WO1989006001A1

WO1989006001A1 - A method of, and apparatus for, reducing or eliminating speckling in reproduction of an image from a hologram

Info

Publication number: WO1989006001A1
Application number: PCT/GB1988/001109
Authority: WO
Inventors: William Nevil Heaton Johnson; Nicholas John Phillips
Original assignee: Scientific Applied Research SAR PLC
Current assignee: Scientific Applied Research SAR PLC
Priority date: 1987-12-15
Filing date: 1988-12-13
Publication date: 1989-06-29
Anticipated expiration: 1990-06-15
Also published as: GB8729212D0; KR900700933A

Abstract

In order to eliminate ''speckling'' in the reproduction of a holographic image using laser light, there is interposed between the source of the laser light and the hologram (12), a screen (16) comprising an array of microlenses, the screen (16) being subjected to small transverse movements in its own plane, during exposure of an imaging arrangement (14). The effect is to average out, over the exposure period, the effects of the ''defects'' in the optical system used in preparing the original hologram, and thereby to eliminate such speckling in the image formed in the imaging arrangement (14).

Description

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Title: "A method of, and apparatus for, reducing or eliminating speckling in reproduction of an image from a hologram"

It is known that in the reproduction of an image by a technique utilising the illumination of a hologram by laser light, a "speckling" effect can occur in the image, i.e. patches or spots of light occur in the image which have no counterpart in the original object. This "speckling" effect is attributable to imperfections in the optical system used, to project the laser beam onto the hologram for the reproduction of the image, "imper¬ fections" including specks of dust on lenses, irregularities in lens edges and so forth, the "speckling" effect is generally more pronounced the smaller the optical aperture of the respective optical system.

The effect is observable in holographically pro¬ duced images viewed, in coherent light, by the naked eye, (where the aperture defined by the iris is definitive) and in images produced by hologram and recorded photo¬ graphically. Such "speckling" presents a drawback to the use of holograms in applications requiring a precisely predetermined light image to be formed on an image plane, for example in certain photo-etching and the like techniques, and where such use would otherwise present the substantial advantage of providing a relatively inexpensive alternative to very expensive precision lens systems and associated indexing equipment. It is known to reduce such "speckling" by inter¬ posing between the laser source and the hologram some means of continually changing the path length between the ^• laser source and the image, such as a continually rotated reflective ball or a continually agitated ground glass screen. Unfortunately, these techniques substantially reduce the amount of light reaching the image,- to counteract which it is necessary either to increase the power of the laser, or to increase the exposure time, both of which introduce practical difficulties which result, for diverse reasons not discussed in detail here, in a reduction in resolution of the image obtained. It is an object of the present invention - to provide an improved method of, and apparatus for, reducing such "speckling" in the reproduction of holographic images.

According to the invention, there is provided a method of reducing or eliminating "speckling" in repro¬ duction of an image by a technique involving the illumination of a hologram by light from a laser source, the method comprising the interposition, between the laser source and the hologram, of a microlens screen,-and subjecting the screen to transverse movements' in the plane of the screen, during exposure of an imaging arrangement.

An embodiment of the invention is described below by way of example with reference to the accompanying drawing which is a schematic illustration of an arrange-' ment embodying the invention.

Referring to the drawing, a laser beam 10 is shown, incident upon a transmission hologram 12 at an angle, with a resultant real image (planar in the example shown) being formed at 14, for example on a photo¬ sensitive layer on a planar support. Extending trans¬ versely across, the beam 10, in the path of the light directed towards the hologram 12, and generally normal to the beam axis, is a light transmitting microlens screen 16.

^' The screen comprises a transparent sheet which carries an array of lenslets or microlenses, formed integrally in or on said sheet and having their principal axes substantially normal to the plane of the sheet.

Preferably said lenslets or microlenses are formed by a method including a photographic imagaging step, carried out upon a photopolymer.

Preferably the screen 16 is formed by providing on a transparent substrate a material having localised variations in refractive index so as to afford said microlenses or lenslets in the form of graded refractive index lenses.

