STRUCTURED SURFACE ARTICLES CONTAINING GEOMETRIC STRUCTURES WITH COMPOUND FACES AND METHODS FOR MAKING SAME

Abstract: A process for making molds for retroreflective sheeting and like articles includes making a substrate having a structured surface of geometric structures such as cube corner elements. The structured surface is in one embodiment partially replicated from a previous generation structured surface, and partially machined in the substrate. At least one of the faces of the structured surface is a compound face that has one machined portion and one non-machined portion, such as a replicated portion. The process can be used to manufacture substrates with desirable geometry cube corner elements, such as cube corner elements having a hexagonal outline in plan view, without requiring the use of pin bundling techniques or laminae. The process can also be used to manufacture articles having cube corner elements in which at least one face of the cube corner element has two constituent faces disposed on opposed sides of a transition line that is nonparallel to a nondihedral edge of such cube corner element.

Description: STRUCTURED SURFACE ARTICLES CONTAINING GEOMETRIC STRUCTURES WITH COMPOUND FACES AND METHODS FOR MAKING SAME Background The present invention relates generally to structured surfaces fabricated using microreplication techniques. The invention has particular application to structured surfaces that comprise retrorefiective cube corner elements. The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein. It is known to use microreplicated structured surfaces in a variety of end use applications such as retrorefiective sheeting, mechanical fasteners, and abrasive products. Although the description that follows focuses on the field of retroreflection, it will be apparent that the disclosed methods and articles can equally well be applied to other fields that make use of microreplicated structured surfaces. Cube corner retrorefiective sheeting typically comprises a thin transparent layer having a substantially planar front surface and a rear structured surface comprising a plurality of geometric structures, some or all of which include three reflective faces configured as a cube corner element. Cube corner retrorefiective sheeting is commonly produced by first manufacturing a master mold that has a structured surface, such structured surface corresponding either to the desired cube corner element geometry in the finished sheeting or to a negative (inverted) copy thereof, depending upon whether the finished sheeting is to have cube corner pyramids or cube corner cavities (or both). The mold is then replicated using any suitable technique such as conventional nickel electroplating, chemical vapor deposition, or physical vapor deposition to produce tooling for forming cube corner retrorefiective sheeting by processes such as embossing, extruding, or cast-and-curing. U.S. Pat. No. 5,156,863 (Pricone et al.) provides an illustrative overview of a process for forming tooling used in the manufacture of cube corner retrorefiective sheeting. Known methods for manufacturing the master mold include pin- bundling techniques, laminate techniques, and direct machining techniques. Each of these techniques has its own benefits and limitations. In pin bundling techniques, a plurality of pins, each having a geometric shape such as a cube corner element on one end, are assembled together to form a master mold. U.S. Patent Nos. 1.591,572 (Stimson) and 3,926,402 (Heenan) provide illustrative examples. Pin bundling offers the ability to manufacture a wide variety of cube corner geometries in a single mold, because each pin is individually machined. However, such techniques are impractical for making small cube corner elements (e.g. those having a cube height less than about 1 millimeter) because of the large number of pins and the diminishing size thereof required to be precisely machined and then arranged in a bundle to form the mold. In laminate techniques, a plurality of plate-like structures known as laminae, each lamina having geometric shapes formed on one end, are assembled to form a master mold. Laminate techniques are generally less labor intensive than pin bundling techniques, because the number of parts to be separately machined is considerably smaller, for a given size mold and cube corner element. However, design flexibility suffers relative to that achievable by pin bundling. Illustrative examples of laminate techniques can be found in U.S. Patent No. 4,095,773 (Lindner); International Publication No. WO 97/04939 (Mimura et al.); and U.S. Application Serial No. 08/886,074, "Cube Corner Sheeting Mold and Method of Making the Same" , filed July 2, 1997. In direct machining techniques, series of groove side surfaces are formed in the plane of a planar substrate to form a master mold. In one well known embodiment, three sets of parallel grooves intersect each other at 60 degree included angles to form an array of cube corner elements, each having an equilateral base triangle (see U.S. Patent No. 3,712,706 (Stamm)). In another embodiment, two sets of grooves intersect each other at an angle greater than 60 degrees and a third set of grooves intersects each of the other two sets at an angle less than 60 degrees to form an array of canted cube corner element matched pairs (see U.S. Patent No. 4,588.258 (Hoopman)). Direct machining techniques offer the ability to accurately machine very small cube corner elements in a manner more difficult to achieve using pin bundling or laminate techniques because of the latter techniques' reliance on constituent parts that can move or shift relative to each other, and that may separate from each other, whether during construction of the mold or at other times. Further, direct machining techniques produce large area structured surfaces that generally have higher uniformity and fidelity than those made by pin bundling or laminate techniques, since, in direct machining, a large number of individual faces are typically formed in a continuous motion of the cutting tool, and such individual faces maintain their alignment throughout the mold fabrication procedure. However, a significant drawback to direct machining techniques has been reduced design flexibility in the types of cube corner geometries that can be produced. By way of example, the maximum theoretical total light return of the cube corner elements depicted in the Stamm patent referenced above is approximately 67% . Since the issuance of that patent, structures and techniques have been disclosed which greatly expand the variety of cube corner designs available to the designer using direct machining. See, for example, U.S. Patent Nos. 4,775,219 (Appledorn et al.); 4,895,428 (Nelson et al.); 5,600,484 (Benson et al.); 5,696,627 (Benson et al.); and 5,734,501 (Smith). Some of the cube corner designs disclosed in these later references can exhibit effective aperture values well above 67% at certain observation and entrance geometries. Nevertheless, an entire class of cube corner elements, referred to herein as "preferred geometry" or "PG" cube corner elements, have up until now remained out of reach of known direct machining techniques. A substrate incorporating one type of PG cube corner element is shown in the top plan view of FIG. 1. The cube corner elements shown there each have three square faces, and a hexagonal outline in plan view. One of the PG cube corner elements is highlighted in bold outline for ease of identification. The highlighted cube corner element can be seen to be a PG cube corner element because it has a non- dihedral edge (any one of the six edges that have been highlighted in bold) that is inclined relative to the plane of the structured surface, and such edge is parallel to adjacent nondihedral edges of neighboring cube corner elements (each such edge highlighted in bold is not only parallel to but is contiguous with nondihedral edges of its six neighboring cube corner elements). Disclosed herein are methods for making geometric structures, such as PG cube corner elements, that make use of direct machining techniques. Also disclosed are articles manufactured according to such methods, such articles characterized by having at least one specially configured compound face. Brief Summary Structured surface articles such as molds or sheetings are disclosed in which a geometric structure has a plurality of faces. At least one of the faces is a compound face comprising a machined portion and a non-machined portion. The non-machined portion can be formed by, for example, replication from another substrate or embossing with a suitable tool. A transition line may separate the machined portion from the non-machined portion. The geometric structure can of course comprise faces arranged to form a cube corner element. Cube corner elements, and structured surfaces incorporating an array of such elements, are disclosed wherein at least one face of the cube corner element terminates at a nondihedral edge of such element, the face comprising two constituent faces disposed on opposed sides of a transition line that is nonparallel to the nondihedral edge. The cube corner element can comprise a PG cube corner element, and exactly one, exactly two. or all three faces of such element can comprise two constituent faces disposed on opposed sides of a transition line that is nonparallel to the respective nondihedral edge. In an array of neighboring cube corner elements, each cube corner element in the array can have at least one face configured as described above. Further, the cube corner elements can be made very small (well under 1 mm cube height) due to the direct machining techniques employed. Molds are disclosed in which the structured surface comprises pyramids disposed proximate to at least one cavity, the cavity being formed by non- machined faces and the pyramids being formed at least in part by machined faces. Methods are disclosed for making a structured surface article comprising at least one geometric structure. The method includes providing a prepared substrate having a non-machined face, and removing material from the prepared substrate to form a machined face such that the machined face and the non- machined face together form one of the faces of the geometric structure. Methods are also disclosed for forming in a substrate a structured surface that extends along a reference plane and contains PG cube corner elements. The method includes providing a prepared substrate, and forming groove side surfaces in the prepared substrate that extend along axes that are substantially parallel to the reference plane. Faces formed by the groove side surfaces together with other faces incorporated in the prepared substrate combine to form the PG cube corner elements. Various methods are disclosed for providing the prepared substrate. One such method begins by forming an array of non-machinable protrusions (or cavities) in a first initial substrate, whether by mechanical, chemical, electromagnetic, or other suitable means. A negative copy of the first initial substrate is made in a second initial substrate composed of a material suitable for machining. Upper portions of the protrusions in the second initial substrate are machined to form pyramids. A negative copy of the second initial substrate is then made to form the prepared substrate. The prepared substrate includes cavities corresponding to the pyramids formed in the second initial substrate, and also includes protrusions between the cavities. In some embodiments, the pyramids are cube corner pyramids and the cavities are thus cube corner cavities. With such a prepared substrate, groove side surfaces are then formed by selectively machining the protrusions in such a way that the machined faces formed in the protrusions are in substantial alignment with faces ("non-machined faces") of neighboring cube corner cavities, which faces had been replicated from the second initial substrate. At least one face of the PG cube corner elements is a compound face that includes both one of the machined faces and one of the non-machined faces. The compound face may include a transition line that separates the replicated face from the machined face. Retrorefiective sheeting or other cube corner articles can then be replicated from