The polarizer is mounted in a high precision computer controlled rotary stage from PI-Instruments and controlled by the C842 DC controller card. The decoder signals are differentially processed which provides a secure transmission of the signal even in a noisy environment. This eliminates the common problem of step losses which occur quite frequently with stepping motors or for DC-cards without differential processing of the signal. The decoder of the DC motor provides an active feedback of the actual position of the rotor and is not just counting voltage pulses. Furthermore the system is equipped with a Hall sensor to get an accurate absolute home position.
A Glan-Thompson Prism is used as a polarizer. Glan-Thompson prisms are the most efficient polarizers. The polarizer used is made of a specially selected Calcit and possesses an extinction ratio better than 10-7. A beam propagating in a birefringent crystal splits into two beams of mutual perpendicular polarizations. The extraordinary beam is totally reflected whereas the ordinary beam is transmitted.