WO2011070350A1

WO2011070350A1 - Compact holographic printer

Info

Publication number: WO2011070350A1
Application number: PCT/GB2010/052038
Authority: WO
Inventors: Stanislovas Zacharovas; Ramunas Bakanas; David Brotherton-Ratcliffe
Original assignee: Geola Technologies Ltd.
Priority date: 2009-12-07
Filing date: 2010-12-07
Publication date: 2011-06-16
Also published as: GB0921432D0; GB201211387D0; GB2489148A

Abstract

A compact 1-step holographic printer is provided comprising one or more diode- pumped pulsed lasers (101). Inexpensive and compact micro-lens arrays or equivalent holographic optical elements (109) are used in conjunction with one or more spatial light modulators (108) to generate one or more object beams encoded with suitable digital image data. One or more variable-angle reference beams are used to write simultaneously one or more holographic pixels. Use of photopolymer recording material (110) enables rapid printing of high quality 1-step full-colour reflection holograms within a compact unit with no chemical processing.

Description

COMPACT HOLOGRAPHIC PRINTER BACKGROUND TO THE PRESENT INVENTION

The present invention relates to the field of holography and digital holographic printing. The preferred embodiment relates to a holographic printer and a method of holographic printing.

Digital Holographic Printers based on pulsed lasers producing high-quality f u II- colour reflection holograms have been commercially available for some years. Both 1 -step and 2-step printers have been produced. Generally, current 1 -step printers are relatively slow. For example, a 1 m x 1 m full-colour reflection hologram may take around 1 day to print at a holopixel diameter of 0.8 mm. 2-step printers are some ten to a hundred times faster but are less developed and rather more complex.

Current full-colour digital holographic printers use flash-lamp based lasers and complex and high numerical aperture optics for the formation of the object and reference beams (e.g. US-7009742).

It is desired to provide an improved holographic printer. SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided a holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

a laser source arranged to produce a laser beam at a first wavelength, the laser beam being split, in use, into an object beam and a reference beam which is mutually coherent with the object beam;

a spatial light modulator wherein, in use, the object beam illuminates the spatial light modulator;

a holographic optical element placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the holographic optical element being arranged to diffract, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium thus forming a first holographic pixel at a given plane;

a positioning device for positioning, in use, a photosensitive medium at the plane; and

a steering device for bringing the reference and object beams into intersection at the plane.

The laser source is preferably arranged to additionally produce laser beams at second and third wavelengths, wherein the first, second and third wavelengths each differ from one another by at least 30 nm.

The holographic printer preferably further comprises a second holographic optical element for use at a second wavelength. The holographic optical element preferably diffracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels to second, third or further common areas on the photosensitive medium thus forming second, third or further holographic pixels at the plane.

The holographic optical element preferably comprises a composite transmission holographic optical element.

The composite transmission holographic optical element is preferably composed of elements each of which corresponds to a pixel or a group of pixels on the spatial light modulator.

According to another aspect of the present invention there is provided a holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

a composite micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the composite micro-lens array being arranged to refract, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium thus forming a first holographic pixel at a given plane;

The holographic printer preferably further comprises a second composite micro-lens array for use at a second wavelength.

The composite micro-lens array preferably refracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels to second, third or further common areas on the photosensitive medium thus forming second, third or further holographic pixels at the plane.

According to an aspect of the present invention there is provided holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

a spatial light modulator wherein, in use, the object beam illuminates the spatial light modulator; an optical element comprising a rectangular or other matrix of Fresnel lenses placed in contact, in close proximity with, adjacent to or downstream of the spatial light modulator, the matrix of Fresnel lenses being arranged to direct, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a

photosensitive medium thus forming a first holographic pixel at a given plane;

a positioning device for positioning a photosensitive medium at the plane; and a steering device for bringing the reference and object beams into intersection at the plane.

The holographic printer preferably further comprises a second matrix of Fresnel lenses for use at a second wavelength.

The matrix of Fresnel lenses preferably directs, in use, light from second, third or further group of spatial light modulator pixels to second, third or further common areas on the photosensitive medium thus forming second, third or further holographic pixels at the plane.