the prepared substrate as modified by the direct machining operations. Another method for providing the prepared substrate begins with the prepared substrate having a substantially flat working surface. An array of cube 5 corner cavities is then formed deep in the working surface by embossing with a hardened tool. Intermediate poπions of the working surface forming protrusions between the cube corner cavities are left unfinished. Groove side surfaces are then formed selectively in the protrusions to form machined faces, the machined faces and the embossed faces of the cube corner cavities together forming the l o array of cube corner elements . Brief Description of the Drawings FIG. 1 is a plan view of a structured surface comprising one type of PG cube corner element array, known from the PRIOR ART; FIG. 2 is a perspective view of a prepared substrate; 15 FIG. 3 is a perspective view of the substrate of FIG. 2 after machining some groove side surfaces; FIG. 4 is a perspective view of the substrate of FIG. 2 after machining all of the groove side surfaces; FIG. 5 is a top plan view of FIG. 4; 0 FIGS. 6-8 are top plan views of structured surfaces having canted PG cube corner elements, such surfaces being capable of fabrication using the methods discussed in connection with FIGS. 2-5; FIG. 9 is a perspective view of a masked substrate; FIG. 10 is a perspective view of the substrate of FIG. 9 after forming 5 protrusions therein with the aid of the mask; FIG. 11 is a perspective view of a substrate having a non-machinable structured surface replicated from the substrate of FIG. 10; FIG. 12 is a perspective view of the substrate of FIG. 11 after directly machining groove side surfaces in the protrusions thereof to form cube corner 0 pyramids therein; FIG. 13 is a perspective view of a copy of the substrate of FIG. 12, forming a prepared substrate analogous to that of FIG. 2; FIGS. 14a-d are schematics showing 2-dimensional representations of the progression from a structured surface article closely approximating a desired geometry to an article or articles that have the desired geometry; FIGS. 15a-d depict the tips of different cutting tools that can be employed in forming groove side surfaces; FIGS. 16a-f are schematic sectional views that show in magnified fashion the region where two constituent faces of a compound face come together, demonstrating different types of possible transition lines; FIG. 17 is a plan view of a PG cube corner cavity in which each transition line between constituent faces has a finite width; FIG. 18a is a plan view of an initial substrate having an array of four- sided protrusions, and FIG. 18b is a sectional view thereof as indicated; FIG. 19 is a plan view of the substrate of FIGS. 18a-b after forming groove side surfaces in the upper portions of the protrusions; FIG. 20a is a plan view of a substrate obtainable by making a negative copy of the substrate of FIG. 19 and machining groove side surfaces in upper portions of such negative copy; FIG. 20b is a sectional view as indicated in FIG. 20a; FIG. 21 is a perspective view of a group of laminae; FIG. 22 is an endwise elevational view of the laminae in a tilted position and having a set of grooves machined in the working surfaces thereof; FIG. 23 is a side elevational view of the laminae of FIG. 22; FIG. 24 is a magnified endwise elevational view of the laminae after tilting them back into alignment and machining their working surfaces further; FIG. 25 is a plan view of a structured surface of a prepared substrate, the structured surface being a negative copy of the structured surface formed by the laminae of FIG. 24; FIG. 26 is the substrate of FIG. 25 after forming groove side surfaces therein; FIG. 27 is a top plan view of an initial substrate after forming groove side surfaces in upper portions of protrusions; FIG. 28 is a plan view of a substrate obtainable by making a negative copy of the substrate of FIG. 27 and machining groove side surfaces in upper portions of such negative copy; FIG. 29 is a plan view of the substrate of FIG. 28 after machining an additional set of parallel grooves therein; and FIGS. 30-32 show an alternative embodiment in an analogous fashion to FIGS. 27-29. In the drawings, the same reference symbol is used for convenience to indicate elements that are the same or that perform the same or a similar function. Detailed Description of the Illustrative Embodiments In Fig. 2. a prepared substrate 10 is shown enlarged in perspective view. A description of how prepared substrate 10 can be fabricated is deferred for later discussion below. Substrate 10 has a structured surface 12 that generally defines a reference plane x-y. Structured surface 12 includes faces 16 arranged in groups of three that form cube corner cavities 18. Interspersed between cube corner cavities 18 on structured surface 12 are protrusions 20. The protrusions as shown each have three side surfaces and a top surface, and are of triangular cross section. Depending on the procedure used to make the prepared substrate, the side surfaces of the protrusions 20 can be inclined to a greater or lesser extent away from the vertical. In the figure, reference points 22 located on the top surfaces of protrusions 20 are shown for ease of description. It is not critical that the tops of protrusions 20 pass through the locations marked by reference points 22, nor is it critical that the side surfaces or top surface have a well defined shape, orientation, or surface finish. This is because outer portions of protrusions 20 are removed in subsequent direct machining operations. FIG. 3 depicts the prepared substrate 10 at an intermediate stage during such a direct machining operation. Cutting tools 24a,24b,24c (collectively referred to as 24) move along structured surface 12, whether by motion of the cutting tools or the substrate or both, to form groove side surfaces 26a (hidden from view from the perspective of FIG. 3), 26b,26c respectively. Each tool 24 is depicted as a so-called "half-angle" tool, which produces only one groove side surface as it progresses through the material rather than a pair of opposed groove side surfaces, although this is not necessary. Consistent with direct machining procedures, cutting tools 24 move along axes 28a, 28b, 28c that are substantially parallel to the x-y reference plane, thus ensuring that the respective groove side surfaces also extend along axes that are substantially parallel to the reference plane. Preferably, the axes 28a.28b, 28c are carefully positioned and the tool orientation carefully selected so that the groove side surfaces are substantially aligned (translationally and rotationally) with adjacent faces 16 of the cube corner cavities 18. Relatively small misalignments, discussed infra, may however be tolerated or may even be desirable as a function of the desired optical characteristics of the cube corner article and the precision of available cutting machinery. Regardless of the degree of alignment, transition lines 30 separate groove side surfaces from faces 16 of cavities 18. As seen in FIG. 3, a protrusion 20 that has been machined by all of the cutting tools forms a protrusion referred to as a geometric structure 20a, which structure includes one face from each groove side surface 26a, 26b, 26c. In the case where the three faces of structure 20a are substantially aligned with adjacent faces 16 of cavities 18, and where such cavities 18 have a common orientation, the three faces of structure 20a (when considered separately) form a "truncated" cube corner pyramid. Such a pyramid is characterized by having exactly three nondihedral edges that form a "base triangle" in the plane of the structured surface. Substrate 10 is composed of a material that can be scribed, cut, or otherwise machined without significant post-machining deformation and without substantial burring. This is to ensure that the machined faces, or replications thereof in other substrates, can function as effective optical reflectors. Further discussion on suitable substrate materials is given below. It should be noted that although three cutting tools are shown in FIG. 3, a single cutting tool can be used. The cutting tool can be made of diamond or other suitably hard material. The machined faces can be formed by any one of a number of known material removal techniques, for example: milling, where a rotating cutter, spinning about its own axis, is tilted and drawn along the surface of the substrate; fly-cutting, where a cutter such as a diamond is mounted on the periphery of a rapidly rotating wheel or similar structure which is then drawn along the surface of the substrate; ruling, where a nonrotating cutter such as a diamond is drawn along the surface of the substrate; and grinding, where a rotating wheel with a cutting tip or edge is drawn along the surface of the substrate. Of these, preferred methods are those of fly-cutting and ruling. It is not critical during the machining operation whether the cutting tool, the substrate, or both are translated relative to the surroundings. Full-angle cutting tools are preferred when possible over half-angle tools because the former are less prone to breakage and allow higher machining rates. Finally, cutting tools having a curved portion or portions can be used in the disclosed embodiments to provide non-flat (curved) surfaces or faces in order to achieve desired optical or mechanical effects. FIG. 4 is a view of substrate 10 after all groove side surfaces have been formed by direct machining operations. As shown, all of the protrusions 20 previously seen in FIGS. 2 and 3 have been modified to form geometric structures 20a. Each geometric structure 20a comprises three machined faces, one each from groove side surfaces 26a. 26b, and 26c, configured approximately mutually perpendicular to each other to form a truncated cube corner pyramid. Each of these three faces is machined to be substantially aligned with the nearest face of an adjacent cube comer cavity 18. Because of this, new cube comer cavities 32 are formed, each new cube comer cavity 32 comprising one cube comer cavity 18 and one face each of its neighboring geometric structures 20a. Reference numeral 32a shows in bold outline one such cube corner cavity 32. A given face of one of the cube comer cavities 32 comprises one face of a cube comer cavity 18 and one of the machined faces 26a, 26b, or 26c. As will be discussed infra, faces 16 of the cube comer cavity 18 are non-machined faces. Therefore, each cube corner cavity 32 comprises a compound face made up of a non-machined portion and a machined portion. The transition lines 30 separate the machined from the non-machined portions. One can also identify new cube comer pyramids 34 formed on the structured surface shown in FIG. 4. Each cube comer pyramid 34 comprises one geometric stmcture 20a, which is a cube corner pyramid, and one face each of its neighboring cube corner cavities 18. Each face of one of the pyramids 34 is a compound face comprising a non-machined face 16 from one of the cavities 18 and a machined face from stmcture 20a. Reference numeral 34a shows in bold outline one such cube comer pyramid 34. Note that the reference points 22 locate the uppermost extremities or peaks of the pyramids 34. Both cube comer pyramids 34 and cube corner cavities 32 are PG cube comer elements, since both have a face terminating at a nondihedral edge of the cube comer element, such nondihedral edge being nonparallel to reference plane x-y. FIG. 5 shows a top view of the structured surface of FIG. 4. Transition lines 30 are drawn narrower than other lines to aid in identifying the PG cube comer elements, i.e. cube comer cavities 32 and cube comer pyramids 34. The compound faces of such PG cube comer elements have a machined and non- machined poπion disposed on opposed sides of transition lines 30. In the embodiment of FIGS. 2-5, all transition lines 30 lie in a common plane referred to as a transition plane, which in the case of this embodiment is coplanar with the x-y plane. The non-machined faces of the structured surface are disposed on one side of the transition plane and the machined faces are disposed on the other side. Although structural differences between machined and non-machined surfaces are subtle, such differences can generally