The invention relates to a method and to an assembly for calibrating devices 11 for detecting blood or blood components in a liquid, in particular dialysate, which devices comprise a light transmitter 17 and a light receiver 18, and an evaluation unit 20 that receives the signal from the light receiver 18 and is designed such that blood or blood components in the liquid are detected on the basis of the weakening of radiation passing through the liquid. The method according to the invention is based on the fact that the calibration of the devices 11 for detecting blood or blood components is carried out without the use of blood. The calibration is carried out using an absorption standard 30, which has predetermined optical properties in relation to the absorption of the light in blood, the absorption standard 30 being arranged in the beam path 19 between the light transmitter 17 and the light receiver 18. The absorption standard 30 makes it possible to identify defined spectral weakening in the light depending on the components of the blood, in particular haemoglobin. Since, by contrast with blood, the absorption standard 30 does not bring about any scattering, meaning that the beam path is influenced in a different way from blood, the calibration is also carried out using a scattering standard 36, which has predetermined optical properties in relation to the scattering of the light in blood. The assembly also comprises a beam deflection unit 22 for coupling out light for a spectral measurement of the light transmitter 17 using a spectrometer 27.
