Bar code is a symbol composed of a group of black-and-white parallel lines with different width according to a specific format and spacing. It is usually printed on goods or printed matter. It can replace all kinds of text information, and can read data at any time through photoelectric readout device. Ean bar code system is composed of bar code symbol itself, bar code reading device, interface and computer, which completes the input and output of commodity information. There are two kinds of bar code recognition symbols. Bar code symbols are rectangular line graphics. The information reading of optical scanner is mainly to read and recognize these bar code symbols. Digital symbols are numbers and letters outside the line, including 0-9 numbers and A-Z letters, which can be directly recognized by the naked eye. Generally, they are 8-16 digits. The number of digits varies with the code system. The arrangement, width and number of bar code lines shall be determined by the users themselves, and the meaning of coding shall be determined. Generally, there are marks at both ends for beginning and end reading, and some are printed with numbers under the line, which can be directly identified or interpreted by optical character reader, so it can also carry out commodity counting, statistics and management.

The reading performance of the bar code system, that is to say, whether the bar code system can be used normally, mainly depends on the reading ability of the system and the printing quality of the bar code. As a kind of coding information, bar code is a specific language for communication between people and computers. Bar code is a line symbol of black and white, thick and thin. The thick black line is used as 1 in the computer, and the thin black line represents 0. Through logical conversion, it can be expressed as Arabic numerals and arrays of 0-9. Therefore, a reading device must be used to read. The reading system mainly includes scanner and decoder. Scanner is a component that directly contacts the bar code to read in the signal. It is composed of light transmitter, photoelectric detector and optical lens. It can read the information abbreviated by bar code at a very fast speed. When scanning, when the light beam from the light emitter shines on the bar code, the photoelectric detector responds according to the light intensity reflected from the bar code. When the scanning light spot is scanned on the white paper or in the blank space between the two black lines, the reflected light intensity causes the detector to output a large current; when scanning into the black line, the reflected light is weak, the detector outputs a small current, and The response time is different according to the width of the black line. With the change of the light and shade of the bar code, the current signals of different sizes are transformed into current signals, which are amplified and then transmitted to the decoder. Through the decoder, the signal is translated into data for local inspection and display, and connected with the keyboard and sent to the computer for data processing. Therefore, the printing quality of bar code is related to whether it can be read normally. The basic quality requirement of bar code printing is that the ink color is uniform, the layout is not dirty, and the lines are clear.