In an interferometric apparatus the path of the output beam of a rotary Optical Path Difference scanning assembly leading to an electrically responsive detector is maintained substantially fixed in space for any scan angle within predetermined limits, thus obviating a serious drawback of a prior art...http://www.google.com/patents/US4684255?utm_source=gb-gplus-sharePatent US4684255 - Interferometric optical path difference scanners and FT spectrophotometers incorporating them
