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Other than all other forms of energy, electromagnetic emission, including within the optical range, can be sent on purpose for charging applications, as outlined in the introduction of this book by Joe Paradiso. This section focuses on the use of radio frequencies (RF) based on antennas (radio frequency harvesting).

In a simplified model, the efficiency ε_RF of these systems is calculated from

(2.14)

The output power thus depends on the emitted power, its distance to the RF harvester, and amount of damping in between. How much power the harvester can receive and convert to usable electric power, depends on the specific antenna design, its damping, and the rectification losses. Recent reviews on this topic have been provided by Serdijn et al. and Cansiz et al. [39, 40].

The inductive charging of electric toothbrushes is a daily life example of such systems.