Method and apparatus for reading invisible symbol
An article including a substrate and an invisible symbol on the substrate is disclosed. The invisible symbol is formed by a compound which includes a cyano group and has an infrared absorption wavelength apart from that of the substrate when heated.
1. An article comprising:
a substrate; and
an invisible symbol on the substrate wherein the invisible symbol is formed by a compound which includes a cyano group and has an infrared absorption wavelength apart from that of the substrate when heated.
2. The article of claim 1 , wherein the compound has a polymer including a cyano group.
3. The article of claim 1 , wherein the substrate is made of paper, polymer or cloth.
4. The article of claim 1 , wherein the compound emits infrared light when heated to 50 degree in Centigrade or more.
5. The article of claim 1 , wherein the invisible symbol is a barcode having minimum bar width about 250 micro-meter.
6. The article of claim 1 , wherein the wavelength of infrared emission is around 4.5 micro-meter.
7. The article of claim 1 , wherein the invisible symbol is printed on the substrate using a laser beam printer.