Abstract
A small-angle measuring device utilizing moiré fringe has been constructed for the absolute measurement of angular displacements smaller than 0.1 µrad. Applying an optical filtering technique, the intensity of the moiré fringe changes sinusoidally as a measuring mirror rotates; the apparatus is self-calibrated with an accuracy of 1%. A light emitting diode is used as the light source in order to reduce heat generation in the apparatus. The electrical noise in the output of the apparatus is about 10 µV which corresponds to an angular displacement of 60 prad. The short-term stability is about 0.6 nrad when the temperature changes are within 0.01°C, and the drift is 3 nrad/h. The measuring range is 4.5 µrad with an accuray of 1%.