There are two forms of 'calibration' service.
1. Calibration confirmation aka Confidence check.
The service provider owns a number of Calibrated Black Body thermal sources. The camera is checked against the sources and an accuracy report produced. No actual calibration is carried out so the camera remains unchanged.
This is what I carry out on my cameras using my own Black Body thermal sources.
2. Full Calibration including repair and certification.
The service provider runs calibration software, specific to a camera, and a set of Calibrated Black Body thermal sources to calibrate the camera. Such full calibration checks and adjusts the cameras calibration tables against known accurate sources. It will also often include updating of the dead pixel maps and NUC tables. After such full calibration, the camera performance is certified as meeting the OEM original specification for that model of camera.
The full calibration is time consuming and so expensive. Many users opt for the simpler and so cheaper confidence check and associated report.
As a side note, the FLIR and NEC industrial thermal cameras that were available in the late 1990's have an excellent reputation for meeting OEM specified calibration accuracy over many years, and even decades. This may be due to the quality of components used and temperature stabilisation employed on the microbolometer. More recent offerings in the thermal camera market are apparently less stable in terms of calibration and an annual calibration check is recommended.
I have a TESTO thermal camera that has developed three dead microbolometer pixels. Having spoken to several calibration houses it is apparent that the only way to get the dead pixels updated in the dead pixel map is to have a full and expensive calibration by a TESTO service agent. Most generic calibration houses do not have access to the required calibration and dead pixel map editing software
The full calibration routine for an NEC AVIO industrial thermal camera is quite tedious. The calibration software forces the technician to go through a set routine with appropriate thermal sources and only then can the NUC and dead pixel map be updated. Updating the maps is done manually using pixel co-ordinates. A long winded and time consuming task. Hence it is expensive and why confidence checks are normally carried out rather than a full calibration.
Fraser