Secondly, for the measurements described here, we have used a laser wavelength of 654.8 nm (see Fig ). However, by substituting the laser module, it is possible to carry out measurements in the approximate range 400 nm to 700 nm without any other changes to the component list. The useable wavelength range is limited by the transmission window of the polarisers (400 to 700 nm) and the sensitivity window of the amplified photodiodes (350 nm to 1050 nm), and appropriate component replacements may permit measurements outside the stated operating window without any other changes to the system design.