CA1100459A - Retroreflective marking tape - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A retroreflective marking material is disclosed comprising a sheet having at least some light-transmitting portions, a light-refracting surface on one side, and a plurality of trihedral light-reflecting units of three mutually perpendicular facets on the other side adapted to be adhered to a working site.
The light-reflecting units are oriented to reflect light having relatively high angles of incidence with respect to the sheet. The marking material may be used as a tape for marking roads and is effective whether wet or dry.
Optionally, the sheet or tape has light-receptors which rise above the tape and are spaced apart along one direction of the tape, and the trihedral light-reflecting units are substantially opposite the light-receptors. Preferably, the light-reflecting units have the shape of a triangular-based pyramid, and the marking material retroreflects light in two opposite directions.

Description

This invention relates to a retroreflector which may be used wherever light reflection is desired. A leading application of the retroreflector is as a marlcing ~ape for roadways, pave~ents, airport runways, and the like, and there~ore it is described with respect to this use.
Light-reflecting units o~ the rectangular parallelepiped type, as hPrein described, are disclosed in United States pal:ent 4,073,568 issued February 14th 1978 and United States patent 4,076,383 issued February 28th 1978, both patents being in the name of James H. ~easley.
Retroreflective tapes or strips are mounted on the sur~ace of a road-way, such as along its center line or shoulders, to delineate paths or lanesor traffic, or at intersections to define stopping lines or cross-lanes for traffic, both vehicular and pedestrian. Markers of this type are mounted in spaced apart relation and serve to guide traffic in following or traversing a roadway, or in following a curve or grade in the roadway. Part:icularly to assist a driver of a vehicle a~ night, these markers have light reflectors which catch and return incident beams of light from vehicular headlights back toward the source of the light. Since automobiles of recent vintage have quite power-ful headlights, the use of marking tapes has become more widespread. Markin~
tapes contribute to traffic safety such as when roads are wet from rain. Under certain conditions~ such as fog, ligh~-reflecting road marking me~ls can be the only way of orienking a driver to a changing direction of a road.
Several forms of retroreflective marking tapes have been suggested.
They suer from one or more limitations, such as reflecting too small a pro-portion of incident light while an approaching vehicle is still at an appreciable distance; or being susceptible excessively to exposure to the weather~ particu-larly rain alone or with subsequent freezing. 7he practice has been to place light-reflecting elements at or even above the upper level of the road marking tape ~ut at this location, the light-reflecting units are not only subject to wear and even breakage from vehicular and oot traffic~ but as well to erosion - ' ,:
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and the damaging ef:fects due to entry of water ancl the like. Further, the accuracy and intensity of light-reflection of such marking tapes are often adversely affected when wet by rain.
For example, in United States Paten-t 3,5~7,~15 to Eigenmann a marking tape is disclosed having a plurality of spaced elements arranged on and partly embedded in the top of a base s~rip, the elements being of a composition hav-ing a substantial amount of microspheroids to provide light reElection.
United States Patent 3,920,3~6 to Wyckoff discloses a marker strip provided with spaced wedges having embedded centrally therein an upwardly oriented retroreflective material, such as glass beads, in righ-t-angled posi-tion with respect to a roadway. rn such a construction, there is no effort to orient refracted light with the retroreflective mateTial so as to obtain maximum reflectivity. F.ven more serious, should a wedge become broken for any reslson, water and like debris have easy entry and access to the retrore:Elec-tive material to destroy its effectiveness.
United States Patent 3,935,365 to Eigenmann discloses a tape material having a twin-layer structure including a lower relatively thin support layer slnd an upper principal thicker layer formed of a composition having hard crys-tal-line particles and light-reflecting particles em~edded therein at various levels.
SUMMARY OF THE lNVENTION
The invention provides a retroreflective marking material comprising in combination:
~a) a substantially continuous sheet having at least some light-trans-mittlng portions, a light-refracting surface on one side adapted to be exposed, and a plurality of trihedral light-reflecting units hav-ing three mutually perpendicular facets on the other side~

