A transmitter (41) is arranged to produce and focus a beam (54) on a remote focal point (55). A small proportion of resultant light (58) is back-scattered from the focal point (55) by particles or aerosols in air flowing through point (55) and returns to a receiver (42) where it is collected A detector combines resultant light (58) and reference light (72), splits the resultant light (58) and reference light (72) equally between two arms and detects which arm leads the other in phase thereby indicating the flow direction of air through focal point (55).