A measuring apparatus for measuring a spectrum (SPEC1) of a gaseous sample (GAS1), the apparatus comprising: - a tunable laser light source (LS1) to provide an illuminating light beam (LBO), - a sample cell for containing the gaseous sample (GAS1), the cell comprising a reflective inner surface (SRF1) to provide transmitted light (LB2) by reflecting light of the illuminating light beam (LBO) such that the transmitted light is transmitted through the gaseous sample (GAS1), - a detector (DET1) to detect intensity (I LB2 ) of the transmitted light (LB2) 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 (GAS1) by modulating the spectral position (λ LB0 ) of the illuminating light (LBO), and by detecting the intensity (I LB2 ) of the transmitted light (LB2) at two or more different spectral positions (λ 0 , λ P1 ) of the illuminating light (LBO), characterized in that the apparatus further comprises a pressure control system (VAC1) to maintain an absolute pressure (p GAS1 ) of the gaseous sample (GAS1) smaller than 50 kPa inside the sample cell so as to reduce spectral widths (Δλ P1 ) of spectral features (P1) of the gaseous sample (GAS1), wherein the total reflectance of the inner surface (SRF1) is greater than 80% at the wavelength (λ LB0 ) of the illuminating light beam (LBO), wherein the specular reflectance of the inner surface (SRF1) is smaller than 50% such as to scramble the phase of the transmitted light (LB2), wherein a longitudinal dimension (L SRF1 ) of the inner surface (SRF1) is in the range of 10 to 100 times a minimum diameter (d SRF1 ) of the inner surface (SRF1), and wherein the divergence (θ LB0 ) of the illuminating light beam (LBO) in a transverse direction is greater than 30° so as to cause multiple consecutive reflections from the inner surface (SRF1). - a tunable laser light source (LS1) to provide an illuminating light beam (LBO), - a sample cell for containing the gaseous sample (GAS1), the cell comprising a reflective inner surface (SRF1) to provide transmitted light (LB2) by reflecting light of the illuminating light beam (LBO) such that the transmitted light is transmitted through the gaseous sample (GAS1), - a detector (DET1) to detect intensity (I LB2 ) of the transmitted light (LB2) 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 (GAS1) by modulating the spectral position (λ LB0 ) of the illuminating light (LBO), and by detecting the intensity (I LB2 ) of the transmitted light (LB2) at two or more different spectral positions (λ 0 , λ P1 ) of the illuminating light (LBO), characterized in that the apparatus further comprises a pressure control system (VAC1) to maintain an absolute pressure (p GAS1 ) of the gaseous sample (GAS1) smaller than 50 kPa inside the sample cell so as to reduce spectral widths (Δλ P1 ) of spectral features (P1) of the gaseous sample (GAS1), wherein the total reflectance of the inner surface (SRF1) is greater than 80% at the wavelength (λ LB0 ) of the illuminating light beam (LBO), wherein the specular reflectance of the inner surface (SRF1) is smaller than 50% such as to scramble the phase of the transmitted light (LB2), wherein a longitudinal dimension (L SRF1 ) of the inner surface (SRF1) is in the range of 10 to 100 times a minimum diameter (d SRF1 ) of the inner surface (SRF1), and wherein the divergence (θ LB0 ) of the illuminating light beam (LBO) in a transverse direction is greater than 30° so as to cause multiple consecutive reflections from the inner surface (SRF1). <EOS>