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, ?~5~3 ~b) light-garnering and re~racting receptors of polygonal cross-section elevated above and spaced along said one side of the sheet, and said receptors being free of light-reflecting units on their exposed sides ~hich are co-extensive with said exposed one side of the sheet and adapted to direct light toward said ligh~-reflecting units on said other side of the sheet, ~c) said light-reflecting units being oriented to reflect light striking said exposed side of said sheet at relatively high angles of inci-dence, and (d~ said sheet being adapted to be directly adhered along said other side to a mounting surface.
The material is preferably provided in sheet or tape form to give a retroreElector of relatively simple design which has efficient retroreflect-ivity and which is particularly well sllited :~or use as a retroreElective mark-ing tape for a roadway or ~he like. A desirable feature is that the light-reflecting units are shielded by the tape rather than exposed by it, so that the units are protected against wear and destruction by the elements such as rain water.
In one form~ the retroreflective marking material is preferably fab-ricated from an organic polymeric resin. As a modification, light-receptors may rise above one side of the sheet and be spaced along one direction of the sheet, but in the present construction the trihedral light-reflecting units are shielded by being placed on the other side of the sheet or at the inter-face between the sheet and the light-receptors. The light-reflecting units are preferably coated with metal to aid in their reflecting function and can be arrang-d to be bidirectional with respect to retroreflecting light.

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Although the lig~ht-re-flecting units must he trihedral and therefore compriscd three mutually perpendicular facets, the si~e of the facets and their optical orientation may vary. For exa~nple, such units may be cube cor-ners or rectangular parallelepipeds. A preferred form is that o~ a triangular-based pyramid which is well suited for use in bidirectional reflection.
Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:-Figure 1 is a fragmentary, perspective view of a roadway having twoforms of the present retroreflective marking tape adhered thereto;
Figure 2 is an enlarged cross-section of Figure 1 on the line 2-2;
Figure 3 is an enlarged cross-section of Figure 1 on the line 3-3;
Figure ~ is a cross-section like Figure 3 ~nd illustrates light-reflecting units at the interface of light-receptors and the body of a marking tape;
Figure 5 is a cross-sec~ion like Figure 3 and shows an al~ernate shape or a light-receptor as well as an alternate location of the light-reflecting units;
Figure 6 is a cross-section like Figure 3 and illustrates one arrange-ment for bidirectional light-re~lection;
Figure 7 is a cross~section like Figure 3 and shows the use of an elastically deformable light-receptor;
Figure 8 is a section of Figure 7 on the line ~-8;
Figure 9 is a fragmentary, enlarged, longitudinal section of a present marking tape and a section of Figure 10 on the line 9-9, and shows the use of cube corners as light-reflecting units;
Figure 10 is a view of Figure 9 on the plane of the line 10~
~i~ure 11 is a fragmentary, enlarged longitudinal section of a present marking tape and illustrates the use of rectangular parallelepipeds as the light-reflecting units;
Figure 12 is a view of Figure ll on the plane of the line 12-12;
Figure 13 is a fragmentaryJ enlarged, bottom plan view of a marking tape having triangular-based pyramids as light-reflecting units;
Figures 1~ and 15 are sections of Figure 13 on the lines 1~-14 and 15-15, respectively; and ~igure 16 is a perspective view of triangular-based pyramidal light-reflecting units in which the outlines of the bases of the pyramids are isosceles triangles.
Several embodiments of the general structure of the retroreflective marking tape are described, after which different light-reflecting units are considered, any of which may be use~ with a tape. Any one embodiment or modification may be used with any one or more other embodiments or modifications.

