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Size-of-source effect in radiation thermometers

The size-of-source effect (SSE) is a significant contributor to error and uncertainty in radiation thermometry measurements in both laboratory standards and industrial temperature measurement.

The SSE originates from diffraction and scattering of radiation, and from imperfections and aberrations within the optical system of the radiation thermometer. The result is that the exact field of view of the thermometer is not well defined – some radiation from within the nominal field of view is lost and some radiation from outside the nominal field of view is detected. Thus, the measured signal is dependent on the size of the target and the radiance distribution surrounding the target.

The SSE can be measured by determining the increase in measured signal as the size of a uniform-radiance source is progressively increased. Two methods, known as the direct and indirect methods, have traditionally been used for carrying out these measurements. They are both limited in the amount of detail they can provide about the SSE of a radiation thermometer, particularly if the SSE extends out to large distances beyond the nominal field of view.

Recently, MSL has developed a new SSE measurement technique based on scanning the thermometer across a uniform source. The method is an adaptation of an idea suggested by the Netherlands Measurement Institute, and provides high resolution SSE data with a minimum of experimental effort. It is capable of measuring the SSE out to effectively infinite distance, which is essential for absolute temperature determination using radiometric techniques.

We have also developed a model of the SSE based on the bi-directional scattering distribution function. This model has allowed us to reconcile the two traditional SSE measurement methods (thought to be equivalent but not definitively shown to be) and to derive the exact relationship between them. We have also applied the SSE model to our new scanning method to derive the relationships of this method to the other two methods. This research has established a framework to facilitate further SSE studies.


M Bart, E van der Ham, P Saunders, “A new method to determine the size-of-source effect“, International Journal of Thermophysics, 28, 2111–2117, 2007.

P Saunders, H Edgar, “On the characterisation and correction of the size-of-source effect in radiation thermometers”, Metrologia, 46, 62–74, 2009.