Spectrally analyzing an unknown sample (10A) for the existence of a characteristic includes (i) analyzing a first known sample (10C) having the characteristic and a second known sample (10D) not having the characteristic to identify less than fifty diagnostic spectral features, each diagnostic spectral feature being present at a different diagnostic wavelength in a mid-infrared spectral region; (ii) directing a plurality of interrogation beams (16) at the unknown sample (10A), each of the interrogation beams (16) having a different interrogation wavelength, and each interrogation wavelength corresponding to a different one of the diagnostic wavelengths; (iii) acquiring a plurality of separate output images (245) of the unknown sample (10A), wherein each of the output images (245) is acquired while the unknown sample is illuminated by a different one of the interrogation beams (16); and (iv) analyzing less than fifty output images (245) with a control system (28) to determine whether the characteristic is present in the unknown sample (10A).