of the invention the screen may be designed tilt around a horizontal axis providing a first screen position characterized by substantially downward output light directions and a second screen position characterized by substantially upward output light directions

The objects of the invention are achieved in one embodiment comprising an image rotator as described above and a diffusing screen. Said embodiment eliminates the multiple reflection lens. The wide-angle lens incorporates an image inverter.

In yet further embodiments of the invention at least one light control material may be disposed within the TIR gap of the image inverter. Each said light control material may provide at least one of reflection, light transmission or light absorption

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings wherein like index numerals indicate like parts. For purposes of clarity details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. IA is a schematic three-dimensional view of an operational configuration of then invention. FIG. IB is a schematic three-dimensional view of an operational configuration of then invention.

FIG.2A is a schematic side elevation view of a first embodiment of the invention.

FIG.2B is a schematic rear elevation view of a first embodiment of the invention.

FIG.2C is a schematic side elevation view of an image inverter that may be used in the invention. FIG.2D is a schematic front elevation view of an image inverter that may be used in the invention.

FIG.2E is a schematic side elevation view of another image inverter that may be used in the invention. FIG.2F is a schematic side elevation view of a further image inverter that may be used in the invention.

FIG.2G is a schematic side elevation view of a further image inverter with that may be used in the invention.

F1G.2H is a schematic side elevation view of a yet further another image inverter that may be used in the invention.

FIG.3 is a schematic rear elevation view showing the central and peripheral regions of the displayed image.

FIG.4 is a schematic side elevation view showing the propagation of rays in a first embodiment of the invention. FIG.5 is a schematic side elevation view of a second embodiment of the invention.

FIG.6 is a schematic side elevation view showing the propagation of rays in a second embodiment of the invention.

FIG.7 is a schematic side elevation view of a further embodiment of the invention.

FIG.8 is a schematic side elevation view of a yet further embodiment of the invention. FIG.9 is a schematic side elevation view of a yet further embodiment of the invention.

FIG.10 is a schematic side elevation view of a detail of a first embodiment of the invention.

FIG.l 1 is a schematic side elevation view of an optical surface used in a further embodiment of the invention.

FIG.12C is a schematic side elevation view of an image inverter that may be used in the invention. FIG.12D is a schematic front elevation view of an image inverter that may be used in the invention.

FIG.12E is a schematic side elevation view of another image inverter that may be used in the invention. FIG.12F is a schematic side elevation view of a further image inverter that may be used in the invention.

FIG.12G is a schematic side elevation view of a further image inverter with that may be used in the invention.

FIG.12H is a schematic side elevation view of a yet further another image inverter that may be used in the invention.

FIG.13 is a schematic side elevation view of a yet further embodiment of the invention.

FIG.14 is a schematic side elevation view of a yet further embodiment of the invention.

FIG.15C is a schematic side elevation view of an image inverter that may be used in the invention. FIG.15D is a schematic front elevation view of an image inverter that may be used in the invention.

FIG.15E is a schematic side elevation view of another image inverter that may be used in the invention.

FIG.15F is a schematic side elevation view of a further image inverter that may be used in the invention.

FIG.15G is a schematic side elevation view of a further image inverter with that may be used in the invention.

FIG.15H is a schematic side elevation view of a yet further another image inverter that may be used in the invention. FIG.13 is a schematic side elevation view of a yet further embodiment of the invention. FIG.14 is a schematic side elevation view of a yet further embodiment of the invention

FIG.16 is a schematic side elevation view of a yet further embodiment of the invention.

FIG.17 is a schematic side elevation view of a yet further embodiment of the invention.

FIG.18A is a schematic side elevation view of a particular embodiment of the invention. FIG.18B is a schematic side elevation view of another particular embodiment of the invention.

FIG.18C is a schematic side elevation view of another particular embodiment of the invention.

FIG.18D is a schematic side elevation view of another particular embodiment of the invention.

FIG.19A is a schematic side elevation view of a stray light control method for use with the invention.

FIG.19B is a schematic side elevation view of a stray light control method for use with the invention. FIG.19C is a schematic side elevation view of a stray light control method for use with the invention.

FIG.19D is a schematic side elevation view of a stray light control method for use with the invention.

FIG.19E is a schematic side elevation view of a stray light control method for use with the invention.

FIG.20A is a schematic side elevation view of an embodiment of the invention using an image inverter and a single lens element.

FIG.20B is a schematic side elevation view of another embodiment of the invention using an image inverter and a single lens element. FIG.20C is a schematic side elevation view of a further embodiment of the invention using an image inverter and a single lens element.

FIG.21A is a schematic side elevation view of an embodiment of the invention using an image inverter and a single lens element with a screen comprising two Fresnel surfaces. FIG.21B is a schematic side elevation view of another embodiment of the invention using an image inverter and a single lens element with a screen including a Fresnel surface.

FIG.21C is a schematic side elevation view of a further embodiment of the invention using an image inverter and a single lens element with a screen including a Fresnel surface.

FIG.22A is a schematic side elevation view of an embodiment of the invention using an image inverter and a single lens element with a screen comprising a curved refracting surface and a Fresnel surface.

FIG.22B is a schematic side elevation view of an embodiment of the invention using an image inverter and a single lens element with a screen comprising a curved refracting surface and a planar surface. FIG.23 is a schematic side elevation view of an image inverter incorporating Fresnel input and output surfaces.

FIG.24 is a schematic side elevation view of another image inverter incorporating Fresnel input and output surfaces.

FIG.25 is a schematic side elevation view of an image inverter incorporating materials for stray light control.

FIG.26 is a schematic side elevation view of another image inverter incorporating materials for stray light control.

FIG.27A is a schematic side elevation view of another embodiment of the invention using an image inverter and a single lens element with a screen including a Fresnel surface. FIG.27B is a schematic side elevation view of another embodiment of the invention using an image inverter and a single lens element with a screen including a Fresnel surface.

FIG.28 is a schematic side elevation view of one embodiment of the invention incorporating ancillary illumination. FIG.29 is a schematic side elevation view of another embodiment of the invention incorporating ancillary illumination.

FIG.30 is a schematic side elevation view of one embodiment of the invention incorporating an ancillary illumination source operated by a remote controller.

FIG.31 is a schematic side elevation view of another embodiment of the invention incorporating ancillary illumination and a light sensor.

FIG.32 is a schematic side elevation view of another embodiment of the invention incorporating an ancillary illumination source and a light sensor.

FIG.33 is a schematic side elevation view of another embodiment of the invention incorporating two way audio communication apparatus. FIG.34 is a schematic side elevation view of another embodiment of the invention incorporating two way audio communication apparatus, an ancillary light source and a remote controller.

FIG.35 is a schematic side elevation view of another embodiment of the invention operative to scavenge room light for illuminating the subject. FIG.35 is a schematic side elevation view of another embodiment of the invention incorporating a tilted screen.

DETAILED DESCRIPTION OF THE INVENTION

The basic concept of a door viewer according to the principles of the invention is shown in FIG.1. FIG.1 A shows a schematic three-dimensional view of a door viewer. In a first embodiment of the invention the door viewer comprises a wide-angle lens system 10, multiple reflection lens system 20 and a viewing screen element 30. Said wide-angle lens system comprises at least an image inverter which will be described in more detail below and an optical interface to said multiple reflection lens systems. The wide-angle lens system may further comprise additional lens elements. FIG. IB shows a schematic side elevation showing the door viewer in a typical operational configuration. The wide-angle lens system is inserted into a cylindrical hole in the door 40. In FIG.1A the input rays are generally indicated by 1000 and the output rays are generally indicated by 2000. It should be noted that FIG.l is provided only for the purposes of showing the approximate appearance of the invention in a typical operational configuration. The details of the optical system are discussed in the descriptions of the embodiments of the invention given below.

