Because different colors reflect different wavelengths of visible light, white objects can reflect various wavelengths of visible light, while black objects absorb various wavelengths of visible light. Therefore, when the light emitted by the barcode scanner light source passes through the diaphragm and the convex lens 1 and shines on the black-and-white bar code, the reflected light is focused by the convex lens 2 and shines on the photoelectric converter, so that the photoelectric converter receives reflected light signals with different intensities corresponding to the white bars and the black bars. It is converted into a corresponding electrical signal and output to the amplification and shaping circuit. The widths of white bars and black bars are different, and the corresponding electrical signal duration is also different. However, the electrical signal corresponding to the bar code and space output by photoelectric converter is generally only about 10mV, so it cannot be used directly. Therefore, the electrical signal output by the photoelectric converter should be sent to the amplifier for amplification first. The amplified electrical signal is still analog. In order to avoid false signals caused by defects and stains in bar codes, it is necessary to add a shaping circuit after the amplification circuit to convert analog signals into digital electrical signals for accurate interpretation by the computer system. The pulse digital signal of the shaping circuit is converted into digital and character information by the decoder. It can distinguish the coding system and scanning direction of bar code symbols by identifying the start character and the end character. By measuring the number of pulse digital electric signal 0, 1, the number of bars and spaces can be determined, and by measuring the duration of signal 0, 1, the width of bars and spaces can be determined. Therefore, the number of bars and spaces for bar code symbols and the corresponding width and code system can be obtained. According to the coding rules corresponding to the coding system, bar code symbols can be converted into corresponding digital and text information, and sent to the computer system for data processing through the interface circuit.