be detected by inspection of the underlying material. Some suitable inspection techniques include examination of the grain or crystalline stmcture, molecular orientation, or variations in the amount of trace elements such as chromates or lubricants. The directly machined cube comer article of FIGS. 2-5 can itself function as a retrorefiective article, both with respect to light incident from above (by virtue of cube comer cavities 32) and. where the substrate is at least partially transparent, with respect to light incident from below (by virtue of cube comer pyramids 34). In either case, depending upon the composition of the substrate, a specularly reflective thin coating such as aluminum, silver, or gold can be applied to the stmctured surface to enhance the reflectivity of the compound faces. In the case where light is incident from below, reflective coatings can be avoided in favor of an air interface that provides total internal reflection. More commonly, however, the directly machined prepared substrate of FIGS. 2-5 is used as a mold from which end-use retrorefiective articles are made, whether directly or through multiple generations of molds, using conventional replication techniques. Each mold or other article made from the directly machined prepared substrate will nevertheless contain cube comer elements having at least one face terminating at a nondihedral edge of the cube corner element, the at least one face comprising two constituent faces disposed on opposed sides of a transition line, the transition line being nonparallel to such nondihedral edge. As seen from FIGS. 4 and 5, transition lines 30 lie in the transition plane coincident with the x-y plane, whereas nondihedral edges shown in bold for both PG cube comer cavity 32 and PG cube comer pyramid 34 are inclined relative to the x-y plane. It is also possible to fabricate surfaces where the transition lines do not all lie in the same plane, by forming groove side surfaces at different depths in the substrate. A wide variety of structured surfaces can be fabricated using the direct machining technique described above. The PG cube comer elements of FIG. 5 each have a symmetry axis that is perpendicular to the x-y reference plane of the stmctured surface. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the element roughly along the symmetry axis. The amount of light retroreflected by a cube comer element generally drops as the incidence angle deviates from the symmetry axis. FIG. 6 shows a top plan view of a stmctured surface 36 similar to that of FIG. 5, extending along the x-y plane, except that the PG cube comer elements of FIG. 6 are all canted such that their symmetry axes are tilted with respect to the normal of the stmctured surface. The symmetry axis for each PG cube corner cavity 46 in FIG. 6 lies in a plane parallel to the y-z plane, having a vertical component in the +z direction (out of the page) and a transverse component in the -t-y direction. Symmetry axes for the PG cube comer pyramids 48 of FIG. 6 point in the opposite direction, with components in the -z and -y directions. In fabricating surface 36. a prepared substrate (see FIG. 13, infra) is used wherein the protmsions of generally triangular cross-section are isosceles in shape, rather than equilateral as in FIG. 2. Further, the non-machined cube comer cavity faces are arranged to have a similarly shaped isosceles triangle outline. Four distinct types of cube corner elements are present on the structured surface 36: tmncated cube comer cavities having non-machined faces and a triangular outline in plan view; tmncated cube corner pyramids having machined faces and triangular outline; PG cube corner cavities having compound faces and a hexagonal outline; and PG cube comer pyramids, also having compound faces and a hexagonal outline. A representative non-machined cube comer cavity is identified in FIG. 6 by bold outline 40, and a representative machined cube comer pyramid is identified by bold outline 42. Transition lines 44 separate machined from non-machined faces, and all such lines 44 lie in a transition plane parallel to the x-y plane. In other embodiments, the transition lines may lie parallel to a transition plane but not be coplanar. Selected faces of cavities 40 and pyramids 42 form canted PG cube corner elements, in particular canted PG cube comer cavities 46 and canted PG cube comer pyramids 48. Reference points 22, as before, identify localized tips or peaks disposed above the x-y plane. FIG. 7 shows a stmctured surface 36a similar to that of FIG. 6, and like features bear the same reference numeral as in FIG. 6 with the added suffix "a". PG cube comer elements of FIG. 7 are canted with respect to the normal of stmctured surface 36a, but in a different direction compared to that of the PG cube comer elements of FIG. 6. The symmetry axis for each PG cube corner cavity 46a is disposed in a plane parallel to the y-z plane, and has a vertical component in the +z direction and a transverse component in the -y direction. FIG. 8 shows a stmctured surface similar to that of FIGS. 6 and 7, and like features bear the same reference numeral as in FIG. 6 with the added suffix "b". PG cube comer elements in FIG. 8 are also canted, but, unlike the PG cube corner elements of FIGS. 6 and 7, the degree of cant is such that the octagonal outline in plan view of each PG cube corner element has no mirror- image plane of symmetry. The cube comer cavities of FIG. 8 each have a symmetry axis that has components in the +z, +y, and -x direction. It will be noted that the triangles formed by transition lines 44 (FIG. 6) are isosceles triangles each having only one included angle less than 60 degrees; triangles formed by lines 44a (FIG. 7) are isosceles triangles each having only one included angle greater than 60 degrees; and triangles formed by lines 44b (FIG. 8) are scalene triangles. Representative values in degrees for the included angles of such triangles are, respectively: (70, 70, 40); (80, 50, 50); and (70, 60, 50). With the aid of FIGS. 9-13, methods for fabricating a prepared substrate suitable for use in the invention will now be described. For illustrative purposes a stmctured surface useful for producing the stmctured surface of FIG. 6 is described. The same principles can however be straightforwardly applied to other embodiments. In brief, a stmctured surface comprising an array of protmsions is formed in a first substrate (FIGS. 9-10), by a process other than direct machining. A negative copy of the stmctured surface is then made in a second substrate (FIG. 11) composed of a substance capable of being machined. Upper portions of protmsions in the stmctured surface of the second substrate are then directly machined to form cube comer pyramids (FIG. 12). Finally, a negative copy of the stmctured surface of the machined second substrate is made in a third substrate (FIG. 13) to form the prepared substrate, in which an array of cube comer cavities (having replicated faces) is intermeshed with an array of protmsions. The machined second substrate can if desired be used thereafter as a master from which a large number of identical prepared substrates can be electroformed or otherwise replicated. Many variations of the procedure are possible. In one, the first and second substrates are bypassed, and the desired stmctured surface is imparted directly into the prepared substrate using an embossing tool. In another, the first substrate is composed of a machinable material so that cube comer pyramids are formed in the first substrate and the third substrate is replicated directly from the first substrate rather than from an intermediate second substrate. As discussed below, machinable laminae can also be used in manufacturing the prepared substrate. Turning to FIG. 9, a substrate comprising flat lower and upper layers 50, 51 is shown carrying a patterned masking layer 52. Masking layer 52 is patterned in the form of intermeshing arrays of triangles, the size and shape of which are selected to closely approximate the network of transition lines appearing in the finished mold (see FIG. 6). Triangular windows 52a have been formed so that layer 52 comprises an array of triangular-shaped islands 52b. One way of forming such a patterned masking layer 52 is with conventional photoresist and well known photolithographic techniques. For example, layer 52 can initially be applied to the substrate as a uniform layer of metal. Then, a layer of positive or negative photoresist is applied on top of layer 52. Portions of the photoresist are selectively exposed to light using a mask that bears the triangular array pattern, or its inverse, and subsequently exposed or unexposed portions of the photoresist are chemically removed . This opens triangular- shaped windows in the photoresist, so that a second chemical applied to the surface can selectively attack exposed portions of layer 52. After removing the remaining photoresist, only triangular islands of layer 52 remain on the substrate surface. In FIG. 10, protmsions 56 having the desired cross-sectional shape have been formed in the substrate by exposing the masked surface to an anisotropic etching agent and etching completely through upper layer 51. Lower surfaces are shown shaded in FIG. 10 and in subsequent FIGS. 11-13. Although it is possible for layers 50 and 51 to be composed of the same material, in a more preferred approach upper layer 51 is composed of a polymeric material, layers 50 and 52 comprise a metal such as copper that functions as an etch stop, and the etching agent is an intense electromagnetic beam illuminating the entire upper surface. Exposed regions of layer 51 are ablated by the electromagnetic beam, and regions protected by islands 52b are left intact. The electromagnetic etching agent proceeds through the material substantially only along an axis approximately normal to the surface, rather than isotropically. This behavior avoids undercutting substrate material beneath masking layer islands 52b, yielding reasonably well-formed cavities with highly sloped walls and substantially flat bottoms, the cavities defining an array of complementary protmsions 56. After etching completely through layer 51 a depth D into the substrate, the anisotropic etching agent is removed and the remaining masking layer is subsequently eliminated. Depth D is preferably selected to be equal to or greater than the cube height of the PG cube comer elements in the finished mold. In another approach, a substrate similar to that of FIG. 9 is used except that it includes no patterned masking layer 52. Triangular cavities are instead formed by exposing the upper layer 51 to an electromagnetic beam that is itself patterned by a conventional mask and optical system such that areas corresponding to 52a are illuminated and areas corresponding to 52b are not. An example of this approach was demonstrated on a substrate in which lower layer 50 was copper and upper layer 51 was a;0.125 mm thick piece of Kapton H film sold by E. I. Du Pont de Nemours and Company, although Kapton V film and other polyimides are also useable. The electromagnetic etching agent was a pulsed krypton-fluoride laser having a wavelength of 248 nm and an energy density between about 0.5 and 1.2 Joules/cπr. Following the ablation procedure, side walls of the protmsions were found to be sloped at about a 5 degree angle measured from the normal, inclined in such a way that the bases of the protmsions were slightly larger than the tops. Alternative techniques for producing the surface shown in FIG. 10 will be readily apparent to those skilled in the microfabrication arts, techniques such as ion milling, knurling, chemical etching, and even hot melting where the substrate is composed of a soft, low melting point material such as wax. The technique known as LIGA, described for instance in Chapter 6 of Fundamentals of Microfabrication by Marc Madou, (CRC Press 1997), is another available technique. In general, the substrate of FIG. 10 might not be composed of a material suitable for directly machining smooth, accurate surfaces therein. Therefore, the surface features of FIG. 10 may need to be replicated or otherwise copied in another substrate, shown in FIG. 11 with reference numeral 58, such substrate 58 composed of a material suitable