CA1096832A - Retroreflector - Google Patents

Abstract

747 Comb.

ABSTRACT OF THE DISCLOSURE

A retroreflector of improved efficiency is disclosed comprising, in one form, a light-transmitting sheet adapted in use to be disposed in an angled position such that a normal to the sheet is at an angle of about 5° to about 85° from an incident beam of light. The sheet has front and back opposed, substantially parallel faces. The front face is substantially smooth and defines a light-refracting surface. The back face has a plurality of light-reflecting units. Each unit comprises three mutually perpendicular surfaces defining a trihedral angle of a rectangular parallelepiped and positioned with respect to the front face that the body diagonal of the rectangular parallelepiped is within an angle of 15° of incident light refracted by the front face. A multi-sided retroreflective body also is disclosed having at least two retroreflective faces adapted to intercept light. Each reflective face is tilted in the same general direction about a lower portion angularly away from a vertical plane and is angularly related in a horizontal plane with respect to another vertical plane disposed substantially at right angles to the direction of such light.

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 retro-reflective element of a roadmarker to provide directional guidance, and therefore it is described with respect to this use.
Roadmarkers are mounted on the surface of a roadway, such as along its center line or shoulders, to delineate paths or lanes for 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. Particularly to assist a driver of a vehicle at night, these markers have light reflectors which catch and return incident beams of light from vehicle headlights back toward the source of the light. Since automobiles of recent vintage have quite powerful headlights, the use of roadmarkers has become more widespread. Roadmarkers contribute to traffic safety such as when roads are wet from rain. Under certain conditions, such as fog, roadmarkers can be the only means of orienting a driver to a changing direction of a road.
Many forms of light reflectors have been suggested.
They usually suffer from one or more limitations, such as reflecting too small a proportion of incident light while an approaching vehicle is still at an appreciable distance.
~; ~ As a result, reflecting markers are often noticed too

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late by a driver to be of substantia] help.
Further, in order to avoid making a roadmarker an obstruction on the road, the marker preferably iS
designed to protrude only a slight amount from the road.
This requirement augments problems of light reflection.
Plainceramic or plastic markers have been used, but they tend only to scatter the light. Light scattering is self-defeating in that it is accompanied by loss of intensity of the reflected light which materially reduces the effectiveness of the marker.
An effective reflecting system is a well known triple mirror reflex reflecting principle, and which is referred to in the art as acube-corner structure. While a cube-corner structure provides satisfactory performance as to light striking perpendicularly against an array r or strip of cube-corners, that is, generally parallel to the axes of the cube-corners, this performance falls off rapidly as incident light enters at angles away from the normal to the surface of the cube-corner array.
A principal object of the present invention is to provide a retroreflector of relatively simple design which provides efficient retroreflectivity and represents an improvement over the known cube-corner structure in that better retroreflectivity is obtained for incident light t implnging on the retroreflector at wider angles of incidcnce. In the present retroreflector, the entering beams of incident light can be reflected sequentially from three mutual]y per~endicular surfaces which are so 1$~6~332 arranged that little or no light is lost.
~ nother object of the present invention is to provide a multi-sided retroreflector of re]ative]y simple design which is durable and yet provides efficient retroreflectivity.
~ ccording to the present invention, there is provided a retroreflector comprising a light-transmitting sheet adapted in use to be disposed in an angled position such that a normal to the sheet is at an angle of about 5 to about 85 from an incident beam of light, said sheet havlng front and back, opposed, substantially parallel faces, the front face being substantially smooth and defining a light-refracting surface, and the back face having a plurality of light-reflecting units, at least some reflecting units comprising three mutually perpen-- dicular surfaces defining a trihedral angle of a .
rectangular parallelepiped and positioned with respect to said front face that the body diagonal pf the rectangular parallelepiped is within an angle of about 15 to incident light refracted by said front face.
~ccording to another feature of the present in-- vention, the retroreflector is incorporated into a multi-sided body having at least two retroreflective substantially planar faces adapted to intercept light that is to be retroreflected, each of said two reflectiv~ faces being tilted in the samc general direction about a lower portion angularly away from a vertical plane and being angularly relateA in a horizon-tal plane with respect to another vertical planc ; ' .

