This Phase II SBIR project will continue the development of an optical sorter that will be used to estimate drop sizes in sprays. The optical sorter will determine the size of drops in the 0.1 to 10 microns range. Drops in this size range are prevalent in the automobile industry, where the fuel injection pressures have increased tremendously over the past two decades. These newer injectors provide higher fuel efficiency and lower pollutant emissions. However, their continued development is hampered by the lack of a drop sizer to validate numerical models of the spray generation.The feasibility of estimating drop sizes using the optical sorter was demonstrated during the Phase I project. The primary objective of the Phase II work is to develop a prototype drop sizer for the automotive industry. The four tasks that are required to achieve this objective are: 1) determine the optimal configuration for the optical sorter, 2) design and fabricate a drop sizing instrument based on the optimal configuration, 3) develop a Graphical User Interface for the prototype instrument, and 4) evaluate the prototype system under laboratory conditions. The prototype system will be provided to NIST after the validation is complete.