Device for determining the dew-point temperature of a test gas

The invention relates to a device for determining the dew point temperature of a measurement gas, having a light guide, a condensation surface located on the light guide and whose reflectivity is dependent on the condensation of the measurement gas, a light source for emitting light through the light guide onto the condensation surface, a light sensor for determining the light intensity reflected back into the light guide by the condensation surface and means for adjusting the temperature of the condensation surface, which has a semihydrophobic construction.