2. Each time the scanner scans an item, it performs a calculation by counting the ‘check digit’. Eg: 34536563 add every odd---(3+5+6+5+3 = 22) If the check digit it calculates is different from the check digit it reads, the scanner knows that something went wrong and the item needs to be rescanned. A barcode reader is an electronic device for reading printed barcodes. it consists of a light source, a lens and a light sensor translating optical impulses into electrical ones. Additionally, nearly all barcode readers contain decoder circuitry analyzing the barcode's image data provided by the sensor and sending the barcode's content to the scanner's output port.