Method and apparatus for reading invisible symbol

Abstract

An invisible symbol reading apparatus includes a heating unit for heating an invisible symbol formed on a sample and containing a material which emits infrared light when heated, a detecting unit for detecting infrared light emitted from the invisible symbol, and an arithmetic operation unit for binarizing a detection signal from the detecting unit. The arithmetic operation unit calculates a differential coefficient of the detection signal, that corresponds to a position on the sample. On the basis of upper and lower threshold values set for the differential coefficient, the arithmetic operation unit determines a maximum value of the differential coefficient in a region exceeding the upper threshold value and a minimum value of the differential coefficient in a region smaller than the lower threshold value. The arithmetic operation unit binarizes the detection signal by using the maximum or minimum value as a loading or trailing edge of a binary function.

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 containing 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.