Retroreflective sheeting material, a method of its production and its use

A retroreflective sheeting material comprising at least one retroreflective optical system (200, 300, 400) consisting of an entrance transmission optical element (201, 301) for receiving and focusing incident electromagnetic radiation from an irradiation source, and a reflective optical element (205, 405) for reflecting the incident electromagnetic radiation back toward the irradiation source; said reflective optical element being positioned in or near the effective focal point of the transmission optical element and the space between the optical elements optionally being constituted by a spacing material (202); wherein at least one of the optical elements (205, 301, 405) is a diffractive optical element, the reflective optical element (205, 405) for substantially all angles of incidence sends at least a part of the incident electromagnetic radiation back along the direction of the incoming chief ray (204), and the reflective optical element does not exclusively consist of a specular mirror type reflective optical element; methods of making such a retroreflective sheeting material; and use of the retroreflective sheeting material for the manufacture of a sign, a marker, or a decoration all of which exhibit retroreflective reflexes upon illumination.

BACKGROUND OF THE INVENTION 
The present invention relates to a retroreflective sheeting material; 
methods of making this retroreflective sheeting material, and its use for 
the manufacture of signs, markers, and decorations. 
1. The Technical Field 
For the visible light region of the electromagnetic spectrum, 
retroreflective sheeting materials are used in a broad range of 
applications including but not limited to signs and markers of the reflex 
type. These reflex type materials provide a greater visibility at night 
compared to ordinary reflective materials such as mirror type or diffusing 
type reflectors because the retroreflected light is automatically returned 
toward the light source in a concentrated, more narrow cone in the field 
of viewing than ordinary reflected light. 
Retroreflective sheeting materials are not limited to applications in the 
visible region of the electromagnetic spectrum. They may also be applied 
e.g. in the infrared region of the spectrum to retroreflect guidance light 
for aeroplanes. Therefore, retroreflective sheeting materials may not only 
comprise retroreflective optical systems reflecting light 
retroreflectivily but may generally comprise retroreflective 
electromagnetic systems reflecting electromagnetic radiation 
retroreflectively. 
"Micro Glass Sphere-type Retroreflective Sheeting Material" 
A known retroreflective sheeting material of the "open surface" type is 
based on micro glass spheres the exposed transparent surface of which is 
not covered with a protective layer. This material is prepared by 
cascading transparent micro glass spheres with a high refractive index and 
ranging in diameter from 70 to 100.mu. onto a carrier sheet which is 
covered with a heat-softenable layer of low-density polyethylene; 
thermally sinking the micro glass spheres to about 50% of their diameters 
by passing the carrier through a tunnel oven; and coating the exposed 
surfaces of the micro glass spheres with a vapor coated aluminium layer. 
The exposed parts of the micro glass spheres are then bonded into a 
polymeric bead bond layer on a sheeting material such as textile or 
adhesive paper, and the carrier liner is stripped away. 
Functionally, these retroreflective optical systems consist of an entrance 
transmission optical element for receiving and focusing incident 
electromagnetic radiation from an irradiation source which is specifically 
constituted by the optionally transparent solid covering in combination 
with the transparent spheres refracting the incident light, and they 
further consist of a reflective optical element for reflecting the 
incident electromagnetic radiation back toward the irradiation source 
which is constituted by the transparent spacer film in combination with 
the back reflector; said reflective optical element being positioned in or 
near the focal point of the entrance transmission optical element, i.e. 
the focal point defined by spheres and spacing film in combination is 
located in or near the back surface of the spheres embedded in a 
reflective binder, or in or near the back reflector. 
Generally, a spherical type retroreflective sheeting material has a good 
retroreflectivity for small as well as for large angles of incidence. 
However, maximum retroreflectivity is obtained for an angle of incidence 
close to zero, i.e. when the light is incident normal to the plane of the 
sheeting material. For increasing angles of incidence the 
retroreflectivity slowly decreases. 
A disadvantage of the spherical type retroreflective optical system is that 
the angle of reflected rays is scattered significantly due to spherical 
aberration. Thus, a spot diagram obtained by raytracing a spherical type 
retroreflector having a refractive index of 1.9 showed that the angle of 
reflected rays scattered.+-.10 degrees for an angle of incidence of zero 
degrees. 
Ideally, a perfect retroreflection without spherical aberration can only be 
obtained for paraxial rays incident on a transparent sphere having an 
index of refraction equal to two. However, transparent materials of e.g. 
glass or plastic having an index of refraction of 2.0 are not readily 
available. 
Another disadvantage of retroreflective sheeting materials based on the 
spherical type retroreflective optical system is that the 
retroreflectivity does not vary substantially with the wavelength except 
for the chromatic aberration whereby a retroreflection of a given 
wavelength cannot directly be obtained by illumination with white light or 
with diffuse daylight. Instead, either a transparent color film or coating 
has to be applied on the top of the transparent covering on the spheres or 
on the flat front face, or the individual spheres have to be coated with a 
transparent color film or coating. 
Also, it is a disadvantage that such retroreflective sheeting materials 
comprising several retroreflective optical systems designed for different 
wavelengths cannot easily be produced. 
A further disadvantages of retroreflective sheeting materials based on the 
spherical type retroreflective optical system is that the spheres cannot 
be packed infinitely close whereby the efficiency of retroreflectivity of 
the sheeting material becomes limited by the packing density of the 
spheres. 
Further disadvantages comprises requirements of high purity of the 
transparent material of e.g. glass or plastic for the spheres, and 
requirements of the accuracy of the spherical shape of the spheres and 
their fixation in the sheeting material all of which requirements increase 
the costs of the final product. 
Also, the method of making the final retroreflective sheeting material is 
complicated and it involves laborious know how and unique equipment for 
each step which makes the final product even more expensive. 
"Micro Prisms Embossed Retroreflective Sheeting Material" 
Another type of retroreflective sheeting material is based on micro prisms 
embossed closely spaced and parallel to the front face of a plastic foil 
of e.g. polyester or polyvinylchloride. This material comprises a 
retroreflective optical system consisting of a 60 degrees three-sided 
pyramid which provides a perfect reflection for a small angle of 
incidence, but the reflection efficiency of which declines rapidly at 
greater angles. 
Also, there is no symmetry of rotation of the reflection whereby the 
material exhibits an uneven retroreflected energy pattern. 
A further disadvantage is that a high efficiency of reflection can only be 
obtained when the embossed prisms have perfectly shaped angles which is 
difficult to achieve unless the embossing is performed in a stepwise 
manner which allows the plastic foil to cool from a temperature of about 
200.degree. C. at the beginning of embossing until the mould can be 
released without injuring the embossments. 
"Fresnel Zone Plates Embossed Retroreflective Sheeting Material with 
Specular Reflecting Back Surface" 
Still another type of retroreflective sheeting material is based on Fresnel 
zone plates embossed in the surface of a transparent medium, the back 
surface of which has a specular reflecting coating. 
This retroreflective material comprises a diffractive entrance optical 
element in form of a Fresnel zone plate and a specular reflecting mirror 
on the back surface of the transparent medium. 
A disadvantage is that even for relative small angles of incidence, the 
specular reflecting mirror will give rise to a large loss of 
electromagnetic radiation, because the specular reflection guides a large 
fraction of the reflected electromagnetic radiation in directions 
different from that of the retroreflective direction. 
