WO2001096933A1

WO2001096933A1 - Alignment of holographic diffraction elements

Info

Publication number: WO2001096933A1
Application number: PCT/IB2001/001195
Authority: WO
Inventors: C. David Nabors
Original assignee: Digilens, Inc
Priority date: 2000-06-14
Filing date: 2001-06-14
Publication date: 2001-12-20
Also published as: GB0014365D0; AU7442901A

Abstract

Laser light from an illuminated reticle (21) is projected by a lens assembly (25) as a collimated beam (26) towards an assembly (28) of switchable holographic diffraction elements (29, 30) and (31) carried on a mounting device (27). Each of the elements (29, 30, 31) when activated diffracts the beam (26) towards a reflector (34), which retro-reflects the beam back to the assembly (28) where it is diffracted once again. The beam is then focussed by the lens assembly (25) to form an image of the first reticle (21) on a second reticle (37). As each element (29, 30, 31) is activated, the reflector (34) is positioned at a location corresponding to the designed direction of diffraction of that element, and the position of the element within the assembly (28) is adjusted until the image of the first reticle (21) is coincident with the second reticle (37).

Description

Title: Alignment of Holographic Diffraction Elements

Field of the Invention

This invention relates to a method of and apparatus for aligning a plurality of holographic diffraction elements.

Description of the Relevant Art

Holographic diffraction elements are designed so as to deflect light rays that pass therethrough or that are reflected therefrom. As such, these elements can have focusing power in the manner of a conventional lens. As can be seen in Figure 1A of the accompanying drawings, when a conventional lens L is displaced laterally (i.e. transversely to its optical axis) by an amount x, a light ray passing therethrough will be displaced by an angle equal to x/f radians, where f is the focal length of the lens. Similarly, as depicted in Figure IB of the accompanying drawings, atransmissive holographic diffraction element HDE when employed in the manner of a lens, will also displace a light ray when moved laterally. Accordingly, when laminating together a plurality of holographic diffraction elements to form a so-called application specific integrated lens (ASIL), it is important that each element is laterally aligned with the other elements in the assembly to a highly accurate degree (typically with a tolerance of less than 50 microns). Moreover, it is desirable that this alignment is effected without direct reference to the final application of the ASIL.

An example of a typical ASIL is shown in Figure 2 of the accompanying drawings, and is composed of three holographic diffraction elements A, B and C which are laminated together to form a stack. Each of the elements A, B and C is electrically switchable between an active, diffracting state and an inactive, non-diffracting state. When activated, the element A acts to deflect light rays as indicated at A', the elements B and C being deactivated at this time. Similarly, the elements B and C when individually activated are operative to deflect the light rays as indicated at B¹ and C respectively. Thus, the ASIL as a whole can be used to direct an incident beam of light in different directions by selectively activating the individual elements, A, B and C. Alternatively, in a further arrangement (not shown) the three elements A, B and C can be arranged respectively to act on wavelengths in three different areas of the spectrum (typically in the red, green and blue visible light regions), with the ASIL then being used to focus light of those wavelengths selectively and/or in sequence.

ASIL's are typically used in optical displays (particularly colour-sequential displays), camera- type imaging applications, or to direct light beams in optical switching or multi-channel detectors.

Accurate alignment of conventional optical elements is often performed using an autocollimating telescope or autocollimator, an example of which is shown in Figure 3 of the accompanying drawings. In its normal usage, such an autocollimator is employed to establish flat optical surfaces and to align these perpendicularly to the optical axis of the autocollimator, and thus parallel to one another, hi the example shown in Figure 3, the autocollimator comprises a white light source 10 which illuminates a first or projection reticle 11 comprising a mask having therein a cross-shaped slit 11a. Light passing through the reticle 11 is then collimated by an objective lens assembly 12, so as to project an image of the slit 11a to infmity. This light then impinges upon and is reflected by a particular component 13 to be aligned. The reflected light after passing back through the lens assembly 12 is deflected sideways by a beamspliter 14 and is focused onto the plane of a second reticle 15 which comprises orthogonal crosshairs. The resultant image and the crosshairs can be viewed by a user of the autocollimator (whose eye is indicated at E) by way of an eyepiece 16. The user adjusts the position of the component 13 until the image of the cross-shaped slit 1 la is exactly aligned with the crosshairs of the reticle 15, at which point the light reflected back from the component 13 is exactly anti-parallel to the light emerging from the autocollimator, i.e. the surface of the component 13 is exactly orthogonal to the optical axis of the autocollimator.

