Note: Descriptions are shown in the official language in which they were submitted.
2161082
WO94/~15 ^ ` l PCT/GB94/008~
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HOLOGRAMS
The present invention relates to holo~.ams and
a method of making holograms, and apparatus for making
holograms.
Conventional holograms produce a three
dimensional image such that the viewer will see a
different aspect of the image depending on where he or
she is stAn~ing in relation to the image. For certain
applications it would be advantageous to provide a
hologram which produced an image which presented the
same information, however it was viewed, such a hologram
would be useful for display signs for example. The
hologram would typically have different parts of a
photo-sensitive plate separately exposed to interference
fringes formed by different parts of the object to be
imaged.
Thus, if the object is a word, i.e. two
dimensional letters, the word may be divided into a
series of letters or parts of letters and each letter or
part used as a separate object for the purpose of
forming interference fringes. The resulting fringes are
recorded on a specific portion of a photographic plate
so that each portion of the plate relates to a different
portion of the word.
UK-A-1292110 discloses a display made in
accordance with this process. The intention of this UK
patent application is to provide a road sign which can
display one of a number of different pieces of
information either as a static display or as an
alternating display between two words. The problem with
this previous proposal is that whatever message is
displayed is fixed in space and the change in the
message displayed is under the control of the apparatus
as it re~uires different illuminating light sources to
be activated so as to change the angle of illumination
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j.,
and hence the message displayed.
Another display arrangement is disclosed in
WO-A-89/08304 where a sign is disclosed which comprises
an array of elements each ~ rising a first lens and a
second lens and an optical`robject in registry with the
first and second lenses such that the virtual image of
the optical object formed in the second lens lies on the
focal surface of the first lens. The optical objects
consist of a representation of the information which the
sign is to present. The optical objects are substantial
identical to each other and the image of the optical
objects seen by a viewer moving past the sign thus
appears to be stationary with respect to the viewer.
There is no disclosure in this earlier document of the
use of holographic material as the optical object and
consequently the advantages to be obtained by using
holographic material are not suggested by this earlier
document.
Holographic material has been used in a number
of different types of optical systems and UK-A-1233242
discloses an optical system utilizing a holographic
material for creating an array of identical images in
two ~ime~ions from a single hologram by illuminating
the hologram with reference light sources at varying
angles depending on the spacing of the array of images.
The application suggested for this apparatus is the
production of semiconductor devices which require
extremely accurate images to be transferred to a photo-
resist layer in a contactless manner so as to avoid
imperfections and loss of resolution which would
otherwise occur if a contact process were utilized.
The present invention provides a visual
display device comprises a plurality of identical
elements, each element consisting of a hologram having a
plurality of multiplexed patterns corresponding
respectively to a plurality of different images stored
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WO94/~K15 ~ PCT/GB941008
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by the hologram. ~ ~
The present invention f~rther provides a
method of forming a holographic display device
comprising illuminating a first part of an object to be
viewed, recording on different parts of a first hologram
element first interference patterns corresponding to
respective different views of the first part of the
object, illuminating a further part of the object to be
viewed, recording on further different parts of the
first hologram element further interference patterns
corresponding to respective different views of the
further part of the object, creating further hologram
elements with recorded first and further interference
patterns identical to the patterns recorded on the first
hologram element, and assembling hologram elements thus
formed into a display.
Certain embodiments of the invention will now
be described by way of example only and with reference
to the accompanying drawings in which:
Figure 1 is a schematic diagram illustrating
how a conventional hologram is formed;
Figure 2 is a schematic diagram illustrating
the formation of a conventional multiplexed hologram;
Figures 3a and 3b are schematic diagrams
illustrating a first method of making holograms
according to the present invention;
Figure 4 is a schematic diagram illustrating a
master hologram made by the method shown in Figure 3;
Figures 5a and 5b show how a transfer hologram
may be formed from a master hologram;
Figure 6 shows an array of secondary holograms
made from the master hologram in Figure 4;
Figures 7a to 7e show various holographic
images seen by viewing the array of Figure 6 from
different positions;
Figures 8 to 11 show various arrangements of
W094/~Kl5 ~ 21610 82 PCT/GB94/008~
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equipment for producing a hologram-as shown in Figures 6
and 7;
Figures 12a and 12b are schematic diagrams
illustrating a further method of making holograms
according to the present invention;
Figure 13a shows an array of secondary
holograms made from the master hologram shown in Figure
12a;
Figures 13b to 13h show various holographic
images seen by viewing the array of Figure 13a from
different positions;
Figures 14 and 14b are schematic diagrams
illustrating a further method of making holograms
according to the present invention; and
Figures lS and 15a to 15e show a further
method for producing a master hologram according to the
present invention.
