An arrangement for optical measurement of at least one process variable in a medium, comprising: at least one light source; at least one light receiver; an optical sensor element at least one data processing unit; and a light conductor. The light conductor connects the light source with the optical sensor element and the optical sensor element with the light receiver. The light conductor is embodied with at least three arms, wherein the first arm is arranged at the light source, the second arm is arranged at the light receiver and the third arm is arranged at the optical sensor element first arm and the second arm combine to form the third arm. The invention relates further to a measuring device comprising an above described arrangement.