A first embodiment of the door viewer is illustrated schematically in FIG.2. According to FIG.2A the door viewer comprises a wide-angle lens system 1 a multiple reflection lens system 2 and a diffusing screen 4. The wide-angle lens system comprises at least a front refracting surface 11 and a surface 12 that provides the entrance surface to the multiple reflection lens system. Surface 12 may be an internal surface of the wide-angle lens.

Alternatively, surface 12 may be the rear surface of the wide-angle lens, said rear surface being either in contact with or air-separated from the multiple reflection lens system. Alternatively, the wide-angle lens may form part of the multiple reflection lens system, with surface 12 corresponding to a virtual surface separating the wide-angle and multiple reflection lens systems. The multiple reflection lens system comprises the entrance surface 12, the curved reflecting surfaces 21a, 21b, a central curved surface portion 22, the curved reflective surface portions 23a, 23b and the curved transmitting surface portions 24a, 24b. In a preferred embodiment of the invention surfaces 21a, 12, 21b form a first single continuous surface and surfaces 24a, 23a, 22, 23b, 24b form a second single continuous surface. Said first and second surfaces enclose at least one refracting medium. Desirably the refracting medium is an optical plastic. Alternatively the refracting medium many be an optical glass. For the purposes of describing the invention the lenses will be assumed to be axi-symmetric and the invention will be discussed in terms of rays confined to the meridional plane intersecting the points AA'. It will also be understood that that curved reflecting surfaces 21a, 21b and 23 a, 23b and curved transmitting surface portions 24a, 24b represent intersection of annular surface areas with said meridional plane. Hence, the multiple reflection lens system comprises the entrance surface 12, the curved reflecting surfaces 21a, 21b, a central curved surface portion 22, the curved reflective surface portions 23a, 23b and the curved transmitting surface portions 24a, 24b. FIG.2B provides a rear elevation vide view of the rear surface of the multiple reflection lens system showing the disposition of the actual surface portions corresponding to meridional section surfaces 24a, 23a, 22, 23b, 24b. The reflecting surfaces of the multiple reflection lens system may rely on total internal reflection. Alternatively, the reflecting surface may use mirror coatings. Each surface of the multiple reflection lens system may be characterized by one of a spherical, Fresnel, diffractive or aspheric optical surface form. Each surface of the multiple reflection lens system may have an anamorphic surface form. Each surface of the multiple reflection lens system may have a conical surface form. The wide-angle lens 1 has two main functions. The first function is to collect light over a large field angle. Typically the wide-angle lens collects light from 0 degrees to greater than ±60 degrees. Since the multiple reflection lens effectively inverts the image it is necessary to apply an opposite inversion. Accordingly, the second function of the wide-angle lens is to reverse the orientation of the image in at least the vertical direction. The wide-angle lens therefore incorporates an image inverter which will be described in the following paragraphs.

The wide-angle lens system may incorporate one or more lens elements. The lens may be designed to provide an internal aperture stop. Alternatively the lens may be designed to have an external stop as, for example, in a landscape lens. The wide-angle lens system may include at least one of spherical, aspherical, diffractive and other surface forms known to those skilled in the art. The invention is not limited to any particular type of wide-angle lens configuration.

The image inverter is an optical device operative to invert the input image in at least the vertical sense. In one embodiment of the invention the image inverter comprises a pair of identical upper and lower optical components. Said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence. Each said optical component comprises an input surface that admits light from the external scene, a planar reflecting surface and an exit surface. The reflecting surfaces of said components are disposed back-to-back substantially overlapping and parallel to each other. Incident ambient light passes through the input surface is then reflected by said reflecting surface and finally passes through the exit surface. The reflecting surface may be mirror coated or may alternatively be a total internal reflection surface. Desirably, said reflecting surfaces are aligned parallel to the horizontal viewing plane. In the case where the reflecting surfaces rely on total internal reflection, said surfaces would be separated by a small air gap. Where a mirror coating is used the reflecting surfaces may be in contact. Said input and exit surfaces may be planar. Alternatively, at least one of said input and exit surfaces may be curved.

Each said optical component is divided into first and second optical elements disposed in sequence along the light path from the external scene and separated by an air gap. The purpose of the air gap is to block the propagation of stray external light by means of total internal reflection. Stray light is defined as any external light following paths that do not intersect with one of the reflecting surfaces described above. The first element has a first surface that provides the input surface of the image inverter and a second surface. The second element has a first surface of the same shape as the second surface of the first element and a second surface that provides the exit surface of the image inverter. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that will not result in a reflection at the planar reflecting surface. Incident ambient light that is not directed out of the inverter by the second surface of the first element passes through the input surface is reflected by said reflecting surface and finally passes through the exit surface. At least one of said first surface of said first element and said second surface of said second element may be curved. At least one of said first surface of said first element and said second surface of said second element may be tilted surfaces.

FIGS.2C-2GH show examples of preferred operational embodiments of the image inverter. FIGS 2C and FIGS 2E to 2H show schematic side elevation views. The basic principles of the image inverter may be understood by referring first to FIGS .2C and FIG.2D where FIG.2D is a front elevation view of the image inverter shown in FIG.2C. The image inverter incorporates at least one air gap designed to block the propagation of stray external light by means of total internal reflection. The configuration and function of the air gap will be illustrated in FIGS.2E-2H and is not shown in FIG.2C and FIG.2D. The inverter 50 comprises a pair of identical elements 51,52. The element 51 comprises an input surface 51a, a reflecting surface 51 b and an exit surface 51c. The reflecting surface 51b may be mirror coated or may alternatively be a total internal reflection (TIR) surface. The reflecting surfaces would be separated by a small air gap in the case of a TIR configuration. Where a mirror coating is used the reflecting surfaces may be in contact. The image inversion process is indicated in a schematic fashion by the rays 1101,1102,1103,1104. A virtual surface 13 marked by a dashed line represents the effective aperture of the image rotator. The surface 13 may lie outside the image rotator as shown. The surface 13 may coincide with the exit surface of the image rotator. The surface 13 may coincide with the entrance surface 12. Alternatively, the surface 13 may be separated from the entrance surface 13 by an air gap. Alternatively, the surface 13 may be an internal surface of the multiple reflection lens system.

As shown in the front elevation schematic view of FIG.2D the image inverter may have a rectangular cross section. FIG.2D shows the front surfaces 51a, 52a of the elements 51,52 illustrated in FIG.2C. The image inverter may have other cross section shapes according to the application and the available physical aperture. For example the cross section may be designed to fit inside a circular aperture.

FIGS.2E shows an example of an image inverter 60 similar to the one shown in FIG.2C. The inverter comprises a pair of identical components 61,62. The component 61 comprises a planar input surface 61a, a planar reflecting surface 61b, a planar exit surface 61c and an outer surface 6 Id. The component 61 includes an air gap 61e bounded by the planar surfaces 6 If, 6 Ig. The exit surface and the reflecting surface form a right angle. The air gap divides the component 61 into the two elements 61A,61B. . Since the component 62 is configured in an identical fashion to the component 61, the details of the component 62 are omitted from the drawing. The image inversion process is indicated in a schematic fashion by the ray path indicated by the rays 1201,1202,1203,1204. The path of a ray that undergoes total internal reflection at an air gap is indicated by 1205,1206. Surfaces 63a,64a and surfaces 63b,64b may represent planar surfaces. In alternative embodiments surfaces 63a,64a and surfaces 63b,64b may represent curved surfaces. In alternative embodiments surfaces 63a,64a and surfaces 63b,64b may represent cross sections of conical surfaces.

