Optically active materials and articles and systems in which they may be used

1. A method of manufacturing a retroreflective article, comprising:

obtaining a retroreflective sheeting; and

positioning an infrared-reflecting multi-layer optical film on the retroreflective sheeting to form a pattern;

wherein the infrared-reflecting multi-layer optical film is transparent and not significantly visible in the visible light spectrum; and

wherein the retroreflective sheeting causes incident light to retroreflect via a retroreflective light path, and wherein the infrared-reflecting multi-layer optical film causes incident infrared light to be reflected from the retroreflective light path.

2. The method of claim 1 , wherein the article is a street sign, a signage used for traffic control purposes, a license plate, a validation sticker, a usage permit, an identification label, or a tag.

3. The method of claim 1 , further comprising:

affixing the retroreflective sheeting to a license plate base.

4. The method of claim 1 , further comprising illuminating the retroreflective sheeting with an infrared light source to facilitate reading of the pattern by an infrared sensor.

5. The method of claim 1 , wherein the retroreflective sheeting is one of prismatic sheeting or beaded sheeting.

6. The method of claim 1 , wherein the pattern includes at least one of machine readable information, graphics, images, alphanumeric characters, bar code, or other indicia.

7. A retroreflective article comprising:

retroreflective sheeting; and

an infrared-reflecting multi-layer optical film disposed on at least a portion of the retroreflective sheeting, wherein infrared-reflecting multi-layer optical film is not significantly visible in the visible light spectrum; and

wherein the retroreflective sheeting retroreflects incident light via a retroreflective light path, and wherein the infrared-reflecting multi-layer optical film causes incident infrared light to be reflected from the retroreflective light path.

8. The retroreflective article of claim 7 , wherein the infrared-reflecting multi-layer optical film forms a pattern.

9. The retroreflective article of claim 8 , wherein the pattern includes at least one of machine readable information, a graphic, an image, an alphanumeric characters, a bar code, or other indicia.

10. A retroreflective sign comprising the retroreflective sheeting of claim 7 .

11. A retroreflective sign comprising:

retroreflective sheeting including covert indicia, wherein the covert indicia in the form of a pattern is infrared-reflecting multi-layer optical film and not significantly visible in the visible light spectrum;

wherein the retroreflective sheeting retroreflects incident light via a retroreflective light path, and wherein the infrared-reflecting multi-layer optical film causes infrared light to be reflected from the retroreflective light path.

12. The retroreflective sign of claim 11 , wherein the covert indicia forms a pattern that can be read by an infrared sensor.

13. The retroreflective sign of claim 11 , wherein the covert indicia includes at least one of an alphanumeric character, a sinusoidal curve, a bar code, a design, an image, a symbol, a machine-readable information, a graphic, or information selected from the group consisting of signage material lot number, installation date, reorder information and product life expectancy.

14. The retroreflective sign of claim 11 , wherein the retroreflective sign is one of a highway sign, a traffic sign or a street sign.