According to an aspect of the present invention there is provided a holographic printer for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising:

a holographic optical element placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the holographic optical element being arranged to diffract, in use, in one dimension only transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium at a given plane;

a one-dimensional diffuser placed in contact with or adjacent to and in front of the photosensitive medium at the first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium and/or a diffraction direction, thereby creating a first column of holographic pixels;

The holographic printer preferably further comprises a second holographic optical element for use at a second wavelength. The holographic optical element refracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels through second, third or further one- dimensional diffusers to second, third or further common areas on the photosensitive medium thus forming second, third or further columns of holographic pixels at the plane.

The composite transmission holographic optical element is preferably composed of elements each of which corresponds to one or more columns of pixels on the spatial light modulator.

a laser source arranged to produce a laser beam at a first wavelength, the laser beam being split, in use, into an object beam and a reference beam which is mutually coherent with the object beam ;

a micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the micro-lens array being arranged to refract in one dimension only the transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium at a given plane;

a one-dimensional diffuser placed in contact with or adjacent to and in front of the photosensitive medium at the first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium and/or a refraction direction thereby creating a first column of holographic pixels;

The laser source is preferably arranged to additionally produce laser beams at second and third wavelengths and wherein the first, second and third wavelengths each differ from one another by at least 30 nm.

The holographic printer preferably further comprises a second micro-lens array for use at a second wavelength.

The micro-lens array refracts, in use, light from a second, third or further group of spatial light modulator pixels through second, third and further one-dimensional diffusers to second, third or further common areas on the photosensitive medium thus forming second, third or further columns of holographic pixels at the plane.

The micro-lens array is preferably composed of elements each of which

corresponds to one or more columns of pixels on the spatial light modulator.

The steering device preferably comprises a second composite optical element to direct the reference beam to one or more holographic pixels at a variable angle. The second composite optical element comprises a second holographic optical element, a micro-lens array, a Fresnel lens, a rectangular or other matrix of Fresnel lenses, a transmission holographic optical element or a reflection holographic optical element.

The spatial light modulator preferably comprises a transmission device.

The spatial light modulator preferably comprises a liquid crystal display.

The laser source preferably comprises a pulsed laser.

The pulsed laser preferably comprises a diode pumped solid state ("DPSS") laser.

According to an aspect of the present invention there is provided a method of directly writing 1 -step white-light viewable holograms, comprising:

providing a laser source arranged to produce a laser beam at a first wavelength, the laser beam being split into an object beam and a reference beam which is mutually coherent with the object beam;

providing a spatial light modulator wherein the object beam illuminates the spatial light modulator;

providing a holographic optical element that is placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the holographic optical element being arranged to diffract transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium at the plane; and

bringing the reference and object beams into intersection at the plane.

providing a micro-lens matrix that is placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the micro-lens matrix being arranged to refract transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium at the plane; and

bringing the reference and object beams into intersection at the plane.

providing a spatial light modulator wherein the object beam illuminates the spatial light modulator; providing a rectangular or other matrix of Fresnel lenses that is placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the Fresnel lens matrix being arranged to direct transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium at the plane; and

bringing the reference and object beams into intersection at the plane.

According to an aspect of the present invention there is provided a method for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising: providing a laser source arranged to produce a laser beam at a first wavelength, the laser beam being split into an object beam and a reference beam which is mutually coherent with the object beam;

providing a holographic optical element placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the holographic optical element being arranged to diffract in one dimension only transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium at a given plane;

providing a one-dimensional diffuser placed in contact with or adjacent to and in front of the photosensitive medium at the first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium and/or a diffraction direction, thereby creating a first column of holographic pixels;

positioning a photosensitive medium at the plane; and

bringing the reference and object beams into intersection at the plane.

providing a micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of the spatial light modulator, the micro-lens array being arranged to refract in one dimension only the transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium at a given plane; providing a one-dimensional diffuser placed in contact with or adjacent to and in front of the photosensitive medium at the first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium and/or a refraction direction thereby creating a first column of holographic pixels;

positioning the photosensitive medium at the plane; and

bringing the reference and object beams into intersection at the plane. A new holographic printer is disclosed based on red, green and blue diode pumped solid state ("DPSS") lasers. A new type of optical scheme that is both inexpensive, easy to implement and lends itself naturally to a compact construction and operation using photopolymer material requiring no chemical processing is also disclosed. The preferred embodiment allows the construction of a desktop holographic printer that produces instant digital holograms in an analogous manner to a dot-matrix printer producing normal printed output.

