Abstract: Method for detecting an undesired object or flaw in relation to a background, wherein radiation with a first characteristic is cast by a first radiation source onto and close to the background and the location where the undesired object or flaw is to be expected, wherein radiation generated by the radiation source and reflected, dispersed, diffracted or transmitted by the undesired object is sensed by one or more radiation sensors, and wherein radiation is cast onto the location by a second radiation source with a second characteristic, which is disposed and/or has a second radiation characteristic such that apparent flaws can be removed relatively easily from the image signal sensed by the sensors.