A sensor for monitoring a monitored area, having a transmitting unit which emits radiation, wherein the sensor is capable of detecting objects in the monitored area. According to the invention, the sensor comprises means, using which the instantaneous and/or average power per unit area applied to a detected object of energy incident on the object, such as electromagnetic radiation of the transmitting unit, can be determined. In addition, adaptation means are provided in order to not let the instantaneous and/or average power per unit area of energy incident on the object, such as electromagnetic radiation of the transmitting unit, exceed a predetermined value during the detection of an object in the monitored area, the sensor being configured to first measure whether objects are located in a predetermined proximal area to the transmitter unit using a comparatively low power which can create no or no noticeable hazard potential for people.