Cube corner article with altered inactive areas and method of making same

A reflective article has a structured surface which includes geometric structures each having at least three specularly reflecting faces which converge at an apex or other extremity. The article is marked with a plurality of spots located between the extremities, the spots having different reflectivity characteristics than the specularly reflecting faces. The geometric structures can comprise cube corner elements. The spots can be diffusely reflecting and distributed uniformly on the structured surface or distributed to define a particular pattern. The article can have a plurality of first active areas at a first illumination geometry, and the spots can be sized and positioned such that they avoid the first active areas. The article can have first inactive areas adjacent the first active areas, and each spot can cover a majority of one inactive area. Alternatively, at least some of the spots can be decentered within their respective inactive areas such that they are visible in retroreflected light only at selected illumination geometries, thereby forming a directional image.

BACKGROUND 
The present invention relates generally to reflective articles. The 
invention has particular application to retroreflective sheeting 
fabricated using microreplication techniques. 
"Retroreflective" as used herein refers to the attribute of reflecting a 
light ray in a direction antiparallel to its incident direction, or nearly 
so, such that it returns to the light source or the immediate vicinity 
thereof 
Two known types of retroreflective sheeting are microsphere-based sheeting 
and cube corner sheeting. Microsphere-based sheeting, sometimes referred 
to as "beaded" sheeting, employs a multitude of microspheres typically at 
least partially imbedded in a binder layer and having associated specular 
or diffuse reflecting materials (e.g., pigment particles, metal flakes, 
vapor coats) to retroreflect incident light. Illustrative examples are 
disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 
5,066,098 (Kult). Due to the symmetrical geometry of beaded 
retroreflectors, microsphere-based sheeting exhibits a relatively 
orientationally uniform light return with respect to rotations about an 
axis normal to the surface of the sheeting. In general, however, such 
sheeting has a lower retroreflective efficiency than cube corner sheeting. 
U.S. Pat. No. 4,708,920 (Orensteen et al.) discloses a modified beaded 
sheeting wherein a set of axial markings are formed in the sheeting by 
laser irradiation at a specific angle, each marking being located at the 
rear of a microlens. The sheeting thus bears a directional half-tone image 
composed of the axial markings and viewable, in only a selected cone of 
observation, in retroreflected light. Directional images such as this are 
widely used as anticounterfeiting measures for motor vehicle license 
plates. 
Cube corner retroreflective sheeting comprises a body portion typically 
having a substantially planar front surface and a structured rear surface 
comprising a plurality of cube corner elements. Each cube corner element 
comprises three approximately mutually perpendicular optical faces that 
intersect at a cube apex or, where the cube apex is truncated, that 
otherwise converge at an uppermost portion. Examples of various cube 
corner designs include those of U.S. Pat. Nos. 1,591,572 (Stimson), 
4,588,258 (Hoopman), 4,775,219 (Appledorn et al.), 5,138,488 (Szczech), 
5,450,235 (Smith et al.), and 5,557,836 (Smith et al.). It is known to 
treat the structured surface with a specularly reflective coating to 
improve performance at high entrance angles. It is also known to apply a 
seal layer to the structured surface in a regular pattern of closed 
polygons which form isolated, sealed cells to keep contaminants away from 
individual cube corners. Heat and pressure used to form the cells destroys 
or deforms cube corner elements located along the polygon boundaries. 
Cube corner sheeting typically has a much higher retroreflectance than 
beaded sheeting, where retroreflectance is expressed in units of candelas 
per lux per square meter. Cube corner sheeting therefore typically appears 
brighter than beaded sheeting in retroreflected light. However, certain 
graphics applications require not only high retroreflectance but high 
daytime "whiteness". The whiteness of an object is sometimes described in 
terms of the second of the tristimulus coordinates (X,Y,Z) for the object, 
and thus is referred to as "cap-Y". The cap-Y scale ranges from 0 for a 
perfectly black object to 100 for a perfectly white object. The whiteness 
of an object is also sometimes described in terms of its "Luminance 
Factor", ranging from 0 to 1. If the daytime whiteness of cube corner 
sheeting could be increased, without substantially reducing 
retroreflectance, such sheeting could find much broader application in 
graphics applications. Cube corner sheetings which have an aluminum or 
other metal vapor coat applied to the structured surface tend to have a 
somewhat grayish appearance, and could particularly benefit from an 
increase in whiteness. 
