GB2214651A

GB2214651A - Colour Holograms

Info

Publication number: GB2214651A
Application number: GB8801339A
Authority: GB
Inventors: Jon Andreassen
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-01-21
Filing date: 1988-01-21
Publication date: 1989-09-06
Also published as: GB8801339D0

Abstract

Colour holograms are manufactured by a method using red, green and blue (R, G, B) light for a true colour hologram. A reference beam (4) and an object beam (3) are formed for each colour from a respective source (S1, S2, S3) generating object and reference beams through optical fibres. For each colour in turn the object beam (3) illuminates the object (9) and then intersects with the respective reference beam (4) and is incident on areas of a holographic recording medium (12) which are predetermined by an apertured mask (10) for a predetermined exposure time. The mask (10) is moved between exposure so that no area of the recording medium (12) is exposed twice. Thus an array of mini holograms is produced (Fig. 1). To reconstruct a true colour image from the hologram (Fig. 3) it is illuminated with reconstruction beams which correspond to the reference beams, a stationary apertured mask being employed to ensure that light from any one reconstruction beam is only incident on those mini holograms recorded at that wavelength. By exposing successive frames (11) of holographic recording material a holographic motion picture film can be obtained. <IMAGE>

Description

HOLOGRAMS This invention relates to colour holograms and methods of recording and reconstructing colour holograms.

Conventional colour holograms may involve use of a white-light source. In one variety of white light transmission holography a colour hologram is constructed by sequentially illuminating an object using three primary colour (red, green, blue) coherent sources and thus three corresponding reference beams, each orientated at different angular positions about the object beam (white light) axis. By making three primary colour exposures, a multiplexed image plane hologram is encoded. This is a thick volume transmission hologram in which three holograms are superimposed, leading to a reduction in efficiency by a factor of 3, since each area of the hologram is exposed three times.To reconstruct an image from the hologram the latter is illuminated by reconstruction beams corresponding to the reference beams used for the recording stage and orientated at the same angles and these may be derived from a white light source. The red beam will only reconstruct the red hologram (not the green or blue holograms) and the green beam will only reconstruct the green hologram, and the blue beam will only reconstruct the blue hologram. The main disadvantage of this method is that the hologram efficiency is reduced by a factor of 3 and that the equipment required for reconstruction (playback) is bulky because of the need for large angular separation of the three reconstruction beams.

For this known method thick (volume) transmission holograms must be recorded. This is disadvantageous for two reasons. Firstly, it requires a higher resolution silver halide film (assuming a pan-chromatic silver halide film is used) which must also be thicker, thus reducing the sensitivity to light and therefore requiring longer exposure times during recording and/or higher powered lasers. Secondly, copying the recorded hologram onto a blank medium can only be achieved by using the original arrangement of three colours at the original configuration and can only be recorded onto media sensitive to all the wavelengths used. Such media are generally not those able to provide high efficiency holograms. Maximum efficiency would be reduced by a factor of 3 (if 3 colours were used).

According to one aspect of the present invention there is provided a method of manufacturing a colour hologram using light of at least two colours including the steps of forming a teference beam and an object beam for each colour from a respective source, and for each colour in turn causing the respective object beam to illuminate an object or scene and then to intersect with the respective reference beam and be incident on a respective area of a holographic recording medium for a predetermined exposure time.

According to another aspect of the invention there is provided a method of manufacturing a true colour hologram of a three-dimensional object and reconstructing an image of the object from the hologram, including the steps of forming a reference beam and an object beam from each of three sources whose colours are red, green and blue respectively, causing each colour of object beam in turn to illuminate the object and then to intersect the reference beam of the same colour and be incident on a respective area of a holographic recording medium for a predetermined exposure time, the respective area being defined by an apertured mask which is moved after each exposure whereby areas of the recording medium are only exposed once, processing the thus exposed holographic recording medium to form said hologram, and illuminating the hologram with reconstruction beams, which correspond to all of the reference beams, via a second apertured mask which serves to ensure that light from any one reconstruction beam is only incident on the part of the hologram recorded by light of that colour/wavelength.