FIG.2F shows another example of an image inverter 70 that may be used in the invention. The inverter comprises a pair of identical optical components 71,72. The component 71 comprises a planar input surface 71a, a planar reflecting surface 71b and a planar exit surface 71c. The component 71 includes an air gap 71 e bounded by the planar surfaces 7 If, 7 Ig. The input and exit surfaces are both tilted with respect to the reflecting surface. The air gap divides the component 71 into the two elements 71A,71B. . Since the component 72 is configured in an identical fashion to the component 71, the details of the component 72 are omitted from the drawing. The image inversion process is indicated in a schematic fashion by the rays 1301,1302,1303,1304. The path of a ray that undergoes total internal reflection at an air gap is indicated by 1305,1306. Surfaces 73a,74a and surfaces 73b,74b may represent planar surfaces. In alternative embodiments surfaces 73a,74a and surfaces 73b,74b may represent curved surfaces. In alternative embodiments surfaces 73a, 74a and surfaces 73b,74b may represent cross sections of conical surfaces. In principle the multiple reflection lens could be designed to provide all of the optical power necessary to form the final real image. However with aberration control in mind it would be advantageous to incorporate some degree of optical power within the wide-angle lens. A wide-angle lens incorporating the planar surface image inverter shown in FIGS.2C-2F would therefore require additional lens elements to provide optical power. The need for further lens elements may be avoided by including curved surfaces in the image inverter.

FIG.2G shows one example of an image inverter 80 that also provides optical power. The inverter comprises a pair of identical optical components 81,82. The component 81 comprises a curved input surface 81a, a planar reflecting surface 81b and a curved exit surface 81c. The component 81 includes an air gap 81e bounded by the planar surfaces 81f,81g. The air gap divides the component 81 into the two elements 81A,81B. . Since the component 82 is configured in an identical fashion to the component 81, the details of the component 82 are omitted from the drawing. The image inversion process is indicated in a schematic fashion by the ray path indicated by the rays 1401,1402,1403,1404. The path of a ray that undergoes total internal reflection at an air gap is indicated by 1405,1406. Surfaces 83a,84a and surfaces 83b,84b may represent planar surfaces. In alternative embodiments surfaces 83a,84a and surfaces 83b, 84b may represent curved surfaces. In alternative embodiments surfaces 83 a, 84a and surfaces 83b,84b may represent cross sections of conical surfaces.

FIG.2H shows another example of an image inverter 90 that also provides optical power. The inverter comprises a pair of identical optical components 91,92. The component 91 comprises a curved input surface 91a, a planar reflecting surface 91b and a planar exit surface 91c. The component 91 includes an air gap 9 Ie bounded by the planar surfaces 91f,91g. The air gap divides the component 91 into the two elements 91A,91B. . Since the component 92 is configured in an identical fashion to the component 91, the details of the component 92 are omitted from the drawing. The image inversion process is indicated in a schematic fashion by the ray path indicated by the rays 1501,1502,1503,1504. The path of a ray that undergoes total internal reflection at an air gap is indicated by 1505,1506. Surfaces 93a,94a and surfaces 93b,94b may represent planar surfaces. In alternative embodiments surfaces 93a,94a and surfaces 93b,94b may represent curved surfaces. In alternative embodiments surfaces 93a,94a and surfaces 93b,94b may represent cross sections of conical surfaces.

Many other image inversion schemes similar to those described above will be apparent to those skilled in the art.

The screen 4 is fabricated from a rear projection screen material having a suitable diffusion angle. The diffusion angle will be determined from consideration of the required range of viewing distances and viewer heights. As shown in FIG.2A the screen comprises a central portion 41 and outer annular portion represented by 42a, 42 according to the earlier defined geometrical convention. The central portion 41 and the outer portions 42a, 42b of the screen may be designed to provide different diffusion characteristics. For example, the central and outer positions may be fabricated from different materials. Alternatively, the central and outer portions may have different structures. One or both of the screen portions may incorporate diffractive structures, which are designed to have combined light bending and diffusing properties. The screens may be based on Fresnel surfaces. One or more portions of the screen may employ holographic light shaping diffusers. The screen may be physically separated from the surfaces of the lens 3 and the multiple reflection lens 2. Alternatively the screen may abut the surfaces the lens 3 and the multiple reflection lens 2. The screen may be curved. The screen may be implemented on one or both of the outer surfaces of the multiple reflection lens 2 as a thin layer of scattering material deposited onto said outer surfaces or a surface relief structure formed in said outer surface. The screen may be designed to direct light into vertical directions that are substantially downwards.

The formation of the viewed image according to the first embodiment of the invention using the apparatus of FIG.2 is now explained with reference to FIG.3 and FIG.4. FIG.3 illustrates the geometrical characteristics of the image displayed on the screen. A central circular image portion 300 is formed as a result of low incidence angle light propagating through surface 11 of the wide-angle lens system element, the virtual interface 13, surface 22 of the multiple reflection lens system, lens 3 and screen element 41. An annular image region 400 substantially abutting the circular region is formed as a result of high incidence angle light propagating through surface 11 of the wide angle lens system and the virtual interface 13, undergoing reflections at surfaces 23a, 23b and 21a, 21b, and propagating through transmitting surfaces 24a, 24b and screen elements 42a, 42b. The effect of any visible join between the central and annular regions 300,400 can be minimized by careful optical design. However, a visible boundary is likely to acceptable for most applications. The formation of the image regions 300 and 400 will now be explained in more detail with reference to FIG.4.

FIG.4 shows the propagation of incident light rays in the meridional plane. We consider a low incidence angle ray 100 and a high incidence angle ray 200. The ray 100 is the limiting ray that corresponds to the edge of the circular region 300. In other words rays with incidence angles equal to or less than that of the ray 100 will be imaged in the circular region 300. The ray 200 is the limiting ray that defines the inner edge of the outer annular region 400. In other words rays with incidence angles equal to or greater than that of the ray 200 will be imaged in the annular image region 400. In practice the precise ray paths through the wide-angle lens system will depend on the optical configuration of the wide-angle lens and the type of image rotation mechanism incorporated therein. In order to simplify the explanation of the invention the ray paths through the wide-angle lens have not been shown in detail. It may be assumed that the wide-angle lens incorporates an image inverter based on similar principles to the ones shown in FIGS .2C to2H. Accordingly, the paths of the rays inside the wide- angle lens system are represented by the dashed lines 101, 201. After propagation through the wide-angle lens the rays 101, 201 enter the multiple reflection lens as the rays 102 and 202 respectively.

We first consider the propagation of the incident ray 200 after propagation through the wide- angle lens and into the multiple reflection lens system 2. The ray 202 intercepts the first reflective surface 23 a and is reflected in the direction 203 towards the second reflective surface 21a where it is reflected into the direction 204. The reflected ray 204 impinges on the refracting surface 24a where it is refracted into the direction 205 towards the screen element 42a. The ray is scattered at the screen element 42a into the diffuse ray directions generally indicated by 206.

We now consider the propagation of the incident ray 100 after propagation through the wide- angle lens and into the multiple reflection lens system 2. The ray 102 intercepts the central refracting surface 22 where it is refracted into the ray direction 103 towards the diffusing screen element 41. The screen element 41 is designed to bend rays emerging from the central portion of the multiple reflection lens into a viewing direction substantially normal to the screen surface. Finally the ray 103 is scattered by the screen element 41 into the diffuse directions generally indicated by 104. A second embodiment of the proposed wide angle-viewing device is illustrated schematically in FIG.5. The viewing device comprises the wide-angle lens system 1 and multiple reflection lens system 2 and the diffusing screen 4 and a further lens system 3. Since the characteristics of the wide-angle lens and multiple reflection lens systems are similar to those of the embodiment shown in FIGS.2-4 the same labels have been used to describe the surface elements. The screen 4 may be based on any of the surface types discussed in relation to the embodiments shown in FIGS.2-4. The screen comprises a central portion 41 and an outer surrounding portion represented by 41a, 41b. Said inner and outer portions may have substantially different scattering properties.

