You're not really seeing the thermal IR part of the spectrum in your kodak HIE images. Thermal infrared involves much longer wavelengths (far IR) that this film simply isn't sensitive to. I think HIE drops off in sensitivity somewhere in the mid 900nm range, which is in the near-IR range. In the grand scheme of EM radiation it's practically visible light.
The human body heat shows up around the 10 to 15 micron range [10,000nm to 15,000nm] well beyond the range of IR film.
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I leave the digital work for the urologists and proctologists.