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 first few hydrogen atom electron orbitals shown as cross-sections with color-coded probability density. ... In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks. ... The velocity of an object is simply its speed in a particular direction. ... In physics and in vector calculus, a spatial vector is a concept characterized by a magnitude, which is a scalar, and a direction (which can be defined in a 3-dimensional space by the Euler angles). ... In physics, power (symbol: P) is the rate at which work is performed. ... Area is a physical quantity expressing the size of a part of a surface. ... A wave is a disturbance that propagates through space, often transferring energy. ... A schematic representation of hearing. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. ...

In physics, the word "intensity" is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match. Amplitude is a nonnegative scalar measure of a waves magnitude of oscillation, that is, magnitude of the maximum disturbance in the medium during one wave cycle. ...

If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity drops off in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies. 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, i.e. 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. (4πr² 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.

Photometry and radiometry

In photometry and radiometry, intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where "intensity" can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists. Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. ... In telecommunication and physics, radiometry is the science of radiation measurement. ... A solid angle is the three dimensional analog of the ordinary angle. ... Table of Opticks, 1728 Cyclopaedia Optics ( appearance or look in ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter. ... In physics, intensity is a measure of the time-averaged energy flux. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. ... Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. ...

See also

• Sound intensity

[edit] The sound intensity, I, (acoustic intensity) is defined as the sound power Pac per unit area A. The usual context is the measurement of sound intensity in the air at a listeners location. ...

[edit] Cover of brochure The International System of Units. ... luminous energy : visible radiant energy, visible light ; units are lumen*seconds or Talbots (T = lm*s) ; lumen = candela*steradian ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... In photometry the Talbot (T) is a nonstandard unit of luminous energy. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The candela (symbol: cd) is the SI base unit of luminous intensity (that is, power emitted by a light source in a particular direction, with wavelengths weighted by the luminosity function, a standardized model of the sensitivity of the human eye). ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... This article is about the unit of measure. ... Illuminance is the total luminous flux incident per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... In photometry, illuminance is the total luminous flux incident on a surface, per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... Luminous efficacy is a measure of the proportion of the energy supplied to a lamp that is converted into light energy. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... Luminous flux or luminous power is the measure of the perceived power of light. ...