FIR rays are a kind of electromagnetic wave. Near infrared (NIR) frequencies approach those of visible light, while FIR frequencies approach those of invisible light at between four and 14 microns.
紅外線是一種電磁波，近紅外線的波長較接近可見光，遠紅外線的則接近不可見光，是波長 4～14 微米的「生育光波」。