Page 225 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Measurand Modulation

218 Chapter Ten

1 2

PD1

1' 2'

Source L short Detector
Tube Tube
L long
Figure 10.4 Rotating eccentric tubes allow in situ sample
path-length modulation.

(a) (b)
Sample
fluid
Detector

Source
Reference
Fluid-prism Sample fluid
translation fluid
Figure 10.5 Moving prismatic sample containers (a) modulate
the absorption. Dual cells allow pure sample referencing and
a parallel optical path.

errors due to the synchronous intensity changes of nonuniform contamination
on the transparent cylinders. These are expected to be small in comparison with
the absolute intensity changes, and good performance is claimed.
Translated prismatic sample cells (Fig. 10.5) can alternatively be used where
a linear translation is simpler to arrange than a rotation. The movable plane
window cell (Fig. 10.5a) can either contain the sample fluid, or be evacuated
or gas-filled and immersed in the sample solution. Dual cell systems (Fig.
10.5b) allow the use of a reference fluid related to the sample itself. For example,
to detect small absorbances due to organic chemicals in water, a chemical-free
water sample could be used as the reference. This has an advantage where
we need to detect a small additional absorption in an already absorbing
benign matrix. It is the situation in much of the optical testing of drinking
water. The closer the match between the two liquids is, the easier it is to
detect very small changes. If necessary the two prisms should be thermally
coupled.

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