Page 219 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

212 Chapter Nine

mately spherical, but are used in whatever cubic, cylindrical or arbitrarily-
shaped, more or less enclosed cavities, as the application requires. They are also
used to determine the optical absorption of solid, liquid, and gas samples.
Absorption can be measured by placing the sample within the cavity. With the
introduction of an absorbing object or material, the average power-density in
the cavity is reduced, and this reduction can therefore be measured to deter-
mine the object’s total volume ¥ absorption coefficient product. As the radia-
tion is diffuse, the power-density reduction is very similar, independent of the
shape or spatial distribution of the absorption, and independent of any loss-less
scatter caused by particles in the fluid. Only the volume of absorbing material
plays a role.
These characteristics are ideal for measuring the absorption of a falling
stream. The shape of the stream plays little role; it is only necessary to control
the flow-rate to maintain a given volume of material in the sphere. By using a
diffuse light technique in this way we measure absorption independent of par-
ticle scatter, with sensitivity increased by the multiple path absorption, and
without the inevitable fouling of a material flow-cell. Light-loss through the
fluid entrance and exit holes reduces the optical gain due to the high surface
reflectivity, but nevertheless good performance can be obtained with very “dif-
ficult” industrial process media.

9.6 Summary

Fouling is often considered to be an uninteresting annoyance to be solved with
some bolt-on, low-tech cleaning contraption, and because of this it gets little
attention in university research labs and publications. This is a pity. It is my
feeling that solutions to fouling, through optoelectronic compensation, elegant
cleaning techniques, new materials or total avoidance of contact with the
sample is the most important aspect of many industrial measurements. Most
on-line instruments are maintained far less often than the designer believes or
expects, so it is important to design enough cleverness and innovation into the
instruments to provide reliability and repeatability, even over a lifetime of
misuse.

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