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; - an optical sensor adapted to receive the laser light and to produce a sample specific response light signal; - an optical reference filter, wherein the optical reference filter is a passband filter having a passband peak wavelength and a narrow frequency passband, wherein the apparatus is adapted to feedback the laser light passing the optical reference filter to lock the excitation wavelength of the laser light generated by the tunable laser light source to the peak wavelength of the optical reference filter; - at least one optical measurement filter adapted to pass the sample specific response light signal produced by the optical sensor, wherein the at least one optical measurement filter is a passband filter comprising a passband peak wavelength and a narrow frequency passband; - a photo detector, wherein the photo detector is adapted to convert the sample specific response light signal into an electrical sample specific response signal; and - a controller, wherein the controller is adapted to receive the electrical sample specific response signal and analyze it to provide analytical results, - 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 (6,7), wherein the optical reference filter and the at least one optical measurement filter belong to the same thermal group, so that they are subject to a same temperature and related drift. - a tunable laser light source adapted to generate a laser light with an excitation wavelength; - an optical sensor adapted to receive the laser light and to produce a sample specific response light signal; - an optical reference filter, wherein the optical reference filter is a passband filter having a passband peak wavelength and a narrow frequency passband, wherein the apparatus is adapted to feedback the laser light passing the optical reference filter to lock the excitation wavelength of the laser light generated by the tunable laser light source to the peak wavelength of the optical reference filter; - at least one optical measurement filter adapted to pass the sample specific response light signal produced by the optical sensor, wherein the at least one optical measurement filter is a passband filter comprising a passband peak wavelength and a narrow frequency passband; - a photo detector, wherein the photo detector is adapted to convert the sample specific response light signal into an electrical sample specific response signal; and - a controller, wherein the controller is adapted to receive the electrical sample specific response signal and analyze it to provide analytical results, - 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 (6,7), wherein the optical reference filter and the at least one optical measurement filter belong to the same thermal group, so that they are subject to a same temperature and related drift.