The screen 16 is preferably formed by a method which utilises a compound applied to said substrate in monomeric form and which, upon selective exposure to ultraviolet light, will polymerise selectively in such a way as to afford said graded refractive index lenses and wherein the layer of monomer is initially exposed, to ultraviolet light, at each of an array of spots over the surface of the layer, to provide the desired microlenses by consequent polymerisation of the material, the material subsequently being subjected to a blanketing exposure of ultraviolet light to complete poly¬ merisation.

More preferably, the material is raised to its softening temperature intermediate the selective exposure and the blanketing exposure, to enhance the refractive index variation within the regions of the microlenses. Similar graded refractive index lenses may be formed in a layer of dichromated gelatin (DCG) . Alternatively, the microlenses or lenslets may be formed by localised shaping of the surfaces of: sai sheet in the region of the or each., microlens, for example to give the microlens the form of a conventional refractive lens. A sheet of the last-noted character may be formed for example, by casting, moulding or pressing an appro¬ priate plastics material in or upon a "master" mould or die having a surface of complementary configuration. Such a "master" mould or die may, for example, be fashioned using laser and/or holographic techniques, or by etching techniques or even mechanically.

Thus, for example, such a master may be formed by coating a glass sheet with a photosensitive . resist, exposing the resist to light whilst the resist layer is in contact with a screen bearing an array of opaque dots, carrying out a developing procedure in the course of which the unexposed resist layer in the region of said- dots is removed (to assist which, after the removal of. the bulk of the unexposed resist, the resist layer may be^* subjected to ion bombardment to remove residual resist from the unexposed areas and thereby- "sharpen" :the. image), and etching the resist coated surface of the glass with hydrofluoric acid for a predetermined time.

The layer in which the microlenses are formed may be of the order of 25μ to 50μ thick, with the diameters of the lenses and the pitch between adjacent lenses in the array, being of the same order.

In operation of the layout described, in the absence of the screen 16 and the operational measures to be described, the image formed at 14 is typically characterised by "speckling", as a consequence of which, for example features in the image which should have straight edges, may have ragged or ill-defined edges. The screen 16 is, as noted above, a transparent sheet bearing a closely packed array of integral lenslets or microlenses, each microlens being a lens with positive power having its axis substantially corresponding with that of the beam 10. The power of the microlenses need only be low, as compared with the values readily obtainable using the techniques described. The screen 16 is mounted by means, (not shown), arranged in operation to apply a transverse movement to the screen, such as an oscillating movement in the directions of arrows 18, for example, an oscillating or vibratory movement, of the order of a millimetre or less in amplitude. The effect of the transverse displacement of the screen is to move the "speckle" features back and forth across the image plane 14 to an extent large in relation to the size of the speckle features, whilst leaving the portions of the image due to the hologram substantially stationary.

The continual movement of the speckle features during exposure of the photosensitive layer at 14 results in an "averaging out" over the exposure period, over the image surface, of the "speckle" illumination, and thus in the elimination of the corresponding "speckling" in the photographic image.

A similar technique is known using, for example, a ground glass screen. However, in this known technique there is substantial scattering of light from the laser beam in directions such as to miss the hologram 12 altogether, and this leads to a significant reduction in the effective power of the laser in such a known arrangement. Such scattering can be largely eliminated by the use of the microlens screen, which can, indeed, be "tailored" to afford a desired maximum angle of deflection or effective "scattering" by arranging the focal lengths of the microlenses accordingly.

Claims

1. A method of reducing or eliminating "speckling" as ^' herein defined, in reproduction of an image by a technique involving the illumination of a hologram by light from a laser source, the method comprising the interposition, between the laser source and the hologram, of a microlens screen, as herein defined, and subjecting the screen to transverse movements in the plane of the screen, during exposure of an imaging arrangement.

2. Apparatus for use in the photographic repro¬ duction of an image by a technique involving the illumination of a hologram by light from a laser source, the apparatus including a laser source, a support for a hologram to be illuminated by said source, a microlens screen, as herein defined, supported in a position in which said screen is interposed between the laser source and the hologram, and means for imparting to said screen transverse movements in the plane of the screen.