FIG.6 shows the propagation of incident rays in the meridional plane. The rays are defined in a similar fashion to the rays 100,200 of FIG.4. We consider a low incidence angle ray 110 and a high incidence angle ray 210. The paths of the rays inside the wide-angle lens systems are represented by the dashed lines 111, 211 where once again the illustration of the light propagation inside said lens has been simplified for the purposes of explaining the invention. It may again be assumed that the wide-angle lens will incorporate an image inverter similar to the ones shown in FIGS.2C to2H. ^• After propagation through the wide-angle lens the rays 111, 211 enter the multiple reflection lens as the rays 112 and 212 respectively.

We first consider the propagation of the incident ray 210 after propagation through the wide- angle lens and into the multiple reflection lens system 2. The ray 212 intercepts the first reflection surface 23 a and is reflected in the direction 213 towards the second reflection surface 21a where it is reflected into the direction 214. The reflected ray 214 impinges on the refracting surface 24a where it is refracted into the direction 215 towards the screen element 42a. The ray 215 is scattered by the screen element 42a into diffuse ray directions generally indicated by 216.

We next consider the propagation of the incident ray 110 after propagation through the wide- angle lens and into the multiple reflection lens system 2. The ray 112 intercepts the central refracting surface22 where it is refracted in the ray direction 113 towards the lens element 3. The lens element 3 directs the ray 113 into a direction 114 towards the diffusing screen region 41. Desirably the ray 114 is substantially normal to the screen region 41. Finally, the ray 114 is scattered by the screen element 41 into the diffused directions generally indicated by 115.

In a further embodiment of the invention, similar to the first embodiment, the wide-angle lens system and the multiple reflection lens system may be separated as shown in FIGS.7. The wide-angle lens system 1 comprises at least a front refracting surface 11 and a rear surface 12. The wide-angle lens may also incorporate an image rotator as discussed earlier. One advantage of having a separation between the wide-angle lens and the multiple reflection lens is that two curved surfaces are available for optimization. The basic imaging properties of the embodiment of FIG.7 are similar to those of the embodiment shown in FIG.2-4.

In a yet further embodiment of the invention similar to the first embodiment, shown in FIG.8, the multiple reflection lens system may be divided into two elements having opposing separated surfaces 25 and 26 as shown in FIG.8. Such an arrangement would provide a further two surfaces separated by an air gap for design optimization. Said surfaces may have any of the surface forms discussed earlier. Surfaces 25 and 26 may each be continuous composite surfaces comprising more than one surface form. For example, said composite surfaces may have central circular portions and outer annular portions. Alternatively, surfaces 25 and 26 may have identical but opposite curvatures such that there is no air gap between the two elements. Surfaces 25 and 26 may be planar as shown in FIG.8. Dividing the multiple reflection lens into two thinner elements may offer cost benefits if moulding processes are used to fabricate the lens elements. The basic imaging properties of the embodiment of FIG.8 are similar to those of the embodiment shown in FIG.2.

FIG.9 shows a further embodiment of the invention in which the multiple reflection lens system is divided into two elements having the opposing separated curved surfaces 27 and 28. Said surfaces may have any of the surface forms discussed earlier. For example, 27 and 28 may each be continuous composite surfaces comprising more than one surface form. Said composite surfaces may have central circular portions and outer annular portions.

It will be clear from consideration of the ray paths shown in FIG.4 that portions of light beams originating from field of view zones close to the transition between high and low incident angle lights may propagate through the central portion 22 and the first reflection region 23a at the same time. In other words a point in the ambient scene may give rise to light is imaged in to the circular region 300 and the annular region 400 at the same time. This effect could give rise to a visible join between the central and annular regions 300,400. Although a visible boundary is likely to acceptable for most applications it would be advantageous to reduce said boundary to a minimum. Referring again to FIG.4, the ray 100 is the limiting ray that corresponds to the edge of the circular region 300. In other words rays with incidence angles equal to or less than that of the ray 100 will be imaged in the circular region 300. The ray 200 is the limiting ray that defines the inner edge of the outer annular region 400. In other words rays with incidence angles equal to or greater than that of the ray 200 will be imaged in the annular image region 400. FIG.10 shows portions of surfaces 21, 22 and 23a of the multiple reflection lens and a portion of the screen 4. We consider the propagation of light rays from an ambient source that are close to the limiting ray. In particle we consider the rays 102a, 102b, 102c,102d. As shown in FIG.4, rays 102a, 102b are reflected towards surfaces 21a as the rays 302a, 302b by the reflecting surface 23 a. Rays 302a, 302b are reflected towards the surface 23 a as the rays 402a, 402b and are refracted towards the screen 4 into the directions 502a, 502b by surface 23a forming a focal spot at the point 40. At the same time rays 102c, 102d are transmitted through the surface 22 towards the screen 4 as the rays 103a 103b forming a focal spot near to the point 40.

FIG.11 shows an example of a type of surface that may be used to perform the functions of the surface 22 and 23 in FIGS .4 and FIG.10. In FIG.l 1 the surface reflects light 105a incident at an angle 110a greater than that of the limiting ray. Light 105b incident at an angle 110b less than or equal to that of the limiting ray is transmitted through the surface 25 into the ray direction 130. It should be noted that the incident light propagates in a refractive index medium. The surface 25 has optical characteristic that allow reflection to take place at angle less than the critical angle defined by the interface between said refractive index medium and air. The surface 25 may be a diffractive or holographic surface. Alternatively the surface may be a Fresnel surface.

In an alternative embodiment of the invention the image inverter schemes illustrated in FIG.2 are replaced by the alternative inverter schemes shown in FIG.12. From consideration of FIG.12 it will be seen that said alternative inverter schemes each have air gaps orientated in an opposite direction to those of FIG.2. Specifically, the changes are as follows. In FIG.12E the component 61 now incorporates an air gap 6 Ih bounded by the planar surfaces 6 Ii, 6 Ij. The air gap divides the component 61 into the two elements 61C,61D. . Since the component 62 is configured in an identical fashion to the component 61, the details of the component 62 are omitted from the drawing. In FIG.12F the component 71 now incorporates an air gap 71h bounded by the planar surfaces 7 Ii, 71j. The air gap divides the component 71 into the two elements 71C,71D . Since the component 72 is configured in an identical fashion to the component 71, the details of the component 72 are omitted from the drawing.. In FIG.12G the component 81 now incorporates an air gap 8 Ih bounded by the planar surfaces 8 Ii, 81j. The air gap divides the component 81 into the two elements 81C,81D. Since the component 82 is configured in an identical fashion to the component 81, the details of the component 82 are omitted from the drawing. In FIG.12H the component 91 now incorporates an air gap 91h bounded by the planar surfaces 9 Ii, 9 Ij. The air gap divides the component 91 into the two elements 91C,91D. Since the component 92 is configured in an identical fashion to the component 91, the details of the component 92 are omitted from the drawing.

