An apparatus for optical measurement of substance concentrations has at least one transmitter arranged in or on a housing and at least one receiver for optical radiation, and a deflection device, which is at a distance from the at least one transmitter and the at least one receiver and is arranged within the substance when the apparatus is being used correctly, for deflection of the optical radiation from the at least one transmitter to the at least one receiver. The distance between the deflection device and the at least one transmitter and/or the at least one receiver can be varied by means of an adjusting device.