Tiled retroreflective sheeting composed of highly canted cube corner elements

Claims

What is claimed is:

1. A retroreflective article comprising a substrate having a base surface and a structured surface opposite the base surface, the structured surface comprising a plurality of arrays of cube corner elements, including:

a first array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction at an angle measuring between about 12.degree. and about 30.degree. from an axis normal to the base surface; and

a second array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction by an angle measuring between about 12.degree. and about 30.degree. from an axis normal to the base surface, the second array of cube corner elements being oriented approximately perpendicular to the first array so that the retroreflective article provides a minimum total light return of about 5% across about a 360.degree. range of orientation angles at an entrance angle of about 40.degree..

2. The retroreflective article of claim 1 wherein the first array and the second array occupy roughly equal areas of the structured surface of the retroreflective article.

3. The retroreflective article of claim 1 wherein the cube corner elements are generally trihedral structures comprising three mutually perpendicular triangular optical faces that intersect at a peak and a triangular base, the base being approximately coplanar with the base surface of the article.

4. The retroreflective article of claim 1 wherein the cube corner elements are generally polygonal structures comprising three mutually perpendicular optical faces including two tetragonal optical faces and a third optical face that intersect at a peak, and a tetragonal base.

5. The retroreflective article of claim 1 wherein the symmetry axes of the cube corner elements in the first are tilted in a backward direction at an angle measuring between about 14.degree.0 and about 20.degree. from an axis normal to the base surface.

6. The retroreflective article of claim 1 wherein the symmetry axes of the cube corner elements in the first array are tilted in a backward direction at an angle measuring about 14.degree. from an axis normal to the base surface.

7. The retroreflective article of claim 1 wherein the symmetry axes of the cube corner elements in the second array are tilted in a backward direction at an angle measuring between about 14.degree. and about 20.degree. from an axis normal to the base surface.

8. The retroreflective article of claim 1 wherein the symmetry axes of the cube corner elements in the second array are tilted in a backward direction at an angle measuring about 14.degree. from an axis normal to the base surface.

9. The retroreflective article of claim 1 wherein the second array is oriented at an angle between about 85.degree. and about 95.degree. relative to the first array.

10. The retroreflective article of claim 1 wherein the cube corner element opposing pairs comprise physically adjacent cube corner elements.

11. The retroreflective article of claim 1 wherein the cube corner element opposing pairs each comprise a different retroreflection pattern.

12. The retroreflective article of claim 1 wherein the cube corner element opposing pairs comprise matched pairs.

13. The retroreflective article of claim 1 wherein the article provides a generally uniform total light return about a 360.degree. range of orientation angles for entrance angles of greater than about 40.degree..

14. The retroreflective article of claim 1 wherein the article is capable of a minimum total light return of about 5% across about a 360.degree. range of orientation angles at an entrance angle of about 50.degree..

15. The retroreflective article of claim 1 wherein the article is capable of a minimum total light return of about 5% across about a 360.degree. range of orientation angles at an entrance angle of about 60.degree..

16. The retroreflective article of claim 1 wherein the article is capable of a minimum total light return of about 10% across about a 360.degree. range of orientation angles at an entrance angle of about 40.degree..

17. The retroreflective article of claim 1 wherein the article is capable of a minimum total light return of about 10% across about a 360.degree. range of orientation angles at an entrance angle of about 50.degree..

18. The retroreflective article of claim 1 wherein the article is capable of a minimum total light return of about 10% across about a 360.degree. range of orientation angles at an entrance angle of about 60.degree..

19. The retroreflective article of claim 1 wherein the substrate and the cube corner elements are formed as a unitary article from a light transmissible material having a refractive index of between 1.3 and 1.7.

20. The retroreflective article of claim 1, wherein the substrate comprises a body layer comprising a light transmissible material having an elastic modulus less than about 7.times.10.sup.8 pascals, and the cube corner elements comprise a light transmissible material having an elastic modulus greater than about 16.times.10.sup.8 pascals.

21. The retroreflective article of claim 1 wherein a plurality of the cube corner elements incorporate minor deviations from perfect orthogonality to thereby alter the light distribution in the emerging cone of retroreflected light.

22. The retroreflective article of claim 1 wherein portions of the first and second arrays of cube corner elements are coated with a specularly reflective substance.

23. The retroreflective article of claim 1 further comprising a sealing medium disposed adjacent the first and second arrays of cube corner elements.

