Abstract: 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.

Abstract: A submersible fluorometer of the type having a source transmitting light pulses into the fluid in which the fluorometer is submersed to cause material in suspension to fluoresce and a fluoroescence detector, wherein a second detector is used to measure the intensity of light output of the source during each pulse of light, the outputs of the fluorescence detector and the second detector are sampled during each pulse of light and a ratioing circuit is used to determine the intensity of the fluorescence relative to the intensity of the light causing fluorescence. This arrangement compensates the output for both short and long term drift effects particularly in the light source. Preferably both the light transmitted to the said fluid and the light received therefrom is filtered, in the first case to remove light having wavelengths equal to or longer than that of the fluorescence and in the second case to remove light having wavelengths shorter than that of the fluoroscence.