What is the threshold for radiant?

Excuse me, could you please elaborate on the term "threshold for radiant"? It seems to be a somewhat unconventional phrase, and I'm curious to understand what context it's being used in. Is it referring to a specific scientific or technical measurement related to radiation? Or perhaps it's a metaphorical term used in a particular field or discipline? Could you provide some context or examples to help clarify its meaning? Thank you for your time.