The non-contact sensors of the M18T series are mainly passive receivers. The thermal radiation of an object within a wave length range of 8 to 14 µm is transformed into an electrical signal via a thermopile and then conditioned as an output signal. The D:S (distance: spot) ratio, which specifies the measuring field diameter at a defined distance, is important in this context. Optimal configuration is achieved if the measuring field is completely covered by the target surface, the temperature of which is to be monitored