OPTICAL ABSORPTION: Integrating sphere tests water quality

In a drinking-water treatment plant, it is important to measure both particles and dissolved materials to ensure that the water meets quality standards. Many dissolved materials can be detected by their optical absorption; however, absorption measurement is made difficult by interference from scattering caused by particles. In addition, the cuvette windows of a standard measurement system are often fouled, which reduces instrument reliability.

Researchers at North West Water (Warrington, England) have developed a method that overcomes both problems. In the technique, the water flows in an unconstrained stream through an integrating sphere. Within the sphere, optical absorption is measured independent of scattering, and this combination of window-free and scattering-independent absorption measurement offers a highly reliable industrial instrument.

Optical absorption—used to assess the concentration of dissolved matter—could be measured through a standard spectrometer arrangement using an input light source and a detector behind the sample. Difficulty arises if the sample also contains particles and bubbles. Scattering from these contribute to the reduction in the light reaching the detector. The light loss through scattering out of the beam path is indistinguishable from absorption in a simple system.

It is possible to avoid this interference by making the measurement in an integrating sphere. The sphere collects the scattered light and keeps it within the measurement system; the light bounces around the cavity and will eventually contribute to the detected signal. The only practical restriction is that the input beam makes at least one reflection off the surface of the sphere before reaching the detector; if not, the directly transmitted light would dominate the detected power.

The integrating sphere, therefore, allows a scattering-independent absorption measurement, which leaves just the problem of window fouling. In the new technique, the researchers have used a free-falling stream passing through the integrating sphere and have found that they can produce a perfect "rod" of water, free from surface ripples (a rough stream or streams would also give reproducible results). The only criterion for reproducibility is that a constant volume of water is contained inside the sphere.

In the prototype system put together by the team at North West Water, the light source is a gallium nitride light-emitting diode operating at 430 nm, and the integrating sphere is machined out of a polymer block (see figure). The system was first calibrated against the normal spectrometer using a scattering-free solution of cobalt chloride. In the region of practical interest the calibration curve is a straight line.

Comparing the spectrometer measurement to the integrating-sphere measurement when silica slurry is added to the cobalt chloride solution shows that the scattering introduced by the silica hardly affects the absorption measured in the sphere. The team has also made measurements on samples of processed drinking water, comparing the results with filtered samples of the same water measured in a standard spectrometer. The results show that the new technique is indeed reliable for the water industry.

Team leader Mark Johnson says, "We have produced a combination of well-known techniques that together provide a reliable and practical industrial measurement method. Although it has been developed for clean-water coagulation processes and distribution systems, it may also be useful in noncontact measurement of waste water streams, where particle and bubble scattering is also a problem, and in other process industries."