In banknote readers of petrol stations or video games a series of optical sensors come into operation. The parameters recognized by these are length, density of the paper and color. In the latter case the banknote is analyzed using infrared light sources. The data is compared with that of the sample banknotes to decide whether the note is true or false. In the US banknotes are printed with a magnetic ink that facilitates automatic reading. On European banknotes this ink is present only in traces and the reading is therefore less reliable.