Page 205 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

198 Chapter Nine

9.3.3 Turbidity correction
A widely practiced instance of this compensation is found in water absorption
measurements. When measurements are performed at blue wavelengths, large
errors are often caused by attenuation from scattering from particles. The vari-
ation of the attenuation due to scatter with wavelength is quite repeatable, as
long as the size and shape distribution of scattering particles is constant. By
measuring at short and long wavelengths (typically 470nm and 880nm), some
correction for the turbidity is possible. At 880nm there will be attenuation due
to particle scatter, but negligible absorption. At 470nm there will be both
absorption and scatter. The 880nm attenuation measurement can therefore
be used to estimate the attenuation due to scatter at 470nm, and subtracted
from the total attenuation, leaving just the 470nm absorption. Compensation
is not perfect, but is a help with samples with relatively well-known and stable
properties.
As an aside, note that even pure, particle-free liquids show some light scatter,
due to temperature and density fluctuations at a molecular level. Far from being
negligibly small, it is easy to see this turbidity by shining a laser pointer through
a glass of clean liquid in a darkened room. No matter how pure is the sample
chosen, the scatter is visible to the eye. It also limits the lowest measurable tur-
bidity of a continuously measuring instrument to about 0.02 NTU (nephelo-
metric turbidity unit). (Good quality drinking water has a typical turbidity at
production of 0.1 NTU). Lower turbidities can be measured by restricting the
volume of water sensed, whereby the signal is detected via the change in inten-
sity as a particle moves through the detectors field of view. This is an example
of sensitivity enhancement due to measurand modulation described in more
detail in Chap. 10.

9.3.4 Multiple-wavelength measurements
Knowing the target spectrum of the indicator of the lower curve of Fig. 9.6, it
is quite easy to see the same feature in the uppermost curve, despite the poor
S/N. Common sense suggests that the more interesting features there are in the
target species spectrum, and the less in the contamination, the easier it will be
to detect the target, and the better will be the compensation for fouling. That
is, we should use as many wavelength channels as possible. This is a useful
approach in detection of some colorimetric indicators, as they often exhibit a
series of well-defined absorption peaks across the visible and UV regions. Then
a selection of optical sources placed on the known absorption bands, and another
selection arranged to measure between the absorptions, suitably divided, will
improve compensation for (slowly) varying spectral characters of the fouling
(Fig. 9.8). In practice, with any more than four or five sources, it may be more
convenient to collect a full spectrum of 256 or more wavelength “bins” using
one of the miniature spectrometer modules from Zeiss, Ocean Optics, Microparts
and others. Selection of on-band and off-band wavelength channels, together
with all fouling compensation, can then be done in software. Optimally choos-
ing the on- and off-band wavelength regions is tricky by hand. There are formal

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