Note: Descriptions are shown in the official language in which they were submitted.
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32380CAN3A
WIDE-ANGLE-REFLECTIVE CUBE-CORNER
RETROREFLECTIVE SHEETING
~I~LD or ~ NV~N51~Y
5This invention relates to cube-corner
retroreflective sheetinq.
BACKGROUND
Cube-corner retroreflective sheeting provides
the brightest reflectivity of any known retrcreflective
sheeting, and yet it has found only limited uses. One
reason for this limited use is that cube-corner
retroreflective sheeting reflects within only a narrow
range of incidence or entrance angles (the angle between
the path of incident li~ht and a line normal to the
sheeting). For example, the "half-brightness" angle
(i.e., the incidence angle at which the reflection is half
as bright as the reflection of perpendicular, or zero-
incidence-angle, light) for "conventional" acrylic-based
cube-corner retroreflective sheeting (i.e., sheeting as
illustrated in Figure 1 of U.S. Pat. 3,712,706, which uses
cube-corner retroreflective elements having equal-sized
faces and an optical axis perpendicular to the front face
of the sheeting) is about 21, which compares with the
half-brightness angle of about 45 or more for glass-
microsphere-based retroreflective sheetings. Also, the
retroreflection fram a cu~e-corner retroreflective
- sheeting is dependent on the rotational angle at which
light is incident on the sheeting, so that from certain
30 rotational angles a viewer receives a limited reflection.
Various proposals have been made to overcome the
limited angularity of cube-corner retroreflectors, In some
versions of molded cube-corner retroreflectors such as
used for reflectors on automobiles, a portion of the
individual cube-corner elements of the reflector are
tilted, so that the tilted elements will reflect light
that strikes the reflector at a higher incidence angle
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(see U.S. Pats. 3,541,606; 3,923,379; 3,926,420 and
4,066,331). However, tilting of the cube-corner
reflective elements in this manner requires a loss in
other properties, such as a reduction in '`head-on"
reflection, i.e., reflection of zero-incidence-angle
light, which strikes the reflector on a path that is
perpendicular to the reflector. Also, the listed patents
are directed to reflectors with large cube-corner
retroreflective elements, made by molding procedures that
are not adapted to formation of very small cube-corner
retroreflective elements such as used in reflective
sheeting for traffic siqns.
In the sheeting field, novel cube-corner
retroreflective elements have been developed (see U.S.
Pat. 4,349,598) which retroreflect light that strikes them
at a large incidence angle, but sheeting made with such
elements does not reflect low-incidence-angle light well;
the narrow range of angularity is simply shifted to a
different segment of incoming light.
U.S. Pat. 3,450,~59 also sqeks to provide
reflection of high-incidence-angle light, but it uses a
multi-faceted retroreflective element that is not adapted
to manufacture in sheeting form and that further is not
demonstrated to provide any real improvement in
angularity.
U.S. Pats. 3,140,340 and 4,303,305 teach
articles in which a second retroreflector (a glass-
microsphere reflector in the '340 patent and a cube-corner
~ ~ reflector in the ~305 patent) is positioned behind a
; 30 transparent cube-corner retroreflective retroreflector,
and the assembly is found to have increased angularity.
However, such an assembly is thick (the first cube-corner
retroreflector must use substantially larger-sized cube-
corner retroreflective elements), which leaves it with
limited utility.
A different approach to improved angularity is
to coat the rear of reflecting surface of cube-corner
lZ9Z9Q3
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elements with a specularly reflective material such as
aluminum. However, while such a c~ating achieves an
increase in angularity, it also absorbs light, and the
overall effect is a significant reduction in "head-on"
reflection. Also, the coating has other disadvantages
(e.g., it gives the retroreflective sheetin~ a gray color
in contrast to the desired white or other colors).
