The filtration means filter the exciter radiation (EX) in a first forward direction from generator means (LS) to the specimen (EC), by allowing a second spectral band (BZ) of predetermined spectral width (l) and centered on a selected frequency corresponding to the wavelength of the exciter radiation (L0) to pass and stopping a first spectral band (FZ) complementary to the second spectral band (BZ), while substantially simultaneously with this first filtration operation, these filtration means further filter the analysis radiation (ON) in a second direction that is the reverse of the first direction, by allowing the first spectral band (FZ) to pass and stopping the second spectral band (BZ).