3~
disposed substantially at right angles to the direction of said light, all four of said angles being so inter-related so as to make said reflective faces substantially optically equivalent, such that light retroreflected by said at least two reflective faces is directed in return paths substantially parallel to that of the intercepted light.
In the accompanying drawings:
Figure 1 is a perspective view of a roadmarker 10 containing in sheet form a retroreflective element of the present invention;
Figure 2 is a cross-section of Figure 1 on the line 2-2;
Figure 3 is a greatly enlarged, fragmentary view of the retroreflective sheet of Figure 2 and illustrates light-reflecting units in stepped rows or tiers;
Figure 4 is a view of Figure 3 on the plane of the line 4-4;
Figure S is a view similar to Figure 3 and shows the 20 retroreflective route a beam of light may take with that embodiment;
~ Figure 6 is a greatly enlarged, fragmentary view, :~ similar to Figure 3, of a modified form of the present ~: retroreflector;
Figure 7 is a view of Figure 6 on the plane of line 7-7; and Figure 8 is a vlew similar to E'igure 6 and shows the retroreflective route a beam of light may take with that emobidment. r .

Figures 9, 10 and 11 are plan, front and side elevational views, respectively, of a roadmarker embodying one form of the present invention having two cooperating retroreflective, substantially planar surfaces.
Figures 12 and 13 are views of Figure 9 on the plane of the lines 12-12 and 13-13, respectively;
Figures 14, 15 and 16 are plan, front and side elevational views, respectively, of a roadmarker embodying another form of the present invention having two sets of three cooperating retroreflective, substantially planar surfaces;
Figures 17 and 18 are views of Figure 14 on the planes of the lines 17-17 and 18-18;
Figure 19 is a cross-section of Figure 14 on the line 19-19.
Referring to the drawings and initially to the embodiment of Figures 1 through 5, a roadmarker comprises a generally truncated pyramidal body 10 having cut-away portions 11 at two opposite sides to form a slope 12 (Figure 2) and adjoining sidewalls 13 at each opposite side in which retroreflective elements 14 of the present invention are seated. The body 11 and retroreflective element 14 may be fabricated from any suitable material, such as a ceramic or synthetic resinous plastic material, although the retroreflective element must be sufficiently clear to transmit light. Body 11 may be suitably molded from any known ceramic, glazed and pigmented if desired to impart color, or from any other durable, weather resistant material. The retroreflective element 14 may .

also be fabricated from any durable, light-transmitting, weather-resistant material, such as glass, but preferably is made from synthetic resins such as polycarbonates and especially from the acrylates like polymethacrylate and polymethylmethacrylate resins. Re-trorcflective element 14 may be tinted, if desired~ to reflect red, yellow or other light, especially if used in a roadmarker.
- Referring more particularly to the retroreflective element of Figures 1 thxough 5, this component is in the form of a sheet havlng front and back, opposed, sub-stantially parallel faces indicated at 15 and 16, respectively. Front face lS is substantiall~r smooth and - defines a ]ight-refracting suxLace. Back face 16 has a plurality of light-reflecting units genera]ly represented at 17 which preferably are formed directly into the back face by a suitable mold, forming dies, or the like from an original planar faceindicated by the broken, imaginary line 18 in Figure 3, such that preferab~y the outer corners of the units 17 are coplanar with line 18 as - 20 illustrated. Here and elsewhere in the drawing, it will ~e appreciated that the light reflecting units are shown ! greatly oversize to facilitate their illustration an~
description.
To aid in their reflecting fw~ction, light-reflecting units 17 may be coated with metal or metalized in a manner known in the art to for~ a metallic layer l9 (Figure 2). Aluminum is the preferred metal for this purpose. An adhesive 20 fills the volume between slope 3?~

