edVirtus Reference

What Are Incident Power Density and Incident Energy Density?

How Do These Quantities Describe RF Energy Arriving at the Body?

Incident power density describes the radio-frequency power arriving at a surface per unit area and is expressed in watts per square meter (W m⁻²). Incident energy density describes the incident energy accumulated over a stated interval per unit area and is expressed in joules per square meter (J m⁻²).

Incident quantities describe the external field before interaction with the body. They must not be confused with absorbed power density or absorbed energy density, which describe power or energy absorbed at or near the body surface and are internal or surface dosimetric quantities.

Incident energy density is the time integral of incident power density. If incident power density remains constant, incident energy density equals power density multiplied by exposure time. The stated interval is therefore essential when an energy-density value is reported.

In a uniform far-field plane wave, incident power density can be related to electric- and magnetic-field strength through the wave impedance of free space. These relationships may be unsuitable in the reactive or radiating near field, where the fields can vary independently and rapidly with position.

Exposure standards specify the frequency range, averaging area, averaging time, and treatment of brief or pulsed exposure for these quantities. A point maximum, a spatial average, a long-term average, and a short-interval energy density are not interchangeable results.

Incident power density and incident energy density may be used as reference-level quantities because external fields can often be measured or calculated more readily than absorption in the body. Exceeding a reference level normally requires further assessment and does not by itself prove that a basic restriction has been exceeded.

A reported value should identify frequency, polarization, direction, evaluation surface, spatial sampling and averaging, time interval, operating conditions, source contributions, method, and uncertainty. These details determine whether the result can be compared validly with the applicable exposure restriction.