Test apparatus for value sheets (e.g. currency notes) comprises optical sensors (16; 116), a path along which the sheet is transported past the sensors, and reference devices (46, 146) carrying reference surfaces (48; 148) having predetermined optical characteristics, for example, for calibration of the optical sensor. Each reference device is rotatable to facilitate rapid movement of the sheet past the optical sensors. The transportation path may be defined by entry and exit guides. Alternatively it may be defined by upper and lower guides (20, 22) of spaced apart wires (24, 32) which do not substantially obscure the surfaces of sheet or the reference surfaces, from the optical sensors. Guide rollers (54, 58, 154, 158) project through spaces between the wires to advance the sheet. An automatic cleaning device is operable to clean the reference surfaces.