Abstract: A system for reducing wavelength error caused by angular motion of the moving mirror of an interferometer, such as a Michelson interferometer having fore-optics introducing a beam of radiant energy for impingement on the moving mirror, teaches reducing, without necessary loss of energy, the width of the beam before impingement on the moving mirror; other advantages also accrue to use of the reduced width beam, such as facility in use of absorption cells and multiplying clear path length, when desired.

Abstract: System for simplifying adjustment of Michelson type interferometers using white light beam to establish point of zero optical path difference in data beam, includes simultaneous adjustment of the white light beam and the data beam with a unitary reflecting surface at the M.sub.1 position, and phasing of the white light beam relative to the data beam by interposition of a transparent optical plate in the white light beam but out of the data beam and providing varying of optical thickness of the white light beam relative to the data beam through rotation of the optical plate.