In the first embodiment of the invention described above stray light is eliminated by providing air spaces and critical angle surfaces in then image inverter. The objects of the invention are achieved in further embodiments of the invention in which the air gaps and critical angle surfaces are provided within the multiple reflection lens of the first embodiment. Examples of such alternative embodiments are shown in FIG.13-14. The embodiments shown in FIG.13 and FIG.14 are identical to the embodiments of FIG.5 and FIG.7 respectively except that the multiple reflection lens of the first embodiment is divided into first and second optical elements separated by small air gaps. The first element of the MLR has a first surface portion that admits light from the wide-angle lens and a second surface portion defined by 25 a and 25b. The second element has a first surface portion defined by 26a and 26b of the same shape as the second surface of the first element and a second surface. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that did not result in a reflection at the planar reflecting surface. Incident ambient light that is not directed out of the multiple reflection lens by the second surface of the first element is imaged according to the principles of the first embodiment of the invention. Referring to FIG.13 and FIG.14 it will be seen that air gaps are provided between the surfaces 25a,26a and the surfaces 25b,26b. Surfaces 25a and 26a have substantially similar forms. Surfaces 25b and 26b have substantially similar forms. Surfaces 25a,26a and the surfaces 25b,26b shown in FIGS 13-14 may represent planar surfaces. In alternative embodiments surfaces 25a,26a and the surfaces 25b,26b shown in FIGS13-14 may represent curved surfaces. In alternative embodiments surfaces 25a,26a and the surfaces 25b,26b shown in FIGS13-14 may represent cross sections of conical surfaces. Apart from the provision of the said air gaps the function of the multiple reflection lens is identical to that of the first embodiment of the invention as discussed above

Note that in the alternative embodiments of the invention shown in FIGS.13- 14 the image inverter comprises a pair of identical optical components, each further comprising an input surface, a reflecting surface and an exit surface. The image inverter does not include air gaps. Typical examples of image inverters without air gaps for use in said alternative embodiment are shown in FIG.15. Apart from the elimination of air gaps the image inverters shown in FIG.15 are identical to the ones illustrated in FIG.2 or FIG.12. In an alternative embodiment of the invention illustrated in FIGS.16-17 the air gaps are be orientated in an opposing direction to those illustrated in FIGS.13-14. Specifically, in FIGS.16- 17 the multiple reflection lens now incorporates an air gap bounded by air separated surfaces 27a, 27b and 28a,28b where said air separated surfaces have substantially similar forms.

We next consider embodiments of the invention that do not use the above described multiple reflection lens.

The objects of the invention are achieved in particular embodiments comprising a wide-angle lens 1 and a diffusing screen 4. Said particular embodiments of the invention are illustrated schematically in the side elevation views of FIG.18. In each case the wide-angle lens incorporates an image inverter 8. The image inverter in said particular embodiments is substantially the same as the one used in the embodiments of FIG.2. The image inverter comprises a pair of identical upper and lower components. Said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence. Each component further comprises an input surface that admits light from the external scene, a reflecting surface and an exit surface. The reflecting surfaces of said components are disposed back-to-back substantially overlapping and parallel to each other. Each component is divided into first and second optical elements disposed in sequence along the light path from the external scene. The first element has a first surface that provides the input surface of the image inverter and a second surface. The second element has a first surface of the same shape as the second surface of the first element and a second surface that provides the exit surface of the image inverter. The second surface of the first element and the first surface of the second element are separated by a small air gap. The second surface of the first element operates as a total internal reflection surface for light from the external scene incident in directions that will not result in a reflection at the planar reflecting surface. Incident ambient light that is not directed out of the inverter by the second surface of the first element passes through the input surface is reflected by said reflecting surface and finally passes through the exit surface. At least one of said first surface of said first element and said second surface of said second element may be curved. At least one of said first surface of said first element and said second surface of said second element may be tilted surfaces. The wide-angle lens may further comprise at least one separated optical element disposed between the inverter and the screen. The wide-angle lens may further comprise at least one separate optical element disposed between the external scene and the inverter.

Since the details of the image inverters that may be used in the invention have already been illustrated in FIG.2 and FIG.12 the details of the image inverter are not shown in the schematic illustrations of FIG.18. In the embodiment of the invention shown in the side elevation view of FIG.18 A the wide- angle lens comprises an image inverter 8 of the type illustrated in FIGS.2G-2H in which optical power is provided by the image inverter. In the embodiment of the invention shown in the side elevation view of FIG.18B the wide angle lens comprises an image inverter 8 of the type illustrated in FIGS.2E-2H and a further lens element 13 disposed between said inverter and said screen..

In the embodiment of the invention shown in the side elevation view of FIG.18C the wide angle lens comprises an image inverter 8 of the type illustrated in FIGS.2E-2H and a further lens element 14 disposed between said inverter and the eternal scene.

In the embodiment of the invention shown in the side elevation view of FIG.18D the wide angle lens comprises an image inverter 8 of the type illustrated in FIGS.2E-2H, a lens element 15 disposed between said inverter and the external scene and a further lens element 16 disposed between said inverter and said screen.

In any of the embodiment shown in FIGS.18A-D the lens elements used in conjunction with the image inverter the wide angle lens may be replaced by multi element lens systems, mirrors, diffractive optical elements or any other type of imaging components used in the design of wide-angle lens systems.

It should be noted that the lenses have been illustrated as single lens elements for the purposes of simplicity. In each case the single element lens may be replaced by a multi element lens system. The use of more than one element offers more scope for the control of optical aberrations. In most consumer applications however it is desirable to minimize the number of lenses leading to some trade-offs between image quality and cost. The lens elements of FIG.18 may have any surface form currently used in lens design and known to those skilled in the art. In particular the lens elements may use spherical, aspherical, convex, concave, Fresnel or diffractive surfaces.

The embodiments of FIG.18 may suffer from the problem of stray light from the lower half of the field of view passing through the image inverter without being inverted and forming an inverted image in the upper portion of the screen. Such non-inverted light would be superimposed onto the image of the correctly inverted light from the upper half of the field of view. A similar effect may arise in the case of stray light from the upper half of the field of view. Since the upper portion of the field of view may contain illumination sources such as the sky or room lights there is a risk that the stray light may overcome the image from the lower field of view portion. Such stray light paths may arise due to the failure of TIR within the prisms due to imperfections in optical surfaces and relative misalignments of the image inverter elements. Stray light paths leading to the above effects may also arise from multiple reflection paths within the image inverter, which may occur at certain incidence angles even when the TIR surfaces are correctly aligned and free from surface errors.

Schemes for eliminating such stray light are illustrated in the series of schematic side elevation views shown in FIG.19. FIGS.19A-19D are essentially identical to the embodiments of FIGS18A-18D but incorporate light block baffles. In FIG.19A there is provided a horizontal baffle 110 between the image inverter 160 and the screen 140. In FIG.19B there is provided a horizontal baffle 111 between the lens 131 and the screen 140. In FIG.19C there is provided a horizontal baffle 112 between the image inverter 160 and the screen 140. In FIG.19D there is provided a horizontal baffle 113 between the lens 134 and the screen 140. Desirably, in the embodiments illustrated in FIGS.19A-19D the baffle is in contact with the surfaces of its bounding optical elements. However, in practice small gaps may need to be provided for assembly purposes. The embodiments of FIGS 19B and 19D suffer from the problem that stray light paths exist within lenses 131 and 134. FIG.19E illustrates an alternative embodiment similar to the one shown in FIG.19D where an additional baffle 114 is provided between the image inverter and the lens 134 and a portion of the baffle 114 is disposed within a slot cut into lens 134. It will be clear from consideration of FIG.19E that that more complete stray light blocking may be achieved by dividing the lens 134 into two equal portions and replacing baffles 113,114 with a single baffle extending from the image inverter to the screen.

