WO1989006811A1

WO1989006811A1 - Reflective material

Info

Publication number: WO1989006811A1
Application number: PCT/GB1989/000032
Authority: WO
Inventors: William Nevil Heaton Johnson; Nicholas John Phillips
Original assignee: Scientific Applied Research (S.A.R.) Plc
Priority date: 1988-01-13
Filing date: 1989-01-13
Publication date: 1989-07-27
Also published as: GB8800710D0; KR900700897A; AU2931789A

Abstract

A light reflective material comprising an array of cube corner reflectors is made by moulding or pressing a plastics sheet using a mould or press tool configured to produce, in the surface of the plastics sheet, an array of cube corner recesses. The surface of the sheet is then metallised, to make the recesses into cube corner reflectors, and a transparent layer, filling the recess, is applied to the metallised surface of the plastics sheet. The mould or press tool is derived, by an electrotyping procedure, from a master formed by indexing a deformable plastics sheet in a scanning mode past an indenting punch comprising a diamond tip cut to cube corner form. The punch is carried by a piezo-electric element which is subjected to an electrical pulse at each indexing step to drive the cube corner diamond tip into the deformable plastics at each indexing step to form a corresponding cube corner recess.

Description

DESCRIPTION OF INVENTION

Title: "Reflective material"

THIS INVENTION relates to a reflective material which is particularly, but not exclusively, useful for the production of such items as cinema projection screens, motorway signs, advertising signs, reflective vehicle number plates and the like.

More particularly, the invention relates to the manufacture of a reflective sheet material having a light reflective surface which is formed with a plurality of cube- corner recesses, as herein defined, whereby light striking said material will be reflected back in the direction from whence it has come.

Several forms of reflective sheet material are already known. One class of such materials comprises a mass of transparent spheres embedded in a cement matrix applied as a coating to the surface which is intended to be reflective, such as a flexible film. However, the retroreflectivity, i.e. the efficiency of such a material, in terms of the percentage of the light striking the material which is reflected back along the path of incidence, is low, for the exposed portions of the spheres actually form a relatively small proportion of the total surface of the medium and only a part of the light entering each sphere is reflected back along its path of incidence. Furthermore there is a substantial scattering of light in directions other than the direction directly opposite to the direction of incidence.

Reflective materials are also known comprising a metallic or quasi-metallic reflective surface formed with a regular array of cube-corner recesses. Whilst materials of this character exhibit a substantially greater retro- reflectivity than materials of the first-mentioned class, it has, hitherto, been difficult to provide such materials in the form of relatively thin sheets, for if the material is in the form of a thin sheet, this implies that the individual reflective cube-corner recesses shall be of correspondingly small size and it has hitherto been difficult to provide accurately formed recesses of such small size economically. One known material of this type is formed from a moulding or pressing operation using tools derived from a master formed by scoring a surface with three sets of parallel equally spaced grooves, the three sets being mutually at 120° with respect to each other, using a grinding wheel or other tool having a triangular-section edge such that the finished workpiece comprises a corres¬ ponding array of projections of cube-corner form. However, the technique used results in unintentional scorings, pro¬ duced on the flanks of the corner-cube projections by the tool, being reproduced in the "faces" of the reflective recesses of the finished retro-reflective material, leading to a reduction in retro-reflectivity and a substantial increase in light-scattering from the material in directions other than the direction of incidence of the rays or the opposite direction.

According to one aspect of the invention there is provided a method of making a light reflective material comprising an array of cube-corner reflectors, as herein defined, comprising forming a master by indexing a body of deformable material, in a scanning mode, relative to a tool so that a surface of the deformable material is advanced in steps past said tool, said tool comprising a punch element having a cube corner head, as herein defined, of a hard material, such as diamond, capable of receiving an optically smooth surface finish, said punch element being mounted for movement towards and away from said surface, the punch element being connected with piezo-electric motor means whereby, at each indexing step, the punch element, more particularly said head of the punch element, can be propelled into said deformable material to form a respective cube corner recess, as herein defined, in said surface of the material, whereby in a succession of indexing steps and scans, an array of such cube-corner recesses is formed in said surface of the material, the material, bearing said array of recesses, subsequently being used as a master for the production of a mould or press tool having a comple¬ mentary surface, the method further comprising manufactur¬ ing said reflective material comprising an array of cube corner reflectors, by a process including using said mould or press tool to produce a product having cube-corner indentations corresponding to those of the master.

The term "cube-corner recess" as used herein is intended to denote a recess defined by three mutually perpendicular planar surfaces intersecting along three orthogonal axes meeting in a vertex of the recess. Corres¬ pondingly, the term "cube corner head" as used herein in relation to a punch element denotes a punch element head defined by three mutually perpendicular planar faces meeting in respective edges terminating in an apex or point. The term "cube-corner reflector" as used herein signifies a cube-corner recess wherein said mutually perpendicular planar surfaces are light-reflective.

