Method and probe for determining displacement

In order to determine displacement, axial and radial in a rotating component, an optical displacement probe is used incident upon a target formed from surface features in a rotating component surface. The surface features create a target which is axially variable in terms of reflectivity, but substantially consistent for the same circumferential band incident position. In such circumstances, differences in responses can be determined by a controller deducting time period ΔT differences between peaks in one axial position from the time period in a different axial position in order to provide an axial signal proportional to axial displacement. In terms of determining radial displacement, an angularly presented probe part in the form of an optical fiber is used so that variation of the time period is proportional to radial as well as axial displacement. In such circumstances by deducting the axial displacement determined through compounding a perpendicularly presented probe it is possible to determine radial displacement. The targets created may comprise stripes, wedge or parallel bands of surface features which have reflectivity from background activity as determined by the probes for signal processing by a controller as described.