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The Use of Filter in the Counterfeit Detector

The currency detector is an optoelectronic instrument for checking the authenticity of banknotes, involving many fields such as machinery, electricity, light, and magnetism. The counterfeit identification methods used by the currency detector are usually three methods: fluorescence detection, magnetic analysis, and infrared penetration. Among them, the fluorescence detection method uses filters to distinguish light and eliminates the interference of ambient light.


1. Detection principle

Banknotes are made of special paper. Counterfeit banknotes are usually made of bleached ordinary paper. The bleached paper will have a fluorescent reaction under ultraviolet light (blue light with a wavelength of 365nm), while real banknotes have no fluorescent reaction.


2. Fluorescence detection method

The first method is to irradiate the surface of the banknote with ultraviolet light and measure the intensity of the reflected fluorescence. In the reflected fluorescence spectrum with a center wavelength of 460nm, the fluorescence intensity of the real banknote is basically zero. The fluorescent light intensity of counterfeit banknotes is much greater than that of real banknotes, so when detecting 460nm wavelength fluorescence, the 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 center wavelength of 550nm to produce a fluorophore visible to the naked eye. Compared with the previous method, this method can improve the identification accuracy.


3. Photoelectric signal system for fluorescence detection

A purple mercury lamp with a center wavelength of 365nm is used as the laser light source, and 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 composite filter in front of the photodiode. The combination has a transmittance of about 40% for 550nm light and can effectively filter other stray light. The reflected light passes through the filter, and the photodiode converts the light signal into an electrical signal input circuit, and the authenticity of the banknote is confirmed by comparing the voltage.


Nanjing Hyperion Optics can provide you with filters for currency detectors. If you need relevant optical components, please feel free to contact us.

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