TABLE OF CONTENTS The multibeam referencing and intensity compensation configurations introduced in the last chapter are of enormous help with another problem that affects the long-term stability of an optical measurement. In this Chapter we will think about some of the problems involved in transferring a high performance optical instrument, well designed in consideration of photon budgets and temperature drift performance, to an industrial environment where it is up against a different set of stresses, the greatest of which is so called fouling. This is the inevitable contamination that builds up on the optical surfaces and windows of an instrument in service. Several of the techniques presented here are related to those used for stability improvement in Chap. 8, although the emphasis is more on environmental effects rather than temperature variations. We will look at some real measurement problems, and the designs of a few instruments which have been used successfully in difficult industrial sensing applications.
Large numbers of optical sensors used for industrial diagnostics and control systems operate on-line (i.e., a sample of fluid is brought to the instrument in dedicated pipe-work) or even in-line (where the sensor is immersed in a flowing process fluid stream or installed in a section of process piping). In both cases the optoelectronic elements, sources and detectors, are separated from the fluid by robust transparent windows, for instance of plastic, glass, fused-quartz (amorphous SiO 2) or sapphire (single-crystal Al 2O 3). Although it is...
