Apparatus for displaying an image

Abstract: An imaging system for the generation of an apparent three-dimensional (3-D) image suspended in space (19) for direct viewing (21) includes a beamsplitter (7) and a retroreflector (5a, 5b). The optical properties of reflecting light back along its original path enable the system to generate such an apparent 3-D image (21) based on the perception of depth cues present in the original scene (3) for viewing.

Description: The present invention relates to apparatus for displaying an apparent three-dimensional image of an object or original image, the apparatus comprising at least one array of retroreflectors and at least one beamsplitter, arranged to produce from the object or original image a real image suspended in space for viewing as the apparent three dimensional image. A known apparatus of this type is disclosed in WO-A-8303019. It is an object of the present invention to provide apparatus to produce a real image suspended in space for direct viewing and derived from an original image such as a two dimensional scene, or from an object, for perception as three dimensional. This is achieved by reducing the depth cues present in the original image which contribute to its perception as being flat or two dimensional, for example, stereoscopic binocular disparity, convergence, accommodation and motion parallax. It is a further object of the present invention to provide a method of producing a real image in space, derived from an object or original image such as a two dimensional scene, for perception as three dimensional. According to a first aspect of the present invention there is provided apparatus as defined in the first paragraph above characterised in that the beam splitter is in the form of a polarisation splitter. Preferably, the retroreflector is in the form of an array of corner-cubes formed as protrusions or depressions on a major surface of a sheet of suitable material, such as a plastics material. Corner-cubes formed as protrusion on the surface of a transparent material may optionally be coated with a reflective metal, such as aluminium. This allows for enhanced resolution of the image suspended in space. Alternatively, an arrangement with the corner-cubes formed as protrusions on the surface of a transparent material may simply make use of total internal reflection within the solid transparent material. If the corner-cubes are formed as depressions on the surface of the sheet, they should be made reflective by coating with a reflective metal such as aluminium. This construction has the advantage that undesirable specular reflections from the front surface of the sheet are avoided and that any beam spreading of a retroflected ray, due to manufacturing tolerances in making the corner-cube array, is not exaggerated by refraction when emerging from the transparent material. Alternatively, the array of retroreflectors may be in the form of a sheet of the glass bead type and may include such beads as have a graded index of refraction such as are present in Luneberg lenses. By the use of small diameter glass bead retroreflectors, retroreflective properties for a large range of angles of incidence, down to almost grazing incidence, are possible, and because this allows for a very large numerical aperture, large viewing angles are available. By the use of retroreflectors within the apparatus to create the real image suspended in space, the flatness cues in the original image are sufficiently weakened that perception of the depth cues recorded pictorially in the real image dominates and 3-D perception is achieved. Solid objects may also pass through the real image without hindrance. Thus the invention also provides for the formation of an apparent three dimensional real image suspended in space from a conventional television receiver without the need for any adjustment or modification of the image displayed on the television screen except, possibly, for increasing its brightness. Furthermore, any suitable two dimensional scene may be employed such as video or computer-generated image. According to a second aspect of the present invention there is provided a method for displaying an apparent three dimensional image of an object or original object or image, the method comprising projecting light beams from the original image via at least one polarisation beam splitter so as to strike at least one retroreflector array and reforming retroreflected light beams thereby to produce a real image suspended in space for viewing as the apparent three dimensional image. The invention will now be described, by way of example only, with reference to the accompanying drawings of which:- Figure 1 illustrates, in schematic form, the optical property of a retroreflector utilised in the present invention and, Figure 2 shows schematically an optical arrangement of the present invention, and, Figure 3 shows schematically a further arrangement of the present invention, Figure 4 illustrates an exploded view of yet a further arrangement of the present invention, and; Figure 5 illustrates an alternative arrangement to that of Figure 4. The invention is based on the perception of depth by humans based on the assimilation of depth cues perceived in the original scene. Many depth cues exist, only two of which - stereo-disparity and convergence - fundamentally require the use