Abstract: The internal atomization characteristics of a practical gas turbine fuel injector are investigated comprehensively using various laser flow diagnostic techniques. The injector consists of a pilot spray surrounded by a primary swirl air, with the spray impinging on a venturi prefilmer to form a liquid film that accumulates into a rim at the venturi tip, which in turn is sheared by a counter-rotating secondary swirl air. The atomizer geometry is modularized into different configurations to access the internal flow field. The quantitative measurements made are with a phase Doppler interferometer at different stages of the injector and time-resolved volume or planar laser induced fluorescence (TR-V/PLIF) imaging. The different internal atomization characteristics such as the liquid film thickness variation, size distribution of craters formed by primary spray droplets impinging on the liquid film, rim thickness at the venturi tip, and droplet size and velocity distribution at different stages are measured. It is found that the film dynamics is influenced by the droplet splashing and the shearing action of the primary air. The liquid rim at the venturi tip is the major source of droplets at the injector exit rather than the liquid film. The unsteady evolution of the multiphase flow inside the injector is dictated by the precessing vortex core of the primary swirl. However, the atomization process is mainly influenced by the central toroidal recirculation zone of the primary swirl flow and the counter-rotating shear layer acting on the accumulated liquid rim at the venturi tip. Graphical abstract: [Figure not available: see fulltext.]. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.