According to a further aspect of the invention there is provided a method of manufacturing a colour hologram of a three dimensional object or scene using light of at least two colours including the steps of forming a reference beam and an object beam for each colour from a respective source, and for each colour in turn causing the respective object beam to illuminate the object or scene and then to intersect with the respective reference beam and be incident on a respective area of a frame of holographic recording medium for a predetermined exposure time, the position of an apertured mask disposed in front of the frame of holographic recording medium being changed after each exposure to ensure that the areas of the frame of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of mini holograms over the whole area of the frame.

According to yet another aspect of the invention there is provided a method of manufacturing a colour holographic stereogram from two two-dimensional transparencies of objects or scenes as viewed from two different angular positions using light of at least two colours, including the steps of forming a reference beam and an object beam for each colour from a respective source, covering one half of a frame of holographic recording medium, and for each colour in turn directing the respective object beam through one transparency toward the uncovered half of the frame for intersection thereat on predetermined areas of the frame with the respective reference half determined by an apertured mask beam for a predetermined exposure time, the position of the apertured mask which is disposed in front of the frame being changed after each exposure to ensure that the areas of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of miniholograms over the uncovered half of the frame, covering the other half of the frame and for each colour in turn directing the respective object beam through the other transparency toward the then uncovered half of the frame for intersection thereat on predetermined areas of the frame with the respective reference beam for a predetermined exposure time, the position of the apertured mask being changed after each exposure, the apertures in the mask and the movement thereof serving to produce a mosaic of miniholograms over the then uncovered half of the frame, processing the thus exposed holographic recording medium to form said holographic stereogram.

According to a still further aspect of the invention there is provided a method of manufacturing a colour holographic stereogram from a plurality of two-dimensional transparencies of objects or scenes as viewed from different angular positions using light of at least two colours, including the steps of forming a reference beam and an object beam for each colour from a respective source, and wherein for each transparency a respective strip of a frame of holographic recording medium is uncovered and for each colour in turn the respective object beam is directed through the transparency toward the uncovered strip for intersection thereat on predetermined areas of the frame strip determined by an apertured mask with the respective reference beam for a predetermined exposure time, the position of the apertured mask which is disposed in front of the frame being changed after each exposure to ensure that the areas of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of mini holograms over the uncovered strip of the frame, repeating the exposure procedure for each transparency employing a corresponding strip of the frame, and processing the thus exposed holographic recording medium to form said holographic stereogram.

According to another aspect of the invention there is provided a true colour hologram motion picture film including a plurality of frames each comprising a colour hologram of a moving object or changing scene at a respective point in time and manufactured by a method comprising the steps of forming a reference beam and an object beam from each of three sources whose colours are red, green and blue respectively, causing each colour of object beam in turn to illuminate the object or scene and then to intersect the reference beam of the same colour and be incident on a respective area of a frame of holographic recording medium for a predetermined exposure time, the respective area being defined by an apertured mask which is moved after each exposure whereby areas of the recording medium are only exposed once and repeating the exposure process for each of said plurality of frames, processing the thus exposed holographic recording medium to form said hologram motion picture film, images of the moving object or changing scene being reconstructed from the film by illuminating each frame thereof in turn with reconstruction beams, which correspond to all of the reference beams, via another apertured mask which serves to ensure that light from any one reconstruction beam is only incident on that part of the hologram recorded by light of that colour/wavelength.

