Optical scanning head with improved resolution

Abstract

The optical scanning head comprises two or more LEDs which are oriented to emit light at different angles so that a fan of light is created. A 'dark room' encloses the LEDs, a lens assembly, and the spacing between the lens assembly and the detector, a linear CCD array. The portion of the dark room that extends between the lens assembly and the detector has a plurality of baffles formed therein to capture scattered light, preventing this scattered light from becoming noise at the detector. The lens assembly includes, along with a plurality of focusing lenses, a combination of a generally elliptical aperture, or 'cat's eye, and a rectangular obscuration. The combination prevents the central lobe of the Airy disk generated by the incoming light from being transmitted to the detector. This results in the incoming information being carried in the outer lobes or rings, increasing the angular separation of the lines of the bar code. Waveshaping software is incorporated in the decoding processor to compensate for the limitations of the imaging system, which include convolution distortion and the resulting loss of image contrast. This software is used to determine the scan parameters including global threshold for the next scan. As an alternative to use of a global threshold, a sliding frame buffer is used to generate pseudo-pixel numbers to fill in the space between physical pixels in the detector, providing a greater number of effective pixels to obtain better resolution.