System For The Automatic Analysis Of Interferograms Obtained By Holographic Interferometry

Interferometry is a very precise measurement technique which has the great advantage of not disturbing the phenomenon being studied. The basis for measurement is generally a photographic print on which the interference fringes are recorded. The measurement itself is derived from the displacement of the fringes between two consecutive interferograms. So the operator must evaluate this displacement by fixing the position of the same fringe in the two photographs. A method of automatic analysis has been developed which uses a television camera linked to a microcomputer, in a system whose overall cost remains relatively modest. The interferogram is formed directly on the vidicon of the T.V. camera. At a moment fixed either by the operator or by a timer program, the signal delivered by the camera is digitalized for the space of one half-image, then stored in memory. The interface linking the camera to the computer was built up around an analogue-digital converter. The encoding distinguishes 16 shades of grey (4 bits), which are sufficient for an interferogram and allow a saving in memory space. An original program for picking out the position of the fringes provides a refinement in accuracy up to one hundredth of the fringe spacing, since it uses all the information contained in each "line" of the T.V. image, when fixing the position of the fringe. This system of analysis has been applied and tested with the holographic interferometry bench designed for studying solar heat collectors.