According to still another aspect of the present invention there is provided a playback machine for reconstructing an image of an object from a colour hologram thereof produced by a method according to one of the preceding paragraphs, the machine comprising a box having a window at which a hologram to be reconstructed can be disposed, an apertured mask disposed in said box, and means contained within said box for illuminating the hologram with reconstruction beams, which correspond to all of the reference beams, via the apertured mask in the box which serves to ensure that light from any one reconstruction beam is only incident on the part of the hologram recorded by light of that colour/wavelength.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a part of a frame of holographic emulsion having a numbler of mini holograms recorded with three different light wavelengths; Fig. 2 illustrates the recording of a colour hologram of an object according to the present invention; Fig. 3 illustrates the reconstruction (playback) stage for the hologram recorded as per Fig. 2; Fig. 4 illustrates schematically a holographic video playback machine; Fig. 5 illustrates the recording of a colour hologram according to the present invention where the object is a transparency (two dimensional object); Fig. 6 illustrates the viewing of a holographic stereogram recorded as per Fig. 5, the view being one taken from above, and indicates various dimensions thereon;; Fig. 7 illustrates one step in the production of a holographic stereogram; Fig. 8 illustrates viewing of a holographic stereogram; Fig. 9 illustrates viewing through the centre of the strips and through the edges of the strips; Fig. 10 illustrates viewing of a holographic stereogram (the view being one taken from above) and indicates various dimensions thereon, and Figs. 11 and 12 illustrate AN=4 and AN=2 respectively.

The frame 1 of holographic emulsion only part of which shown in Fig. 1, is effectively divided up into a number of areas 2 (squares as shown). For any threedimensional object or scene to be recorded, that object or scene is illuminated in turn with each of three different wavelengths of light (red, green, blue). This results, due to a masking process which is described hereinafter, in selective recording of a large number of mini holograms at the areas 2 of the object or scene.

The mini holograms are arranged in groups of three, one recorded by red light, one recorded by green light and one recorded by blue light (R, G, B). The exact relative position of the mini holograms in a group depends on the masking and the reference source position as will be further discussed below. The mini holograms may be square with a side dimension of 2 to 3mm, for example.

The recording of such a hologram consisting of a number of mini (sub) holograms will now be described with reference to Fig. 2. During recording of the hologram of an object or scene 9, an apertured mask 10 is disposed in each of three possible positions in turn in front of a frame 11 of holographic emulsion 12 provided on a backing film 13. The mask has one position for each recording light wavelength used, for example red, green, blue. The mask is apertured such that it uncovers those areas to be exposed by one wavelength, green for example, whilst it covers those areas to be exposed by the two other wavelengths, red and blue for example. For three recording wavelengths, the mask, in the limit, covers two-thirds of the frame at any one time whilst allowing the remaining third to be exposed.The apertures in the mask are preferably square, to produce square close-packed mini holograms for maximum efficiency. With three mini holograms in a group they may be arranged in an L-shape as indicated in Fig. 1 or in a straight line.

In order to record a hologram, two recording beams, an object beam 3 and a reference beam 4, of the same wavelength are required. They generally originate from the same source, for example a laser, but are directed along different paths. The object beam 3 is caused to be incident on the object whose hologram is to be recorded before being caused to be incident on those areas of the holographic emulsion exposed by the mask together with the reference beam 4, whereby to produce an optical interference pattern therein. The two recording beams (reference and object) 3 and 4 are required to be in phase and of equal polarisation.

In the arrangement illustrated in Fig. 2 the two recording (reference and object) beams 3 and 4 of three wavelengths LXII 73 (red, green, blue respectively) are conveyed by optical fibres 14 (reference) and 15 (object) indicated partially in dotted lines from respective sources shown schematically as S1, S2 and S3. The optical fibres are preferably but not necessarily monomode.