12 of the roadmarker and light-reflecting units 17 to secure the retroreflective element 14 in place in cut-away portions 11. A wide variety of adhesives may be used for this purpose SUCIl as natural adhesives like glue, bitumen, etc., or resinous adhesives like epoxy, polyester or polyurethane resins. Indeed, the same adhesives can be used to secure roadmarker 10 to a surface o a road, although catalyzed thermosetting adhesives are preferred for this purpose.
Considering now in greater detail the retro-; reflective element itself, it will be apparent that the element can be used alone in sheet form as illustrated by Figures 3, 4 and 5, or as part of any support, such as a roadmarker, sign, or the like, from which retro-reflection of light is desired. In order to provide the improved retroreflection of the present invention, . . ~,, .
tlle retroreflective sheet must be angled with respect to approaching incident pencils or beams of light. The ~.
light itself may be traveling in any direction and in 2~ any plane. To relate the relative position of the retroreflective sheet 14 to approaching light which is generally considered to travel in straight lines, the retroreflective sheet of the present invention should be .
~ ` ` disposed in an angled position such that a normal, that ' . .
~` is a line perpendicular to the sheet, is at an angle of ~- about 5 to about 85, and preferably from about 30 : to about 85, from an incident beam of light. ~ccord-ingly, if line 21 in Figu e 3 represents a normal to the - , :

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.
front face lS of the retroreflective sheet, the sheet is in position to receive and retroreflect light approaching the sheet within the angle ~ which represents an angle of about 5 to about 85 from line 21.
In practice, the retroreflector is usually positioned to receive and retroreflect light traveling generally in a horizontal plane, such as in a road sign or a roadmarker. While the above described angulation is important and paramount in all forms of the present invention, as a further indication of the angulation involved and when the retroreflector is used to intercept horizontally traveling light, the angle B sheet 14 makes with the horizontal may be within the range of about 5 to about 60. When the retroreflector is part of a road-marker the angle B sheet 14 makes with the horizontal may be within the range of about 15 to about 45, since a roadmarker is normally lower in elevation with respect to the approaching light. However, these angulations are secondary to the angulation described in the proceeding paragraph which controls in all cases.
The back face 16 of retroreflective sheet 14 com-prises light-reflecting unita~ generally represented at 17 in the embodiment of Figure 3, which, in order to achieve the improved efficiency of retroreflection afforded by the present invention, cover an appreciable area of the back face and preferably are coextensive with that face.
The resulting array of light-reflecting units provides a more even distribution of light reflection with little or no blind spots. The array of all the light reflecting .

~Q~6832 units forms a multifaceted reflecting surface which totally retroreflects light in a particular direction.
In a preferred formr the array of light-reflecting units are stacked to form a series of steps or rows 22 of units which extend transversely across back face 16.
As shown in Figure 3, when the retroreflective sheet is .in use and angled as previously described, rows 22 are laterally spaced from one another due to thei~r generally vertical disposition. It is, therefore, not merely a matter of stacking roWs 22 atop one another; rather, they must be laterally offset with respect to each other as shown. While size lS~ not critical~ ght-reflectlng units 17 have been shown oversized in the drawing for purposes of~lllustratlon. In one embodiment, each row was about 1/16~:inch in height:~a~nd the;~rows were~spaced laterally (or:~ho~rizontally~as~vlewed~in~;Flgure 3) about~ 1/16 inch.
The embodlment~of~:FIgures~3, 9 and 5 lllustrate the preferred~f~orm~;o2 11ght-reflecting~units, while the embodiment~o:f~Figures~6,;~7~and 8~represents a modlfied 20~; -form..~The llght-reflecting~unlta of both~embodiments `may~génerally~be~considered to comprise units of three ` ~ ually~perpendicular~surfaces~defining a trihedral angle~of~a~:rectangular~paral:lelèpiped, just as though a co~rner~of~a~rectangular~paralle~l~epiped was pressed against the~baGk:;.~face~of;the~retroreflective sheet while it was deformàble in order:to~:form the unit. In the preferred pra~`tice,~ s:uch:a~corneI~penetrates lnto the~sheet until the~remo`te~edges of the two~vertically disposed sides 1~4~32 .

of the rectangular parallelepiped reach the back face of sheet.
If a polyhedron is a solid bounded by planes, and a prism lS 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 parallelograms. A
right parallelepiped, then, is a paxallelepiped with edges perpendicular to the bases. As used here and in the claims, the term "rectangular parallelepiped" means a right parallelepiped whose bases are rectangles.
Of the three surfaces of the light reflecting units of all illustrated emobodiments,one surface is horizon-tally disposed when the retroreflectoris in the angled position of about 5 to about 85 from an incident beam of light, and the other two of the surfaces are . ~
vertically disposed and intersect each other in a direction toward the rear face of the retroreflector to form an intersection line. As used here and in the claims, the term "horizontally disposed" is taken to mean generally horizontal, that is, more horizontal than vertical, and not an exact, true horizontal direction.
Similarly, as used here and in the clai~s, the term "vertically d~sposed" is taken to mean generally vertical, that is, more vertical than horizontal and not an exact, true vertical direction.
For example, in the embodiment of Plgures 3, ~ and 5, .
6~3~