Such a conventional type of autocollimator is not suitable for use in aligning the elements of an ASIL composite because the focusing power required for most ASIL applications is too large. Also, use of a white light source is sometimes not appropriate because the individual holographic diffraction elements are highly dispersive.

Object of the Invention

It is an object of the present invention to provide a method of and apparatus for aligning a plurality of holographic diffraction elements, in which these problems are obviated or mitigated.

Summary of the Invention

According to a first aspect of the present invention, there is provided a method of aligning a plurality of holographic diffraction elements in an assembly of such elements, comprising:

collimating light from an illuminated first reference marker into a collimated beam, projecting said collimated beam onto said assembly of holographic diffraction elements, such that said beam is diffractively deflected by one of said elements, positioning a reflector in a predetermined position to receive the deflected beam, said predetermined position being calculated in accordance with a desired position of said one of said elements, said reflector being operative to reflect said beam back to said assembly such that the reflected beam is again diffractively deflected by said one of said elements to form a return beam, focusing said return beam to form an image of said first reference marker on or adjacent to a second reference marker, adjusting the position of said one of the holographic diffraction elements until said image of said first reference marker bears a pre-determined relationship to said second reference marker, and, repeating the process for each of the other holographic diffraction elements in said assembly.

It is preferred that, prior to adjusting the individual holographic diffraction elements, the collimated beam is surface reflected from said assembly, the reflected beam is focused to form an image of the first reference marker, and the position of said assembly is adjusted until said image of the first reference marker bears a predetermined relationship to the second reference marker.

Conveniently, the assembly of holographic diffraction elements is carried by mounting means which holds a first of said elements fixed relative to the mounting means but which permits movement of the other element or elements transversely to a main optical axis of said assembly, and the position of said first of the elements is adjusted by moving the mounting means.

Advantageously, the mounting means comprises a first mounting which carries said assembly of holographic diffraction elements, and a second mounting which carries the first mounting, the first mounting being moveable relative to the second mounting.

Preferably, the position of each of the holographic diffraction elements is adjusted until the image of said first reference marker is coincident with the said second reference marker.

Conveniently, the first and second reference marker each comprise a cruciform reticle. The first reference marker can comprise a cross-shaped slit, whilst the second reference marker can take the form of crosshairs.

Desirably, each of said holographic diffraction elements has optical power such that it possesses an effective focal length, and said reflector is located within said effective focal length.

Preferably, said first reference marker is illuminated by means of light from a monochromatic or substantially monochromatic light source, such as a laser. In a particular arrangement, the first reference marker can be selectively illuminable by means of light from a plurality of monochromatic or substantially monochromatic light sources which emit light at respective different wavelengths. Desirably, the holographic diffraction elements are fixed in their adjusted positions by providing a curable adhesive between each said element and its neighbour or neighbours, and by curing said adhesive after the positions of said elements have been adjusted. Conveniently, the adhesive is curable by means of ultra-violet radiation.

Advantageously, each of said holographic diffraction elements is switchable between an active, diffracting state and an inactive, non-diffracting state, and is individually activated during the adjustment of its position.

In one particular example, said assembly comprises holographic diffraction elements that are respectively adapted to diffract light of differing wavelengths or wavelength bands, for example in the red, green and blue regions of the visible spectrum.