Conventional commercial digital holographic printers require at least two and usually three expensive high NA objectives to form the object beams. They also require large and expensive high NA optics to form the required variable-angle reference beams. In addition they employ pulsed lasers based on flash-lamp pumping and for full-colour output they use Silver halide materials requiring chemical processing. Conventional printers have only demonstrated the writing of one RGB holographic pixel at a time. Given these constraints, known holographic printers do not lend themself to a "table-top" 1 -step instant holographic printer.

A significantly different approach to a 1 -step digital holographic printer is disclosed. By using computer-designed composite refractive or holographic diffractive elements it is possible to eliminate the complex, large and expensive conventional components. The preferred embodiment allows a single SLM to write many holopixels at the same time. In addition, by using strategically delayed laser pulses from red, green and blue pulsed diode- pumped lasers it is possible to produce high diffractive efficiencies using currently available photopolymer materials. With these improvements it is possible to produce a relatively inexpensive "table-top" 1 -step "instant" holographic printer.

The holographic printer according to the preferred embodiment therefore represents a significant advance in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described, by way of example only, and with reference to the accompanying figures in which:

Fig. 1 shows a holographic printer according to an embodiment of the present invention;

Fig. 2 shows a close up view of a LCD and an adjacent micro-lens matrix or holographic optical element;

Fig. 3 shows a close up view of an element of the holographic printer; and

Fig. 4 shows an object beam illuminating a LCD according to another embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Monochromatic holographic printer Fig. 1 shows an embodiment of the present invention. A diode-pumped laser 101 is arranged to produce highly stable laser radiation pulses (3 mJ, 40 ns) at a frequency of 120 Hz. Each pulse is TEM00 SLM 532 nm having a coherence length of greater than 50 cm. The laser output energy is controllable by a computer 1 12.

The laser beam passes through a half-waveplate 102 before striking a Brewster angle polarising beam-splitter 103. The beam is split into a reference beam and an object beam having orthogonal polarizations. The ratio of reference to object energies is controlled by element 102. The object beam continues to element 104 which comprises a half waveplate which is used to tune the polarisation to a desired value.

The object beam continues to a mirror 105 and then on to a diverging lens 106 which forms a collimating telescope with a lens 107. The collimated object beam is arranged uniformly to illuminate a LCD panel 108 having a resolution of 200 x 300 pixels and a size of 10 x 15 cm. Attached to this panel is a transmission holographic optical element 109 that acts to form an approximate image of each of the LCD pixels at a location 1 19 on a holographic material 1 10. According to another embodiment the holographic optical element 109 may be replaced by a matrix of micro-lenses.

The reference beam is directed from the beamsplitter 103 via mirrors 1 13,1 14 to a half-waveplate 1 15 and then on to a gimble-mounted 2-axis rotating mirror 1 16 controlled by motors 1 17. The function of the mirror 1 16 is to direct the reference beam onto an element 1 18 at a desired position. Element 1 18 preferably comprises either a

transmission holographic optical element with laminated polarizer or a specially designed polarizing micro-lens matrix. The element 1 18 preferably directs the reference beam to form a desired, preferably square, footprint required at location 1 19 where interference will occur between the object and reference beams to form a holographic pixel. Other less preferred embodiments are contemplated wherein the element 1 18 may comprise a reflection holographic optical element.

Each time a new holopixel is written, the computer 1 12 preferably advances the holographic material 1 10 using motors 1 1 1 so that a matrix of preferably square holopixels are arranged to cover the whole hologram uniformity in an abutting x-y grid. The preferred holopixel size is 0.5 mm. It is important that the holographic material is held precisely as the object and reference beams must properly and accurately overlap at all times.

The waveplates 1 15,104 are preferably aligned so that the polarisations of the object and reference beam at 1 19 correspond such that interference is maximal and reflection of the reference beam from 1 10 is minimal. Likewise, the beam energy and object/reference ratio is preferably tuned according to the requirements of the

photomaterial used.

Each time a new holopixel is written the computer 1 12 preferably updates the motors 1 17 such that the mirror 1 16 rotates to a position which then causes the reference beam to be directed onto the next segment of the element 1 18 causing the reference beam to then be incident at 1 19 from a slightly different angle.

By controlling the motors 1 17 each time a holopixel is written a hologram is preferably synthesised that requires illumination by a point-source at a given optimum distance and angle. Conventional variable reference beam systems are much more complicated and expensive. In addition, they have been intrinsically large. By contrast, element 1 18 is according to an embodiment 10 cm x 10 cm and preferably comprises an array of 200 x 200 elements each preferably 0.5 mm in diameter according to an embodiment of the present invention. This allows 40000 different reference beam angles to be encoded which for small format holograms (smaller than 20 cm x 30 cm) is more than sufficient. Larger holograms will require correspondingly larger versions of element 1 18 as the individual element size is limited by diffraction for a given holopixel size.

