Infrared Heat is the therapeutic warmth you feel from the Infrared Energy or Light of the sun.
Radiant heat is a form of energy that heats objects directly through a process called conversion, without heating the air in between. Radiant heat is also called infrared energy or IR.
The electromagnetic spectrum is divided into three segments by wavelength, measured in microns or micrometres.
The Far Infrared segment of the electromagnetic spectrum occurs just below, or "Infra" to, red light as the next lowest energy band. The "Far" part of infrared light is the longer wavelengths.
This band of light is not visible to human eyes but can be seen by special cameras that translate infrared into visible colours. We can, however, feel this type of light, which we perceive as heat.
The sun produces most of its energy in the infrared segment of the spectrum. Our atmosphere has a "window" in it that allows infrared rays, in the 7 to 14 micron range, to reach the Earth's surface safely. When warmed, the earth radiates infrared rays in the 7 to 14 micron bands with its peak output at 10 microns.