SUMMARY OF THE INVENTION
10 The present invention provides a new cube-corner
retroreflective product which provides half-briqhtness
angles in a cube-corner retroreflector approaching those
achieved in glass-microsphere retroreflective sheeting,
while still maintaining and even enhancing other desired
properties of cube-corner retroreflectors. In brief
summary, a cube-corner retroreflector of the invention
comprises a transparent layer configured on its rear
surface with cube-corner retroreflective elements which
retroreflect light beamed against the front of the layer,
and a specularly reflective layer having a specularly
reflective surface which is a negative of the configured
rear surface of the transparent layer and which is
interfitted with and closely spaced from said rear
surface, with the adjacent and matching portions of the
specularly reflective and rear surfaces being
substantially parallel to one another.
In contrast to previous spécularly coated cube-
corner retroreflectors, there is no reduction in
retroreflection by retroreflectors of the present
invention for light within the range of angles where
conventional uncoated cube-corner retroreflectors reflect
efficiently. Such a reduction is avoided because the
specularly reflective layer is spaced from the cube-corner
elements. Much of the light reflected by the cube-corner
reflective element in a reflector of the invention never
reaches the specularly reflective layer and therefore is
not absorbed by that layer. Only light wh~ch passes
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through the surfaces of the cube-corner retroreflective
element, and therefore would ordinarily not be reflected,
strikes the specularly reflective layer, and only a
portion of this light, which was previously totally lost,
is absorbed hy the specularly reflective layer. Also, the
color of the reflector is substantially unaffected by the
specularly reflective layer, so that bright white or other
colored retroreflectors can be obtained.
The results achieved by the invention are
remarkable. For example, we have made cube-corner
retroreflective sheeting having a reflective brightness of
1718 candela per lux per square meter at a _4D incidence
angle, with an observation angle of 0.2, and a rotational
angle of 0 (measurements made at a rotation angle of 0
are defined as measurements in a plane parallel to a
groove or valley in the configured rear surface of the
transparent body of a retroreflector of the invention, as
will be described more fully in the following discussion),
and brightnesses of 1016 and 425 candela per lux per
squàre meter at incidence angles of 30 and 45,
respectively, By contrast, a conventional sheeting
(having a transparent layer or body of the same
canfiguration and composition of the transparent body of
the tested sheeting of the invention, but without a
specularly reflective layer of the invention) exhibited a
reflective brightness of only 485 and 168 candela per lux
per square meter at incidence angles of 30 and 45. Al60,
as another comparison, a version of the conventional
: sheeting directly vapor-coated on its rear surface with
aluminum exhibited a reflective brightness of 716 candela
: per lux per square meter at a -4 incidence or entrance
angle, observation angle of 0.2 and rotational angle of
~: . 0, and of 445 and 197 candela per lux per square meter at
incidence angles of 30 and 45. As will be seen, sheeting
of the invention was brighter than the coated sheeting,
not only at 0 but also at the larger incidence angles.
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A different advantage of sheeting of the
invention is an increase in rotational uniformi~y, i.e.,
an increase in the uniformity of reflective brightness of
the sheeting at high incidence or entrance anqles as the
rotational orientation of the sheeting is changed. With
conventional cube-corner retroreflective sheeting, the
reflective brightness of the sheeting will vary
slgn~ficantly as the sheeting is r-.~ated thro~qh an an~le
of 0 to 90 around an axis perpendicular to the sheeting.
Sheeting of the present invention exhibits a much smaller
variation in reflective brightness for high-incidence-
angle light during such rotation, mainly because light is
not lost by transmission. The result is that it is less
significant that sheeting of the invention be attached to
a sign board in a particular orientation.
DESCRIPTION OF THE DRAWINGS
The invention will be explained with reference
to the drawings, wherein:
Figure 1 is a sectional view through a portion
of an illustrative retroreflective sheet material of the
inven~lon;
Figure 2 is a sectional view through a portion
o~ an intermediate product ~ormed in the course o~ making
retroreflective sheet material af the invention;
Figure 3 is a sectional view through a portion
o~ a s~eet assembly which may be ma~e in the cou~se of
making sheet material of the invention;
Figu~e 4 is a bottom view of a portion (i.e.,
the transparent body portion) of the sheet material of the
invention shown in Figure 1;
Figures5 and 6 are enlarged sectional views
through a sheet material of the invention along the plane
5-5 marked in Figure 4;
Figure 7 is a bottom view of a different variety
of cube-corner retroreflective element ùseful in the
invention; and
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1Z92903
Figure 8 is a plot of reflective brightness at
different angles of rotation for a representative
retroreflective sheeting of the invention in comparison to
a similar sheeting without the specularly reflective layer
included in the sheeting of the invention.
