Reflection holograms

A reflection hologram is recorded in a photosensitive film on a substrate held in a support by interference between an illuminating beam and its return from a back reflector. A scanning device moves the beam from a laser angularly about a fixed apparent beam source position which remains stationary relative to the film to give improved hologram formation with the back reflector spaced from the film.

BACKGROUND OF THE INVENTION 
This invention concerns improvements in or relating to reflection holograms 
and relates more particularly to a method of and apparatus for making 
reflection holograms (sometimes conversely called holographic reflectors), 
and reflection holograms made by use of such method and apparatus. 
Reflection holograms can provide efficient colour reflective filters which 
reflect a narrow waveband of incident light. A reflection hologram can be 
made by scanning a light beam, usually from a laser, over the surface of a 
photosensitive film behind which is a mirror. The incident beam and the 
beam reflected back from the mirror produce interference fringes in the 
film which, usually after suitable processing, provide corresponding 
variations in refractive index that give rise to the hologram. Such a 
method is disclosed in UK Pat. No. GB2071866B in which scanning of the 
laser beam is effected by two mirrors arranged to rotate about respective 
axes so as to produce horizontal and vertical movement. However, there is 
some limitation on the forms of hologram which can be satisfactorily 
produced by use of such mirror arrangement. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided a method of making a 
reflection hologram including the steps of directing an illuminating beam 
towards a photosensitive film from a substantially fixed apparent beam 
source position, angularly moving the illuminating beam relatively to the 
film in a manner such that said substantially fixed apparent beam source 
position remains substantially stationary relative to the film, and 
reflecting the illuminating beam back to the film after passage 
therethrough from a reflector spaced from the film so as to cause 
interference in the film between the illuminating beam and its reflection 
back. 
Such a method is distinguished from the use of a double scanning mirror 
arrangement as disclosed in UK Pat. No. 2071866B by the requirement for a 
substantially fixed apparent beam source position. With the backing mirror 
almost in contact with the photosensitive film to produce fringes lying 
parallel to the film as described in that patent, motion of the 
illuminating beam apparent source position does not generally affect the 
hologram formation adversely to a significant extent. However when the 
backing reflector is required to be spaced from the film, for example to 
make a hologram in which the interference fringes do not lie wholly 
parallel to the film, then apparent source movement can mar or prevent 
satisfactory hologram formation as more fully explained later. 
The present invention further provides apparatus for use in making a 
reflection hologram, the apparatus comprising support means for supporting 
an element carrying a photosensitive film, a light emitter, light 
directing means for directing light from the emitter towards an element 
supported by the support means as an illuminating beam from a 
substantially fixed apparent beam source position and for angularly moving 
the illuminating beam relatively to the film in a manner such that said 
apparent beam source position remains substantially stationary relative to 
the film, and a reflector at a location spaced from the film on an element 
supported by the support means and arranged to reflect the illuminating 
beam back to the film after passage therethrough. 
The illuminating beam movement relative to the film is preferably a 
scanning movement. The movement may be effected by, and said means for 
directing light may therefore comprise, an optical device with means for 
moving the device about mutually orthogonal axes which intersect at said 
substantially fixed apparent beam source position. For example, there may 
be a mirror and means for moving the mirror about mutually orthogonal axes 
which intersect at a point on the mirror surface from which incident light 
is reflected as the illuminating beam so that said point constitutes said 
substantially fixed apparent beam source position. Alternatively there may 
be first and second mirrors, means for moving the first and second mirrors 
about respective mutually orthogonal axes which intersect at a point on 
the second mirror surface from which light is reflected as the 
illuminating beam so that said point constitutes said substantially fixed 
apparent beam source position, and means for directing light reflected 
from the first mirror to the second mirror, the light being directed for 
example by reflection from a curved mirror. In an alternative arrangement 
there may be an optical fibre and means for moving an end portion of the 
optical fibre about mutually orthogonal axes which intersect at a point so 
that light issuing from the fibre end provides the illuminating beam and 
that point constitutes said substantially fixed apparent beam source 
position which may be forward of, at, or behind the fibre end. A lens may 
be associated with the fibre end to cause a desired convergence or 
divergence of the illuminating beam. If desired the illuminating beam may 
be collimated by collimating means, such as a lens, held stationary at a 
position between the light directing means and the element supported by 
the support means. 
