the invention relates to a method and a device for determining a modification at one object or a modification caused by an object , wherein a measurement of the light propagation time is also possible . according to the invention , an amplitude control regulates a transmitter light source and a reference light source on the basis of a control value in such a way that the signals from the corresponding associated light paths have approximatively the same value at the input of a comparator . as a result , an alternative detection of extraneous light is possible . in addition , a phase regulation for measuring the light propagation time can be provided .