Abstract: An apparatus for characterizing energy and direction dependence of intensity for an electromagnetic signal uses spectral analysis and has particular application in the field of surface plasmon resonance. An energy dependent filter is located in an imaging space of the signal and separates the signal in an energy dependent manner. A first portion of the signal output from the filter is limited to a predetermined range of narrow energy bands and is directed to a photodetector. The photodetector receives the first signal portion and detects signal intensities across the photodetector surface, each of the signal intensities corresponding to a specific wavevector direction and energy band within the predetermined range. The filter provides said energy dependent selection for each of a plurality of different ranges of energy bands so as to create a three-dimensional dataset indicative of the energy and direction dependence of the signal intensity.