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Referring ~o Fi~ure 1, a roadway 10 which may be of any known con-struction has retroreflective marking tapes 11 and 12 adhered thereto which represent two different embodiments and which are shown in enlarged cross-section by Figures 2 and 33 respectively. Any embodiment of the ~ape may be in the form of a straight strip, continuous or discontinuous, or arranged in the form of circles, polygons, arrows, letters, symbols, and the like. Preferably, the tape is flexible to facilitate handling and application.
In Figure 2, tape 11 has a refracting surface 13 and a plurality of trihedral light-reflecting units 14, having three mutually perpendicular facets~
on the other side. An adhesive 15 secures tape 11 to roadway 10~ Ihe composi-tions used for tape 11 and adhesive 15 may be the same for all embodiments.
The body of tape 11, for exmaple, must at least have some portions tha~ are light-transmitting through which light reaches light-reflecting units 14 and ma~ be fabricated from any suitable durable, weather resistant material, such as ceramic, glass, or synthetic resinous plastic materials, such as polyvinyl chloride, the polycarbonates like the polycarbonate of bisphenol A, and especially the acrylates like polymethacrylate and polymethylmethacrylat0 resins~ Such compositions may be glazed or pigmented, if desired, to impart colors.
Adhesive 15 may be any suitable material sufficiently adhesive to hold tape 11 with respect to roadway 10. Examples of suitable adhesives include natural ones like glue or bitumen, or synthetic resinous adhesives like epoxy, polyester, or polyurethane resins. Thermosetting cemen~s such as ~ubber hydrochloride may also be used. Still other useful adhesives are described in the cited United States Patents No. 3,585,415 and No. 3,935,365 to Eigenmann.
The trihedral light-relecting units 14, ha~e three mutually perpendicular ~ace~s and may be any of those hereinaf~er more ~ully deseribed.

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In the embodiments of Figure 2, units 14 extend cont:inuously along the underside of tape ll. Because o the positioning of the marking tape on a roadwc~y, that is, appreciably below normal eye level, units 14 must be oriented to reflect light having relatively high angles of incidence with respect to the tape.
This is illustrated in Figure 2 by an incident ray of light represented at 16 which strikes tape 11 at an angle of incidence A that becomes almost a glancing angle. Ray 16 is refracted by surface 13, retroreflected by units 14, and returned in substantially the same direction, as illustrated.
As used herein, the optical axis of a light-reflecting ~mit 14 is that axis of the trihedral configuration along which the unit is maximally retroreflective. In the present tape, the optical axes of units 14 are preferably substantially parallel to the light they receive, that is, to the direction ray 16 takes after being refracted by surface 13. However, satis-factory perormance can still be achieved if the optical axes deviate from the direction of the refracted light, for example, up to about lS degrees. To aid in their reflecting function, light-reflecting units 14, and the units of any embodiment, may be coated with metal or metalli~ed in a manner known in the art to form a metallic layer 17. Aluminum is the preferred metal for this purpose and can be applied by vapor deposition. Adhesive lS fills the volume between the l~wer, embossed side of tape 11, having the light-re1ecting units 14, and roadway 10 to secure tape 11 to the roadway.
The size of the tape is not critical. In one instance, a marking t~pe was 4 inches in width and about 2 mils to about 10 mils in thickness. In the embodiment of Figure 2, the body of tape ll actually protects light-reflecting units 14 from wear and exposure to the elements. The tape is operative whether wet or dry.
Figures 3 through 8 illust~ate modified forms of the marking tape which include the use of light~receptors and optior.al positioning of the light-_7_ - . - ' ' , . ' .' , ~ ~ .: ' ' . .
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reflecting units. It is with;n the contemplation of the present tape to in corporate light-receptors ~hich rise above the tape and are spaced apart along one direction. The light-receptors may have any polygonal or spherical cross-section and are used to garner rays of incident light and assist in focusing them tot~ard the light-reflecting units. The latter can be placed at different stations below the light-receptors while still constituting an integral part of the tape and still being protected by an overlying segment of the tape.
For exampleJ in Figure 3, a tape 12 has light-receptors 1~ which rise above intervening flat areas 20 so as to be spaced lengthwise of the tape.
Light-recep~ors 19 are triangular in cross-sectlon in the longitudinal direction of tape 12. Light-reflecting units 14 extend continuously along the underside of tape 12 althoughJ if preferredl they can extend along only that section of the underside which is substantially opposite light-receptors 19. An adhesive 15 secures tape 12 to roadway 10. The use of a light-receptor 19 permits a slightly different orientation of li~ht-reflecting units 1~ in aligning their optical axes with respect to the direction of the refracted light. For example, an incident ray of light 21 may be refracted and retroreflected as illustrated in Figure 3.
It is possible to place light-reflecting units at the interface of light receptors and the body of the tape. In this case, the light-receptors may be separately fabr;cated and secured to the body of the tape by an adhesive.
Figure ~l illustrates a triangular light-rec~ptor 23 having a plurality of light-reflecting units 2~ formed as by embossing along its flat base. An adhesive 25 fills the cavities left by the light-reflecting units and secures light-receptor 23 to a strip 26 defining the marking tape, and adhesive 15 secures strip 26 to roadway 10. Adhesive 25 may be any of those previously described for adhesive 15. The separat0 fabrication of light-receptor 23 and strip 26 enables each to contain a different coloring pigment. For exa~ple, strip 26 , ,, ,, ~ .