It would also be desirable to incorporate directional images, such as are 
currently produced in beaded sheeting, in cube corner sheeting, even 
though conventional cube corner sheeting does not incorporate any lens- or 
microlens-type structure. 
Cube corner sheeting is typically composed of a polymeric material formed 
using a precision negative mold having a mold structured surface which is 
the inverse or complement of the desired structured surface of the 
sheeting. There are two ways of obtaining the precision negative mold. The 
standard way begins with fabricating a "master mold" by a very expensive 
and time-consuming process involving precisely machining microscopic 
angled surfaces in a substrate such as a directly machinable substrate, 
individual pins, or one or more lamina. See, e.g., U.S. Pat. No. 4,478,769 
(Pricone et al.). A second way, which is possible only if a sample is 
available having the desired structured surface in an undisturbed 
condition, is to use such a sample itself as a "master" from which 
replicas are made, thereby bypassing the expensive procedure of 
fabricating the master mold. 
It would be desirable to mark cube corner sheeting in some way, preferably 
without substantially reducing retroreflective performance, in order to 
deter unscrupulous copyists from duplicating the structured surface of the 
sheeting without investing in the tooling and machining required to 
produce a master mold. 
BRIEF SUMMARY 
Disclosed herein is a reflective article having a structured surface which 
includes at least one geometric structure each having at least three 
specularly reflecting faces which converge at an extremity of the 
respective geometric structure. The article also has a plurality of spots 
spaced apart from each extremity, the spots having different reflectance 
characteristics than the specularly reflecting faces. In one embodiment 
the specularly reflecting faces comprise a cube corner element. The spots, 
which are preferably diffusely reflecting to enhance whiteness of the 
article, can be distributed uniformly on the structured surface or 
distributed to define a particular pattern. 
In one embodiment the article has a plurality of first active areas at a 
first illumination geometry, and the spots include a first group of spots 
sized and positioned such that they avoid the first active areas. The 
article can have first inactive areas adjacent the first active areas, and 
each spot in the first group can cover a majority of one inactive area. 
Alternatively, at least some of the spots can be decentered within their 
respective first inactive areas such that they are visible in 
retroreflected light only at selected illumination geometries. 
A method is disclosed for making a marked cube corner article. The method 
includes providing an article having a structured surface which has a 
plurality of first active areas and first inactive areas associated with a 
first illumination geometry. The method also includes marking the 
structured surface in localized regions positioned to overlap the first 
inactive areas more than the first active areas. The marking step can 
include selectively altering the structured surface at spots confined to 
one or more first inactive areas. The article can comprise a mold, so that 
once the mold has been marked as desired, a multitude of marked cube 
corner articles such as cube corner sheeting can be formed from the marked 
mold using known microreplication techniques. Alternately, the article can 
comprise cube corner sheeting.

In the drawings, the same reference symbol is used for convenience to 
indicate elements which are the same or which perform the same or a 
similar function. 
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
FIG. 1 shows a magnified plan view of a structured rear surface 2 of a 
retroreflective sheeting layer as seen through a front surface thereof 
Three sets of parallel grooves 4,6,8 are formed in the structured surface 
2, defining geometric structures 10 and 12 which each have three faces 
that converge at an apex 10a, 12a respectively. Apexes 10a, 12a are the 
rearmost extremities of structures 10, 12, and the "bottom" or "vertex" of 
grooves 4,6,8 (the frontmost portion, where opposed groove side surfaces 
intersect) define triangular-shaped bases of structures 10, 12. The faces 
of structure 10 comprise groove side surfaces 4a,6a,8a, and the faces of 
structure 12 comprise groove side surfaces 4b,6b,8b. For ease of 
illustration, only some of the side surfaces of grooves 4,6,8 are shown in 
FIG. 1. 
The groove sets intersect each other at about 60 degree included angles. 
The geometric structures 10, 12 as shown are cube corner elements, meaning 
that three side surfaces of each structure are approximately mutually 
perpendicular. The faces of the cube corner elements are substantially 
smooth and are characterized by high specular reflectivity and small or 
negligible diffuse reflectivity. If desired, some or all of the apexes 
10a, 12a can be truncated to allow the structured surface to be partially 
transmissive. In such case the faces of the geometric structures would 
still converge at extremities of such structures, the extremities then 
being the truncated peaks. 
An imaginary datum mark 14 is also shown in FIG. 1 as a reference from 
which angles can be defined to describe various illumination geometries. 
