Has anyone used the thermocouple attach point? My understanding is that it is a way to measure the temperature.
I havent tried using a thermocouple but I have experimented with a very accurate infrared thermometer. First thing I noticed is that you can't get the temperature of the surface of the cob when it is turned on because it does create some infrared light of its own. You have to switch it off and take the temp immediately.
Another way to estimate the junction temp is by measuring the ambient temp and forward voltage at ambient and then letting the junction temp rise and remeasuring the forward voltage. If you have an accurate voltmeter, that gives you a good estimation of the junction temp if you know the coeeficient, which we do. For the CXA3070, for every .1 volts the vF falls, the junction temp has increased by 1 degree Celsius over ambient.
The Tc (case temp) is assumed to be about 30C lower than the Tj (junction temp). You can see this demonstrated in the Cree manual on this graph. They set the Tj 85C, 1925mA test current as 100% on the graph and the 100% intersection on the graph is at Tc 55. Therefore junction temp in this case is 30C above the case temp.
Of course if you are running softer the difference between case temp and junction temp would be much less. In my case I am aiming for Tj of 50C or less. Since it is winter my ambient temps are very reasonable ~21C and my Tj has been as low as 40C. Hopefully I can stay under 50C during the warm season.