The exposure technique of the recording step involves three exposures, one for each colour (wavelength) of the recording beams. Fig. 2 illustrates exposure with the green (lah2) recording beams. In between exposures the position of the mask 10 is changed. Moreover, the 'point1 source position of the diverging reference beam is also changed. As illustrated, this can be achieved simply by transmitting the three possible reference beams 4 along respective fibres 14 with each optical fibre output end having a different and well-defined position. As illustrated there are also three optical fibres 15 conveying the light to illuminate the object or scene 9.The three different positions of mask 10, having an array of square apertures, are such that the apertures can expose substantially the whole of the area of the one frame 11 of the holographic film but with each section of the holographic emulsion being exposed only once. In the illustrated example of Figs. 2 and 3 the mask 10 may have three positions along a first axis on which it is moved (arrow B) thus producing red, green, blue, red, green, blue, and so on, holograms in corresponding lines on the holographic emulsion. Following exposure the holographic emulsion is processed (developed and fixed) in the appropriate manner to the type thereof. The use of optical fibres, to produce point sources of light, and movable masks in the manufacture of holograms is referred to in our earlier GB Patent Application No.

8629389 (Serial No. ) (J. Andreassen 3).

On reconstruction (see Fig. 3) all of the three colours, that is all of the diverging beams from optical fibres 14, which were termed reference beams 4 for the recording stage but are termed reconstruction beams 5 for the reconstruction stage, act to reconstruct the frame of the processed holographic emulsion simultaneously. Obviously, different fibres or point sources can be used but the relative arrangement thereof and the wavelengths must be the same as that for the recording stage. A second mask 16, which is stationary, is placed between the reconstruction beam sources (the output ends of optical fibres 14) and the patchwork of mini-holograms making up the one frame.Mask 16 is such as to ensure that light from any one reconstruction beam source is only incident on that part (third) of the holographic frame which was recorded with light of that wavelength, resulting in an aberration-free holographic image for each colour. The resultant 'colour' illusion to a viewer disposed to the left of the film in Fig. 3 is that of a three-dimensional colour image 9'.

By having the holographic emulsion on an elongate backing film, in the manner of a motion picture film, and exposing each successive frame with all of the three colours of light in turn using masking as described above, a holographic image 'motion picture" can be achieved of a moving three-dimensional object or changing scene. For reconstruction the successive frames of recorded film will be moved through the reconstruction area, as indicated by arrow A in Fig. 3, to produce a virtual, true colour holographic "motion picture" or moving visual image. The reconstruction stage may be performed in a unit, equivalent to a combined video playback machine and television set, which comprises a single table-top or desk-top colour holographic video playback machine.Particular applications envisaged include, personalised entertainment and battlefield simulations, although these are not the only applications.

Fig. 4 illustrates schematically a view from above of a playback machine. The machine employs a cartridge 17 containing holographic film 18. A drive mechanism (not shown) is coupled to shafts 17a in order to drive the film through a reconstruction area, i.e. in front of a window 19a of a box or housing 19, the latter containing three laser sources, S1, S2, S3 for three different wavelengths ) 2 )3 optical fibres 14 and mask 16. Instead of three laser sources, two laser sources with an output of three colours could be used.

The holographic film material may be, for example, a silver halide emulsion sensitive to all three of the colours used during the recording step. Such emulsion is very sensitive but the thus recorded holograms are preferably transferred such as onto a dichromated gelatin material which can reconstruct the holographic images more effectively. This transfer can be achieved by exposing dichromated gelatin with blue light through the recorded hologram, which thus acts as a master and facilitates copying of the holographic movie. The recorded hologram is a thin amplitude hologram whereas that produced in the dichromate gelatin is a thick phase hologram.

The invention thus provides a hologram recording and playback technique which is particularly but not necessarily applicable to the recording and playback of a colour holographic 'motion picture". As will be appreciated, an analogy can be drawn between a shadow mask TV tube, with its three electron guns (red, green and blue), apertured shadow mask and triad of phosphor dots (red, green, blue) on the screen, and the laser light sources, mask 16 and mask 10 (together with the holographic film), respectively of the present technique.

Whereas in the above the use of three colours red, green and blue are referred to for the light source, and this will result in a true colour effect, more or less than three colours and appropriately apertured and controlled masks for the recording stage can be employed to produce other colour effects.

