Abstract: The spatial or temporal characteristic of a beam of light or other electromagnetic radiation or a beam of sub-atomic particles is measured by an array of detectors. The array comprises the layer 1 of photosensitive material on which there are positioned pairs of electrodes 1a, 2a and 1b, 2b etc., with gaps between the electrodes of a pair. The gaps are spaced apart linearly and the material is chosen so that incident light affects current flow between the electrodes defining a gap. Changes in current flow between the individual pairs of electrodes are measured. For measurement of spatial profile layer 1 has a resistivity which varies linearly with the amplitude of incident light. For measurement of temporal profile the beam is split with the split beams being directed on to the array with different angles of incidence and the material is chosen to have a two-photon conductivity.