TABLE OF CONTENTS Until now we have restricted ourselves to single-channel optical transmission systems, and their optimization with low-noise design and narrow-band synchronous 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.
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 permanganate. 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 predominantly 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...
