The optical system being used represents the start of the measurement chain. A lens system focuses the ੂthermal radiation emitted by an object onto a detector. The important thing here is to ensure that the measurement object is larger or the same size as the sensor’s field of view. The optical resolution of the measuring system E:M is defined by the ratio of the size of the measuring spot to the distance between the detector and the measurement object (E:M = distance from measurement object to measurement device: measurement spot diameter). The optical resolution and, consequently, the local temperature resolution improves as the ratio increases.