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can be colored white for daylight marking, while light-receptor 23 can be tinted red or green or still other color. In this case only light-receptor 23 need be light-transmitting.
Figure 5 illustrates a light-receptor 27 having a quadrilateral cross-section and also a further modification in that the wide bot~om side of receptor 27 is flat. In this case, light-reflecting units 2~ are formed in an upper surface 30 of a strip 31 defining the tape. An adhesive 32 fills the cavities of the light-reflecting units 28 to secure receptor 27 and strip 31 together, and adhesive 15 secures strip 31 to roadway 10. However, since in this case adhesive 32 fills units 28 and is therefore interspersed between light-receptor 27 and units 28, it is necessary that adhesive 32 be light-transmitting. Light-transmltting adhesives which can be used for this purpose are known in the art and include transparent epoxy resins, transparent polyurethanes~ solvent-based synthetic rubbers, and the like. In the embodiments of Figures 4 and 5, strips 26 and 31 need not be light-~ransmitting and can be opa~ue. Por this purpose such strips can be colored white, red, green, or still other colors.
It is also possible to make the marking tape bidirectional, that is, to receive ~nd re~roxeflect light from either of two opposite directions.
Figure 6 illustrates one embodiment of this modification in conjunction with a light-receptor and the use of light-reflecting units disposed only sub-stantially opposite a light-receptor, although these structural features are not essential to bidirectional retroreflection. In the embodiment of Figure 6J a tape 33 has a triangular shaped light-receptor 34 with light-reflecting units embossed along the underside of the tape, as in the embodiment of Figure 3. However, in this instance the units extend along the underside a distance which corresponds generally to the length of receptor 34 along the tape and, further, a~e oriented so as to be bidirectional. Thus, light-reflecting units 35a on the left hand side as viewed in Figure 6 have their optical axes _ g_ - : .: .~ .. . . , . . .. : - .

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arranged to be preferably parallel or to deviat0 no more than about 15 degrees from light approaching from the right side of the figure. The retroreflection of a ray of incident light 36 occurs therefore as illustrated in Figure 6.
Light approaching from the left side receives little retroreflectivity from units 35a. Conversely, light-reflecting units 35b on the righ~ sid~ as viewed in Figure 6 have their optical axes arranged to be ]preferably parallel or to deviate no more than about 15 degrees from light approaching from the left.
The retroreflection of a ray of incident light 37 oscurs therefore as illustrated in Figure 6. Light approaching from the right receives little retroreflection from units 35b. If desired, that portion of the tape contain-ing units 35a may be tinted one color such as green, and that portion of the tape containing units 35b can be tinted another color such as red.
Marking tapes, and especially those having upward projections like the present light-receptors, are subject to rugged wear and treatment because of being almost constantly over-ridden by vehicular traffic, such as automotive tires, snow plows, and the like. In one form, a light-receptor of the marking tape may be elastically deformable and thereby adapted ~o return substantially to its original position after deformation. In this manner, the light-receptor resiliently yields to over-rolling vehicular traffic and prolongs its useful life. Yet, after deformation, the light-receptor immedia~ely returns sub-stantially to its original position where it is useful as before. For this purpose, the light-receptor can be fabricated from various elastomeric, resin-ous synthetic plastics. Polyvinyl chloride is one such elastomer that may be so used.
Figures 7 and 8 illustrate a marking tape having an elastically defo~nable light-receptor as ~Jell as an adaptior. of this form for bidirectional light retroreflection. Preferably~ the light-receptor is hollow and th0 light-reflecting units are disposed along an inner side of the light-receptor. In ' ' ~ '' ... .

