What's FIR (Far Infrared Ray) heating? This is a type of electro-magnetic light energy from the sun. It falls within the same family of infrared rays in the light spectrum, but due to its longer wavelengths, FIR cannot be seen by human eyes, just like the ultraviolet rays and x-rays. This radiant energy is absolutely essential for human beings, and all livings things in the world. The research on biology shows that the FIR rays wavelengths between 5.6um and 15um are best absorbed by the human body.