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Abstract

A system for measuring the angle of a beam of light is arranged so that a periodically varying pattern is formed on a photodetector, with a waveform property such as wavelength or phase of the pattern varying with the angle of the light. This may be done by creating interference between the light beam to be measured and a reference light beam having a fixed angle, or by forming a shadow on the detector array of one or more masks having a periodically varying transmission characteristic. One convenient way of processing the output of the photodetector array is to perform a fast Fourier transform, obtain therefrom one or more spatial frequency components, and extract therefrom a part which provides a measure of the angle of the light beam. The use of a periodically varying pattern on the photodetector array enables a substantial length of the array to be used in any particular measurement operation, and averages out individual performance differences between different elements of the photodetector array. The use of waveform properties enables convenient signal processing to be carried out, for example using well known fast Fourier transform techniques.

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First Claim