A barcode is an optically readable representation of data. Barcodes represent data in the width and the spacing of parallel lines—these are commonly referred to as linear or 1D (1 dimensional) barcodes or symbologies. Barcodes also come in patterns of squares, dots, hexagons and other geometric patterns within images which are commonly referred to as 2D (2 dimensional) matrix codes or symbologies. Barcodes can be read by optical scanners called barcode scanners or barcode readers or scanned from an image using specialized software.

Symbologies

The mapping between messages and barcodes is called a symbology. The specification of a symbology includes the encoding of the single digits/characters of the message as well as the start and stop markers into bars and spaces, the size of the quiet zone required before and after the barcode, and the computation of a checksum.

Linear symbologies can be generally classified by two properties:

• Continuous versus discrete
  
  Characters in continuous symbologies usually abut, with one character ending with a space and the next beginning with a bar, or vice versa. Characters in discrete symbologies begin and end with bars; the inter-character space is ignored as long as it isn't wide enough to look like the code ends.
  
  

• Two-width versus many-width
  
  Bars and spaces in two-width symbologies are wide or narrow; how wide a wide bar is exactly has no significance as long as the symbology
  requirements for wide bars are adhered to (usually two to three times wider than a narrow bar).
  
  Bars and spaces in many-width symbologies are all multiples of a basic width called the module; most of these codes use four widths of 1, 2, 3, and 4 modules.
  
  There is a large variety of 2D symbologies. The most common are matrix codes, which feature square or dot-shaped modules arranged on a grid pattern. 2D symbologies also come in a variety of other visual formats. Aside from circular patterns, there are several 2D symbologies that use steganography by hiding an array of different sized or shaped modules within a user-specified image.

Linear symbologies are optimized to be read by a laser scanner, which sweeps a beam of light across the barcode in a straight line, reading a slice of the barcode light-dark patterns. Imaging doesn't require moving parts like a laser scanner does. In 2007, linear imaging was surpassing laser scanning as the preferred scan engine for its performance and durability.

2D symbologies cannot be read by a laser because typically there isn't a sweep pattern that can encompass the entire symbol. 2D symbologies must be scanned by an image-based scanner using a Charge Coupled Device (CCD) or other digital camera sensor technology; however 2D scanners have the capability of reading 1D barcodes. In addition, imaging doesn't require moving parts like a laser scanner does.

Barcode scanner connections to a computer

Barcode scanners can be classified into three categories based on their connection to the computer:

• RS-232 barcode scanner (older type)
  
  RS-232 barcode scanners require special programming to transfer input data to the application program.
  
  

• Barcode scanner that connects to a computer and its PS/2 or AT keyboard via an adapter cable
  
  

• USB barcode scanner
  
  Like the keyboard interface scanner, this barcode scanner type doesn't need any code or program to transfer input data to the application program. When you scan the barcode, its data is sent to the computer as if it had been entered on the keyboard.