Fig. 2 shows a close up view of the LCD 108 and an adjacent micro-lens matrix (or HOE) 109. Every element of the LCD (e.g. 201 ) is preferably directed onto the location 1 19 on the holographic material 1 10 by a corresponding tilted "lenslet" or HOE (e.g. 202) on the element 109. Since the LCD 108 acts to rotate the polarisation of the incoming radiation, a polarising film is preferably incorporated on element 109 to achieve amplitude modulation.

Element 109 may be created using moulding of plastic or other materials including glass. A "lenslet" size of around 0.5 mm is preferred. The shape of each "lenslet" may be calculated by simple ray-tracing given the constraint that the holopixel footprint must be of a certain shape (usually square) and size. The shape of each "lenslet" is therefore a highly aspheric quasi-planar tilted surface. Different elements may be designed to give different holopixel sizes and different holopixel shapes. Each lenslet preferably both refracts the rays emanating from a given SLM cell to a common area at the holopixel and also preferably approximately collimates the rays from a given SLM cell into a pencil beam. Diffraction will counter such collimation and the preferred embodiment will work sub- optimally if the SLM pixel size becomes too small. An approximation of a micro-lens matrix is a Fresnel lens. A Fresnel lens in close proximity to the SLM will be unable to control the divergence of each pencil beam emanating from a given SLM cell even if it does refract all the pencil beams to the common holographic pixel location at the photosensitive substrate. Such lack of control will limit the minimum holopixel size and will also decrease the angular resolution of the hologram. Nonetheless, a matrix of Fresnel lenses may be used according to a less preferred embodiment of the present invention.

Element 109 may be created using a digital holographic printer where a composite transmission hologram (HOE) is preferably created. One or more holographic pixels then effectively does the same work as the "lenslets" described above (refraction being replaced by diffraction). In the case of a HOE, however, elements 108,109 may be tilted at an angle if the diffraction efficiency is not sufficient such that the zeroth order transmitted light does not impinge upon the holographic material.

Fig. 3 shows a close-up view of the holographic optical element 1 18. Holographic optical element 1 18 is almost identical to element 109 except that it is designed to function with a diverging incident beam. Again, given the desired footprint at 1 19 simple ray-tracing defines each of the "lenslets" in element 1 18. Different elements can therefore be designed to produce different footprints and different ranges of convergence angle. Full Colour Printer

The above monochromatic embodiment may be extended to three colours. In addition to a green pulsed Nd:YAG laser, a red Nd:YAG diode-pumped laser at 660 nm (SLM, TEM00, single pulse at 120 Hz, 3 mJ, 40 ns) and a blue Nd:YAG diode-pumped laser at 473 nm (SLM, TEM00, single pulse at 120 Hz, 3 mJ, 40 ns) may be provided. However, someone skilled in the art will clearly understand that other types of pulsed, multi-pulse or even CW lasers may be used.

The arrangement shown in Fig. 1 is preferably repeated for each of the three colours. This system leads to three sets of object/reference beams and three spatially separated holopixel writing locations, one for each colour.

This system works well for many photomaterials. However, for at least some photopolymers it may be necessary to write all three colours at the same location and at approximately the same time. This requires a single writing location for a white object and white reference beam. This can be arranged by the known optical techniques used in SLM projectors where images of three LCD or LCOS panels are combined using different colours of light. Red, green and blue beams may, therefore, be mixed to form a white reference beam. For the object beam the red, green and blue channels may be used to illuminate separate LCD or LCOS panels and then mix these images at 109. Both elements 109 and 1 18 can be made to function properly with white light if they are made as micro- lens matrices. HOE implementations in this case are more complex but are still possible (i.e. three thick gratings must be used or parasitic beams blocked).

In addition to the above solution, intrinsically three-colour spatially multiplexed SLM panels are now available. Less preferably this enables a white reference beam and a white object beam to be mixed and then proceed with elements 106 through to 1 10 (object beam) and 1 16 to 1 10 (reference beam).

For many materials it has been noted that writing with a white beam leads to greater diffractive efficiency. When using pulsed lasers, delaying the two final colours by slightly different small delays (usually microseconds) can further optimise diffractive response particularly when using photopolymers.

