The Influence of Distance and Focus on the Calibration of Infrared Radiation Thermometers

摘要

If a radiation thermometer is calibrated by measuring the temperatures of two cavities having different geometries, sometimes discrepancies arise between them, even though their emissivities are close to that of a blackbody. The origin of such discrepancies may result from the size-of-source effect, and in the distance-to-target effect for those thermometers that offer focusing capability. Examples include: (a) out-of-focus image changes the reading: different focus settings produce different results and (b) measurements taken at different distances produce different results. These effects are discussed, their contribution to the measurement uncertainty is evaluated, and some recommendations are made for practical blackbody cavities or radiators to reduce such effects.