The above described recording technique involved a three-dimensional object 9, however the threedimensional object can be replaced as indicated in Fig.

5 by a two-dimensional transparency 20 of an object illuminated from behind via a diffuser 21, for example of ground glass. Moreover, if transparencies are used, a three-dimensional effect can still be perceived by the viewer of the replayed holographic film. For each frame 22 of holographic film recorded, two transparencies, i.e. films from two cameras corresponding to a left and right view of the same object or scene are required and these are recorded by the triple exposure technique described above. The left half 22a of the frame 22 of holographic film is recorded with one of the transparencies (left view) as the object whilst the right half 22b of the frame 22 of holographic film is recorded with the other transparency (right view) as the object.For such recording as well as a mask 23 corresponding to mask 10 of Fig. 1 an additional mask 24 is required which has two possible positions, corresponding to recording the left view or the right view transparency. The light sources are omitted from Fig. 5, however the output ends of the object beam fibres 15 and the reference beam fibres 14 are indicated.

On reconstruction of a frame of the holographic film recorded as described with reference to Fig. 5, both views are generated simultaneously but the wields of view of each of the two images is different.

Ideally the left eye of a viewer is in one field of view, that is of the left viewpoint of the image, and the right eye of the viewer is in the other field of view. This is illustrated in Fig. 6. The illusion is three-dimensional provided that the two different images that are recorded do in fact match up with the two images that the viewer would expect to see in real life. A car standing say 30 metres away cannot be viewed from two angles differing by for example 400, so two transparencies depicting this, if used in the recording stage would not give a three-dimensional effect when viewed during reconstruction through the hologram/holographic aperture/window.

The dimensions of the apparatus, and the viewer, are important as well as the distance of the viewer from the holographic film and the distance of the object from the holographic film during recording. In Fig. 6, which is a view from above the apparatus, D is the distance between the viewer and the holographic film 30, Y is the distance between the object transparency and the holographic film, E is the minimum distance between the viewer's eyes, X is half the width of one frame of the holographic film, W is the approximate width of the transparency/object and its image, and H is the tangential distance from the centre of the holographic film to the centre of the transparency/ object. The apparatus is designed such that the following relations (1) and (2) hold.

Relation (1) E/D = W/y (1) If E = 4cm, then D = 4y/W.

If the viewed image is 12cm wide (W), then D = 4y/12 and D = y/3.

If the viewed image appears 30cm behind the hologram (y) then D = 30/3 = 10cm.

Thus in this example the viewer is only 10cm from the hologram and is focussing at (10 + 30)cm ahead onto the image. This distance is very small and this example would therefore possibly be in the form of personalised holographic TV/video machine, for example mounted on a helmet, and may have military applications such as for training/simulation exercises of 3-D terrain/combat.

Relation (2) The distance perpendicular to the viewing direction over which the whole of, for example, the left image can be viewed by the left eye is given by F, where F = D/y [X - W] + X (2) If, as before, D = y/3, the minimum distance between the eyes being 4cm and the viewed image being 12cm wide, F = 1/3 [X - W] + X It should be noted that if W > X then F is less than X and decreases with increased distance of the viewer from the hologram. If W > X then F is more than X and increases with increased distance of the viewer from the hologram.

As indicated before, a reasonable/minimum value for W, which is the approximate width of the object viewed, is 12cm. It is very unlikely that the half width of one frame of film will be greater than this, therefore W is greater than X. Twice X can be regarded as the "window' through which the viewer observes the image. Combining equations (1) and (2).

F = D/y [X - W1 + X where D = E.y/W Therefore, F = (Ey/W)/y [X - W] + X = E/W [X - W] + X = EX/W - E + X or F = X - E + EX/W (3) If E = 4cm and W = 12cm F = [X - 4 + X/3] cm If X = 5cm, that is the width of the film frame is 10cm, F = 1 + 5/3 = 2.7cm that is each eye has to be placed within an area which is 2.7cm wide in order to see the whole holographic image by viewing through the 'hologram' window.