The incoming light will be focused by the Fresnel zone plate on the 
specular reflective back surface, but the law of reflection for this 
specular reflecting back surface will even for small angles cause the 
reflection of the incoming focused light to miss the Fresnel zone plate 
through which it entered and probably be sent in a direction towards a 
neighbour Fresnel zone plate positioned nearby. Generally, only 
electromagnetic radiation exiting through the same Fresnel zone plate as 
it entered will be diffracted by this Fresnel zone plate back towards the 
irradiation source, i.e. the retroreflective direction. Therefore, this 
retroreflective material has a significantly lower efficiency for angles 
of incidence which are not very close to the optical axis. 
2. Prior Art Disclosure 
U.S. Pat. Nos. 2,354,018 and 2,354,049 disclose the basic prior art reflex 
reflectors based on transparent spheres and a flat back reflector. 
U.S. Pat. No. 2,407,680 discloses an optical sheet adapted to be associated 
with and produce reflex light reflection from a reflecting surface, 
including a light-returning layer formed of a large number of contigous 
small transparent spheres whose back extremities are optically exposed for 
rearward passage of lights rays, and a continous overlying transparent 
solid covering united and conforming to the front extremities of said 
spheres and having a flat front face; said spheres having a refractive 
index at least 1.15 times that of said transparent covering. 
U.S. Pat. No. 3,758,192 discloses a reflex-reflecting structure comprising 
a plurality of layers in which the outermost layer comprises a monolayer 
of glass beds, preferable having diameters within the range of 25 to 250 
microns and a refractive index of at least 1.8. The beads are 
approximately hemispherically embedded in a binder material such as a 
substantially clear and colorless resin. In order to provide an efficient 
reflex-reflective material which tends to approach the efficiencies 
obtained by using aluminium as reflective material and at the same time 
avoid undesirable "off-colors", the binder contains specularly reflective 
nacreous pigment particles which have a maximum dimension size falling 
within the range of 8 to 30 microns but less than the diameter of the 
glass beads and a thickness within the range of 25 to 200 nanometers, the 
binder preferably containing at least about 15% by weight of pigment based 
on the total vehicle solids. 
U.S. Pat. Nos. 4,244,683; 4,332,847; 5,171,624; and GB Patent Application 
No. 2 245 194A disclose retroreflective microprismatic materials, and 
methods and apparatus of making the same, e.g. by compression moulding. 
Nothing is indicated or suggested about the retroreflective optical 
systems of these materials being based on diffractive optical elements. 
U.S. Pat. Nos. 4,036,552 and 3,993,401 disclose a retroreflective material 
including a transparent medium containing a plurality of diffraction 
elements comprising light interference fringe patterns and a method of 
making such a retroreflective material comprising providing a stamper for 
embossing an array of phase modulated Fresnel zone plates having a given 
focal length; embossing phase modulated Fresnel zone plates at one surface 
of a transparent copy medium having a thickness equal to said focal 
length; and coating a specular reflective surface on the opposite surface 
of said copy medium. 
DISCLOSURE OF THE INVENTION 
Objects of the Invention 
It is the object of the present invention to provide a retroreflective 
sheeting material comprising a retroreflective electromagnetic system 
which selectively returns the incident radiation in an angle which is 
substantially the angle of incidence but which returned radiation 
propagates in the opposite direction; particularly a retroreflective 
optical system which has a reduced spherical aberration for a given 
wavelength with respect to a micro glass spherical-type retroreflective 
sheeting material, and improved retroreflection for angles far from the 
optical axis with respect to Fresnel zone plate embossed retroreflective 
sheeting materials with specular reflecting back surfaces. 
It is another object of the present invention to provide such a 
retroreflective sheeting material for which the reflectivity is optimized 
for a desired angle of incidence and not necessarily for an angle normal 
to the sheeting material as for conventional retroreflective sheeting 
materials. 
It is still another object of the present invention to provide such a 
retroreflective sheeting material which comprises several retroreflective 
optical systems designed for different wavelengths. 
It is a further object of the invention to provide such a retroreflective 
sheeting material for which the retroreflective optical system can be 
positioned in a controlled manner and with a higher packing density than 
for spherical type retroreflective sheeting materials. 
It is a still further object of the invention to provide such a 
retroreflective sheeting material which is cheaper and easier to produce 
than conventional retroreflective sheeting materials. 
It is also the object of the present invention to provide methods of making 
such retroreflective sheeting materials and use thereof. 
Further objects appear from the description. 
A. Retroreflective Sheeting Material 
Solution and Advantageous Effects of the Invention 
According to the invention, these objects are fulfilled by providing a 
retroreflective sheeting material comprising at least one retroreflective 
optical system consisting of: 
a) an entrance transmission optical element for receiving and focusing 
incident electromagnetic radiation from an irradiation source, and 
b) a reflective optical element for reflecting the incident electromagnetic 
radiation back towards the irradiation source; 
the reflective optical element being positioned in or near the effective 
focal point of the transmission optical element; and the space between the 
optical elements optionally being constituted by a spacing material; 
characterized in 
c) that at least one of the optical elements is a diffractive optical 
element, 
d) that the reflective optical element for substantially all angles of 
incidence sends at least a part of the incident electromagnetic radiation 
back along the direction of the incoming chief ray, and 
e) that the reflective optical element does not consist exclusively of a 
specular mirror type reflective optical element. 
It is intended that the term "retroreflective optical system" should be 
interpreted broadly to cover retroreflective electromagnetic systems not 
only in the visible range of the electromagnetic spectrum but also in the 
far ultraviolet and far infrared regions. 
It has surprisingly turned out that spherical aberration of the 
retroreflective optical system of spherical type retroreflective sheeting 
materials can advantageously be improved by wholly or partially applying 
diffractively based optical elements instead of refractively based optical 
elements. 
According to the invention, if at least one of the optical elements i.e. 
the entrance transmission optical element, the reflective optical element, 
or both, is a diffractive optical element, it is obtained that spherical 
aberration of the retroreflective system can be completely reduced for 
small angles of incidence and significantly reduced for larger angles. 
Since the diffraction efficiency of a diffractive optical element can be 
optimised for a given angle of incidence, it is possible to optimize the 
retroreflective system comprising at least one diffractive optical 
focusing element to a given angle of incidence. 
Retroreflection is ensured for substantially all angles of incidence by the 
fact that the reflective optical element sends the incident 
electromagnetic radiation back through the entrance transmission optical 
element by non-specular reflection. 
Several retroreflective optical systems designed for different wavelengths 
can be incorporated in the same sheeting material whereby e.g. reflex 
signs in different colors can be provided in the same retroreflecting 
sheeting material; said reflex signs either being reflected in different 
colors at a given angle of incidence, or being reflected in different 
colors at different angles of incidence. This cannot be obtained by prior 
art retroreflective optical systems. 
Further, the retroreflective optical systems can be produced with a 
hexagonal, squared, or triangular shape which provides for a close packing 
density. 
Particularly for a pure diffractive retroreflective optical system, 
refractive optical elements can be avoided whereby the requirements of 
e.g. micro beads of high refractive index and well-defined shape is 
avoided which makes the retroreflective materials according to this 
embodiment of the invention cheaper than conventional retroreflective 
sheeting materials. 
