The invention relates to a system for the transcutaneous measuring of the concentration of a substance, preferably of alcohol, in blood, comprising a light source (1), a detector device (15), an optical assembly (7, 7) having an inlet (5), an outlet (49) and a discharge opening (35), and a contact surface element (37) having a contact surface (41), wherein: the contact surface element (37) is arranged in such a way that measuring radiation emitted from the discharge opening (35) falls through the contact surface element (37) the contact surface (41) is provided on the side of the contact surface element (37) facing away from the optical assembly (7, 7) the inlet (5) of the optical assembly (7, 7) is arranged in such a way that measuring radiation emitted from the light source (1) falls on the inlet (5) of the optical assembly (7, 7) the optical assembly (7, 7) is designed in such a way that measuring radiation emitted from the light source (1) and entering into the inlet (5) passes through the discharge opening (35) and is focused on a measuring point (39) at a predetermined distance from the contact surface (41) on the side of the contact surface element (37) facing away from the optical assembly (7, 7) the optical assembly (7, 7) is designed in such a way that scattered radiation generated in the measuring point (39) and entering into the discharge opening (35) is focused in an outlet point (19) at the outlet (49) and the detector device (15) is arranged in such a way that it detects the scattered radiation focused at the outlet point (19).
