Only one-dimensional bar codes in one direction (usually horizontal direction) expression, whereas in the vertical direction is not expressed any information, a certain height is usually for ease of reading of alignment.
The one-dimensional bar code application can increase the speed of information entry and reduce error rate, but one-dimensional bar code, there are still some shortcomings :
* Data smaller capacity : about 30 characters
* Contain only letters and numbers
* Barcode relatively larger size (less space utilization)
* Barcode not been damaged after reading
The horizontal and vertical two-dimensional spatial orientation of the bar code information storage, called two-dimensional bar code (2-dimensional bar code).

With one-dimensional bar code, two-dimensional bar code has many different coding method, or code system. These codes on the system of coding theory, can generally be classified into the following three types
1. Linear stacked two-dimensional codes in one-dimensional bar code coding theory on the basis of a number of one-dimensional vertical stacking yards and returned. Typical code system such as : Code 16K, Code 49, such as PDF417.
2. Matrix is a two-dimensional code in a rectangular space through the black and white pixels in the matrix of the different distribution of coding. Typical code system such as : Aztec, Maxi Code, QR Code and Data Matrix.
3. Through various postal code length of the code, mainly for e-mail coding, such as : Postnet, BPO 4-State.

In many types of two-dimensional bar code, the code used in the system are : Data Matrix, Maxi Code, Aztec, QR Code, Vericode, PDF417, Ultracode, Code 49, Code 16K, which :
* Data Matrix mainly for the electronics industry marking small parts, such as Intel's Pentium processor printed on the back of this code.
* Maxi Code by the United Parcel Service (UPS) developed by the company for the parcel sorting and tracking.
* Aztec by the United States 1,100 (Welch Allyn) launched, can accommodate a maximum of 3,832 or 3,067 digital alphanumeric characters or 1,914 gigabytes of data.

Now, we PDF417 code as an example, the two-dimensional bar code on the traits and characteristics.

1) Introduction PDF417

PDF417 code is studied Chinese Wang Yin King (sound), Dr invention. PDF is the way English Portable Data File three words of the first letter acronym meaning "portable data files." Because the composition of each bar code symbols are characters from four and four of the air composition, will be formed if the bar code or the most narrow space as a module, the above-mentioned four and four of the total space module for a certain number of 17, said the 417 yards or PDF417 code.

2) the characteristics of PDF417

1. Information content PDF417 code can be expressed except letters, numbers, ASCII characters, but also a few binary expression. In order to enable more compact code, improve information density, PDF417 code in three formats :
* The extension of the alphabet digital compression format can accommodate 1,850 characters;
* Binary / ASCII format can accommodate 1,108 bytes;
* Digital compression format can accommodate 2710 figures.
2. To correct errors in a peacekeeping capacity with the bar code checking function normally in order to prevent the wrong time, once defaced occurred bar code reading will be refused. While two-dimensional bar code can not only prevent errors and mistakes to be corrected, even partially damaged bar codes, but also can restore the right message out.
3. Print modest ordinary printing equipment can be printed, faxed document can be read.
4. Various reading equipment can be used to read PDF417 code can be used with the grating laser reader, linear and facial image-scanning reader reading.
5. Size adjustable to suit different space Print
6. Code has developed an open system of international standards, China has also developed a 417-yard GB.

3) error correction function PDF417

Two-dimensional bar code error correction function is to repeat some of the information indicated (redundant) to achieve. For example, in the PDF417 code, in addition to a line of the information contained profession, there are some other location to reflect on the character (error correction code) information. Thus, even when a part of the bar code damage, can also exist in other locations error correcting codes will be reduced from its information.

PDF417 capacity according to the FEC error correcting codes to the number of words divided into a total of 8 0 ~ 9, see figure 4, the highest level, the correct code word more, the more correct, the greater the bar code. When the correct rating for 8:00, even though 50% of bar codes defaced also be read out correctly, as illustrated in figure 5.


4) Several deformation PDF417

Figure 6, there are several PDF417 code deformation of the system forms :
* Truncated PDF417 code in a relatively "clean" environment, the possibility of damage to the bar code small, may be the right line indicator omitted and reduce termination website.
* Micro PDF417 codes to further reduce the PDF code.
* Acer PDF417 code when the document is too long and could not be a code PDF417 said, includes a number of available (1 ~ 99,999), the bar code block Acer PDF417 code to indicate.

The advantages of two-dimensional bar code

From the above presentation can be seen, one-dimensional bar code and two-dimensional bar code than there is a clear advantage can be summarized in the following areas :
1) greater data capacity
Map of PDF417 code text box contains all the characters.
2) beyond the limits of the digital alphabet
3) relatively small size barcode
4) ability to resist damage

The application of two-dimensional bar code

1) The application of the transport industry

A typical transport business process normally experience : suppliers --> freight forwarders, freight forwarders --> freight forwarders, shipping companies --> clients several processes, each process has involved shipping document processing. Shipping document contains a lot of information, including : the sender of information, the consignee information, the list of goods, transport and so on. Document processing is the prerequisite of data entry, manual keyboard entry of the existence of low efficiency, high error rate, which has been unable to meet the requirements of modern industry.
In this two-dimensional bar code provided a good solution to the content of the document into a two-dimensional bar code, print the shipping document, transport operations in various aspects of the use of two-dimensional bar code reader to scan a bar code, then input the information into the computer management system, both fast and accurate.
In the United States, despite the innovative application of EDI business processes to the core, but unfortunately it is a process overlooked the key role -- the shipping companies. Many EDI messages for cargo operators always run late and so could not be confirmed timely and accurate single shipment information to affect the transport of goods and customer documentation generation.
The United States Air Cargo Association (ATA) has put forward a paper EDI system. EDI to transmit information into a PDF417 barcode labels to cargo operators, bar code scanning, information immediately imported cargo operators in the computer system. All this happened at the right time and the right place, making the whole process of transport efficiency is greatly enhanced.


2) identification card application

The U.S. Department of Defense has military identification cards printed on the PDF417 code. The cardholder's name, rank, photographs and other personal information are compiled into a PDF417 code printed on the card. The card has been used as an important venue management and access management to hospitals.
The application of the advantage of the real-time data acquisition and low cost of implementation, cards damage (such as shooting) will also read and security nature.
Chinese residents of Hong Kong SAR also adopted the identity PDF417 code. Other applications, such as business licenses, driving licenses, passports, the movement of China's urban population temporary residence permit, medical insurance cards also are good application direction.

3) documents and application forms

Japan Seimei insurance agent in each of their clients have met with notebook computers. Each policies and agreements made in the computer and print out. When they returned to the office after office need to manually enter data into the company's mainframe.
In order to improve the accuracy of data entry and speed in the production office at the same time to compile a policy as PDF417 bar code, print the documents, so they can use two-dimensional bar code reader to scan a bar code to mainframe data entry.
Other similar applications include : customs declaration form, tax returns, government departments of various types of applications and so on.

4) Asset Tracking

Steel Tube Company in the United States all over different types of pipeline need to preserve. To track every pipes, they will pipe the number, location number, manufacturer, length, class, size, thickness and other information into a PDF417 bar codes, labels made after affixed to the tube on. When the tube removed or installed, the operator scanned bar code labels, the database information will be updated.
Factories can use two-dimensional bar code tracking production equipment; Hospitals and clinics can be used in two-dimensional bar code labels to track equipment, computers and surgical instruments.