An interrogation device for detecting luminescent light produced by analytes in a sample excited by multiple excitation light beams each having individual spectral contents, comprising a plurality of light sources each generating an excitation light beam; at least one detector for detecting the luminescent light produced by the sample; and an optical assembly defining distinct and fixed excitation light paths for each of the excitation light beams from the light sources to a common excitation site on the sample and defining a shared luminescence light path for the luminescent light from the excitation site on sample to the at least one detector, the excitation light paths and the luminescence light path being on a same side of the sample, the optical assembly including sample-side optics projecting the excitation light towards the sample and collecting luminescent light from the sample.