Abstract: A device for optically detecting in transmission nanoparticles moving in a fluid sample includes a light source for emitting a spatially incoherent beam for illuminating the sample; an imaging optical system; and a two-dimensional optical detector. The imaging optical system includes a microscope objective. The two-dimensional optical detector includes a detection plane conjugated with an object focal plane of the microscope objective by said imaging optical system. The two-dimensional optical detector allows a sequence of images of an analysis volume of the sample to be acquired, each image resulting from optical interferences between the illuminating beam incident on the sample and the beams scattered by each of the nanoparticles present in the analysis volume during a preset duration shorter than one millisecond.