The study of sprays of droplets issuying from cars injectors requires fast diagnostics synchronized on very short jet appearance. Particle sizer must furthermore be able to measure very small drops in high pressure chambers with restricted access. An instrument with capability for measuring droplet size by Fraunhofer diffraction technique with high frequency acquisition was developed. The temporal variation of the size distribution within a single diesel injection may be now recorded. An inversion program involves a generalized Lorenz Mie theory, developed in the laboratory. With this program, the scattered light of a droplet in a gaussian laser beam can be calculated. The effect of illumination profile (i.e: ratio of the laser beam diameter on drop size) on the scattered light distribution is evaluated; the smallest beam size for a higher spatial resolution with no bias on measurement may then be calculated.
By use of a standard Malvern detector, measurements of droplet size larger than one micrometer are performed in a pressurised chamber with thick windows. Under the present configuration a special lens with short focus length and large diameter is used. An example of application to a Diesel spray is showed.