Does the spot meter use average of pixels, and how many?
The spot meter uses the value from one pixel only, the center of the spot marker. It is not a calculated average of several pixels. This is the case both in the cameras and in analysis software like ThermaCam Reporter.
Due to optical dispersion, radiation from a very small area will not give one detector element enough energy for correct value. We recommend making sure that the hot area where the spot value is requested is at least 3×3 pixels. In many cameras and PC softwares, the spot marker has a shape to help estimating required size.
The same principle is valid also for measureing area max: A hot spot smaller than 3×3 pixels will give too low result.