Variableangle scattering meters

The scattering properties of natural waters are best determined by directly measuring the scattered light. The general principle is that a parallel beam of light is passed through the water, and the light scattered from a known volume at various angles is measured. In the ideal case, the volume scattering function, b($), is measured from 0 to 180 this provides not only the angular distribution of scattering for that water but also, by integration, the total, forward and backward scattering coefficients (§1.4). Such measurements are in reality difficult to carry out and relatively few natural waters have been completely characterized in this way. The problem is that most of the scattering occurs at small angles (typically 50% between 0 and 2-6 °) and it is hard to measure the relatively faint

Quadrupole Mass Analyzer
Fig. 4.5 Optical system of low-angle scattering meter (after Petzold, 1972). The vertical dimensions of the diagram are exaggerated.

scattering signal so close to the intense illuminating beam. We shall consider in situ scattering meters first.

An instrument developed by Petzold (1972) for very low angles (Fig. 4.5) uses a highly collimated beam of light traversing a 0.5 m pathlength in water and then being brought to a focus by a long-focal-length lens. Light that has been neither scattered nor absorbed comes to a point in this plane. Light that has been caused to deviate by scattering arrives at this plane displaced a certain distance (proportional to the scattering angle) to one side. A field stop is placed in the focal plane, opaque except for a clear annular ring that allows only light corresponding to a certain narrow (scattering) angular range to pass through and be detected by a photomultiplier behind the stop. Three such field stops are used, each with the annular ring a different radial distance from the centre: these correspond to scattering angles of 0.085,0.17 and 0.34 To measure the intensity of the incident beam a fourth stop is used, which has a calibrated neutral-density filter at the centre.

Kullenberg (1968) used a He-Ne laser to provide a collimated light beam traversing a pathlength of 1.3 m in water. At the receiving end of the instrument, the central part of the beam was occluded with a light trap and a system of conical mirrors and annular diaphragms was used to isolate the light that had been scattered at 1, 2.5 or 3.5 °.

Bauer and Morel (1967) used a central stop to screen off the collimated incident beam and all light scattered at angles up to 1.5 Light scattered between l.5 and 14 ° was collected by a lens and brought to a focus on a photographic plate: over this angular range was determined by densitometry.

The LISST-100X particle size analyzer, referred to earlier, measures light scattering at 32 forward angles over a range depending on which

Fig. 4.6 Schematic diagram of optical system of general-angle scattering meter (after Petzold, 1972).

version of the instrument is used. In the Type B instrument the range is 0.1 to 18 The light source is a solid-state laser operating at 670 nm over a 5 cm pathlength, and there are 32 ring detectors whose radii increase logarithmically.816

Instruments for measuring scattering at larger angles are constructed so that either the detector or the projector can rotate relative to the other. The general principle is illustrated in Fig. 4.6. It will be noted that the detector 'sees' only a short segment of the collimated beam of light in the water, and the length of this segment varies with the angle of view. Instruments of this type have been developed by Tyler and Richardson (1958; 20-180 °), Jerlov (1961; 10-165 °), Petzold (1972; 10-170 °) and Kullenberg (1984; 8-160 °). The recently described volume scattering meter, developed at the Marine Hydrophysical Institute at Sevastopol (Ukraine) in collaboration with Satlantic in Canada, measures the volume scattering function over the range 0.6 to 177.3 with an angular resolution of 0.3 °.786 This instrument differs from the normal design in that the positions of the light source and detector are fixed. The measurement angle is modified by rotation of a special periscope prism. The initial version was monochromatic, but there is now a multispectral model, operating at seven wavelengths - 443, 490, 510, 532, 555, 590 and 620 nm.104

The scattering properties of natural waters can be measured in the laboratory but, at least in cases in which scattering values are low, as in many marine waters, there is a real danger that the scattering properties may change in the time between sample collection and measurement. In the case of the more turbid waters commonly found in inland, estuarine and some coastal systems, such changes are less of a problem but it is still essential to keep the time between sampling and measurement to a minimum and to take steps to keep the particles in suspension. Commercial light scattering photometers were developed primarily for studying macromolecules and polymers in the laboratory, but have been adapted by a number of workers for the measurement of b($), in natural waters.83,1278 The water sample is placed in a glass cell illuminated with a collimated light beam. The photomultiplier is on a calibrated turntable and can be positioned to measure the light scattered at any angle within the range of the instrument (typically 20 to 135 °). Laboratory scattering photometers for very small angles have been developed40,1281 and commercial instruments are available.

From the definition of volume scattering function (§1.4) it follows that to calculate the value of b(0) at each angle it is necessary to know not only the radiant intensity at the measuring angle, but also the value of the scattering volume 'seen' by the detector (this varies with angle as we noted above), and the irradiance incident upon this scattering volume. In the case of laboratory scattering meters, scattering by a water sample can be related to that from a standard scattering medium such as pure benzene.937

Once the volume scattering function has been measured over all angles, the value of the scattering coefficient can be obtained by summation (integration) of 2pb(^) sin0 in accordance with eqn 1.40. The forward and backward scattering coefficients, bf and bb, are obtained by integration from 0 to 90 and from 90 to 180 °, respectively.

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