Technology and Engineering of Barcode Scanners

Technology and Engineering

laser scanners : typical scanners utilise a 650nm laser diode, leveraging on the economies of scale of diodes manufactured for CD/DVD drives. The diode is normally housed in a metal casing for heat dissipation. The laser light emitted is focused through a lens and reflected off a rotating polygonal mirror, or an oscillating mirror. This produces a line or raster pattern which may be aimed at a barcode. The light reflected off the barcode is captured onto a photodiode whereby the current is amplified and interpreted by the decoding circuit. The signal obtained is the modulation of the laser spot (whose profile approximates that of a Gaussian beam), over the alternating black and white lines of the barcode. The performance of the scanner is a function of:
the signal to noise ratio (determined by the laser power, the size of the laser spot, reflectivity/transmittivity of the optical parts, receiving light collection area, distance to the barcode, the level of optical noise such as sunlight, fluorescent light as well as the ability to filter out that noise, etc). The laser power is usually limited by CDRH Class I/II safety requirements.
the scan speed of the device. Basically a trade-off between decode speed and the size of the decode zone.
the barcode (determined by difference in refectance of the barcode's alternating black & white stripes--print contrast ratio, defects, the size of the barcode, and the barcode's orientation in relation to the scanner). Generally, the laser has to pass a certain amount of the 'quiet zone' on the barcode before it hits the line patterns, in order to decode.
the decoding circuit's ability to decode and to handle errors in the barcode, optical noise as well as the range of frequencies at which the signal is modulated.