Further embodiments

It has been described above how reflection holograms may be made according to embodiments of the present invention. The preferred embodiment may also extended to create digital transmission holograms and digital holographic optical elements. All types of image date may be encoded leading to both single and double-parallax holograms.

A further useful embodiment of the screen 109 is to divide the screen 109 up into a matrix of e.g. 2 x 2 or 3 x 3 elements. A larger resolution LCD may then be used. For example, the screen may be split into a matrix of 3 x 3 rectangular elements and used with an LCD of 600 x 900. Each LCD pixel preferably measures 0.2 mm corresponding to a LCD panel size of 12 cm x 27 cm. This is shown in Fig. 4 where the object laser beam 401 illuminates the LCD 409. A polarizer 403 is preferably laminated to the LCD 409. The composite HOE screen 408 is divided into nine rectangular sections, the first of which is 403. Each HOE element in the area 403 directs the transmitted light to the holopixel 405 on the photosensitive substrate 404. Adjacent areas of the screen produce holopixels 406 and 407. In total this scheme allows nine holopixels to be written at the same moment in time. This increases the print speed by nearly an order of magnitude.

Another embodiment of the screen 109 is to arrange that each "lenslet" covers a matrix of LCD pixels (e.g. 5 x 5 or 10 x 10). Each lenslet can then be designed so that an approximate "image" of this matrix is formed at the holographic pixel (the term "image" is used here as indicating that the intensity distribution produced at the holopixel surface approximately replicates that at the LCD surface). By having a sufficient LCD resolution (e.g. a LCD of 200 x 300 pixels according to the preferred embodiment but in general a higher resolution may be used), sufficient object beam angle resolution (i.e. a sufficient number of LCD pixel groups) may be maintained at the same time that the holopixel intensity distribution can be altered digitally. In this way differently shaped and sized holopixels may be automatically created without having to change element 109. The main constraint here is diffraction from the "lenslet" or from the smaller group of pixels making up a pixel group. The diffraction angle: δθ = λ Ι ά

(1 ) and determines the smallest possible holopixel diameter H.

If the distance from element 109 to the holopixel is D, then the approximate smallest holopixel is given by: H = Όδθ

(2)

If the diameter of the "lenslet" is d = 0.5 mm and it is assumed that green light at 0.5 microns and L = 0.1 m, then H = 0.1 mm.

Since holopixel sizes of less 0.4 mm diameter are not expected to be required, this gives sufficient room for designing useful systems. Where the holopixels size are restricted by darkening the outer LCD pixels within a group, the diffractive formula must be applied to this reduced size rather than the "lenslet" size. This essentially introduces a more severe dependence which limits the minium holopixel size. Nevertheless, this restriction still admits a useful range for the control of the holopixel diameter.

The same concept can be applied to colour control. Here, a single LCD panel is illuminated by white light. However, each LCD pixel may be covered by a filter of either red, green or blue. As a result, all colour information is treated digitally on the same panel. A given "lenslet" would then either smear all the coloured subpixels onto the same white holopixel footprint or it may be made to form an approximate image at the holopixel. In the latter case (which is more demanding from a resolution and diffraction point of view leading to greater constraints on minimum holopixel size and angular resolution of the final hologram) both pixel size and colour may be controlled digitally using only a single LCD panel.

If only single parallax (i.e. Horizontal Parallax Only "HPO") holograms are required to be written then the screen 109 may be composed of an array of cylindrical optical elements instead of a matrix of roughly rectangular optical elements. The cylindrical elements may then direct the transmitted light from each LCD pixel horizontally to a common area on the photosensitive substrate whilst preserving the vertical propagation.

This leads to the formation of a vertical line on the holographic substrate the same height as the LCD. By placing a vertical diffuser in contact with the photosensitive material a vertical line of holopixels may be effectively written at one time. Several hundred to several thousand holopixels may then be written at the same time.

The technique of using a vertical diffuser to write HPO holograms may be used in conjunction with the technique of Fig. 4 thereby further increasing the print speed.

Although the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.

Claims

Claims 1 . A holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

a laser source (101 ) arranged to produce a laser beam at a first wavelength, said laser beam being split, in use, into an object beam and a reference beam which is mutually coherent with said object beam;

a spatial light modulator (108) wherein, in use, said object beam illuminates said spatial light modulator (108);

a holographic optical element (109) placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said holographic optical element (109) being arranged to diffract, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane;

a positioning device for positioning, in use, a photosensitive medium (1 10) at said plane; and

a steering device for bringing the reference and object beams into intersection at said plane.

