Measurement unit (10",100) for measuring the gas content in the head space of a closed container (30", 130), a portion of said container at least at the head space being made of optically transparent material, comprising: - a partially open compartment for housing the closed container (30",130) at least at the height of said head space of said container made of optically transparent material at least at the head space; - a laser source for emitting a laser beam at a wavelength tunable to an absorption wavelength of a gas contained in the head space of the closed container; - the compartment comprising a portion (16",116) with high optical scattering coefficient, the portion (16",116) with high optical scattering coefficient facing towards the laser source , wherein a housing space is defined between said portion (16",116) with high optical scattering coefficient and said laser source , and wherein the laser source is positioned so as to direct the laser beam towards said portion (16",116) with high optical scattering coefficient; and - at least one detector positioned so as to detect at least one portion of the laser beam emitted by the laser source once it has passed through the housing space , has been scattered by the portion with high optical scattering coefficient (16",116), and has undergone a plurality of passes through the housing space , whereby the detector provides output data representative of an absorption spectrum of the gas, wherein the partially open compartment consists of one single wall , said one single wall comprising said portion (16",116) with high optical scattering coefficient, and the detector is positioned close to the laser source and faces in the same direction as the laser source . - a partially open compartment for housing the closed container (30",130) at least at the height of said head space of said container made of optically transparent material at least at the head space; - a laser source for emitting a laser beam at a wavelength tunable to an absorption wavelength of a gas contained in the head space of the closed container; - the compartment comprising a portion (16",116) with high optical scattering coefficient, the portion (16",116) with high optical scattering coefficient facing towards the laser source , wherein a housing space is defined between said portion (16",116) with high optical scattering coefficient and said laser source , and wherein the laser source is positioned so as to direct the laser beam towards said portion (16",116) with high optical scattering coefficient; and - at least one detector positioned so as to detect at least one portion of the laser beam emitted by the laser source once it has passed through the housing space , has been scattered by the portion with high optical scattering coefficient (16",116), and has undergone a plurality of passes through the housing space , whereby the detector provides output data representative of an absorption spectrum of the gas, wherein the partially open compartment consists of one single wall , said one single wall comprising said portion (16",116) with high optical scattering coefficient, and the detector is positioned close to the laser source and faces in the same direction as the laser source . <EOS>