In their normal habitat, the natural stone provides plenty of low-level radiation that allows them to see in that environment as if in bright daylight. Obviously, when travelling outside of this terrain, they will want to take with them a mildly radioactive substance as a "lantern" (they are more resistant to radiation than other species).
The primary benefit is that they would be able to somewhat see through any material through which radiation will pass, which means that it will be harder to hide from them. Also, it means that they could see in pitch black, given the right "light source".
What limitations can you foresee? I'm pretty sure a vision mode like this means that the race would not even be able to see something like a computer monitor or television (Bit of a long story on why this is relevant. In summary, there's an advanced alien species in my game that periodically experiments on the party by abducting them and forcing them to fight strange creatures and species that were also abducted. The party doesn't know it yet, but most of the dungeons they have gone through are purely mental simulations).
I'm asking this because I'd like to make them a playable race, but my knowledge of radiation is limited. I don't just want to give them darkvision and be done with it.
EDIT: I think I am going to include a cheap magic item that "translates" visible light into a form visible to the race. This should help overcome any of the major problems.
