A fast, solid-state, two-dimensional position detector based on photodiodes as a means of image conversion is described. The detector uses conventional optics to project an image onto a matrix of photodiodes; the resulting electric pulses are encoded both in amplitude and time duration and merged into a one-channel signal which can be stored in a fast digital storage oscilloscope (DSO). The stored oscillograms can then be analyzed to reconstruct the trajectory of the moving target. The maximum tracking speed of the device is of the order of tens of nanoseconds. An application in which the device tracks the propagation of a fast electrical discharge is also described. © 1995.