According to a second aspect of the present invention, there is provided apparatus for aligning a plurality of holographic elements in an assembly of such elements, comprising;

first and second reference markers, illumination means operative to illuminate said first reference marker, collimation means operative to collimate light from the illuminated first reference marker into a collimated beam, and to project said collimated beam towards said assembly of holographic diffraction elements, such that the beam is diffractively deflected thereby to form a deflected beam, mounting means operative to mount said assembly of holographic diffraction elements so as to permit relative movement between said elements, a reflector positioned so as to receive said deflected beam from said assembly and operative to reflect said light back to said assembly so that said light is again diffractively deflected thereby to form a return beam, said reflector being angularly moveable relative to said mounting means, and focusing means operative to focus said return beam to form an image of said first reference marker on or adjacent to said second reference marker, the arrangement being such that said image of said first reference marker can be brought into a predetermined relationship with said second reference marker by adjusting the position of each holographic diffraction device within the assembly.

Preferably, the mounting means comprises a first mounting which is adapted to carry said assembly of holographic diffraction elements in such a manner as to permit relative movement therebetween in a direction transverse to a main optical axis of said assembly, and a second mounting on which said first mounting is carried in such a manner as to be moveable relative thereto. Desirably, said first mounting is carried on said second mounting in such a manner as to be capable of both translation and rotation movement relative thereto.

Conveniently, the mounting means is adapted to carry said assembly in such a manner that one of said holographic diffraction elements is fixed relative thereto.

Desirably, each of the holographic diffraction elements has optical power such that it possesses an effective focal length, and said reflector is located within said effective focal length.

Preferably, the illumination means comprises a monochromatic or substantially monochromatic light source, such as a laser. In one particular arrangement, the illumination means can comprise a plurality of selectively operable monochromatic or substantially monochromatic light sources which emit light at respective different wavelengths.

Conveniently, the first and second reference markers each comprise a cruciform reticle. The first reference marker can comprise a cross-shaped slit, whilst the second reference marker can take the form of crosshairs.

Desirably, the reflector is moveable relative to said mounting means about an arc centred generally at the position of said assembly on the mounting means.

Brief Description of the Drawings

The invention will now be further described, by way of example only, with reference to the remaining figures of the accompanying drawings, in which:

Figure 4 is a schematic side view of a first embodiment of apparatus according to the present invention for aligning a plurality of holographic optical elements;

Figure 5 is a diagram showing part of the apparatus in use; and

Figure 6 is a schematic side view of a second embodiment of apparatus according to the present invention.

Detailed Description

Referring to Figure 4, the apparatus shown therein comprises an autocollimator 20 of the same general type as shown in Figure 3. More particularly, the autocollimator includes a first reticle 21 having therein a cross-shaped slit 22 which is illuminated by means of a laser 23. Light from the illuminated reticle 21 passes through a beamsplitter 24 and is received by a collimator lens assembly 25 which converts the light into a collimated output beam 26. The autocollimator 20 is desirably a model as manufactured by Edmond Scientific, which is preferred because of its convenient access to the illumination port. The laser 23 is conveniently a 3-m W-class frequency-doubled Nd:YAG laser as manufactured by Laser Power Optics. The curvature of the incident light upon the reticle 21 is such that a clear, cruciform pattern corresponding to the slit 21 is formed approximately 20cm from the lens assembly 25.

The apparatus also -includes a mounting device 27 on which an assembly 28 of holographic diffraction elements can be mounted. In the illustrated embodiment, the assembly 28 comprises three such elements which are referenced 29, 30 and 31, respectively. However, it is to be understood that the invention is equally applicable to assemblies containing any plurality of holographic diffraction elements. The mounting device 27 is composed of a translation mounting 32 which carries the assembly 28 in such a manner as to permit relative movement between the individual holographic diffraction elements 29, 30 and 31 in a direction transverse to the main optical axis of the autocollimator 20, as indicated by arrows X. In the particular example shown, the translation mounting 32 is adapted to mount the element 29 in a fixed position whilst allowing transverse translation of the elements 30 and 31. The mounting 32 is in turn carried by a mounting stage 33 which can be translated along three orthogonal axes whilst also being rotatable about three orthogonal axes, thereby enabling the assembly 28 to have six degrees of freedom in its overall movement.

