  Quantitative thermal imaging has the potential of reliable temperature
measurement across an entire field-of-view. This non-invasive technique has
applications in aerospace, manufacturing and process control. However, robust
temperature measurement on the sub-millimetre (30 {\mu}m) length scale has yet
to be demonstrated. Here, the temperature performance and size-of-source
(source size) effect of a 3 {\mu}m to 5 {\mu}m thermal imaging system have been
assessed. In addition a technique of quantifying thermal imager non-uniformity
is described. An uncertainty budget is constructed, which describes a
measurement uncertainty of 640 mK (k = 2) for a target with a size of 10 mm.
The results of this study provide a foundation for developing the capability
for confident quantitative sub-millimetre thermal imaging.
