  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.
