An apparatus for characterizing luminescent entities by excitation comprising: • a substrate (6) being in contact with a solution comprising luminescent entities; • a source of electromagnetic radiation (4) providing at least a primary beam of radiation (8); an objective (5); a first optical element (1) capable of transforming the intensity profile of the primary beam (8) into an arbitrary secondary intensity profile (distribution) (9); a second optical element (2) capable of separating (discriminating) radiation by wavelength; and a detector (7), where the arbitrary secondary intensity profile has at least an off-center circular continuous intensity distribution (33) focused on the back focal plane (12) of the objective forming a collimated beam (10) capable of creating an evanescent field on the side of the substrate where the solution comprising luminescent entities are located, where the evanescent field excites the luminescent entities thereby creating emission radiation separated by the second optical element (2) and captioned by the detector (7). The invention also relates to an apparatus comprising two optical elements providing a final third intensity profile (distribution) which is the convolution of two mathematical transformations corresponding to each of optical element one and four, respectively.