Page 217 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

210 Chapter Nine

The falling stream can equally be used to improve the reliability of fluores-
cence and Raman detection, and a number of fluorescence-detection systems is
available commercially. One system uses a low-pressure mercury vapor lamp
emitting predominately at 254nm in the ultraviolet to excite 270–300nm fluo-
rescence in a range of aromatic compounds such as benzene, toluene, ethyl-
benzene and xylene. This group (BTEX), with its origin in fuel-oils, is found
widely in water-courses and environmental samples. It is hence a very useful
marker to trace pollution effects, with their source in many thousands of
leaking fuel-tanks.
Multiple reflections can also be put to good use to increase the sensitivity of
turbidity, fluorescence and Raman instruments. The incidence angle for total
internal reflection is 49° for an air-water interface, so the water thread can func-
tion as a high NA waveguide. Light scattered or emitted within the liquid thread
will be guided to each end, where it can be detected. This can increase the effec-
tive detector capture angle, and hence the overall detection sensitivity. Figure
9.17 shows one configuration of fluorescence instrument, in which both the exci-
tation light and the fluorescence are guided up the water thread to a detector.
With a 200-mm long approximately one-millimeter wide thread a gain of six
times was obtained above the unguided case.

Collimated
source

Water
inlet
Detector

Header
tank
Falling
water
thread

To waste
Figure 9.17 Axial propagation in a
falling liquid thread is useful for
non-contact scatter, fluorescence and
Raman detection.

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