1. A photometric device which comprises: an array of radiation sources that are spaced apart from one another, and which are operable to generate radiation that differs from that generated by at least one other radiation source in the array, and a lens arrangement for directing radiation from the radiation sources to a region of space, and at least one radiation detector for receiving radiation from the region of space; the device further comprising signal processing means for determining a size distribution of a sample of particles in said region of space on the basis of scattering data of the particles, the signal processing means operable to: (a) determine a scattering vector z for scattering of radiation of a number of different wavelengths and polarisation states by the sample of particles at a number of different scattering angles; and (b) apply Bayesian inference to the scattering vector to determine the posterior probability distribution of the mean over a period of time of the particle size distribution from prior information regarding the likely particle size distribution and the Mie scattering equations for determining the scattering vector z from the size distribution.