Reflective article with concealed retroreflective pattern

• US 6,024,455 A
• Filed: 01/13/1998
• Issued: 02/15/2000
• Est. Priority Date: 01/13/1998
• Status: Expired due to Term

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Wheatley, John A., O'Neill, Mark, Weber, Michael F., Benson, Olester Jr., Jonza, James M., Hanson, Gary B.
• Primary Examiner(s): Spyrou, Cassandra
• Assistant Examiner(s): Schuberg, Darren E
• Application Number:
  US09/006,292
• Time in Patent Office: 763 Days
• Field of Search: 359/494, 359/498, 359/529, 359/530, 359/584, 359/589, 359/590, 283/72, 283/85, 283/87, 283/91
• US Class Current: 359/530
• CPC Class Codes: G02B 5/124 plural reflecting elements forming part of a unitary plate or sheet

Abstract

A reflective article has a multilayer film covering a patterned retroreflective layer. In one embodiment the patterned retroreflective layer includes a separate indicia layer covering a retroreflective substrate. The indicia are substantially hidden under normal viewing conditions but easily detectable under retroreflective lighting conditions. Various types of retroreflective layers are disclosed. The article can also include microstructured relief patterns which produce holographic images under ambient lighting but which disappear under retroreflective conditions. The multilayer film includes alternating layers of at least a first and second polymer, and has a relatively high reflectivity for normally incident light within a first spectral range which is approximately coextensive with the visible spectrum, and a relatively low reflectance for normally incident light within a second spectral range. The second spectral range can include a transmission window of 15% or less fractional bandwidth located at least partially in the visible spectrum. Film constructions are disclosed which achieve sharper spectral transitions than previously attainable. Film constructions are also disclosed which preserve such sharp transitions even at high entrance (tilt) angles.

Claims

1. A reflective article comprising a multilayer film covering a patterned retroreflective layer, the multilayer film comprising alternating layers of at least a first and second material, the alternating layers being configured to exhibit a relatively high reflectance for normally incident light within a first spectral range and a relatively low reflectance for normally incident light within a second spectral range.

2. The article of claim 1, wherein the multilayer film is a multilayer polymeric film and wherein the first material comprises a first polymer and the second material comprises a second polymer.

3. The article of claim 2, wherein at least the first polymer layers are birefringent.

4. The article of claim 3, wherein, over a wavelength range of interest, at least two adjacent layers in the film have indices of refraction along an axis perpendicular to a plane of the film which differ by no more than 50% of the maximum in-plane refractive index difference between the two adjacent layers.

5. The article of claim 4, wherein a plurality of pairs of adjacent layers in the film have indices of refraction along the axis perpendicular to the plane of the film which differ by no more than about 20% of the maximum in-plane refractive index difference between the respective pairs of adjacent layers over the wavelength range of interest.

6. The article of claim 5, wherein substantially every pair of adjacent layers in the film has indices of refraction along the axis perpendicular to the plane of the film which are substantially the same.

7. The article of claim 3, wherein the first polymer is selected from the group consisting of:

polyethylene naphthalate, and copolymers and blends thereof based upon naphthalene dicarboxylic acid;

polyethylene terephthalate, and copolymers and blends thereof based upon terephthalic acid;

polybutylene naphthalate, and copolymers and blends thereof based upon naphthalene dicarboxylic acid; and

polybutylene terephthalate, and copolymers and blends thereof based upon terephthalic acid.

8. The article of claim 2, wherein the multilayer polymeric film comprises a plurality of polymeric layers defining optical repeating units (ORUs) with associated optical thicknesses, the ORUs arranged in a sequence of decreasing optical thickness along a given direction adjacent a sequence of non-decreasing optical thickness.

9. The article of claim 8, where the sequence of ORUs arranged in a non-decreasing optical thickness has an optical thickness profile selected from the group consisting of a flat, graded linear, and curved profile.

10. The article of claim 8, wherein the ORUs are arranged in a first and second set of ORUs each having a monotonically decreasing optical thickness profile and at least one set of ORUs having a non-decreasing optical thickness profile.

11. The article of claim 10, wherein the at least one set of ORUs has a curved optical thickness profile.

12. The article of claim 8, wherein, over a wavelength range of interest, at least two adjacent layers in the film have indices of refraction along an axis perpendicular to a plane of the film which differ by no more than 50% of the maximum in-plane refractive index difference between the two adjacent layers.

13. The article of claim 2, wherein the multilayer polymeric film has at least about 50% reflectance over at least 75% of the spectral range from 400 to 700 nanometers for normally incident light.

14. The article of claim 13, wherein the multilayer polymeric film has at least about 80% reflectance over at least 75% of the spectral range from 400 to 700 nanometers for normally incident light.

15. The article of claim 2, wherein the first spectral range includes substantially all of the spectral range from 400 to 700 nm.

16. The article of claim 2, wherein the multilayer polymeric film has a percent transmission greater than about 50% for at least some wavelengths in the second spectral range for normally incident light.

17. The article of claim 2, wherein the multilayer polymeric film has an absorption of no greater than about 1% over the first and second spectral ranges.

18. The article of claim 2, wherein the second spectral range includes at least one transmission band having a fractional bandwidth of no more than about 15%.

19. The article of claim 18, wherein the at least one transmission band has a fractional bandwidth of no more than about 5%.

20. The article of claim 18, wherein the at least one transmission band at least partially overlaps the visible spectrum.

21. The article of claim 2, wherein the patterned retroreflective layer comprises an indicia layer and a retroreflective layer.

22. The article of claim 21, wherein the indicia layer comprises portions which preferentially transmit selected wavelengths of light.

23. The article of claim 2, wherein the patterned retroreflective layer includes a structured surface having a first and second distinguishable adjacent zone defining a pattern.

24. The article of claim 23, wherein the first zone comprises retroreflective elements and the second zone is substantially unstructured.

25. The article of claim 23, wherein the first zone comprises a regular array of retroreflective elements and the second zone comprises retroreflective elements randomly tilted with respect to each other.

26. The article of claim 2, wherein the patterned retroreflective layer exhibits retroreflectance in at least one plane of incidence but not in at least one other plane of incidence.

27. The article of claim 26, wherein the patterned retroreflective layer comprises prisms extending along an axis substantially perpendicular to the at least one plane of incidence.

28. The article of claim 26, wherein the patterned retroreflective layer comprises pyramids having two distinct pairs of orthogonally-oriented opposed faces.

29. The article of claim 2, wherein the patterned retroreflective layer includes retroreflective elements having individual markings associated therewith which are detectable in retroreflected light at a selected illumination geometry.

30. The article of claim 2, wherein the patterned retroreflective layer includes beaded retroreflective sheeting.

31. The article of claim 30, wherein the beaded retroreflective sheeting includes an overcoat layer, a spacing layer, and a metallic coating.

32. The article of claim 31, wherein the overcoat layer includes an opaque binder layer.

33. The article of claim 2, further comprising a microstructured relief pattern producing a holographic image.

34. The article of claim 33, wherein the microstructured relief pattern is disposed in front of the alternating layers.

35. The article of claim 33, wherein the microstructured relief pattern is formed in a transparent layer which covers the multilayer polymeric film.

36. The article of claim 2, wherein the multilayer polymeric film is laminated to the patterned retroreflective layer.

37. The article of claim 2, wherein at least a portion of the patterned retroreflective layer is cast and cured onto the multilayer polymeric film.