This paper is devoted to the development and application of two spatiotemporally resolved optical techniquescapable of liquid film thickness measurements in downward annular (co-current) gas-liquid flows, namely PlanarLaser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF). A single laser sheet is used to excite the liquid film, which has been doped with a fluorescent dye, along a longitudinal/vertical planenormal to the pipe wall. Two cameras (one for each technique) are placed at different angles to the plane of the lasersheet in order to recover independently the shape of the gas-liquid interface along this section. The effect of theangle between the laser sheet and the PLIF camera axis is also investigated. It is found that, at film regions wherethe gas-liquid interfaceis smooth, the conventional approach used for interpreting PLIF data is vulnerable to totalinternal reflection of the fluorescent light at the free surface, which leads to an overestimation of the film thicknessthat increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features suchas light intensity maxima or minima can often be located within the fluorescent signal that correctly identify theinterface, which in these conditions also coincides with the BBLIF film-thickness measurement. The BBLIFmeasurement, on the other hand, can lead to localized overestimation of the film thickness at flow regions withsignificant wave activity, i.e. steep slopes or agitated films, and around gas bubbles entrained into the liquid film, or an underestimation inside the gas bubbles. Correction procedures are developed to compensate distortions causedby both methods that would make these techniques more accurate for standalone employment. The simultaneousapplication of both techniques is highly recommended to attain the most reliable information.