A photometric device for investigating a sample, 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 the other radiation sources in the array. The device includes a lens arrangement for focusing the radiation at a region of space where a sample may be located for example by means of a sample holder, and at least one detector for receiving radiation from the region of space. Preferably, a number of detectors are employed that are spaced apart from one another, and especially about an axis of the device, so that one radiation detector can detect radiation transmitted by the sample and the other detectors can detect radiation scattered by it. The radiation sources may be time division multiplexed so that in each time slot the detectors receive radiation originating from each radiation source. In an alternative embodiment, the radiation from the region of space may be transmitted to the sample via a beam homogeniser, for example an optical waveguide in the form of an optical fibre, which may be used to reduce or remove speckle where laser diodes are employed as the radiation sources. The device may be used to determine the particle size distribution of particles in a sample by a method employing Bayesian inference.