The illuminating beam is preferably a laser beam and said light emitter is 
therefore preferably a laser. It is to be understood that the illuminating 
beam light need not necessarily be in the visible part of the 
electromagnetic spectrum but could be infra-red or ultraviolet and the 
terms `light`, `illuminating` and the like are to be construed 
accordingly. 
The invention further provides a reflection hologram made by a method 
and/or by use of apparatus as set forth above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 schematically illustrates the passage of a coherent monochromatic 
illuminating beam (usually a laser beam) through a transparent substrate 1 
carrying a photosensitive film 2 to a reflector 3 which reflects the beam 
back towards the film 2. The illuminating beam and its reflection back 
interfere and a reflection hologram can be derived from the interference 
pattern recorded in the film 2. The illuminating beam can be moved 
relatively to the film 2, in particular in a scanning motion, but 
satisfactory hologram formation requires that the interference pattern in 
each exposed area of film should remain substantially constant and 
stationary. Change or movement of the interference pattern in a particular 
film area will tend to mar or prevent hologram formation in that area. 
When, as in FIG. 1, the back reflector 3 is spaced from the film 2, 
movement or change of the interference pattern can be caused by movement 
of the apparent beam source position. For a given angular movement of the 
apparent source position the effect on the hologram is dependent on the 
number of wavelengths separating the film from the backing reflector and 
the cosine of the angle of incidence at the reflector. 
Specifically, the number N of nodes in the standing wave pattern between 
the film 2 and the back reflector 3 is given by d/s where `d` is the 
distance between the film and the back reflector and `s` is the separation 
of the nodes. If .lambda. is the illuminating beam wavelength, `n` is the 
index of refraction of the medium between the film and the back reflector, 
and .theta. is the angle of incidence in that medium, then: 
EQU s=.lambda./(2n Cos .theta.) 
Hence 
EQU N=d/s=(2nd Cos .theta.)/.lambda. 
To avoid degradation of the hologram N needs to remain substantially 
constant throughout the exposure of each part of the film and therefore 
Cos .theta. must remain constant to the same degree. With a single node 
between the film and the back reflector (i.e. N=1) then constancy within 
about 10% is usually sufficient so Cos .theta. should also remain constant 
within 10%. However, the sensitivity to angle is proportional to N so that 
the permissible variation of Cos .theta. is generally 10/N%. 
The medium between the film 2 and the back reflector 3 may conveniently be 
a liquid with a refractive index matching that of the substrate 1, which 
may conveniently be glass. Thus, by way of example, with a medium of 
refractive index 1.5, a separation `d` of 1 mm and an illuminating 
wavelength .alpha. of 0.6.times.10.sup.-3 mm, N is 5.times.10.sup.3 Cos 
.theta. and the permissible change in Cos .theta. is 0.1/5.times.10.sup.3 
Cos.theta.. If, for example, .theta. is 20.degree. in the medium 
(equivalent to 31.degree. in air), then .theta. must remain constant to 
about 12 seconds of arc. Such constancy requires a substantially fixed 
apparent beam source position, which remains stationary relative to the 
film so that all the light reaching any point on the film appears to come 
from the same fixed source position. 
As indicated above, a reflection hologram can be constructed conveniently 
by a scanning motion of the illuminating beam, the area over which the 
angular tolerance requirement must be maintained then being dependent on 
the beam diameter at the film and the degree of overlap between successive 
scans. Hologram construction using a scanning motion of the illuminating 
beam can overcome problems arising from a Gaussian beam intensity profile 
in which the intensity falls off radially outwardly, and `beam noise` 
resulting from diffraction fringes caused by imperfections in the 
generating optics (which usually comprise a small pinhole illuminated by a 
laser via a microscope objective). 