We next consider further practical embodiments in which the apparatus comprises an image inverter a single element lens and a screen. As indicated above such embodiments are attractive because of their low component count and ease of fabrication. For the purposes of simplicity the baffles will not be considered. The embodiments are similar to the one illustrated in FIG.18B, which represents the simplest practical embodiment of the invention. Essentially we consider embodiments comprising an image inverter according to the principles of the embodiments of FIG.2 indicated by 160 a single element lens and a screen. Desirably, the lens is fabricated from optical plastics. The image inverter may also be fabricated from optical plastics.

In the embodiment of FIG.20A the lens comprises a piano spherical lens with the curved surface facing the screen. In alternative embodiments the curved surface may have an aspheric form. The screen 141 is fabricated from an optical plastic such as acrylic or polycarbonate. The screen may have one polished surface and one diffusing surface. Alternatively, the screen may have two diffusing surfaces. In the embodiment of FIG.20B the lens is a biconvex lens. The surfaces may have spherical or aspheric forms.

In the embodiment of FIG.20C the lens has a meniscus form. The surfaces may have spherical or aspheric forms.

Image brightness uniformity may be improved by incorporating Fresnel surfaces within the screen. The embodiments of FIGS.21A-21C use screens incorporating at least one Fresnel surface. The embodiments of FIGS.21 A-21C may employ any of the lens types described above. For simplicity the lens in each case is represented by a lens element 138 which may be taken to represent any of the lens types discussed above. In the embodiment of FIG 21A the screen 141 provides two Fresnel surfaces 141A, 141B. In the embodiment of FIG.21B the screen 142 provides one planar surface 142A and one Fresnel surface 142B with the Fresnel surfaced facing the viewer. In the embodiment of FIG.21B the screen 143 provides one planar surface 143B and one Fresnel surface 143 A with the planar surfaced facing the viewer. The planar surfaces 143A and 143B may have diffusing characteristics.

Image brightness uniformity and resolution may be improved by using an optical element comprising at least one curved surface as a screen. The embodiment of FIGS.22A comprises an image inverter 160 a lens 138 and a piano convex lens 145 comprising a curved surface 145 A and a planar surface 145 facing the viewer. The lens 138 represents any of the lens types discussed above. The curved surface 144A may have diffusing characteristics. In an alternative embodiment illustrated in FIG.22B the lens 145 is replaced by the lens 144 comprising the curved surface 144A and a Fresnel surface 144B facing the viewer. At least one of the curved surface 145 A and the planar surface 145B may have diffusing characteristics.

The image inverter 160 may incorporate Fresnel surfaces to assist in the control of aberrations and illumination uniformity. For example, FIG.23 shows a schematic side elevation view of an inverter similar to the one illustrated in FIG.2C in which the upper element comprises the input surface 11 Ia, a reflection surface 1 Ib an output surface 111c and a surface 11 Id wherein the input surfaces I lia and output surfaces 11 Ic are Fresnel surfaces. The surfaces of the lower element 112 are symmetrically disposed to the surfaces of element 111.

FIG.23 is a schematic side elevation view of an inverter similar to the one illustrated in FIG.2E in which the upper elements comprises the input surface 121a, a reflection surface 121b an output surface 121c and a surface 12 Id and air spaced parallel surfaces indicated by 121e wherein the input surfaces 121a and output surfaces 121c are Fresnel surfaces. The surfaces if the lower element 122 is symmetrically disposed to the surfaces of element 121.

It will be clear from consideration of the embodiments of FIG.2 that the scope for introducing Fresnel surfaces into the image inverter is not limited to the schemes illustrated in FIGS.23- 24. In principle any of the surfaces used in the inverters of FIG.2 may be replaced by Fresnel surfaces. Advantageously, Fresnel surfaces may be used in at least one of the input and output surfaces of the image inverter In alternative embodiments of the invention any of the surfaces used in the image inverters of FIG.2 may be replaced by refractive optical surfaces

In alternative embodiments of the invention any of the surfaces used in the image inverters of FIG.2 may be replaced by diffractive optical surfaces designed to compensate for aberrations such as chromatic aberrations and dispersion. Diffractive optical surfaces may also be used to control illumination uniformity.

In yet further embodiments of the invention the image inverter may incorporate materials for light control within the TIR gap. FIG.25 is a schematic side elevation view of an image inverter similar to the one illustrated in FIG.2F. The air gap 61d contains optical media indicated by 6 Ie, 6 If, 6 Ig. At least one of the media may be a light-absorbing medium. At least one of the media may be a light-transmitting medium having the same refractive index as the prism elements of the image inverter. At least one of the media may be a light- transmitting medium having a refracting index higher or lower than that of the prism elements of the image inverter. At least one of the media may be a light reflecting material. Although FIG.25 shows three optical media, the number of media may be higher or lower. From consideration of FIG.25 it will be appreciated that be controlling the number and optical characteristics of the optical media it is possible to control the paths of light from different incidence angles through the prisms to minimize the problems of stray light.

In a further embodiment of the invention similar to the one of FIG.25 there are further provided light control layers 6 Ij, 6 Ik applied to the outer surfaces 6 Id. Symmetrically disposed light control layers would normally be provided in the second image inverter component. The purpose of the light control layers is to block or divert stray light from imaging light paths. The light control layers are preferably light absorbing. In certain embodiments of the invention the light control layers may comprise reflective materials design to reflect stray light towards a light stop.

It should be noted that that the improvements to the image inverter illustrated in FIGS.25-26 might be applied in any of the embodiments of the invention

FIG.27A is a schematic side elevation view of an embodiment of the invention based on the principle of FIG.20 comprising the image inverter of FIG.2F, a single element lens 135 and a screen 140. FIG.27B is a schematic side elevation view of an embodiment of the invention comprising the image inverter of FIG.2F a single lens element 135 and a screen 142 incorporating a Fresnel surface and a diffusing surface as illustrated in FIGS.21B-21C. The embodiment of FIG.27B further comprises a stray light baffle based on the principles of FIG.19B.

Further embodiments of the invention directing at enhancing image quality and ease of use of the apparatus will now be discussed with reference to the series or schematic side elevation views shown in FIGS.28-36. For the sake of simplicity the following embodiments will be described in relation to the embodiment of FIG.18B, which represents the simplest embodiment of the invention. Accordingly, it will be assumed that the viewer comprises at least the image inverter 8, lens 31 and screen 140. It is further assumed that the apparatus is inserted into a door whose cross section is indicated by 50 and that the viewer is packaged within a housing generally indicated by 51. The screen is mounted in a holder indicated by 100. The housing, which may be fabricated from metal or plastic, incorporates mounting fixtures, which are not illustrated, for securing the prisms, lens and screen elements in position. The housing may have a substantially conic shape for ease of insertion into the door. In many applications such as, for example, hotels the housing will need to meet strict security and fire safety standards. A metal housing is very desirable with regard to satisfying fire safety regulations. The invention does not assume any particular design of or method of fabricating the housing.

In certain embodiments of the invention ancillary light sources may be provided for illuminating the subject to assist in face recognition and/or to provide a brighter image. The sources may be LED, laser or incandescent. However a LED would normally be the preferred option with regard to cost, lifetime, safety and power consumption. In one embodiment of the invention illustrated in FIG.28 at least one ancillary light source such as the ones indicated by 105A,105B may be integrated within the viewer in such a way that the illumination light path from each sources to the subject traverses the input light path or at least a portion thereof in reverse. Hence the source 105 A provides upwardly directed illumination light 300 IA and the source 105B provides downwardly directed illumination light 3001B. In the embodiment of the invention shown in FIG.28 the illumination sources are operated by the room occupant by means of a control device 106 mounted in proximity to the viewer screen. Typically, the controller would provide an on/off switch and/or means for controlling the level of illumination. The control device may also incorporate a power supply for the illumination sources. The power supply may be mains driven or may rely on batteries.