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'; ' , `, ~:: ' the illustrated embodimentJ a marking tape 71 has a light-receptor 72 of tri~
angular cross-section in the longitudillal direct;on of the tape. Light-receptor 72 is hollow and may be open from the underside of tape 71 as illustrated. Adhesive 15 secured tape 71 to roadway 10~ Li~ht-reflecting units are conventionally formed along an inner side of light-recep~or 72.
The illustrated embodiment is bidirectional. Light-re1ecting units 73a and 73b are formed along inner and opposite sides of light-receptor 72.
Thus, an incident ray of light 74 approaching from the left as viewed in Figure 7 is refracted by the left hand side of the triangular shaped light-receptor 72 and retroreflec~ed by light-reflecting units 73a as illustrated.
An incident ray of light 75 approaching from the right is refTacted by the right hand side of triangular light-receptor 72 and retrore1ected by light-reflecting units 73b. In use, light-receptor 72 can be smashed flat and immediately bounce back to its original station. Alternati~ely, hollow-light-receptor 72 may be filled with a compressible material, such as foamed elastomeric material, which is secured inside the hollow-pocket formed by light-receptor 72 and/or to roadway 10.
The light-reflecting units, such as those illustrated at 14, 24, 28, ; 35a, 35b, 73a and 73b have been previously described as trihedral light-reflecting units having three mutually perpendicular facets. Figures 9 through 16 illustrate some of the confi~rations such trihedral units may take. These may be summarized as cube corners, rectangular parallelepipeds, truncated rectangular parallelepipeds, and ~runcated cube corners. In all of the figures, it will be appreciated that the light-reflecting units are shown greatly oversize to facilitate their illustration and description.
Figures 9 and 10 illustrate light-reflecting units which are cube corners, sometimes referred to as triple reflectors, in which the light-.

reflecting unit comprises three square faces which are mutually perpendicular.
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Since the sides are square and equal in size and thus simulate a cube, such a reflecting unit has come to be called a "cube corner". In Figures 9 and 10, the cooperating sides o a single cube corner unit are illustrated at 39, 40, and 41. Such a unit is repeated to for~ a row 42, cmd likewise such rows are repeated in parallel ~ashion to form a light-reflecting array along the under-side of a marking tape ~3. By ~he very nature of cube corners, it is difficult if not impossible to avoid optically inactive bands or blind spots 44 ~Figure 10) in forming the cube corners by pins. In a commercial production, rows of such pins are somewhat longitudinally offset with respect to each other unavoidably forming optically inactive bands 44 in the light-reflecting array. These bands represent loss of reflec~ive area and accordingly diminish the effectiveness and light-reflecting intensity of the array. Also, while a cube corner structure provides satisfactory reflectance to light striking perpendicularly against an array or strip of cube corners, that is, generally parallel to the axes of the cube corners, its performance falls off fairly rapidly as incident light strikes the array at angles away from the normal to the surface of the cube corner array.
A more prefèrred trihedral light-reflecting unit is illustrated by ~igures 11 and 12 and is herein termed a rectangular parallelepiped. If a polyhedron is a solid bounded by planesl and a prism is a polyhedron of which two faces are congruent polygons in parallel planes, and the other faces are parallelograms having two of their sides in the two parallel planes, a parallelepiped may be broadly defined as a prism whose bases are parallelo-grams. A right parallelepiped, then, in a parallelepiped with edges perpendi-cular to the bases. The term ''rectc~ngular parallelepiped" can mean a right parallelepiped whose bases or sides are rectangles. However, some authorities broadly define a rectangle as a parallelogram whose fo~lr angles are right angles. This definition generically includes a square as a rec~angle. At ~ -12-.
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: , least two of the three rectangular faces which form the present rectan~llar parallelepiped light-reflecting unit cannot be square. Each oF the at least two faces must be rectangular under the more generally used defirlition that two adjacent edges of a rectangle are unequal. As used here and in the claims, the term "rectangular parallelepiped" means a right parallelepiped of which at least two faces or face~s are rectangular in the sense ~hat each such rectangle has two adjacent unequal edges.
The rectangular parallelepiped herein contemplated may be regarded as the volume occupied by a six-faceted box, all six facets being a quadrilateral with four right angles. In an actual light~reflecting unit, only ~hree of the facets which meet in a point are actually exposed on the underside of the tape, at least two of those facets being rectangular in which two adjacent edges of each of the two rectangular facets are unequal. Preferably, the two rectangular acets, as defined, are equal in size or area, and the third facet is a square.
The three acets of the rectan~llar parallelepiped may also be considered as forming the sides of a trihedral angle with two facets being rectangular as defined.
Referring to Figures 11 and 12, a marking tape 45 has a plurality of light-reflecting units 46 along its underside comprising three mutually perpen-dicular facets 47, 48, and 49 which define a trihedral angle of a rectangular parallelepiped as described. Facets 47 and 48 intersect each other in a direc-tion toward refracting surface 51 of tape 45 to form an intersecting line 52 and define the two rectangular facets of the rectangular parallelepiped pre-viously described. Facet 49 is a square in the preferred embodiment~ A light-reflecting unit 46 is preferably so positioned ~lith respect to refracting face 51 that a body diagonal of the rectangular parallelepiped ~hich is also its optical axis is preferably substantially parallel to incident light refracted by surface 51, although the body diagonal can deviate ~herefrom up to about lS

