A combined theoretical and experimental study of the parameters affecting the accuracy of Planar Droplet Sizing (PDS) measurements is presented. The principle of the PDS technique relies on the assumption that the intensity emitted by a fluorescent dye added to a liquid is proportional to the volume of a resulting droplet during atomisation and that the scattered light intensity is proportional to its surface area, allowing measurement of Sauter Mean Diameter (SMD) by taking the ratio of these intensities. A geometrical optics light scattering approach was extended to calculate the fluorescence intensity emitted by a droplet, in addition to providing the scattered light intensity integrated over the collection aperture. The theoretical approach quantified the influences of scattering angle, refractive index, droplet size and dye concentration on the PDS technique. Experiments with monodisperse droplet streams confirmed the calculations in terms of dependence of the scattered and fluorescence intensities. The limitations of the technique have been established together with an appropriate calibration procedure for application in dense sprays.