The apparatus further comprises a reflector 34 in the form of a plane mirror which is mounted by way of a tip/tilt mounting upon a rotatable turntable 35, whose rotation is controlled by a computer control 36. The reflector 34 is thereby moveable along an arc whose axis passes approximately through the assembly 28. The turntable 35 is preferably a high precision rotation stage as manufactured by Newport Corporation, which is accurate to within 0.001 degrees.

The collimated light beam 26 emerging from the autocollimator 20 is incident upon the assembly 28 of holographic diffraction elements, and is diffractively deflected towards the reflector 34. The reflector 34 then retro-reflects this light such that it is incident upon the assembly 28 once again and is diffractively deflected for a second time, back towards the autocollimator 20. The lens assembly 25 of the autocollimator 20 then projects this light towards the beamsplitter 24, which deflects the light sideways to a second reticle 37 having orthogonal crosshairs 38. By this means, an image of the cruciform slit 22 of the reticle 21 is focused on to the reticle 37, and can be viewed by an observer (whose eye is indicated at 39) by means of an eyepiece lens 40. As will be explained in detail later, each of the holographic diffraction elements 29, 30 and 31 is aligned in turn, and when correct alignment has been achieved the image of the slit 21 is arranged to be coincident with the crosshairs 38 of the reticle 37.

The holographic diffraction elements 29, 30 and 31 are essentially holograms that have been pre-recorded into a medium. These can be thin phase holograms (that is, holograms which conform to the Raman Nath regime) or they can be volume holograms also known as thick or Bragg holograms. Use of the latter is preferred, because they offer high diffraction efficiencies for incident beams whose wavelengths are close to the theoretical wavelength satisfying the Bragg diffraction condition, and which are within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition.

The recording medium is typically a polymer-dispersed liquid crystal mixture which undergoes phase separation during the hologram recordal process, creating fringes comprising regions densely populated by liquid crystal micro-droplets interspersed with regions of clear polymer. These fringes can effectively be erased by applying an electric field to the hologram, which changes the natural orientation of the liquid crystal molecules and reduces the refractive index modulation of the fringes, thereby causing the diffraction efficiency to drop to a very low level. Such an electric field can be applied by means of electrodes deposited on opposite surfaces of the substrate that is used to encapsulate the hologram. By using a control 41 to apply an electrical potential across the electrodes, it is thus possible to switch each element 29, 30 and 31 between an active, diffracting state and an inactive, non-diffracting state. Using such a system, it is possible to achieve very fast switching rates, typically with a switching time of less than 150 microseconds, and perhaps as low as a few microseconds.

The substrate can be composed of glass, plastics or composite materials which can be flexible or rigid and flat or curved. The electrodes can be composed of transparent conducting material, such as ITO or electrically-conducting polymers, and can be provided with anti- reflection coatings. It is also possible for the switching circuitry for the electrodes to be deposited on the substrate as well.

The apparatus is initially set up by setting the main optical axis of the autocollimator 20 so that it is exactly perpendicular to the rotation axis of the turntable 35, and by adjusting the reticle 21 so that the slit 22 is square to this same reference. A zero-angle of the reflector 34 is then determined by using the autocollimator 20 in a conventional manner, i.e. by reflecting the collimated output beam 26 directly from the reflector 34 and by adjusting the latter until the image of the slit 22 is exactly coincident with the crosshairs 38. The computer control 36 is then pre-programmed to adjust the position of the reflector 34 to values corresponding to specific design diffraction angles (for normal light incidence) for the various holographic diffraction elements in the assembly 28.

