The liquid core length of diesel spray is important information for understanding and modeling the spray break-up process.However,standard optical diagnostic methods are failing to visualize the spray determining break-up zone due to the high optical density of diesel spray.The optical connectivity method(OCM)is a promising approach to address this problem.Here,the liquid phase of the spray is internally illuminated by laser light coming from the inside of the nozzle.In former works,this light for this was coupled into the nozzle sac by an optical fiber.While optical fibers can easily withstand the temperature of ambient air,they cannot be used in hot conditions such as in the hot charge of a high-temperature high-pressure injection chamber.To avoid this shortcoming,the remote optical connectivity method(ROCM)was developed.Here,the optical path of the OCM was redesigned in order to avoid as much as possible heat-exposed fiber surface(Figure 1).For a first validation study,atmospheric measurements were conducted by sampling images of the liquid core by a high-speed camera equipped as well with a photographic lense as with a long distance microscope(LDM).Here,the break-up length under various injection pressures could be quantitatively measured.The typical trend of a shortening of the break-up length with increasing injection pressures was observed.Also,the measurement results showed that the liquid core length was nearly constant at the steady state of the injection.Especially at low injection pressures,a remarkable waviness of the break-up zone was observable.