at least some of the li.ght-reflecting units 17 comprise three mutually perpendicular surfaces defining a tri-hedral angle of a rectangular parallelepiped as described.
One surface 23 is horizontally disposed when retro--reflective sheet 14 is in the described, operational~
angled position, and two surfaces 24 and 25 are vertically disposed and intersect each other in a direction toward back face 16 to form an intersection line or peak 30- A
light-reflecting unit 17 ls so-posit.ioned with respect to ~ront face 15 that a.body diagonal of-a rectangular parall- --elepiped as illustrated in:.phantom at 28 in Figures 4 and 5,is preferably substantially parallel to and at least : within an angle of about 15 of incident light refracted by face 15. The body diagonal is a straight line drawn from the trihedral angle formed by surfaces 23, 24 ; and 25 to the opposite trihedral angle of the rectangular ... : parallelepiped.
While the light-reflecting units of any embodiment may be spaced from one another along a given row and rows may likewise be spaced from one another, it is , preferred that the light-reflecting unlts adjoin one another in a row without spacing therebetween and that consecutive rows be contiguous to each other wlthout .
spacing therebetween to avoid possible blind spots in the retroreflection. Where the units within a row have no : ~ spacing therebetween, the vertically disposed surfaces, such as surfaces 24 and 25 of the embodiment of Figures - 3,.4 and 5, intersect vertically disposed surfaces of . -12-, ~ ', .

lQ~S~32 adjoining lightorelecting units 17 in a direction towards front ~ace 15 to form a second intersection line or depres~
sion 26. This line 26 is not only substantially parallel to the first mentioned intersection line 30 but, in the embod-iment of Figures 3, 4 and 5, is substantially aligned with an intersection line 30 at the peak of an adjacent lower row 22.
Fi~ure 5 illustrates the retroreflective route of an isolated beam of liyht represented at 31 for the embod-iment of Figure 3. The beam is first refracted by front face 15 and directed toward light-reflecting units 17. Upon striking any one o~ the three contiguous faces 23, 24 or 25 (shown as first striking a horizontally disposed surface 23 in Fi~ure 5~, beam 31 is reflected in turn by ~he three faces ~nd returned substantially parallel to its incident direction. In a special case, if sheet 14 is adapted to rec~ive horizontally directed light and makes an angle B
with the horizon, surface 23 is a square, surfaces 24 and 25 are identical rectangles, each row 22 has a vertical height H in inches ~Figure S~, the overall hori~ontal length of two re~lecting units of two adjacent rows is L in inches, and sheet 14 has an index of refraction of light n, in the ideal situation these values have substantially the re~ation:

cos B = n cos ~ n~l ~ ~

When these values are exactly met, ~he path of beam ' .

., 31 of light in Figure 5 within the retroreflective sheet 14 is exactly parallel to the body diagonal 28 of the rectangular parallelepiped. However, it will be apparent ", that deviations from one or more of these values may be - taken without losing the'advantages of the present invention.
Figures 6, 7 and 8 illustrate a modified form of the invention. This form differs from that of Figures 3, 4 and 5 principally in that the rows of light-reflecting units are spaced farther apart in a horizontal direction as viewed in Figure 6, so that a land or continuous plane is formed between adjacent rows which extends transversely across the back of the retroreflective sheet 'More particularly', the retro~reflecting sheet 32 of Figure 6 is positioned in use, like the embodiment of Figures 3, 4;and 5, at an angle of about 5 to about 8~5 and preferably from about 30 to about 85 from an incident beam of~light. Sheet 32 has front and back, opposed,~ substant1ally parallel faces shown at 33 and 34, 20~respectlvély,~back 34 being formed along the plane of the line bearing~this reference numb~ir. Front face 33 is substantlally smooth and defines a~light-refracting sur-aae,~while~back face 34 has a plurality of llght-refracting~unit~s ,formed into that face and generally represented at 35.~ Rows 36 of units 35 are formed into the ~s~ back~fa'ce~;and extend transversely across the back of sheet 32. At least some of the reflecting units comprise three mutua~lly~perpendicular surfaces deflning a trihedral angle ,' ' '~ ' lQ~6~33~

