Abstract
The more and more intensive use of Fabry-Perot velocimeters to study impact phenomena is leading to more and more films to be processed. Thus, engineers having to deal with this increasing plenty were asking for automatic tools in order to analyse optical records. A software package named ARIDEL has been written so as to fulfil this requirement. When looking at the DLI films, one can notice several points : a) Traces are very bright and large at the beginning of the film, and become less and less bright and therefore less and less large when time is going on. b) Traces vanish, most of the time on one side of the fringes and sometimes on both sides ; they may appear again later on. c) There are artifacts from place to place, leading to false detections. d) Optical density of the background may not be constant, especially at the beginning of the film. e) Velocity variations can make a new fringe appear or a previous one disappear. f) At the very beginning of the dynamic part of the record, it is impossible to know the exact jump of fringes without any outcoming information. Analysing computational errors due to discrete data (round-off noise), one can find out a sampling criterion allowing to get the maximum information from the film at the lowest computer memory allocation. After the film has been digitized and the image stored in the computer memory, ARIDEL software computes a new but binary image using a self adaptative threshold. The ring widths on this new image are too large and have to be skeletonized, assuming that for each side of every fringes the luminous flux is symmetrical. The static part of the record (reference rings) is detected, the axis of symmetry computed and the diameter of the reference rings measured. At this point, the program can find out the origin of the dynamic part of the record or allow the user to fix it by himself. Estimation of the initial velocity value has to be given. Most of the time, the program is working with the two innermost rings. From each line of the image, ARIDEL removes all the points that do not satisfy two sets of conditions : the first one in order to ensure coherence between them, thanks to analytical relations connecting the fringes diameters, the second one using continuity conditions with the previous lines. So, it is possible to take into account the appearance of a new innermost fringe or its disappearance. By this way, the computation al error of measured velocity is being kept at its lowest possible value. Today, ARIDEL prints out velocity vs time, visualize the results on graphic screens and on plotters. Routines computing other physical quantities are to be added to the main program, depending on the users' requests.
