Abstract: An apparatus for tracking a rotating blade to determine parameters associated with the rotation of that blade. In one embodiment, the apparatus includes a detector in which at least two sensing photodiodes define substantially fan shaped detector planes and are arranged to sense an illuminated point at a known location on the blade passing through these planes. The time taken for the point on the blade to pass between the planes is measured and parameters associated with the rotation of the blade are calculated therefrom. In another embodiment, the detector defines at least two divergent detector fields, and a reactive radiation source responsive to the passage of the blade through the detector fields is provided on the blade. The reactive radiation source is arranged to radiate a signal such as a light signal which is detected by the detector.