  The measurements of size distribution of small particles (e.g. dusts,
droplets, bubbles, etc.) are critical for a broad range of applications in
environmental science, public health, industrial manufacturing, etc. Laser
diffraction (LD), a widely used method for such applications depends on
model-based inversion with underlying assumptions on particle properties.
Furthermore, the presence of sampling biases such as velocity differentials are
often overlooked by the simple ex-situ calibrations, which introduces as an
additional source of error. In contrast, digital inline holography (DIH), a
single camera coherent imaging technique, can both measure particle size
distributions without the need for a model-based inversion and can directly
provide information on the shape characteristics of the particles. In this
study, we evaluate the performance of an LD system in characterizing
polydisperse droplets produced in a flat fan spray using in-situ DIH based
imaging as a reference. The systematic differences in the two techniques are
examined. A droplet-trajectory-based correction for the LD-inferred size
distributions is proposed to compensate for the observed differences. We
validate the correction using NIST standard polydisperse particles undergoing
differential settling, and then apply the correction to polydisperse spray
droplet measurements. The correction improves agreement between LD and DIH size
distributions for droplets over two orders of magnitude, but with LD still
underestimating the fraction of droplets at sizes above ~1 mm. This
underestimation is possibly linked to the complex oscillatory and rotational
motion of droplets which cannot be faithfully captured by measurement or
modelled by the correction algorithm without additional information.
