The optical resolution of a pyrometer, infrared temperature sensor or infrared thermometer is usually specified in the measuring distance to spot ratio or D:S, whereby a specification is made either in concrete mm specifications or in a ratio (e.g. 25:1).

An optical resolution of 100:10mm (e.g. with the IR502G sensor) means that at a measuring distance of 100mm from the sensor optics, a measuring spot with a diameter of 10mm is given. Other designations are also measuring distance to measuring spot size and optical detection range of the sensor.

When selecting the appropriate sensor optics, make sure that the measurement object completely fills the measurement spot of the sensor.