Details are given of a system for making measurements of a jet flow using interferometry, schlieren or shadowgraph. The system is relatively inexpensive and should be generally applicable for any high-speed flows where density gradients are significant. Particular features of the system are the ease of conversion between modes of operation, the stability of the arrangement and the solution of problems of vibration. A computerised technique for analysis of interferograms is described and the results compare well with independent measurements.