Barcode readers all work by measuring the intensity of light on some kind of light sensor. Many use laser light sources, some use non-lasing light-emitting diodes (LEDs) and two newer designs are able to use abmient light alone. Regardless of the light source or type of sensor, barcode scanners operate by using some device which employs the photoelectric effect to translate the pattern of light intensity of the barcode to a pattern of higher and lower voltages, which are interpreted by a processor into stock-keeping units (SKUs).
Older barcode readers require passing the device at a slow, constant rate past the barcode to ensure that each bar is translated by a photodiode to a long enough interval of lower-intensity reflected light to measure the scanning time for each bar in the correct proportion to the others. If the movement is too fast, the width of some bars could be too little for the response time of the photodiode.
Other designs employ a prism or mirror, moving at a fixed rate, to cause a stationary laser to scan a beam of light across the barcode. Because the timing at which light falls upon the photodiode is constant, this type of barcode reader only requires the user to aim the reader at the barcode long enough for the beam to scan across the barcode once. This design is the once commonly chosen for retail sales, and is easy enough to use that a few retailers have introduced check-out lanes which combine ATM technology with these scanners, allowing customers to process their purchases without waiting for a clerk to be available.
Two other barcode reader designs do not include their own light sources at all, and depend on ambient light for the photons they detect. A charge-coupled device reader (CCD reader) uses a row of hundreds of photodiodes to sense ambient light, and a camera-based reader uses hundreds of rows, each containing hundreds of photodiodes, making them similar to digital cameras. They have only become feasible relatively recently, as devices became available that are sensitive enough to identify barcodes in any lighting conditions bright enough for their human users. Also, because of the significant image-processing requirements of camera-based readers, these are too expensive to be competitive as mere barcode readers, although their technology has the potential to serve that purpose. They are, however, of interest to police departments and security companies for license plate and facial recognition, applications which less expensive barcode readers cannot accomplish.
