Long invisible rays, below the visible red end of the colour spectrum, used to produce heat in body tissues in the treatment of traumatic and inflammatory conditions.
An invisible heat ray from beyond the red end of the spectrum. Infrared wavelengths range from 7700 angstrom units (A.U.) to 1 mm. Longwave infrared rays (15,000 to 150,000 A.U.) are emitted by all heated bodies and exclusively by bodies of low temperature such as hot water bottles and electric heating pads; short-wave infrared rays (7,200 to 15,000 A.U.) are emitted by all incandescent heaters. The sun, electric arcs, incandescent globes, and so-called infrared burners are sources of infrared rays.