Mark 14 is shown parallel to groove set 4. 
FIG. 2 is a depiction of the same view of the structured surface 2 of FIG. 
1, but where the groove side surfaces are not shown. This is so that 
"inactive areas" 16, which are shaded, and "active areas" 18, which are 
unshaded, can more easily be viewed. It will be understood that inactive 
areas 16 are located at each point of intersection 20 of the three groove 
sets 4,6,8, whether or not shown as such in FIG. 2. 
The term "inactive area" can be defined as follows: a light ray striking 
the structured surface in an "inactive area" does not go on to strike both 
of the other two reflecting faces of the geometric structure. For a cube 
corner element, this condition is generally exhibited by a dark appearance 
in retroreflected light. An "active area" is defined in an opposite 
manner: a light ray striking the structured surface in an "active area" 
does go on to strike both of the other two reflecting faces. Active areas 
generally appear bright in retroreflected light. 
Since the active and inactive areas are defined in terms of the incident 
light, these areas can and do change as a function of the "illumination 
geometry", i.e., the direction of the incident light with respect to the 
sheeting or article. The incident light direction is typically described 
in terms of entrance angle .beta. and orientation angle .omega. (see 
glossary and FIG. 7, both infra). FIG. 2 depicts the situation for light 
incident perpendicular to the sheeting, i.e. for .beta.=0. For 60-60-60 
degree (non-canted) cube corner elements, and for normally incident light, 
the inactive areas comprise approximately 1/3 of the total area of the 
structured surface. 
In FIG. 3, structured surface 2 has been altered to include diffusely 
reflecting spots 22 which are dispersed among the geometric structures and 
located within inactive areas 16. The spots 22 have a roughened texture or 
quality which scatters a light ray incident on them into many different 
directions. This is in contrast with surfaces of structured surface 2 that 
are adjacent spots 22, such as the unaltered portions of faces 
4a,4b,6a,6b,8a,8b. Such faces are highly specularly reflective to minimize 
losses for light rays that reflect off of three of the faces, thereby 
maintaining high retroreflective performance. Spots 22 can comprise spots 
of paint, patterned photoresist, or other suitable diffuse material 
applied to the structured surface 2, depending upon the size of the 
geometric structures. Such materials will frustrate total internal 
reflection (TIR) from the affected areas to produce the desired diffuse 
quality without actually disturbing the smooth surface finish of the 
affected area. Spots 22 can also comprise localized regions where the 
surface finish of structured surface 2 is roughened relative to adjacent 
regions. A method of producing such physically roughened regions using 
concentrated laser energy is described below. 
The spots 22 of FIG. 3 enhance the whiteness of retroreflective sheeting to 
which they are applied, since they reflect incident light in all 
directions. Advantageously, the spots 22 do not degrade retroreflective 
performance to the extent they are confined to inactive areas. Larger 
spots which completely cover the inactive areas 16 and which cover a small 
portion of active areas 18 can also be used where retroreflectance can be 
sacrificed for greater increases in whiteness. Maximum whiteness at 
maximum retroreflectance is achieved when the spots fill the inactive 
areas without impinging on the active areas. 
It is apparent from a comparison of FIGS. 1, 2, and 3 that the spots 22 
occupy positions which do not overlap with apexes 10a, 12a. Spots 22 are 
located between apexes 10a, 12a. 
FIG. 4 shows the same plan view of structured surface 2 shown in FIG. 2 but 
now with collimated light incident at .beta..apprxeq.30 degrees, 
.omega..apprxeq.0 degrees. This illumination gives rise to inactive areas 
24 (shown shaded) and active areas 26 (shown unshaded) different from 
areas 16,18 respectively. 
FIG. 5 is similar to FIG. 4 except that light is incident at 
.beta..apprxeq.30 degrees, .omega..apprxeq.80 degrees. This illumination 
produces inactive areas 28 (shown shaded) and active areas 30 (shown 
unshaded) different from those of FIGS. 2 and 4. 
