I just saw a TI paper (ref below), and my summary from that (similar to the points above):
- order of accuracy (paper date 2016): IR camera, fluor-optic probe, thermocouple, IR gun (max field of view 4mm diameter)
- thermocouple: 36 to 40 gauge J or K wire. It is important for wire to be thin, 36+ gauge.
- it is better to attach the thermocouple with a thermally conductive epoxy, taping is not recommended.
- when using an IR camera or IR gun, reading should be corrected for emissivity of the surface.
More info on section 3.1 Case Temperature Measurement of :
https://www.ti.com/lit/an/spra953c/spra953c.pdf