Good morning Daniel. I'll offer the following suggestions with two disclaimers:
1) my climate chamber work is cold, not warm; and
2) though I work with heat and cold and use imaging as one of my sensors, I'm not a thermographer.
That said, I'd suggest that, if you haven't already done so, you discuss your specific application with the manufacturer of your imager. I came to understand that each imager deals with intrinsic sources of temperature error differently, and knowing what happens as you approach the ends of your imager's published envelope can affect how you set up your test.
In one of our low temperature cases, we had to leave the imager powered on in the conditions for 15 minutes to stablise and then had about a 10 minute window where we could use it before exceeding the manufacturer's recommendation for exposure to temp. The concern expressed wasn't damage in my case, it was degradation of the data acquired by the imager.
Second, our chamber itself was rich with with heat and cold sources whose reflections needed to be managed. The smooth metal walls of the chamber didn't help.
I don't know how emissive your target is - in our case, we had bare metal plates for part of things, which required some creativity for dealing with reflections, establishing points of known temperature and high emissivity to 'calibrate' the image in post processing. We developed a combination of targets and logging thermistors that were time sequenced with the imager for some of the more difficult cases and were happy with the result.
As I said, others with better experience and training will have better advice. I hope this helps is some way.
Cheers
-john