I understand certain phosphors will absorb ultraviolet light and then re-emit in the visible spectrum, for example "glow in the dark" paint and toys. Are there any material that go the other way around ? If yes, are any of them strong enough to produce visible reactions solely based on the amount of radiation emitted by the human body ?
If the emitted photon has more energy, the energy difference is called an anti-Stokes shift; this extra energy comes from dissipation of thermal phonons in a crystal lattice, cooling the crystal in the process. Yttrium oxysulfide doped with gadolinium oxysulfide is a common industrial anti-Stokes pigment, absorbing in the near-infrared and emitting in the visible portion of the spectrum. (https://en.wikipedia.org/wiki/Stokes_shift)