A theoretical explanation is given of a technique based on Mie scattering interferometry (MSI)I obtained by defocusing of the collecting optics, to size droplets. The originality of this study is the development of a droplet sizing method by planar laser light scattering for the case of a scattering angle range close to 90 degrees. The feasibility of this method and its limitations are fully described. The dependence on intensity levels and refractive index variations can be neglected. After discussion of some practical details about particle size, imaging and camera constraints, the results obtained in the combustion chamber of a spark ignition (SI) engine, near the spark plug, prior to ignition and for different injection timings an described and discussed. It can be concluded that the implementation of the MSI method in this experimental set-up has been realized successfully to provide droplet distributions in an SI engine. To allow the easier use of the technique, image processing software will be developed in the Matlab environment.