Beam splitter and spectrometer containing the beam splitter

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.