Topics in Particle and Dispersion Science

Measuring attenuation of light: Implications of the optimum pathlength

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In natural waters, the attenuation coefficient ranges from (order-of-magnitude) 0.1 to 1 m^-1. This would imply the optimum pathlength in water of between 10 and 1 m respectively. Although long-pathlength (several meters) measurements were attempted (for example, Anassontzis EG et al 1992, Bradner H and Blackinton 1984), the pathlength, and thus the size of the transmissometer is frequently limited by the requirement of the ease-of-handling. Hence, the medium thickness can be as small as 0.25 m (for example, Capone A et al 2002).

In the atmosphere, the attenuation coefficient is much smaller, on the order of 10^-4 m^-1 (for example, Husar RB et al 2000). Hence the optimum distance would be on the order of 10 km. Aside from long-pathlength measurements with telephotometers, multipass cells have been used for measurements of transmission of light by atmospheric aerosols (for example, Gerber HE 1979, 10 x 2 m) and gases (for example, water vapor: Carleer M et al 1999, 12 x 50 m) to reduce the size of the instrument. Note that open-air long-pathlength measurements with telephotometers are influenced also by the atmospheric turbulence (for example, Malm WC et al 1986)

Hawe E et al (2007b, 2007a) used a small (z = 5 cm) integrating sphere (for example, Labsphere Inc 2007, Goebel DG 1967) to obtain an effective pathlength ~10z to analyse substantial absorption of light by CO₂ in the near infrared. This use of an integrating sphere is different from that aimed at the angular integration of light scattered by the medium (for example, Haardt H and Maske 1987).

CITATION: Swanson N. L., Jonasz M. 2007. Measuring attenuation of light: Implications of the optimum pathlength (www.tpdsci.com/Tpc/AtnCfMsOptmPlenImp.php). In: Top. Part. Disp. Sci. (www.tpdsci.com).

HISTORY: Published: 21-Nov-2007 Modified: 21-Jan-2008 Peer-reviewed: 22-Dec-2007

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