A note skew detector, for use in a note transport mechanism of an Automated teller Machine (ATM) is described. The detector comprises a light source and a single optical sensor, optically coupled via a pair of optical wave-guides arranged to have an air gap there between so as to provide a note transport path between the said wave-guides. The wave-guides are further arranged to provide a first optical path and a second, distinct, optical path between said light source and said sensor.