preferably used near infrared range nir windows consisting of quartz or sapphire may be used the optical window 44 is enclosed by a coupling structure 45 which corresponds with the coupling structure 25 of the sensor head 10 i e in the present example it is configured according to the tri clamp standard of course it is also possible to use any other type of coupling which in particular ensures rigid coupling between the optoelectronic module 40 and the sensor head 10 screw and bayonet coupling systems may be mentioned purely by way of example rigid coupling becomes particularly important in the preferred cases of free beam input and output of light respectively to and from the sensor head such a case is outlined in fig3 a light source 48 powered by an electrical supply unit 46 and in particular with an emission spectrum in the nir generates illumination light which is guided to the optical window 44 by beam optics in the case represented by means of a deflection mirror 50 and shines through it the illumination light strikes the reflector disk 16 which is preferably formed as a lambertian scattering disk but may also be configured in a different way for example as a mirror the light cast back by the reflector disk 16 passes through the optical window 44 once more this time in the opposite direction and is received in the solid angle 52 to the numerical aperture of the of the fiber optic entry 54 of a spectrometer 56 this structure allows a transflection measurement of a medium which lies between the optical window 44 and the reflector disk 16 in the final assembled state the flow chamber 34 of the sensor head 10 lies at precisely this.