Cavitation has been known to occur in injector nozzles. It is known that the mass flow rate from a nozzle and occurrence of cavitation in nozzle holes are dependent on injection pressure and back pressure together with the nozzle geometry and needle effects. Interestingly, injected mass from sharp inlet hole nozzles under these conditions is independent of back pressure for a range of pressures under cavitating flows (similar to choking). In the past, information on the change in the spray parameters and the Sauter mean diameter (SMD) for these varying isolated back pressures has not been available.In this study spray visualizations, SMD measurements and momentum estimates for sharp and rounded inlet nozzles were performed for a range of upstream and back pressures while keeping the ambient density constant. A Live Digital Light Extinction Technique (LDLET) was developed and applied as the optical technique for spray Sauter mean diameter measurements. Discharge coefficient estimates were obtained by direct measurements of nozzle sac pressures of a high-pressure common rail injection system using sharp and rounded inlet hole nozzles. Results were compared and classifications of spray mixing were proposed for the sharp and rounded inlet hole nozzles, which otherwise had the same nozzle geometry.