A commonly used technique is Laser Diffraction (LD). Laser Diffraction is popular method for particle size analysis because it provides a large dynamic range and fast measurement time. Laser Diffraction is, by definition, an ensemble technique.
Laser Diffraction consists of scattering laser light off an assembly of particles and collecting the scattered light using a spacial array of detectors. The signal from the detectors is really a pattern of scattered/diffracted light vs. angle. This pattern results from many particles being illuminated by the laser light source at the same time, where all of their individual scattered/diffracted light rays mix together at each detector element. This is shown schematically below.
The scattered light pattern requires a complex mathematical algorithm to obtain an approximate representation of the particle size distribution (PSD). Because the problem of converting the scattered light pattern to a particle size distribution is fundamentally ill-conditioned, Laser Diffraction devices have serious limitations in terms of the kind of information that they can provide. In general, Laser Diffraction devices have low resolution and low sensitivity.