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.

Abstract: A heterodyne interferometer to achieve simultaneous and independent measurements of amplitude and phase includes a beam splitting device, BS which splits of the incident light beam into two parts with equal amplitudes and an angular deviation, .alpha.; imposes two unequal frequency shifts F.sub.1 and F.sub.2 on the two beams and amplitude modulates the two beams in phase quadrature at frequency F.sub.s. The two beams emerging from the beam splitter BS are focused by a lens L1 normally on to the surface of an object O. Upon reflection from the object surface, the two beams traverse through the lens and beam splitter a second time, where they are recombined. Both the frequencies and the amplitudes of the two beams will be shifted (or modulated) again after the second passage. The recombined beam is then diverted towards a photodetector PD via the beamsplitter.

Abstract: A scanning optical microscope which comprises a source of optical radiation and means for focussing radiation from the source into an interrogating spot on a surface under examination. The spot is deflected about a point on the surface, and the surface topography is measured by measuring the amplitude and/or phase of the radiation reflected from the surface at the spot position.