Method for operating a water-conducting household appliance with a drum for receiving items (22, 34) to be treated, which has at least one perimeter with at least a radiation passage (30, 35), a control facility, a spectrometer, which comprises at least one radiation source (5, 27), at least one radiation detector (7, 28) and a microcomputer, wherein microcomputer and control facility are designed to exchange information with one another so that data can be exchanged between spectrometer and control facility, characterised in that it comprises the following steps: (a) rotating the drum with a predetermined rotational speed Dz, wherein a frequency f, in respect of the number of rotation cycles of radiation passes (30, 35) per time unit, is defined as the product of the rotational speed Dz with the number of radiation passes (30, 35); (b) continuously emitting radiation from the at least one radiation source (5, 27) of the spectrometer in a wavelength range λ 1 tο λ 2 through the at least one radiation passage (30, 35) of the rotating drum, so that a radiation interrupted periodically with the frequency f is produced within the drum, and the items (22, 34) in the rotating drum are subjected periodically to interrupted radiation with the frequency f and the at least one radiation detector (7, 28) is arranged such that the radiation reflected by the items (22, 34) is registered directly as a measuring signal without passing through the radiation passage (30, 35) and without being interrupted by the frequency f through the radiation passage (30, 35) and a periodic function, with a period length 1/f, in particular a sinus or rectangular function, which is only dependent on the household appliance, is produced by the control facility as a reference signal, wherein the reference signal is determined substantially by the number of radiation passages (30, 35), the geometry thereof, i.e. the size and shape thereof, and the rotational speed of the drum; (c) transmitting the measuring signal and the reference signal to the microcomputer, evaluating the measuring signal and its comparison with the reference signal with an evaluation routine stored in the microcomputer and transmitting the evaluated measuring signal to the control facility; and (d) selecting and/or adjusting an operating program by means of the control facility in relation to the evaluated measuring signal, wherein the steps (b) to (d) are carried out once or repeatedly. (a) rotating the drum with a predetermined rotational speed Dz, wherein a frequency f, in respect of the number of rotation cycles of radiation passes (30, 35) per time unit, is defined as the product of the rotational speed Dz with the number of radiation passes (30, 35); (b) continuously emitting radiation from the at least one radiation source (5, 27) of the spectrometer in a wavelength range λ 1 tο λ 2 through the at least one radiation passage (30, 35) of the rotating drum, so that a radiation interrupted periodically with the frequency f is produced within the drum, and the items (22, 34) in the rotating drum are subjected periodically to interrupted radiation with the frequency f and the at least one radiation detector (7, 28) is arranged such that the radiation reflected by the items (22, 34) is registered directly as a measuring signal without passing through the radiation passage (30, 35) and without being interrupted by the frequency f through the radiation passage (30, 35) and a periodic function, with a period length 1/f, in particular a sinus or rectangular function, which is only dependent on the household appliance, is produced by the control facility as a reference signal, wherein the reference signal is determined substantially by the number of radiation passages (30, 35), the geometry thereof, i.e. the size and shape thereof, and the rotational speed of the drum; (c) transmitting the measuring signal and the reference signal to the microcomputer, evaluating the measuring signal and its comparison with the reference signal with an evaluation routine stored in the microcomputer and transmitting the evaluated measuring signal to the control facility; and (d) selecting and/or adjusting an operating program by means of the control facility in relation to the evaluated measuring signal, wherein the steps (b) to (d) are carried out once or repeatedly.