It is known property of a cube-corner reflector, as herein defined, that a light beam entering the recess from any direction will be reflected back therefrom in the precisely opposite direction.

Consequently, a reflective material made by a method embodying the invention and comprising an array of such cube-corner reflectors, forms a highly efficient reflector for use in situations where the material will be viewed in substantially the same direction as that in which light is directed to the material to illuminate the same, for example in motorway signs intended to be observed by the drivers of vehicles having headlamps illuminating the signs, or for reflective vehicle number plates or in relation to cinema projection screens.

A further application of such a material, particularly where the material comprises a dense array of relatively minute indentations, is in relation to certain optical and holographic and laser techniques utilising, for example, the fact that a plate of such material, because it effectively reverses each light ray falling thereon, can be utilised, in principle, to provide a simultaneous real image of an object, coincident with the object itself, or, if a semi- transparent/semi-reflective screen is utilised as a "beam splitter", located at a position spaced from the object.

In a preferred embodiment of the invention, in order to manufacture a retroreflective material, a master is first manufactured by indexing a sheet of deformable plastics material supported by an indexing table, in a scanning mode, step-by-step past a tool so that, relative to the plastics sheet, the tool effects a scanning movement above the surface of the plastics. The tool comprises a punch element having a cube-corner head and motor means, in the form of a piezo-electric element, for propelling said punch, at each indexing step, into the deformable plastics material to form a respective cube corner recess in the surface of the material. Thus, the punch element is mounted for movement towards and away from the exposed surface of the deformable material, for example by being directly secured to one end of the piezo electric motor element mounted by its opposite end in a suitable support, so that the elongation of the piezo electric element on the application of a voltage pulse thereto automatically moves the stylus or head towards and into the deformable material.

The punch element has the head thereof formed by a tip or stylus of diamond or like hard material capable of receiving an optically smooth surface finish, said tip or stylus being formed with three mutually perpendicular highly polished planar facets defining the cube corner, said facets being also equally inclined to the direction of movement of the punch element into and out of the deformable material. The piezo electric motor element is energised periodically by electrical pulses at successive indexing steps whereby during an entire scan, the surface of deformable plastics material is formed with a close array of such right pyramidal recesses, each recess being formed, at a res¬ pective indexing step, in a respective single electrical pulse applied to the piezo-electric motor element.

The punch element is also mounted for rotation about an axis parallel with the last-noted direction of movement, relative to the table, whereby two interspersed sets of cube-corner recesses forming the array can be formed, one set being rotated through 180° relative to the other set (as seen looking down on the material), so that the recessed surface is formed entirely by said cube-corner recesses, without surface regions of finite width between adjacent said recesses which do not form part of the cube-corner surfaces.

The punch element may be rotated at every step, in alternate senses, so that in successive indenting steps each recess formed is oriented at 180° relative to that formed in the preceding step. It will be appreciated, however, that this arrangement also necessitates a slight displacement of the tool, or the indexing table, relative to the line scan direction, in one sense or the opposite sense, at each indexing step, and it will generally be preferable, in the first of every two successive scanning lines, to form a first row of similarly oriented cube-corner recesses, and in the second scanning line, after turning the punch element through 180° and advancing the indexing table appropriately perpendicularly to the scanning lines, to form a second row of oppositely oriented cube corner recesses each located between a respective pair of adjacent recesses in the first row, the apparatus being so controlled as to ensure that the necessary half-pitch displacement between two lines of intermeshed recesses is maintained.

The means provided for moving the indexing table in steps during each scanning line, and for moving the indexing table by the appropriate amount in a perpendicular direction between successive scanning lines, and also the piezo¬ electric motor means for the tool, and further means provided for rotating the punch about said axis, are all preferably controlled by microprocessor. The depth of the recesses is determined principally by the voltage of the electrical pulse applied to the piezo-electric element at each indexing step, the appropriate voltage to produce recesses corresponding in size to the indexing steps being determined by experiment for the respective deformable material used. In order to minimise errors due to variations in thickness of the deformable material, or in discrepancies between the indexing plane and the plane of the supporting surface of the indexing table, the apparatus may comprise a sensor, possibly constituted by the piezo electric element itself, arranged to provide a signal when the corner-cube head touches the material, and the arrange¬ ment may be such that, at each indexing step, the electrical pulse applied comprises a relatively slowly rising voltage applied to the piezo electric element to bring the cube corner head into contact with the material, and a pre- determined voltage increment which is then added rapidly to the voltage applied to the element, to drive the head to a predetermined extent into the material. Alternatively a further motor means may be provided to effect approach of the tool to the material prior to contact and for retraction of the tool after indenting of the material. It is preferred to use, as the deformable sheet material a plastics material, such as PVC (polyvinyl- chloride) or polyethylene or a polyester, which is mechanic¬ ally deformable but has minimal recovery or flow character- istics so that the impressions of the tool head will be retained therein without distortion. However, as an alternative, the deformable material may be a deformable oxidation-resistant metal, such as aluminium, silver or gold.

