Page 224 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

Measurand Modulation 217

perature coefficient issues of Chap. 8 addressed, this is a difficult and expensive
requirement.
Figure 10.2b shows a scheme in which the absorption coefficient is somehow
100 percent modulated. Synchronous detection can still be used, although the
detected signal is now much smaller (in this example about 10 times smaller).
4
However, by making this change we have removed the requirement for unreal-
istic levels of source and gain stability. Measurand modulation greatly eases the
requirements on scale-factor stability. If the gain changes by 1 percent, this only
affects the measured peak-to-peak amplitude of the small modulation signal;
the change in static intensity is not seen. Hence, for 0.01 percent resolution of
the absorption, we only require that the noise of the transmitted signal is ade-
quately small, not that the gain is stable to 0.01 percent. This is an altogether
more realistic requirement. Now we will look at some of the means to modu-
late the measurand parameter of interest.
For instance we might vary the length of the transmission cell (Fig. 10.3),
recording the change in transmitted intensity at minimum and maximum cell
lengths, or even continuously as a function of sample length. Some care will be
required in collimating the beam, in translating the cuvette window to avoid
beam wobble and in choosing beam and detector sizes to avoid even small
changes in capture efficiency. We cannot escape the requirement to reduce
modulation-induced changes in detected intensity caused by these parasitic
effects. Synchronous intensity changes caused by such errors have to be reduced
to the full required relative precision, that is, to well below 0.01 percent.
Nevertheless, the system intrinsically downgrades slow source power and detec-
tor sensitivity variations, especially caused by temperature changes, to signal
scale-factor errors. Window contamination is also compensated.
The approach has been used by Horiba for in-situ absorption measurements
in wastewater. In an elegant design, a fixed source and a detector are immersed
in the fluid to be measured (Fig. 10.4). Eccentrically-mounted cylindrical silica
tubes then rotate together, varying the sample path-length L over a large frac-
tional change. The system suffers from a use of different window surfaces
during different parts of the measurement cycle, which opens up additional

Contamination R f

-
+A
Signal
PD1
LED
source Variable-length cell:
- same contamination
- different path length
Figure 10.3 Variable cell-length gives a modulated
intensity, with slow changes due to window contam-
ination compensated.

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