of both eyes. All the other depth cues - and they are well known to those skilled in the art - can be perceived with one eye only. Thus many of these other depth cues are able to be captured by a video camera or on a photograph or the like and many can be generated by a computer or artist and displayed in a photograph, painting or the like. Examples of such pictorial depth cues are: perspective, foreshortening, interposition, accommodation, motion parallax, and kinetic interposition. Those skilled in the art will appreciate that many more depth cues exist, and the above are given by way of example only. Since some or many of these pictorial depth cues are inherent within any representation or picture of a scene, in principle they can be used in order to create a perception of depth within that picture. However, humans perceive, for example, a television screen image as being flat or two dimensional because either there are too many flatness cues, (for example, stereoscopic disparity, convergence, accommodation and framing effects) present in the image or that the number and strength of depth cues is too small. The apparatus of the present invention tends to weaken flatness cues sufficiently for the human eye/brain combination to utilise the pictorial depth cues and to perceive depth within the image formed. This weakening of the flatness cues derives from a number of contributory factors, for example, the absence of reflections or scattering from the real image as it is suspended in space, its detachment from any surrounding frame and its unusual occurrence suspended in front of the retroreflectors or superimposed on the real world beyond. This is achieved by the use of retroreflectors. The principles of retroreflectors are shown diagrammatically in Figure 1. It can be seen that light incident upon a retroreflector is substantially reflected back along its original path, whatever the angle of incidence (with limits defined by the type of retroreflector employed). By referring now to Figure 2, it will be seen that the present invention involves creating a real image suspended in space. In this preferred embodiment, the apparatus includes a conventional television set 1 arranged to lie so that its screen 3 is in a horizontal plane. Directly above the screen 3 and also in a horizontal plane is positioned a retroreflector 5a. Orthogonal to this retroreflector 5a and in a vertical plane there is arranged a further retroreflector 5b. The type of retroreflector chosen in this apparatus is of the corner-cube type such as is manufactured by the Reflexite Inc. company. Positioned between and at 45° to both retroreflectors 5a, 5b, is a beam splitter 7. Considering now a pixel 9 on the screen 3. Light from the pixel 9 will be emitted in divergent rays and strike the beam splitter 7. In Figure 2, three such rays only 11, 13 and 15 are shown for clarity. The light is then split by the beam-splitter 7 wherein some light is reflected to retroreflector 5b and some light is transmitted to retroreflector 5a. Both retroreflectors 5a, 5b reflect the light rays impinging on them back along their respective original paths i.e. to the beam-splitter 7. These light rays are once more split by the beam splitter 7, but because the rays have been retroreflected when diverging, they now become convergent and so combine or reform to come to a focus 17, and form a suspended real image of the pixel in space. Whilst the example above refers to a single pixel 9, a television screen image is simply an array of such pixels, and as such a projected image of the whole television screen image may be viewed by an observer 21, the image of the television screen 3 appearing suspended in space at 19. Because many depth cues were present in the original image, they are also present in the real image suspended in space. However, because this real image is suspended in space, it has the appearance of viewing a scene through a window, and the flatness cues become weakened and allow the observer to use the image depth cues to create an effect of three dimensional viewing. Furthermore, by utilising retroreflective surfaces as opposed to the large mirrors or lenses described in the prior art, a large numerical aperture is obtained which allows a wide viewing angle, and the high costs of large lenses and curved mirrors and their associated aberrations and distortions are avoided. Retroreflective sheets are available commercially in large sizes and at modest cost. Their positioning and orientations are not critical matters, in strict contrast to the use of lenses or curved mirrors which may distort the image produced if they are not accurately aligned and oriented. With reference now also to Figure 4, it will be seen how an increased viewing angle is obtained. The apparatus of Figure 4 is similar to that of Figure 2, but with the inclusion of two plane mirrors 23a, 23b. The mirrors 23a, 23b, flank both retroreflectors 5a, 5b and the beamsplitter 7. The mirrors 23a, 23b are arranged such that they fit against both retroreflectors 5a, 5b and beamsplitter 7. In Figure 2, in order for an apparent 3-D image to be formed, each retroreflector 5a, 5b is necessarily in a direct optical line with the pixel 9 and beamsplitter 7, i.e. light from pixel 9 emitted at an elevated angle to the plane of the paper such that it passed through the beamsplitter 7 