Optical system for angle measurement by interferometry

Claim 1

An optical system for angle measurement by interferomety comprising: a. two, similar beam pathlength multipliers each consisting of a fixed retroreflector and a movable retroreflector in the reflecting path of the corresponding fixed retroreflector to form the beam pathlength multiplier therewith, b. fixed mounting means mounting both of the fixed retroreflectors with their inversion planes in a common plane which is normal to beams to be reflected back and forth between each fixed retroreflector and the corresponding movable retroreflector, c. rotatable mounting means mounting both of the movable retroreflectors for rotation together about a single axis at right angles the said beams, and with the movable retroreflectors at a predetermined distance apart, d. a beam splitter adjacent a first one of the fixed, retroreflectors, for splitting a primary beam of electromagnetic energy, from a source thereof, into two secondary beams, and directing one secondary beam into the reflecting path of the beam pathlength multiplier containing the said first fixed retroreflector, and the other secondary beam in front of the reflecting faces of a second one of the fixed retroreflectors, e. a beam folder adjacent the said second, fixed retroreflectors for reflecting the said other secondary beam to beam pathlength multiplier consisting of the said second, fixed retroreflector and the corresponding movable, retroreflector thereto, f. two retroreflectors each adjacent one of the fixed retroreflectors, and each for reflecting one of the said secondary beams back along paths between the adjacent, fixed retroreflector and the corresponding movable retroreflector towards the beamsplitter where interference between the secondary beams will occur, forming patterns of fringes, whereby, g. when a primary beam of electromagnetic energy is directed from a source thereof to the beam splitter, angular displacements of the rotatable mounting means about said axis of rotation can be deduced from the pre-determined distance that the ridges of the movable retroreflectors are apart and a count of the number of fringes generated at the beam splitter by rotation of the rotatable mounting means.