Page 198 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
9

Contamination and Industrial Systems

9.1 Introduction

The multibeam referencing and intensity compensation configurations intro-
duced 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 differ-
ent 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 instru-
ment 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.

9.2 Transfer to the Industrial Arena
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 often
assumed that the window is just a passive, protecting interface between sample
and sensor, over time it can become a significant variable element in its own
right. In particular, windows can become coated with chemical deposits, often
aided by biological films. As the fouling layer builds up, optical transmission
decreases and fluorescence can increase, leading to measurement errors.

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