for direct machining. Substrate 58 is shown to have a stmctured surface that is a negative copy of the stmctured surface of substrate 50. Thus, cavities 60 correspond substantially to protmsions 56, and protmsions 62 correspond substantially to cavities in substrate 50 that are bounded by protmsions 56. A substrate having a positive copy of the stmctured surface of substrate 50 could equally well be used in place of substrate 58. The upper portions of protmsions 62 are then directly machined with a suitable cutting tool such as a full- or half-angle tool. The tools are guided along axes that are parallel to the substrate and precisely aligned with borders of the triangular protmsions in order to form groove side surfaces 64a, 64b, 64c as shown in FIG. 12. The depth of the cutting tool is limited to some fraction, preferably about one-half, of the dimension D. The groove side surfaces are cut at angles such that tmncated cube corner pyramids 66 are formed on the upper halves of protmsions 62. As a consequence of machining only the upper portions of protmsions 62, the lower portions of cavities 60 remain intact, such lower portions referred to as reduced cavities 60' . The reduced cavities have a depth D', which preferably satisfies the following relationship: D' > (desired PG cube height) - (cube corner pyramid 66 cube height) Finally, to obtain the prepared substrate, a negative copy of the stmctured surface of substrate 58 is made by a suitable replication technique in a substrate 68, which is the prepared substrate in which the PG cube corner elements shown in FIG. 6 are later formed. Substrate 68 has a stmctured surface 70 comprising cube corner cavities 72 corresponding to cube comer pyramids 66. Significantly, the faces of each cube comer cavity 72 have been formed not by machining surface 70 but instead by replicating surface 70 from another stmctured surface. Therefore, the faces of cube comer cavities 72 are referred to as non-machined faces. Protmsions 74 are also included on stmctured surface 70, the protmsions corresponding to reduced cavities 60' and hence having a height D'. Substrate 68 is composed of a machinable material so that precision groove side surfaces can later be machined in the prepared substrate in registration with the non-machined faces of cavities 72 to yield the desired stmctured surface geometry. Regarding canting of cube comer elements, it will be apparent that if canted cube corner elements are desired in the final article for a given optical effect, the cube comer pyramids 66 will be configured to be canted so that the corresponding cube comer cavities 72 as well as the PG cube comer cavities 46 (see FIG. 6) will likewise be canted. Alternatively the cube corner elements need not be canted. As was discussed in connection with FIG. 10, alternative techniques capable of producing a prepared substrate such as shown in FIGS. 2 or 13 will be readily apparent to those skilled in the microfabrication arts. In one such alternative technique, a single pin of triangular cross-section and having a cube comer pyramid formed on one end thereof can emboss into a substrate of a suitable plastic material an array of recessed cube corner cavities using a step- and-repeat process. If the plastic material is machinable, it can then function as the prepared substrate, and in a later operation groove side surfaces can be machined into protmsions between the cube comer cavities. In this case the resulting geometric structures will have at least one compound surface having a machined portion and a non-machined portion, the non-machined portion being an embossed face of the cube comer cavity formed by the pin. If the substrate is not machinable, then a prepared substrate can be formed by producing a positive copy of the stmctured surface in such a machinable material. An even number of conventional replication steps can be used to produce the positive copy. A variation of the pin embossing technique just described is where a plurality of pins are held together and simultaneously emboss the cube comer cavities to a common depth in an initially flat surface. Prepared substrates can take on a variety of forms. In each embodiment shown above, the prepared substrate has a stmctured surface comprising an array of cube comer cavities and protmsions, each protrusion having steeply inclined side walls and a large flattened top surface. When groove side surfaces are later formed in the protmsions, the cutting tool removes a relatively large amount of material because the angle between the steeply inclined side wall and the subsequent machined face is often in excess of 10 degrees, typically ranging from about 10 to about 45 degrees. It may be desirable to provide a modified prepared substrate that has a stmctured surface more closely resembling the finished mold, for example, a substrate whose individual faces are no more than a few degrees, and preferably in the range of about 2 to about 0.5 degrees, from the desired orientation. Groove side surfaces can then be formed in such a modified prepared substrate by removing much less material from the protmsions during the final groove side surface formation, thereby reducing tool forces which could detrimentally cause distortions. Another benefit is less wear on the cutting tool. A modified prepared substrate can also be used as a master from which a family of differently configured daughter molds can be made. FIGS. 142 -d depict schematically three-dimensional stmctured surfaces hi two dimensions for simplicity. It is to be understood that such stmctured surfaces would commonly comprise geometric structures comprising at least three-sided pyramids and at least three-sided cavities. In FIG. 14a, a modified prepared substrate 76 is provided with a stmctured surface 78 comprising actual faces 80a-d as shown. Transition lines 82 (shown as points), lying in a transition plane 84, separate faces 80a,b from faces 80c, d. Reference faces 86a-d are also shown, in broken lines, to represent the desired position of some or all faces in the stmctured surface of the final mold, which also terminate at transition lines 82. Where the final mold is intended to be retrorefiective, reference faces 86a, 86d are parallel to each other and perpendicular to faces 86b, 86c. Several features distinguish modified prepared substrate 76 from previously described prepared substrates such as substrate 10 of FIG. 2. First, faces 80c, are more inclined than faces 86c, d, yielding cavities that are slightly deeper than desired. Such deeper cavities can be formed for example by replicating pyramids that are higher than ordinarily required. Second, protmsions formed by faces 80a. b likewise deviate only slightly from the desired configuration. Faces 80a, b are more inclined than faces 86a,b, yielding protmsions that are slightly higher than desired. Significantly, the actual faces intersect the respective reference faces in transition plane 84. along transition lines 82. The poπions of substrate 76 disposed above plane 84 can then be machined, shaving off the small amount of material necessary to produce protmsions having the desired faces 86a, 86b. More preferably, however, substrate 76 is not itself machined but instead is left intact for use in making additional molds. In such case a negative copy of stmctured surface 78 is first made in another substrate 90 (FIG. 14b). Features of substrate 90 that correspond to features of substrate 76 are given the same reference number, but with the addition of a prime symbol. Faces 80c',80d', formed by replication from faces 80c, 80d, are not shown in FIG. 14b because a subsequent direct machining step has formed groove side surfaces in the protmsions so that they terminate at desired faces 86c '.86d'. Note that cavities formed by faces 80a', 80b' are deeper than desired, because the protmsions from which they were replicated are higher than desired. Note that the stmctured surface 92 of substrate 90 contains cavities and pyramids comprising compound faces, one compound face made up of faces 86d' and 80a' and another compound face made up of faces 80b' and 86c'. Constituent faces 86d' and 80a' are only slightly misaligned with each other, as are constituent faces 80b ',86c'. Hence, stmctured surface 90 has the interesting property that it contains cube corner cavities (represented by compound face 86d'-80a' and compound face 80b'-86c') in which the poπion of the cavities on one side of plane 84' comprise mutually perpendicular constituent faces and the poπion on the other side of plane 84' comprise constituent faces that are not mutually perpendicular, although they are nearly so. The cube comer pyramids formed on stmctured surface 90 can be characterized in the same way. Proceeding to FIG. 14c, a substrate 94 is shown having a stmctured surface 96 that is replicated from stmctured surface 92. Double primes have been appended to the reference numerals which otherwise correspond to the designations in FIGS. 14a and 14b. As with stmctured surface 92, stmctured surface 96 contains compound face cube comer elements in which the poπion of the elements on one side of plane 84" comprise mutually perpendicular constituent faces and the poπion on the other side of plane 84" comprise constituent faces that are not mutually perpendicular, although they are nearly so. Substrate 94 shown in FIG. 14c can serve as a prepared substrate for the final mold shown in FIG. 14d. Groove side surfaces are directly machined in the substrate by a cutting tool that stays on the upper side of plane 84" and removes small amounts of material from the protmsions to form machined faces 86a" and 86b", which are in substantial alignment with replicated faces 86d" and 86c" respectively. The cube corner elements disclosed herein can be individually tailored so as to distribute light retroreflected by the aπicles into a desired pattern or divergence profile, as taught by U.S. Pat. No. 4,775,219 (Appledom et al.). For example, compound faces that make up the PG cube corner elements can be arranged in a repeating pattern of orientations that differ by small amounts, such as a few arc-minutes, from the orientation that would produce mutual orthogonality with the other faces of cube comer element. This can be accomplished by machining groove side surfaces (both those that ultimately become the faces in the finished mold below the transition plane as well as those that become faces in the finished mold above the transition plane) at angles that differ from those that would produce mutually oπhogonal faces by an amount known as a "groove half-angle error" . Typically the groove half-angle error introduced will be less than ± 20 arc minutes and often less than ± 5 arc minutes. A series of consecutive parallel groove side surfaces can have a repeating pattern of groove half-angle errors such as abbaabba... or abcdabcd..., where a, b, c, and d are unique positive or negative values. In one embodiment, the pattern of groove half-angle errors used to form faces in the finished mold above the transition plane can be matched up with the groove half-angle errors used to form faces in the finished mold below the transition plane. In this case, the machined and non-machined poπions of each compound face will be substantially angularly aligned with each other. In another embodiment, the pattern used to form one set of faces can differ from the pattern used to form the other, as where the faces below the transition plane incorporate a given pattern of nonzero angle eπors and faces above the transition plane incorporate substantially no angle errors. In this latter case, the machined and non-machined poπions of each compound face will not be precisely angularly aligned with each other. Advantageously, a substrate such as substrate 76 discussed in connection with FIG. 14a can serve as a master substrate from which a whole family of daughter molds can be made, all having the same general shape of cube comer element in plan view but having slightly different face configurations. One such daughter mold can incorporate cube comer elements that each have compound faces whose constituent faces are aligned, the compound faces all being mutually perpendicular to the remaining faces of the cube corner element (see e.g. FIG. 14d). Another such daughter mold