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This paper is devoted to the simultaneous application of two spatiotemporally resolved optical techniques capable of liquid film thickness measurements, namely Planar Laser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF), to co-current downward annular gas-liquid flows. A single laser sheet is used to excite the liquid film, which has been seeded with a fluorescent dye, along a longitudinal/vertical plane normal to the pipe wall. Two cameras, one for each technique, are placed at different angles to the plane of the laser sheet in order to recover, independently by the two techniques, the shape of the gas-liquid interface along this section. The effect of the angle between the laser sheet and the PLIF camera axis is also investigated. In film regions where the gas-liquid interface is smooth and flat, the conventional approach used for interpreting PLIF data is affected by total …