The optical layout used here is similar to the one described in our previous paper (see Fig. ), except to reduce costs we have eliminated one plane mirror, one optical density filter, and one 50:50 beam-splitter. In the new design, the two glass slides BS1 and BS2 reflect approximately four percent of the incident beam intensity into the reference and object beam, respectively, while the transmitted light is dumped into a beam-stop. The chosen design benefits from an extreme economy of specialist parts. Optically, it requires just three thin-film polarisers and two plane mirrors. Mechanically, it uses a single hollow-shaft brushless gimbal motor -an off-the-shelf accessory used for tilting or panning a photographic camera. And, electronically, it uses a laser-diode, a motor driver, two amplified photodiodes, a light-to-frequency converter, and a 32-bit microcontroller with two built-in ADCs. Importantly, the two-beam measurement technique employed here is quite insensitive to jitter in light intensity and motor speed, which means it is possible to use a cheap laser pointer (~1 mW, 654.8 nm) for the light-source and a hobbyist radio control speed controller for the motor driver without noticeably compromising performance. The total cost of all functional components is just £150 (~$200), see Table .