This is however not the only method by which a three-dimensional image can be perceived by a viewer when in fact only a collection of two-dimensional images (transparencies, slides) are used in the recording stage.

As before the three-dimensional object is replaced by a set of two-dimensional transparencies illuminated from behind via a diffuser. However the holographic film is split up into several vertical strips, for example ten strips, and the distance of the final holographic film from the viewer is much greater than in the previous method. A standard technique of producing a holographic stereogram involves such vertical strip division of a holographic film and this technique may be incorporated with the triple exposure using masks and optical fibres to produce colour holograms described above, as indicated below.

For each position of a mask 40 (Fig. 7) exposing a strip of a holographic film 41 a different transparency/slide/drawing 42 is placed in front of a diffuser 43 such that the left most holographic strip is recorded with the left most view of the object/scene etc, where each view of the object takes up a full frame. According to the present invention a further mask (not shown) will be needed in order to expose with separate red, green and blue beams different areas of the holographic film, as a result of the triple exposure process of the present invention. On viewing a final processed holographic frame of film (10 strip hologram 44) a 3-D illusion results as indicated in Fig. 8. The brain fuses the points AL and AR in the image plane 45 as seen by the left and right eyes to give the impression of a point at A, and similarly for B L and BR to give B.When a point is viewed through the centres of the strips it appears in the correct position. If either eye looks through the edge of a strip then the image will be displaced from the correct position. Both the correct position 46 (intersection of solid lines) and the displaced position 47 (intersection of dashed lines) are indicated in Fig. 9. The area of each frame of the holographic film acts as a window through which the object is seen.

The criterion for creating/observing holographic stereograms will now be discussed with reference to Fig. 10, which shows a view from above of a hologram 50 having vertical strips of holograms each of the same object but from a different viewpoint, the image plane and a viewer. The strips are each of width X. E is the distance between the eyes of the viewer, y is the distance from the plane of the hologram 50 to the image plane 51, D is the distance from the observer's (viewer's) plane 52 to the hologram plane. The same region of an object is seen from two different angles via two different hologram strips, separated by a distance AN.X, that is the distance between the left view and right view hologram strips. In the example illustrated the hologram strips concerned are adjacent and bun = 1.

For E and x D or y, then the following relation holds/must be observed.

E - #N.X = ## = #left = #left view -# right view D D+y

HenceAN.X = D+y If AN=1 as in Fig. 10, then the adjacent strips of holograms must hold the correct left view and right view perspectives as would be viewed by the observer in real life.

If however AN = 4, for example, as indicated in Fig. 11, then pairs of strips 4 strips apart must hold the correct left view and right view perspectives.

Moreover, every such possible pair of strips that are 4 strips apart must hold the correct left and right view perspective but from a different average viewing angle.

It may, however, be possible to enhance the stereoscopic effect by in fact observing two views which represent a difference in viewing angles which is greater than that which one would observe in real life.

As above,N.X = Ey/(D+y) If D=4y, then AN.X = E (1/5) and if X =-E/10, then AN = E (1/5) (10/E) = 2 In this case the pairs of strips 2 strips apart must hold the correct left view and right view perspectives, as indicated in Fig. 12.

The inventive method of manufacturing colour holograms is such that a thin transmission hologram (as opposed to a thick one of the prior art) can be recorded, so that a thinner holographic film of coarser particle sizes (in the case of silver halide emulsion/films) is required, giving increased sensitivity to light during the recording stages. In the inventive method since the "master" copy is a thin hologram, particularly a thin, amplitude variation hologram, meaning that it modulates the amplitude of an incident beam, it can be more readily copied onto blank film/media either by direct or "contact" printing, or by recording thick volume holograms from the master thin holograms. For example, the patterns/fringes comprising the recorded holograms can be directly copied onto a second (blank) film by exposing the blank film to blue light transmitted through the recorded hologram. The inventive method of colour hologram manufacture enables colour holograms and copies thereof to be produced without an intrinsic loss of efficiency. The playback machine can be smaller than hitherto because the three reconstructing sources are close together, enabling a compact design.

