Fibre evanescent field absorption (FEFA): an optical fibre technique for measuring light absorption in turbid water samples

La Trobe University Faculty of Science, Technology and Engineering Murray Darling Freshwater Research Centre

MDFRC item.

An optical fibre technique for measuring the absorption of water-borne pigment in the present of significant suspended sediment concentration has been evaluated. Based on the absorption of the evanescent field of light propagating down a single glass (silica) fibre, the fibre evanescent field absorption (FEFA) technique has been demonstrated to be approximately 10-fold less sensitive to absorbing species than traditional bulk absorption methods. However, unlike traditional optical absorption measurements, the FEFA technique is insensitive to scattering by the suspended particles for particle concentrations expected in typical inland waters. A simple calculation estimates that this insensitivity persists for sediment concentrations up to 2000-fold those expected in Australian inland rivers. In addition to experimental results, a discussion of the potential operational use of this technique in measuring optical absorption properties of water containing suspended sediment is presented.