Monitoring of clearance gaps between a rotating component and a surrounding cowling surface is difficult in view of the general packaging constraints. Furthermore, when such operation occurs at high temperature there may be distortion in view of natural expansion and contraction over thermal gradients of the monitoring arrangement. By provision of a diffractive optical element with light guided for reflection against an aperture end and a part of a rotating component, it is possible through shift in the reflective wavelength frequencies to monitor and determine clearance gaps between that aperture end and incident parts of the rotating component.