DETAILED DESCRIPTION
The illustrative sheet material of the invention
10 shown in Figure 1 comprises a transparent sheet, body
or layer ll and a reflector sheet or layer 12 closely
spaced from the transparent sheet. The transparent sheet
11 is configured on its rear or bottom surface with cube-
corner retroreflective elements 13, and t~e reflector
sheet 12 is configured on its front or top surface with
lS projections 14 that mate with the cube-corner
retroreflective elements 13 on the transparent sheet. In
other words, the top or front surface of the underlying
reflector sheet is a negative of the rear or bottom
surface of the transparent sheet, with the pyramidal-
shaped projections 14 on the reflector sheet fitting intovalleys between the pyramidal-shaped cube-corner
retroreflective elements, and vice versa. The top
configured surface of the reflector sheet 12 carries a
coating 15 of specularly reflective material, thus
providing a specularly reflective surface that mates with
and is closely spaced from the rear or bottom surface of
the cube-corner retroreflective elements, with the
adjacent and matching portions (e.g., 16 and 17 in Figure
1) of the specularly reflective and rear surfaces being
substantially parallel to one another.
The transparent sheet 11 is typically made by
casting or embossing procedures using a configured
embossing or casting surface typically generated as an
electroform from a machined surface. For example, as
taught in U.S. Pat. 3,712,706, a metal plate may be
grooved on a ruling machine using a V-shaped diamond tool
h~ld at appropriate angles. A first groove is cut, then
~292903
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the tool is offset a precise microscopic amount to achieve
proper spacing, a second groove cut, and so on across the
surface of the plate. Next, the plate is rotated 60 and
a second set of grooves is cut to form a second surface of
the cube-corner retroreflective elements. Finally, the
plate is again rotated 60 and a third set of grooves is
cut forming the third surface of the cube-corner
retroreflective elements. The metal plate is typically
used to generate negative and positive electroforms, and
cast~ng or embossing suraces are then ormed from the
electroforms.
The transparent sheet may be formed as a
monolithic sheet or as a composite sheet, for example,
with a base flat-surfaced sheet which carries
retroreflective elements cast onto the flat-surfaced
sheet. The cast cube-corner retroreflective elements may
or may not have the same index of refraction as the flat-
surfaced sheet, and may or may not be made from the same
material as the flat-surfaced sheet.
Figures 2 and 3 illustrate one possible method
for manufacture of the reflector sheet 12 included in the
sheet material of the invention shown in Figure 1. In
this method, the transparent sheet 11 is coated as shown
in Figure 2 with a specularly reflective layer 15, as by
vapor-depositing or chemically depositing a metal such as
aluminum, silver, or nickel, or by vapor-depositing a
dielectric reflective layer (see U.S. Pat. 3,700,305).
Next, the material for a backing for the reflector sheet
12 is coated or extruded onto the specularly reflective
layer 15 and solidified as by cooling or crosslinking.
After solidification, the backing material may be stripped
away from the transparent sheet, as shown in Figure 3.
The backing material and process conditions are chosen so
that the backing material has a greater affinity to the
specularly reflective layer 15 than to the transparent
sheet 11. Accordingly, the specularly reflective layer 15
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trans~ers with the backing and separates f rom the
transparent sheet.
As an alternative method, a reflector sheet can
also be formed by casting or embossing techniques similar
to those used to form the transparent sheet 11. For
example, a casting or embossing surface may be generated
from the machined plate descrihed above but instead of
using a negative of that machined surface such as may be
used as a casting or embossing surface for the transparent
sheet, a positive of the machined surface may be generated
by a two-step electroforming process. A backing for the
reflector sheet is cast or embossed and coated with a
specularly reflective material.
The reflector sheet should be slightly spaced
from the rear or bottom surface of the transparent sheet
in order to achieve the advantages of the invention. If
the two layers are in close contact with no air space, the
sheeting becomes the same as one with a specularly
reflective layer applied directly to the back surfaces.
