Beam transmissometers

In the absence of absorption, the scattering coefficient could in principle be determined by measuring the loss of intensity of a narrow parallel beam passing through a known pathlength of medium. If absorption as well as scattering occurs, then the parameter measured by the instrument would in fact be the beam attenuation coefficient, c, rather than the scattering coefficient. If it is possible also to measure the absorption coefficient, a, of the water at the appropriate wavelength, then the scattering coefficient, b, may be calculated (b = c — a).

Beam attenuation meters - or beam transmissometers as they are more commonly called - for the in situ measurement of c have long been important tools in hydrologic optics. In principle all they have to do is measure the proportion, c, of the incident beam that is lost by absorption and scattering in a pathlength, r, the beam attenuation coefficient being equal to -[ln(1 - C)]/r (§1.4). In practice, the construction of instruments that accurately measure c is difficult. The problem is that most of the scattering by natural waters is at small angles. Therefore, unless the acceptance angle of the detector of the transmissometer is very small (<1 °), significant amounts of scattered light remain in the beam and so attenuation is underestimated.


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Fig. 4.4 Beam transmissometer optical system (adapted from Austin and Petzold, 1977). To confine measurements to a narrow spectral waveband, a filter can be included in the light path within the receiver.

The principles of design of beam transmissometers and the variation of the size of the error with the design parameters have been discussed by Austin and Petzold (1977). One possible optical system is shown schematically in Fig. 4.4. Most beam transmissometers are monochromatic, but spectral instruments also exist.135 One of the simpler monochromatic instruments is the WET Labs C-Star, which uses an LED light source to provide a narrow bandwidth centred on 650 nm. This is focused and collimated with an aperture and a lens to produce a parallel beam, which passes through a pathlength of either 10 cm or 25 cm in the water. At the receiver end the light is brought to a focus with another lens, and its intensity measured with a silicon photodiode. The acceptance angle in water is 1.2 °.141 Laser light sources have the advantage for beam trans-missometry that the beam is already highly collimated. The Sequoia Scientific LISST-100X, which is a multi-angle scattering meter used for measuring particle size distribution, and the volume scattering function (see below), uses a laser light source at 670 nm and provides values of beam attenuation coefficient in addition to the other data. Its pathlength is 5 cm and it has an acceptance angle in water of 0.0269 °.141

The WET Labs ac-9 instrument package includes a spectral beam transmissometer, together with an absorption meter.6 Both instruments operate at nine wavelengths over the range 412 to 715nm. Light from an incandescent source passes through a 1 mm aperture, and is then colli-mated with a 38 mm lens followed by a 6 mm aperture. Approximate monochromaticity is achieved by passing the beam through a rotating wheel containing nine 10-nm full width half maximum (FWHM) filters. The collimated monochromatic beam passes through a flow tube 25 cm (or 10 cm) long, containing the surrounding water in which the instrument


Fig. 4.4 Beam transmissometer optical system (adapted from Austin and Petzold, 1977). To confine measurements to a narrow spectral waveband, a filter can be included in the light path within the receiver.

is immersed. Whereas in the absorption meter the tube is, as discussed earlier (§3.2), a reflective quartz cylinder surrounded by an air layer which ensures that most sideways-scattered photons are not lost, in the trans-missometer the tube has a blackened surface so that sideways-scattered photons are removed. At the receiving end a 30 mm lens re-focuses the light onto a detector behind a 1 mm aperture. The acceptance angle in water is 0.93 Another WET Labs instrument, the ac-s, is broadly similar to the ac-9 but uses a linear variable filter to achieve spectral coverage over the range 400 to 730 nm with ~ 80 wavebands of 15.5 nm FWHM.1498 The acceptance angle is 0.75 ° in water.

Although in situ measurements of c are to be preferred, it is possible to measure the beam attenuation coefficient in the laboratory with a spec-trophotometer, provided that a very small acceptance angle and long-pathlength cells are used.

If, in any water body, the vertical attenuation coefficient for downward irradiance, Kd, and the beam attenuation coefficient are both measured for the same spectral waveband, then using certain empirical relations that have been found to exist between Kd, b and c, or Kd, a and c, it is possible to estimate b and thus determine a (from c = a + b), or estimate a and thus determine b. Gordon (1991) has described a calculation procedure by means of which, from near-surface measurements of c, Kd and irradiance reflectance (R), it is possible to estimate a, b and the backscattering coefficient, bb. This method, like the others, makes use of empirically established relationships between these quantities, but has the advantage that it makes no assumption about the shape of the scattering phase function.

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