24. The retroreflective article of claim 1 wherein a sealing medium is bonded to the structured surface by a network of intersecting bonds to define a plurality of cells within which the cube corner elements are hermetically sealed.

25. The retroreflective article of claim 1 wherein a sealing medium maintains an air interface with the structured surface such that the cube corner elements retroreflect according to the principles of total internal reflection.

26. The retroreflective article of claim 1 wherein the cube corner elements comprise full cube corner elements.

27. A retroreflective article comprising a substrate having a base surface and a structured surface opposite the base surface, the structured surface comprising a plurality of arrays of cube corner elements, including:

a first array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction at an angle measuring between about 12.degree. and about 30.degree. from an axis normal to the base surface; and

a second array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction by an angle measuring between about 12.degree. and about 30.degree. from an axis normal to the base surface, the second array of cube corner elements being oriented approximately perpendicular to the first array so that the retroreflective article provides a generally uniform total light return about a 360.degree. range of orientation angles.

28. A retroreflective article comprising a substrate having a base surface and a structured surface opposite the base surface, the structured surface comprising a plurality of arrays of cube corner elements, including:

a first array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction at an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface; and

a second array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction by an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface, the second array of cube corner elements being oriented approximately perpendicular to the first array.

29. A mold assembly suitable for use in forming retroreflective sheeting, the mold assembly comprising a substrate having a base surface and a mold surface opposite the base surface, the mold surface comprising in roughly equal proportions:

a first array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction at an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface; and

a second array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the array being tilted in a backward direction by an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface, the second array of cube corner elements being oriented approximately perpendicular to the first array.

30. The method of claim 29 wherein the first and second arrays of cube corner element opposing pairs are tilted by an angle measuring between about 15.1.degree. and about 20.degree..

31. A method of making a retroreflective article, comprising:

providing a mold assembly suitable for forming retroreflective articles, the mold assembly comprising a substrate having a base surface and a mold surface opposite the base surface, the mold surface comprising in roughly equal proportions:

a first array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the first array being tilted in a backward direction at an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface, and

a second array of cube corner element opposing pairs, the symmetry axes of the cube corner elements in the second array being tilted in a backward direction by an angle measuring between about 15.1.degree. and about 30.degree. from an axis normal to the base surface wherein the second array of cube corner elements is oriented approximately perpendicular to the first array;

forming a replica of the mold, the replica of the mold including a surface having a negative image of the mold; and

forming a retroreflective article in the surface of the replica.

32. The method of claim 31 wherein the first and second arrays of cube corner element opposing pairs are tilted by an angle measuring between about 15.1.degree. and about 20.degree..

FIELD OF THE INVENTION

The present invention relates generally to cube corner retroreflective sheeting that is capable of returning a significant percentage of incident light at relatively high entrance angles regardless of the rotational orientation of the sheeting about an axis perpendicular to its major surface.

BACKGROUND OF THE INVENTION

Retroreflective materials are characterized by redirecting incident light back toward the originating light source. This property has led to the wide-spread use of retroreflective sheeting in a variety of conspicuity applications. Retroreflective sheeting is commonly applied to flat, rigid articles such as, for example, road signs and barricades to improve their conspicuity in poor lighting conditions. Retroreflective sheeting is also used on irregular or flexible surfaces. For example, retroreflective sheeting can be adhered to the side of a truck trailer, which requires the sheeting to cover corrugations and protruding rivets, or the sheeting can be adhered to a flexible body portion such as a road worker's safety vest or other such safety garment.

Many conspicuity applications are governed by specific performance standards for retroreflective sheeting. Manufacturers must demonstrate that their retroreflective sheeting is capable of meeting the relevant performance standards to be considered as a supplier in the marketplace. A body of standards exists both for describing retroreflection (see ASTM Designation E808-93b, Standard Practice for Describing Retroreflection) and for measuring retroreflectors, (see ASTM Designation E809-94a, Standard Practice for Measuring Photometric Characteristics of Retroreflectors).

Two known types of retroreflective sheeting are microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, employs a multitude of microspheres typically at least partially embedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes or vapor coats, etc.) to retroreflect incident light. Illustrative examples are disclosed in U.S. Pat. Nos. 3,190,178 (McKenzie), 4,025,159 (McGrath), and 5,066,098 (Kult). Due to the symmetry of beaded retroreflectors, microsphere-based sheeting exhibits relatively uniform entrance angularity when rotated about an axis normal to the surface of the sheeting. Therefore, the retroreflective performance of beaded sheeting is relatively insensitive to the orientation at which the sheeting is placed on the surface of an object. In general, however, microsphere-based sheeting exhibits relatively low retroreflective efficiency. Beaded retroreflective sheeting typically exhibits a total light return of approximately 5% to 15% in an observation cone angled about 2.degree..

Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar base surface and a structured surface comprising a plurality of cube corner elements opposite the base surface. Each cube corner element comprises three mutually substantially perpendicular optical faces that typically intersect at a single reference point, or apex. The base of the cube corner element acts as an aperture through which light is transmitted into the cube corner element. In use, light incident on the base surface of the sheeting is refracted at the base surface of the sheeting, transmitted through the respective bases of the cube corner elements disposed on the sheeting, reflected from each of the three perpendicular cube corner optical faces, and redirected toward the light source.

One aspect of many performance standards requires retoreflective sheeting to return specified percentages of light incident on the face of the sheeting at various entrance angles. The total light return characteristic of a retroreflective sheeting as a function of the entrance angle of incident light is generally referred to in the art as the `entrance angularity` of the sheeting. A retroreflective sheeting capable of returning a significant percentage of light incident at relatively high entrance angles can be characterized as having strong or wide entrance angularity, such as disclosed in the isobrightness curves in U.S. Pat. No. 4,588,258 (Hoopman).

By contrast, retroreflective sheeting with poor entrance angularity loses its retroreflective brightness (total light return decreases) rapidly as the angle of incidence deviates from 0.degree.. Moreover, entrance angularity typically varies about a 360.degree. range of orientation angles (orientational uniformity), requiring proper alignment of the retroreflective sheeting for each application. The entrance angularity and orientational uniformity of a retroreflective sheeting is a significant performance factor because it materially affects the ability of a driver to see an object such as a traffic sign or a safety barrier in poor lighting conditions at various orientations.

The symmetry axis, also called the optical axis, of a cube corner element is the axis that forms an equal angle with the three optical surfaces of the cube corner element. Cube corner elements typically exhibit the highest optical efficiency in response to light incident on the base of the element roughly along the optical axis. The amount of light retroreflected by a cube corner retroreflector drops as the incidence angle deviates from the optical axis.

Cube corner elements offer the advantage of being significantly more efficient retroreflectors than beads. The terms `active area` and `effective aperture` are used in the cube corner arts to characterize the portion of a cube corner element that retroreflects light incident on the base of the element. A detailed teaching regarding the determination of the active aperture for a cube corner element design is beyond the scope of the present disclosure. One procedure for determining the effective aperture of a cube corner geometry is presented in Eckhardt, Applied Optics, v. 10, n. 7, July, 1971, pp. 1559-1566. U.S. Pat. No. 835,648 (Straubel) also discusses the concept of effective aperture. At a given incidence angle, the active area can be determined by the topological intersection of the projection of the three cube corner faces onto a plane normal to the refracted incident light with the projection of the image surfaces for the third reflections onto the same plane. The term `percent active area` is then defined as the active area divided by the total area of the projection of the cube corner faces. The retroreflective efficiency of retroreflective sheeting is directly proportional to this percent active area. The maximum theoretical total light return of truncated cube corner elements commonly used in retroreflective sheeting is approximately 67%, while in practice cube corner retroreflective sheeting exhibits a maximum total light return of approximately 35%, due to sealing, front surface losses, and reflection losses at the cube faces.

Predicted total light return (TLR) for a cube corner matched pair array can be calculated from a knowledge of percent active area and ray intensity. Ray intensity can be reduced by front surface losses and by reflection from each of the three cube corner surfaces for a retroreflected ray. Total light return is defined as the product of percent active area and ray intensity, or a percentage of the total incident light which is retroreflected. A discussion of total light return for directly machined cube corner arrays is presented in U.S. Pat. No. 3,712,706 (Stamm).

The light return profile of the basic cube corner element is inherently asymmetric in nature. The breakdown of total internal reflection (TIR) is the most significant cause of this asymmetry in non-metallized cube corner retroreflectors. Coating the reflecting faces with a specular reflector substantially reduces the asymmetry in the reflection pattern. Metallized cube corner arrays, however, are typically not white enough for daytime viewing, such as on signing applications. The durability of the specular vapor coat may also be inadequate. Finally, a portion of the asymmetry is d