However, the spacing can be quite narrow or close, for
example, as thin as about five wavelengths of light.
Spacings greater than about one-fourth the height of the
cube-corner retroreflective elements generally cause too
great a reduction in the amount of retroreflection from
the sheet material of the invention, because some light
exiting the transparent sheet and reflected by the
reflector sheet will not reenter the sheet at a point from
which the sheet will return the light toward the source of
the light. Preferably, the spacing is less than 5 percent
of the cube-corner height.
Needed spacing can be obtained inherently when a
reflector sheet is laid against the transparent sheet,
because air trapped between the two sheets holds the
sheets apart. The reflector sheet and transparent sheet
can be adhered together in an assembled relation, e.g., by
adhering together edge portions of the shee t s i n the
manner described in U.S. Pat. 3,140,340, ot by forming a
9 1292903
network of bonds or bonding walls between the two sheets
in the manner described in U.S. Pat. 3,924,929 or
4,025,159. Spacing and adhesion can also be achi!eved by
coating adhesive material onto the surface of one of the
transparent or reflector sheets, e.g., in a network-like
pattern, pressing the two sheets together, and solidifyinq
the adhesive material. Still another alternative is use
of exterior means such as clamps or insertion within an
envelope formed by transparent sheets adherediat their
edges.
The effect achieved by the invention can be
further illustrated by reference to Figures 4-6 of the
attached drawings. Figure 4 shows, in greatly enlarged
view, from the bottom, a part of a typical or
representative transparent layer from a retroreflector of
the invention, normally two adjacent cube-corner
retroreflective elements 20 and 21 from the layer.
Figures 5 and 6 show a cross section through the
representative retroreflective element taken along the
A 20 line~ 5-5 in Figure 4. In Figures 5 and 6, the portion of
the reflector sheet 22 that mates with the cube-corner
retroreflective elements 20 and 21 shown in Figure 4 is
also included in the drawing. The cube-corner
retroreflective elements 20 and 21 are transparent bodies
having a front face 23 and three mutually perpendicular
surfaces 24, 25 and 26 such as obtained at the corner of a
cube. A light ray 27 entering the front face 23 of the
transparent cube-corner retroreflective element 20 travels
to a first surface 24 of the element where, if the angle
at which the light ray strikes the surface is more than
the critical angle, the light is reflected by internal
reflection from the surface. (The critical angle depends
upon the index of refraction for the material from which
the element is made; a representative index of refraction
for an acrylic polymeric material is about 1.5, and the
critical angle for such a material is about 42.) The
light reflected from the surface 24 thereupon travels to a
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second surface of the cube-corner retroreflective element,
where it is again reflected if the critical angle is
exceeded, this time to the third surface. At the third
surface it can be again reflected, and this third
reflection directs the liqht back along a path
substantially parallel to the path that the light traveled
to the reflector (in the two dimensional representation of
Figure 5, which is commonly used, the ray 27 is shown
being returned toward the source of the light after being
reflected from only two surfaces, because of the
difficulty of representing reflection from a third
surface).
The narrow angularity of cube-corner
retroreflective elements or sheeting arises because highly
inclined light, e.g., the light ray 28 in Figure 6 having
a large incidence angle to the front surface 23 of the
element 20, will strike the first surface 24 of the
element 20 at an angle that is less than the critical
angle for that surface. Because it strikes the surface at
an angle that is less than the critical angle, the light
ray 28 passes through the surface 24, and in conventional
cube-corner retroreflective sheeting that light is lost
and not retroreflected.
However, in reflectors of the invention, much of
the light that passes through the surfaces of the cube-
corner retroreflective element strikes an underlying
spaced specularly reflective layer, whose surface is
substantially parallel to the surface through which the
light passed, i.e., the surface 29 in Figure 6, and is
reflected back into the cube-corner retroreflective
element. After re-entry into the cube-corner
retroreflective element, the light can be reflected by a
second surface, or an underlying spaced specularly
reflective layer and then similarly by a third surface or
underlying specularly reflective layer back along a path
substantially parallel to the the path that the light
originally traveled to the element.
