Patent ID: 8190393

Claim:
A position detector for detecting the position of a helicopter blade comprising: a sensor arranged to generate a signal in dependence on the position of the helicopter blade with respect to the sensor; and a data processing module arranged to receive the signal from the sensor and generate an output indicative of the helicopter blade position from the received signal, the sensor comprising: a mask having a plurality of angularly divergent slits formed therein; a lens assembly having a field of view and being arranged to direct incident light onto a focal plane of the lens assembly at which the mask is located; and a light detector arranged to detect the level of illumination passing through each of the plurality of slits in the mask and provide a signal indicative of the illumination level to the data processing module, wherein the data processing module is arranged to receive the signal for each slit and sum the signals to generate a combined signal, wherein the ratio of combined signal amplitude when a helicopter blade is in the field of view of the lens assembly to when a helicopter blade is not in the field of view is indicative of blade height in relation to the sensor position.