GB1521351A - Methods and apparatus for measuring variations in distance to a surface - Google Patents

Abstract

1521351 Measuring displacement photoelectrically NATIONAL RESEARCH DEVELOPMENT CORP 6 Jan 1977 [19 Jan 1976] 02057/76 Heading G1A The position of a surface 1,1', Figs. 1 and 2, is measured by: illuminating the surface 1,1' with an array 4 of spaced sheets of light; imaging, 12, the surface on a photo-detector 13 (PIN diode or photo-multiplier) having a field of view so dimensioned as to give a variable output if the imaged sheets move relative to the detector; shifting the sheets 4 cyclically to cause the detector output to include a sinusoidal component whose phase represents the position of the surface 1, 1'; and using the phase as a measure of the surface position. Components of a beam from a laser 5, split at 6, are redirected by mirrors 7, 10 so angled that the components emerge from the splitter at a small angle to each other, where by the composite beam comprises an array of fringes forming parallel uniformly-spaced sheets of light. These, magnified if necessary by a telescope 8, 11 are directed to the surface 1, 1' at a substantial angle to a measuring axis defined by the lens 12 and detector 13. Lateral oscillation of the sheets is provided by modulating the phase of at least one constituent beam with a Pockels cell 9 driven by a ramp generator 14 which also supplies signals to phase detectors 16, 17 arranged to feed pulses representing the direction and magnitude of surface movement to an up/down counter 20. The oscillation amplitude of the light-sheets is an integral number of sheet spaces, preferably one. The photo-detector may have a number of sensitive areas spaced in conformity with the light sheets. Both constituent beams may be modulated at high frequencies, so as to cause the light sheets to oscillate at a difference frequency.