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The elements ~0 and 21 shown in Figure 4 can be
described as truncated cube-corner retroreflective
elements, and contrast with a different useful
retroreflective element shown in Figure 7 which ls
commonly called a full cube-corner retroreflective
element. The full cube-corner retroreflective element as
shown in Figure 7 has sq~lare side surfaces and because of
that larger surface area can reflect larger proportions of
light striking the element. However, because of their
full shape, such cube-corner elements nest together in a
way that does not permit manufacture of a cube-corner
surface by grooving techniques and, therefore, makes
manufacture of microsized cube-corner elements difficult.
The truncated form shown in Figure 4, on the other hand,
can be readily made by the grooving techniques described
above. Retroreflectors or sheets of the invention can use
either the retroreflective element shown in Figure 4 or 7.
Incidentally, Figures 1-3 show product that is
sectioned approximately along the line 1-1 in Figure 4.
Sample materials for the transparent sheet or
layer are acrylic polymers such as polymethylmethacrylate,
polymethylmethacrylate and polyacrylate blends,
polycarbonate, polyethylene terephthalate, and vinyl
polymers, or reactive materials such as taught in United
Kingdom Pat. 2,027,441.
The backing of the reflector sheet is preferably
a reacted or crosslinked material so as to provide
dimensional stability. Useful materials of that type are
polyacrylates, urethane oligomers, silicone elastomers,
and epoxy resins. Thermoplastic polymeric materials such
as polyvinylacetate can also be used. In a different
approach, the specularly reflective sheet is formed
directly from metal, as by embossing a metal foil or a
laminate of metal foil and polymeric material.
The adhesive materials for adhering the
transparent and reflector sheets together are preferably
reactive materials, usually reacted through radiation such
-12- 1 ~ Z 9 0 3
as ultraviolet or electron-beam radiation or by
application of heat.
Although the invention has been described in
terms of sheeting, where it makes several contributions,
the invention is also useful for larger cube-corner
reflectors such as molded, perhaps more rigid, polymeric
or glass plates. Such molded larger cube-corner
retroreflectors of the invention exhibit improved wide
angularity, bright retroreflectivity, good color, reduced
orlentation sensitivity, etc. The cube-corner
retroreflective elements in molded or larger cube-corner
retroreflectors are often of a shape as shown in Figure 7.
The microsized cube-corner retroreflective elements in
thin sheeting typically have triangular surfaces, since
the grooving operation forms such surfaces. Also, the
cube-corner retroreflective elements used in reflectors of
the invention can be tilted, e.g., as taught in U.S. Pat.
4,349,598, or 3,926,420. It is desirable to use a
specularly reflective layer that overlaps substantially
all of the mutually perpendicular surfaces of the cube-
corner retroreflective elements.
The invention will be further illustrated by the
following examples.
_ XAMPLE 1
A transparent sheet like the sheet 11 shown in
Figure 1 was formed from an extrusion-grade polycarbonate
polymer using a negative embossing tool generated from a
machined metal plate as described above. The individual
cube-corner retroreflective elements had a base width of
0.318 millimeter (0.0125 inch) and a height of 0.150
millimeter (0.0059 inch). The configured rear surface of
the prepared transparent sheet was vapor-coated with
aluminum, after which the material for the backing of a
reflector sheet was coated onto the aluminum. This
material was comprised of an epoxy resin, i.e., a
particle-filled diglycidyl ether of bisphenol A. After
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coating onto the al~minum layer, it was crosslinked by
heating at 32C (90F), and stripped away from the
transparent sheet. The aluminum layer transferred with
the backing, forming a specularly reflective surface that
was a negative of the configured rear surface of the
transparent sheet. The reflector sheet was then
reassembled in close spacing to the transparent sheet by
light hand pressure, and the reflective properties of the
assembly were measured. using the rotation angle that
achieved maximum head-on brightness, the reflectivity of
the assembly was measured with a retroluminometer
described in Defensive Publication T 987,003 and was found
to have retroreflective brightness values as in the
following table.
TAsLE I
Retroreflective Brightness
Incidence Angle Candella/Lux/Square Meter
4 1650
2030 1365
40 880
450 576
The assembly was determined to have a half-brightness
angle of about 41 at the rotation orientation of the
test.