of a rectan~ular parallelepiped as previously described.
One surface 37 is hori ontally disposed when sheet 32 is in the angled position, and the other two surfaces 38 and 39 are vertically disposed and intersect each other . in a direction away from front face 33 to form an inter-section line or peak 43. In t~is case, however, the hori-æontally disposed sur~aces 37 o~ each unit are continuous ~ith respect to each other in a given row (Figure.7), so that a land or continuous plane indicated at 42 is formed.
The ~ertically disposed surfaces 38 and 39 of at least some of the light~reflecting units 35 can be spaced apartl but pre~erably they intersect ~ertically disposed surfaces of adjoining light-reflecting units 37 in a direc-.. . .
tion towards front face 32 to form a second intersection line or depxession 41 that.is su~stantially parallel to the first mentioned intersection line or peak 43. As shown ... especialLy in Figure 6~ the second intersection line or depression 41 of one row 36 is spaced laterally of the first mentioned intersection line or peak 43 of an adjacent, higher row.
Figure 8 illustrates the retrore~lective route of an isolated beam of light represented at 4~ for the embodi-ment of Figures 6 and 7. The beam is first refracted by ~ront face 33 and directed toward light-reflecting units 35. Upon striking any one of the three contiguous, mutually perpendi-culax surfaces 37, 38 or 39 ~shown as first striking a horizontall~ disposed surface 37), beam 45 is re~lected in turn by the three surfaces and returned substantially parallel to its incident direction. In a . - 15 -special case, if the retroreflective sheet 32 forms an angle B with the horizontal, each row 36 has a v~r~ical height ~l in inches (Figure 8), the horizontal length of each reflecting unit is D in inches, and the overall horizontal length of two reflecting units of two adjacent rows 36 is S in inches, in the ideal situation these values have substantially the relation:

tan B = H
S-D
.
When these values are exactly met, the retro-reflective path of the beam of light 45 is exactly parallel to the incident beam of liyht 45, if the retro--reflective sheet 32 is disposed so as to receive the incident beam of light within angle A as described for Figure 3. However, it will be appreciated that ; deviations from one or moreof these values may be taken without losing advantages of the present invention.
~ .
Neither light-reflective units 17 nor 35 have re-entrant surfaces and therefore are easily molded. A
pro3ection of the array of units 17 or 35, that is, of just the units alone, forms a like array of hexagons ! ' . filling the projection plane. Accordingly, tools for forming molds to shape the reflecting surfaces can be made from pins of hexagonal cross-section having three mutually perpendicular planar faces machined on the end of each pin. At least in the embodiment of Figure 3, these plandr faces are mutually perpendicular to a body diagonal 28 o~ a rectangular parallelepiped which is 3~

parallel to the lateral edges of such pins under the ideal situation where diagonal 28 is exactly parallel to the refracted incident light.
It will be apparent that light-reflecting units 17 and 35 can, if desired, be metallized to aid in their reflecting function as described in connection with Figure 2. In Figures 3 through 8, this metallization has not been shown to facilitate illustration of the structure of the light~reflecting units.
10Increased durability for roadmarkers can be obtained by eliminating sharp edges and corners. In order to accomplish this without sacrificing optical performance~ all faces of the roadmarker which intercept light, as from oncoming headlights, must be as maximally retroreflective as feasible. A further desideratum is that the same tooling be used for forming all reflective faces in order to reduce expenses and speed production. This can be accomplished by arranging the - retroreflective surfaces in such a way that an angle between a normal to the surface and an incident ray of light is the samefor each surface.
- The present retroreflective body satisfies these conditions by rendering the associated, substantially planar faces optically equivalent. As used here and in the claims, the term "optically equivalent" and forms thereof are taken to mean that the faces receive and redirect incident light in return paths that are sub-stantially parallel to that of the intercepted light.

