Geometrical and physical parameters for electromagnetic relativistic sources

The electromagnetic field is typically measured by the charged particle motion observation. Generally in the experiments, position, velocity and other physical parameters concerning relativistic particle beams, are estimated evaluating the actual distance between the moving source and the detector, at even given time (typically appointing the reference time to the minimum distance between the two points). So to describe faithfully a physical scenario, fixing a spatial reference system is a very troublesome act as well as evaluating the geometrical parameters and relative time evolution. In the present note, within a simple experimental context, avoiding as much as possible complex mathematical formalisms, the intensity field evolution is predicted at a given point, using fundamental geometrical assertions. The result is consistent with the special relativity law.