Claims

CLAIMS:

1. A method of manufacturing a colour hologram using light of at least two colours including the steps of forming a reference beam and an object beam for each colour from a respective source, and for each colour in turn causing the respective object beam to illuminate an object or scene and then to intersect with the respective reference beam and be incident on a respective area of a holographic recording medium for a predetermined exposure time.

2. A method as claimed in claim 1 wherein an apertured first mask is disposed in front of the holographic recording medium and including the step of moving the first mask between exposure for the different colours whereby areas of the holographic recording medium are only exposed once.

3. A method as claimed in claim 1 or claim 2, wherein for each colour a portion of a beam output from the respective source is employed to provide the respective object beam and is launched into one end of a first optical fibre, the other end of the first optical fibre acting as a point source for the respective object beam, and wherein for each colour another portion of the beam output from the respective source is employed to provide the respective reference beam and is launched into one end of a second optical fibre, the other end of said second optical fibre acting as a point source for the respective reference beam, the other ends of the fibres being disposed so that the point sources are at fixed positions.

4. A method as claimed in claim 3 wherein the optical fibres are monomode at the respective source wavelengths.

5. A method as claimed in claim 3 as appendant to claim 2 and for providing a true colour hologram, including the use of light of three colours, red, green and blue, and a first mask movable to three positions.

6. A method as claimed in claim 5, wherein the colour hologram comprises an array of closely-packed mini holograms arranged in groups of three, each mini hologram of a group being recorded by light of a different wavelength, and wherein the first mask is movable between said three positions whereby to expose areas corresponding to each mini hologram of the groups in turn to the respective wavelengths.

7. A method as claimed in any one of the preceding claims and including the step of processing the thus exposed holographic recording medium to form said hologram.

8. A method of manufacturing a true colour hologram substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

9. A colour hologram manufactured by a method as claimed in any one of the preceding claims.

10. A method of reconstructing an image of an object from a colour hologram thereof produced by a method according to any one of the preceding claims, including the step of illuminating the hologram with reconstruction beams, which correspond to all of the reference beams, via a second apertured mask which serves to ensure that light from any one reconstruction beam is only incident on the part of the hologram recorded by light of that colour/wavelength.

11. A method of manufacturing a true colour hologram of a three-dimensional object and reconstructing an image of the object from the hologram, including the steps of forming a reference beam and an object beam from each of three sources whose colours are red, green and blue respectively, causing each colour of object beam in turn to illuminate the object and then to intersect the reference beam of the same colour and be incident on a respective area of a holographic recording medium for a predetermined exposure time, the respective area being defined by an apertured mask which is moved after each exposure whereby areas of the recording medium are only exposed once, processing the thus exposed holographic recording medium to form said hologram, and illuminating the hologram with reconstruction beams, which correspond to all of the reference beams, via a second apertured mask which serves to ensure that light from any one reconstruction beam is only incident on the part of the hologram recorded by light of that colour/wavelength.

12. A method of manufacturing a colour hologram of a three dimensional object or scene using light of at least two colours including the steps of forming a reference beam and an object beam for each colour from a respective source, and for each colour in turn causing the respective object beam to illuminate the object or scene and then to intersect with the respective reference beam and be incident on a respective area of a frame of holographic recording medium for a predetermined exposure time, the position of an apertured mask disposed in front of the frame of holographic recording medium being changed after each exposure to ensure that the areas of the frame of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of mini holograms over the whole area of the frame.