The assembly 28 is then mounted on the mounting device 27 and is brought into an orientation exactly perpendicular to the main optical axis of the autocollimator 20 by adjusting the mounting stage 33 whilst observing the image of the slit 22 as reflected by a front surface of the top element 31 in the assembly 28. The computer control 36 is then operated to move the reflector 34 to an angular position corresponding to the design angle for the bottom element 29 in the assembly, which (as noted previously) is fixed relative to the translation mount 32. The entire mounti ng device 27 is then translated until the light diffracted by the element 29, retroreflected by the reflector 34 and diffracted again by the element 29 is brought into coUimation, i.e. until the image of the slit 22 becomes exactly aligned with the crosshairs 38. During this exercise, the holographic diffraction element 29 is activated whilst the elements 30 and 31 are de-activated by means of the control 41. The light diffractively deflected by the element 29 is shown as dotted line R, in figure 4.

The use of the laser 23 as the light source for the autocollimator 20 means that the light projected from the illuminated reticle 21 appears as a readily visible cross pattern on the element 29. When autocollimation has been achieved in the manner described above, the cross pattern is located on the element 29 at exactly the position corresponding to the design angle.

The procedure is then repeated for each of the other holographic diffraction elements 30 and 31 in turn. More particularly, the computer control 36 is operated to set the reflector 34 at an orientation corresponding to the design angle for the element 30, the element 30 is activated whilst the elements 29 and 31 are de-activated by the control 41, and the position of the element 30 is laterally adjusted until autocollimation is achieved. The light diffractively deflected by element 30 is indicated by solid line R₂ in Figure 4. The reflector 34 is then set at an orientation corresponding to the design angle for the element 31, the element 31 is activated whilst the elements 29 and 30 are de-activated by the control 41, and the lateral position of the element 31 is adjusted until autocollimation is achieved. The light diffractively deflected by the element 31 is indicated by broken line R₃. By this means, the three elements 29, 30 and 31 will have been set into an appropriate alignment whereupon their positions can be permanently fixed. A preferred way of achieving this is to pre-insert a curable adhesive between the facing surfaces of the elements 29 and 30 on the one hand, and the elements 30 and 31 on the other, and by bonding the elements together by curing the adhesive with ultra-violet radiation.

Figure 5 shows a typical pattern that will be seen by the observer through the eyepiece lens 40 of the autocollimator 20. In this Figure, the image of the slit 22 is designated as 42 and has two arms 42a and 42b. In the illustrated situation, it is assumed that autocollimation has not yet been fully achieved, because the arms of the image 42 are displaced from the crosshairs 38, showing both vertical and horizontal mis-alignment. Abberations in the holographic diffraction elements cause the arms 42a and 42b to appear bent.

In the above embodiment, the holographic diffraction elements 29, 30 and 31 have been described as being individually activated during the alignment process. This enables each element to have a diffracting power that is unaffected by the other elements, and avoids potential ambiguities caused by stray beams. However, it is not essential that this should be the case. For example, where the assembly 28 is composed of elements which are adapted to act on respective different wavelengths (for example in the red, green and blue regions of the visible spectrum), the wavelength and angle selectivity of the Bragg holograms may be sufficient to prevent significant cross-talk between the elements, in which case it can become unnecessary to de-activate those elements that are not being adjusted at a given time. Indeed, under these circumstances, the invention can be applied also to holographic diffraction elements which are non-switchable.

The above description also assumes that the assembly 28 is composed of transmissive holographic diffraction elements. However, the invention is equally applicable to assemblies which comprise reflective holographic diffraction elements, as is illustrated in Figure 6. In this Figure, the same reference numerals are used as in Figure 4 to designate similar parts. In this arrangement, however, the reflector 34 is now positioned to receive light diffractively reflected from the holographic elements 29, 30 and 31, and the light retro-reflected by the reflector 34 is diffractively reflected by the elements back towards the autocollimator 20.

