Page 213 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

206 Chapter Nine

Detector
Water Surface

10-50mm

Detail:
Inverted meniscus
hanging below hole Fiber-coupled
source

Figure 9.14 Holey cup. Surface tension is useful to support macro-
scopic liquid samples above a small hole, allowing noncontact optical
transmission measurements through large sample thicknesses.

water without difficulty, although the water will bulge down into the hole to
form a positive lens.
Again we have a macroscopic sample of liquid, bound on opposite sides by an
air-water interface—just what we need for windowless transmission measure-
ments. The small hole is certainly big enough to allow convenient transmission
of light from LEDs (polished down to expose the bare semiconductor chip) or
optical fibers. The meniscus curvature is even useful to collimate the diverging
light. With the small size, surface tension forces dominate gravity, so that the
meniscus is robust, and its shape is almost independent of orientation. This
makes the technique useful for 90° scattering experiments also, when holes in
the vertical container walls can be used.
The 40mm path length is the same as in the longest cuvettes used with most
spectrometers, and would be quite adequate for optical measurements in 96-,
384-well and other micro-titre cell arrays. Single holey-cups could be useful as
very low cost disposable cuvettes for high volume measurements, even at wave-
lengths below 300nm where expensive silica windows are typically needed. The
hole makes a pretty good, smooth-surface window for use from the hard UV to
the far IR.
In highly fouling media, even the holes will become blocked by settling debris,
and eventually bridged by biofilms. Hence we need occasionally to blow the
holes clean. This can be arranged, and further advantages gained, from the con-
figuration of Fig. 9.15. Instead of leaving the underside of the supporting menis-
cus free to the atmosphere, we encapsulate it in a small-volume cavity with a
conventional, heated window. In this way we can share support of the water
column between surface tension at the hole periphery and pressure over the

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