The beam splitter comprises an optical, parallel sided, plate having partially reflecting coatings applied on each half side in alternate halves and having opposite alternate uncoated half sides covered by respective matching transparent wedge plates which are optically adhered to the parallel sided plate and arranged thereon in respective opposite directions. A spectrometer for analysing vacuum ultra-violet light comprises a vacuum tank containing the beam splitter and a pair of cat's eye retroreflectors which limit and define the optical paths of the rays of light reflected and transmitted by the beam splitter, one of which retroreflectors is translated along the respective paths at constant velocity during the course of each measurement. A laser beam of different wavelength to that being examined is reflected into separate and parallel paths and the recombined laser beam is detected, the signal therefrom providing a precise representation of the velocity of the translated retroreflector.

An interferometer having beamsplitting and reflective coatings on the same surface of a beamsplitter member. The fixed length path of the interferometer includes a compensator matched to the beamsplitter member, a Cat's Eye Retroreflector, the reflective coating and a retro-mirror. The variable length path of the interferometer includes a moving coil mirror, a movable Cat's Eye Retroreflector and the retro-mirror.

To reduce the data rate of a digitalized measuring signal produced in Fourier spectroscopy, the analog signal representing the interferogram is mixed with a second electrical signal whose frequency is modulated in proportion to the deviation of the instantaneous actual speed of the moving mirror of a double beam interferometer from its constant desired speed. Only the difference frequencies produced as a result of the frequency mixing are digitalized and supplied to an associated computer for analysis of the interferogram. The digital measuring signal thus obtained is distinguished by a substantially reduced data rate without giving rise to additional errors in the frequency transformation. To produce the second frequency modulated electrical signal, a laser source is used which supplies a laser beam into the interferometer parallel to the measuring beam. The interferogram of the laser beam produced in the interferometer is converted by a second optoelectrical converter into an electrical signal frequency modulated in a desired manner and is supplied to a frequency mixer.

An improved mirror scan control for driving a movable mirror in an interferometer with a constant scan velocity comprises a closed loop servo control which provides constant velocity mirror scan in response to a phase comparison of a signal derived from the beat frequency of a two frequency laser beam passing through the interferometer and a selected frequency reference signal, wherein the difference in frequencies or beat frequency of the two frequency laser beam is stabilized at a specified difference in frequencies. The mirror scan control employs a phase lock control loop which locks the frequency of the referenced signal with the frequency of the signal derived from the laser beam to provide precise mirror velocity control.

A high resolution interferometer is provided which is insensitive to slight misalignment of its elements, avoids channeling in the spectrum, generates a maximum equal path fringe contrast, produces an even two-sided interferogram without critical matching of the wedge angles of the beamsplitter and compensator wedges, and is optically phase tunable. The interferometer includes a mirror along the path of each beam component produced by the beamsplitter, for reflecting the beam component from the beamsplitter to a corresponding retroreflector and for reflecting the beam returned by the retroreflector back to the beamsplitter. The fact that the mirror "covers" the retroreflector, so that the mirror reflects the beam component before and after it reaches the retroreflector, results in the system being insensitive to slight tilting of the mirror. A wedge located along each beam component path, is large enough to "cover" the retroreflector, so that each beam component passes through the wedge during movement towards the retroreflector and away therefrom, to assure that all portions of a beam component pass through the same thickness of glass of the wedge.