but did not strike either retroreflector 5a, 5b would not then form an apparent 3-D image. The arrangement of Figure 4 obviates this problem because light reflected from either mirror 23a, 23b which would otherwise not then contribute to forming the apparent 3-D image will now necessarily be reflected toward either the beamsplitter 7 or retroreflector 5a or 5d dependent upon whether the point at which the light strikes the mirror is before or after passing through beamsplitter 7. It will be apparent that the mirrors 23a, 23b need not be orthogonal to each other but may be angled and still allow an increased viewing angle. It will be apparent to those skilled in the art, by consideration of simple optical geometry that the mirrors 23a, 23b will also enable a greater viewing angle than the embodiment shown in Figure 2 by the same principles as above. It has also been found that by shielding the television screen from the view of the observer 21 a more dramatic 3-D effect is observed for the real image suspended in space. By referring to Figure 3, it may be seen that this is achieved by covering the screen 3 with a louvre-screen 23 or the like. This louvre screen allows light to pass through it but only over a limited range of angles. Thus near normal light may pass through to reach the retroreflectors and form the real image, whilst obliquely emitted light from the original image, namely the screen 3, is absorbed and the original image cannot be seen directly. Whilst in the example given hereinbefore, two retroreflective surfaces have been used, it will be apparent to those skilled in the art that any number or arrangement of retroreflectors will enable the successful working of the invention such as a retroreflective surface formed on the inside surface of an apertured sphere. Indeed, if one of the above mentioned retroreflectors, 5a or 5b is removed from the system, then a viewable image 19 will continue to be produced, but will be reduced in brightness by approximately half. Also, by removing one of the retroreflectors in the above example, it is possible to combine another image, real or virtual, with the apparent 3-D image 19, thus enabling the 3-D image to be overlaid over a background or foreground scene. An increase in brightness of the image 19 is achieved by using a beam-splitter 7 with a polarisation splitting layer or a circularly dichroic beam splitter such as one or more layers of circularly dichroic cholesteric or smectic liquid crystals or liquid crystal polymers and optionally, if required, quarter wave retarders placed in front of the retroreflective walls. Transmission or reflection of right-hand circularly polarised or left-hand circularly polarised light respectively by the layer is achieved, the light being reversed in polarisation upon being retroreflected (through the quarter wave retarder, if required,) and then respectively reflected and transmitted so that it recombines to give an unpolarised image having greater brightness than if a semi-reflective mirror had been used. With reference now to Figure 5, it will be seen that an apparent 3-D image suspended in space may be formed by an alternative arrangement to those detailed herebefore and which employs the properties of such polarisation splitter. Screen 3 is overlaid by a circular polariser 25 such that light from a pixel 9 having passed through polariser 25 is circular polarised in one sense, say, right hand circular polarised. The divergent right hand circularly polarised light then passes through a half-silvered retroreflector 27 and on to strike a circularly dichroic beamsplitter in the form of a cholesteric liquid crystal layer 29. Layer 29 has the optical property of transmitting light polarised in one circular sense, say left, and reflecting light of the other sense, right. Hence, the right circularly polarised light which strikes layer 29 will be reflected as a divergent beam towards the half-silvered retroreflector 27. On striking retroreflector 27 a portion of the light will be reflected back along its original path but (as will be apparent to those skilled in the art). Consequently, the sense of circular polarisation of the retroreflected light will have changed from right to left. Thus when the now left hand circularly polarised light impinges on layer 29 it will be transmitted and so from an apparent 3-D image for viewing at 21. Similar embodiments are envisaged using a non-corner cube type retroreflector which separates orthogonal linearly polarised components of the incident light and may also, optionally if required, use quarter wave retarders. It will be appreciated that if the distance between the screen 3 and beamsplitter 7 is increased, a corresponding increase in the distance between the beam splitter 7 and the suspended image 19 will ensue, in line with conventional optics. The observer 21 may interact with the image 19 or even walk right through it. It is envisaged that this effect will be of value in advertising displays or special effects. Furthermore, the sheets of retroreflectors may be made very large and used to cover the walls and ceiling of a room in order to bring the real image of a movie or video recording of a life-size artiste out into the room for perception as three dimensional, this image may then be approached and even walked through, or viewed through a lens, e.g. a magnifying