METHOD AND APPARATUS FOR MONITORING RELATIVE ANGLES

摘要

A light source (16) projects light through a fixed light polarising filter (18) and through a second light polarising filter (20) mounted on a pendulum (14). An upper window (24) of a twin windown photo diode (12) receives light from the first filter (18) only and the sensed light is compared to a datum value and the strength of the light source (16) is adjusted in order to ensure that the reading given at the lower window (22) of the diode (12) (which receives light which has passed through both the first and second filter) provides an accurate indication of the relative angle between the two filters.