can incorporate cube comer elements that also have compound faces whose constituent faces are aligned, but the compound faces can differ from orthogonality with remaining faces of the cube corner element. Still another such daughter mold can incorporate cube comer elements that have compound faces whose constituent faces are not aligned (see e.g. FIGS. 14b, 14c). All such daughter molds can be made from a single master mold with a minimal amount of material removed by machining. Transition Lines In the preceding figures, transition lines between constituent faces of a compound face have been illustrated as simple lines or dots. Transition lines can in general take on a great variety of forms, depending upon details of the cutting tool used and on the degree to which the motion of the cutting tool is precisely aligned with other faces in the process of forming groove side surfaces. Although in many applications transition lines are an aπifact to be minimized, in other applications they can be used to advantage to achieve a desired optical result such as a paπially transparent aπicle. FIG. 15 shows greatly magnified schematic profiles of some possible cutting tools useable with disclosed processes. FIGS. 15(a) and (b) depict half- angle tools, and FIGS. 15(c) and (d) depict full-angle tools. Flattened tips are provided in the tools of FIGS. 15(a) and (c). Sharp-edged tips are provided in those of FIGS. 15(b) and (d). Other possibilities include tools having a rounded or radiused tip. Each of these tools can comprise a synthetic diamond or other suitable hard material as is known to those of skill in the an. FIG. 16 shows, also in greatly magnified fashion, schematic sectional views depicting the region where two constituent faces of a compound face come together, demonstrating different types of possible transition lines. Each view is along the axis of the respective transition line. In FIG. 16(a), a machined face 104 is formed in near perfect registration with a non-machined face 106, yielding a nearly imperceptible transition line 108a of minimal width. The line may be detectable only by observing a difference in microscopic surface texture between face 104 and 106. In FIG. 16(b), a flat-tipped cutting tool positioned too far into the substrate material produces a small horizontally-disposed flat surface making up transition line 108b. In FIG. 16 (c), a cutting tool positioned too far away from the substrate material leaves a small veπically-disposed remnant of the protmsion wall to form transition line 108c. In FIG. 16(d), a sharp-tipped tool positioned too far into the substrate material and too deep produces a jagged transition line 108d. In FIG. 16(e), a sharp-tipped tool that is misaligned when forming groove side surfaces both in the prepared substrate and in a predecessor substrate (from which the prepared substrate is replicated) produces an even more jagged transition line 108e. Transition line 108f of FIG. 16(f) is like line 108e, except that line 108f is made using flat-tipped cutting tools. FIG. 17 shows the effect of having transition line 108b (see FIG. 16) in place of idealized transition lines 30 in the stmctured surface shown earlier in FIGS. 4 and 5. FIG. 17 shows a top plan view of one of the PG cube comer cavities 32. Constituent faces 16 and 26a, 26b, 26c are separated by flat transition lines 108b. Assuming the faces are highly reflective so that cavity 32 is retrorefiective, reflections of each transition line will be visible and are shown as 108b'. (Reflections of the three dihedral edges of PG cube comer cavity 32 will also be visible, but are not shown to avoid confusion.) The area taken up by the three lines 108b as well as their counterpaπs 108b' detracts directly from the area that is effective for retroreflection. Thus, if retrorefiective active area is to be maximized, the width of the transition lines will be minimized by carefully controlling the cutting tool position. It may be desirable however to make an aπicle that is not only retrorefiective but also behaves like a simple flat minor, or that is paπially transparent. In such cases a transition line such as 108b may be used to achieve such results. — Additional Embodiments — An Embodiment That Is Not Fully Retrorefiective In FIGS. 18a and 18b, a substrate 110 having an array of four-sided protmsions and cavities is shown. Protmsions are defined by upper surfaces 112 and adjoining sloped side surfaces 114, while cavities are defined by lower surfaces 116 and the adjoining surfaces 114. Depressed flat surfaces are shown shaded in the top plan view of FIG. 18a, as well as in FIG. 19. The surface of substrate 110 can be formed in the same way as that of substrate 58 discussed previously in connection with FIG. 11. As shown in FIG. 19, groove side surfaces 118, 120 are formed in the upper poπions of the protmsions by the action of cutting tools moving along axes 122, 124 respectively. The groove side surfaces form four-sided pyramids in the protmsions. The pyramid peaks are identified by dots 126. Note that lower surfaces 116 and the lower poπions of side surfaces 114 are left intact. A negative copy of the machined substrate 110 is made using standard replication techniques in a substrate 128. Peaks 126 in substrate 110 produce pits 126a in substrate 128, the pits 126a lying at the bottom of downwardly inclined faces 118a, 120a which correspond respectively to upwardly inclined groove side surfaces 118, 120 of substrate 110. Flat-topped protmsions are formed in substrate 128 by lower surfaces 116, but are shown in FIGS. 20a and 20b after forming groove side surfaces 130, 132 therein by action of a cutting tool along axes 134, 136 respectively. Groove side surfaces 130, 132 are in substantial alignment with adjacent faces 118a, 120a respectively, and form pyramids with pyramid peaks 138. Transition lines 140 separate the machined faces from replicated faces 118a. 120a. Adjacent machined and replicated faces, each of which are three-sided, combine to form four-sided compound faces. Such compound faces define extended geometric structures (both cavities centered at pits 126a and pyramids centered at peaks 138, see outline 142 of a representative extended cavity) that are both wider and deeper than either of the simple pyramids formed from triangular faces 118, 120 in substrate 110 or from triangular faces 130, 132 in substrate 128. Substrate 128, or positive or negative replicas thereof, can be used for a variety of purposes. If faces 118a are made mutually perpendicular, and likewise for faces 120a, 130, and 132, then the aπicle can function as a so-called flashing retroreflector which is retrorefiective only in selected planes of incidence. Such aπicle is retrorefiective only for a light source whose direction of incidence lies in a plane perpendicular to the plane of FIG. 20a and parallel either to axes 134 or to axes 136. Such aπicle can be illuminated either from the stmctured surface side or from the flat side opposite the stmctured surface, depending upon the details of construction:' Substrate 128 can also be used as an abrasive surface, or as a mold to produce abrasive replicas. For such an application, faces 118a can be made mutually perpendicular, as can faces 120a, 130, and 132, or they can be oriented at smaller or larger angles to yield desired abrasive propeπies. An Embodiment Making Paπial Use of Laminae In embodiments disclosed above, the direct machining technique is used together with other techniques such as etching, embossing, replicating, and so on to produce stmctured surfaces never before associated with direct machining. The example that follows shows how it is possible for the direct machining technique to also be used in conjunction with non-unitary techniques such as the laminate technique. In brief, a prepared substrate is produced by replicating a stmctured surface formed by a group of laminae whose working surfaces have been machined. The prepared substrate is then directly machined to yield a finished stmctured surface. In FIG. 21, a plurality of individual laminae 146 each having a working surface 148 are held together in a fixture (not shown) that defines a base plane 150. Each lamina is composed of a material suitable for machimng smooth burr- free surfaces. The laminae are disposed on and perpendicular to the base plane in FIG. 21. FIG. 22 shows an end elevational view looking down axis 152 after the laminae are tilted or rotated in the fixture about axis 152 by an angle θ, and after a set of adjacent v-shaped grooves is formed in the working surfaces 148 by a cutting tool moving parallel to an axis 154 that is perpendicular to axis 152 and parallel to plane 150. The grooves have groove side surfaces 155, 157 that intersect at upper edges 156 and lower edges 158. FIG. 23 shows a side elevational view of the laminae as viewed along the axis 154. Adjacent groove side surfaces 155, 157 are approximately mutually perpendicular. The laminae are then set upright as in FIG. 21 and an additional groove side surface 160 is formed in the working surface of each laminae by action of a cutting tool moving parallel to axis 152. Surface 160 is formed approximately mutually perpendicular to surfaces 155, 157, thereby defining a row of cube comer pyramids in the working surface of each laminae, each cube corner pyramid comprising one each of surfaces 155, 157, and 160. Finally, every other laminae is offset in a direction parallel to axis 152 such that edges 156 of one lamina line up with edges 158 of its adjacent laminae. A magnified endwise elevational view of three laminae as just described is shown in FIG. 24, the view otherwise corresponding to that of FIG. 22. A negative copy of the stmctured surface produced by the laminae is then made in a unitary substrate 162, a top plan view of which is shown in FIG. 25. Faces and edges corresponding to those of FIG. 24 are identified with the same reference number with the addition of a prime. Since the working surface of the laminae defines cube comer pyramids, the stmctured surface of substrate 162 has cube comer cavities formed by replicated faces 155', 157', and 160'. Both the cube comer pyramids of FIG. 24 and the cube comer cavities of FIG. 25 are PG cube corner elements because at least one nondihedral edge (158 or 158') is inclined relative to the plane of the stmctured surface and is parallel to an adjacent nondihedral edge of a neighboring cube comer element. Substrate 162 can be considered a prepared substrate, with an array of cube corner cavities interspersed with an array of protmsions 164, only one of which is outlined in broken lines in FIG. 25. Each protmsion 164 has a triangular base that bounds three triangular faces: an upper poπion of face 155' ; and upper poπion of face 157'; and a substantially veπical face shown at 166 that joins the other two faces. Direct machining is then performed on prepared substrate 162 to form groove side surfaces 168 in protmsions 164, the groove side surfaces being in substantial alignment with faces 160' and extending along axes that are parallel to the plane of the stmctured surface. Transition lines 170 also extend along such axes and separate machined faces 168 from replicated faces 160'. Tips or peaks 172 are formed in protmsions 164 at the intersection of machined faces 168 and the remaining poπions of replicated faces 155', 157'. Adjacent faces 168 and 160' form a compound face. A comparison of FIG. 26 with FIG. 5 reveals that the stmctured surface of substrate 162 has PG cube comer elements, both PG cube corner pyramids and PG cube comer cavities, just as substrate 10 does. Also apparent is the fact that whereas cube comer elements of substrate 10 included three compound faces, each comprising two constituent faces disposed on opposed sides of a transition line, cube comer elements of substrate 162 have only one compound face. Finally, the stmctured surface of FIG. 26 is seen to comprise an array of three-sided pyramids 164 having one machined face 168, and an array of cavities defined by non-machined faces, the cavities comprising the remainder of the stmctured surface (the poπions of faces 155', 157', 160' that lie below the plane defined by transition lines 170), and each pyramid disposed proximate to and at least paπially extending above one of the