EXAMPLE 2
Example 1 was repeated with different samples as
follows:
In Sample 2-A, the transparent sheet was a
polyacrylate having a refractive index of 1.49 and the
specularly reflective layer was chemically deposited
silver. In Sample 2-B, the transparent sheet was a
polycarbonate having a refractive index of 1.57, with a
vapor-deposited aluminum specularly reflective layer.
Retroreflective brightness for each sample was
measured for the following different forms of each sample:
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129Z903
i) Cube-corner sheet with no specular layer
present;
ii) Cube-corner sheet with specular layer
adhered to the sheet;
iii) Dual layer construction of the invention
with the specular layer spaced from the
rear sucface of the first cube-corner
sheet.
Reflectivity measurements were made at 0.2 observation
angle and at the entrance angles and rotational angles
indicated below with the following results
(candella/lux/square meter).
TABLE I I
0~ ROTATION 90ROTATION
SAMPLE 2 - A
(Polyacrylate/silver)
Entrance angle -4 30 45 -4 30 _45
i ) 2018 14524 2880 754 217
ii ) 1464 646307 1045 808 296
iii ) 2038 848394 2873 1005 341
.
SAMPLE 2-B
(Polycarbonate/aluminum)
: 30
i ) 1333 49729 1604 485 168
ii ) 570 294138 716 445 197
iii) 1420 6781801718 1016 425
EX~MPLE 3
Retroreflection was measured on the following
samples:
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` ` ~zg2903
-15- 60557-3234
Sample 3-A: A conventional sheet of large (2.4 m~llimeter
(0.095-inch) cube-corner size) full acrylic
cuhe-corner retroreflective elements such as
used for a highway delineator.
Sample 3-B: Sheeting as described in Example 2-B but
without a specularly reflective layer.
Sample 3-C: Sample 3-A with a Sample 3-B type behind it
(as described in U.S. Pat. 4,303,305).
Sample 3-D: Sheeting of the invention using Sample 3-B
with a silver specularly reflective layer
spaced closely behind the rear surface of the
Sample 3-~ sheet.
Reflectivity measurements were nlade at a 0.2 observation
angle and at the entrance anqles indicated. Rotational
angles were chosen to give maximum and minimum readings
rather-than taken at 0 and 90.
TABLE III
Candella/Lux~Square Meter at 0.2 ohservation angle
-4 entrance 45 entrance 45~ as %
min. max. av~. min. max. avg. of -4
Ex. 3-A457 907 6~2 3 51 27 4%
25 Ex. 3-B1116 1457 128719 250 135 11%
Ex. 3-C457 907 682 21 2B1 151 22%
Ex. 3-D943 1687 1315240 540 390 30%
The data plotted in Figure 8 illustrates the
reduced rotational sensitivity for retroreflectors or
sheet material of the invention for high-incidence-angle
light. The dotted-line plot shows retroreflective
brightness (plotted along the ordinate in candlepower per
lux per square meter) for a polycarbonate cube-corner
retroreflective sheet without specularly reflective layer
(for example, like the Sample 3-8 product in Example 3
above), and the solid-line plot is for a retroreflective
, - .
-16- 1292~03
sheet of the invention (like the Sample 3-D product in
Example 3) having a silver specularly reflective layer
spaced behind the polycarbonate transparent sheet.
Retroreflective measurements were made at various
rotational angles from 0 to 360. The observation angle
was 0.2, and the entrance angle was 45. As will be
seen, at certain rotational anqles the polycarbonate sheet
without specularly reflective layer had very low
reflective brightness. While the sheet of the invention,
shown by the solid-line plot, also had variation in
retroreflective brightness depending on rotational angle,
the range between maximum and minimum retroreflective
brightness was much less than for the conventional
retroreflective sheet without specularly reflective layer;
and the minimum values for the sheet of the invention
approached the maximum values for the conventional cube-
corner retroreflective sheet.
. Various modifications and alterations of this
invention will become apparent to those skilled in the art
without departing from the scope and spirit of this
invention.
:~ 35
