A measuring apparatus for measuring a spectrum (SPEC1) of a gaseous sample, the apparatus comprising: - a tunable laser light source to provide an illuminating light beam, - a sample cell for containing the gaseous sample, the cell comprising a reflective inner surface to provide transmitted light by reflecting light of the illuminating light beam such that the transmitted light is transmitted through the gaseous sample, - a detector to detect intensity (I LB2) of the transmitted light guided through the sample cell, wherein the measuring apparatus is arranged to measure one or more spectral transmittance values (I LB2 (λ)/I 0 (λ)) of the sample by modulating the spectral position (λ LB0) of the illuminating light, and by detecting the intensity (I LB2) of the transmitted light at two or more different spectral positions (λ 0, λ P1) of the illuminating light, characterized in that the apparatus further comprises a pressure control system to maintain an absolute pressure (p GAS1) of the gaseous sample smaller than 50 kPa inside the sample cell so as to reduce spectral widths (Δλ P1) of spectral features of the gaseous sample, wherein the total reflectance of the inner surface is greater than 80% at the wavelength (λ LB0) of the illuminating light beam, wherein the specular reflectance of the inner surface is smaller than 50% such as to scramble the phase of the transmitted light, wherein a longitudinal dimension (L SRF1) of the inner surface is in the range of 10 to 100 times a minimum diameter (d SRF1) of the inner surface, and wherein the divergence (θ LB0) of the illuminating light beam in a transverse direction is greater than 30° so as to cause multiple consecutive reflections from the inner surface. - a tunable laser light source to provide an illuminating light beam, - a sample cell for containing the gaseous sample, the cell comprising a reflective inner surface to provide transmitted light by reflecting light of the illuminating light beam such that the transmitted light is transmitted through the gaseous sample, - a detector to detect intensity (I LB2) of the transmitted light guided through the sample cell, wherein the measuring apparatus is arranged to measure one or more spectral transmittance values (I LB2 (λ)/I 0 (λ)) of the sample by modulating the spectral position (λ LB0) of the illuminating light, and by detecting the intensity (I LB2) of the transmitted light at two or more different spectral positions (λ 0, λ P1) of the illuminating light, characterized in that the apparatus further comprises a pressure control system to maintain an absolute pressure (p GAS1) of the gaseous sample smaller than 50 kPa inside the sample cell so as to reduce spectral widths (Δλ P1) of spectral features of the gaseous sample, wherein the total reflectance of the inner surface is greater than 80% at the wavelength (λ LB0) of the illuminating light beam, wherein the specular reflectance of the inner surface is smaller than 50% such as to scramble the phase of the transmitted light, wherein a longitudinal dimension (L SRF1) of the inner surface is in the range of 10 to 100 times a minimum diameter (d SRF1) of the inner surface, and wherein the divergence (θ LB0) of the illuminating light beam in a transverse direction is greater than 30° so as to cause multiple consecutive reflections from the inner surface.