An alternative embodiment of the invention directed at providing illumination light is illustrated in the schematic side elevation view of FIG.28. At least one ancillary light source such as the one indicated by 107 may be attached externally near the viewer entrance port. The illumination source is operated by the room occupant- by means of a control device 108 mounted in proximity to the viewer screen. Typically, the control device would provide an on/off switch and/or means for controlling the level of illumination. The control device may also provide a power supply for the illumination sources. The power supply may be mains driven or may rely on batteries.

In an embodiment of the invention shown in FIG.30, the above described illumination control functions may be performed by remote control using a hand-held control device such as the one indicated by 113 in FIG.30. The hand held controller provides a modulated infrared beam indicated by 114 which is detected by an infrared receiver incorporated within the control device. Desirably, the control device incorporates drive circuitry for operating the light sources. Desirably the control device also incorporates a power supply for supplying power to the infrared receiver and the illumination sources. In one embodiment of the invention the hand held controller may be configured as a general purpose remote controller capable of operating other types of electronic equipment such as televisions. In one embodiment of the invention illustrated in FIG.31 an externally mounted light sensor 117 may be used to match illumination light provided by any of the above methods to the external light level. The light detector is operated by a control device 116. Desirably, the control device incorporates drive circuitry for operating the light sources and light sensor. Desirably, the control device also incorporates a power supply for supplying power to the light sensor and the illumination sources.

In one embodiment of the invention illustrated in FIG.32 the illumination may be initiated automatically by detecting activity and presence in proximity to the door using a passive infrared sensor 119. The infrared sensor 119 and the illuminator 107 are operated by a control device 118. Desirably, the control device incorporates drive circuitry for operating the light sources and infrared sensor. Desirably, the control device also incorporates a power supply for supplying power to the infrared sensor and the illumination sources.

In one embodiment of the invention illustrated in FIG.33 an interface for providing two way audio communication between the subject and the occupant may be provided. The apparatus of FIG.33 incorporates an audio transmitter/receiver 101 operative to transmit and receive audio signals indicated by 102A,102B mounted in proximity to the viewer entrance port. The apparatus of FIG.33 further incorporates an audio transmitter/receiver 103 operative to transmit and receive audio signals indicated by 104A,104B mounted in proximity to the viewer screen. The audio transmitter/receivers may be switched on/off by the occupant using a control device mounted near to the viewer screen or using a hand-held remote control device similar to the one used for illumination control in FIG.30. Any of the embodiments illustrated in FIGS. 28-32 could be operated in conjunction with the audio device. For example in the embodiment of FIG.34 the apparatus comprises the illumination apparatus of FIG.28 providing illumination light indicated by 3002A,3002B. The apparatus of FIG.34 further incorporates an audio transmitter/receiver 101 operative to transmit and receive audio signals indicated by 102A,102B mounted in proximity to the viewer entrance port. The apparatus of FIG.34 further incorporates an audio transmitter/receiver 103 operative to transmit and receive audio signals indicated by 104A,104B mounted in proximity to the viewer screen. The apparatus of FIG.34 further incorporates a hand held controller 116 which provides a modulated infrared beam indicated by 117. The infrared beam is in turn detected by an infrared receiver integrated within a control device 116. Desirably, the control device incorporates drive circuitry for operating the audio transmitter/receivers and the infrared receiver. Desirably the detector module also incorporates a power supply. The apparatus of FIG.34 may further comprise ancillary lighting of the type discussed above. In one embodiment of the invention the hand held controller may be configured as a general purpose remote controller capable of operating other types of electronic equipment such as televisions.

In one embodiment of the invention illustrated schematically in FIG.35 the subject may be illuminated by scavenging room light and traversing at least part of the input light path in reverse. It should be noted that in the numbering the letter A refers to light entering the upper portion of the viewer screen following at least a portion of the input light path in reverse and emerging from the input port into the upper portion of the field of view. On the other hand the letter B refers to light entering the lower portion of the viewer screen following at least a portion of the input light path in reverse and emerging from the input port into the lower portion of the field of view. In FIG.35 room light vindicated by 3003A and 3003B is captured through the viewer screen propagate through the lens 31 to provide the light 3004A,3004B which In an alternative embodiment of the invention directed at scavenging room light the illuminated light may be captured via ports distributed around the periphery of the viewer screen and following similar reverse paths to those indicted in FIG.35.

In one embodiment of the invention illustrated schematically in FIG.36 the screen may incorporate a tilt facility to improve visibility of the screen for persons outside the design height range. In one embodiment of the invention the screen 140 may be designed tilt around a horizontal axis providing a first screen position 140A characterized by substantially downward output light directions and a second screen position 140B characterized by substantially upward output light directions. It should be noted that in the numbering the letter A refers to light entering the upper portion of the viewer screen following at least a portion of the input light path in reverse and emerging from the input port into the upper portion of the field of view. On the other hand the letter B refers to light entering the lower portion of the viewer screen following at least a portion of the input light path in reverse and emerging from the input port into the lower portion of the field of view. Suitable optical path compensation is provided within the imaging optics. In the embodiment of the invention shown in FIG.36 the optical path compensation is provided by tilting the lens 31 into positions 3 IA, 3 IB corresponding to screen positions 140A, 140B . When the screen is in a first position indicated by 140A and the lens is in a first position indicated by 3 IA, input light 3006A,3006B is inverted by the image inverter 6 to provide light 3007A,3007B which is focused onto the screen as light 3008A,3008B by the lens to provide output light 3009A,3009B.When the screen is in a second position indicated by 140B and the lens is in a second position indicated by 3 IB, input light 3016A,3016B is inverted by the image inverter 6 to provide light 3017A,3017B which is focused onto the screen as light 3018A,3018B by the lens to provide output light 3019A,3019B.

Advantageously, the screen tilt angles may be computed using principles based on the well known Scheimpflug correction described in British patent number GB 1196/1904 issued in 1904 to Theodor Scheimpflug. Further details are provided in a book by Rudolf Kingslake entitled "Lenses in Photography", published by Case-Holt Corporation for Garden City Books, Garden City, New York, 1951. However, other methods well known to those skilled in lens design may be used to determine the optical tilt angles for the lens and screen.

The basic invention is not restricted to door security viewers. Possible applications include viewers for use in vehicles and process monitoring. The invention could be used to provide visual access in many application domains where cost factors, hazardous environments or privacy requirements preclude the use of windows. The invention may also be configured to operate at much closer object conjugates than those required for security. For example, the invention may provide a magnifier for the inspection of textiles, printed materials. In many applications it may be advantageous to incorporate an illumination sources such as an LED, laser or incandescent lamp.

Image formation by the door viewer has been described in terms of rotationally symmetric optical surfaces. However, the viewer may also use optical elements on based other forms such as cylindrical elements or anamorphic optical elements. The optical elements discussed in FIGS2-8 may be fashioned to provide elliptical cross sections. Alternatively, portions of the optical elements may be removed to provide rectangular cross sections. In a typical door viewer application the subject being viewed is likely to be in line with or below the optical axis of the viewer. Hence, the emergent rays corresponding to the centre of the subject will typically be along the optical axis or at some angle above the optical axis. It is therefore desirable that the viewing screen should have asymmetrical diffusion properties such that light hitting the screen is bent towards the nominal viewing position.

The design of the door viewer will require careful optimization to maximize light throughput and minimize aberrations and distortions. For example, chromatic aberration may be traded off against image distortion.

