The paper is devoted to development and validation of film
thickness measurement techniques in interfacial gas-liquid
flows. The specific flow investigated here is that of downwards
(co-flowing) annular flow in a vertical pipe, however, many of
the observations and findings are transferable to similar flow
geometries. Two advanced spatially resolved techniques,
namely planar laser-induced fluorescence and brightness-based
laser-induced fluorescence , are used simultaneously in the same
area of interrogation. A single laser sheet is used to excite
fluorescence along one longitudinal section of the pipe, and two
cameras (one for each method) are placed at different angles to
the plane of the laser sheet in order to independently recover
the shape of the interface along this section. This allows us to
perform a cross-validation of the two techniques and to analyse
their respective characteristics, advantages and shortcomings.