An apparatus adapted to perform spectrometric measurements, said apparatus comprising a tunable laser light source adapted to generate a laser light with an excitation wavelength supplied to an optical sensor which produces a sample specific response light signal; an optical reference filter adapted to measure laser light with the excitation wavelength fed back as a reference signal to provide wavelength calibration of the tunable laser light source; at least one optical measurement filter adapted to measure the sample specific response light signal produced by the optical sensor, wherein the optical reference filter and the at least one optical measurement filter are thermally coupled to maintain a constant wavelength relationship between the filter characteristics of the optical filters.