cavities. Embodiments Having Optically Opposed Cube Comer Pyramids Without Cube Comer Cavities Another embodiment is shown by the sequence of FIGS. 27-29. FIG. 27 shows a plan view of an initial substrate 180 having a stmctured surface comprising an array of protmsions 182 and cavities 184 (shown shaded), the cavities 184 being bordered by substantially veπical walls of the protmsions 182. Substrate 180 is similar to the substrate of FIG. 12, except that the protmsions and cavities of substrate 180 are four-sided diamond shapes in horizontal cross section rather than triangles. Groove side surfaces a. b as well as c, d have been formed in the substrate by the action of cutting tools along axes 186 and 188 respectively. Axes 186, 188 are parallel to the plane of the stmctured surface, thus ensuring that side surfaces a, b, c, d all extend along axes parallel to such plane. The geometry of the cutting tool is selected to configure surfaces "a" substantially perpendicular to surfaces c, and to configure surfaces b substantially perpendicular to surfaces d. Each protmsion 182 thus has four faces a, b, c, d that are inclined relative to the plane of the stmctured surface and that meet at an elevated peak identified by a dot 189. Note that faces a, b, c, d do not form a cube comer element. A negative copy of this stmctured surface is then, by electroforming or other suitable means, made in a substrate 190 referred to herein as a prepared substrate. Faces a-d of initial substrate 180 form replicated faces a'-d' in prepared substrate 190. Cavities 184 of substrate 180 form protmsions 192 in substrate 190, such protmsions shown in FIG. 28 after groove side surfaces e, f, g, h have been formed therein by cutting tools moving along axes 194, 196. The cutting tools are controlled to form such surfaces substantially parallel to and in substantial registration with the respective replicated faces so that pairs of individual faces a' and f, b' and e, c' and h, and d' and g form compound faces, respectively designated face a'f, face b'e, face c'h, and face d'g. The faces a'f are substantially perpendicular to faces c'h, and faces b'e are substantially parallel to faces d'g. Dots 198 locate the peaks of pyramids formed by faces e, f, g, h. Transition lines 200, all disposed substantially in a common transition plane parallel to the plane of the figure, separate machined faces e-h from non- machined faces a'-d'. FIG. 29 depicts substrate 190 after forming therein a set of parallel grooves comprising opposed groove side surfaces i and j by action of a cutting tool along axes 202 as shown. In the embodiment shown, surfaces i and j are inclined at the same angle relative to the normal to the stmctured surface, although this is by no means required. Such grooves extend deeper into substrate 190 than transition lines 200, preferably extending to a depth about equal to the local minima disposed at the intersection of faces a', b', c', d'. The cutting tools remove the highest poπions of the stmctured surface, shifting the uppermost peaks from points 198 (FIG. 28) to points 204. Surface i is configured to be substantially perpendicular to compound faces b'e and d'g, thus forming one group of PG cube comer pyramids labeled 206. Surface j is configured to be substantially perpendicular to compound faces a'f and c'h, forming another group of PG cube comer pyramids labeled 208. Pyramids 206, 208 are matched pairs of cube comer elements because one corresponds to a 180 degree rotation of the other about an axis perpendicular to the stmctured surface, and because there is a one-to-one correspondence of pyramids 206 to pyramids 208. Note that each pyramid 206, 208 has exactly two faces that are compound. Note also that the stmctured surface contains no cube comer cavities. However, tmncated non-machined faces a', b', c', d' do form cavities, and pyramids formed by machined faces e, g, and i, or by machined faces h, f , and j , are aπanged on the stmctured surface such that a plurality of such pyramids border a given cavity. The sequence of FIGS. 30-32 depict a variation of the embodiment just described. In FIG. 30, an initial substrate 210 has an array of protmsions 212 and cavities 214 (shown shaded) therein, with groove side surfaces a, b, c, d formed in the upper poπions of the protmsions by cutting tools acting along axes 216, 218. Lower poπions of the protmsions have substantially veπical walls, not visible in top plan view. Cavities 214 extend to and are bordered by such veπical walls of a large number of protmsions because such protmsions do not form a fully interconnected array in plan view, as in previous figures, but rather only a paπially interconnected aπay. Cavities 214 can thus be referred to as "open" cavities rather than "closed" cavities, as they are not substantially entirely bounded by protmsion walls on all sides. As before, surfaces "a" are perpendicular to surfaces c, and surfaces b are perpendicular to surfaces d. Points 220 locate the peaks of the pyramids formed by surfaces a, b, c, d. FIG. 31 shows a negative copy of the stmctured surface in another substrate 222. Faces a-d produce replicated faces a'-d'. Cavities 214 produce extended protmsions 224, on which have been formed groove side surfaces e, f, g, h by action of one or more cutting tools along axes 226, 228, the cutting tools controlled to form surfaces e, f, g, h coplanar with surfaces b', a', d', c' respectively thus forming compound faces a'f, b'e, c'h, and d'g. Transition lines 230 separate machined faces e-h from non-machined faces a'-d'. Points 232 locate the peaks of pyramids formed by these compound faces. FIG. 32 shows substrate 222 after forming a set of three different parallel grooves therein along axes 234, 236, 238. The groove along axis 236 is formed by a sharp-edged tool such as that of FIG. 15b or FIG. 15d and comprises opposed groove side surfaces i and j . The grooves along axes 234 and 236 are formed with tools having flattened tips such as thai of FϊG. 15a or 15c. Opposed groove side surfaces k and 1 incline upwardly from flat groove bottom m, as do groove side surfaces n and o from flat groove bottom p. Surfaces i, k, and n are parallel to each other and perpendicular to compound faces b'e and d'g. Surfaces j, 1, and o are parallel to each other and perpendicular to compound faces a'f and c'h. This geometry results in six different types of PG cube comer pyramids on the stmctured surface: pyramids 240, having faces k, b'e, and d'g; pyramids 242, having faces 1, a'f, and c'h; pyramids 244, having faces i, b'e, and d'g; pyramids 246, having faces j, a'f, and c'h; pyramids 248, having faces n, b'e, and d'g; and pyramids 250, having faces o, a'f, and c'h. Pyramids 240 and 242 are matched pairs, as are pyramids 244, 246, and pyramids 248, 250. The depth of the final set of parallel grooves is adjusted so that the peaks of the cube comer pyramids 240, 242, 244, 246, 248, and 250 are all disposed at the same elevation, although this is not required. Thus, the groove along axis 236 has the deepest groove bottom, the groove along axis 234 has the shallowest groove bottom (m), and the groove along axis 238 has a groove bottom (p) of intermediate depth. The use of grooves having different groove bottoms as shown impacts the optical performance of retrorefiective aπicles made from the stmctured surface of substrate 222. First, flat groove bottoms m, p produce flat features in the retrorefiective aπicle that can act as windows, making the aπicle paπially transparent. Second, the various groove geometries affect the so-called aspect ratio of the cube comer elements, which in turn impacts retrorefiective performance as a function of viewing angle. Aspect ratio as used herein relates to the degree of elongation of the outline of a cube comer element seen in top plan view. For example, the PG cube corner pyramid 244 shown in bold outline has a left and a right edge that are separated by a given width, and an upper and a lower edge that are separated by a given length. The ratio of length to width is the aspect ratio for that paπicular cube comer element. It can be readily seen that the PG cube corner pyramids of substrate 222 all have the same width but that pyramids 244 and 246 have the greatest length, pyramids 240 and 242 have the shoπest length, and pyramids 248 and 250 have an intermediate length. Adjustment of the aspect ratio of the cube comer apertures is desirable because it can tailor the divergence profile (for a fixed source position, the amount of retroreflected light as a function of viewing angle) and the entrance angularity (for a fixed viewing angle, the amount of retroreflected light as a function of source position) of the cube corner article. Providing grooves with flat groove bottoms as shown in FIG. 32 have an additional benefit when such flat groove bottoms are deeper in the substrate than the sharp recessed points or edges located at the intersection of other faces. In such case, negative copies of the substrate, which can be joined together to scale- up a larger mold, will be more robust and less prone to damage because the highest features on the surface of the negative copies will be flat-topped ridges. Placing such a negative copy face-down on a flat surface, the flat-topped ridges will themselves experience little damage because of their large surface area, and further will protect sharp points or ridges formed by neighboring faces from damage due to contact with such flat surface. Cube comer elements of FIGS. 29 and 32 can be canted or uncanted as desired. Producing cube comer elements that are canted to a greater or lesser degree is accomplished by tailoring the shape of the diamond-shaped protrusions (FIGS. 27, 30) and then the orientation of the groove side surfaces (a, b, c, d, e, f, g, h, i, j, etc.) to be in conformance with the desired degree of canting. It has already been noted that the techniques used to make the embodiments of FIGS. 29 and 32 produce matched pairs of optically opposed cube comer elements. If canting is used, then such matched pairs can, in keeping with principles discussed in U.S. Patent Nos. 4,588.258 (Hoopman), 5,812,315 (Smith et al.), and 5,822,121 (Smith et al.), give rise to widened retrorefiective angularity so that an aπicle having the structured surface will be visible over a widened range of entrance angles. Turning again to FIG. 32, the stmctured surface shown there can be viewed, similar to the surfaces of FIGS. 4, 5, 6, 7, 8, 12, 14b, 14d, 19, 20a, 20b, 26, 27, 28, and 29, 30, 31, as comprising cavities formed by non-machined faces and pyramids formed at least in pan by machined faces, each of the pyramids disposed proximate to at least one of the cavities. The tmncated non- machined faces a', b', c', d' in FIG. 32 form the cavities, and the pyramids are formed by machined faces e, g, and one of k, i. or n, or by machined faces h, f, and one of 1, j, or o. Discussion The working surface of the mold substrates can have any suitable physical dimensions, with selection criteria including the desired size of the final mold surface and the angular and translational precision of the machinery used to cut the groove surfaces. The working surface has a minimum transverse dimension that is greater than two cube comer elements, with each cube comer element having a transverse dimension and/or cube height preferably in the range of about 25 μm to about 1 mm, and more preferably in the range of about 25 μm to about 0.25 mm. The working surface is typically a square several inches on a side, with four inch (10 cm) sides being standard. Smaller dimensions can be used to more easily cut grooves in registration with non-machined surfaces over the whole stmctured surface. The substrate thickness can range from about 0.5 to about 2.5 mm. (The measurements herein are provided for illustrative purposes only and are not intended to be limiting.) A thin substrate can be mounted on a thicker base to provide rigidity. Multiple finished molds can be combined with each other e.g. by welding in known tiling arrangements to yield a large tiled mold that can then be used to produce tiled retrorefiective products. In the manufacture of retrorefiective articles such as retrorefiective sheeting, the structured surface of the machined substrate is used as a