Measurement Of Hidden Bar Code With Confocal Optical Sensor

Based on the light to light conversion properties of the infrared up-conversion(IRUC) material, the characteristics of IRUC ink, the function of optical confocal system and the detecting method of hidden bar code in principle have been discussed and investigated in this paper.Infrared up-conversion material is a new sort of optical function material made of rare earth. Up-conversion is an optical linear phenomenon, which IRUC material with high concentration ions emits the short wavelength light when activated by the long wavelength light due to the interaction between ions in the rare earth material. The essence of the IRUC is anti-stokes principle. We mix the IRUC powder with the transparent ink to form a type of infrared-ink (ER-ink) as the printing ink for bar code. The bar code printed with IR ink is invisible in the normal day light condition, and named as hidden bar code. Not same as the normal bar code printed in the blank area, the hidden bar code can be printed on the surface with color or patterns. Therefore, the hidden bar code takes not only the same advantage as the normal bar code technique, such as quick input speed, high reliability and agility and practicality etc., but also it can be as a new counterfeit tool. Of course, because the conversion efficiency of IR ink from infrared light to visible light is very low, it is very difficult to detect the IR ink.Since the detection of hidden bar code is the weak signal detecting against strong noise, a small confocal optical sensor(CFOS) has been designed and fabricated to activate IR ink by using a laser beam with 980nm infrared light. The structure and performance of the optical confocal system with CFOS have been analyzed. By achieving a small focal spot of infrared laser beam onto the surface of IR ink, the conversion efficiency of up conversion material is greatly increased. The experimental results show that the CFOS can collect more excited visible light from IR ink, and prevent the environmental light to enter the detector. Therefore, the optical confocal system has an excellent feature in detecting weak signal against strong noise, and is proved to have rehable detecting performance when the up-conversion material content in IR ink is more than 0.5%. The scanning resolution of optical confocal system has already been achieved over 0.4mm, the method of improving the resolution of the system is given. The feasibility of hidden bar code has been proved by the experimental results. Hidden bar code can be as an important implement to normal bar code technique.