In the case of thermographic measurements, however, the problem is much more serious: since only part of the radiation reaches the sensor, generally only a smaller amount of radiation is used to calculate the temperature and therefore overall lower temperatures are measured than the actual ones. Local maximum values are therefore generally lower than the real maximum object temperatures. However, radiation from pixels with higher temperatures also reaches surrounding pixels with lower temperatures due to poor focusing. For these pixels, a temperature value is calculated based on a part of the own radiation and a part of the radiation of the warmer neighbouring pixel, which can therefore also be higher than the actual pixel temperature.
Effect of inaccurate focusing on the temperature readings:
Measurement Error due to Non-observance of the Geometric Resolution
As already mentioned with the camera parameters, in addition to the thermal resolution, the geometric resolution also has a considerable influence on the image quality and accuracy of the measurement results that can be achieved with a thermographic system.
The size of the individual measuring spot assigned to each pixel can be determined for a given object distance using the numerical information for the parameter IFOV "Instantaneous Field of View". However, since it is not known where exactly this measuring spot is located, there is a risk that it is only partially located on the object. To determine the smallest possible measurable object size, the above value of the measuring spot dimension must therefore be doubled.
The above consideration assumes that the individual detectors join each other on the detector matrix without gaps (this would correspond to a fill factor of 1). However, the FPA detectors have a gap between the individual detectors due to technological necessity. Smaller errors in the optical imaging must also be expected. Therefore, for safety, instead of doubling, one applies a tripling of the single spot diameter to determine the minimum object size.