According to a first embodiment of the
invention a master hologram is made by a technique
similar to that used in for so-called "multiplexed
holograms". To facilitate an understanding of this
embodiment, conventional holograms and then multiplexed
holograms will first be briefly described.
Figure 1 is a schematic diagram illustrating
the process for production of a conventional hologram,
which is formed by photographically recording an
interference pattern between coherent radiation Es
scattered from an object 10 and a reference beam of
coherent radiation, ER. The photo-sensitive plate 11 is
developed and suitably illuminated to reconstruct an
mage .
In Figure 1 the reference beam and scattered
object beam are produced from the same source, the
reference beam being reflected from a mirror 12.
Figure 2 illustrates diagrammatically the
formation of a multiplexed hologram. A multiplexed
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hologram is formed using a series of slides 13 or other
suitable transparencies, projected using laser light
onto a diffusion or projection screen 14, showing
different views of the object instead of the original
three dimensional object. The interference pattern from
each slide is exposed onto an appropriate portion lla of
a photo-sensitive plate 11 while the rest of the plate
is masked off. The mask is then moved to the adjacent
portion of the photo-sensitive plate (in direction D),
and the photo-sensitive plate is exposed to the next
projected image. Once developed and appropriately
illuminated the plate has recorded a continuous series
of stereographic images creating a three ~imeP~ional
illusion.
A conventional or multiplexed hologram of the
type described above is referred to as a master
hologram.
A master hologram may be used to make further
holograms by various techniques. The further holograms
are variously referred to as copy holograms, secondary
holograms and transfer holograms.
Figures 3 to 6 illustrate a very simple
example of a method of making a hologram according to
the present invention. In this example the desired
holographic image is a sign displaying the words DANGER.
The reference beams have been left out for the sake of
clarity, although they are essential for forming the
interference patterns with the object light (Es).
As shown in Figure 3, using a technique
similar to the multiplexed hologram technique described
a~ove, different parts of a photographic plate are
exposed to different interference fringes so that the
final complete pattern represents the whole word.
Thus, as shown in Figure 3a, a portion of a
photo-sensitive plate 20 is exposed to interference
fringes formed by scattering laser light (Es) through an
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object 21 in the shape of a letter "D". (The letters
are reversed in Figure 3, as they are read from the
direction of the photographic plate.) The object may be
a stencil or a letter pain~te~ on a transparent plate for
example. The remainde~ ~f the plate 20 is protected by
a baffle 22. Then, i'n Figure 3b a second portion of the
plate is exposed to the patterns formed by the letter
"A" object 23 using a baffle 24 and so on through to the
end of the word.
Once the master hologram 20a has been made and
is re-illuminated the viewer sees, at position (X), the
letters D, A, N, G, E, R when looking from positions 1,
2, 3, 4, 5, 6 respectively (Figure 4). Or conversely
the letters are seen from angles a, b, c, etc.
Using the master hologram (hereafter Hl), a
transfer or a copy hologram (hereafter H2) is made by
traditional methods.
For example, in Figure 5a the Hl hologram is
illuminated using laser light Es. This creates an
optical image of the original objects. A second photo-
sensitive plate H2 is placed at the position of this
image and "referenced" with a secondary laser beam ER
causing an interference pattern at the H2 which is then
developed to form the copy hologram.
When the H2 hologram is illuminated (Figure
5b), the image of the original Hl floats in space in
front of the H2.
When the viewer looks through his Hl image at
the H2 from the relative positions al, a2, a3, a4, a5,
a6, the letters appear D, A, N, G, E, R etc respective ly
(Figure 5b).