The master formed by the above described treatment of the deformable material is used to provide, by an electro¬ type or other process, known per se, a relatively hard and durable mould or die part affording a complementary surface and which can be used, in a moulding or pressing operation, to provide a plastics sheet, for example of PVC having a surface configuration corresponding with that of the master. This plastics sheet may then be provided, on its surface afforded by the cube-corner recesses, with a thin reflective metallic layer, for example by vacuum deposition of the metal over the recessed surface, the layer being very thin, for example 10 to 100 atoms thick, and conforming closely with the plastics surface.

Preferably the cube corner embossed, metallised sur¬ face is then coated with a layer of transparent plastics, such as clear PVC of a melting point lower than the base material, which fills the cube-corner recesses and affords an optically smooth flat surface, remote from the metallisation, which lies parallel with the notional surface in which lie the edges of the cube-corner recesses, and which is parallel with the flat surface of the embossed or moulded reflective sheet which is remote from the cube- corner recesses. This covering layer of transparent plastics, securely bonded to the metallisation, not only protects the reflective cube corner recesses, for example by preventing foreign material from lodging in the recesses and protecting the metallisation from oxidation or other chemical attack, but also serves to refract any light ray, incident on the reflective sheet at an angle to the perpendicular, so that it enters the respective reflective recesses at an angle closer to the perpendicular to the reflective sheet than it would otherwise have done, whereby the range of angles of incidence over which the reflective material is fully effective is increased.

In view of the small size and spacing of the cube corner recesses, perceptible diffraction effects might arise if the array of recesses were completely regular. To avoid such diffraction effects, the microprocessor controlling the apparatus may be programmed to apply a limited random or pseudo-random variation to the recess size and/or spacing and/or the angular orientation of the recesses about axes perpendicular to the general plane of the deformable sheet material. Such variation necessarily entails leaving a small proportion of the reflective surface of the finished product which is not part of any cube-corner recess and which may therefore reflect an incident ray in a direction other than that opposite to that in which it reached the material. That is to say the randomisation referred to brings with it a degree of scatter which represents a trade-off against the reduction or elimination of diffraction effects. It should be noted that if the variations referred to are of a pseudo-random, rather than a strictly random nature, and are according to a predetermined pattern, the characteristic pattern of variation may be used to determine whether a given sample of the material emanates from a manufacturer using that specific pattern of variation, or was derived by "reverse engineering" from that manufacturer's product. Thus, the pattern of variation from strict regularity may be used to serve a security function.

In a variant of the method, instead of forming the master from a generally planar sheet of deformable material supported by an indexing table, a cylindrical member having its surface provided by an appropriate deformable material may be rotated and traversed stepwise, past a deforming tool such as described above, with a view to producing, ultimately, a cylindrical roller which can be used in the production, on a continuous basis, of a continuous length of the reflective material, by passing an appropriate sheet plastics material, in strip form, through the nip between the cylindrical roller, (formed with surface projections complementary in form to the desired right pyramidal recesses), and a counter roller, to produce the indented sheet which is then metallised and coated as before.

The preferred method described allows the economical production of a retroreflective material, comprising an array of reflective cube corner recesses in which the size of the recesses is substantially smaller and the recess density, in terms of number of recesses per unit area, substantially greater, than has been possible hitherto. It will be appreciated that the edge of each recess, where the planes of the three mutually perpendicular planes defining the recess meet the notional plane containing the uppermost or outermost part of the surface defined by the recesses, has substantially the form of an equilateral triangle. The length of each side of each such triangle, in the preferred embodiment of the invention, may be from 30 to 100 microns, preferably in the range of from 30 to 50 microns. Whilst this is substantially smaller than has been possible by using techniques known hitherto, it is sufficiently large to avoid significant diffraction effects. Assuming the three facets of the diamond head of the punch element are accurately formed and appropriately finished, the reflective surfaces in each cube corner recess in the finished material conform much more closely to the ideal optically flat reflective surfaces than materials used hitherto and consequently, despite the small size of the cube corner recesses, scatter from the reflective material made by the above-noted method embodying the invention, (as opposed to strict retroreflection) is substantially smaller than with the known retro-reflective materials.

It will be appreciated that the lack of scatter may afford substantial advantages even in applications such as road signs, car number plates etc. particularly in fog where such scatter might have unpredictable effects.