FIG. 6 is a sectional view through a reflective sheeting or article 32 
having the structured surface 2 opposite a front surface 34. The views of 
FIGS. 1-5 are of article 32 as seen from the left side of FIG. 6. Arrow 36 
represents the direction of incident light associated with FIG. 4, with 
arrows 38 representing light retroreflected from active areas 26. Arrow 36 
makes an angle .beta..sub.1 .apprxeq.30 degrees with respect to an axis 40 
which is normal to front surface 34. Arrow 42, also making an angle 
.beta..sub.2 .apprxeq.30 degrees with respect to axis 40, represents the 
incident light direction associated with FIG. 5. Arrows 44 represent 
retroreflected light from active areas 30. The angles .beta..sub.1, 
.beta..sub.2 are not drawn to scale in FIG. 6. 
FIG. 7 shows a perspective view of sheeting 32 being illuminated with light 
from a direction 46 characterized by an entrance angle .beta. and 
orientation angle .omega.. 
Turning now to FIGS. 8A-C, one of the intersection points 20 is shown for 
the three illumination geometries {.beta.,.omega.} of {0,0}, {30,0}, and 
{30,180} degrees respectively. A diffusely reflecting spot 48 covers most 
(more than half the surface area) of the inactive zone 16, which comprises 
inactive portions of three neighboring structures 10 and three neighboring 
structures 12. A broken line is shown to define the border of the 
aggregate inactive area in FIG. 8A and in the other figures. The spot 48 
can but does not necessarily obliterate edges between angled faces on the 
structured surface such as the edges between faces 4a,4b,8b,8a,6a,6b (see 
FIG. 1). FIGS. 8B and 8C show that for off-axis illumination directions 36 
and 42, respectively, active areas 26 and 30 encroach upon spot 48. Of 
course, the portions 26a,30a of areas 26,30 which overlap spot 48 become 
at least partially inactive since a light ray striking those locations 
will be diffusely reflected in many different directions and hence will 
not be efficiently retroreflected. Due to the overlap portions 26a,30a, 
the retroreflected light for illumination directions 36,42 is slightly 
darkened. Spot 48, therefore, in addition to enhancing the whiteness of 
the retroreflector, also can be used to provide a directional pattern or 
image since it degrades retroreflectance at some illumination angles and 
not at others. A plurality of spots 48 placed at selected intersection 
points 20 act as individual pixels to make up the desired pattern or 
(half-tone) image. 
The amount of darkening or reduction of retroreflected light at a given 
illumination geometry is a function of (1) the extent to which specular 
reflection in the altered areas has been reduced, and (2) the fractional 
active area which becomes inactive due to the presence of the spots. The 
second factor can be greater than the percent of the original active area 
covered by the spots, if other portions of the active area not covered by 
a spot nevertheless would normally cooperate with the covered area in the 
production of retroreflected light. An extreme example of this is where 
nearly all of one entire face of the cube corner element is altered by a 
triangular-shaped spot. In such case the other two faces, even if left 
unaltered, will stop retroreflecting and become inactive since they cannot 
cooperate with the altered face to produce retroreflected light. 
Where maximum whiteness is not required or desired, some or all of the 
spots discussed herein can have a reflective characteristic other than 
diffusely reflecting and still produce a discernable pattern. All that is 
required is that the region of the spot have a reflective characteristic 
which is different from that of neighboring regions on the structured 
surface. For example, a spot can be an absorbing paint or other substance 
applied selectively to the article such that specular reflectance at the 
spot is reduced. 
FIGS. 9A-C show an intersection point 20 for the illumination conditions of 
FIGS. 8A-C respectively. A spot 50 is decentered within inactive area 16, 
located at its upper boundary. Since there is no overlap between active 
area 18 and spot 50, there will be no reduction in retroreflectance for 
normally incident light, as was the case for spot 48. However, with all 
other factors being equal, spot 50 will give a smaller improvement in 
whiteness because it has a smaller area than spot 48. The shape and 
placement of spot 50 is such that it remains completely within the 
inactive area for off-axis illumination direction 36 (FIG. 9B). For the 
opposite illumination direction 42, however, (see FIG. 9C) it 
substantially overlaps active area 30 at portion 30b as shown. In 
retroreflected light, spot 50 is substantially invisible for incident 
light directions ranging from direction 36 to normal incidence, and then 
becomes noticeable for light directions between normal incidence and 
direction 42. An image composed of a plurality of spots 50 will be 
viewable in retroreflected light in an observation cone which is 
asymmetric with respect to the perpendicular axis 40. 
FIGS. 10A-C show the same sequence of illumination geometry as FIGS. 9A-C. 