The physical structures of the various embodiments of the drawing are first described; then the inter-related angulation of the reflecting surfaces inter se;
and finally the structures and retroreflection operation of the reflecting elements themselves.
The embodiment of Figures 9 through 13 represents a roadmarker generally represented at 110 in form of a truncated pyramid of hexagonal cross-section of which - two contiguous, substantially planar faces 111 and 112 comprise retroreflective elements. The retroreflection of the embodiment of Figure 9 is unidirectional, that is, it is designed to receive and retroreflect light coming from one general direction, namely, in the general direction of arrow-113 so that faces 111 and 112 intercept the light. The body of roadmarker 110 may be fabricated from any suitable durable, weather-resistant material, such as ceramic, glass, or synthetic resinous plastic material. Such material may be glazed or pigmented, if desired, to impart colors.
The retroreflective faces 111 and 112 may be present in the form of sheets or wafers suitably adhered in place onto the roadmarker, as by natural or synthetic adhesives, or in matching recesses designed to receive .
the sheets. The retroreflective elements defining faces lll or 112 may also be fabricated from any durable, light-transmitting, weather-resistant material, such as glass. But preferably such elements are made from synthetic resins such as polycarbonates and especially . . ~

6~32 from the acrylates like polymethacrylate and poly-methylmethacrylate resins. The retroreflective elements may be tinted, if desired, to reflect red, yellow or other light, especially if used in a roadmarker. Possible structures of the retroreflective faces 111 and 112 are hereinafter more fully described collectively in connection with the embodiment of Figures 14 through 19.
If desired, the embodiment of Figure 9 can be bi-directional, that is, receive and retroreflect light coming from either or both o~ two opposite directions.
In this case, opposed faces I14 and 115 are also - optically equivalent, substantially planar retroreflective faces like faces 111 and 112.
Each of retroreflective faces 111 and 112 have an angular relationship with a different vertical plane - and wlth respect to each other in a horizontal plane.
Face 111 is tilted about a lower portion angularly away from a vertical plane 116 through an angle Al (Figure 13).
Similarly face 112 is tilted about a lower portion angularly away from a vertical plane 117 through an angle A2 (Figure 12). Face 111 makes an acute angle B] in a horizontal plane (Figure 9) with a second vertical plane 118 which is substantially at right angles to the direction of the approaching light as indicated by arrow 113.
Face 112 likewise makes an acute angle B2 in a horizontal plane with the second vertical plane 118. The four indicated angles are interrelated so as to make faces 111 and 112 optically equivalent as described.

l~q6~32 Although in the embodiment of Figures 9 through 13, angle Al equals angle A2 and angle Bl equals angle B2, this is not essential. These angles can substantially deviate from one another in value as long as the optical equiv-alents of faces 111 and 112 is maintained. As a rule, angle Al and angle A2 normally lie within the range of about 40 to about 75.
If a retroreflector is made with n reflective surfaces and all of these surfaces are to be formed with the same tooling and to retroreflect light in the same general direction, then the angular relation can be expressed by the following equation, using the angles of Figures 9, 12 and 13:

, Thls equation represents ideal conditions. Sub-stantial deviation in any value for any angle can occur without departing from the advantages of the invention.
For example, one or more of the angles of the equation may have a value lying within + 10~ of the value expressed by the equation.
Figures 14 through 19 illustrate a preferred , embodiment of the present multi-sided retroreflector.
A roadmarker generally represented at 120 is in the form of a truncated pyramid of octagonal cross-section of which six substantially planar faces 121, 122, 123, 124, 125 and 126 contain retroreflective elements. Faces 121, 122 and 123 cooperate with one another to form one .

` lQq6832 set of retroreflective faces, while 124, 125 and 126 cooperate to define another set of retroreflective elements. In this manner, roadmarker 120 can receive and return incidental light approaching the roadmarker from either or both of two opposite directions.
Roadmarker 120 can be fabricated from the same materials described for roadmarker 110. Figure 19 ill-- ustrates an alternatlve construction in which an outer, one-plece shell 127 of a light-transmitting synthetic resin of the type previously disclosed is filled or potted with a relatively rigid filler material in the - form of a solid core 128. The core completely fills the interior of shell 127 and contacts its inner surfaces.
Core 128 which may be of any solid, weather-resistant material, such as glass, ceramics, synthetic resins, part1cularly thermosetting resins, reinforces the shell and provides a solid, rugged structure to withstand forces applied to roadmarker 120 as by tires of vehicular traffic.
The inside surfaces of shell 127 forming faces 121 through 20~ ~126 have 1ight-reflectlng units hereinafter more fully described. In this instance, an adhesive between shell 127~and~core 128 is not usually employed, the material of core~12~8 provlding lts own adhesion to the shell.
; T~e retroreflective operation of faces 124, 125 and 126~1s the~ same as~that of faces 121, 122 and 123 and, therefore, only the~ latter~ set of faces is described in detail. The embodiment of Figure 14 is a special case of that of Figure 9 in which there is a frontal sub-a~tlally~planar face~ and two other substantlally .''' '" ' ' ' ' ' ' .