Reflective material embodying the invention may also, for example, have applications in advertising. Thus, for example, instead of utilising a surface coating layer of neutral-toned transparent plastics, the metallised reflect¬ ive material might, in effect, be overprinted with "ink" of differently coloured but transparent plastics, to afford a poster or the like of striking brightness. Alternatively the neutral transparent coating layer applied directly over the reflective recesses may be retained and the transparent coloured "inks" printed onto the coating layer. On a smaller scale, the reflective material embodying the invention, particularly having a small individual recess dimension and high recess density, might be used as a backing for a photographic print material similar to that sold under the trade mark "CIBA-CHROME" in which light transmitting layers in the medium are overlaid on a highly reflective backing material, the use of material made in accordance with the invention in such an application providing prints, affording an even higher brightness with high contrast combined with a full range of tone gradations.

Material embodying the invention may also, for example, be manufactured in the form of an adhesive plastics tape, provided with a permanently tacky adhesive on the side thereof remote from the reflective side so that, for example, pieces of such tape might be applied to existing road signs to modify the same or might be applied directly to the road surface, to provide temporary road signs.

Claims

1. A method of making a light reflective material comprising an array of cube-corner reflectors, as herein defined, comprising forming a master by indexing a body of deformable material, in a scanning mode, relative to a tool so that a surface of the deformable material is advanced in steps past said tool, said tool comprising a punch element having a cube corner head, as herein defined, of a hard material, such as diamond, capable of receiving an optically smooth surface finish, said punch element being mounted for movement towards and away from said surface, the punch element being connected with piezo-electric motor means whereby, at each indexing step, the punch element, more particularly said head of the punch element, can be propelled into said deformable material to form a respective cube corner recess, as herein defined, in said surface of the material, whereby in a succession of indexing steps and scans, an array of such cube-corner recesses is formed in said surface of the material, the material, bearing said array of recesses, subsequently being used as a master for the production of a mould or press tool having a comple¬ mentary surface, the method further comprising manufactur- ing said reflective material comprising an array of cube corner reflectors, by a process including using said mould or press tool to produce a product having cube-corner indentations corresponding to those of the master.

2. A method according to claim 1 wherein said mould or press tool is used to produce a body of plastics material forming said product having cube-corner indentations, and wherein the indented surface of said product is subsequently metallised, to render it light reflective.

3. A method according to claim 1 or claim 2 wherein such punch element is rotated about an axis extending in the direction of movement of the punch element towards and away from said deformable material, in opposite senses at alternate indexing steps whereby in the row of cube corner recesses formed in each scanning line, the triangular edge of each recess shares a respective side with the trangular edges of the cube corner recesses on either side.

4. A method according to claim 1 or claim 2 wherein said punch element is rotated about an axis extending in the direction of movement of the punch element towards and away from said deformable material, in opposite senses at alternate scanning lines, whereby in every other scanning line, each of the cube corner recesses formed is located between a respective pair of adjacent such recess formed in the preceding scanning line.

5. A method according to any preceding claim wherein said reflective material comprising said array of cube-corner reflectors is provided with a coating of a transparent material which fills said cube-corner recesses and affords an optically smooth outer surface parallel with the rotational surface containing the edges of said recesses, whereby light rays incident on the reflective material at an angle are refracted by said transparent material so as to pass into said reflectors closer to perpendicularity with respect to said notional surface.

6. A reflective material made by the method of any of claims 1 to 5.

7. A material according to claim 6 wherein the dimensions of said cube corner recesses, and the spacing between adjacent said recesses is in the range 30 to 100 microns.

8. A material according to claim 6 wherein the dimensions of said cube corner recesses, and the spacing between adjacent said recesses is in the range 30 to 50 microns. - 13-

9. An advertising display comprising a backing of reflective material according to claims 6, 7 or 8 and picture, text or the like applied to the reflective side o the reflective material as a coating of transparent bu variously coloured plastics material.

10. A photographic print having a reflective materia according to any of claims 6 to 8 as a light-reflective bas layer.

11. Apparatus in the form of a mould or press tool for us in manufacturing a light reflective material comprising a array of corner cube reflectors, the apparatus having bee formed by a method comprising forming a master by indexing body of deformable material, in a scanning mode, relative t a tool so that a surface of the deformable material i advanced in steps past said tool, said tool comprising punch element having a cube corner head, as herein defined of a hard material, such as diamond, capable of receiving a optically smooth surface finish, said punch element bein mounted for movement towards and away from said surface, th punch element being connected with piezo-electric moto means whereby, at each indexing step, the punch element more particularly said head of the punch element, can b propelled into said deformable material to form a respectiv cube corner recess, as herein defined, in said surface o the material, whereby in a succession of indexing steps an scans, an array of such cube-corner recesses is formed i said surface of the material, the material, bearing sai array of recesses, subsequently being used as a master fo the production of said mould or press tool having a comple mentary surface, for the manufacturing production of sai reflective material.