A particular example of `fixed source` scanning arrangement is 
schematically shown in FIG. 2. It comprises a single front surface 
scanning mirror 4 which is angularly moveable about two mutually 
orthogonal axes of rotation which lie in the plane of the mirror and 
intersect at a point where light providing the illuminating beam is 
reflected from the mirror. The incident light is focussed onto the mirror 
at this point so that the illuminating beam for the hologram formation 
always appears to diverge from it. That point therefore constitutes a 
substantially fixed apparent beam source position which remains stationary 
relative to the film over which the beam is scanned. 
In FIG. 2 a laser beam is schematically shown as being focussed by a lens 5 
onto the front surface of mirror 4, the rotational axes X and Y lying in 
that front surface and intersecting at the centre where the focussed laser 
beam is incident. The mirror is mounted in a gimbal arrangement having two 
servo motors which effect the respective angular movements about the X and 
Y axes. Thus one (line scan) motor 6 angularly moves a shaft 7 on which 
the mirror 4 is carried to rotate the mirror about the Y axis. The other 
(frame scan) motor 8 angularly moves a frame 9 across which the shaft 7 
extends to rotate the mirror about the X axis. Suitable bearings are of 
course provided to permit sufficiently precise movement of the frame 9 and 
of the shaft 7 relatively to the frame 9, and suitable controls are 
provided for the motors 6 and 8 to effect the required scanning motion of 
the illuminating laser beam emanating from the centre point of the mirror 
4. Also, the laser and associated optics are aligned to ensure that the 
incident beam focussed onto the mirror 4 strikes it with sufficient 
accuracy at the centre point where the rotational axes intersect. It will 
be appreciated that the accuracy required is dependent on the required 
beam diameter at the photosensitive film, and generally a small diameter 
beam can be used so that the film area exposed to the beam at any one 
instant corresponds to a very small angular movement of the mirror 4. The 
beam diameter at the film must, of course, be sufficient to provide an 
overlap between the incident and reflected (from the reflector 3 in FIG. 
1) illuminating beams so as to produce the interference pattern by which 
the hologram is generated. By way of example, an illuminating beam 
diameter of 10 mm at the film would permit a distance (`d` in FIG. 1) 
between the film and the reflector of up to about 5 mm. At a working 
distance (between the film and the centre point of the scanning mirror) of 
500 mm the 10 mm beam diameter would subtend an angle of 1.degree. at the 
centre point of the scanning mirror. This represents the angle over which 
the `fixed source` criterion will apply, which requires an angular 
movement of the scanning mirror of only 0.5.degree.. Over this angle of 
movement a lateral displacement of the initial beam incident on the 
scanning mirror of 1 mm from the centre point would result in an apparent 
source movement of the order of 4 arc seconds. This would generally be 
acceptable (assuming other errors were negligible) for distances `d` 
between the film and the reflector of up to about 3 mm but for longer 
distances alignment of the initial incident beam would usually be more 
critical. Also the accuracy requirements would of course be more relaxed 
for greater working distances and more critical for closer working 
distances. 
Another example of `fixed source` scanning arrangement is schematically 
illustrated in FIG. 3. This comprises an optical fibre 10 having an input 
end which receives light emitted by a laser 11 and an output end 12 from 
which the illuminating beam emanates. The fiber output end 12 is mounted 
in a manner such that the emergent beam can be moved angularly about a 
substantially fixed apparent beam source position which is shown in FIG. 3 
as being at the fibre end. Movement of the fibre end to effect a required 
scanning motion of the illuminating beam can be achieved by means 13 which 
may, for example, comprise two motors operable to cause angular movement 
about orthogonal axes which intersect at the fiber end position. For a 
raster scan one motor can govern the line scan and the other the frame 
scan. The fibre end position remains substantially stationary relative to 
the film as the beam is scanned thus providing a substantially fixed 
apparent beam source position. 
