|Monday, 11 September 2006|
A barcode label may be read by a scanner that detects reflected and/or refracted light from the bars and spaces that comprise the bar code characters. Generally, a barcode symbol is a coded pattern of indicia comprised of a series of bars of various widths separated by spaces of various widths, the bars and spaces having different light-reflecting characteristics. A typical one dimensional (1D) barcode includes a series of parallel lines and spaces of varying widths which in accordance with known conventions may be used to represent desired information relating to the item. A two dimensional (2D) barcode provides more complex lines and spaces in two orthogonal axes for representing even more information regarding an item than is available in 1D barcodes. A bar code label comprises a series of parallel dark bars of varying widths with intervening light spaces, also of varying widths. The information encoded in the bar code is represented by the specific sequence of bar and space widths, the precise nature of this representation depending on the particular bar code symbology used. Barcode scanning systems electro-optically transform the graphic indicia into electrical signals. The system decodes these signals into alpha-numerical characters that provide some information about the article. Such characters are typically represented in digital form, and are used as input to processing systems associated with applications such as point-of-sale and inventory control. Various optical readers and optical scanning systems have been developed heretofore for reading bar code symbols appearing on a label or on the surface of an article. There're several types of bar code readers available including the pen type readers (bar code wands, pencil wands, contact scanners), laser bar code scanners, CCD (charge couple devices) bar code readers and camera-based bar code readers used for most two dimensional (2D) bar codes which contain much more information than standard vertical line bar codes. Each of these types uses a slightly different technology for reading and decoding a bar code.
Pen type readers scan or read a bar code by emitting visible red or infrared light such as, for example, from an LED which strikes the bar code while the scanner is moved across the bar code in a linear direction with the tip of the scanner touching the bar code label. Pen type bar code readers have a light source and a photo diode placed next to each other in the tip of a pen or wand. To read a bar code, a user drags the tip of the pen across all the bars in a steady even motion. The photo diode measures the intensity of the light reflected back from the light source and generates a waveform corresponding to the widths of the bars and spaces in the bar code. The bar code reader sends the waveform to the decoder, which decodes the waveform and sends it to the computer in a traditional data format. CCD bar code scanners use an array of tiny light sensors lined up in a row in the head of the bar code reader. Voltage waveform corresponding to the bars and spaces of the bar code is generated and sent to the decoder, which decodes the data and sends it to the computer. CCD scanners simultaneously illuminates all of the bars and spaces of a bar code symbol with light of a specific wavelengths in order to capture an image thereof for recognition/decoding purposes. Laser bar code reader uses a focused light beam, typically a focused laser beam, to sequentially scan the bars and spaces of a bar code symbol to be read. This type of bar code symbol scanner is commonly called a flying spot scanner. In general, laser bar code symbol scanners are subclassified further by the type of mechanism used to focus and scan the laser beam across bar code symbols. The camera-based bar code readers used for the majority of 2D bar codes which are becoming more popular due to increased data carrying ability, use a small video camera to capture an image of a bar code. The bar code reader then transmits that information to a computer and uses sophisticated digital image processing techniques to decode the bar code. The main difference between a CCD bar code scanner, a pen type bar code scanner, and laser bar code scanner is that the CCD bar code scanner measures emitted ambient light from the bar code whereas pen or laser bar code scanners measure reflected light of a specific frequency originating from the scanner itself. Wand scanners tend to be the least expensive, followed by CCD scanners and laser scanners (the most expensive).
A typical laser barcode scanner is configured to scan an outbound laser beam across a barcode, and decode the inbound back scattered light therefrom. The laser beam is segmented into scan lines by reflection from a mirrored spinner, which scan lines are projected against the barcode either directly or upon reflection from one or more pattern mirrors. As the laser beam scans across the barcode, light is reflected from the white spaces and absorbed by the dark spaces to modulate the back scattered light, which may then be suitably demodulated or decoded. In a laser barcode scanner, a rotary spinner having a plurality of mirrored facets divides the laser beam into a corresponding number of segments which are reflected off a plurality of differently oriented pattern mirrors to form scan lines which project in an intersecting pattern once per revolution of the spinner. This maximizes the ability to scan a barcode placed at any orientation within the field of view of the scanner. For the bar code to be read clearly the laser beam must be focused at or adjacent the bar code. Laser beams used to illuminate bar codes have a limited depth of field over which they are focused. The scanner generates a scanner signal based on light that enters the scanner as a result of a reflection of the scan pattern from a bar code. The scan pattern is generated by reflections of a laser beam produced by a scanner laser, with the laser beam produced by the scanner laser being of a particular wavelength. In order to produce a scan pattern, a scanner directs a laser beam from a laser source to a mirrored polygonal spinner which is rotated by an electric motor. The sides of the polygonal spinner may be referred to as facets. The spinner directs light to one or more mirrors in order to produce a scan pattern which is directed to and emerges from an aperture. The majority of laser scanners in use today employ lenses and moving (i.e. rotating or oscillating) mirrors in order to focus and scan laser beams across bar code symbols during code symbol reading operations. A barcode scanner of a laser scanning system uses a laser diode as a light source.
Optical scanners are available in a variety of configurations, some of which are built into a fixed scanning station and others of which are portable. In the fixed mode of operation, objects with bar codes thereon are moved to or past a stationary bar code scanner for scanning. The fixed type bar code reader is provided with a mechanism (a scanning/converging optical system) for scanning the bar code in a multiplicity of directions by irradiating the bar code with light beams for reading so that an operator is capable of reading the bar code without being so aware of a bar code direction. A portable bar code scanner is typically oriented and/or moved to the bar code label to be read. For purposes of this description, the term bar code scanner shall henceforth denote a scanner of the spot scanning type, wherein an illumination spot is moved across a bar code. The portable scanners provide a number of advantages, including the ability to inventory products on shelves and to track portable items such as files, documents, or small equipment. A number of portable scanners use lasers which permit the user to scan the bar code symbols at variable distances from the surface on which the bar code is imprinted.