2. A holographic printer as claimed in claim 1 , wherein said laser source (101 ) is arranged to additionally produce laser beams at second and third wavelengths, wherein said first, second and third wavelengths each differ from one another by at least 30 nm.

3. A holographic printer as claimed in claim 1 or 2, further comprising a second holographic optical element for use at a second wavelength.

4. A holographic printer as claimed in claim 1 , 2 or 3, wherein said holographic optical element (109) diffracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels to second, third or further common areas on said

photosensitive medium (1 10) thus forming second, third or further holographic pixels at said plane.

5. A holographic printer as claimed in any preceding claim, wherein said holographic optical element (109) comprises a composite transmission holographic optical element.

6. A holographic printer as claimed in claim 5, wherein said composite transmission holographic optical element (109) is composed of elements each of which corresponds to a pixel or a group of pixels on said spatial light modulator (108).

7. A holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

a composite micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said composite micro-lens array being arranged to refract, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane;

8. A holographic printer as claimed in claim 7, wherein said laser source (101 ) is arranged to additionally produce laser beams at second and third wavelengths, wherein said first, second and third wavelengths each differ from one another by at least 30 nm.

9. A holographic printer as claimed in claim 7 or 8, further comprising a second composite micro-lens array for use at a second wavelength.

10. A holographic printer as claimed in claim 7, 8 or 9, wherein said composite micro- lens array refracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels to second, third or further common areas on said photosensitive medium (1 10) thus forming second, third or further holographic pixels at said plane.

1 1 . A holographic printer for directly writing 1 -step white-light viewable holograms, comprising:

an optical element comprising a rectangular or other matrix of Fresnel lenses placed in contact, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said matrix of Fresnel lenses being arranged to direct, in use, transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane; a positioning device for positioning a photosensitive medium (1 10) at said plane; and

12. A holographic printer as claimed in claim 1 1 , wherein said laser source (101 ) is arranged to additionally produce laser beams at second and third wavelengths, wherein said first, second and third wavelengths each differ from one another by at least 30 nm.

13. A holographic printer as claimed in claim 1 1 or 12, further comprising a second matrix of Fresnel lenses for use at a second wavelength.

14. A holographic printer as claimed in claim 1 1 , 12 or 13, wherein said matrix of Fresnel lenses directs, in use, light from second, third or further group of spatial light modulator pixels to second, third or further common areas on said photosensitive medium (1 10) thus forming second, third or further holographic pixels at said plane.

15. A holographic printer for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising:

a holographic optical element placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said holographic optical element being arranged to diffract, in use, in one dimension only transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) at a given plane;

a one-dimensional diffuser placed in contact with or adjacent to and in front of said photosensitive medium (1 10) at said first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium (1 10) and/or a diffraction direction, thereby creating a first column of holographic pixels;

a positioning device for positioning a photosensitive medium (1 10) at said plane; and

16. A holographic printer as claimed in claim 15, wherein said laser source (101 ) is arranged to additionally produce laser beams at second and third wavelengths, wherein said first, second and third wavelengths each differ from one another by at least 30 nm.

17. A holographic printer as claimed in claim 15 or 16, further comprising a second holographic optical element for use at a second wavelength.

18. A holographic printer as claimed in claim 15, 16 or 17, wherein said holographic optical element refracts, in use, transmitted light from a second, third or further group of spatial light modulator pixels through second, third or further one-dimensional diffusers to second, third or further common areas on said photosensitive medium (1 10) thus forming second, third or further columns of holographic pixels at said plane.

19. A holographic printer as claimed in any of claims 15-18, wherein said holographic optical element comprises a composite transmission holographic optical element.

20. A holographic printer as claimed in claim 19, wherein said composite transmission holographic optical element is composed of elements each of which corresponds to one or more columns of pixels on said spatial light modulator (108).

21 . A holographic printer for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising:

a micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said micro-lens array being arranged to refract in one dimension only the transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) at a given plane;

a one-dimensional diffuser placed in contact with or adjacent to and in front of said photosensitive medium (1 10) at said first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium (1 10) and/or a refraction direction thereby creating a first column of holographic pixels;

22. A holographic printer as claimed in claim 21 , wherein said laser source (101 ) is arranged to additionally produce laser beams at second and third wavelengths and wherein said first, second and third wavelengths each differ from one another by at least 30 nm.