1~6~32 planar faces laterally and rearwardly disposed from the frontal face. These faces also have an angular relation both from a vertical plane and with respect to each other. In particular, face 122 is tilted about a lower portion away from a vertical plane 30 through an angle X (Figure 16), the plane being adapted to be disposed substantially at right angles to the approaching direction of incident light. Each of faces 121 and 123 is similarly tilted about a lower portion away from vertical planes 131 and 132 through angles Yl and Y2 respectively. Face 121 makes an acute angle Zl in a horizontal plane with vertical plane 130, and face 123 makes an acute angle Z2 in a horizontal plane with vertical plane 130 (Figure 14).
g , Yl, Y2, Zl,and Z2 are interrelated so as to make faces 121, 122 and 123 optically equivalent as herein defined. In a preferred and special case ~which is not essential to the invention), angle Yl equals angle Y2 and angle Zl equals angle Z2' In this arrangement and representing ideal conditions, the relationship among such angles is represented by the ~ equation.

; Cos X = Cos: Y x Cos Z

All faces 121, 122, and 123 can retroreflect light in the same general direction and can be made with the same tooling, even though there is substantlal deviation in any value for any angle from this equation, without departing from the advantages of the invention. For . .

1~683%

example, one or meLeof said angles may have a valuelying within + 10~ of the value stated in the equation, although deviations exceeding even this value are permissible in one or more angles as long as the optical equivalence of faces 121, 122, and 123 is maintained.
As a rule, and as a basis for a starting calculation, angle A normally lies within the range of about 40 to about 75.
Considering next the structure, itself, of the retroreflective, substantially planar faces, the follow-ing applies for any of the faces of any of the embodiments, whether it be for face 111, 112, 121, 122, 123, 124, 125, or 126. In keeping with the advantage of the present invention that the same forming tool to - be used to form all retroreflective faces and yet achieve retroreflection in substantially the same direction from those same faces, it is preferred although not essential that the retroreflective faces have a plurality of light-reflecting units comprising three mutually perpendicular surfaces. Conveniently, the retroreflective element comprises a layer or sheet having such light-reflecting units formed in its back side or face.
Light-reflecting units of three mutually perpen-dicular surfaces may include those in which the three surfaces define a trihedral angle of a rectangular parallelepiped or such light-reflecting units may comprise cube-corners.

The light-reflecting units in the form of strips, wafers, films, sheets, and the like of the same construction as the slanting sides of shell 127 in Figure 19 can be used to form faces 111 and 112 of the embodiment of Figure 9 and the faces 121, 123, 124, 125 and 126 of the embodiment of Flgure 14.
The light-reflecting units of any substantially planar retroreflective faceof the present retro-reflector may comprise cube-corners as in a cube-corner array, The term "cube corner" is an art recognized term and refers to a well known triple mirror reflecting principle. If three reflécting surfaces are arranged at right angles to each other and intersect at a common point, they form the inside corner of a cube. A beam of light incident on such a cube-corner - is reflected from surface to surface and then back along the same general direction taken by the arrivins light beam. Such a construction may also be termed a central triple reflector.
Each cube-corner has an axis and the axes of all the cube-corners are generally parallel to one another.
Although such axes are preferably parallel to each other, this does not mean that the axes must be normal to a front face as herein defined or to an array of cube-corners.
Although the foregoing describes several embodiments of the present invention, it is understood that the ; invention may be practiced in still other forms within the scope of the following claims.

