Page 240 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Source: Photodetection and Measurement

Chapter
11

Multiple Channel Detection

11.1 Introduction

Until now we have restricted ourselves to single-channel optical transmission
systems, and their optimization with low-noise design and narrow-band syn-
chronous detection. In Chapter 8 a second, reference channel was added to reduce
instability caused by source temperature fluctuations and aging. However, this
still only allows the measurement of a single optical channel. For many kinds of
optical instrumentation, the single channel measurement is too restricting, and
we must consider how to configure multiple channels in a convenient way.

11.2 Chemical Analysis and Optical Filters

Consider an application from chemical analysis, in which the concentration
of a liquid colorimetric reagent in an environmental water sample is to be
determined from its optical absorption. This is essentially a realization of the
two-wavelength schemes of Fig. 9.5. The background water is absorbing,
and this absorption varies from day to day as rainfall modifies the source’s
flow and content. It is decided to measure at two (or more) wavelengths, chosen
from an investigation of the reagent’s absorption spectrum. Figure 11.1
shows a UV/visible absorption spectrum of a dilute solution of potassium per-
manganate. The primary absorption peak of this reagent lies around 540nm.
A readily available pure-green LED emitting at 525nm is a reasonable match
to the absorption feature. Additionally a 430nm GaN LED is used to see pre-
dominantly the reagent’s low absorption region. Both these wavelengths are
conveniently addressed using readily available LEDs, and with the choice of
devices it is often possible to find useful combinations. If not, color filters could
be used, either in addition to the LEDs or with chopper-modulated incandes-
cent sources.
The chemical concentration is calculated from the ratio of attenuations at
the two wavelengths, or the difference in absorbance values, which hopefully

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