Medical thermography has become ever more accessible to hospitals, medical research, and clinical centers with the new generation of thermal cameras, which are easier to use and lower in cost. Some diagnostic techniques using thermal cameras are now regarded as standardized, such as the cold challenge test for Raynaud’s phenomenon. The future for medical thermography appears to be improved accuracy, standardization, and establishment as a mainstream medical imaging methodology. Medical thermography standardization, quantitative measurements, image comparison, and multi-center research trials all require thermal cameras to provide a demonstrably traceable, accurate, and reliable temperature output. To this end, the National Physical Laboratory (NPL) has developed a multi-fixed-point source that serves as an in-image calibration system, thereby providing a reliable means for radiometric image validation. An in-field-of-view fixed-point validation system for thermal imaging has successfully been developed, tested, and validated at NPL and has undergone field trials at three clinical centers in the UK. The sources use the phase change plateaux of gallium–zinc eutectic, gallium, and ethylene carbonate. The fixed-point sources have an estimated cavity emissivity of greater than 0.998, a plateau longevity of nominally 3 h at ambient conditions, a stability of 0.1°C, or better, over that period, a repeatability of 0.1°C or better, and an estimated temperature uncertainty of ±0.4°C (k = 2). In this article, the source specifications and design as well as testing, validation, and field trial results are described in detail.