Preferred Embodiments 
"Entrance Transmission Optical Element" 
According to the invention, the entrance transmission optical element can 
be any suitable transmission optical element which is able to receive the 
incident electromagnetic radiation and to focus the received radiation 
onto the reflective optical element. 
In specific embodiments according to the invention, the entrance 
transmission optical element can be a refractive spherical or aspherical 
optical element, or a diffractive focusing optical element such as a 
Fresnel diffractive structure like a Fresnel zone plate, a Fresnel phase 
plate, or a blazed Fresnel phase plate. 
In a preferred embodiment according to the invention, the entrance 
transmission optical element is a focusing transmission diffractive 
optical element. 
"Reflective Optical Elements" 
According to the invention, the reflective optical element can be any 
suitable reflective optical element which is able to reflect the incident 
electromagnetic radiation back towards the irradiation source, so that for 
substantially all angles of incidence the reflective optical element sends 
the incident electromagnetic radiation back through the entrance 
transmission optical element by non-specular reflection. 
Such reflective optical elements comprise diffractive optical elements 
which consists of a substrate having a diffractive pattern either embedded 
therein or disposed thereon and a reflective layer or coating attached 
either directly or spaced thereto by a spacing material. 
When a spacing material is applied between the diffractive optical element 
and the reflective layer, the thickness of the spacing material is 
preferably chosen such that the distance from the diffractive optical 
element to the reflective layer plus the distance from the reflective 
layer to the entrance transmission optical element equals the effective 
focal length of the diffractive optical element. 
In a preferred embodiment according to the invention the reflective optical 
element consists of a diffractive optical element having a reflective 
optical coating coated on its back boundary. 
In another preferred embodiment according to the invention, the reflective 
optical element is a transmission diffractive optical element having said 
reflective optical coating coated on its back boundary. 
In another embodiment according to the invention, the reflective optical 
element is a diffractive optical element consisting of a transmission 
diffractive element exhibiting a significant reflection; said element 
being composed of a material having a refractive index differing from that 
of the front and back boundaries of the element. 
In still another embodiment according to the invention, the reflective 
optical element is a refractive spherical or aspherical mirror, or a 
diffractive mirror such as a Fresnel diffractive structure like a Fresnel 
zone plate, a Fresnel phase plate, or a blazed Fresnel phase plate. 
In a preferred embodiment, the reflective optical element is a diffuse 
reflective optical element. 
A uniform diffuse reflective optical element has the advantage in the 
production setup that no alignment of the entrance transmission optical 
element with respect to the diffuse reflective optical element is 
necessary. If only the thickness of the spacing material is near the 
effective focal length of the focusing entrance transmission optical 
element retroreflection will be ensured. 
"Diffractive Optical Elements" 
According to the invention, the diffractive optical element can be any 
suitable diffractive optical element known in the art such as, but not 
limited to, amplitude or phase holograms, or surface relief holograms. 
Further, the diffractive optical elements can have suitable diffraction 
patterns generated by any known method in the art such as, but not limited 
to, 
i) methods of producing amplitude holograms interferometrically in a 
photographic film such as a silverhalogenide-film without bleaching; 
ii) methods of producing phase holograms interferometically in a 
photographic film such as a silverhalogenide-film with bleaching, or in a 
photopolymer; or 
iii) methods of producing surface relief (phase) holograms lithographically 
or interferometrically in a suitable recording medium such as positive or 
negative resists, e.g. electron resists or photoresists; 
the latter methods of producing surface relief holograms being generally 
preferred, because surface relief holograms can be mechanically replicated 
by e.g. thermo-mechanical embossing of a thermoplastic film from an 
electroformed (nickel) master (hologram) or casting of a fluid 
thermoplastic or an UV--or thermosetting plastic, or other suitable 
precursor therefor, wherein the master forms a part of the moulding form; 
see e.g. R. A. Bartolini et al., "Replication of Relief-Phase Holograms 
for Prerecorded Video", J. Electrochem. Soc.: Solid-State Science and 
Technology, Vol. 120, No. 10, October 1973, 1408-1413, the contents of 
which is hereby incorporated by reference. 
Plates carrying the master holograms can be given a cylindrical shape and 
be mounted onto embossing rolls which allows for continuous embossing into 
softened suitable plastics such as, but not limited to, vinyl, 
polycarbonate, mylar or cellulose esters; see e.g. J. J. Cowan and W. D. 
Slafer, "Holographic Embossing at Polaroid: The Polaform Process", SPIE 
Vol. 100, Progress in Holographic Applications, 1985, 49-56. 
The master holograms can alternatively be given a flat shape and be mounted 
in a plate press that allows for a semicontinuous embossing, see e.g. U.S. 
Pat. No. 4,244,683 disclosing apparatus for compression moulding of 
retroreflective sheetings having a retroreflective optical system of the 
micro prism type. 
In a preferred embodiment according to the invention, the diffractive 
optical elements comprise surface relief diffraction patterns. 
"Positioning of the Entrance Transmission Optical Element with Respect to 
the Reflective Optical Element" 
According to the invention, the reflective optical element is positioned in 
or near the effective focal point of the transmission optical element. 
Accordingly, the pair of optical elements, i.e. the entrance transmission 
optical element and the reflective optical element, are designed for a 
given mutual distance between the elements which in turn depends on the 
desired thickness of the sheeting material. 
The optical elements may be orientated in any suitable direction, i.e. the 
orientations of the center axes of the two optical elements may be 
designed to have any suitable directions. 
In preferred embodiments according to the invention, the orientation of the 
center axis of the entrance transmission optical element with respect to 
the orientation of the center axis of the focusing reflective optical 
element is selected from the group of axis orientations consisting of 
non-parallel axes, substantially parallel axes, or substantially 
coinciding axes. 
It is preferred that the axes are substantially parallel and closely 
spaced, particularly substantially coinciding. 
"Spacing Materials" 
The spacing material may be any suitable spacing material known in the art. 
In a preferred embodiment according to the invention, the space between the 
optical elements, the space between the transmission diffractive optical 
element and the reflective coating of the reflective optical element, or 
both, are constituted by a spacing material selected from the group 
consisting of dielectric or substantially dielectric materials such as 
glass, hard plastic, soft plastic, air, or combinations of these. 
"Dye Additives" 
The retroreflective sheeting material may be colored by addition of any 
suitable dye known in the art. 
In a preferred embodiment according to the invention, at least one of the 
entrance transmission optical element, the reflective optical element, the 
diffractive element with reflective optical coating, the reflective 
optical coating, the diffuse reflective optical element, or the spacing 
materials contain one or more dyes. 
Also, the carrier of the optical elements and/or the spacing material can 
be pre-dyed in any transparent color or fluorescent transparent color. 
When it is desired to produce multi colors or images, the surface of the 
sheeting material can be printed imagewise by screen-printing or 
flexoprint. When a protective layer is applied, this material can be 
pre-printed in any desired pattern. 
If the colors are to be reflective by night appearance, the ink has to be 
transparent. When only colored day time appearance is wanted, an ordinary 
non-transparent ink can be used. 