A bar code scanner typically has an optical system with two subsystems: an illumination subsystem which produces an illumination beam and a collection subsystem which collects and detects light. The illumination subsystem, typically comprising a light source, a focusing lens, and a scan engine, focuses an outgoing light beam to a minimum diameter, known as the waist, and generates a scan pattern so that the illumination beam, or spot, is likely to be scanned across a bar code. The collection subsystem, which typically includes a collection lens, or alternatively a concave collection mirror or functional equivalent thereof, and a photodetector, collects at least some of the light scattered and/or reflected from the bar code illuminated by the illumination beam and focuses the same onto the detector. The photodetector produces an analog signal having an amplitude determined by the intensity of the collected light. The photodetector is usually positioned in the scanner such that it has a field of view which extends across and slightly past a symbol within the pattern. The signal from the photodetector is analyzed by a computing device which attempts to produce valid bar codes from the incoming signals. Optical scanners generally employ a laser diode, the light from which is focused and collimated to produce a scanning beam. A mirrored polygon directs the beam against a plurality of stationary mirrors, and collects the beam after it is reflected by an item bearing a barcode label. A motor rotates the mirrored polygon, and a detector receives the returning beam. A barcode scanner may also employ a set of scan pattern generating optics to produce a multiplicity of scan lines in various directions from the scanner and at varying orientations, thereby allowing barcodes to be read over a large angular field of view and over a wide range of orientations. The scan pattern generating optics typically comprise a set of mirrors aligned at varying angles, each of which intercepts the illumination beam during a portion of its motion and projects it into the region in front of the barcode scanner, hereinafter referred to as the scan volume. Each mirror in the set, in conjunction with the scan engine, produces a scan line at a particular position and at a particular orientation.
In general, methods for reading bar codes may comprise generation of an electronic signal wherein a signal voltage alternates between two preset voltage levels, one representing a dark bar and the other representing a light space. Typically, bar codes are read by a bar code scanner by illuminating the bars and spacings in a sequential manner, with the bars absorbing light and the background spacings reflecting light. The bar code may be read by having a light beam translated across the bar code and a portion of the light illuminating the bar code is reflected and collected by a scanner. The intensity of the reflected light is proportional to the reflectance of the area illuminated by the light beam. In order to detect and decode a bar code, the scanner employs a detector including a photodiode or other device that produces the scanner signal upon being struck by the reflections of the scan pattern from the bar code. The photodetector may generate a high voltage when a large amount of light scattered from the bar code impinges on the detector, as from a light space, and likewise may produce a low voltage when a small amount of light scattered from the bar code impinges on the photodetector, as from a dark bar. The scan pattern is made up of light produced by reflections of the scanner laser, and this light has a particular wavelength. The wavelength of light to be detected in order to produce the scanner signal is the wavelength of light produced by the scanner laser. Accuracy of a bar code reader reading a bar code much depends on the quality of a bar code to be read, the accuracy of an optical system in a bar code reader reading a bar code, and the performance of decoding processing of a bar code reader. The performance of a bar code scanner, the efficiency and accuracy of detection and decoding of bar codes, improves as total scan line length increases. A longer total scan line length provides a greater amount of light per unit of surface area when scan lines strike a surface, and also provides a greater area of coverage.
Bar code readers employ decoding circuitry to interpret the signals produced by a photodetector receiving the reflected light from the bar code symbol. As part of a barcode reading system, a decoder can be defined as an electronic package that receives a signal from the scanner, performs an algorithm to interpret the signal into meaningful data, and provides the data to other devices of the system. The light reflected from the barcode typically follows the reverse scanning path through the pattern mirrors and spinner to a collection mirror that focuses the light onto a photodetector to produce a corresponding electrical signal which is decoded in a decoder. There are 3 main types of decoders: wedge decoders, serial decoders and software decoders. Wedge decoders are external devices that generally wedge between a keyboard and terminal. With keyboard wedge decoders, the data appears as though it was manually typed or keyed directly into the computer. Serial decoders are also external devices that connect into a communications port of the personal computer. Likewise, expensive decoded scanners (i.e., scanners in which the decoder is integrated, usually into the handle of the scanner) usually connect via a keyboard wedge or via a serial port. A digitizer included in the scanner processes the analog signal from the photo detector to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and the threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer is applied to a decoder. The decoder first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyzes the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard.
The use of bar code scanners in a great many aspects of everyday life is commonplace. The use of bar code scanners in retail transactions decreases the time required to enter a transaction, and decreases the chance of an error during manual entry of data. Bar code scanners provide increased efficiency in the use of labor, increased customer satisfaction due to reductions in waiting time and incorrect charges, and an improved quality of operation that comes from the reduction of pricing errors. Bar code scanning allows for instant, accurate updating of inventory, making for a greatly increased efficiency of operation. The use of bar code scanners has dramatically increased the speed at which many commonplace transactions can be completed. Bar code symbols are widely utilized in many commercial environments such as point-of-sale (POS) stations in retail stores and supermarkets, inventory and document tracking, and various data control applications. Bar code scanners are found in many different types of facilities, including supermarkets, airport security, check-in and boarding areas, stadiums, libraries, test centers, conference centers, and many other places.