Distortion of Bar Code Signals in Laser Scanning

Abstract

One of the most common methods of reading bar codes is to scan them with a laser beam and process the received optical signal. The received light is proportional to the integral of the beam profile multiplied by the region of the bar code covered by the beam. The finite spot size of the laser beam acts as a filter on the original bilevel bar code signal. This filtering can cause intersymbol interference between adjacent edges of the bar code signal. An edge detector attempts to reconstruct the original bilevel signal from this filtered signal. The edges detected in the filtered waveform are in different locations than those in the original bilevel bar code signal resulting in a distortion of the bar code signal. If this distortion is too large, it can prevent the bar code from being properly decoded. Thus, there is a limit on the spot size of the laser beam that can be used to scan a given density bar code.