The refracting and reflecting surfaces of the door viewer may employ spherical, aspherical, and diffractive and other optical surface forms known to those skilled in the art. Diffractive optical surfaces in particular may play a key role in optimizing the performance. The use of diffractive optical surfaces will offer considerable form factor benefits, including reducing the required door hole size and minimizing the distance of the viewer screen from the door surface. Any of the optical surfaces used in the viewer may incorporate diffractive forms for the purposes of color correction. Further benefits of using diffractive surface forms include improving the image resolution of the image and compensating for chromatic aberrations. Other benefits of using diffractive surfaces will be familiar to those skilled in the art of optical design.

It will be clear to those skilled in the art of optics that the invention could also be applied with the directions of the ray paths reversed. In other words the screen could be used to provide an input image surface and the wide angle lens and image inverter would provide a means for projecting said image onto a remote surface in the external scene. It will be clear from consideration of FIGS.2C-2G that in each case half the image inverter may be used for the purposes of forming images of either the upper or lower portions of the field of view.

It will be clear from consideration of the drawings that since the display apparatus is symmetrical around the plane of image inverter reflection surfaces a display using either the upper or lower portions of the apparatus may be provided.

Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention.

Claims

CLAIMSWhat is claimed is:

1. An apparatus for displaying an image comprising: an image inverter(50;60;70;80;90;8,160); a lens(3,13,16,131, 134,135,138); and a screen(4, 140,141, 142,142B, 143,144,145), characterised in that one surface of said screen has a Fresnel form

(141A.141B.143A144B), wherein said image inverter and said lens together form an erect image of an external scene at said screen, wherein said image inverter comprises upper and lower components(51,52;61,62;71,72;81,82;91,92), wherein said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence, wherein each said component comprises an input surface (51a;61a;71a;81a;91a) that admits light from said external scene, a reflecting surface(51b;61b;71b;81b;91b) and an exit surface(51c;61c;71c;81c;91c), wherein said components are symmetrically disposed about an axis of symmetry of said apparatus wherein said reflecting surfaces are disposed back to back and substantially overlap, characterized in that each said component is divided into first and second optical elements(61A,61B;71A,71B; 81A,81B;91A,91B; 61C,61D;71C,71D; 81C,81D;91C,91D;) disposed in sequence, wherein said first element has a first surface(61a;71a;81a;91a) that provides said input surface and a second surface(61f;71f;81f;91f; 61i;71i;81i;91i), wherein said second element has a first surface (61g;71g;81g;91g; 61j,71j,81j,91j), of the same shape as said second surface of the first element and a second surface(61c;71c;81c;91c) that provides said exit surface, wherein said second surface of said first element and said first surface of the second element are separated by an air gap(61e;71e;81e;91e; 61h;71h;81h;91h), wherein said second surface of said first element operates as a total internal reflection surface for light from said external scene incident in directions that do not intercept said reflecting surface.

2. The apparatus of claim 1 wherein at least one of said first surface of said first element and said second surface of said second element are curved.

3. The apparatus of claim 1 wherein at least one of said first surface of said first element and said second surface of said second element are tilted planar surfaces.

4. The apparatus of claim 1 wherein said reflecting surface is a total internal reflection surface.

5. The apparatus of claim 1 wherein said reflecting surface is divided into two portions by said first and second elements.

6. The apparatus of claim 1 wherein one surface of said screen is a diffuser.

7. The apparatus of claim 1 wherein said lens has at least one aspheric surface.

8. The apparatus of claim 1 wherein a first horizontal light blocking baffle (110,113) is provided between said lens element and said screen, wherein said baffle is disposed in a plane parallel to the plane of symmetry of the image inverter.

9. The apparatus of claim 1 wherein a second horizontal light blocking baffle (114) is provided between said image inverter and said lens element, wherein said baffle is disposed in a plane parallel to the plane of symmetry of the image inverter.

10. The apparatus of claim 1 wherein at least one of said first surface of said first image inverter element and said second surface of said second image inverter element have Fresnel forms (11 Ia₅11 Ic, 121a, 121c).

11. The apparatus of claim 1 wherein said screen may be tilted with respect to at least a vertical plane.

12. An apparatus for displaying an image comprising: an image inverter(50;60;70;80;90;8,160); a lens(3,13,16,131,134,135,138); and a screen(4, 140,141, 142.142B, 143,144,145), characterised in that one surface of said screen has a Fresnel form (141A,141B,143A144B, wherein said image inverter and said lens together form an erect image of an external scene at said screen, wherein said image inverter comprises upper and lower components(51,52;61,62;71,72;81,82;91,92), wherein said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence, wherein each said component comprises an input surface (51a;61a;71a;81a;91a) that admits light from said external scene, a reflecting surface(51b;61b;71b;81b;91b) and an exit surface(51c;61c;71c;81c;91c), wherein said components are symmetrically disposed about an axis of symmetry of said apparatus wherein said reflecting surfaces are disposed back to back and substantially overlap, characterized in that each said component is divided into first and second optical elements(61A,61B;71A,71B; 81A,81B;91A,91B; 61C,61D;71C,71D; 81C,81D;91C,91D;) disposed in sequence, wherein said first element has a first surface(61a;71a;81a;91a) that provides said input surface and a second surface(61f;71f;81f;91f; 61i;71i;81i;91i), wherein said second element has a first surface (61g;71g;81g;91g; 61j,71j,81j,91j), of the same shape as said second surface of the first element and a second surface(61c;71c;81c;91c) that provides said exit surface, wherein said second surface of said first element and said first surface of the second element are separated by an air gap(61e;71e;81e;91e; 61h;71h;81h;91h), wherein said second surface of said first element operates as a total internal reflection surface for light from said external scene incident in directions that do not intercept said reflecting surface, characterised in that said apparatus further comprises an ancillary illumination source (105A, 105B 107).

13. An apparatus for displaying an image comprising: an image inverter(50;60;70;80;90;8,160); a lens(3,13,16,131, 134,135,138); and a screen(4, 140,141, 142,142B, 143,144,145), characterised in that one surface of said screen has a Fresnel form

(141A,141B,143A144B, wherein said image inverter and said lens together form an erect image of an external scene at said screen, wherein said image inverter comprises upper and lower components(51,52;61,62;71,72;81,82;91,92), wherein said upper component is operative to provide a vertical inversion to light incident above a predetermined angle of incidence and said lower component is operative to provide a vertical inversion to light incident below a predetermined angle of incidence, wherein each said component comprises an input surface (51a;61a;71a;81a;91a) that admits light from said external scene, a reflecting surface(51b;61b;71b;81b;91b) and an exit surface(51c;61c;71c;81c;91c), wherein said components are symmetrically disposed about an axis of symmetry of said apparatus wherein said reflecting surfaces are disposed back to back and substantially overlap, characterized in that each said component is divided into first and second optical elements(61A,61B;71A,71B; 81A,81B;91A,91B; 61C,61D;71C,71D; 81C,81D;91C,91D;) disposed in sequence, wherein said first element has a first surface(61a;71a;81a;91a) that provides said input surface and a second surface(61f;71f;81f;91f; 61i;71i;81i;91i), wherein said second element has a first surface (61g;71g;81g;91g; 61j,71j,81j,91j), of the same shape as said second surface of the first element and a second surface(61c;71c;81c;91c) that provides said exit surface, wherein said second surface of said first element and said first surface of the second element are separated by an air gap(61e;71e;81e;91e; 61h;71h;81h;91h), wherein said second surface of said first element operates as a total internal reflection surface for light from said external scene incident in directions that do not intercept said reflecting surface, characterised in that said apparatus further comprises a two way audio communication link (101,103) between said external scene and an observer viewing said screen.

14. The apparatus of claim 13 further comprising a light sensor (117,119).

15. The apparatus of claim 13 further comprising an ancillary illumination source (105 A, 105B 107).