A preferred ink used by the serigraphic industry and supplied in 
transparent as well as non-transparent form consists of cyclohaxanon, 
1-methoxy-2-propylacetate, 3-methoxy-n-butylacetate, 
ethyl-3-ethoxypropionate, and C.sub.9 -C.sub.10 aromates. It is sold under 
the trademark Bargoscreen.TM. 2K-6500. 
"Protective Layer" 
The retroreflective sheeting material may be covered by a protective layer 
consisting of any suitable transparent material. 
In a preferred embodiment according to the invention, a protective material 
is coated on the front boundary of the entrance transmission optical 
element, the back boundary of the reflective optical element, or both. 
When a protective layer is applied to surface relief diffractive optical 
elements, it is preferred that the diffractive pattern is deeper than the 
diffractive pattern when a protective layer is not applied. The depth of 
the diffraction pattern can be optimized by the skilled person either 
imperically or by diffraction efficiency calculation according to 
"Diffraction Analysis of Dielectric Surface Relief Gratings", M. G. 
Moharam and T. K. Gaylord, Journal of the American Optical Society, Vol. 
72, No. 1, Nov. 1982, 1385-92 the contents of which is hereby incorporated 
by reference. 
"Carrier Material" 
The retroreflective optical system may be supported by any suitable carrier 
known in the art such as textile web or adhesives, optionally 
self-sticking adhesives. 
In many applications the carrier is the sheeting material itself. 
B. Methods of Making Retroreflective Sheeting Materials 
In another aspect, the present invention also related to methods of making 
retroreflective sheeting materials. 
"Joining Two Sheeting Substrates" 
The optical elements may be provided on the respective first and second 
sheeting substrates by any method known in the art as mentioned above. 
Then the first and second sheeting substrates are joined by any suitable 
joining technique in the art comprising but not limited to adhesion. 
It is preferred that the reflective optical element is a diffractive 
optical element or a diffuse reflective optical element. 
"Processing of One Sheeting Substrate" 
According to this method the optical elements are provided on the first and 
the opposite side of a sheeting substrate. 
It is preferred that the reflective optical element is a diffractive 
optical element or a diffuse reflective optical element. 
"Embossing of Surface Relief Patterns in Opposite Surfaces of a Sheeting 
Substrate" 
It is preferred that the surface relief pattern of the second embossing 
matrice is a diffracting pattern or a diffuse reflection pattern. 
According to this method the surface relief diffraction pattern is provided 
in the desired optical element by any suitable method known in the art, 
see the explanations of "Diffractive Optical Elements" above and "Sheeting 
Substrate Materials" to follow. 
Further, the embossing matrices of the master holograms of the surface 
relief diffraction patterns are provided by any suitable method known in 
the art such as electroplating by a suitable metal, e.g. nickel, to 
produce a hard material which can be used as an embossing matrix, e.g. a 
cylindrical embossing roll or a flat embossing plate press. 
Embossing of the relief diffraction patterns of the entrance transmission 
optical element and of the reflective optical element in the sheeting 
material may take place either simultaneously or subsequently. 
In a preferred embodiment according to the invention, the embossing step 
consists of thermo-mechanical embossing the pairs of surface relief 
diffractive patterns simultaneously in opposite surfaces of the sheeting 
material. 
In a preferred embodiment according to the invention, the embossing step 
consists of a continuous roll-to-roll process. 
The embossing in the sheeting material may be carried out at any suitable 
temperature dependence on the sheeting materials. 
"Positioning Control" 
In replicating two surface relief diffractive optical elements on opposite 
sides of a sheeting substrate it is important to provide an accurate 
positioning of the surface relief diffraction patterns with respect to 
each other. 
For a pair of surface relief diffractive optical elements having a diameter 
of 340 .mu.m and an embossed profile depth of 1 .mu.m, and a 
retroreflective sheeting polymer foil having a thickness of 100 .mu.m, the 
accuracy must be better than 10 .mu.m, preferably about 5 .mu.m. 
In order to obtain this positioning accuracy any suitable position 
controlling method known in the art may be applied. 
In a preferred embodiment according to the invention, the position control 
comprises: 
a) embossing position markers in the sheeting substrate; 
b) measuring relative displacements in predetermined directions of the 
position markers; and 
c) adjusting the positions of the embossing matrices, the sheeting 
substrate, or both, when the measured relative displacements of the 
position markers exceed predetermined limits. 
In a preferred embodiment according to the invention, the position markers 
are in form of triangles. 
In still another preferred embodiment according to the invention said 
embossing matrices form the parts of a moulding form wherein the sheeting 
substrate in form of a fluid thermoplastic or an UV- or thermosetting 
plastic, or any suitable precursor therefor, is moulded. 
"Sheeting Substrate Materials" 
According to the invention, the sheeting substrate material can be any 
suitable sheeting material known in the art such as UV-curable polymers. 
Such materials comprise, but are not limited to, polyethylene, 
polymethylmethacrylate, polyvinylchloride, polycarbonates, polyesters 
including acrylated epoxy oligomers, polyurethanes and acrylated 
urethanes. 
Further, suitable sheeting substrate materials are disclosed in e.g. GB 
Patent Application No. 2,245,194A and U.S. Pat. No. 4,576,850, the 
contents of which are hereby incorporated by reference. 
C. Use of Retroreflective Sheeting Materials 
Still another aspect of the invention is the use of retroreflective 
sheeting material according to the invention for suitable purposes 
involving retroreflection, particularly retroreflection with a reduced 
spherical aberration for a given wavelength and retroreflection optimized 
for a desired angle of incidence. 
In a preferred embodiment according to the invention, the retroreflective 
sheeting material is used for the manufacture of a sign such as a road 
sign, a marker such as a sticker, or a decoration, said sign, marker or 
decoration exhibiting retroreflective reflexes upon illumination. 
Further signs or markers include but are not limited to signs for highway, 
traffic, and runway signs; post and barrels markers; and warning signs. 
Definition of Expression 
Within the present context it is intended that the term "electromagnetic 
radiation" designates electromagnetic radiation that can be diffracted by 
a suitable diffractive element, particularly electromagnetic radiation 
generally having a wavelength in the range from 10 nm to 100 .mu.m, 
preferably in the range from 100 to 1000 nm, particularly in the visible 
range of the electromagnetic spectrum from about 400 to about 700 nm, and 
which can be diffracted by a suitable diffractive optical element designed 
to function at desired wavelengths. Hence the concept "diffractive optical 
element" should be interpreted broadly to include diffractive elements 
suitable for electromagnetic radiation of other wavelengths than those 
typically considered to be in the "optical" range. 
Within the present context, it is intended that the term "retroreflection" 
designates reflection of incident electromagnetic radiation in a direction 
which is substantially opposite to the direction of the incident 
electromagnetic radiation; said direction being either coinciding or 
parallel with the direction of incidence. Thus, retroreflection 
selectively returns the incident radiation in an angle which is 
substantially the angle of incidence but which reflected radiation is 
propagating in the opposite direction. This selective reflection of 
electromagnetic radiation back in the angle of incidence is different from 
the reflection of mirror type reflectors reflecting the incident light 
specularly so that the angle of incidence equals the angle of reflection. 
Also, this selective reflection is different from the reflection of the 
diffusing type reflectors reflecting the incident radiation in all 
directions. 