Spot 52 is sized and positioned so as not to overlap with active areas in 
each of the geometries shown, and therefore will not be noticeable in 
retroreflected light at any of these illumination geometries. Spot 54 
overlaps partially at portion 18a for normal incidence, has no overlap in 
FIG. 10B, and overlaps completely at portion 30c of active area 30 in FIG. 
10C. Within the range of angles shown, spot 52 only contributes to 
whiteness with no reduction in retroreflectance. Spot 54 contributes to 
whiteness and selectively reduces retroreflectance as a function of 
illumination orientation. 
FIG. 11A shows a front magnified view of a retroreflective sheeting having 
a structured rear surface 56 comprising parallel groove sets 58,60,62. The 
groove sets intersect each other as shown at included angles .theta..sub.1 
=55 degrees, .theta..sub.2 =55 degrees, .theta..sub.3 =70 degrees. The 
groove spacing and side angles are as specified in U.S. Pat. No. 4,588,258 
(Hoopman) to produce geometrical structures which are canted cube corner 
elements 64,66. As in FIG. 1, it is understood that groove side surfaces 
58a,58b,60a,60b,62a,62b extend indefinitely along the respective grooves. 
The grooves intersect at intersection points 68. The faces of the cube 
corner elements converge to cube peaks 64a,66a. Datum mark 69, parallel to 
groove set 58, is shown for reference purposes. 
FIG. 11B shows the approximate location of inactive areas 70, shown shaded, 
and active areas 72, shown unshaded, on the structured surface 56 for 
normally incident light (.beta.=0). Spots 74 and 76, dispersed among the 
cube corner elements and located between cube peaks 64a,66a, are provided 
on the structured surface. Spots 74,76 are sized to occupy a major portion 
of an inactive area region 70 for increased whiteness. Light 
retroreflected from normal incidence is not diminished due to spots 74,76. 
FIG. 11C shows shifted active areas 78 and inactive areas 80 for light 
incident from the right and tilted 30 degrees from the normal (.beta.=30 
degrees, .omega.=0 degrees from datum mark 69). Spots 76 are seen to 
remain within inactive areas. Spots 74 are seen to overlap with active 
areas at two locations 78a,78b. For light incident at the same tilt angle 
from the left (.beta.=30, .omega.=180 degrees), it will be apparent that 
spots 74 will be totally within inactive areas and spots 76 will overlap 
active areas in two locations similar to locations 78a,78b. Two 
independent directional patterns or images can be formed by including a 
set of spots 74 distributed across the face of the article as pixels 
defining a first pattern and including a separate set of spots 76 
independently distributed to define the second pattern. The first pattern 
will be visible in one off-axis observation cone and the second at a 
second off-axis observation cone on the opposite side of the normal. Such 
patterns will not be visible in retroreflected light at normal incidence 
and all of the individual spots will contribute to the whiteness of the 
article. 
Turning now to FIG. 12, a method of fabricating a cube corner article 
marked with a plurality of diffuse spots is shown. A machinable substrate 
82 made of copper, brass, or other suitable material that resists burring 
is ruled or fly-cut along parallel sets of grooves at step 84 to produce a 
master mold 82a with a structured surface 85. Cube corner elements on 
structured surface 85 have apexes which point out of (away from) the 
structured surface. The master mold is then replicated at step 86 using 
known replication techniques to produce a nickel "stamper" 88. Stamper 88 
has a structured surface 89 which is the inverse or complement of surface 
85. Cube corner elements on structured surface 85 have apexes which point 
into surface 85, making stamper 88 itself retroreflective for light 
impinging directly on its structured surface 85. 
At step 90, stamper 88 is replicated using known replication techniques 
such as electroforming to produce a nickel "mother" mold 92. Mother mold 
92 has a structured surface 93 which is a negative copy of structured 
surface 89 and a positive copy of surface 85. Cube corner elements on 
surface 93 thus have apexes which point out of surface 93, and surface 93 
is not generally retroreflective to light incident upon it from above. At 
step 94, selected areas of surface 93--predominantly those areas 
associated with inactive areas on an article produced from the mold 92, at 
a given illumination orientation--are irradiated with high peak power 
pulsed laser light to produce modified mother mold 92a with roughened or 
textured spots 96. Where the surface to be altered is a structured surface 
and highly reflective at the laser wavelength, it is advantageous to 
irradiate the surface in a way which is non-retroreflecting, such as by 
illuminating surface 85 of master mold 84 or surface 93 of mother mold 92 
from above, to avoid sending high energy laser light back to the laser. 
