Power Density (W/m²)

Power density, often denoted S, is the rate at which electromagnetic energy flows through a unit area perpendicular to the direction the wave is travelling. The standard SI unit is watts per square meter (W/m²); milliwatts per square centimeter (mW/cm²) is also widely used in U.S. regulatory documents (1 mW/cm² = 10 W/m²).

For a plane wave in the far field, power density relates to the electric field strength E (V/m) and the impedance of free space (Z₀ ≈ 377 Ω) by S = E² / Z₀. Equivalently, S = H² · Z₀, where H is the magnetic field strength in A/m. These conversions are valid only in the far field, where E and H are in phase and perpendicular to each other and to the propagation direction.

Power density is the regulatory metric used for far-field RF exposure assessment. The FCC's general-population exposure limit for the cellular and Wi-Fi bands is on the order of 1 mW/cm² (10 W/m²), with the precise value frequency-dependent. ICNIRP's 2020 guidelines specify reference levels in W/m² that scale with frequency between 100 kHz and 300 GHz.

Power density is also the natural quantity for applying the inverse square law: as distance from a point source increases, power density falls as 1/r². Doubling the distance from a source quarters the power density.

For near-field exposure — typically within a wavelength of the source — power density is not a clean metric, because the field structure is dominated by reactive components rather than radiating ones. SAR (W/kg) is used instead for near-field compliance, particularly for handheld devices.

RADIHALT designs EMF protection blankets built around woven copper-nickel Faraday fabric. The terminology on this page — from Faraday-cage physics through attenuation figures and ICNIRP exposure limits — is what underpins the engineering and the claims we publish about our products.

We try to keep our marketing language tied to the same vocabulary regulators and physicists use. If a definition on this page conflicts with anything on a RADIHALT product page, the glossary entry is the source we hold ourselves to.