The invention relates to methods and apparatus for detecting properties of heterogeneous samples, including detecting properties of particles or fluid droplets in industrial processes. Embodiments disclosed include a particle characterization method, comprising: providing a fluid containing suspended particles; causing at least a first subset of the suspended particles to flow past a first two-dimensional array detector (24); illuminating the first subset of suspended particles as they flow past the first two-dimensional array detector (24) in the fluid; acquiring a plurality of images of the first subset of particles as they flow past the first two-dimensional array detector (24) in the fluid; and automatically counting the particles in the images.