Claims (19)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A retroreflector comprising a light-transmitting sheet adapted in use to be disposed in an angled position such that a normal to the sheet is at an angle of about 5° to about 85° from an incident beam of light, said sheet having front and back, opposed, substantially parallel faces, the front face being substantially smooth and defining a light-refracting surface, the back face having a plurality of light-reflecting units, at least some reflect-ing units comprising three mutually perpendicular surfaces defining a trihedral angle of a rectangular parallelepiped, two of said three mutually perpendicular surfaces being rectangular and said two surfaces being vertically disposed, the third of said surfaces being horizontally disposed, and positioned with respect to said front face such that the body diagonal of the rectangular parallelepiped is within an angle of about 15° to incident light refracted by said front face, the said mutually perpendicular surfaces forming a peak at the back face of the sheet at least two of said three mutually perpendicular surfaces being rectangular and said two surfaces being vertically disposed.

2. The retroreflector of claim 1 in which the exposed areas of at least some of said reflecting units are coated with metal to aid in their reflecting function.

3. The retroreflector of claim 1 in which said sheet comprises a light-transmitting organic polymeric resinous material.

4. The retroreflector of claim 1 in which said sheet is adapted in use to be disposed in an angled position such that a normal to the sheet is at an angle of about 30° to about 85° from an incident beam of light.

5. A retroreflector body containing as a retroreflec-tive element the retroreflector of claim 1.

6. The retroreflector of claim 1 in which said plur-ality of light-reflecting units includes rows of said units extending transversely across said back face and formed over an appreciable area of said face.

7. The retroreflector of claim 6 in which said rows are contiguous to each other without spacing therebetween.

8. The retroreflector of claim 6 in which said light-reflecting units of each row adjoin one another without spacing therebetween.

9. The retroreflector of claim 6 in which said angled position said sheet makes an angle B with the horizontal, said rows have a vertical height of H in inches, the horizontal length of each of reflecting unit is D in inches, the overall horizontal length of two reflecting units of two adjacent rows is S in inches, and said values have sub-stantially the relation:
tan B =

10. The retroreflector of claim 1 in which one of said three mutually perpendicular surfaces of said at least some reflecting units is horizontally disposed when the retroreflector is in said angled position, and the other two of said surfaces are vertically disposed and intersecting each other in a direction toward said front face of the retroreflector to form an intersection line, said vertically disposed surfaces of at least some of said light-reflecting units intersect vertically disposed surfaces of adjoining light-reflecting units in a direction away from said front face to form a second intersection line that is substantially parallel to the first mentioned intersection line.

11. The retroreflector of claim 10 in which said second intersection line of one row is spaced laterally of the first mentioned intersection line of an adjacent higher row.

12. The retroreflector of claim 10 in which said second intersection line of one row is substantially aligned with the first mentioned intersection line of an adjacent higher row.

13. A retroreflector comprising a light-transmitting sheet adapted in use to be disposed in an angled position such that a normal to the sheet is at an angle of about 30°
to about 85° from an incident beam of light, said sheet having front and back opposed, substantially parallel faces, the front face being substantially smooth and defining a light-refracting surface, the back face having a plurality of light-reflecting units formed into that face, the outer corners of said units being substantially coplanar with said back face, at least some reflecting units comprising three mutually perpendicular surfaces defining adjacent sides of a rectangular parallelepiped, one of said surfaces of each unit being horizontally disposed when the retro-reflector is in said angled position, and the other two of said surfaces being rectangular and vertically disposed, at least some of said units being positioned with respect to said front face that a body diagonal of a rectangular parallelepiped serves as the optical axis of that unit and is substantially parallel to incident light refracted by said front face.

14. The retroreflector of claim 13 in which the exposed areas of at least some of said reflecting units are coated with metal to aid in their reflecting function.

15. The retroreflector of claim 13 in which said sheet comprises a light-transmitting organic polymeric resinous material.

16. The retroreflector of claim 13 in which said plurality of light-reflecting units including rows of said units extending transversely across said back face and formed over an appreciable area of said face.

17. The retroreflector of claim 16 in which said rows are contiguous to each other without spacing therebetween.

18. The retroreflector of claim 16 in which the light-reflecting units of each row adjoin one another without spacing therebetween.

19. The retroreflector of claim 13 in which said angled position the retroreflector is adapted to receive horizontally directed light, said horizontally disposed surface of a light-reflecting unit is a square and the other two surfaces are identical rectangles, said sheet makes an angle B with the horizontal, said units including rows of units extending transversely across said back face, each row having a length L in inches, said sheet has an index of refraction of n, and said values have substantially the relation:

cos B = n cos