In physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light).

The word "intensity" in physics is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is physical nonsense.

If a point source is radiating energy in three dimensions, and there is no energy lost to the medium, then the intensity drops off as the distance from the object squared. The reason for this is one part physics, one part geometry. The physics comes from the conservation of energy (i.e. if the energy isn't being lost to the medium, it can't just disappear). The consequence of this is that the net power coming from the source must be constant, thus:

where P is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. That P is a constant. If the source is radiating uniformly, that is, the same in all directions, and we take A to be a sphere centered on the source (so that I will be constant on its surface), the equation becomes:

where I is the intensity at the surface of the sphere, and r is the radius of the sphere (note: enclosed in parentheses is the expression for the surface area of a sphere). Solving for I, we get:

Anything that can carry energy can have an intensity associated with it.

If the medium is damped (i.e. both sound and light in air slowly lose energy), then the intensity drops off more quickly than the above equation suggests.

"No energy can get lost", not even slowly.

See also

SI light units
Quantity	SI unit	Symbol	Notes
Luminous energy	joule	J
Luminous flux	lumen or ( candela · steradian )	lm	also called Luminous power
Luminous intensity	candela	cd
Luminance	candela / square metre	cd/m2	also called Luminosity
Illuminance	lux or (lumen / square metre)	lx
Luminous efficacy	lumens per watt	lm/w

SI radiometry units
Quantity	SI unit	Symbol	Notes
radiant energy	joule	J	energy
radiant flux	watt	W	radiant energy per unit time, also called radiant power
radiant intensity	watts per steradian	W · sr-1	power per unit solid angle
radiance	watts per steradian per square metre	W &middot sr-1 · m-2	power per unit solid angle per unit area
irrradiance	watts per square metre	W · m-2	power per unit area, also called radiant emittance or radiant exitance
spectral radiance	watt per steradian per cubic metre	W · sr-1 &middot m-3	commonly measured in W sr-1m-2nm-1
spectral irradiance	watt per cubic metre	W · m-3	commonly measured in W m-2 nm-1