Question

in the process called Fluorescence, some substances absorb photons of one kind of radiation, and give out photons of a different kind. When infrared radiation falls on these materials, visable light is not emitted. Why not?

is it to do with the degree of which the photon increases the fluorescence energy level?

The energy given out has to be less or equal to what was received. long wave light energy of less energy than infrared cannot be seen because it is below the energy spectrum our eyes are sensitive too.

Thank you