It has been mentioned above that the alignment of the holographic diffraction elements needs to be achieved with a high degree of accuracy, for example with a tolerance of less than 50 microns. To achieve such a tolerance, it is possible to design the apparatus such that the steering of light rays is 1.4 deg/rnm on the transmitted beam, with a net angle on retro- reflection (after double passage through the holographic diffraction elements) of 1.2 deg/mm. The discrimination of the autocollimator 20 is judged to be a few arcmins when reasonable care is taken, which enables a positional of accuracy of within 40 microns to be achieved. In order to obtain the best quality for the image of the slit 22 and the most discrimination in the alignment of this image with the crosshairs 38, it is preferred that the reflector 34 is located within the effective focal length of the holographic diffraction elements 29, 30 and 31.

Whereas the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention. For example, instead of comprising a single laser, the light source for illuminating the reticle 21 can comprise a plurality of co-aligned lasers of differing wavelengths. Where the application requires, these lasers can then be selectively operated. Also, it is possible to automate the above-described process by means of machine vision and automatic/robotic control of items such as cameras, computers and activators, particularly to eliminate the need for a human observer to judge when autocollimation has been achieved.

Claims

1. A method of aligning a plurality of holographic optical elements (29, 30, 31) in an assembly (28) of such elements, comprising:

collimating light from an illuminated first reference marker (21) into a collimated beam (26);

projecting said collimated beam (26) onto said assembly (28) of holographic diffraction elements, such that said beam is diffractively deflected by one of said elements;

positioning a reflector (34) in a predetermined position to receive the deflected beam, said predetermined position being calculated in accordance with a desired position of said one of said elements, said reflector (34) being operative to reflect said beam back to said assembly (28) such that the reflected beam is again diffractively deflected by said one of the elements to form a return beam;

focussing said return beam to form an image of said first reference marker

(21)on or adjacent to a second reference marker (37);

adjusting the position of said one of the holographic diffraction elements (29, 30,31) until said image of the first reference marker (21) bears a predetermined relationship to the second reference marker (37); and

repeating the process for each of the other holographic diffraction elements

(29, 30, 31) in said assembly (28).

A method as claimed in claim 1, wherein prior to adjusting the individual holographic diffraction elements, the collimated beam (26) is surface reflected from said assembly (28), the reflected beam is focussed to form an image of the first reference marker (21),and the position of said assembly is adjusted until said image of the first reference marker (21) bears a predetermined relationship to the second reference marker (37).

3. A method as claimed in claim 1 or 2, wherein the assembly (28) of holographic diffraction elements (29, 30, 31) is carried by mounting means (27) which holds a first of said elements (29) fixed relative to the mounting means but which permits movement of the other element or elements (30, 31) transversely to a main optical axis of said assembly, and the position of said first of the elements (29) is adjusted by moving the mounting means (27) .

A method as claimed in claim 3, wherein the mounting means (27) comprises a first mounting (32) which carries said assembly of holographic diffraction elements, and a second mounting (33) which carries the first mounting (32), the first mounting (32) being movable relative to the second mounting (33).

5. A method as claimed in any preceding claim, wherein the position of each of the holographic diffraction elements (29, 30, 31) is adjusted until the image of said first reference marker (21) is coincident with said second reference marker (37).

A method as claimed in any preceding claim, wherein the first and second reference markers (21, 37) each comprise a cruciform reticle.

7. A method as claimed in claim 6, wherein the first reference marker (21) comprises a cross-shaped slit (22).

. A method as claimed in claim 6 or 7, wherein the second reference marker (37) comprises crosshairs (38).

A method as claimed in any preceding claim, wherein each of the holographic diffraction elements (29, 30, 31) has optical power such that it possesses an effective focal length, and the reflector (34) is located within said effective focal length.

10. A method as claimed in any preceding claim, wherein the first reference marker (21) is illuminated by means of light from a monochromatic or substantially monochromatic light source, such as a laser (23).

11. A method as claimed in claim 10, wherein the first reference marker (21 ) is selectively illuminable by means of light from a plurality of monochromatic or substantially monochromatic light sources which emit light at respective different wavelengths.