13. A method of manufacturing a colour holographic stereogram from two two-dimensional transparencies of objects or scenes as viewed from two different angular positions using light of at least two colours, including the steps of forming a reference beam and an object beam for each colour from a respective source, covering one half of a frame of holographic recording medium, and for each colour in turn directing the respective object beam through one transparency toward the uncovered half of the frame for intersection thereat on predetermined areas of the frame with the respective reference half determined by an apertured mask beam for a predetermined exposure time, the position of the apertured mask which is disposed in front of the frame being changed after each exposure to ensure that the areas of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of miniholograms over the uncovered half of the frame, covering the other half of the frame and for each colour in turn directing the respective object beam through the other transparency toward the then uncovered half of the frame for intersection thereat on predetermined areas of the frame with the respective reference beam for a predetermined exposure time, the position of the apertured mask being changed after each exposure, the apertures in the mask and the movement thereof serving to produce a mosaix of miniholograms over the then uncovered half of the frame, processing the thus exposed holographic recording medium to form said holographic stereogram.

14. A method of manufacturing a colour holographic stereogram from a plurality of two-dimensional transparencies of objects or scenes as viewed from different angular positions using light of at least two colours, including the steps of forming a reference beam and an object beam for each colour from a respective source, and wherein for each transparency a respective strip of a frame of holographic recording medium is uncovered and for each colour in turn the respective object beam is directed through the transparency toward the uncovered strip for intersection thereat on predetermined areas of the frame strip determined by an apertured mask with the respective reference beam for a predetermined exposure time, the position of the apertured mask which is disposed in front of the frame being changed after each exposure to ensure that the areas of the holographic medium are only exposed once, the apertures in the mask and the movement thereof serving to produce a mosaic of mini holograms over the uncovered strip of the frame, repeating the exposure procedure for each transparency employing a corresponding strip of the frame, and processing the thus exposed holographic recording medium to form said holographic stereogram.

15. A colour hologram motion picture film including a plurality of frames each comprising a colour hologram of an object or scene manufactured by a method according to any one of claims 1 to 7.

16. A true colour hologram motion picture film including a plurality of frames each comprising a colour hologram of a moving object or changing scene at a respective point in time and manufactured by a method comprising the steps of forming a reference beam and an object beam from each of three sources whose colours are red, green and blue respectively, causing each colour of object beam in turn to illuminate the object or scene and then to intersect the reference beam of the same colour and be incident on a respective area of a frame of holographic recording medium for a predetermined exposure time, the respective area being defined by an apertured mask which is moved after each exposure whereby areas of the recording medium are only exposed once and repeating the exposure process for each of said plurality of frames, processing the thus exposed holographic recording medium to form said hologram motion picture film, images of the moving object or changing scene being reconstructed from the film by illuminating each frame thereof in turn with reconstruction beams, which correspond to all of the reference beams, via another apertured mask which serves to ensure that light from any one reconstruction beam is only incident on that part of the hologram recorded by light of that colour/wavelength.

17. A true colour hologram motion picture film substantially as herein described with reference to the accompanying drawings.

18. A colour holographic stereogram manufactured by a method as claimed in claim 13 or claim 14.

19. A playback machine for reconstructing an image of an object from a colour hologram thereof produced by a method according to any one of claims 1 to 9, the machine comprising a box having a window at which a hologram to be reconstructed can be disposed, an apertured mask disposed in said box, and means contained within said box for illuminating the hologram with reconstruction beams, which correspond to all of the reference beams, via the apertured mask in the box which serves to ensure that light from any one reconstruction beam is only incident on the part of the hologram recorded by light of that colour/wavelength.

20. A machine as claimed in claim 19, and for use with colour hologram motion picture film, the machine including drive means for indexing the film past the window for reconstruction of each frame thereof in turn.

21. A machine as claimed in claim 20 and wherein the colour hologram motion picture film is disposed in a film cartridge, the box and the cartridge being adapted to one another.

22. A colour hologram playback machine substantially as herein described with reference to and as illustrated in Fig. 4 of the accompanying drawings.