1. Field of the Invention
The present invention relates generally to a bar code detector or acquisition system and method for detecting the presence and location of a bar code pattern in a field of view. The bar code pattern detector of the subject invention is not designed to decode a bar code pattern, but merely to be sensitive to and detect the presence of a bar code pattern so that a conventional bar code scanner can then be activated and perform a decoding operation thereon.
More particularly, the subject invention pertains to a bar code detector or acquisition system and method as described which is based upon the production of Moire patterns, and includes an electro-optic system for detecting and locating Moire patterns and thereby detecting and locating the bar code pattern or patterns causing their generation. The electro-optic system is designed to operate in an imaging mode in a work environment which is diffusely illuminated by ambient light.
2. Discussion of the Prior Art
The increased use of bar code symbols to identify products, particularly in retail businesses, has resulted in the development of various bar code reading systems. Many users of bar code readers require portable hand-held scanners which place a premium on small size, lightweight and low power consumption requirements for the devices. One such system is a laser scanning bar code reading system as described in U.S. Pat. No. 4,496,831, commonly assigned to the same assignee as the present patent application.
The laser scanning system disclosed in U.S. Pat. No. 4,496,831 includes a portable hand-held scanning head which may be embodied in various shapes but preferably has a gun shaped housing made of lightweight plastic. A handle and barrel portion are provided to house the various components of the scanning head therein. Within the barrel portion are mounted a miniature light source, a miniature optic train including focusing lenses and a scanning system for directing light from the light source across a bar code symbol, and miniature sensing means for detecting reflected light from the bar code symbol being scanned.
The miniature light source can comprise a laser tube such as a coaxial helium neon laser tube, or an LED, or preferably a semiconductor laser diode which is considerably smaller and lighter than a laser tube, thus reducing the required size and weight of the scanning head and making the scanning head easier to handle and more maneuverable. Light generated by the light source passes through the optic train which directs the beam onto the scanning system which is mounted within the barrel portion of the scanning head. The scanning system sweeps the laser beam across the bar code symbol and comprises at least one scanning stepping motor or resonating or vibrating bar or scanner for sweeping the beam lengthwise across the symbol, and may comprise two motors wherein the second motor sweeps the beam widthwise across the symbol. Light reflecting mirrors are mounted on the motor shaft or shafts to direct the beam through the outlet port to the symbol.
A sensing circuit then detects and processes the light reflected from the symbol and generally comprises a photodetector element such as a semiconductor photodiode. The user positions the hand-held unit so the scan pattern traverses the symbol to be read, and the photodetector element produces serial electrical signals to be processed for identifying the bar code. A signal processing circuit for a bar code produces a signal which is directed to a bar pattern decoder circuit for decoding the bar pattern.
The reader unit can have a movable trigger employed to allow the user to activate the light beam and detector circuitry when pointed at the symbol to be read, thereby conserving battery life if the unit is self-powered. The lightweight plastic housing contains the laser light source, the detector, the optics, signal processing circuitry, a CPU and a battery. The reader is designed to be aimed at a bar code symbol by the user from a position where the reader is spaced from the symbol, i.e., not touching the symbol or moving across the symbol. Typically, this type of hand-held bar code reader is specified to operate in the range of perhaps several inches.
Alternatively, the scanning can be performed automatically without any human interface or intervention.
One disadvantage of such prior art bar code detection systems is that the laser scanner is frequently triggered on when a bar code pattern is not present in the field of view of the bar code scanner, which results in a shortened operational lifetime for the laser of the laser scanner.