Retroreflective sheeting and license plate with reduced retroreflectivity at high entrance angles

• US 9,110,235 B2
• Filed: 10/12/2010
• Issued: 08/18/2015
• Est. Priority Date: 10/16/2009
• Status: Active Grant

• Current Assignee: 3M Innovative Properties Company (3M Company)
• Original Assignee: 3M Innovative Properties Company (3M Company)
• Inventors: Muramoto, Akira, Orensteen, Bruce D., Nakajima, Toshitaka, Yokoyama, Masami
• Primary Examiner(s): Doak, Jennifer L.
• Application Number:
  US13/502,056
• Publication Number:
  US 20120200709A1
• Time in Patent Office: 1,771 Days
• US Class Current: 1/1
• CPC Class Codes: G02B 5/128 transparent spheres being embedded in matrix

Abstract

The present application relates to retroreflective sheeting that is capable of use in a license plate and that can be accurately read and/or detected by an ALPR system. The retroreflective sheeting is a microsphere-based retroreflective sheeting that provides reduced retroreflectivity at high entrance angles. The retroreflective sheeting can also be a microsphere-based retroreflective sheeting that exhibits reduced retroreflectivity of incident infra-red light from high entrance angles and substantially unaffected retroreflectivity of incident visible light from entrance angles in the vicinity of the normal.

Claims

1. A retroreflective sheet, comprising:

a binder layer;

microsphere lenses at least partially buried in the binder layer;

a spacer layer;

a reflector layer adjacent to the spacer layer; and

an infra-red non-transmissive material, wherein the infra-red non-transmissive material is disposed in at least one of a layer between the spacer layer and the reflector layer or the binder layer.

2. The retroreflective sheet of claim 1, wherein the binder layer is a bead bonding layer.

3. The retroreflective sheet of claim 1, wherein the microsphere lenses form a monolayer.

4. The retroreflective sheet of claim 1, wherein the microsphere lenses have a refractive index between 1.7 and 2.3.

5. The retroreflective sheet of claim 1, wherein the spacer layer is contoured so as generally to follow a backside of the microsphere lenses.

6. The retroreflective sheet of claim 1, wherein the reflector layer includes one of a mirror-surfaced reflection layer and a multi-layer optical film.

7. The retroreflective sheet of claim 1, wherein the retroreflective sheet is one of microsphere-enclosed, microsphere-encapsulated, and microsphere-exposed.

8. The retroreflective sheet of claim 1, further including a protective layer.

9. The retroreflective sheet of claim 1, wherein the binder layer has a thickness and the microsphere lenses have an average microsphere diameter, and wherein the binder layer thickness to average microsphere diameter ratio is greater than or equal to 1% and less than or equal to 99%.

10. The retroreflective sheet of claim 1, wherein the infra-red non-transmissive material is in the binder layer.

11. The retroreflective sheet of claim 1, wherein the infra-red non-transmissive material is in a separate, infra-red non-transmissive layer.

12. A license plate utilizing the retroreflective sheet according claim 1.

13. An ALPR system, comprising:

a license plate according to claim 12;

a light source that directs light at the license plate; and

a machine capable of photographing license plate.

14. A retroreflective sheet, comprising:

a protective layer;

a bead bonding layer;

microsphere lenses at least partially buried in the bead bonding layer;

a spacer layer contoured so as generally to follow a backside of the microsphere lenses;

a reflector layer adjacent to the spacer layer; and

an infra-red non-transmissive layer disposed between the spacer layer and the reflector layer.

15. The retroreflective sheet according to claim 14, wherein the bead bonding layer has a thickness and the microsphere lenses have an average microsphere diameter, and wherein thickness of the bead bonding layer relative to average microsphere diameter is greater than or equal to 1% and less than or equal to 99%.

16. The retroreflective sheet according to claim 14, further comprising:

a transparent resin layer partially burying the respective microsphere lenses disposed between protective layer and the bead bonding layer; and

the backside face of the bead bonding layer having a multiplicity of aperture parts conforming to the spherical faces of the microsphere lens.

17. A license plate utilizing the retroreflective sheet according to claim 14.

18. An ALPR system, comprising:

a license plate according to claim 17;

a light source that directs light at the license plate; and

a machine capable of photographing license plate.

19. A retroreflective sheet, comprising:

a protective layer;

a bead bonding layer including an infra-red non-transmissive material;

microsphere lenses at least partially buried in the bead bonding layer;

a spacer layer transparent to visible light and contoured so as generally to follow the backside faces of the microsphere lenses; and

a reflector layer covering the backside face of the spacer layer.

20. The retroreflective sheet according to claim 19 wherein the bead bonding layer has a thickness and the microsphere lenses have an average microsphere diameter, and wherein thickness of the bead bonding layer relative to average microsphere diameter is greater than or equal to 1% and less than or equal to 99%.

21. The retroreflective sheet according to claim 19 further comprising:

a transparent resin layer partially burying the respective microsphere lenses disposed between protective layer and the bead bonding layer; and

the backside face of the bead bonding layer having a multiplicity of aperture parts conforming to the spherical faces of the microsphere lens.

22. A license plate utilizing the retroreflective sheet according to claim 19.

23. An ALPR system, comprising:

a license plate according to claim 22;

a light source that directs light at the license plate; and

a machine capable of photographing license plate.