A system for monitoring the refractivities of fluids in the DELPHI RICH detectors in the VUV region

An overview of the instrumentation developed for the quality control of the optical properties of fluid components of RICH detectors is presented. An on-line data acquisition and control system for operation of a refractometer in the UV and VUV region is presented. The possibility of simultaneous measurement of transmittance and refractivity using a Fabry-Perot refractometer in the UV and VUV region is discussed. The accuracy of the refractometer in the n − 1 determination of atmospheric air is presently 0.3% at 255 nm and about 1% at 185 nm. It is expected to improve when measuring the refractivities of the fluorocarbons used as radiators in RICH detectors.