12. A method as claimed in any preceding claim, wherein the holographic diffraction elements (29, 30, 31) are fixed in their adjusted positions by providing a curable adhesive between each said element and its neighbour or neighbours, and by curing said adhesive once the positions of said elements have been adjusted.

13. A method as claimed in claim 12, wherein the adhesive is curable by means of ultraviolet radiation.

14. A method as claimed in any preceding claim, wherein each of said holographic diffraction elements (29, 30, 31) is switchable between an active, diffracting state and an inactive, non-diffracting state, and is individually activated during adjustment of its position.

15. A method as claimed in any preceding claim, wherein said assembly (28) comprises holographic diffraction elements (29, 30, 31) that are respectively adapted to diffract light of differing wavelengths or wavelength bands.

16. A method as claimed in claim 15, wherein said wavelengths or wavelength bands are respectively in the red, green and blue regions of the spectrum.

17. Apparatus for aligning a plurality of holographic diffraction elements (29, 30, 31) in an assembly (28) of such elements, comprising:

first and second reference markers (21, 37);

illumination means (23) operative to illuminate said first reference marker

(21);

coUimation means (25) operative to collimate light from the illuminated first reference marker (21) into a collimated beam (26), and to project said collimated beam (26) towards said assembly (28) of holographic diffraction elements(29, 30, 31), such that the beam (26) is diffractively deflected thereby to form a deflected beam;

mounting means (27) operative to mount said assembly (28) of holographic diffraction elements (29, 30, 31) so as to permit relative movement between said elements; a reflector (34) positioned so as to receive said deflected beam from said assembly (28) and operative to reflect light back to said assembly (28) so that said light is again diffractively deflected thereby to form a return beam, said reflector (34) being angularly moveable relative to said mounting means (27); and

focussing means (25) operative to focus said return beam to form an image of said first reference marker (21) on or adjacent to said second reference marker (37);

the arrangement being such that said image of said first reference marker (21) can be brought into a predetermined relationship with said second reference marker (37) by adjusting the position of each holographic diffraction device (29, 30, 31) within the assembly (28).

18. Apparatus as claimed in claim 17, wherein the mounting means (27) comprises a first mounting (32) which is adapted to carry said assembly (28) of holographic diffraction elements (29, 30, 31) in such a manner as to permit relative movement therebetween in a direction transverse to a main optical axis of said assembly (28), and a second mounting (33) on which said first mounting (32) is carried in such a manner as to be moveable relative thereto.

19. Apparatus as claimed in claim 18, wherein said first mounting (32) is carried on said second mounting (33) in such a manner as to be capable of both translation and rotation movement relative thereto.

20. Apparatus as claimed in claim 17, 18 or 19, wherein the mounting means (27) is adapted to carry said assembly (28) in such a manner that one of said holographic diffraction elements (29) is fixed relative thereto.

21. Apparatus as claimed in any one of claims 17 to 20, wherein each of the holographic diffraction elements(29, 30, 31) has optical power such that it possesses an effective focal length, and said reflector (34) is located within said effective focal length.

22. Apparatus as claimed in any one of claims 17 to 21, wherein the illumination means (23) comprises a monochromatic or substantially monochromatic light source, such as a laser.

23. Apparatus as claimed in claim 22, wherein the illumination means(23) comprises a plurality of selectively operable monochromatic or substantially monochromatic light sources which emit light at respective different wavelengths.

24. Apparatus as claimed in any one of claims 17 to 23, wherein the first and second reference markers (21, 37) each comprise a cruciform reticle.

25. Apparatus as claimed in claim 24, wherein the first reference marker (21) comprises a cross-shaped slit (22).

26. Apparatus as claimed in claim 24 or 25, wherein the second reference marker (37) comprises crosshairs (38).

27. Apparatus as claimed in any one of claims 17 to 26, wherein the reflector (34) is moveable relative to said mounting means (27) about an arc centred generally at the position of said assembly (28) on the mounting means (27).