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The best light-reflecting unit found to date for the marking tape is that of a triangular based pyramid as illustrated by Figures 13, 14, and 15. This unit is well suited for light having high angles of incidence (grazing incidence) which is needed in a marking tape, and by virtue of its configuration such a ~mit can readily be nested wîth other like units to pro-vide bidirectional retroreflection. More particularly, this type of unit is a truncated rectangular parallelepiped formillg an asymmetrical triangular-based pyramid. The three mutually perpendicular facets are those facets which meet ~he triangular base, one of the facets having a much smaller area than the other two. Such pyramids can be formed, for example, by cutting three parallel sets vf grooves at specific spacings and angles, note Figure 13. The width and depth of the grooves may be varied depending on the retroreflection desired. Each groove forms two adjacent facets of two different pyramids. The resulting reflector has two optical axes and there~
fore is bidirec~ional in light reflection, each axis being at the same angle to the surface normal but in opposite azimut~al directions.
Referring to Figures 13 ~o 15, a marking tape 56 has a plurality of light-reflecting units generally represented at 57. Each unit 57 has three mutually perpendicular facets indicated at 58, 59, and 60. Facets 58 and 59 converge as ~iewed in Figure 13 to an intersecting line 61 and are perpendic-ular to facet 60. The pyramid of the three facets 58~ 59, and 60 define a ,:

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unit 57 which projects from the plane of ~he bottom of underside of tape 56, such that the base of ~he pyramid from which facets 58, 59, and 60 extend actually merges with the body of tape 56. The triangular bases of the pyramids may, therefore, be contained in an imaginary plane indicated in Figures 14 and 15 at 62, plane 62 being essentially parallel to a refracting surface 63 of tape 56.
As shown in Figure 13, light-reflectlng u]nits 57 are placed in juxta-posed rows indicated at 64, each row being placed transversely to the direction of light to be retroreflected. Each row 64 consist~ of pyra~idal units arranged in alternating order, such that the transversely disposed facets 60 and 60a of tNo adjacent pyramids are separated by a point 65 of an inter-vening third pyramid. This alternating positioning enables the array of units 57 to be bidirectional. Thus, as viewed in Figures 13 and 14, facets 58, 59, and 60 of one light-reflecting unit and facets 58a, 59a, and 60a of another similarly disposed light-reflecting unit and still other ~mits simllarly dis-posed receive and retroreflect light coming from the right. In contrast, acets 67, 68 and 69 of one light-reflecting uni~ and facets 67a~ 68a, and 69a of another light-reflecting unit as well as other units similarly disposed receive and retroreflect ligh~ coming from the left. Each row 64 optically behaves the same.
When the pyramidal units 57 are truncated rectangular parallelepipeds, the outlines of the triangular bases of the projecting pyramids are isosceles triangles. When the pyramidal units are truncated cube corners, the outlines of the triangular bases of the projecting pyramids are equilateral triallgles.
As a further guide in illustrating the pyramidal type of light-reflecting unit, Figure 16 illustrates a perspective view of truncated rectangular parallele-pipeds. Figure 16 is taken on the imaginary plane 62 of Figure 14.
In Figure 16, or instance, the plane of the sheet of drawing contains .. . - .. .,, . ' .. .
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the triangular bases of the pyra~lidal light-refLecting units 57. Thus, points A~ BJ and C lie in the plane of the sheet of drawing and point D lies depressed - below the plane of the sheet of drawing. The pyramid defined by points ABCD
is a truncated rectangular parallelepiped, and the outline of base ABC is an isosceles triangle. Similarly, poin~s A, C, and E lie in the plane of the sheet of drawing and point F lies depressed below the plane of the sheet of drawing~ The pyramid defined by points ACEF is also a trlmcated rectangular parallelepiped, and the outline of base ACE is an isosceles triangle. In the case of a truncated cube corner, the outline of base ABC or base ACE is an equila~erial triangle.
It will be apparent that all light-reflecting units can be metallized to aid in their reflecting function as illustrated at 17 in Figure 2. In the other figures, metallization has not been shown to facilitate illustration of the structures.
Although the foregoing describes several embodiments of the present invention, it is understood that the in~ention may be practiced in still other forms within the scope of the following claims~

