A bar code is simply a simple way of encoding characters in a simple machine readable format of black bars, normally printed onto a white background. The characters encoded may be just numbers or both letters and numbers. There are a variety of different codes used to translate between the characters and the black and white bands. These codes are generally known as symbologies. Products in a supermarket are universally coded with an EAN13 or EAN8 symbology. EAN is for the European Article Numbering Association, and the 13 or 8 refers to the number of digits (numbers only) encoded. In America, the equivalent standard is UPC for Universal Product Code and is broadly similar. EAN13 codes can also have an extension code added. These are commonly used for issue identification with magazines.
Bars in the code are distinguished by their width; normally a choice is made between two widths, one unit or two units wide.
There are a large number of other codes available, but the main additional one of interest is code 39. This is used by this program for any bar codes that are printed out. These are printed with a free bar code font provided by Bar Codes, Inc. at http://www.barcodesinc.com/free-barcode-font/ . You are free to use these fonts in other applications. For instance, you might wish to generate bar code labels with a spread sheet program, such as Excel. Any number represented in code 39 needs to have a * character both before and after the number as start and stop codes. That is, if you want a number 1003456, you need to have the string of characters *1003456* printed in the Code 39 font. To see how this works, *1003456* is the same string in the Code 39 font. These need to be printed with a high quality printer; ideally a high resolution laser printer, although success can be achieved with an ink jet printer. A dot matrix printer is not suitable.


It is common practice to print the number that is coded below the actual bar code as this makes it possible to enter a code manually if the scanner fails to read it successfully.
The machine readable part is satisfied by an optical system together with a unit that converts light signals into an electrical signal. There are two main ways of doing this.
The bar code can be illuminated, and then an image of the bar code is converted by a device that is similar in principle to a video camera, except that it only scans a single line, instead of a rectangle. These sensors are based on CCD (charge coupled device) technology and hence the scanners based on them are known as CCD scanners. The illumination is provided by a set or LEDs (light emitting diodes) and such scanners are easily recognised by the pulsing red light that they give off. Red is chosen for illumination, as this is the most sensitive part of the spectrum for the normal silicon based CCD devices used.
The other approach used is to scan the bar code with an intense moving beam of light, and to detect the reflected light with a simple photodetector. The intense beam of light is provided by a solid state visible laser. These devices are robust and have a long life. The power in the light beam is insufficient to cause eye damage, although looking into the beam is not recommended. The effective intensity is high because the laser light is monochromatic, allowing much of the ambient light to be removed by a simple filter before detection. The beam is scanned with a mechanical system of rotating or nutating mirrors, either along a straight line, or in a complex two dimensional pattern for an omni-directional scanner. The use of a mechanical system makes such scanners less rugged overall, but they have the advantage of high readability and fast acquisition. They can read bar codes at a distance, including the larger case codes, and are far less dependent upon the actual distance that the bar code is from the scanner, as image focusing is not involved. Most types will scan continuously or will start scanning when presented with an object at the scanning aperture, allowing hands-free operation. They are generally the preferred option for retail use, but are not the cheapest option. They can be recognised by the sharp red line, or pattern of lines, emitted when in operation.

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