Optical filters in the currency detector
Oct. 15, 2020
The currency detector is an optoelectronic instrument for checking the authenticity of banknotes, involving many fields such as mechanics, electricity, optics and magnetism. The counterfeit identification methods used by currency detectors are usually three methods: fluorescence detection, magnetic analysis and infrared penetration. The method of fluorescence detection uses filters to distinguish light and eliminates the interference from ambient light.
Principle of detection
Banknote is made of special paper. Counterfeit banknotes are usually made of bleached ordinary paper. The bleached paper will undergo a fluorescence reaction to ultraviolet light (blue light with a wavelength of 365 nm). Blue light is diffracted under the excitation of ultraviolet light with a wavelength of 420-460nm. Real banknotes have no fluorescence reaction.
The first method is to irradiate the surface of banknotes with ultraviolet light and measure the intensity of reflected fluorescence. In the reflected fluorescence spectrum with a central wavelength of 460nm, the fluorescence intensity of real banknotes is basically zero. The fluorescent intensity of counterfeit banknotes is much greater than that of real banknotes. When detecting 460nm wavelength fluorescence, the one with higher light intensity is the counterfeit banknote.
Another method is to use ultraviolet light to irradiate the colorless fluorescent ink to excite the fluorescence with a central wavelength of 550nm to produce a fluorophore visible to the naked eyes. Compared with the previous method, this method can improve the identification accuracy.
Photoelectric signals system for fluorescence detection
A purple mercury lamp with a center wavelength of 365nm is used as the laser light source. An ultraviolet filter is added in front of the light source to allow only 365nm ultraviolet light to pass through to ensure the purity of the laser light. Place a pair of filters in front of the photodiode. This combination has a transmittance of about 40% for 550nm light and can effectively filter other stray light. The reflected light passes through filters. The photodiode converts the light signal into an electrical signal input circuit. The authenticity of the banknote is confirmed by comparing the voltage.
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