Within the present context it is intended that the term "spherical 
aberration" designates the focusing defect of not bringing all parallel 
rays to a focus at a unique point. 
Within the present context it is intended that the angle of incidence 
designates an angle of zero degrees when the incident electromagnetic 
radiation propagates normally to the surface of the sheeting substrate. 
Within the present context it is intended that the term "sheeting material" 
designates a continuous thin sheet or film material which is suitable for 
carrying the optical elements of the retroreflective optical system 
according to the invention. 
Within the present context it is intended that the term "chief ray" 
designates the geometrical ray of electromagnetic radiation originating 
from an illumination source and passing through the center of the limiting 
aperture, which in all preferred embodiments is the aperture of the 
entrance transmission optical element.

DETAILED DESCRIPTION 
A. Retroreflective Sheeting Material 
"Prior Art Retroreflective Sheeting Materials" 
Referring to FIG. 1A there is shown a cross-sectional sketch of a 
retroreflective sheeting material 100 of the spherical type according to 
the prior art. 
Glass/plastic spheres 101 are partly covered with a reflective layer 102 
and partly imbedded in a sheeting material 103. The part of the spheres 
not covered with a reflective layer is optionally covered with a 
protective layer 104. Incident light 105 entering the protective layer 104 
is transmitted through the spheres 101 and reflected by the reflective 
layer 102. The reflected light 106 is then (ideally) reflected in the 
opposite direction of the incident light 105. 
For clarity reasons, an opaque or absorbing barrier coating between the 
spheres is not shown. 
Referring to FIG. 1B there is shown a cross-sectional sketch of a 
retroreflective sheeting material 100b of the Fresnel zone plate type with 
a specular reflecting coating. 
Fresnel zone plates 110, 111 are embossed in the front surface of a 
transparent sheeting substrate 112 facing the incident light 113, 113b, 
and a specular reflective mirror 114 is coated on the rear surface of the 
substrate; said substrate having a thickness corresponding to the focal 
length of the Fresnel zone plates 110, 111. 
For the shown angle of incidence, none of the incident light 113, 113b will 
be reflected retroreflectively directly back along the incident chief ray 
113b by the specular reflecting mirror 114. Some of the light 113 will be 
reflected back in ray 115. 
It is also shown that some of the reflected light 115b will miss the 
specific Fresnel zone plate 110 through which it entered and therefore be 
sent back in a direction different from the retroreflective direction. 
"Preferred Embodiment of Retroreflective Sheeting Materials According to 
the Invention" 
Referring to FIG. 2 there is shown a cross-sectional sketch of a section of 
a preferred embodiment of a retroreflective sheeting material 200 
according to the invention comprising an entrance transmission optical 
element consisting of half-spherical lenses 201 embedded in a sheeting 
substrate 202 which at its back side opposite the side of incident light 
203, 204 and vis-a-vis the half-spherical lenses has reflective optical 
elements consisting of a reflective diffractive optical element 205 (or 
reflective holographic optical element), including a diffractive surface 
relief pattern 206 and a reflective coating 207; said reflective coating 
207 may fully or partly cover the back surface of the sheeting substrate; 
said reflective coating 207 may fully or partly cover the back surface of 
the sheeting substrate. The reflected light 208, 209 is reflected back 
towards the irradiation source by having the reflective diffractive 
optical element 205 sending at least a part 208 of the incident 
electromagnetic radiation back along the direction of the incoming chief 
ray 204. The space 210 between the half-spheres or reflective diffractive 
optical elements can have a suitable barrier or diffuse reflecting coating 
(not shown), if desired. 
Referring to FIG. 3 there is shown a cross-sectional sketch of a section of 
a preferred embodiments of retroreflective sheeting material 300 according 
to the invention comprising an entrance transmission optical element 301, 
consisting of a transmission diffractive optical element comprising a 
surface relief diffractive pattern 302 at the front side of a sheeting 
substrate 202 and a reflective optical element consisting of a reflective 
diffractive optical element 205 including a diffractive surface relief 
pattern 206 and a reflective coating 207 at the back side of the sheeting 
substrate 202 opposite to the entrance transmission optical element 301. 
As in FIG. 2 at least a part 208 of the incident electromagnetic radiation 
is sent back along the direction of the incoming chief ray 204. 
Referring to FIG. 4A-D there is shown a cross-sectional sketch of a section 
of a preferred embodiment of a retroreflective sheeting material 400 
according to the invention comprising an entrance transmission optical 
element 301 consisting of a surface relief diffractive pattern at the 
front side of a sheeting substrate 202 and a diffuse reflective optical 
element 405 at the back side of the sheeting substrate 202 opposite to the 
entrance transmission optical element 301. 
The principle of the retroreflective sheeting material is shown for two 
angles of incidence. In FIGS. 4A and 4B the principle is shown for a 
relative small angle of incidence, and in FIGS. 4C and 4D the principle is 
shown for a relative large angle of incidence. The focusing effect of the 
entrance diffractive optical element 301 is illustrated in FIGS. 4A and 4C 
and the diffuse back reflection from the diffuse reflective optical 
element 405 is illustrated in FIGS. 4B and 4D. 
A near Lambertian irradiation distribution is indicated by the ellipsoid 
406 in FIGS. 4B and 4D. This diffuse non-specular reflection ensures that 
at least a part 208 of the incident electromagnetic radiation is send back 
through the entrance optical element 307 along the direction of the 
incoming chief ray 204. 
B. Methods of Making Retroreflective Sheeting Materials 
A preferred total retroreflective optical system of the purely diffractive 
type according to the invention comprises a focusing transmission 
diffractive optical element, a spacing material/sheeting substrate and a 
reflective diffractive optical element. 
In an embodiment the total retroreflective optical system can be produced 
by separately producing the focusing transmission diffractive optical 
element on a first sheeting substrate and the reflective diffractive 
optical element on a second sheeting substrate followed by joining the two 
sheeting substrates, optionally with a spacing material there between. 
A diffractive optical element incorporated in a sheeting material according 
to the present invention can be produced by a lithographic production 
process which is useful for the production of a retroreflective optical 
system for any desired wavelength. 
Also, it can be produced by an interferometric production process which is 
the simplest way of producing retroreflective optical systems for 
wavelengths in the 400-500 nm region. 
When optimum retroreflection is desired for normal incidence, the process 
of production is greatly simplified by the fact that the two diffractive 
elements can be made from the same mathematical calculations (in the 
lithographic method as described below) or in the same geometrical setup 
(in the interferometric method as described below). The diffractive 
structures in the two diffractive elements will be identical except that 
the depth of the surface profiles or otherwise diffractive material will 
be different. The optimum depth of the diffractive structures can be 
achieved by a skilled person either empirically or by using a diffraction 
efficiency calculation as described in "Diffraction Analysis of Dielectric 
Surface-Relief Gratings" by M. G. Moharam & T. K. Gaylord in Journal of 
Optical Society of America, Vol. 72, No. 10, November 1982, p. 1385-1392. 
The production processes of diffractive optical elements described below 
can therefore be used for producing both the focusing transmission 
diffractive optical element and the reflective diffractive optical element 
in the sheeting substrate, with the exception that the optimum depth of 
the diffraction material is different for the two elements. 
