A fluorometric system for on-line monitoring of biological processes uses a plurality of light sources, each with a different spectral width, to illuminate a sample, the light sources selected to produce emission of fluorescent light, and usually also dispersed light, from the illuminated sample. One of the light sources has a wide spectral range. The light sources are operated sequentially or in combination. Spectra acquired from the emitted light are combined and processed to evaluate properties of the sample, such as concentration of the fluorescing components or particle concentration. The system preferably uses an electro-optical probe in which excitation optical energy is transmitted to the sample directly from the light sources, typically LEDs, without an optical waveguide.