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Gas-liquid annular flow is one of many possible two-phase flow regimes that are encountered in the (e.g., parabolic collector) solar fields of direct-evaporation concentrated solar-power (CSP) plants. Conventional planar laser-induced fluorescence (PLIF) has been used previously to investigate the liquid film topology (i.e. film thickness) in annular flows, however, limitations have been found regarding the accurate identification of the gas-liquid interface with this technique, especially when the interface is smooth. Therefore, a novel variation of PLIF, which we refer to as structured planar laser-induced fluorescence (S-PLIF), has been developed to overcome these limitations. In this study, S-PLIF is used to investigate the topology of falling films in a vertical pipe over the range ReL ≈ 150 – 1500. Comparison of S-PLIF at two different angles (70° and 90°) shows that the technique performs better with an observation angle of 70° as this minimizes the distortions caused by the radial liquid film structure. In addition, S-PLIF70 shows good agreement with data from other techniques that have shown reliability when studying smooth films over the same range of conditions.