An optical absorption measuring instrument for determining the percentage of a component in a fluid shall be improved such that changes in the measuring radiation caused by drift or temperature have only a limited effect on the measuring result. Provisions are made for deflecting the measuring beams (10, 11) from a radiation source (4) to a detector (5) by at least two plane mirrors (71, 72), which are positioned such that the measuring beams (10, 11) emitted by the radiation source (4) are deflected to the detector (5). The surface of each plane mirror (71, 72) is dimensioned to be such that its illuminated surface in the area of the detector (5) is larger than the receiving surface (12) of the detector (5).