Furthermore, a reflective layer is preferably applied to the reflective 
diffractive optical element thereby maximizing the diffraction efficiency 
of the reflection orders of diffraction. This reflective layer can be 
applied by sputtering on a metal layer by use of e.g. an equipment as the 
Diode Sputter Coater SC510 supplied by Bio Rad Micro-Science Division or a 
similar supplier. 
The process of transferring the diffractive pattern from the photoresist 
surface to the sheeting substrate is described below in the section 
"Replication of Diffractive Optical Elements". 
"Test Element" 
For illustration of the production of a purely diffractive type 
retroreflective sheeting material, a test element was designed for best 
use with a wavelength of 550 nm. A fused silica substrate having a 
refractive index of 1.46 and a thickness of 200 .mu.m was used. An array 
of 14.times.14 diffractive Fresnel zone patterns each of about 300 
micrometer squared was used both as front and back side optical elements. 
An image of the produced entrance focusing transmission diffractive optical 
element obtained by scanning electron microscopy of a diffractive optical 
element produced by lithographic means is shown in FIG. 5. Only a part 
near the centre of the diffractive optical element is shown. A scale unit 
501 of 1 micrometer indicates the size of the surface relief structures. 
Both the surface relief structure 502 and the substrate material 503 are 
fused silica. 
The test element is not intended for replication applications. Therefore, 
the diffractive optical elements were both incorporated on both sides of 
the fused silica substrate. 
"Substrate Preparation" 
Alignment marks aligned from front to back side of the silica substrate was 
needed in order to achieve the required positioning accuracy in the 
following e-beam writing. The front array of diffractive optical elements 
should preferably be aligned with an accuracy in the range of 5 .mu.m to 
the back array of diffractive optical elements. 
Firstly, a standard photomask with the desired alignment marks was made by 
making a 10 to 1 reduction copy of a simple overhead film with the desired 
alignment marks printed in a LaserJet IIIP from Hewlett Packard. This 
reduction was made in a reduction camera using a standard photographic 
reduction lens. The exposure was made on HDP plates from the company KODAK 
and developed and fixed in accordance with the prescriptions from KODAK. 
The photomask consisted of 2 crosses placed with about 27 mm distance. The 
lines in the crosses were about 5 .mu.m thick near the center of the 
crosses and about 1 mm in the line ends farthest away from the center. 
This gave an easier detection of the alignment marks, but is not essential 
for good results. The marks were transmitting for visible and UV 
wavelengths, while the rest of the photomask was absorbing. 
The substrate was coated on the front side with a photoresist 5206 from the 
company Hoechst. This was done by cleaning, spin coating and baking in 
accordance with prescriptions from the photoresist supplier. This resist 
can be used both as positive and as a negative resist. In this case 
prescriptions for use as a positive resist were used. 
The photomask with the two alignment marks was used as mask plate and the 
photoresist coated substrate as wafer in a mask aligner AL 6-2 from the 
company Electronic Visions Corporation, Scharding, Austria. Exposure was 
performed in accordance with the prescriptions from the manufacturer. For 
a resist thickness of 0.5 .mu.m an exposure time of 15 seconds was found 
to give the best results. After exposure the substrate was developed in a 
Microposit 351 developer from the company Shipley, in accordance with the 
prescriptions from the resist supplier. This gave a transferred pattern of 
alignment marks, wherein the crosses now appear as holes in the resist, as 
the developer has removed resist material. 
The substrate with the now exposed front side was placed in a thermal 
evaporation system from the company AVAC for depositing first a thin 50 
.ANG. layer of Cr and then a 200 .ANG. layer of Au. The Cr was needed for 
binding the Au to the silica substrate, and the Au gave alignment marks 
which later could be seen in the scanning electron microscope mode of the 
e-beam writer. After evaporation the substrate was given an ultrasonic 
bath in acetone, which dissolved the photoresist, giving a lift off 
process, where only the exposed alignment mark crosses were Cr/Au coated. 
The rest of the Cr and Au was lifted off with the dissolved resist. 
This finished the preparation of the first side. The second side was 
prepared in the same way, by exposing in the mask aligner with the second 
side towards the mask and light source, but with the mask aligned to the 
now visible Cr/Au marks on the first side of the substrate. This could be 
done in the AL 6-2 mask aligner as the equipment has the necessary front 
to back alignment system. This was essential for the making of this 
prototype, and it must be noticed that most present mask aligners do not 
have this facility. 
Now, with the substrate with well-aligned alignment marks on front and back 
side, the substrate was prepared for the electron beam and etching 
process. This was done by spincoating front side of the substrate with a 
Novolak based photoresist such as the Microposit 1400 series from Shipley 
and afterwards baking the substrate for 45 minutes at 225.degree. C., 
which gave a very hardbaked photoresist. This layer would later function 
as an etching mask for the etching of the diffractive structures in the 
silica. On top of the hardbaked photoresist of 500 .ANG. layer of Ge was 
evaporated in a vacuum evaporator such as the AVAC evaporator. This layer 
would later serve as a conducting layer in the e-beam process as well as 
an etching mask for etching the diffractive structures in the photoresist. 
Again on top of this a 150 nm layer of e-beam resist such as the SAL 601 
was spincoated. This 3 layer resist/Ge/resist has given the best results 
for achieving deep small feature size etching in fused silica, but other 
materials could be used. After exposure and etching on the front side as 
described below, the 3 layer resist/Ge/resist was coated and evaporated on 
the back side. Exposure and etching of the back side was then made as the 
first exposure on the front side. 
"Data Preparation" 
A Fresnel zone pattern for the e-beam fabrication was prepared using the 
JEOL language. The pattern was made of commands resulting in concentric 
circular rings. The Fresnel zone pattern was chosen to give a Fresnel zone 
diffractive lens having an effective focal length in silica--the used 
sheeting material--equal to the thickness of the silica substrates. This 
was calculated according to standard Fresnel zone plate equations as given 
in "OPTICS", 2nd edition by Eugene Hecht, Adelphi University, ISBN 
0-201-11611-1, p. 445+. 
This Fresnel zone pattern is a diffractive pattern that ensures that for 
substantially all angles of incidence at least a part of the incident 
light is sent back along the direction of the incoming chief ray. 
"E-beam Fabrication" 
After reading the pattern data into the file server attached to the e-beam 
equipment, calibration of the e-beam equipment was done in accordance with 
the prescriptions from the manufacturer. The data was loaded into the 
exposure computer system, and after loading the prepared substrate the 
alignment marks were found on the front (first) side of the substrate by 
using the scanning electron microscope mode of the equipment. By reading 
the absolute positions of the alignment marks on the equipment laser stage 
control and by feeding these to the computer in accordance with the 
manuals of the system, a correct positioning of the pattern on the front 
side could be made automatically by the exposure computer system. The 
exposure was started and in this case with a 5.times.5 mm array or 
diffractive lenses an exposure time of about 5.5 h was needed. After 
exposure the substrate was postexposure baked for 20 minutes at 
110.degree. C. and developed for 2 min in a standard developer from 
Shipley (named "Developer") for Novolak based e-beam resist. This removed 
the non-exposed resist, which leaved the exposed diffractive structures as 
rings of e-beam resist on top of the photoresist/Ge layers on the silica. 