By paneling an array of these H2 holograms,
all identical to each other, (Figure 6) an interesting
effect occurs. Looking from position Yl, in front of an
array of 6 H2 holograms, because of the angular
relationship to each H2 panel, the viewer would see the
letter "D" at H2-31, the letter "A" at H2-32, the letter
WO 94/24615 21 61~082 `~ ~r PCTIGB94/00844
"N" at H2-33, the letter "G" at H2-34, the letter "E" at
H2-35 and the letter "R" at H2-36. In other words the
viewer would see the whole word spelt across six
identical holograms (Figure 7a).
If the viewer then moves sideways towards Y2,
because all holograms are identical to each other and
contain all the information to be conveyed and the
angular relationship to the H2's is preserved, so the
word kinetically moves with the viewer. At Y2 the word
"DANGER" appears between H2-35 and H2-40 (Figure 7b).
Note especially that when the panel H2-35 is viewed from
Yl the viewer sees the letter "E" whereas from Y2 the
viewer sees the letter "D".
If the viewer moves further away from the
array of H2's (for example to position Y3, Figure 6),
the image of "DANGER" will be spread over a greater
number of holograms. The holograms which are totally
seen by the viewer are those at the same angle with
respect to the viewer as when viewed at Yl or Y2, i.e.
from H2-41 to H2-51. This crude example will have a
crude result (Figure 7c), however if the image to be
used was divided into perhaps 1000 parts with 1000
different exposures instead of 6 parts and 6 exposures
(Figure 7d) the resulting image would be much smoother
tFigure 7e). So as the viewer moves closer and further
from the H2's the effect of compression and extension of
the image would take place, due to changes in angle of
view of each hologram.
The following relates to possible arrangements
of equipment for producing holograms according to the
invention.
Figure 8 shows one arrangement for producing
Hl holograms necessary for this invention. The photo-
sensitive plate 80 has a baffle or mask 81 in front of
it. The mask 81 allows exposure of consecutive parts of
the photo-sensitive plate 80 as the mask is moved in
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direction 82 between exposures. A stencil, drawing,
transparency or any image-forming device 83 is placed in
front of a diffusion~s~èen 84. The diffusion screen is
suitably illl-~inA~d by laser light (Es). This light
only needs to cover the area 86 of the diffusion screen
84 which is the size of the single copy holograms, which
will be later grouped into an array. The stencil is
also moved in direction 82 between exposures.
If the length of the image is similar to the
length of the Hl hologram then an optional link 85 can
be used to move mask and stencil in synchronisation with
each other. However images longer or shorter than the
photo-sensitive plate can also be made in which case the
mask and stencil would be moved at different rates over
the Hl production period. In this case the optional
link would not be used.
Figure 9 shows another scenario where the
photo-sensitive plate 90 and its' baffle 91 act
similarly to the example of Figure 8. In Figure 9 the
stencil is replaced by using a projection device 93
through which laser light (Es) is projected through a
transparency 95 onto a diffusion/back-projection screen
94. The baffle 91 would be moved in direction 92 and
the transparency changed to a different image between
exposures. These images would be equivalent to the
moving stencil. It will be appreciated that for clarity
the images in Figure 9 are a crude version. In a smooth
version only consecutive vertical slices of letters,
words or images would be used.
Figure 10 describes the way in which one copy
hologram H2 can be made from the Hl hologram by
traditional means. Figure 10 shows the master hologram
Hl and forming an image of overlapping letters at 116.
A single photo-sensitive plate H2 is then placed close
to the plane of that image and referenced by more laser
light ER. This is then developed to form a hologram H2.
W094/24615 2I ~ g 2 rl ~ PCT/GBg4/00844
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Many H2's would be made by this method and then panelled
together into an array. ~ r
Arrays of H2's could also be made using a
"step and repeat" mechanism as in Figure 11. This is
similar to Figure 10 except that a static baffle 117 is
placed either side of the source image to mask off
unwanted source light Es and H2 reference light ER.
Behind the baffle is a length of unexposed
photo-sensitive plate or film H2 which is moved in
direction 118 between exposures. By this method an
array of secondary holograms can be produced on one
piece of photo-sensitive material.
