What Are Absorbed Power Density and Absorbed Energy Density?
How Is Superficial RF Energy Absorption Expressed?
Absorbed power density and absorbed energy density describe RF energy absorbed in superficial body tissues per unit surface area. They are useful when penetration is shallow and a surface-related quantity represents localized heating more appropriately than a mass-averaged quantity.
Absorbed power density is the rate at which RF energy is absorbed per unit area. It is expressed in watts per square meter (W/m2). Because it is a rate, its assessment includes the specified spatial area and time-averaging interval.
Absorbed energy density is the energy absorbed per unit area over a stated exposure interval. It is expressed in joules per square meter (J/m2). For a constant absorbed power density, absorbed energy density increases in proportion to exposure time.
These absorbed quantities are not the same as incident power density or incident energy density measured outside the body. Some incident energy is reflected, and the fraction entering and being absorbed by tissue depends on frequency, angle, polarization, body shape, and tissue properties.
RF exposure standards can use absorbed power density for time-averaged localized exposure and absorbed energy density for brief or time-integrated exposure. The applicable frequency range, averaging area, exposure duration, and limit must be taken from the relevant standard.
Determining these quantities generally requires dosimetry. Validated computational models, laboratory measurements, or conservative relationships from external reference levels may be used, depending on the source and compliance method.
Clear terminology matters because incident and absorbed quantities have different meanings. A value should always be reported with its units, averaging area, time basis, exposure category, and whether it refers to fields outside the body or energy absorbed by the body.
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