Droplet formation and subsequent evaporation in Diesel sprays occur so fast that conventional diagnostic tools are generally too slow to resolve time dependent features. To overcome this drawback a very fast optical diagnostic system has been developed, providing a time resolution of 4 μs per sampling point over the full duration of automotive type Diesel sprays.In order to accomplish this high speed the simultaneously measurable quantities had to be reduced to the two most essential values: mean droplet size and number of droplets in the test section. A combined extinction-diffraction technique is used to determine the two free parameters in a generalized droplet distribution function which has been derived from a detailed analysis of spray characteristics using photo-graphic recordings of droplet distributions. It is shown that these two parameters are sufficient to characterize with good accuracy the full width of variations occuring in practical droplet distributions.The optical diagnostic system is controlled by a PC-based data aquisition and evaluation system. It is easily calibrated by a series of thin wires covering the measuring range from about 4-80 μm and can be used even in quite dense sprays with extinction signals down to 50%. This is accomplished by taking multiple diffraction automatically into acccount, if the extinction signal falls in the range:(80-50)%.If required the measuring range may be easily shifted to larger sizes by minor alignments in the optics. This will not affect the dynamic range of approximately 1 to 20 in measurable droplet ratios.The theoretical derivation of the mathematical background function, the layout of the optical setup and the calibration scheme are described in detail. The diagnostic is tested on a single hole nozzle using i rent injection pressures. The time dependent profiles of mean droplet diameter and number of droplets in the spray cone have been measured at different distances from the nozzle for injection pressures in the range 200-300 bar into atmospheric air. A good agreement has been achieved between our optical data and published data in the literature.