23. A holographic printer as claimed in claim 22, further comprising a second micro- lens array for use at a second wavelength.

24. A holographic printer as claimed in claim 21 , 22 or 23, wherein said micro-lens array refracts, in use, light from a second, third or further group of spatial light modulator pixels through second, third and further one-dimensional diffusers to second, third or further common areas on said photosensitive medium (1 10) thus forming second, third or further columns of holographic pixels at said plane.

25. A holographic printer as claimed in any of claims 21 -24, wherein said micro-lens array is composed of elements each of which corresponds to one or more columns of pixels on said spatial light modulator (108).

26. A holographic printer as claimed in any preceding claim, wherein said steering device comprises a second composite optical element (1 18) to direct the reference beam to one or more holographic pixels at a variable angle.

27. A holographic printer as claimed in claim 26, wherein said second composite optical element (1 18) comprises a second holographic optical element, a micro-lens array, a Fresnel lens, a rectangular or other matrix of Fresnel lenses, a transmission holographic optical element or a reflection holographic optical element.

28. A holographic printer as claimed in any preceding claim, wherein said spatial light modulator (108) comprises a transmission device.

29. A holographic printer as claimed in any preceding claim, wherein said spatial light modulator (108) comprises a liquid crystal display.

30. A holographic printer as claimed in any preceding claim, wherein said laser source (101 ) comprises a pulsed laser.

31 . A holographic printer as claimed in claim 30, wherein said pulsed laser comprises a diode pumped solid state ("DPSS") laser.

32. A method of directly writing 1 -step white-light viewable holograms, comprising: providing a laser source (101 ) arranged to produce a laser beam at a first wavelength, said laser beam being split into an object beam and a reference beam which is mutually coherent with said object beam;

providing a spatial light modulator (108) wherein said object beam illuminates said spatial light modulator (108);

providing a holographic optical element (109) that is placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said holographic optical element (109) being arranged to diffract transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium (1 10) at said plane; and

bringing the reference and object beams into intersection at said plane.

33. A method of directly writing 1 -step white-light viewable holograms, comprising: providing a laser source (101 ) arranged to produce a laser beam at a first wavelength, said laser beam being split into an object beam and a reference beam which is mutually coherent with said object beam;

providing a micro-lens matrix that is placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said micro-lens matrix being arranged to refract transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium (1 10) at said plane; and

bringing the reference and object beams into intersection at said plane.

34. A method of directly writing 1 -step white-light viewable holograms, comprising: providing a laser source (101 ) arranged to produce a laser beam at a first wavelength, said laser beam being split into an object beam and a reference beam which is mutually coherent with said object beam;

providing a rectangular or other matrix of Fresnel lenses that is placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator

(108), said Fresnel lens matrix being arranged to direct transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) thus forming a first holographic pixel at a given plane;

positioning a photosensitive medium (1 10) at said plane; and

bringing the reference and object beams into intersection at said plane.

35. A method for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising:

providing a laser source (101 ) arranged to produce a laser beam at a first wavelength, said laser beam being split into an object beam and a reference beam which is mutually coherent with said object beam;

providing a spatial light modulator (108) wherein said object beam illuminates said spatial light modulator (108); providing a holographic optical element placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said holographic optical element being arranged to diffract in one dimension only transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) at a given plane;

providing a one-dimensional diffuser placed in contact with or adjacent to and in front of said photosensitive medium (1 10) at said first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium (1 10) and/or a diffraction direction, thereby creating a first column of holographic pixels;

positioning a photosensitive medium (1 10) at said plane; and

bringing the reference and object beams into intersection at said plane.

36. A method for directly writing 1 -step white-light viewable Horizontal-only parallax holograms, comprising:

providing a micro-lens array placed in contact with, in close proximity with, adjacent to or downstream of said spatial light modulator (108), said micro-lens array being arranged to refract in one dimension only the transmitted light from a first group of spatial light modulator pixels to a first common area on a photosensitive medium (1 10) at a given plane;

providing a one-dimensional diffuser placed in contact with or adjacent to and in front of said photosensitive medium (1 10) at said first common area to diffuse the light in an orthogonal direction to the normal of the photosensitive medium (1 10) and/or a refraction direction thereby creating a first column of holographic pixels;

positioning said photosensitive medium (1 10) at said plane; and

bringing the reference and object beams into intersection at said plane.