A method and apparatus for analysing the three-dimensional electromagnetic field resulting from an interaction between a focused illuminating beam and a sample to be observed, by characterising the distribution of the state of polarization of light across a measurement plane, the method comprising the steps of generating a beam of illuminating light; controlling the state of polarization at different positions across the beam width of the light beam; focussing said illuminating light beam to a focus, wherein said focus is a tight focus and said focused light has a suitable three-dimensional vectorial structure at the focus; detecting and measuring the state of polarization of the reflected light at different positions across the width of the measurement plane to retrieve information on the three-dimensional vectorial electromagnetic interaction of the illuminated focused field and the sample.