FIG. 3 schematically illustrates the principle of a fibre-optic scan 
arrangement in simple form but in practice some further complexity may be 
necessary. In particular, unless the optical fibre is of sufficiently low 
numerical aperture to give an acceptably small illuminating beam 
divergence itself, then some form of lens may be provided at or near the 
fibre output end, and the fibre end movement may be adapted to suit the 
appropriate fixed source position. FIG. 4 schematically shows an 
arrangement with a lens 14 at or near the fibre output end 12 which causes 
convergence of the emergent beam to a point 15 constituting the `fixed 
source` position from which the illuminating beam diverges. The angular 
movement of the end portion of the fibre 10 and lens 14 is then about the 
point 15 at which the rotational axes intersect and which is forward of 
the fibre end and lens. FIG. 5 schematically shows an arrangement with a 
lens 16 at or near the fibre output end 12 which gives an illuminating 
beam apparently diverging from a point 17 constituting the `fixed source` 
position and located behind the fibre end and its associated lens. In this 
arrangement the fibre is held stationary at point 17, and the fibre end 
portion and lens 16 are moved angularly about that point to effect the 
required scanning. It will be appreciated that any suitable means may be 
employed for effecting the required movement of the fibre end portion 
consistent with achievement of the necessary accuracy. 
It will also be appreciated that a fibre optic arrangement can facilitate 
change of the illuminating wavelength in that the laser coupled to the 
input end of the optical fibre can readily be changed without disturbing 
the scanning set-up at the output end of the optical fibre. This can be 
especially advantageous if it is required to have superimposed different 
wavelength holograms in the same film. 
It will further be appreciated that the apparent source at the `fixed 
source` position may be virtual or real, and that other arrangements than 
those shown and described by way of example may be used to produce angular 
movement of the illuminating beam about the `fixed source` position. 
FIG. 6 schematically shows a general apparatus set-up for use in making a 
reflection hologram in accordance with the principles previously 
described. It comprises a support member 18 arranged to support an element 
19 (corresponding to the substrate 1 in FIG. 1) carrying a photosensitive 
film 20 (2 in FIG. 1), and a back reflector 21 (3 in FIG. 1) at a location 
spaced from the film 20 on the supported element 19. The spacing between 
the reflector 21 and the film 20 is greatly exaggerated in FIG. 6 for 
purposes of illustration and the space may in practice be filled with an 
index matching liquid as previously mentioned. The apparatus further 
comprises a light emitter in the form of a laser 22, and a light directing 
device 23 which receives light from the laser 22 and directs it towards 
the element 19 as an illuminating beam from a substantially fixed apparent 
beam source position. The light directing device 23 incorporates a 
scanning arrangement whereby the illuminating beam is moved relatively to 
the film in a manner such that the apparent beam source position remains 
substantially stationary relative to the film 20. The device 23 may be of 
a form as described with reference to any of FIGS. 2 to 5, or of any other 
suitable form. 
FIG. 7 schematically shows a modification to the apparatus of FIG. 6 
whereby a collimated illuminating beam can be employed. This modification 
involves the provision of a collimating lens 24 which is held stationary 
in a suitable mounting 25 at a position between the scanning device 23 and 
the element 19. Light directed by the scanning device 23 is used as though 
it were a real point source, the substantially fixed apparent beam source 
position being effectively transferred to and remaining substantially 
stationary at infinity through the action of the stationary collimating 
lens 24. 