glass. The invention may equally well be employed to produce a 3-D image of a 3-D object, for example public display of precious jewellery in a shop window, with the real image brought through the glass and into the street for prospective customers to examine but not remove. Another example where the invention may be adopted is to create a real image, suspended in space, of one or more real lamps or light sources. It should be noted that a three dimensional object is turned inside out in the real image, thus a face produces a real image resembling a hollow mask. Thus it will be apparent that a true apparent 3-D image may be created by having two of the described embodiments arranged in a "back-to-back" format, that is such that the output (apparent 3-D image) of one becomes the input (object) for the other. It will be appreciated that other types of retroreflector may be incorporated into the present invention, such as the glass bead type as manufactured by the 3M company under the trade mark "Scotchlite". Additionally, it is envisaged that other forms of retroreflector may equally well be employed within the present invention. It will be further appreciated that the original object or image to be displayed is not limited to the examples given hereinbefore. Other suitable objects are computer generated pictures, visual display units, movie or cine pictures on a screen or a slide projection. Thus the invention described hereinbefore provides an apparatus and method to produce an apparent 3-D image of an original scene based on the perception of depth cues present in the original scene, and by employing retroreflectors.

Claims: Apparatus for displaying an apparent three-dimensional image of an object or original image, the apparatus comprising at least one array of retroreflectors and at least one beamsplitter, arranged to produce from the object or original image, a real image suspended in space for viewing as the apparent three-dimensional image characterised in that the beamsplitter is in the form of a polarisation splitter. Apparatus according to claim 1 wherein the retroreflector is in the form of a material having an array of corner cubes formed as protrusions or depressions on a major surface of the material. Apparatus according to claim 1 wherein the array of retroreflectors is in the form of a sheet of beads or microlenses. Apparatus according to claim 1 wherein the polarisation splitter includes dichroic cholesteric or smectic liquid crystals. Apparatus according to any one of the preceding claims including louvre means arranged such that the object or original image may not be viewed directly. Apparatus according to any one of the preceding claims including at least one plane mirror arranged for increasing the viewing angle associated with the image suspended in space. Apparatus according to any one of the preceding claims wherein first and second arrays of retroreflectors are arranged to meet at respective ends thereof oriented substantially orthogonal to each other, and one beamsplitter is arranged in between the retroreflectors and at substantially 45° to each retroreflector; such that the object or original image for view is situated substantially orthogonal to the first retroreflector, and the apparent three-dimensional image is viewed from a position substantially orthogonal to the second retroreflector. Apparatus according to claim 1 further including a circular polarising means and wherein the at least one array of retroreflectors is half-silvered. Apparatus according to claim 8 wherein the circular polarising means is arranged directly between the object or original image and a half-silvered retroreflector; such that light from the object or original image which passes through the half-silvered retroreflector is circularly polarised and arranged to impinge upon the polarisation splitter thereby to effect reflection of polarised light of a first circular sense and transmit polarised light of a second circular sense for viewing as the apparent three dimensional image. Apparatus according to any one of the preceding claims wherein the object or original image comprises a television display system. A system according to claim 10 wherein signals applied to the television system include video and/or computer-generated signals. A method for displaying an apparent three-dimensional image of an object or original object or image, the method comprising projecting light beams from the original image via at least one polarisation beamsplitter so as to strike at least one retroreflective array, and reforming retroreflected light beams thereby to form a real image suspended in space for viewing as the apparent three-dimensional image. A method for displaying an apparent three-dimensional image of an object or original object or image, the method comprising projecting light beams from the original image via a circular polarising means and a half-silvered retroreflector so as to impinge upon a polarisation-splitting means thereby to effect reflection of polarised light beams of a first circular sense and transmit polarised light of a second circular sense for viewing as the apparent three-dimensional image.

Inventors: SHANKS IAN ALEXANDER, SHANKS, IAN ALEXANDER
Applicants: EMI PLC THORN, THORN EMI PLC
Application year: 1991
Main classification: H04N13
IPC codes: H04N13, G02B27, G09F19