value u which is proportional to the charge is proportional to the energy of the light on the other hand the signal value u can be determined indirectly if the relation between a measured value and the energy of the light arriving on the photo element during its activation is known for example if the photo element includes a condenser that is discharged via a photodiode during the activation of the photo element the measured value is a voltage that is approximately inversely proportional to the energy of the light arriving on the photo element during its activation detection optics 21 are arranged in front of the photo element 16 in order to image a field of view 24 onto the photo element 16 illumination optics 20 are arranged in front of the light source 15 in order to shape the light emitted by the light source 15 such that an illumination area 23 can be illuminated by the light source 15 the illumination area 23 and the field of view 24 are shaped such that the field of view 24 is substantially completely covered by the illumination area 23 the distance measuring device 14 is adapted such that the light emitted by the light source 15 impinges onto the object 22 located within.