Abstract: Embodiments of the present invention provide 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 a 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 blade position from the received signal, the sensor comprising; a mask (M1) having a plurality of slits (S1-Sm) formed therein; a lens assembly (RXL) having a field of view and being arranged to direct incident light onto a focal plane of the lens assembly; 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 mask is located at the focal plane of the lens assembly and wherein the slits formed in the mask are angularly divergent from one another.

Abstract: A method of detecting the velocity of a moving object comprising the steps of: passing a beam of coherent light through an optical mask having a pattern of alternating opaque and non-opaque regions formed thereon, whereby an image of alternating light and dark fringes is projected along the length of the light beam; causing the moving object to pass through the projected image such that a portion of the light beam is reflected from the moving object as a series of pulses; detecting the reflected light pulses and the frequency of the pulses; and calculating the velocity of the object as a function of the pulse frequency and the known separation of the fringes of the image.

Abstract: Apparatus for determining the velocity and direction of a moving object, the apparatus comprising: a coherent light (L) source arranged to generate an incident beam (B) of coherent light; an optical imaging system arranged to receive light reflected from a moving object passing through the incident beam and to focus the reflected light into a magnified image of the moving image; a first optical mask (M) through which the magnified image is projected, the mask having a first region of alternating opaque and non-opaque areas having a first spacing and second region of alternating opaque and non-opaque areas having a second spacing different from the first spacing; a first light detector (D) arranged to receive the light signal transmitted through the optical mask, the light signal comprising a series of pulses, and to calculate the velocity and direction of the moving object as a function of the pulse frequency and the spacing of the opaque areas of the optical mask.