Abstract: The present invention discloses a method to optically measure the absorption coefficient of a liquid or gas medium, without affecting the medium. Such absorption spectrum can be utilized to quantify the chemical components of the medium, or other metrics such as color or signature of the medium. The method acknowledges the possible drifts in the optical components of the system with time and measures the absorption coefficient independently of the state of the optical components. Therefore, there is no need to calibrate the system or take reference measurements. The method presented can be implemented cost effectively, designed in compact systems, and is energy efficient, thus, meeting the stringent challenges of spaced constrained remote systems such as aging wine barrels.