1. Field of the Invention
This invention relates to a method and apparatus for optical code reading and, in particular, relates to a method and apparatus for code reading that utilizes a micro-electromechanical (MEM) optical resonator having an integral photodetector.
2. Description of the Related Art
Optical code reading systems have found widespread acceptance in a variety of diverse applications such as information handling, consumer checkout, and factory/warehouse automation. In a typical application, a product, package or other physical entity is marked with a code, such as a typical one dimensional bar code comprising a pattern of black bars or lines. A code reader is then used to read the bar code pattern and to thereby identify the physical entity.
There are two general types of architectures employed in code readers: imager based and laser scanning based. Imager based code readers capture an image of the code pattern using a CCD or similar integrated circuit that employs a matrix of pixels. The code pattern is then decoded using image processing techniques. The advantages of this approach include solid state reliability, orientation insensitivity, and code type flexibility. Another advantage of this approach is that the imaging device is usable in conjunction with code patterns formed of low contrast symbols. However, this advantage is of limited benefit in practice since the vast majority of code reading applications use standard bar codes which are formed of high contrast symbols. The disadvantages of this approach include limited depth of field and read range, the need for a separate high energy light source, and limited resolution.
Laser scanning based code readers scan the code pattern with a moving laser spot. The code pattern may then be determined by determining the instantaneous amount of light reflected as a function of time during a given scan interval (or, in practice, during multiple scan intervals for redundancy). The advantages of this approach include high speed, high resolution, integral light source, longer range and larger depth of field. The advantages of the laser scanning approach and the disadvantages of the imaging approach have led to the laser scanning approach being the preferred approach for most warehouse/factory automation applications as well as many other applications.
Nevertheless, existing laser scanning based code readers suffer several disadvantages related to the optical resonator used to provide the moving laser spot. Generally, an optical resonator comprises an oscillating mirror that is used to reflect/scan a light beam from a laser light source onto a target. Currently, optical resonators include rotating motors with polygon mirror facets, oscillating stepper or galvonmetric motors moving a mirror, rotating motors driving a holographic disc, and resonant flexure devices driving a mirror or light source.
The devices that are currently used to construct optical resonators suffer from the following disadvantages. First, existing optical resonators employ structure that is susceptible to mechanical fatigue and failure. Optical resonators constructed using rotating devices have bearings that are susceptible to failure. Optical resonators constructed using resonant flexure devices having bending or rotational spring elements are also susceptible to failure. Generally, any metallic element that experiences a large number of stress cycles (as will be the case in a scanning engine) is susceptible to failure due to imperfections in the metal structure, i.e., grain boundaries at which cracks may initiate and propagate. In short, therefore, existing optical resonators are not as reliable as solid state devices which, lacking structural fatigue mechanisms, have extremely high reliability.
Second, existing optical resonators comprise several to many discrete components, consume significant space and are relatively costly. As previously described, existing optical resonators include rotating motors with polygon mirror facets, oscillating stepper or galvonmetric motors moving a mirror, rotating motors driving a holographic disc, and resonant flexure devices driving a mirror or light source. As the number of discrete components increases, so too does the space consumed by the components and the cost of providing and assembling the components.