Below violet.
"Dark radiant heat begins at absolute zero temperature, and extends through light, chemical rays, actienic rays, and infra-violet rays, up to the dissociation of all molecules to the 63rd octave." (Keely, pre1894) [Law of Oscillating Atoms]
See Also
Infrared
infrasound
Law of Oscillating Atoms
Ultraviolet
Violet
Violet Flame
Violet Ray Machine