The protagonist in my world has its eyes and subsequently its brain altered (genetic alterations from conception) in such a way as to allow it to see into the UV and IR range via the addition of new cones with proteins sensitive to these wavelengths, The IR cones are sensitive to long wavelength IR and it's sensitivity trails off toward medium/short-IR. I don't know the exact wavelengths most suitable for animals and objects in a room. Or say navigating a dark hallway. But you should be able to distinguish enough detail to function normaly with this setup? If IR is your only intake. And anatomical changes to the eyes, mainly placing the photoreceptors on the opposite side they lie in modern humans, removing the species blind spots as the nerves aren't routed through a gap in the photoreceptors and reducing the fluid(I assume) light has to travel through.
Now assuming the brain has been changed to allow it to process these new colors without greatly reducing general intelligence by some means--perhaps limiting visibility of some wavelengths, which may indeed be a separate post, there remains a substantial problem with 'thermal' medium and long wavelength IR vision in a warm blooded biological organism, their own bodies would glow and particularly their own eyes which would make seeing a great difficulty.
Now I've supposed some of solutions of my own. But I find myself unable to properly justify them.
The humanoids body operates slightly below room temperature. How could I justify such a cold human like this if possible and what would the consequences be? a-1. Could it be achieved without greatly slowing the metabolism?
The eyes are cooled more than the rest of the body, below 20°C. While the rest of the body is still cooler. Not sure if this would be possible to specifically target and cool the eyes with biology. I figure this could allow for the body to be warmer if below room temperature is too cold to operate at 24/7.
3 The humanoid has glands similar to the pit viper for detecting the thermal IR wavelengths. Although it would sacrifice resolution. Perhaps located in its faces. I'm still uncertain if the output of these organs would be translated as visual perception on top of its eye output or some separate vision, or something else? There is also a limited amount of space for the organs to be placed, and consequently limited detection.
I've also considered that the 'thermal' range of IR 1,000 nm to 14,000 nm doesn't pass through water very well. And the eyes are filled with it. But I've had a hard time finding a suitable replacement to fill the eye.
I'm not sure if moving the photoreceptors as before mentioned alleviates this problem either.
I know its possible for some humans such as the Australian Aborigines to survive much colder temperatures more comfortably than the average man see: aborigines sleeping at lower temperatures Clearly the body can operate in an 0°C environment with their mutations, but their internal temps the study doesn't say. But this is when sleeping so it might be different than what I'm positing.
If I'm missing any other problems with IR vision disadvantages or something else. I'll update my post accordingly.
TL:DR a biologically plausible explanation for a human that has a colder body temperature than normal. Cold enough to see objects at room temperature in Infrared. The body has to be colder than 20°C perhaps by a few degrees. How much lower I'm not completely sure.