master mold which can be replicated using electroforming techniques or other conventional replicating technology. The stmctured surface can include substantially identical cube comer elements or can include cube corner elements of varying sizes, geometries, or orientations. The stmctured surface of the replica, sometimes referred to in the art as a 'stamper', contains a negative image of the cube comer elements. This replica can be used as a mold for forming a retrorefiective aπicle. More commonly, however, a large number of suitable replicas are assembled side-by-side to form a tiled mold large enough to be useful in forming tiled retrorefiective sheeting. Retrorefiective sheeting can then be manufactured as an integral material, e.g. by embossing a preformed sheet with an array of cube comer elements as described above or by casting a fluid material into a mold. See, JP 8-309851 and U.S. Patent No. 4,601,861 (Pricone). Alternatively, the retrorefiective sheeting can be manufactured as a layered product by casting the cube comer elements against a preformed film as taught in PCT application No. WO 95/11464 (Benson, Jr. et al.) and U.S. Pat. No. 3,684,348 (Rowland) or by laminating a preformed film to preformed cube comer elements. By way of example, such sheeting can be made using a nickel mold formed by electrolytic deposition of nickel onto a master mold. The electroformed mold can be used as a stamper to emboss the pattern of the mold onto a polycarbonate film approximately 500 μm thick having an index of refraction of about 1.59. The mold can be used in a press with the pressing performed at a temperature of approximately 175° to about 200° C. The various mold substrates discussed above can generally be categorized into two groups: replicated substrates, which receive at least pan of their stmctured surface by replication from a prior substrate, and bulk substrates, which do not. The substrate shown in FIG. 9 is an example of a bulk substrate, as are the laminae 146 of FIGS. 21-24. Replicated substrates can be further categorized into those whose stmctured surface is subsequently machined— such as prepared substrate 10 of FIGS. 2-5, substrate 58 of FIGS. 11-12, prepared substrate 68 of FIG. 13, substrate 90 of FIG. 14b, prepared substrate 94 of FIGS. 14c-14d, substrate 110 of FIGS. 18a-b and 19, substrate 128 of FIGS. 20a-b, prepared substrate 162 of FIGS. 25-26, substrate 180 of FIG. 27, substrate 190 of FIGS. 28-29, substrate 210 of FIG. 30, and substrate 222 of FIGS. 31-32— and those whose stmctured surface is not subsequently machined, such as the final mold that is used for embossing or casting-and-curing retrorefiective sheeting. Suitable materials for use with bulk mold substrates are well known to those of ordinaiy skill in the art, and generally include any material that can be machined cleanly without bun formation and that maintains dimensional accuracy after groove formation. A variety of materials such as machinable plastics or metals may be utilized. Acrylic is an example of a plastic material; aluminum, brass, electroless nickel, and copper are examples of useable metals. Suitable materials for use with replicated mold substrates that are not subsequently machined are well known to those of ordinary skill in the an, and include a variety of materials such as plastics or metals that maintain faithful fidelity to the prior stmctured surface. Thermally embossed or cast plastics such as acrylic or polycarbonate can be used. Metals such as electrolytic, chemical, or vapor deposited nickel or nickel alloys are also suitable. Suitable materials for use with replicated mold substrates whose stmctured surface is subsequently machined are also well known to those of ordinary skill in the art. Such materials should have physical properties such as low shrinkage or expansion, low stress, and so on that both ensure faithful fidelity to the prior stmctured surface and that lend such materials to diamond machimng. A plastic such as acrylic (PMMA) or polycarbonate can be replicated by thermal embossing and then subsequently diamond machined. Suitable hard or soft metals include electrodeposited copper, electroless nickel, aluminum, or composites thereof. With respect to retrorefiective sheetings made directly or indirectly from such molds, useful sheeting materials are preferably materials that are dimensionally stable, durable, weatherable and readily formable into the desired configuration. Examples of suitable materials include acrylics, which generally have an index of refraction of about 1.5, such as Plexiglas resin from Rohm and Haas; thermoset acrylates and epoxy acrylates, preferably radiation cured, polycarbonates, which have an index of refraction of about 1.6; polyethylene- based ionomers (marketed under the name 'SURLYN'); polyesters; and cellulose acetate butyrates. Generally any optically transmissive material that is formable, typically under heat and pressure, can be used. Other suitable materials for forming retrorefiective sheeting are disclosed in U.S. Pat. No. 5,450,235 (Smith et al.). The sheeting can also include colorants, dyes, UV absorbers, or other additives as needed. It is desirable in some circumstances to provide retrorefiective sheeting with a backing layer. A backing layer is particularly useful for retrorefiective sheeting that reflects light according to the principles of total internal reflection. A suitable backing layer can be made of any transparent or opaque material, including colored materials, that can be effectively engaged with the disclosed retrorefiective sheeting. Suitable backing materials include aluminum sheeting, galvanized steel, polymeric materials such as polymethyl methacrylates, polyesters, polyamids, polyvinyl fluorides, polycarbonates, polyvinyl chlorides, polyurethanes, and a wide variety of laminates made from these and other materials. The backing layer or sheet can be sealed in a grid pattern or any other configuration suitable to the reflecting elements. Sealing can be affected by use of a number of methods including ultrasonic welding, adhesives, or by heat sealing at discrete locations on the anays of reflecting elements (see, e.g. U.S. Pat. No. 3,924,928). Sealing is desirable to inhibit the entry of contaminants such as soil and/or moisture and to preserve air spaces adjacent the reflecting surfaces of the cube comer elements. If added strength or toughness is required in the composite, backing sheets of polycarbonate, polybutryate or fiber-reinforced plastic can be used. Depending upon the degree of flexibility of the resulting retroreflective material, the material can be rolled or cut into strips or other suitable designs. The retroreflective material can also be backed with an adhesive and a release sheet to render it useful for application to any substrate without the added step of applying an adhesive or using other fastening means. Glossary of selected terms An "anay of neighboring cube corner elements" means a given cube comer element together with all adjacent cube corner elements bordering it. "Closed cavity" means a cavity that is substantially entirely bounded by walls of protmsions or pyramids on all sides. "Compound face" means a face composed of at least two distinguishable faces (refened to as "constituent faces") that are proximate each other. The constituent faces are substantially aligned with one another, but they can be offset translationally and/or rotationally with respect to each other by relatively small amounts (less than about 10 degrees of arc, and preferably less than about 1 degree of arc) to achieve desired optical effects as described herein. "Cube corner cavity" means a cavity bounded at least in part by three faces arranged as a cube corner element. "Cube comer element" means a set of three faces that cooperate to retroreflect light or to otherwise direct light to a desired location. Some or all of the three faces can be compound faces. "Cube corner element" also includes a set of three faces that itself does not retroreflect light or otherwise direct light to a desired location, but that if copied (in either a positive or negative sense) in a suitable substrate forms a set of three faces that does retroreflect light or otherwise direct light to a desired location. "Cube comer pyramid" means a mass of material having at least three side faces arranged as a cube comer element. "Cube height" means, with respect to a cube comer element formed on or formable on a substrate, the maximum separation along an axis perpendicular to the substrate between poπions of the cube comer element. "Dihedral edge" of a cube comer element is an edge of one of the three faces of the cube comer element that adjoins one of the two other faces of the same cube comer element. Note that any paπicular edge on a stmctured surface may or may not be a dihedral edge, depending upon which cube comer element is being considered. "Direct machining" refers to forming in the plane of a substrate one or more groove side surfaces typically by drawing a cutting tool along an axis substantially parallel to the plane of the substrate. "Geometric stmcture" means a protmsion or cavity having a plurality of faces. "Groove" means a cavity elongated along a groove axis and bounded at least in part by two opposed groove side surfaces. "Groove side surface" means a surface or series of surfaces capable of being formed by drawing one or more cutting tools across a substrate in a substantially continuous linear motion. Such motion includes fly-cutting techniques where the cutting tool has a rotary motion as it advances along a substantially linear path. "Non-machinable" means, with respect to a stmctured surface that extends along a reference plane, that such stmctured surface cannot be fabricated simply by drawing a cutting tool along paths substantially parallel to the reference plane. "Nondihedral edge" of a cube comer element is an edge of one of the three faces of the cube comer element that is not a dihedral edge of such cube comer element. Note that any particular edge on a stmctured surface may or may not be a nondihedral edge, depending upon which cube comer element is being considered. "PG cube corner element" stands for "preferred geometry" cube comer element, and is defined in the context of a stmctured surface of cube comer elements that extends along a reference plane. For the purposes of this application, a PG cube comer element means a cube corner element that has at least one nondihedral edge that: (1) is nonparallel to the reference plane; and (2) is substantially parallel to an adjacent nondihedral edge of a neighboring cube comer element. A cube comer element whose three reflective faces are all rectangles (inclusive of squares) is one example of a PG cube comer element. "Prepared substrate" means a substrate that has a plurality of faces corresponding to only portions of a desired or final structured surface. "Protmsion" has its broad ordinary meaning, and can comprise a pyramid. "Pyramid" means a protmsion having three or more side faces that meet at a vertex, and can include a frustum. "Reference plane" means a plane or other surface that approximates a plane in the vicinity of a group of adjacent cube comer elements or other geometric structures, the cube corner elements or geometric structures being disposed along the plane. "Retroreflective" means having the characteristic that obliquely incident incoming light is reflected in a direction antiparallel to the incident direction, or nearly so, such that an observer at or near the source of light can detect the reflected light. "Stmctured" when used in connection with a surface means a surface that has a plurality of distinct faces arranged at various orientations. "Symmetry axis" when used in connection with a cube comer element refers to the axis that extends through the cube comer apex and forms an equal angle with the three faces of the cube comer element. It is also sometimes referred to as the optical axis of the cube comer element. "Transition line" means a line or other elongated feature that separates constituent faces of a compound face. All 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 an will recognize that changes can be made in form and detail without depaπing from the spirit and scope of the invention.

