Abstract
A concise review of the recent developments in some of the standard optical diagnostics applied for primary jet breakup studies has been presented here. Primary breakup is the core breakup of liquid jets and sheets into droplets upon its interaction with the ambient gaseous atmosphere. This phenomenon is encountered in various aerodynamic, fluid dynamic, and combustion situations. The imaging diagnostics reviewed here include photography, high-speed imaging, shadowgraphy, digital holography, ballistic imaging, jet core illumination, thermal imaging, Mie imaging, x-ray phase contrast imaging, and laser-induced fluorescence. The advantages and limitations of each technique, their success, and future developmental trend are discussed.