A further development of this invention would
be to make a master hologram in which the image to be
recorded has been divided up along the vertical axis
(hereafter Y) as well as the horizontal axis (hereafter
X) (Figure 12a). In this example laser light Es is
passed through a diffusion screen 121 and then through
part of a stencil 122. Some of this light forms
interference fringes with the reference light ER when
they meet at the photo-sensitive plate (hereafter Pl).
There is a baffle 123 which has a hole cut in it 124 so
that only the appropriate portion of Pl is exposed. The
stencil 122 and the baffle 123 would be moved between
exposures, firstly in direction X to form a row of
exposed Pl. The stencil and baffle would then be moved
up one row in direction Y and the procedure repeated
until Pl is fully exposed. The diffusion screen 121 and
the photo-sensitive plate Pl do not move throughout the
whole process.
The equipment shown in Figure 12a could be
modified in the same way as Figures 8 and 9.
Portions of the photo-sensitive plate will be
exposed to portions of the stencil in an inverse
relationship. Figure 12b is a schematic diagram showing
'his relationship, so that portion P/l, P/2, P/3, P/4,
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P/5, P/6, P/7, P/8, P/9 etc of the photo-sensitive plate
Pl will be exposed to portions S/l, S/2, S/3, S/4, S/5,
S/6, S/7, S/8, S/9 etc of the stencil 122.
In the above example (Figure 12a), the photo-
sensitive plate wou ~'~e developed by traditional means
to form an Hl master hologram. Many copy holograms
~H2s) would be made by traditional methods or by a step
and repeat method similar to those shown in Figures 10
and 11. These holograms would be tiled together into a
two dimensional array of holograms H2s by various
methods (Figure 13a). For the clarity of the following
examples a picture frame 131 has been placed around the
array of holograms.
Figure 13b shows example viewing positions El,
E2, E3, E4 and E5, all perpendicular to the H2 array
(H2s), and looking towards XYl, XY2, XY3, XY4 and XY5
respectively. Figure 13c shows the stencil image with
border as used in this example 132. Figures 13d, 13e,
13f, 13g and 13h show what the viewer would actually see
from positions El, E2, E3, E4 and E5 respectively.
Figures 13d to 13h are drawn in perspective, meaning
that the frame 131 around the array of holograms (H2s)
is smaller as the viewpoint is further away and larger
as it is closer. It is important to note that the image
132 of the jug always rem~ins the same size relative to
the viewpoint regardless of the distance from the array.
The converse is also true, in that, if the holographic
array of H2s is moved past the viewer and the viewer is
static, then the image does not change (provided that
the viewer is always at 90 degrees to the plane of the
array3.
Figure 14 shows a method of making an XY array
of holograms, without first producing a master hologram,
with a similar result to that of Figures 13a to 13h. In
this case, instead of using a master hologram to project
the image, a stencil (141) and a diffusion screen (142)
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WO941~15 ~ , r PCT/GB94/008~
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are used to deflect/scatter the coherent light (Es)
through the stencil, which is between the diffusion
screen and the photo-sensitive medlum (Pl). Some of the
light (Ess) will reach the photo-sensitive medium (Pl)
through an aperture (144) in the mask (145). A
reference beam of coherent light (ER) is directed to the
same position from an appropriate angle (AR), forming
interference fringe patterns which can later be
developed to produce a hologram. The photo-sensitive
medium (Pl) is moved se~uentially in directions X and Y
with respect to the mask which is stationary so that the
fringe patterns can be exposed to it. The
photo-sensitive medium is in turn developed into an
array of holograms. The hologram could be of the type
known as a "Reflection Hologram" or "Transmission
Hologram" or "Edge-lit Hologram" or indeed any practical
method of making holograms.
Figure 14b is an example of how the resulting
array of holograms might look from a position Vl. Here
the phenomenon of the angular relatio~hi r of image to
surface is maintained, giving similar visual results to
those described here in Figures 13a to 13h. It also
shows how, because of the angular relationship of each
array element to the eye, the resulting image will
appear to be inverted.
In this case, if it were desired to improve
the resulting image ~uality/sharpness, a small aperture
could be specified in the mask (144). The sharpness of
the image will also be greatly effected by the type,
quality and design of the final illumination source.
- An optional lens (not shown), or other
focussing device, could be placed between the stencil
(141) and the aperture (144) to m~xi mi se the amount of
light (Ess) reaching the photo-sensitive plate.