Claims: WHAT IS CLAIMED IS: 1. A geometric stmcture having a plurality of faces and disposed on a stmctured surface, wherein at least one of the faces is a compound face comprising a machined portion and a non-machined portion. 2. The geometric stmcture of claim 1, wherein the geometric stmcture comprises a cube corner element that has as one of its faces the compound face. 3. The geometric stmcture of claim 1, wherein the geometric stmcture is selected from the group consisting of a four-sided pyramid and a four-sided cavity. 4. The geometric stmcture of claim 1, wherein the geometric stmcture is selected from the group consisting of a three-sided pyramid and a three-sided cavity. 5. The geometric stmcture of claim l , wherein the machined portion is substantially aligned with the non-machined portion. 6. The geometric stmcture of claim 5, wherein the machined and non-machined portions have angular orientations that differ by less than 10 degrees of arc. 7. The geometric stmcture of claim 6, wherein the machined and non-machined portions have angular orientations that differ by less than 1 degree. 8. The geometric stmcture of claim 7, wherein the machined portion and the non-machined portion are substantially coplanar. 9. The geometric stmcture of claim 2, wherein the non-machined portion is a replicated portion. 10. The geometric stmcmre of claim 2, wherein the non-machined portion is an embossed portion. 11. The geometric stmcmre of claim 2. wherein the compound face has a transition line separating the machined portion from the non-machined portion. 12. The geometric stmcmre of claim 11, wherein the compound face terminates at a nondihedral edge of the cube corner element, and wherein the transition line is nonparallel to the nondihedral edge. 13. The geometric stmcture of claim 2, wherein the cube comer element has a substantially hexagonal outline in plan view. 14. The geometric stmcmre of claim 2, wherein the cube corner element has a substantially rectangular outline in plan view. 15. The geometric stmcmre of claim 2, wherein exactly one of the faces of the cube corner element is compound. 16. The geometric stmcmre of claim 2. wherein exactly two of the faces of the cube corner element are compound. 17. The geometric stmcmre of claim 2, wherein exactly three of the faces of the cube corner element are compound. 18. A mold comprising a plurality of cube corner elements as set forth in claim 2. 19. The mold of claim 18, wherein at least some of the plurality of cube comer elements are PG cube corner elements. 20. The mold of claim 18, wherein the plurality of cube corner elements have cube heights no greater than about 1 mm. 21. The mold of claim 18, wherein the plurality of cube comer elements are part of a stmctured surface that comprises cavities formed by non-machined faces and pyramids formed at least in pan by machined faces. 22. The mold of claim 21, wherein both the cavities and the pyramids comprise cube comer elements. 23. A retroreflective aπicle made directly or indirectly from the mold of claim 18. 24. A retroreflective aπicle made directly or indirectly from the mold of claim 19. 25. A cube comer element having three faces and a cube height of no greater than about 1 mm, at least two of the three faces each terminating at a nondihedral edge of the cube comer element, and each of the at least two faces comprising two constituent faces disposed on opposed sides of a transition line that is nonparallel to the respective nondihedral edge. 26. The cube corner element of claim 25, wherein each of the three faces terminate at a nondihedral edge of the cube corner element, each of the three faces comprising two constituent faces disposed on opposed sides of a transition line that is nonparallel to the respective nondihedral edge. 27. The cube corner element of claim 25. wherein all of the transition lines are parallel to a reference plane. 28. The cube comer element of claim 25, wherein, for at least one of the at least two faces, one of the two constiment faces has a machined surface and the other of the two constituent faces has a non-machined surface. 29. A cube comer article having a stmctured surface comprising a plurality of cube comer elements as set forth in claim 25. 30. The cube comer article of claim 29, wherein all of the transition lines are parallel to a reference plane. 31. The cube comer aπicle of claim 29, wherein the plurality of cube comer elements comprise PG cube corner elements. 32. The cube comer aπicle of claim 29, wherein the cube corner elements comprise matched pairs of cube comer elements. 33. The cube comer aπicle of claim 31, wherein the stmcmred surface comprises cube comer pyramids and cube comer cavities. 34. The cube comer aπicle of claim 29, wherein the cube comer aπicle comprises retroreflective sheeting. 35. The cube comer article of claim 30, wherein the cube corner article comprises retroreflective sheeting. 36. An array of neighboring cube comer elements each having three faces, wherein, for each cube corner element in the aπay: at least one of the three faces terminates at a nondihedral edge of such cube corner element; and the at least one face comprises two constituent faces disposed on opposed sides of a transition line that is nonparallel to the nondihedral edge. 37. The aπay of claim 36, wherein the cube corner elements define a reference plane, and wherein each transition line in the aπay is substantially parallel to the reference plane. 38. The aπay of claim 36, wherein the at least one face comprises two of the three faces. 39. The array of claim 36, wherein the at least one face comprises all three faces. 40. A cube comer article having a stmctured surface comprising the aπay of neighboring cube comer elements as set forth in claim 36. 41. The cube corner aπicle of claim 40, wherein the cube comer aπicle comprises retroreflective sheeting. 42. A cube comer article having a stmctured surface comprising the aπay of neighboring cube comer elements as set forth in claim 38, and fuπher wherein each transition line in the aπay is substantially parallel to a common reference plane. 43. The cube comer aπicle of claim 42, wherein the cube corner aπicle comprises retroreflective sheeting. 44. The cube comer article of claim 36, wherein the cube comer elements each have a cube height no greater than about 1 mm. 45. The cube corner aπicle of claim 36, wherein the cube comer elements comprise matched pairs of cube corner elements. 46. A mold having a stmcmred surface comprising a cavitiy formed by non- machined faces and a plurality of pyramids bordering the cavity and formed at least in pan by machined faces. 47. The mold of claim 46, wherein the pyramids comprise cube comer pyramids. 48. The mold of claim 46, wherein the pyramids extend at least in pan above the cavity. 49. The mold of claim 47, wherein the cavity comprises a cube comer cavity. 50. The mold of claim 47, wherein the cavity comprises at least four faces. 51. The mold of claim 46, wherein at least some of the machined faces are adjacent to and substantially aligned with at least some of the non-machined faces. 52. The mold of claim 46, wherein at least some of the non-machined faces together with at least some of the machined faces form PG cube comer elements. 53. The mold of claim 46, wherein at least some of the faces are offset from a desired orientation by less than about 10 degrees of arc. 54. The mold of claim 53, wherein at least some of the non-machined and machined faces are offset from a desired orientation by between about 2 and about 0.5 degrees of arc. 55. A retroreflective aπicle made directly or indirectly from the mold of claim 46. 56. A retroreflective article made directly or indirectly from the mold of claim 52. 57. A method of making a stmcmred surface article comprising a geometric stmcmre having a plurality of faces, the method comprising: providing a prepared substrate having a non-machined face; and removing material from the prepared substrate to form a machined face such that the machined face and the non-machined face together form one of the plurality of faces of the geometric stmcture. 58. The method of claim 57, wherein the prepared substrate has at least one additional non-machined face, and wherein the removing step further forms at least one additional machined face, the at least one additional non-machined face and the at least one additional machined face forming at least another of the plurality of faces. 59. The method of claim 57, wherein the relieving step forms the machined face in an orientation offset by less than about 10 degrees of arc from a desired orientation. 60. The method of claim 57, wherein the at least one geometric stmcmre comprises at least one cube comer element, and wherein the one of the plurality of faces is a face of the at least one cube comer element. 61. The method of claim 60, wherein the providing step comprises: providing an initial substrate having an intermediate face; and producing a negative copy of the initial substrate to form the prepared substrate, the non-machined face being a replica of the intermediate face. 62. The method of claim 60, wherein the providing step comprises: embossing the non-machined face into the prepared substrate. 63. The method of claim 60, wherein the non-machined face and the machined face are substantially planar. 64. The method of claim 63, wherein the non-machined face and the machined face are substantially coplanar. 65. The method of claim 63, wherein the non-machined face and the machined face are noncoplanar. 66. The method of claim 65, wherein the removing step forms the machined face in an orientation offset by less than about 10 degrees of arc from that of the non-machined face. 67. The method of claim 60, wherein the at least one cube corner element comprises a PG cube corner element. 68. The method of claim 60, wherein the prepared substrate has a plurality of non-machined faces arranged to form a plurality of cavities, the prepared substrate also having protmsions extending above the cavities. 69. The method of claim 68, wherein the providing step comprises: providing a first substrate having a plurality of intermediate protmsions; removing material from an upper poπion of at least some of the intermediate protmsions to form intermediate pyramids therein; producing a negative copy of the first substrate to form the prepared substrate, wherein the plurality of cavities coπespond to the intermediate pyramids. 70. The method of claim 69, wherein the removing material from the prepared substrate step comprises removing material from an upper poπion of at least some of the protmsions to form a plurality of machined faces. 71. The method of claim 69, wherein the providing a first substrate step comprises providing an initial substrate, modifying the initial substrate to define initial protmsions, and making a negative copy of the initial substrate to produce the first substrate. 72. The method of claim 69, wherein the intermediate protmsions have a triangular cross-section. 73. The method of claim 69, wherein the intermediate protmsions have a diamond-shaped cross-section. 74. The method of claim 69, wherein the intermediate protmsions are substantially fully interconnected. 75. The method of claim 69, wherein at least some of the intermediate protmsions are not fully interconnected. 76. The method of claim 69, wherein the removing material from an upper poπion of at least some of the intermediate protmsions step comprises removing material from an upper ponion of each of the intermediate protmsions to form intermediate cube comer pyramids therein. 77. The method of claim 69, wherein the removing material from the prepared substrate step comprises removing material from an upper poπion of each of the protmsions to form cube comer pyramids therein. 78. The method of claim 77, wherein the removing material from the prepared substrate step forms cube comer pyramids substantially in registration with the cavities to form compound cube corner pyramids that comprise at least poπions of the cube comer pyramids and at least poπions of neighboring cavities. 79. The method of claim 69, wherein at least some of the cube comer elements are PG cube comer elements. 80. The method of claim 68 wherein the at least one cube comer element is a plurality of cube comer elements forming a stmcmred surface, the method funher comprising replicating the stmcmred surface to form retroreflective cube corner sheeting. 81. A method of forming in a substrate a stmcmred surface extending along a reference plane and containing PG cube comer elements, comprising: providing a prepared substrate; and forming in the prepared substrate groove side surfaces that extend along axes that are substantially parallel to the reference plane; wherein poπions of at least some groove side surfaces form at least poπions of the PG cube comer elements. 82. The method of claim 81 , wherein the prepared substrate has an initial stmctured surface that includes a first plurality of faces, and wherein the groove side surfaces define additional faces, the additional faces and the first plurality of faces together forming the PG cube corner elements. 83. The method of claim 82, wherein exactly one of the three faces of each PG cube comer element is a compound face comprising one of the first plurality of faces and one of the additional faces. 84. The method of claim 82, wherein exactly two of the three faces of each PG cube comer element are compound faces comprising one of the first plurality of faces and one of the additional faces. 85. The method of claim 82, wherein all of the three faces of each PG cube comer element are compound faces comprising one of the first plurality of faces and one of the additional faces. 86. The method of claim 82, wherein the first plurality of faces define at least one cube comer cavity that has at least four faces. 87. The method of claim 82, wherein the additional faces define cube corner pyramids. 88. The method of claim 82, wherein a transition plane separates at least some of the first plurality of faces from at least some of the additional faces. 89. The method of claim 81 , wherein the providing step comprises providing a prepared substrate having an exposed surface that is stmctured, and wherein the groove side surfaces are formed in the exposed surface. 90. The method of claim 81, wherein the providing step comprises: providing a first substrate having a plurality of initial protmsions; removing material from an upper ponion of at least some of the initial protmsions to form intermediate pyramids therein; and producing a negative copy of the first substrate to form the prepared substrate, the prepared substrate having a plurality of cavities coπesponding to the intermediate pyramids, the prepared substrate also having secondary protmsions extending above the cavities. 91. The method of claim 81, funher comprising replicating the stmcmred surface to form retroreflective cube comer sheeting.

Inventors: BENSON GERALD M, SMITH KENNETH L, BENSON, GERALD, M., SMITH, KENNETH, L.
Applicants: 3M INNOVATIVE PROPERTIES CO, BENSON GERALD M, SMITH KENNETH L, 3M INNOVATIVE PROPERTIES COMPANY, BENSON, GERALD, M., SMITH, KENNETH, L.
Application year: 1999
Main classification: G02B5
IPC codes: G02B5