1. A lens-free imaging system for detecting particles in a sample deposited on image sensor, comprising: a fluidic chamber for holding a sample and an image sensor for imaging the sample, the image sensor having a light receiving surface and a plurality of photosensitive pixels disposed underneath the light receiving surface, the fluidic chamber formed at least in part by the light receiving surface; and a light source for illuminating the sample to form a shadow image of the sample on at least a portion of the plurality of photosensitive pixels, the light receiving surface being at least partially transmissive to light emitted by the light source, the shadow image including shadows formed by particles in the sample that block at least a portion of the light, distance from the light receiving surface to the plurality of photosensitive pixels, size of particles of interest, and wavelength of the light being such that particles of interest in the sample are identifiable in the shadow image using size-selection criteria.