Another method for producing an array of
holograms in the X and Y axes is quite similar to that
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of Figure 14, and so in this instance we will describe
this new method with reference to Figure 14. In this
version the light (Es) must firstly be spread to
adequately cover the stencil às before. It must then be
redirected through the ste~cil (141), to the desired
position on the photo-sens`itive material (144), by using
a suitable focussing device such as a large, positive
focal length, lens (not shown). The lens would replace
the diffusion screen (142) and so the light would no
longer be scattered (143). Instead each individual ray
of light (Ess) will converge towards the focussing point
(144) at an angle unique to its spatial position. A
suitable reference beam of light (ER) is then required
to form the necessary interferenced patterns, in order
to make an array of holograms as with the previous
example. Again, the directional/angular relationship of
image to surface is maintained and, once the resulting
array of holograms is produced and lit, will give
similar image phenomena to those described here and
shown in Figures 13a to 13h and Figure 14b.
Figure 15 shows a further method for producing
a master hologram, according to the present invention.
In this embodiment a beam of coherent reference light is
directed to the photo-sensitive plate (Pl) at a suitable
angle (AR). Another beam of coherent light is focussed
by a lens (151) so that it converges to the same point
(152) on the photo-sensitive plate (Pl) forming an
interference pattern which can be later developed. The
photo-sensitive plate is moved in the horizontal and
vertical directions, and exposures made consecutively,
forming an array of interference patterns which can be
developed. This array could be similar in position to
the picture cells (or pixels) of a cathode ray tube,
computer or television screen. If selective exposures
are made, i.e. the laser beam is switched on and off so
that not every position in the plate is exposed, designs
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could be built up to produce a master hologram of
unexposed and exposed areas.
It is also true to say that exposures do not
necessarily need to be made in a grid fashion and could
be made consecutively in any direction in the X Y plane
by moving the photo-sensitive plate. This could be
construed as a method of "drawing" the exposures in a
vector format.
Once the photo-sensitive plate is developed,
the resulting master hologram is suitably illuminated by
coherent light. Figure 15a shows such a master which,
for clarity and example, has been sequentially exposed
in the form of the letter "R". Every array element of
the exposed parts of the master will diffract an image
of the lens (Figure 15b). This effectively means that
the light will fan out from each point on the master
hologram, through a broad angle ~153,154). As the light
spreads from each pixel point (Figure 15c), it can be
seen that, at a certain distance from the master tFigure
15d) there are spatial locations (155) which receive
light from every diffracting array element of the master
hologram. This "sub-set" position (155) receives light
from each array element as shown in Figure 15e. This
master hologram can then be used to produce an array of
secondary holograms, hitherto described.
It will be appreciated that a hologram
according to this invention can be made by numerous
methods and is not limited to the
specific examples described above. For example, any
appropriate photo-sensitive material could be used for
the production of the master or transfer holograms.
Alternatively they may be produced by other methods such
as the formation of relief patterns by embossing,
engraving, etching, casting or moulding or any other
optical, electronic, magnetic or storage technique.
An application of this invention could be in
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the sign industry with road signs easily legible from
any distance.
Or signs of arrows which follow the viewer,
kinetically guiding them. For example, a sign with
arrows and text, in which as the viewer looks to the
left he or she can always read the words "Way Out" and
when looking to the right he or she can read the words
"No Exit". This would be a useful form of public flow
control in such places as Airports, Museums, Railway
stations, Public buildings etc.
or in roll-tape form as kinetic de-lineation
markers in crowd~traffic flow control situations.
Or as an optical protractor with markers which
remain at a specific angle to the viewer.
or for use as security measure to prevent
forgery of certain products such as credit cards.
or as a security key/identity system.
Or as part of a product counting/
identification conveyer system. For example, as the
product, with an array hologram on it, passes the viewer
or viewing device at any speed, the image on the
hologram remains static relative to the viewer.
or as a kinetic packaging or gift wrapping, or
a sign which follows any linear route.
Application of the XY field array hologram
include:-
road signs in which the image stays the samesize from any distance;
signs which animate as the viewer gets
closer.
An application might be a "Slow Down" sign of
animated Slow Down chevrons as a driver nears a bend.
