- Electromagnetic (EM) radiation can be thought of as a stream of photons, in which case radiant energy can be viewed as photon energy – the energy carried by these photons.
- Alternatively, EM radiation can be viewed as an electromagnetic wave, carrying energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in quantum field theory (see wave-particle duality).
- Radiant energy includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
- The quantity of radiant energy is measured in terms of radiant flux over time.
- Radiant energy also applies to gravitational radiation. For example, the first gravitational waves ever observed were produced by a black hole collision that emitted about 5.3×1047 joules of gravitational-wave energy.