After transferring these structures to the silica by etching as described 
below the substrates second (back) side was prepared with the 3 layer 
(resist/Ge/resist) system, and exposure in e-beam, baking and development 
was made on this second side in the same way as described above. The 
second side diffraction structures were then transferred to the underlying 
silica in the same way as the first side, as described below. 
"Pattern Transfer" 
The exposed diffractive structures in the e-beam resist were transferred to 
the underlying layers of Ge, photoresist and finally to the silica 
substrate by reactive ion etching. All reactive ion etching was done in a 
Vacu Tec Plasma System installed with CHF.sub.3, SF.sub.6 and O.sub.2 
gasses. The pressure and effect parameters of the system are critical for 
getting a good anisotropic etch and it should be noted that the best 
parameters can be quite different from equipment to equipment. A skilled 
person should make a series of tests to find the best parameters for each 
specific system. It should be noted that a low pressure and a high effect 
gives a very anisotropic etching with vertical sidewalls in the etched 
structures, while a high pressure and a low effect gives a very selective 
etching. 
Firstly, the diffractive structures were transferred from the e-beam resist 
to the 500 .ANG. Ge by 20 sec. reactive ion etching with 6 m Torr 
SF.sub.6, 20 cm.sup.3 /min and an effect of 87 W. Secondly, the 500 nm 
hardbaked photoresist between the Ge and the silica was etched by 250 sec. 
of reactive ion etching with 6 m Torr, 30 cm.sup.3 /min O.sub.2 and an 
effect of 240 W. Finally, the structures were transferred to the silica by 
reactive ion etching with 30 m Torr, 30 cm.sup.3 /min CHF.sub.3. For the 
first side, where a structure depth of about 1 .mu.m was desired the 
etching time for the silica was 39 minutes, and for the back side, where 
the desired depth was about 0.5 .mu.m, the etching time of 19.5 minutes 
was used. This silica etch also removed any remaining Ge. 
As an extra step an oxygen etch similar to the one used for the photoresist 
etching described above was given for 3 minutes to clean the element for 
remaining resist, and cleaning the vacuum chamber. This is not a vital 
step for the use of the retroreflector, but was used to give a pure hard 
silica surface. 
"Back side reflection" 
As the last step in the fabrication the back side was coated with 500 nm 
aluminium, to give a high reflectance. This was done in an evaporator 
similar to the one used for the Au and Cr evaporation. 
"Interferometric Recording of a Diffractive Optical Element" 
Another method of producing a diffractive optical element according to the 
invention is by using an interferometric recording setup as described in 
general holographic literature such as "Optical Holography" by P. 
Hariharan, Cambridge University Press, Cambridge 1984 ISBN 0 521 24348 3. 
For clarity this method is described more thoroughly in the following. 
A holographic setup for recording a focusing diffractive optical element is 
shown in FIG. 6. A coherent laser 601 having a wavelength suitable for the 
recording material is used as light source. Furthermore, the setup makes 
use of standard optical mirrors 602, 603, 604, beam splitters 605, 606 and 
spatial filters 607, 608. 
The holographic recording material 609 is preferably a photoresist such as 
the Microposit 1400 from the company Shipley. For this photoresist a 
suitable laser would be the Argon Ion Laser Model 2030, wavelength 488 or 
457.8 nm, from the company Spectra Physics. This photoresist can be coated 
on a plane glass substrate by spin coating in accordance with the 
prescriptions from the photoresist manufacturer. 
As shown in FIG. 6, the laser light is split by the beam splitter 605 in 
two separate beams 610, 611. The mirrors 603 and 604 direct the beam 611 
through the spatial filter 608, and the beam splitter 605 directs the beam 
610 through the spatial filter 607. Both spatial filters 607, 608 consist 
of a microscope objective and pin-hole for filtering the laser light and 
forming two gaussian beams 612, 613 originating from the pinholes which 
act as point sources. These two gaussian beams 612, 613 are both incident 
on the recording material 609 and the coherent nature of the laser light 
causes an interference pattern in the plane of the recording material. 
This interference pattern is recorded latent in the recording material. 
The recorded diffractive microstructure is then obtained by development of 
the recording material in accordance with the prescriptions from the 
supplier of the recording material. 
The diffractive microstructures obtained by this process are dependent on 
the geometrical setup of the recording scheme. If the distance "a"+"b" 
from the spatial filter 607 to the recording material 609 is large 
compared to the distance "c" from the spatial filter 608 to the recording 
material 609, then the light from the spatial filter 607 can be considered 
a plane wave, and the focal length of the obtained focusing diffractive 
optical element will be equal to the distance "c". The diffractive optical 
element obtained in this process will therefore act as a sperical lens 
with focal length "c" when used in a setup similar to the recording setup 
with respect to plane wave illumination, refractive index, and wavelength. 
This ensures that the diffractive pattern of the reflective diffractive 
optical element for substantially all angles sends at least a part of the 
incident light back along the direction of the incoming chief ray. 
Diffractive elements with these characteristics are used both as entrance 
transmission diffractive optical elements and as reflective diffractive 
optical elements. 
The process of tranferring the diffractive pattern from the photoresist 
surfaces to the sheeting substrate is described below in the section 
"Replication of Diffractive Optical Element". 
"Interferometric Recording of Diffuse Reflecting Diffractive Optical 
Element" 
A holographic setup for recording a diffuse reflective diffractive optical 
element is shown in FIG. 7. A coherent laser with a wavelength suitable 
for the recording material is used as light source. Furthermore, the setup 
makes use of standard optical mirrors, beam splitters, spatial filters as 
well as a glass diffuser such as the "13FSD003" from Melles Griot, 1770 
Kettering Str, Irvine, Calif. 92714. 
The holographic recording material is preferably a photoresist such as the 
Microposit 1400 from Shipley. For this material a suitable laser would be 
the Spectra Physics Model 2030 Argon Ion Laser, wavelength 488 or 457.8 
nm. This material can be coated on a plane glass substrate by spin coating 
in accordance with the prescriptions from the resist manufacturer. 
FIG. 7 shows a geometric setup in which an interferometric recording of a 
diffuse diffractive optical element can be done. Referring to the figure, 
701 designates a laser, 702--mirrors, 703--a beam splitter, 704--spatial 
filters, 705--a transparent diffuser, 706--a collimating lens, and 
707--the holographic recording material coated on a substrate and 708 the 
laser beam. 
As shown in the FIG. 7, the laser light 708 is split by the beam splitter 
703 in two separate beams. The mirrors 702 direct these two beams through 
the spatial filters 704, which consist of a microscope objective and a 
pinhole, thereby filtering the laser light and forming two gaussian beams, 
originating from the pinholes which act as point sources. One of these 
gaussian beams is directly incident on the recording material, acting as a 
reference beam. The other gaussian beam is incident on the glass diffuser 
705, giving a diffuse illumination of the recording material from a given 
direction, .PHI., and comprising a solid angle .DELTA..PHI., acting as an 
object beam. The coherent nature of the laser light causes a complex 
interference pattern in the plane of the recording material 707. The 
desired complex diffusing microstructures can be obtained by developing 
the exposed recording material in accordance with the prescriptions from 
the supplier of the recording material. 
The recorded microstructure pattern is actually an image hologram of the 
diffuser glass plate. The obtained hologram can therefore be reconstructed 
using the conjugated reference beam as a reconstruction illumination. 