A second stamper mold 98 is produced at step 100 by electroforming a 
negative copy of mother mold 92a. The structured surface 99 of mold 98 is 
substantially the same as structured surface 89 of first stamper mold 88, 
except that surface 99 includes the roughened spots 96 thereon. Surfaces 
99 and 89 are retroreflective for light incident from above the respective 
molds. Thus, both stamper molds 88 and 98 can be considered to be 
retroreflective cube corner articles. At step 102, a negative copy of the 
structured surface 99 is produced by conventional microreplication 
techniques in a rear surface 103 of a transparent sheeting 104. Rear 
surface 103 is substantially the same as surface 93 of mother mold 92a. 
Rear surface 103 retroreflects light which impinges on a front surface of 
sheeting 104, opposite the rear surface 103, but does not generally 
retroreflect light incident from the back side of sheeting 103. The spots 
96 are shaped and located such that they overlap with inactive areas more 
than they overlap with active areas, as described above. Preferably the 
spots 96 are confined to inactive areas for normally incident light. Also, 
spots 96 can be provided at each inactive area and each spot 96 can be as 
large as possible within the bounds of the respective inactive area to 
have maximum impact on whiteness. 
It may be desirable in some instances to eliminate two replicating steps by 
forming spots 96 directly on the master mold 82a rather than on the mother 
mold 92. The altered master mold can then be used to form an altered 
stamper mold, which in turn can be used to produce sheeting. Since each 
replication step can potentially introduce imperfections or distortions, 
however slight, into the structured surface, reducing the number of 
replication steps between the master mold and the sheeting in this way has 
the benefit of ensuring a finished product with the fewest imperfections. 
However, disadvantages of this approach include having to machine a new 
master mold if an error in the marking process occurs, and being unable to 
produce different finished products having the same cube corner geometry 
but different spot patterns from the same master mold 85. In contrast, in 
the process shown in FIG. 12, multiple mother molds 92 can be made from a 
single master mold 82a, and each mother mold 92 can be altered in a 
different way (or not altered at all) to produce different finished 
products from the same master mold 82a. 
FIG. 13 shows one setup for altering selected areas of a structured surface 
106 of an article 108. The structured surface 106 is oriented generally 
parallel to an x-y plane in Cartesian coordinates x,y,z. A normal axis 110 
is parallel to the z-axis. Cube corner elements on structured surface 106 
preferably have apexes directed out of structured surface 106 so that 
light directed downward onto surface 106 is not retroreflected by such 
elements. Article 108 can be a master mold (e.g. mold 82a of FIG. 12), a 
mother mold (e.g. mold 92 of FIG. 12), or even a retroreflective sheeting 
(e.g. sheeting 104 of FIG. 12). A substantially collimated light beam from 
a directional source such as a laser is directed along an axis 112. The 
beam has an initial beam diameter 114 and is focused by lens 116 to a spot 
on the surface 106. The spot size can be changed by moving lens 116 
forward or back along arrow 118. It is desirable that lens 116 have a 
relatively long focal length so that the depth-of-focus is about the same 
as the variation in height of surface 106 within areas to be altered. 
Depending upon details of the structured surface 106, it may also be 
desirable to tilt axis 112 relative to normal axis 110 by an angle 120 to 
further avoid reflecting laser light back upon itself. 
Article 108 rests on a translation stage capable of precise movement in the 
x-y plane shown by arrow 122. By proper coordination of stepwise 
repetitive translation of article 108 and regulation of the light source, 
an array of spots is formed on surface 106, each spot characterized by a 
localized surface texture which is roughened or otherwise altered compared 
to neighboring surfaces of structured surface 106. The roughened texture 
of the spot on article 108 and/or on replicas thereof reflects light 
diffusely compared to the neighboring surfaces. 
In FIG. 14, points 124 represent positions of inactive areas on a cube 
corner retroreflective sheeting. Diffusely reflective spots 126 have been 
applied selectively to some of the points 124 to form a macroscopic 
pattern "3M" as shown. Arranging spots 126 in a pattern such as this to 
identify the manufacturer of the sheeting advantageously deters 
competitors from duplicating the sheeting and selling it as their own. 
