Page 209 - Photodetection and Measurement - Maximizing Performance in Optical Systems

202 Chapter Nine

stream. This is particularly convenient in the water industry, where open chan-
nels are used for transporting water, and in environmental sources such as
rivers and lakes. The systems works as follows.
A small modulated diode laser projects a 1mW, 670nm collimated beam 3 to
10m to the water surface. Apart from Fresnel reflection losses (ª2.5 percent),
most of the beam energy refracts into the liquid, where scattering takes place
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at a distribution of particles of different sizes. A small fraction (typically 10 )
of the incident light retraces its path to a collection lens focussed onto a single
photo-diode receiver. For turbidities up to at least 1000 NTU the scattered
intensity increases approximately linearly with turbidity. As the source and
receiver are laterally offset from one another, but arranged with parallel axes,
the laser beam and the acceptance cone of the receiver only overlap beyond a
certain distance. With a 1-mm-diameter photodiode and a 16mm focal length
collection lens the acceptance cone is about 3.5° wide. Based on the principles
developed earlier in the book, and noting that this instrument will need to func-
tion in brightly illuminated industrial halls and also in the open air with intense
sunlight, the source is modulated at 4kHz and synchronously detected. The
received light level is only a few nanowatts, so a high value (ª100MW) tran-
simpedance is chosen. Nevertheless, with some care taken with component