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Claims (15)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A retroreflective marking material comprising in combination:
(a) a substantially continuous sheet having at least some light-transmit-ting portions, a light-refracting surface on one side adapted to be exposed, and a plurality of trihedral light-reflecting units having three mutually perpendicular facets on the other side, (b) light-garnering and refracting receptors of polygonal cross-section elevated above and spaced along said one side of the sheet, and said receptors being free of light-reflecting units on their exposed sides which are co-extensive with said exposed one side of the sheet and adapted to direct light toward said light-reflecting units on said other side of the sheet, (c) said light-reflecting units being oriented to reflect light striking said exposed side of said sheet at relatively high angles of inci-dence, and (d) said sheet being adapted to be directly adhered along said other side to a mounting surface.

2. The marking material of claim 1 in which said light-receptors have a triangular cross-section disposed in said one direction of the sheet.

3. The marking material of claim 1 in which said light-receptors have a quadrilateral cross-section disposed in said one direction of the sheet.

4. The marking material of claim 1 in which said trihedral light-reflec-ting units occur on said other side of the sheet substantially opposite said light-receptors, some of said trihedral light-reflecting units being disposed to retroflect light in one direction and some of said trihedral light-reflec-ting units being disposed to retroflect light in the opposite direction, where-by said marking material is bidirectional with respect to retroreflecting light.

5. The marking material of claim 1 in which said light-receptors are elastically deformable and adapted to return substantially to their original positions after deformation.

6. The marking material of claim 1 in which said light-receptors are hollow and composed of elastically deformable material adapted to return sub-stantially to its original position after deformation, and said light-reflect-ting units are disposed along an inner side of the hollow light-receptors.

7. The marking material of claim 1 in which said light-receptors are hollow and open from said other side of the sheet and composed of elastically deformable material adapted to return substantially to its original position after deformation, said light-reflecting units are disposed along inner and opposite sides of the light-receptors to render them bidirectional with re-spect to retroreflecting light.

8. The marking material of claim 1 in which said trihedral light-reflec-ting units are so disposed that their optical axes are within 15 degrees of the direction of the refracted light.

9. The marking material of claim 1 in which said trihedral light-reflec-ting units are so disposed that their axes are substantially parallel to the direction of the refracted light.

10. The marking material of claim 9 in which said light-reflecting units are metallized.

11. The marking material of claim 1 in which said trihedral light-reflec-ting units are cube corners.

12. The marking material of claim 1 in which said trihedral light-reflec-ting units are rectangular parallelepipeds.

13. The marking material of claim 1 in which said trihedral light-reflec-ting units are triangular-based pyramids.

14. The marking material of claim 13 in which the outlines of the bases of said pyramids are isosceles triangles.

15. The marking material of claim 13 in which the outlines of the bases of said pyramids are equilateral triangles.