Spots 126 can also be arranged to provide other information such as a 
product model number, date of manufacture, and, for canted or otherwise 
orientationally sensitive cube corner elements, sheeting orientation 
information (e.g., "This Side Up.fwdarw." or ".rarw.Vertical.fwdarw."). As 
discussed above, sheeting used for graphics applications can incorporate 
oversized spots 126 at every inactive area for enhanced whiteness, or 
spots 126 arranged in a half-tone image. If the number of inactive areas 
outside the desired image areas are greater than the number of inactive 
areas inside the desired image areas, whiteness can be enhanced by 
producing a negative image, whereby all inactive areas outside the desired 
image area are marked with the diffusely reflective spots and inactive 
areas inside the desired image area are left unaltered. 
FIGS. 15 and 16 show sectional views of two different types of spots 
128,130 positioned predominantly within inactive areas of canted cube 
corner sheeting 132 similar to that disclosed in U.S. Pat. No. 4,588,258 
(Hoopman). Sheeting 132 has a front surface 134 opposite a structured 
surface which includes cube corner elements 136. The faces of elements 136 
converge at cube peaks 137. Sheeting 132 also has an optional specularly 
reflecting vapor coat 138 of aluminum or other suitable metal applied 
uniformly to the structured surface. Spot 128 comprises a layer of 
photoresist which was initially applied uniformly on the structured 
surface prior to vapor coat 138 and then removed using standard 
photolithographic techniques in all areas except for at spot 128. Spot 130 
comprises locally roughened surfaces of cube corner elements 136, formed 
for example by localized heating from a focused laser light source. 
The term "sheeting" generally refers to articles which have a thickness on 
the order of about 1 mm or less and which in large samples can be wound 
tightly into a roll for ease of transportation. Retroreflective sheeting 
can be manufactured as an integral material, e.g. by embossing a preformed 
sheet with an array of cube corner elements or by casting a fluid material 
into a mold. Alternatively, retroreflective sheeting can be manufactured 
as a layered product by casting the cube corner elements against a 
preformed film or by laminating a preformed film to preformed cube corner 
elements. The cube corner elements can be formed on a polycarbonate film 
approximately 0.5 mm thick having an index of refraction of about 1.59. 
Useful materials for making retroreflective sheeting are preferably 
materials that are dimensionally stable, durable, weatherable, and readily 
formable into the desired configuration. Generally any optically 
transmissive material that is formable, typically under heat and pressure, 
can be used. The sheeting can also include colorants, dyes, UV absorbers 
or separate UV absorbing layers, and other additives as needed. As 
discussed earlier, a backing layer sealing the cube corner elements from 
contaminants can also be used, together with an adhesive layer. 
EXAMPLE 1 
Four individual cube corner prisms made of glass, each having triangular 
base entrance faces 1.5 inches (38 mm) on a side, were obtained. The base 
entrance face and the three mutually perpendicular side faces for each 
prism had smooth polished surface finishes. The inactive areas for normal 
incidence (hereinafter, the ".beta.=0 inactive areas") of the prisms were 
identified. For two of the prisms, portions of the side faces near the 
three corners of the base, corresponding to the .beta.=0 inactive areas, 
were chemically etched to produce a frosted surface finish. One of these 
etched prisms and one of the unetched prisms were aluminum coated on all 
faces except the base entrance face. Each of the prisms was then 
illuminated with substantially collimated light incident on the base 
entrance face at .beta.=45 degrees, and the luminance of light reflected 
from the respective prism in a direction normal to the base entrance face 
was measured with a photometer. For reference purposes, a glossy black 
standard and a barium sulfate white standard were also measured. The 
whiteness of the various articles for the illumination geometry is assumed 
to be proportional to the measured luminance of reflected light. A 
comparison of the results for prisms A and B, and for prisms C and D, in 
the table below demonstrate the effect on reflected light, and hence on 
whiteness, of the diffusely reflecting spots covering the inactive areas. 
______________________________________ 
Prism 
Prism Prism Prism Black 
White 
"A" "B" "C" "D" Std Std 
______________________________________ 
Inactive No Yes No Yes (N/A) 
(N/A) 
areas 
etched 
Alum. No No Yes Yes (N/A) 
(N/A) 
Coated 
Luminance 
4 8 5 11 2 50 
(cd/m.sup.2) 
______________________________________ 
EXAMPLE 2 
A nickel mother mold having noncanted (60-60-60 degree base triangle) cube 
corner elements therein was obtained and placed in the setup of FIG. 13. 
