How do the distance and angle affect the thermal imaging temperature measurement?

Without considering atmospheric transmittance and radiative transmission loss,

A.    To the source point, the irradiance of the object at the source point is inversely proportional to the square of the distance. Although the radiation intensity of the source point does not change, the field angle between the source point and the thermal image becomes smaller as the distance between them increases. The figure below shows the relationship between the irradiance of the source point and the distance.

Thus, when the distance increases, the measuring temperature will get obviously lower.

A.    To the plane source object, if there is no angle between the target light source and the thermal imager at the same plane, and the instantaneous field of view in the measurement system is full of radiation energy, the distance change will make no difference in the result of thermal imaging temperature measurement.