Thank you for all information
We still have a doubt about the size of calibrator aperture. The first question is, do you use something to change the aperture of blackbody furnace? When you said “The diameter of a view is 16mm…”, Did you talk about “calibrator aperture” or the measuring area that pyrometer see?
Let me try to explain what would be the “calibrator aperture”, It’s a metal circle placed in front of the hole of blackbody furnace to change the aperture diameter (as shown in the figure below).