The method of operation of the apparatus schematically shown in FIGS. 6 and 
7 will be apparent from the preceding description. Briefly the 
illuminating beam is transmitted through the element 19 and film 20 and 
the back reflector 21 is arranged to reflect it back to the film 20. An 
interference pattern is set up in the film and, by virtue of the 
substantially fixed apparent beam source position which remains 
substantially stationary relative to the film, the interference pattern in 
each particular film area is kept substantially constant and stationary as 
it is recorded in the photosensitive film. The resultant reflection 
hologram derived, usually by appropriate processing of the film which is 
preferably dichromated gelatine although other materials may be employed, 
from the recorded interference pattern is of corresponding quality. It 
will be understood that the expressions `substantially fixed` and 
`substantially stationary` used in relation to the apparent beam source 
position are intended to comprehend the tolerance permissible to achieve 
satisfactory hologram formation. It will further be understood that the 
film 20 and its substrate element 19 need not necessarily be planar but 
could if desired be curved or otherwise shaped and that the back reflector 
21 need not be parallel to the film 20, and need not be planar, but may be 
inclined and/or shaped, for example as a concave or a convex curve, 
possibly to produce a resultant hologram in respect of which the 
interference fringes are correspondingly inclined and/or shaped, e.g. 
curved, and the present invention comprehends a method and apparatus of 
the type schematically illustrated by FIG. 6 or FIG. 7, and a reflection 
hologram made by use of such method and apparatus, whatever the form or 
shape of the film and back reflector and the resultant interference 
fringes. 
FIG. 8 schematically shows an alternative form of optical device to that 
shown in FIG. 2. In the FIG. 8 embodiment there are two front surface 
planar mirrors 26 and 27 rotatable about respective orthogonal axes X and 
Y which intersect at a point on the reflecting face of the mirror 27. A 
stationary curved mirror 28 is located so that light from a laser 29 
incident on the first mirror 26 is reflected to the stationary mirror 28 
and directed by reflection from the curved mirror 28 to the second mirror 
27. Light reflected from the second mirror 27 provides the illuminating 
beam coming from the substantially fixed apparent beam source position 
constituted by said point on the reflecting face of the mirror 27. The 
movable mirrors 26 and 27 are driven by respective motors 30 and 31 and 
together produce a raster scan of the laser beam. One motor effects 
movement of its mirror appropriate to the frame scan, which is a 
relatively slow movement and the mirror is returned at the end of a frame 
to its starting angular position. The other motor effects movement of its 
mirror appropriate to the line scan, which is a relatively fast movement. 
Conveniently, therefore, this mirror is rotated continuously in a single 
angular direction, which is a simple and reliable arrangement but with 
some light wastage. The curved stationary mirror 28 may have cylindrical 
curvature and, depending on the precision required, a further optical 
element may be provided to take account of its anamorphic power so as to 
focus the beam to the point on the reflecting face of the mirror 27. Thus, 
another stationary anamorphic element may be provided in the laser beam 
path to the mirror 26. Alternatively a non-anamorphic focussing element 
may be provided in the laser beam path to the mirror 26 to focus the beam 
on to the reflecting face of that mirror, and the curved mirror 28 may be 
ellipsoidal to refocus the beam on to the reflecting face of the mirror 
27. In practice, for ease of manufacture, a slice cut from a mirror of 
spherical curvature may provide a close enough approximation to an 
ellipsoidal mirror. 
It will be appreciated that an arrangement similar to that of FIG. 8 could 
employ a lens instead of the curved mirror 28, the angularly moveable 
first and second mirrors 26 and 27 being disposed so that the laser beam 
is reflected from the first mirror towards the second mirror and the lens 
being located in the light path between the mirrors to direct the beam 
reflected from the first mirror on to the reflecting face of the second 
mirror at said point. 
Other optical devices and arrangements for effecting movement of the 
illuminating beam about a substantially fixed apparent beam source 
position may occur to those skilled in the art, and any suitable such 
device or arrangement may, like that shown in FIG. 8, be used as the 
scanning device 23 in the apparatus of FIG. 6 and FIG. 7.