Light from a pulsed Q-switched Nd:YAG laser (.lambda.=1.06 .mu.m) was 
expanded to a beam width 114 of about 32 mm and focused onto the 
structured surface of the mold by a plano-convex lens 116 having a focal 
length of 500 mm. The tilt angle 120 was about 5 degrees. It was found 
that positioning lens 116 slightly closer to the structured surface than 
the position which produces minimum spot size, and keeping the pulse 
energy of the laser at no more than about 2 millijoules/pulse resulted in 
a spot surface texture having good light diffusing properties. Each spot 
was formed using one 2 millijoule energy pulse centered on a given groove 
intersection point. These conditions were also found to avoid doing damage 
to areas on the structured surface adjacent the desired spot location. The 
spots were approximately round with diameters of about 0.16 to 0.20 mm. 
The spots were distributed uniformly on the structured surface of the 
mother mold (at each of the groove intersections) in a zone measuring 
about 25 by 27 mm. 
A stamper (a negative copy of the altered nickel mother mold) was made in 
nickel from the marked mother mold. When viewing the stamper in 
retroreflected light, no darkening of the altered zone relative to 
neighboring unaltered areas was visible to the eye for .beta.&lt;20 degrees. 
Some darkening was visible for various values of .beta.&gt;20 degrees and 
various values of .omega.. When the stamper was illuminated with diffuse, 
nondirectional light, the altered zone appeared whiter than its 
surroundings at only certain viewing angles; at other angles it appeared 
the same as its surroundings. When the stamper was illuminated with 
directional light (sunlight), the altered zone appeared whiter than its 
surroundings at most viewing angles. 
A "pressing" (a relatively thin generally flat article having a structured 
rear surface which is a negative copy of the stamper) composed of a 
transparent polymer believed to be polycarbonate was then made from the 
stamper. The cap-Y whiteness of the pressing was measured using standard 
illuminant D65 (simulated daylight) and the 2-degree standard observer, 
both as described in A.S.T.M. Standard No. E308. Measurements were made of 
the altered 25 by 27 mm zone containing the spots, and compared to 
measurements for areas of the pressing outside the zone. On average, the 
altered zone yielded a cap-Y whiteness of 17.33 compared to 15.88 in the 
other areas, demonstrating an average whiteness increase of 1.45 units. 
EXAMPLE 3 
A sample comprising a single acrylic layer having a structured rear surface 
of non-canted cube corner elements with an aluminum vapor coat was 
obtained. The pulsed Q-switched laser mentioned above was focused to a 
relatively large area about 3 mm in diameter, encompassing a large number 
of cube corner elements, and directed through the front of the sample. A 
Faraday isolator protected the laser from backscattered high energy light. 
The reflections caused by the structured surface and the sufficiently high 
peak power laser light cooperated to remove the vapor coat from portions 
of the structured surface. 
When the altered sheeting was observed under diffuse illumination, little 
effect was visible. When viewed in retroreflected light, patterns formed 
from a multitude of spots were visible within limited angular ranges as 
dark areas. 
Glossary of Certain Terms 
Datum Mark: a mark (whether real or hypothetical) on a reflective article 
that is used as a reference to indicate orientation about the reference 
axis. 
Entrance Angle (.beta.): the angle between the illumination axis and the 
reference axis. 
Entrance Half-Plane: a half-plane which originates on the reference axis 
and contains the illumination axis. 
Groove Side Angle: the dihedral angle between a groove side and a plane 
extending parallel to the length of the groove and perpendicular to a base 
surface of the reflective article. 
Illumination Axis: a line segment extending between the reference center 
and the source of illumination. 
Light: electromagnetic radiation, whether in the visible, ultraviolet, or 
infrared portion of the spectrum. 
Orientation Angle (.omega.): the dihedral angle between the entrance 
half-plane and a half-plane originating on the reference axis and 
containing the datum mark. 
Reference Axis: a line segment extending from the reference center away 
from the reflective article, and which is ordinarily perpendicular to the 
reflective article at the reference center. 
Reference Center: a point on or near a reflective article which is 
designated to be the center of the article for specifying its performance. 
Visible Light: light detectable by the unaided human eye, generally in the 
wavelength range of about 400 to 700 nm. 
All U.S. patents and patent applications referred to herein are 
incorporated by reference. Although the present invention has been 
described with reference to preferred embodiments, workers skilled in the 
art will recognize that changes can be made in form and detail without 
departing from the spirit and scope of the invention.