Intensitäts

Intensität is a term used across several scientific disciplines to describe the concentration or density of a quantity flowing through a region of space, typically per unit area or per unit solid angle. Because different contexts treat different quantities as the relevant “flow,” the exact definition of intensity is context-dependent. In general, it refers to how much of something passes through a surface or in a given direction.

In optics and radiometry, several related but distinct quantities share the concept of intensity. Radiant intensity (I) is the power emitted by a source per unit solid angle, with units of watts per steradian (W/sr). Irradiance or radiant flux density is the radiant power incident on a surface per unit area (W/m²). Photometric terms introduce luminous intensity (Iv), measured in candela (cd), which represents the perceived power per unit solid angle, taking human eye sensitivity into account. Luminous flux and its distribution over directions are described by these quantities.

In acoustics, sound intensity is the sound power transmitted per unit area, measured in watts per square meter (W/m²). It relates to acoustic pressure by I = p²/(ρc) for a plane wave, where p is pressure, ρ is medium density, and c is the speed of sound.

Mathematically, intensity often scales with the square of a wave’s amplitude, linking it to energy transport. Because intensity depends on geometry and context, it is not a single universal constant but a broad concept used to quantify how strongly a quantity is delivered or distributed in space.