The diffractive diffuser hereby obtained is characterized in that it only 
directs diffused light in a specific direction comprising a specific solid 
angle of .DELTA..PHI.. The diffuser can therefore have a more energy 
efficient use, as no diffracted diffuse light is scattered in directions 
which are not wanted. Some spectral aberrations can however be seen. The 
geometric setup used for this fabrication should be designed to give the 
best diffuser for use in the desired geometries of the retroreflecttive 
sheeting material. This can be done by a skilled person in holography or 
according to the prescriptions and formulas in "Optical Holography" by P. 
Hariharan, Cambridge University Press, 1984, ISBN 0 521 31163 2. 
The geometric setup should be chosen to ensure that the recorded 
diffractive diffuser sends a part of the incident light back along the 
direction of the incoming chief ray. This can be ensured by controlling 
.PHI. and .DELTA..PHI. in the setup, compensated for different refractive 
index and wavelength from the recording setup to the use of the optical 
element. 
The obtained diffractive diffuser can in a preferred embodiment be 
replicated by embossing to a polymer or plastic sheeting substrate as 
described in the section "Replication of Diffractive Elements". 
"Production of a Retroreflective Sheeting Material" 
In a preferred embodiment the retroreflective characteristics are achieved 
by two diffractive optical elements having a spacing equal to the 
effective focal length of the entrance transmission diffractive element. 
The geometry of the recording setup for the diffractive optical elements 
were therefore chosen to give diffractive optical elements with the 
correct focal length, that is a focal length equal to the desired distance 
between the two diffractive optical elements in the final sheeting 
material. In accordance with the above explanation of FIG. 6 this was 
accomplished by having a geometric setup with a large distance from one of 
the spatial filters to the recording material and having the desired focal 
length as the distance between the other spatial filter and the recording 
material. 
To avoid unnecessary aberrations, the material between the spatial filters 
and the holographic recording material had the same refractive index as 
the spacer material used in the final sheeting material. Alternatively, 
the distances in the recording setup were changed to minimize the 
aberrations. 
The wavelength used in the recording setup will also be the optimum 
wavelength for which the final retroreflective material can be used. Only 
for this wavelength aberrations will be minimized, and severe chromatic 
aberrations can be expected if the retroreflective material is used at 
wavelengths far from the recording wavelength. For use in the visible area 
a wavelength as close to the centre of the visible spectrum would be 
preferred, but the sensitivity of the available recording materials puts 
limitations to the freedom of choice. For known photoresists the 
sensitivity drops sharply for wavelengths above 500 nm therefore optimum 
wavelengths below 500 nm are the only choice available today if 
photoresists are used. Other recording materials such as dichromated 
gelatine, or holographic silverhalide plates such as the AGFA 8E75 gives a 
more wide range of available wavelengths, but there are no known suitable 
methods of mechanical replication of these materials. 
The aperture of the individual diffractive optical element should be large 
enough to avoid annoying diffractive effects from the aperture. For a 
distance between the two diffractive elements of 100 .mu.m and a spacing 
material with a refractive index of 1.5, good results are achieved with an 
aperture of the individual diffractive element of about 150 .mu.m. 
"Replication of Diffractive Optical Elements" 
It is clear that for a large scale production, a holographic or 
lithographic recording of the individual element is not feasible. It will 
be necessary to record a master e.g. by the interferometric method 
described above and use this master in a replication. As both the 
reflective diffractive optical element and the entrance transmission 
diffractive element in the preferred embodiment are of the same kind and 
almost identical, the considerations of replication are the same for both 
these elements. As photoresist was used as the recording material, the 
diffractive patterns were achieved by microstructures in the surface of 
the resist. Replication was then achieved by making a hard metal copy of 
these microstructures by electroforming and using this metal copy as a 
master in an embossing process. 
For achieving the right spacing between the front and back diffractive 
element simultaneous embossing of both sides of the sheeting material of 
the correct thickness was made. 
An alternative replication technique is to emboss two separate sheeting 
materials and afterwards join these two materials together, thereby 
achieving the correct positions and distances of the two diffractive 
elements. 
A description of an embossing replication technique similar to this one can 
be found in "Aztec Surface-Relief Volume Diffractive Structure" by James 
J. Cowan, Polaroid Corporation, Journal of the Optical Society of America, 
Vol. 7, No. 8, Aug., 1990, p. 1529, but careful experiments by a skilled 
person should be made to find the optimum parameters for pressure and 
temperature in the embossing process. 
"Roll-to-Roll Embossing of Surface Relief Patterns in Opposite Surfaces of 
a Sheeting Substrate" 
Referring to FIG. 8, there is shown an illustration of a roll-to-roll 
embossing af a continuously moved sheeting substrate 800. 
The sheeting substrate in the form of a transparent plastic foil is 
introduced between two embossing rolls 801, 802 having thereon two hard 
master holograms of the respective surface relief patterns and embossing 
simultaneously the pair of the surface relief patterns on each side of the 
foil. 
In a preferred embodiment, only 50% of the total embossing takes place by 
rotating the embossing rolls 801, 802 a half rotation and then lifting the 
roll free from the sheeting substrate, whereas the other 50% of the 
embossing takes place by the embossing rolls 803, 804 further down the 
line in a similar manner. It is possible to make adjustments of the 
embossing matrices by means of rolls 805 and 806 so that the surface 
relief patterns are aligned vis-a-vis each other. 
"Positioning Control" 
Referring to FIG. 9, there is illustrated a number of position markers in 
form of triangles 901-906 embossed in two rows in a transparent polymer 
foil 900, the triangles are orientated so that the perpendicular sides 
coincide with the X-axis and the Y-axis, respectively, of the polymer 
foil; said foil being moved longitudinally in the direction of the X-axis. 
Only a section of the polymer foil is shown. The sandwich surface relief 
diffractive optical elements are not shown. 
Laser diode and detector combinations 907, 908 are provided in each row (A) 
and (B). 
When light from the laser diode and detector combinations 907 hits the 
hypothenuse AB of e.g. triangle 902, a Y-timer is stopped and it is 
started, when the light hits the cathetus AC. 
Following the movement of the polymer foil 900, the hypothenuse of the 
triangle 904 of the next position marker stops the Y-timer. If the 
measured timer difference is longer than the predetermined timer 
difference for a correctly moving foil, the foil has moved into the 
positive Y-direction. If it is shorter, the foil has moved into the 
negative Y-direction. 
The timer difference is sent to a process computer which may provide 
signals for the Y-directional adjustment of the position of the embossing 
matrices, the polymer foil, or both, until subsequent Y-timer time 
differences are within acceptable limits. 
For the adjustments of the X-direction an X-timer is started when the light 
from the laser diode and detector combination 907 hits the cathetus AC of 
the tringle 902, and it is stopped when the light from the laser diode and 
detector combination 908 hits the cathetus AC of the triangle 903. If the 
measured time difference is longer than the predetermined time difference 
for a correctly moving foil, the foil on one of the foil webs has moved 
too slowly in the X-direction, and if the time difference is shorter, it 
has moved too fast in the X-direction. The time difference is sent to the 
process computer which may provide signals for the adjustment in the 
X-direction in a similar manner to that described above for the adjustment 
in the Y-direction.