I'm sure it's OP....
this looks like it's cousin..
http://www.amazon.com/Raytek-ST20XB-.../dp/B000MIRFJY
I guess that is why Sigma had a cone made for the tip......
I remember using a metal probe with a mechanical temp gauge, and touching the glow plug area....and then testing it with my raytek..
It was within 2 degrees......
Is the exergen center point?...